**5. Genetic risk factors in sporadic AD**

Among all identified genetic factors involved in the disease, APOE gene has been extensively studied over the past decade or so. APOE is a major risk gene associated with AD, is located on chromosome 19 [75], and encodes for apolipoprotein E, a 34-kDa lipid binding protein involved in triglycerides and cholesterol transport [76–81]. Three ApoE isoforms which differ by single amino acid substitutions have been found in humans: ApoE2, ApoE3, and ApoE4. ApoE4 carriers present with high levels of cholesterol and LDL in the plasma, which predis‐ pose the carriers to cardiovascular disease and AD [82]. ApoE4 carriers contribute to about 50% of AD cases. While ApoE2 decreases the risk and delays the onset of AD [53], ApoE4 multiplies the risk of EOAD and late-onset AD (LOAD) by approximately 3-fold for hetero‐ zygous and 10-fold for homozygous carriers [62]. ApoE4 is shown to increase amyloid plaque formation by altering Aβ clearance [83] and promoting fibrillary aggregations [84].

New methods of genetic mapping of single-nucleotide polymorphisms (SNP) provided new information regarding genes involved in increasing or decreasing the risk of AD. Besides wellestablished AD risk genes described above, nine additional AD-related genes including complement receptor 1 (CR1), bridging integrator 1 (BIN1), clusterin (CLU), phosphatidyli‐ nositol-binding clathrin assembly protein (PICALM), MS4A4/MS4A6E, CD2AP, CD33, EPHA1, and ATP-binding cassette transporter (ABCA7) have been unveiled by genome-wide association and sequencing studies [85, 86].

The CR1 gene polymorphism rs6656401 was the first found to be associated with AD in European population [85]. A study conducted on two Canadian cohorts further showed that polymorphisms in CR1 (rs6656401 and rs3818361), BIN1 (rs7561528), and CD33 (rs3865444) are highly associated with LOAD [87]. The CD33 polymorphism seems to provide neuropro‐ tection against AD through inhibition of CD33 expression with a subsequent decrease in brain Aβ42 levels [88].

Recent studies have also identified a risk gene: the triggering receptor expressed on myeloid cells 2 (TREM2) encodes an immune receptor preferentially expressed in microglia, which are involved in inflammation and phagocytosis [89–91]. TREM2 is localized in cerebral regions where AD pathologies exhibit [90, 92, 93]. Some evidence suggests that activated TREM2 is involved in Aβ clearance during AD [94, 95]. A study reported that TREM2 mutations prevent its physiological function in Aβ clearance [90]. Autosomal recessive loss-of-function mutations of TREM2 are associated with increased risks of AD leading to development of early-onset dementia [96].

TREM2 receptor is found to be cleaved in the ectodomain to release a soluble fragment (sTREM2) that is detectable in cerebrospinal fluid (CSF) [96, 97]. A cross-sectional study reported that sTREM2 levels in the CSF of AD patients were higher than in controls while TREM2 variant (R136Q, D87N, Q33X, or T66M) carriers exhibited lower levels of CSF sTREM2. Interestingly, R47H variant carriers displayed significantly higher levels of CSF sTREM2 than non-carriers, suggesting that this variant different from other variants, increases AD risk through a mechanism not necessarily involving TREM2 protein expression. This study also showed that elevated sTREM2 levels in the CSF were strongly correlated with levels of tau and hyper-phosphorylated tau but not with Aβ42 levels [96, 98]. These observations have been reproduced by another study [97], implicating that elevated CSF sTREM2 levels could be used as a potential biomarker for early symptomatic phase of AD.

ATP-binding cassette subfamily B member (ABCB) gene has been reported to play a role in AD pathogenesis as well. ABCB encodes P-glycoprotein (P-gp) which is essential for Aβ clearance, and its inhibition as a consequence of genetic polymorphisms prevented Aβ clearance in an AD mouse model [99]. Decrease in P-gp expression levels was found to correlate with increased Aβ deposition [100].

Finally, a recent meta-analysis in combination with sequencing study identified five novel genes associated with AD: HLA-DRB5–HLA-DRB1, PTK2B, SORL1, SLC24A4-RIN3, and DSG2 [101]. Their functional roles in disease mechanisms are yet to be characterized.


*Edited by Paul Bracken*

There continue at present many developments in the area of quantum mechanics and quantum dynamics in particular, of a very fundamental nature, all the way from implications for the foundations of physics to the influence of quantum mechanics on emerging technologies, such as the areas of quantum semiconductors and quantum computing, both of which are very important examples. It is hoped that the papers in this volume will be able to provide a much needed resource for researchers with regard to current fields of research in this dynamic area.

ISBN 978-953-51-2485-6

Research Advances in Quantum Dynamics

Photo by BrianAJackson / iStock


**1. Introduction**

Hydrogen can be a promising energy carrier for future energy economy [1,2], which, of course, involves changes in the transport infrastructure [3,4] and the adaptation of the entire energy system to the properties of hydrogen. One of the most important challenges is hydrogen storage [5]. In this context, the solid-state alloys that allow for reversible hydrogen storage [6] belong to the class of materials that can be suitable for application.

The titanium-zirconium-based alloys are the second largest class of solids, for which quasicrystallinity was found, e.g., TiZrNi [7] or TiZrFe [8–10]. The Ti-Zr compounds are promising candidates for many applications such as biomedical [11–13], filler metal [14], medium entropy alloys [15], bulk metallic glasses [16], dental implant [17], shape memory alloys [18], and high-entropy alloys [19]. The TiZrNi quasicrystal seems to be especially suitable for hydrogen storage due to its large hydrogen uptake capacity [20,21]. In this case, the hydrogen atoms are situated preferentially near Ti and Zr atoms in the quasicrystal lattice.

**Citation:** Zywczak, A.; Gondek, Ł.; ˙ Czub, J.; Janusz, P.; Selvaraj, N.B.; Takasaki, A. Physical Properties of Ti45Zr38Fe17 Alloy and Its Amorphous Hydride. *Energies* **2022**, *15*, 4236. https://doi.org/10.3390/ en15124236

Academic Editor: Giovanni Esposito

Received: 29 March 2022 Accepted: 30 May 2022 Published: 9 June 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

The chemical-physical properties of the Ti45Zr38Ni17 compositions can be changed by substituting other elements such as Ag [22], Pd [23–25], V [26], Co [27], Cu [28], Li [29], Ce [30], Mg [31], and Fe [32] in order to increase hydrogen sorption.

Since the discovery of quasicrystals, magnetic properties have not been studied much due to the lack of translational symmetry required for establishing long-range magnetic order. However, some quasicrystals exhibited spin-glass-like behavior [33]. Recently, the quasicrystals showing both ferromagnetic and antiferromagnetic behavior have been studied [34–38]. The magnetism in the TiZrNi quasicrystals was highly demanded [39]. The ferromagnetic signal for the Ti45Zr38Ni17 originated from nickel nano-cluster precipitations in the quasicrystalline alloy [40].

The present work aims at tracking the transformation of the Ti45Zr38Fe17 from the amorphous to the quasicrystalline/crystalline phases by the in-situ neutron diffraction technique and monitoring the magnetic properties. Additionally, our goal is to obtain the amorphous Ti45Zr38Fe17 alloy with the highest hydrogen capacity.


**Abstract**

India is the second most populous country in the world with a population of 1.3 billion; any change in its morbidity and disability pattern is bound to bring change at the Asia level, which is a matter of concern for the developing countries. Disability-free life expectancy (DFLE) and disability-adjusted life years (DALYs) provide summary measures of health across characteristics. The assessments of epidemiological patterns and health system performance of any place and time period display its progress towards the goal of sustainable development goals (SDGs). The main aim of this study is to assess the age and sex pattern of the burden of diseases (mortality and morbidity) and disability in India. The information on disease and deaths was extracted from the 71st round of the National Sample Surveys (NSS) conducted in 2014 (NSS 2014) and the Causes of Deaths Study conducted in the 2010–2013 (RGI 2010–2013) and disability from Census of India 2011 (ORG 2011).

**Keywords:** morbidity, disability, age, sex and India


*3.2.2 Silages*

*Livestock Health and Farming*

*3.1.4 Fodder crops*

SEL during the dry season limits their availability in the rest of the season [29]. Fresh leaves of species such as *C. mopane*, for instance, are high in tannins and lignin [30, 31]. Additionally, indigenous browse species normally attract multiple uses at the livestock-wildlife interface with the more visible, more dominant, and more frequent browse species having more uses than less apparent plants [32]. They are used as sources of firewood, timber, fruits, edible roots, bark and leaves, and human and ethnoveterinary medicines [33–36]. Competitive use increases vulnerability to

Fodder refers to any plants grown specifically as animal feed. They include a variety of pasture grasses like *Panicum maximum*, *Cenchrus ciliaris*, and *Chloris gayana*; pasture legumes such as *Vigna unguiculata*, *Dolichos lablab*, and *Macroptilium atropurpureum*; and fodder trees such as *Leucaena leucocephala*, *Acacia angustissima*, and *Calliandra calothyrsus*. However, most of them do not thrive in semi-arid areas such as the SEL due to high temperatures and low precipitation. Low adoption of fodder crop production is also attributed to lack of extension for farmer training, shortage of labour due to overlapping of the farming calendar with the main crop, high cost and unavailability of seed, and land scarcity. It is therefore important for farmers to

maximise production of those species adaptable to their climatic conditions.

*3.1.5 Conventional supplements, food industry, and agro-industrial by-products*

*Browse trees provide feed during the dry season when both the grazing resource and cereal stovers become*

There are different food industry by-products and agricultural wastes that are alternative dry season livestock feed supplements. These can be of animal and plant origin or of the fermentation industry. Animal by-products include blood, bones, meat and bone offals, fat, intestine and rumen contents, whey, tannery by-products, and poultry manure [1]. By-products of plant origin consist those of the milling industry (e.g. bran, waste flour), oil industry (e.g. soya bean and sunflower cakes), sugar industry (molasses), and citrus and horticulture waste. The fermentation industry produces grain, molasses, and brewer's waste, among a large array of other by-products. By-products of plant origin are the commonly used. For instance, in the SEL, molasses is readily available as the main sugarcane processing factories in Zimbabwe are located in that area. However, high cost of transportation

overutilisation, unsustainable harvesting, and mismanagement.

**108**

**Figure 3.**

*limiting.*

Silage is forage produced from the fermentation process of chopped fresh green material under anaerobic conditions. These materials include fodder or forage grasses. Ensiling maize has been shown to improve feed digestibility and reduce methane gas production by 30% compared to feeding dry maize [37]. However, despite silages being advantageous in areas of water shortages, as well as reducing tannins due to the heat produced during the incubation period, silage production is not common among farmers.


**Abstract**

The article studies a novel approach of inflation modeling in economics. We utilize a stochastic differential equation (SDE) of the form *dXt* ¼ *a X*ð Þ , *t dt* þ *b X*ð Þ , *t dBH t* , where *dBH t* is a fractional Brownian motion in order to model inflationary dynamics. Standard economic models do not capture the stochastic nature of inflation in the Eurozone. Thus, we develop a new stochastic approach and take into consideration fractional Brownian motions as well as Lévy processes. The benefits of those stochastic processes are the modeling of interdependence and jumps, which is equally confirmed by empirical inflation data. The article defines and introduces the rules for stochastic and fractional processes and elucidates the stochastic simulation output.

**Keywords:** inflation, dynamics, modeling, stochastic differential equation, fractional Brownian motion, Lévy process, jump-diffusion


*3.5. Solidifying Mechanism Analysis*

#### 3.5.1. XRD

The XRD pattern of the fly ash used for the test is shown in Figure 3. It can be seen that the main body of the fly ash is amorphous, and its crystalline substances are quartz and mullite, which are the main substances containing silicon and aluminum. The characteristic peaks of quartz appear at 22.53◦, 50.71◦, and 54.69◦; and that of mullite at 17.15◦, 27.33◦, 32.67◦, and 34.82◦. The diffraction peaks in a "bun" shape in the 2θ angle range of 15◦ to 30◦ indicate the presence of vitreous body in the fly ash, from which the activity of the fly ash is mainly derived.

**Figure 3.** XRD pattern of fly ash.

The variations in the mineral phase of the fly ash-based geopolymer and its solidified body are shown in Figure 4. Comparing Figure 4 with Figure 3, the crystalline phases in the fly ash-based geopolymer are mainly mullite and quartz, with a small amount of calcium silicate present at 39.54◦, and the crystalline diffraction peaks are somewhat reduced compared to that in the fly ash XRD pattern, demonstrating that the crystalline substances in the fly ash react to form amorphous substances during the alkali excitation process. Additionally, in the curve of the pure fly ash-based geopolymer, "bun"-shaped dispersion peaks appear in the interval 18◦ to 35 ◦, which are slightly shifted to the right compared to the fly ash curve, indicating that the dissolution of the vitreous body in the fly ash produces amorphous gel-like substance in the fly ash-based geopolymer; the absence of new phases and the disappearance of old phases mean that some of the crystalline particles in the fly ash are dissolved, and some are encapsulated by the gel material in the geopolymer, which corresponds to the SEM results.

**Figure 4.** XRD pattern of the fly ash-based geopolymer and its solidified body.

Compared to the curve in Figure 3, the overall curve in Figure 4 is flatter, which means a relatively high degree of reaction, and a more adequate polymerization of the silica-aluminous material in the raw material with the alkali activator, resulting in a good strength of the geopolymer. Diffraction peaks of crystalline minerals associated with Pb2+ are not found on the curve of the solidified body doped with Pb2+, and no new phases appear, which means that Pb2+ is not involved in the overall internal chemistry of the geopolymer and the Pb3SiO5 phase suggested by Palomo et al. [20] is not detected. It can be assumed that Pb2+ may have been bonded to the internal structural skeleton of the fly ash-based geopolymer, or the solidification of heavy metal ions by the geopolymer solidified body is a physical encapsulation.

### 3.5.2. SEM

Figure 5a shows a micrograph of the fly ash-based geopolymer in the early stage of the reaction. At this stage, the fly ash particles have not fully reacted; half of them are dissolved, while the other half appears partially encapsulated on the surface.

Figure 5b shows a micrograph of the fly ash-based geopolymer at a later stage of the reaction. It can be observed that the fly ash, after polymerization with the alkali activator, produces a very dense geopolymer gel (silica-aluminate gel) with a uniformly dense and amorphous outer surface; the particles in the fly ash are completely dissolved or encapsulated within the formed geopolymer gel; the beads are no longer visible compared to the pre-reaction period, but traces of their internal concavity left by the dissolution reaction can be found. Overall, the internal morphology of the fly ash-based geopolymer is dominated by a dense amorphous structure, which means that the reaction of the materials in this test is sufficient and the polymerization reaction of internal depolymerization and polycondensation is complete. As a result, the fly ash-based geopolymer obtained from the reaction possesses good physicochemical properties.

(**a**) Early stage (**b**) Later stage

**Figure 5.** SEM of the fly ash-based geopolymer in the early and later stages of the reaction.

Figure 6 shows the electron micrographs of the fly ash-based geopolymer during the reaction. Compared to the early stage of the reaction in Figure 5a, the wrapping condition in Figure 6a is more complete, and the progress of the reaction can be clearly observed. Figure 6b is a partial magnification, showing that the gel in the geopolymer slowly grows and extends from the top of the particle downwards until it completely wraps around the geopolymer, forming a kind of core-shell structure. These indicate that a portion of the fly ash particles is encapsulated by the gel into a core-shell structure, except for a portion being dissolved by the alkaline solution. The outer shell of this structure extends until it completely encapsulates the geopolymer and then forms a integral whole with its surrounding dense gel-like substances, which also has a beneficial effect on the mechanical properties of the geopolymer.

(**a**) Before magnification (**b**) After magnification

**Figure 6.** SEM of the fly ash-based geopolymer during the reaction.

Figure 7a shows the state during the solidifying Pb2+ reaction of the fly ash-based polymer, and the fully reacted state is given in Figure 7b. Compared to Figure 7a, a more complete and dense whole is formed in the latter figure, resulting in a good performing solidified body of the fly ash-based geopolymer.

(**a**) During reaction (**b**) After reaction

**Figure 7.** SEM of Pb2+ solidified body of the fly ash-based geopolymer.

#### 3.5.3. FT-IR

The infrared spectrum of the fly ash-based geopolymer and its solidified body is shown in Figure 8.

**Figure 8.** Infrared spectrum of the fly ash-based geopolymer and its solidified body.

According to the figure, the geopolymer exhibits O-H stretching vibration and O-H bending vibration at 3462.5 cm−1 and 1638.7 cm−1, respectively; while the solidified body exhibits O-H stretching vibration and O-H bending vibration at 3471.3.4 cm−1 and 1642.9 cm−1, respectively, which is surmised to be due to the presence of bound water within the geopolymer and the solidified body. The absorption peaks at 1087.3 cm−1 of the geopolymer and 1025.6 cm−1 of the solidified body in the infrared spectrogram correspond to the stretching vibration of Si-O-T (T = Si or Al). The introduction of Pb2+ has an effect on the stretching vibrations of Si-O-T around 1087.3 cm−1 in the geopolymer, indicating that it is bonded into the backbone of the geopolymer. The absorption peaks near 564.8 cm−1 of the geopolymer and 553.2 cm−1 of the solidified body correspond to the stretching vibration of Al-O-Si. The bending vibration of Si-O or Al-O of the geopolymer and its solidified body is located near 461.5 cm−1 and 455.9 cm−1, respectively. The peak at 1374.5 cm−1 occurs only in the solidified body, which is analyzed as being due to the nitrate

contained in the heavy metal salts when Pb2+ is introduced. Overall, the infrared spectrum of the two samples is generally consistent, with no significant differences in the positions of the individual absorption peaks, indicating that the main structure of the solidified body is a complex three-dimensional mesh structure formed by the polymerization of silica-oxygen and aluminum-oxygen tetrahedra, although there are some differences in its physicochemical properties after the introduction of Pb2+.

#### **4. Conclusions**

In this study, the fly ash-based geopolymer was prepared to solidify Pb2+; the mechanical properties and leaching concentration of fly ash-based geopolymer were studied; the effects of different concentrations of Pb2+ on the compressive strength and solidifying effect of the solidified body were analyzed, and were compared with the cement solidified body; the reaction process, structural morphology, and solidifying mechanism were studied by XRD, FT-IR and SEM. The main conclusions are as follows:

(1) The compressive strength of the solidified body decreases compared to that of the pure geopolymer; the higher the concentration of Pb2+ added, the greater the decrease in strength. The solidifying performance of the fly ash-based geopolymer solidified body in a neutral environment is better than that in an acidic environment. Taking into account the compressive strength and leaching concentration, the optimum solidifying concentration of Pb2+ in the fly ash-based geopolymer is 2.0%.

(2) The fly ash-based geopolymer solidified body performs better than the cement solidified body, in terms of both mechanical properties (the compressive strength) and solidifying properties (leaching concentration). At Pb2+ concentration of 2%, the compressive strength of the geopolymer solidified body can still reach around 40 MPa and its leaching concentration is below the limit, whereas the compressive strength of the cement solidified body is only 21.25 MPa, and the leaching concentration is already as high as 8.29 mg·L−1.

(3) The analysis of the mineral composition, microscopic morphology, and chemical structure of the fly ash-based geopolymer and its solidified body by XRD, SEM and FTIR reveals that the fly ash-based geopolymer forms a complete whole inside with a very dense structure, and possesses good physicochemical properties. The solidifying of Pb2+ by the geopolymer is a combination of both chemical bonding and physical encapsulation.

**Author Contributions:** Data curation and investigation, R.T.; writing—original draft preparation, F.L.; writing—review and editing, X.Y.; supervision, B.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work is supported by the National Natural Science Foundation of China [51878116 and 51902270]; Liaoning Province Key Project of Research and Development Plan [2020JH2/10100016]; Dalian Science and Technology Innovation Fund Project [2020JJ26SN060]; the Youth Innovation Team of Shaanxi Universities.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data are not to be shared.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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## **Ultrafast Time‐Resolved Measurements of Hybrid Solar Cells Ultrafast Time**‐**Resolved Measurements of Hybrid Solar Cells**

Kaibo Zheng and Carlito S. Ponseca Kaibo Zheng and Carlito S. Ponseca

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65022

#### **Abstract**

The early time charge carrier dynamics in quantum dot‐sensitized and organo‐metal halide perovskite solar cells are presented in this chapter. Using transient spectroscopy techniques, i.e., absorption, photoluminescence, and photoconductivity, we probed the generation mechanism, charge injection, mobility, and recombination of charges in the time scales of subpicosecond (ps) to a nanosecond. In few ps, electron injection from quantum dot to *n*‐type metal oxide (MO) is complete while hole injection to *p*‐type MO required hundreds of ps. The injection process is dictated by the band alignment, density of states of MO and the charge transfer state at the interface. For organo‐metal halide perovskite material, there is a distribution of exciton binding energy brought about by the nonuniformity in the quality of the sample. As a result, varying amount of exciton and highly mobile charges may be generated depending on the morphology of the film. In the sample presented here, we found that 30% of photo‐generated charges are excitons, which then dissociates within 2–3 ps. The rest of the photons are instantane‐ ously converted into highly mobile charges (*µe* = 12.5 cm2 V‐1 s‐1 and *µh* = 7.5 cm2 V‐1 s‐1), and at the appropriate excitation fluence, the photoconductivity remains constant up to 1 ns. The time scale and mechanism of charge injection from perovskite into organic electrodes are also presented.

**Keywords:** transient absorption, photoluminescence, photoconductivity, THz spectro‐ scopy, mobility

## **1. Introduction**

The emergence of different photovoltaic technologies has been driven by the desire to find an alternative solar cell technology that can be manufactured using simple laboratory processes

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and at lower cost. This has led to conceptualization and development of hybrid solar cells, a structure where an organic molecule is used as light absorber and then attached to a metal oxide (MO). The earliest of this type is the dye‐sensitized solar cell also known as the Graetzel‐type solar cell. Upon light excitation, the dye generate excitons wherein electrons are injected into metal oxide (usually TiO2 ) while holes traverse in liquid iodine electrolyte. This field has branched out to many material substitutions, two of which will be discussed in this chapter. On one hand, organic dyes were replaced by quantum dots, whose absorption spectra highly depends on its miniscule size, have now reached an overall power conversion efficiency (PCE) of 11.3% according to the solar cell efficiency chart of the National Renewable Energy Laboratory (NREL), USA. On the other hand, organo‐metal halide perovskite solar cells adopt a perovskite crystal structure (usually orthorhombic), but has organic molecules, e.g., methyl ammonium, within its unit cell. Since its first discovery about 6 years ago, it has become one of the most serious competitors of silicon solar cells having a PCE 22.1% (NREL).

In this chapter, we present the ultrafast charge dynamics of these materials from the subpico‐ second (ps) to a nanosecond (ns) time scale. Using transient spectroscopy techniques, the evolution of the charges from photoexcitation to recombination will be discussed. This chapter is divided into two main sections. First, results on quantum dot (QD) sensitized solar cell will be presented. This will include the electron injection in an *n*‐type metal oxide (MO) acceptor as well as the hole injection to a p‐type MO. The influence of single layer and multiple layers of QD on excitation transfer will be also examined. The second section will focus on the nature of photogenerated charges in organo‐metal halide perovskite (OMHP), mobility and lifetime of charge carriers, and the mechanism and time scale of charge injection from perovskite material to organic electrodes.

## **2. Dynamics of charge carriers in QD‐sensitized solar cells**

Semiconductor nanocrystals, so‐called quantum dots (QD), are confined quantum objects whose optoelectronic properties are dependent on their sizes [1]. Due to recent progress in chemical solution processing techniques for synthesis of colloidal QDs [2], it has attracted increasing attention on its fundamental properties [3] and applications [4]. The QDs can be utilized as imaging markers [5], as building blocks in light‐emitting diode devices, [6] lasers and light harvesters in solar cells devices [7, 8]. Particularly, the potential application in photovoltaic devices has become the focus of the field over the past decade. One of the reasons for this is the possibility of breaking the Shockley‐Queisser thermodynamic limit of single junction solar cells via multiple exciton generation (MEG) [9] or hot electron transfer (HET) [10]. Besides, the efficiencies of colloidal QD‐based solar cells have been rapidly improving [11]. In this work, we would only discuss colloidal QDs and would refer it simply as QD. The recent progress in the understanding of the photo‐induced dynamic processes in QD‐based solar cell components is summarized in this section. Electron injection dynamics in QD—metal oxide (MO) composites is investigated followed by the studies of hole transfer dynamics and trapping. We also analyze excitation transfer in the films of QDs. The article is mainly based on the studies of Cadmium chalcogenide QDs as well as some progress made for systems such as lead chalcogenide QDs in solar cell application.

#### **2.1. Electron injection from QD to MO**

Electron injection from QD to MO is a process responsible for charge separation in the vast majority of QD‐sensitized solar cells (the so‐called *n*‐type solar cells). The first study can be found already in the 1990s [12]. The idea of using QD‐MO heterojunction in solar cells was directly inspired by the study of dye‐sensitized solar cells. As a key factor in photovoltaic process that would greatly determine its efficiency, electron injection in this system has been widely studied [13–19]. Time‐resolved spectroscopy techniques are commonly used method‐ ology to analyze electron injection dynamics including transient absorption (TA) or time‐ resolved photoluminescence (TRPL). However, combining such experiments with other spectroscopic techniques would be more useful to obtain more thorough picture of the overall injection process. The typical systems to study in QD‐MO hetero‐junction are CdSe QDs attached to a suitable wide band‐gap MO (i.e., TiO2 or ZnO) [20]. Generally speaking, the electron population in the QDs plays a domination role in the TA signal of CdSe QDs in the visible region [20]. Thus, one can easily distinguish the difference between electron and hole dynamics. Other than the CdSe, the PbS and PbSe QDs are also widely studied [21–23]. The band gap of the lead chalcogenide QDs is much narrower allowing it to harvest more photons over the whole sunlight spectrum. The narrower band gap is also favorable in studying the MEG process [24]. It should be noted that in the lead chalcogenide QDs, the transient absorption signal have features from both electron and hole dynamics. Therefore, a more careful identification of the spectral features including both interband and intraband transi‐ tions are highly needed [21].

By using the TA and TRPL measurements, it is possible to track down the density of mean electron population in QDs after excitation [20]. Decrease in the population, however, does not imply electron injection. For instance the electron trapping results in such decay as well. Identification of the electron injection itself can therefore become a complicated issue. The conventional techniques (visible TA or TRPL) can only be used to probe the electron population in QDs. However, one can also monitor electron population in MO by combining other spectroscopy techniques to directly show electron transfer. Blackburn et al. [19] provided a good example in QD‐MO system. In that work, they tracked the population in MO using the TA in far IR region (around 5 μm wavelength) and it can therefore be used to detect the arrival of electrons. However, QDs have features in this spectral region as well. It is therefore necessary to correctly normalize and subtract QD‐MO and QD signals to extract the true injection kinetics. Another suitable probe for the electron injection is a terahertz (THz) light source, which also has been applied previously on the dye‐sensitized MOs. Absorbance in THz wave is correlated to the change in photoconductivity, and therefore we can use it to probe the evolution of the mobile charges in the system. In QDs, the charges are highly localized with rather low THz absorption. However, due to the relatively large mobility of the electrons in MO, the THz absorption would be much larger (especially for ZnO) [25, 26]. In this scenario, we utilized THz spectroscopy to probe the electron injection from QD to MO. By observing

simultaneous depopulation of electrons in QDs (probed by visible TA) with population of mobile charges in the system (probed by THz) one can directly show the electron transfer process and estimate the transfer rate [25]. This is illustrated in **Figure 1(b)**.

**Figure 1.** (A) Comparison of slow TA decay kinetics for bare CdSe QDs in solution (dotted line) and significantly faster decay kinetics of CdSe QD‐ZnO system (solid line) indicates electron injection. (B) Overlapping decays of TA (cyan line) and THz kinetics (black line) demonstrating the electron transfer from QD to MO. (C) Model of the THz kinetics to exclude the possible hetereogeneous injection and confirm the injection via CTS (red line). Figures are reproduced from Ref. [25]. Copyright American Chemical Society, 2012.

In QDs, one cannot ignore the effect of electron‐hole Coulomb interaction— for example, the exciton binding energy in conventional CdSe QDs can reach hundreds of meV, which is much larger than their bulk value [27]. Such Coulomb interaction prevents electron injection process [16]. In dye‐sensitized or polymer solar cells with the similar high exciton binding energy, a so‐called charge transfer state (CTS) has been observed [28]. This CTS formation indicates the build‐up of an electron‐cation bound complex after electron injection. The further movement of the injected electrons would follow the dissociation of such complex. The CTS is therefore important to the charge separation and collection. We have reported this CTS formation in the CdSe QD‐ZnO system whose details can be found in Ref. [25]. The combination of THz and TA spectroscopy provided here is a direct evidence of that type charge transfer. A two‐ component dynamics can be observed in both pump‐probe and the THz spectroscopy. The explanation that assumes injection of the electrons from two classes of QDs (the so‐called heterogeneous injection, HI), cannot simultaneously reproduce both the transient kinetics. In constrast, the injection of electrons via a CTS can fully explain both TA and THz dynamics see **Figure 1(c)**. The formation of the CTS greatly depends on the binding energy. Therefore, in PbS QDs with low exciton binding energy, the CTS would be negligible which explains the faster injection process [21–23].

## **2.2. Hole injection in** *p***‐type solar cells**

In *n*‐type solar cells, the hole transfer from QDs to liquid electrolyte is usually 2–3 orders of magnitude slower than the corresponding electron injection [29]. Thus, the hole dynamics becomes the limiting factor for the photo‐conversion efficiency [30]. To circumvent this, *p*‐type solar cells are becoming popular, wherein QDs are attached to *p*‐type MOs, e.g., NiO where holes are injected and extracted at the electrodes [31]. The lifetime and the pathway of the photo‐generated holes are therefore essential for the photon‐to‐current conversion efficiency. It has been recently reported that the photon‐to‐current conversion efficiency is relatively low, about 17% only [32]. In this section, we give an overview of our studies on the hole dynamics in QDs, specifically the hole trapping process and the injection to *p*‐type MOs.

To rigorously confirm the hole injection rates, complementary analysis using both TRPL and TA is necessary. It should be noted that the DOS of conduction band at the band‐edge transition of CdSe QDs is significantly smaller than the DOS of valence band. Moreover, the hole states are much more closely spaced in QDs [9, 33]. Therefore, the signal of TA bleaching is dominated by the electron filling while hole contribute much less. Moreover, the PL measurements are sensitive to both charge carriers. It is therefore through TA kinetics that we are able to ascertain or preclude the role of electron depopulation in PL quenching as shown in **Figure 2** [34].

**Figure 2.** (a) PL decay kinetics trace of CdSe QDs after continuous laser excitation. The inset shows the evolution of the steady‐state PL intensity. (b) TA kinetics at the band‐edge bleach and PL decay of CdSe QDs with different capping agent attached to QD. Figures are reproduced from Ref. [34]. Copyright American Chemical Society, 2012.

In *p*‐type dye‐sensitized solar cells, the hole injection to MO can occur if it is energetically favorable [35, 36]. In QD‐sensitized solar cells, the hole injection would also be restricted by the fast hole trapping. However, such hole trapping is likely to be greatly self‐passivated. One example is the self‐passivation by continuous light soaking. The effect is commonly explained by surface passivation induced by the chemical changes of the QD surface during photoirra‐ diation [37].

Compared with electron injection in *n*‐type MOs, the hole injection in *p*‐type MOs such as NiO turns out to be much slower reaching hundreds of picoseconds that is due to the weaker electronic coupling, heavier effective mass of holes and less driving force for the charge transfer [37]. The driving force of QD is changed depending on its size, which means it also influences the holes' injection rate. The influence does not come from the difference in energy levels as the valence band of QDs tends to be pinned when attached to MOs [38]. The difference in the driving forces, mainly originate from the size‐dependent Coulomb energy wherein larger driving force is found in larger QDs since Coulomb coupling is weaker.

#### **2.3. Excitation transfer**

Due to large exciton binding energy in CdSe QDs, initial photo‐generated charge species behave like excitons [39]. The motion of such excitons between the QDs occurs via Förster resonant energy transfer (FRET), which is essential for the function of optical devices with densely packed QD films including light emitters and solar cells [40–42].

In QD‐sensitized MOs, it is conventionally believed that electron injection only occurs from QDs directly attached to the MO surface while multilayer QD attachment would hinder the electron collection process. However, in the system of QD‐MO with multiple layer QDs attachement, it is found the excitation transfer (Förster energy transfer) also occurs within the aggregates of QDs [41]. This transfer process can be traced in TA as an additional long‐lived (5 ns) excited states depopulation (see **Figure 3**). Such energy transfer has also been reported in tandem‐layered cadmium chalcogenide QD solar cells, which can be an effective comple‐ ment to improve the solar cell efficiency [40].

**Figure 3.** Energy transfer between indirectly attached QDs in QD‐MO photoanodes. (a) TA kinetics of QD‐ZnO NWs with different sensitization times. The inset illustrates the long‐lived TA component (red dots) extracted from the expo‐ nential fit of the kinetics. The consistence between the TA amplitude and the number of indirectly attached QDs indi‐ cates indirect exciton depopulation with ns time scale. (b) Main photoinduced processes, including direct electron injection and energy. Figures are reproduced from Ref. [41]. Copyright American Chemical Society, 2012.

## **2.4. Outlook**

QDs provide great opportunities for the development of optoelectronic devices. Moreover, the booming of QD solar cell research will surely put forward photo‐induced dynamics questions waiting to be revealed in the future. For example, the band alignment assembly in lead chalcogenide QD devices currently holds the highest record of solar cell efficiency. The general principles of the device have been revealed but the atomistic details are still to be understood. The conventional spectroscopic techniques used in investigating the photo‐induced dynamics are usually very different from the solar illumination conditions. These include the excitation intensity, the loading of circuit, medium conditions, etc. Systematic studies of charge carries dynamics under real solar cell functioning conditions may provide more useful reference for device application. Another issue of future research is to utilizing the "green" elements in the materials to replace the toxic Cd and Pb, which are overwhelmingly used in recent studies. All in all, many challenges remain to be faced before the QD solar cells can be considered as a real viable solar technology. However, recent advances in QD research give ground for optimism.

## **3. Dynamics of charge carriers in organo‐metal halide perovskite solar cells**

From its first use as a light absorber in a dye‐sensitized solar cell, the PCE of organo‐metal halide perovskite (OMHP) has now reached to 22.1%. Such remarkable rise has dazzled scientists and engineers that have now considered OMHP to be one of the most serious competitors to the current solar cell industry leader, silicon.  Details of transient spectroscopy techniques used to unravel these processes can be found in Ref. [25].

## **3.1. Intrinsic properties of OMHP**

Shown in **Figure 4(a)** is the rise of transient photoconductivity of methyl ammonium lead triiodide (MAPbI3 ), MAPbI3 /Al2O3 , and MAPbI3 /TiO2 measured using time‐resolved THz spectroscopy (TRTS). Notice that the rise in MAPbI3 and MAPbI3 /Al2O3 is a two‐step process, one that is instrument limited (about 70% of the total amplitude), while the second is about 2– 3 ps (about 30% of the total amplitude). We note that molecular excitons that are tightly bound are neutral by definition, and therefore would not contribute to the transient photoconduc‐ tivity obtained here. However, if photo‐generated species are either loosely bound or mobile, or both, TRTS would be able to detect it. The instantaneous rise therefore means that highly mobile charges are created within the response of our instrument. The next question is how to explain the additional 2–3 ps rise. This can be understood in two ways. One, is that from the 70% highly mobile charges, these carriers become faster, that is gaining more mobility during that time scale. This would show in the technique as increase in photoconductivity. Another explanation is that there is a distribution of binding energy in the sample. The implication of this is that, there is a nonuniformity in the quality of film wherein in some parts have defect‐ free area that promotes generation of mobile charges. Area of the film with high defect density would tend to have exciton that is more tightly bounded. In fact, it has been reported that binding energy of exciton in these materials vary between few meV [43] and 50 meV [44] depending on the preparation conditions. As a result of heterogeneity in its binding energy, photo‐generated species dissociates at different rates. This explains the two‐component rise in the photoconductivity kinetics shown in **Figure 4(a)**. In this scenario, 70% are directly con‐ verted to mobile charges while the 30% are generated via exciton dissociation. For MAPbI3 / TiO2 , the transient photoconductivity rises in a single‐step instrument‐limited time scale. This means that, unlike the first two samples, mobile charges are readily created. This is reminiscent of our previous results on electron injection from QD to ZnO [25] as well as the injection rate of electrons from RuN3 dye attached to TiO2 [45]. Similarly, we assign the single step rise of the transient photoconductivity as evidence of subpicosecond injection of electrons from the OMHP to TiO2 . There is favorable band energy alignment between the energy of perovskite

and metal oxide that helps in the separation of any bound electron‐hole pair allowing injection in the ultrafast time scale.

**Figure 4.** (a) Transient photoconductivity kinetics of neat MAPbI3 , MAPbI3 /Al2O3 , and MAPbI3 /TiO2 . Normalized to 1 (*λ*pump = 400 nm, *I*exc = 1.7 × 1013 ph/cm2 per pulse). (b) Transient absorption kinetics (*λ*pump = 603 nm, *λ*probe = 970 nm, *I*exc = 6.0 × 1014 ph/cm2 per pulse). (c) Photoluminescence spectra (*λ*pump = 550 nm) of the three samples. Reprint with permis‐ sion from Ref. [46]. Copyright 2014, American Chemical Society.

To verify the above assertions, we also obtained the transient absorption kinetics of the three samples, shown in **Figure 4(b)**. For MAPbI3 and MAPbI3 /Al2O3 , the kinetics is characterized by two‐step decrease whose time scale is identical to that of the rise in the transient photo‐ conductivity. This means that the processes in both kinetics should be at least similar if not identical. Since the absorption kinetics is negative for the two samples, it means that something is being bleached or being emitted. Furthermore, as shown in **Figure 4(c)**, for the same two samples, the emission spectra of the steady state photoluminescence are quite high. The two‐ step stimulated emission in the transient absorption kinetics show that not all charges are created simultaneously while the bright photoluminescence in steady state PL means these charges eventually meet and recombine radiatively. In the case of MAPbI3 /TiO2 , the positive one‐step rise in the transient absorption kinetics supports our conclusion that there is instan‐ taneous charge generation while the very low PL count reiterates our assertion that electrons are injected into TiO2 and does not recombine radiatively with holes left in the perovskite.

We further analyzed and take the transient photoconductivity of the three samples at different excitation fluences and at up to 1 ns. Plotted in **Figure 5(a)** is the transient photoconductivity per photon absorbed per pulse for the first 40 ps. For the first two samples, MAPbI3 and the MAPbI3 /Al2O3 , the obtained mobility is 20 cm2 V‐1 s‐1 while for MAPbI3 /TiO2 , it is 7.5 cm2 V‐1 s ‐1. For the MAPbI3 , both electrons and holes are generated in the perovskite material. This is also true for MAPbI3 /Al2O3 sample, since the alignment of band energies of Al2O3 and perov‐ skite is not favorable for charge transfer, both electrons and holes remain in the perovskite material. Therefore, 20 cm2 V‐1 s‐1 is the sum of the mobility of both charges in the perovskite. For MAPbI3 /TiO2 , where there is an ultrafast electron injection as discussed above, electrons are transferred to TiO2 and its mobility becomes 0.1 cm2 V‐1 s‐1 only, since it adopts the property of the accepting material. The implication is that the measured mobility of 7.5 cm2 V‐1 s‐1 in MAPbI3 /TiO2 should be coming from holes left in perovskite since electrons are already in the TiO2 . Now, knowing that the hole mobility is 7.5 cm2 V‐1 s‐1 and the total mobility is 20 cm2 V‐1 s ‐1, one can concludes that the electron mobility should be 12.5 cm2 V‐1 s‐1 in both MAPbI3 , and MAPbI3 /Al2O3 . This is the first report where both the electron and hole mobilities are measured in OMHP materials [46].

**Figure 5.** (a) Transient photoconductivity of neat MAPbI3 , MAPbI3 /Al2O3 , and MAPbI3 /TiO2 per photon absorbed. (b) Comparison TA and TRTS kinetics for neat MAPbI3 showing that similar decay rates up to 1 ns. (c) Intensity depend‐ ence transient photoconductivity of MAPbI3 /Al2O3 . Reprint with permission from Ref. [46]. Copyright 2014, American Chemical Society.

We then compared the kinetic traces of MAPbI3 using transient absorption and photoconduc‐ tivity up to 1 ns, at very similar excitation conditions, which are plotted in **Figure 5(b)**. From about 3 ps to 100 ps, both of the traces are flat which then started to decay at similar rate until 1 ns showing that the charge dynamics should be identical. Transient absorption monitors the population or depopulation of charge carries while transient photoconductivity measures the product of charge concentration and mobility. From 3 ps to 100 ps, where the two traces are flat means that neither the population of the charges nor the mobility of the carriers is changing. However, for time scale longer than 100 ps, the decay starts to manifest. In this case, the transient absorption kinetics shows that charges are disappearing. Similarly, the decay in transient photoconductivity should manifest the same phenomenon. Ergo, the mobility of charges in perovskite should have remained constant, at least up to 1 ns, otherwise its decay should be more substantial than just the corresponding transient absorption. This is a very important finding since mobility of charges usually decay in emerging photovoltaic materials due to the defects in the film as well as its high exciton binding energy. In the case of OMHP, it seems to suggest that there is at least less defects in these films that favor the charge to maintain its mobility up to 1 ns. For organic solar cell material, we have previously shown that mobility is 50% slower in half of nanosecond [48].

We then want to determine the influence of reduced excitation fluence to the decay of the transient photoconductivity as we have shown previously that there are nonlinear effects at high fluency in organic solar cells [48]. Shown in **Figure 5(c)** is the transient conductivity of MAPbI3 /Al2O3 at three different intensities. For 7.4 × 1013 ph/cm2 per pulse, the mobility is around 5 cm2 V‐1 s‐1 and its decay started early at about 5 ps. For intensity of 1.7 × 1013 ph/cm2 per pulse, a mobility of 20 cm2 V‐1 s‐1 was obtained and the onset of decay is prolonged to around 10 ps. At the lowest excitation condition of 2.0 × 1012 ph/cm2 per pulse, the highest mobility is measured 25 cm2 V‐1 s‐1 and decay did not start until after 300 ps. This shows how essential the excitation intensity dependence measurements in transient spectroscopy. At high excitation conditions, nonlinear effects as such charge pair annihilation or second order nongeminate recombination dominates as the main channel of charge depopulation. This is the reason for the behavior of the transient photoconductivity shown above. At high excitation, mobility is low, i.e., at the earliest time scale, charges recombine right away since the resulting charge density at this condition is quite high. Moreover, the onset of decay is early. On the other hand, at the lowest excitation intensity, charges are rather sparse with each other (low charge density) and the probability of it recombining is low. This is the rationale of the high mobility at the early time scale. For the same reason, recombination is also delayed to at least 300 ps. The difference between the mobility of MAPbI3 (20 cm2 V‐1 s‐1 , **Figure 5a**) and MAPbI3 / Al2O3 (25 cm2 V‐1 s‐1 , **Figure 5c**) can be explained by the better film morphology in MAPbI3 / Al2O3 . The metal oxide Al2O3 acts like a scaffolding creating a more continuous film than the bare MAPbI3 .

Another, interesting feature of this material is the very small differences in the mobility of electrons and holes. In contrast with organic solar cells, this difference in mobility can be few orders of magnitude. Due to this, a built‐in electric field is produced since one of the charge specie, usually the electrons, arrives earlier in the electrode while the holes, being slow, is still traversing the polymer molecule, arriving later in the counterelectrode. As shown in the above results, electrons and holes in OMHP have a difference in the mobility of just about half, i.e., *µ*e = 12.5 cm2 V‐1 s‐1 and *µ*h = 7.5 cm2 V‐1 s‐1. This indicates that both charges arrive in their respective electrodes almost at the same time, avoiding the built‐up electric field. This again is advantageous for solar cell operation since it will be able to collect more charges.

#### **3.2. Injection into organic electrodes**

Similar to other emerging photovoltaic technologies, there are significant efforts on using organic molecules, like PCBM and Spiro‐OMeTAD as electrodes. In this section, we will show the mechanism and time scale of electron and hole injection in these organic electrodes. Plotted in **Figure 6(a)** is the transient photoconductivity of MAPbI3 , MAPbI3 /PCBM, and MAPbI3 / Spiro‐OMeTAD per photon absorbed per pulse. It can be seen that for MAPbI3 mobility obtained is 15 cm2 /Vs and remained flat for the first ns. We note that the difference in the mobility measured here with respect to the MAPbI3 sample discussed above (20 cm2 /Vs). It has been reported by Wang et al. [49] that depending on the preparation conditions, the concen‐ tration and type of defects could differ significantly. In fact, they found that thermal annealing alone should be able to shift the property of perovskite material from an *n*‐type to intrinsic to *p*‐type semiconductor. This means that no two‐perovskite samples are made identical to each other, more so, when prepared by different groups despite following the same recipe. Having said that, the transient photoconductivity measurements of all perovskite samples we studied in the past gave a rather consistent result, i.e., from 15 to 25 cm2 /Vs. We surmise that these differences do not significantly alter the interpretation of the photophysical properties of the materials we presented here.

**Figure 6.** Transient photoconductivity of (a) neat MAPbI3 , MAPbI3 /PCBM, and MAPbI3 /Spiro‐OMeTAD per photon ab‐ sorbed per pulse (*λ*pump = 590 nm) up to 1 ns and (b) of neat MAPbI3 and MAPbI3 /with 7 ns time window. Reprint with permission from Ref. [47]. Copyright 2014, American Chemical Society.

Also plotted in **Figure 6(a)** is the transient photoconductivity of MAPbI3 /Spiro‐OMeTAD. Spiro‐OMeTAD is an organic hole transporting material that is widely used for highly efficient perovskite‐based solar cell material despite its reported very low conductivity, 10‐8 S/cm [50]. The measured mobility from this sample is about 5 cm2 /Vs, which is a third of that obtained from the MAPbI3 only, but it stayed constant for 1 ns. Upon photoexcitation, electrons and holes are generated in the perovskite material. In the presence of an acceptor material that has favorable band energy alignment, charges may be injected. In this case, there is a 0.5 eV difference in the band edge between the valence bands of perovskite and Spiro‐OMeTAD, leading us to conclude that there is an efficient hole injection. The injection rate is ultrafast since the mobility is reduced to three times in the earliest time scale. This is also supported by the fact that decent PCE are reported to these devices, which means that holes are really transferred from perovskite to Spiro‐OMeTAD. Moreover, since holes are injected to the Spiro‐ OMeTAD and the conductivity of this material is very small, this means that the mobility of 5 cm2 /Vs should be the mobility of electrons. This value of electron mobility is again different from the sample presented in the previous section. However, we reiterate that different preparation conditions yield different film quality manifesting as different values in their mobility, among other properties. As to the flat transient photoconductivity trace of this sample, it can be understood such that electrons remain in the perovskite material and did not encounter either defects where they can be trapped or holes which they can recombine with.

The transient photoconductivity trace of MAPbI3 /PCBM is characterized by mobility similar to MAPbI3 , 15 cm2 /Vs, but is decaying to almost a third in 1 ns. The initial value of the mobility implies that both electrons and holes are generated in the perovskite. The decay could then be assigned as due to second order geminate recombination. However, this type of recombination is excitation dependent. If this is the case, both the transient photoconductivity of MAPbI3 and MAPbI3 /Spiro‐OMeTAD should also be decaying at the same rate since the excitation fluence used for these three samples are the same. Since this is not the case, one can discount this possibility. In addition and as shown in **Figure 6(b)**, where a higher excitation fluence is used for MAPbI3 and MAPbI3 /PCBM but for a longer time window of 7 ns, the decay in MAPbI3 / PCBM is faster than MAPbI3 only, showing that there is an additional mechanism that causes the decay other than the second order recombination. It has been reported that the energy difference between the conduction bands of PCBM and perovskite is only 0.2 eV. As such, injection is still possible, but unlike in perovskite/Spiro‐OMeTAD interface, this injection can be slower since the driving force is at least two times less. This slower injection rate of electrons from perovskite to PCBM could be one of the mechanisms of the decay. Furthermore, one should also take into account the electron mobility in PCBM, which is 10‐3 cm2 /Vs [51–55]. This implies that while electrons are slowly injecting into the PCBM, the low mobility of the PCBM causes the electrons to be pinned at the interface. In this scenario, holes that are in perovskite could easily recombine with the electrons pinned at the interface of PCBM which could also have a ns time scale. We surmised that the ns decay of the transient photoconductivity shown in **Figure 6** is a convolution of the electron injection and recombination of the pinned electrons at the interface with the holes left in the perovskite material both occuring in the time scale of few ns.

## **3.3. Outlook**

There are many studies that have reported the very impressive properties of perovskite‐based solar cells and these have inspired material scientists and engineers to pursue this field of study. However, this should be taken as only the start of a longer, more detailed investigations in the future. It is only very recently that evidence of the influence of preparation conditions is getting the attention it deserves. In organic solar cells, preparation routes dictate the morphology, therefore, the quality of the film, which now appear to be very similar to perovskite solar cells.

## **4. Conclusion**

Ultrafast time‐resolved studies of two of the most important hybrid solar cell technologies, quantum dot‐sensitized and perovskite‐based solar cells, were discussed in this chapter. The mechanism and time scale of charge generation, nature of charged species, mobility, injection,

and recombination were obtained using transient absorption, photoluminescence, and photoconductivity measurements. For quantum dot‐sensitized solar cell materials, electron injection is a two‐step process; first, a few ps injection at the interface between QD and ZnO, while the second step is a charge transfer state‐mediated 200 ps injection from the interface to the bulk of the ZnO material. For multiple layers of QDs, excitation transfer is confirmed while hole injection is shown to be dictated by the band alignment at the interface but limited by fast hole trapping. For organo‐metal halide perovskite, almost ideal solar cell characteristics were found, i.e., ultrafast generation charges, high mobility of electrons and holes that is maintained for at least 1 ns, and balanced transport. Using organic electrodes, electron injection from perovskite to PCBM is found to be in the sub‐ns while the sub‐ps hole injection is obtained from perovskite to Spiro‐OMeTAD. The time scale of charge transfer was found to be depend‐ ent on the driving force between the interface of perovskite and organic electrodes. Despite the seemingly convincing time‐resolved measurement results, we surmise that more thorough investigations are warranted. This is in the broader context of further understanding the influence of preparation conditions to the ultrafast charge carrier dynamics of the solar cell technologies discussed here.

## **Acknowledgements**

This work was supported by the Swedish Energy Agency, the Knut and Alice Wallenberg Foundation, and the Swedish Research Council. Collaboration within nanoLund and Lund Laser Center is acknowledged. We also acknowledge support from by NPRP grant # NPRP7‐ 227‐1‐034 from the Qatar National Research Fund (a member of Qatar Foundation) and Laserlab‐Europe (EU‐H2020 654148).

## **Author details**

Kaibo Zheng1,2 and Carlito S. Ponseca1\*

\*Address all correspondence to<EMAIL_ADDRESS>
1 Division of Chemical Physics, Lund University, Lund, Sweden

2 Gas Processing Center, College of Engineering, Qatar University, Doha, Qatar

## **References**

[1] Leutwyler W, Bürgi S, Burgl H Semiconductor clusters, nanocrystals and quantum dots. *Science* 1996; 271:933–937.


**4. Regulation of the use of LAB**

Streptomycin was the first aminoglycoside reported for which resistance appeared in enterococcal strains (concentrations higher than 2000 μg/mL); this resistance is carried out by adenylation of streptomycin, by the action of the enzyme streptomycin adenyltransferase, encoded by the *aadA* gene [35, 41]. Resistance to gentamicin, kanamycin, neomycin, and netilmicin (aminoglycosides as well) is mainly due to the production of the bifunctional enzyme 2′-phosphotransferase-6′ acetyltransferase, which promotes the ATP-dependent phosphorylation of aminoglycosides [41]. Strains of enterococci of clinical origin between 60 and 65% exhibit resistance to tetracyclines, although these antibiotics are not routinely used in the treatment of infections caused by these microorganisms. There are two fundamental mechanisms of resistance to tetracyclines in enterococci: flow pumps and protection of the ribosome, thus preventing the binding of the antibiotic. The *tetK* and *tetL* genes code for proteins associated to flow pumps responsible to remove the antibiotic outside of the cell, while the *tetM*, *tetO*, and *tetS* genes code for proteins that provide resistance to tetracyclines for ribosome protection. The *tetL* and *tetM* genes are the most frequent in the chromosome and mobile determinants [41, 52, 53]. Finally, vancomycin (glycopeptide) is the main cause of concern, since this antibiotic is considered at the last option for antibiotic therapy for the treatment of Gram-positive bacteria. The resistance to vancomycin in enterococci is varied, having described six genotypes called *vanA*, *vanB*, *vanC*, *vanD*, *vanE,* and *vanG,* where the genotype *vanA* is more frequent in the *Enterococcus*

In general, Lactobacilli have a high natural resistance to vancomycin, bacitracin, cefoxitin, metronidazole, nitrofurantoin, and sulfadiazine, as well as antibiotics that inhibit the synthesis of proteins such as chloramphenicol, erythromycin, quinupristin/dalfopristin, lincomycin, clindamycin, and tetracyclines [45]. Guo et al. [54] observed 85% of incidence of vancomycin resistance in food isolated *Lactobacillus* strains, especially in *Lactobacillus plantarum* and *Lactobacillus casei*, with the lower frequency for *Lactobacillus helveticus*, but these resistances are not transferable, as genes are located in the chromosome [54]. In addition, genes that code for resistance to tetracycline and erythromycin have been detected in different *Lactobacillus*

The genus *Lactobacillus* is an excellent receptor for exogenous genes by conjugation, as demonstrated by Abriouel et al. [45] for the conjugative pAMβ1 plasmid found in *Lactobacillus plantarum* that could be obtained from enterococci and streptococci. *Lactobacillus* are commonly susceptible to antibiotics, such as penicillins (ampicillin, oxacillin, and piperacillin), inhibitors of β-lactamase, and cephalosporins (cephalothin and cefuroxime, ceftriaxone and cefoxitin), but in recent years some authors have reported resistance to penicillin G in some strains of *Lactobacillus rhamnosus*, *Lactobacillus reuteri*, and *Lactobacillus plantarum* [45, 56]. Other studies demonstrated that *Lactobacillus rhamnosus* is safe to use as a starter or probiotic culture, despite having resistance genes to vancomycin, as these resistance is encoded into the chromosome [45, 48, 54].

The horizontal gene transfer (HGT) involves the gene interchange between different bacteria through mobile DNA elements such as plasmids, conjugative transposons, integrons, and

genus [41].

*3.2.2. Lactobacillus*

64 Antimicrobial Resistance - A Global Threat

species isolated of probiotics and foods [12, 31, 55].

**3.3. Horizontal transfer of LAB to the intestinal microbiota**

The FDA categorizes microorganisms with the GRAS distinction after being evaluated in general aspects of safety, taxonomy, potential to produce pathogenicity toxins, resistance to antibiotics, and the historical background of food safety. LAB have a broad history of use in fermented foods and usually recognized as safe. However, the dissemination of AR genes puts the GRAS category in another context, especially for bacteria that present mobile genes of transfer such as *Lactobacillus*, since in the US there are still no guidelines that contemplate the type of resistance in microorganism used in food processing [57]. On the other hand, the EU commission regulates the safety of LAB used as starter or probiotic cultures in the European continent, through the EFSA that establishes guidelines for assigning qualified presumption of safety quality to the organisms since 2003. As previously mentioned, the term QPS is based on reasonable and qualified evidence to allow certain restrictions and may be analogous to the GRAS concept but with more rigid guidelines in which the reliable safety of the bacteria is verified, making clear the phrase "from farm to fork" [58]. The QPS status is given to a bacterium, by the EFSA BIOHAZ Panel (Biological Hazards) that must take into account the following aspects (**Figure 3**): (1) the identity of the taxonomic unit at the genus level; (2) documentation related to the LAB safety, based on scientific evidence and history of use; (3) pathogenicity, in which it is evaluated if any species of the genus has pathogenicity factors, if the information is available, the pathogenic strains are excluded; and (4) knowledge of the final use of the microorganism, identifying if the bacteria is part of the food chain or if it is used to produce other products [6, 58].

The list of QPS includes species of *Lactobacillus sakei*, *Lactobacillus curvatus*, *Lactobacillus plantarum*, *Lactobacillus fermentum*, *Lactobacillus brevis*, *Lactobacillus rhamnosus*, *Lactobacillus*

a complementary technique involves the search for AR genes using PCR techniques and microarrays [25, 29, 54]. Also, identifying the location of these genes allows to determine their potential transfer, while their sequencing can provide evidence of their bacterial taxa and

Antibiotic Resistance in Lactic Acid Bacteria http://dx.doi.org/10.5772/intechopen.80624 67

Functional metagenomics is an important approach in the investigation of antibiotic resistance genes (ARG) since it can be used to identify and characterize new ARG, including those not previously associated with antibiotic resistance [48, 61]. It is also one of the most recent techniques in the study of resistance in pure bacterial groups or more complex samples such as food; some works reported in the literature indicate the wide diversity of resistance systems that are present in food, considering the cultivable and not cultivable bacteria. Metagenomic studies help to understand the mechanisms of resistance in such a way that it allows direct applications in the identification of new drugs and the synthesis of novel and active antibiotic molecules [61].

The FEEDAP Panel proposed a scheme to evaluate the resistance present in lactic acid bacteria that can be used as probiotic or starter cultures in food processing; as previously mentioned, it is essential to distinguish between the intrinsic and acquired resistance as part of the food safety of lactic acid bacteria [58, 62]. The correct identification of the bacteria (sequencing and comparison of the 16S rDNA gene in international databases) by molecular taxonomy is essential to evaluate the type of resistance, since the intrinsic resistance is specific for a specie or genus. Once the specie under study has been identified, the MIC (minimum inhibitory concentration)

**Figure 4.** Proposed scheme for the antibiotic resistance assessment of lactic acid bacteria used as probiotic and starter

culture. Adapted from Laulund et al. [58] and EFSA [62].

identity of the genes, which helps to trace the origin of their genomes [29].

**5.1. Procedure to evaluate LAB resistant to antibiotic used in food**

**Figure 3.** Scheme for assessing the suitability for qualified presumption of safety (QPS) status of a BAL. adapted from Laulund et al. [58].

*alimentarius*, *Leuconostoc lactis, Leuconostoc citreum*, *Leuconostoc mesenteroides*, *Leuconostoc pseudomesenteroides*, *Pediococcus acidilactici*, *Pediococcus dextrinicus*, *Pediococcus pentosaceus*, *Lactococcus lactis*, and *Streptococcus thermophilus* [6]. In the case of *Enterococcus*, the QPS category cannot be assigned to all species; each specie must be individually analyzed [6].

### **5. Methods to identify antibiotic-resistant LAB**

Most widely used antibiotic susceptibility testing methods are based on (1) phenotypic detection of antibiotic resistance by measuring bacterial growth in the presence of the tested antibiotic and (2) molecular identification of resistant genotypes through polymerase chain reaction (PCR) [21, 25, 29, 39, 54]. The evaluation of phenotypic susceptibility to antibiotics in lactic acid bacteria should be done using recognized methods that allow the identification of the minimum inhibitory concentration (MIC) for the most commonly used antibiotics. Most LAB species used in food can be evaluated by the method described in ISO 10932: 2010 [59], considering the conditions and culture media for *Bifidobacteria* and LAB that do not belong to the genus enterococci [56, 57]. In case of having strains of *Enterococcus*, it is recommended to use the methods described by the Clinical and Laboratory Standards Institute [21, 60]. Some of the recommended methods to determine the MIC in LAB are the E-test, the Kirby-Bauer test (diffusion method), and the broth microdilution method (MDIL) [43]. In particular the cutoff values are known for the genera *Lactobacillus, Pediococcus*, *Lactococcus*, *Streptococcus*, and *Bifidobacteria*. The MDIL method is widely used to evaluate MIC for a large number of strains and antibiotics, although the method has some limitations, especially for those antibiotics for which a strain could quickly acquire resistance [43]. However, MIC evaluation in LAB is somewhat inconsistent among the researchers, mainly due to the lack of culture media that can ensure proper growth of LAB without interfering with the assay results. Therefore, a complementary technique involves the search for AR genes using PCR techniques and microarrays [25, 29, 54]. Also, identifying the location of these genes allows to determine their potential transfer, while their sequencing can provide evidence of their bacterial taxa and identity of the genes, which helps to trace the origin of their genomes [29].

Functional metagenomics is an important approach in the investigation of antibiotic resistance genes (ARG) since it can be used to identify and characterize new ARG, including those not previously associated with antibiotic resistance [48, 61]. It is also one of the most recent techniques in the study of resistance in pure bacterial groups or more complex samples such as food; some works reported in the literature indicate the wide diversity of resistance systems that are present in food, considering the cultivable and not cultivable bacteria. Metagenomic studies help to understand the mechanisms of resistance in such a way that it allows direct applications in the identification of new drugs and the synthesis of novel and active antibiotic molecules [61].

#### **5.1. Procedure to evaluate LAB resistant to antibiotic used in food**

*alimentarius*, *Leuconostoc lactis, Leuconostoc citreum*, *Leuconostoc mesenteroides*, *Leuconostoc pseudomesenteroides*, *Pediococcus acidilactici*, *Pediococcus dextrinicus*, *Pediococcus pentosaceus*, *Lactococcus lactis*, and *Streptococcus thermophilus* [6]. In the case of *Enterococcus*, the QPS cat-

**Figure 3.** Scheme for assessing the suitability for qualified presumption of safety (QPS) status of a BAL. adapted from

Most widely used antibiotic susceptibility testing methods are based on (1) phenotypic detection of antibiotic resistance by measuring bacterial growth in the presence of the tested antibiotic and (2) molecular identification of resistant genotypes through polymerase chain reaction (PCR) [21, 25, 29, 39, 54]. The evaluation of phenotypic susceptibility to antibiotics in lactic acid bacteria should be done using recognized methods that allow the identification of the minimum inhibitory concentration (MIC) for the most commonly used antibiotics. Most LAB species used in food can be evaluated by the method described in ISO 10932: 2010 [59], considering the conditions and culture media for *Bifidobacteria* and LAB that do not belong to the genus enterococci [56, 57]. In case of having strains of *Enterococcus*, it is recommended to use the methods described by the Clinical and Laboratory Standards Institute [21, 60]. Some of the recommended methods to determine the MIC in LAB are the E-test, the Kirby-Bauer test (diffusion method), and the broth microdilution method (MDIL) [43]. In particular the cutoff values are known for the genera *Lactobacillus, Pediococcus*, *Lactococcus*, *Streptococcus*, and *Bifidobacteria*. The MDIL method is widely used to evaluate MIC for a large number of strains and antibiotics, although the method has some limitations, especially for those antibiotics for which a strain could quickly acquire resistance [43]. However, MIC evaluation in LAB is somewhat inconsistent among the researchers, mainly due to the lack of culture media that can ensure proper growth of LAB without interfering with the assay results. Therefore,

egory cannot be assigned to all species; each specie must be individually analyzed [6].

**5. Methods to identify antibiotic-resistant LAB**

Laulund et al. [58].

66 Antimicrobial Resistance - A Global Threat

The FEEDAP Panel proposed a scheme to evaluate the resistance present in lactic acid bacteria that can be used as probiotic or starter cultures in food processing; as previously mentioned, it is essential to distinguish between the intrinsic and acquired resistance as part of the food safety of lactic acid bacteria [58, 62]. The correct identification of the bacteria (sequencing and comparison of the 16S rDNA gene in international databases) by molecular taxonomy is essential to evaluate the type of resistance, since the intrinsic resistance is specific for a specie or genus. Once the specie under study has been identified, the MIC (minimum inhibitory concentration)

**Figure 4.** Proposed scheme for the antibiotic resistance assessment of lactic acid bacteria used as probiotic and starter culture. Adapted from Laulund et al. [58] and EFSA [62].

in which the LAB is sensitive to the antibiotic analyzed is determined. The bacterium can be considered safe when the MIC is lower than the cutoff level (MIC < cutoff). On the other hand, if the MIC value is above the cutoff value (MIC > cutoff), the bacterium is considered resistant to the antibiotic, and its resistance should be confirmed by molecular methods as PCR [39, 54, 62]. However, the resistance genes not always are expressed but can be transferred to other bacteria if the environmental conditions stimulate the expression of these genes [34]. If the bacteria have intrinsic resistance, it is considered acceptable for use in food. Otherwise, it must be demonstrated whether the acquired resistance is in mobile genetic material or was acquired in the process of mutation in the bacterial chromosome (also acceptable for use in foods). Finally, the bacteria are not accepted by any regulatory body for its application in food if it is demonstrated that the resistance is exogenous and easily transferable (**Figure 4**).

[2] Antimicrobial Resistance in Food and Agriculture. FCC-EMPRES, information sheets. April 2017. Available from: http://www.fao.org/antimicrobial-resistance [Accessed: Jun

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[3] Mermelstein NH. Combating antibiotic resistance. Food Technology. 2018;**72**(3):62-65 [4] Calvo M, Martínez LM. Antimicrobial mechanisms of action. Enfermedades Infecciosas

[5] Kapoor G, Saigal S, Elongavan A.Action and resistance mechanisms of antibiotics: A guide for clinicians. Journal of Anaesthesiology Clinical Pharmacology. 2017;**33**(3):300-305.

[6] Fraqueza MJ. Antibiotic resistance of lactic acid bacteria isolated from dry-fermented sausages. International Journal of Food Microbiology. 2015;**212**:76-88. DOI: 10.1016/j.

[7] O'Neill J. Tackling drug-resistant infections globally: Final report and recommendations. In: The Review on Antimicrobial Resistance. 2016. pp. 10-12. Available from: https://amr-

[8] The FAO Action Plan on Antimicrobial Resistance 2016-2020. Rome: Food and Agri-

[9] Preethi C, Rao-Thumu SC, Halami PM. Occurrence and distribution of multiple antibiotic-resistant *Enterococcus* and *Lactobacillus spp*. from Indian poultry: In vivo transferability of their erythromycin, tetracycline and vancomycin resistance. Annales de

[10] Dowling A, Dwyer JO, Adley CC.Antibiotics: Mode of action and mechanisms of resistence. In Méndez-Vilas A, editor. Antimicrobial Research: Novel Bioknowledge and Educational Programs. 1st ed. Badajoz, Spain: Formatex Research Centre; 2017. 536 p. ISBN (13): 978-

[11] Shao Y, Zhang W, Guo H, Pan L, Zhang H, Sun T. Comparative studies on antibiotic resistance in *Lactobacillus casei* and *Lactobacillus plantarum*. Food Control. 2015;**50**:250-258.

[12] Mathur S, Singh R.Antibiotic resistance in food lactic acid bacteria—A review. International Journal of Food Microbiology. 2005;**105**:281-295. DOI: 10.1016/j.ijfoodmicro.2005.03.008 [13] Erginkaya Z, Turhan EU. Determination of antibiotic resistance of lactic acid bacteria isolated for traditional Turkish fermented dairy products. Iranian Journal of Veterinary

[14] Giraffa G. Overview of the ecology and biodiversity of the LAB. In: Hotzapfel WH, Wood BJB, editors. Lactic Acid Bacteria: Biodiversity and Taxonomy. 1st ed. London:

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**5. Conclusions**

The path to reconciliation entails significant justice. However, it is for Indigenous peoples to determine their part in this path for themselves—with support from allies where possible. This paper is a starting point in identifying what a white settler woman can do in the supporting role of ally. This paper is a conceptual paper based on my forty-year legal and academic practice of justice. Through exploratory reflection utilizing hermeneutical phenomenology, I set the foundation of truth in relation to numbered Canadian treaties and residential schools in Canada. By hearing the truth, the path for achieving Indigenous law and sovereignty is advanced towards finally achieving reconciliation, although it may not be comfortable or a restoration to prior times.

Acting as an ally, I support Indigenous people and their journey. One day, Canada may achieve reconciliation and formal justice. This will be a world without oppression and discrimination; a world where everyone has the same opportunities and life chances. In this world, Indigenous peoples will not be over-incarcerated and under-represented in our most rewarding occupations. This path will not be based on restoring a previous relationship, and it will not necessarily be comfortable. It will involve hearing Indigenous peoples' truth and recognizing Indigenous sovereignty and Indigenous law.

**Funding:** This research was funded by the Canada Research Chairs program file 950-232285.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The author would like to thanks the reviewers for the helpful comments and suggestions.

**Conflicts of Interest:** The author declares no conflict of interest.


**Quality 4.0 in Action: Smart Hybrid Fault Diagnosis System in Plaster Production**

#### **Javaneh Ramezani \* and Javad Jassbi**

Faculty of Sciences and Technology and Uninova CTS, NOVA University of Lisbon, Campus de Caparica, 2829-516 Caparica, Portugal<EMAIL_ADDRESS>
**\*** Correspondence<EMAIL_ADDRESS>or<EMAIL_ADDRESS>
Received: 9 April 2020; Accepted: 21 May 2020; Published: 26 May 2020

**Abstract:** Industry 4.0 (I4.0) represents the Fourth Industrial Revolution in manufacturing, expressing the digital transformation of industrial companies employing emerging technologies. Factories of the future will enjoy hybrid solutions, while quality is the heart of all manufacturing systems regardless of the type of production and products. Quality 4.0 is a branch of I4.0 with the aim of boosting quality by employing smart solutions and intelligent algorithms. There are many conceptual frameworks and models, while the main challenge is to have the experience of Quality 4.0 in action at the workshop level. In this paper, a hybrid model based on a neural network (NN) and expert system (ES) is proposed for dealing with control chart patterns (CCPs). The idea is to have, instead of a passive descriptive model, a smart predictive model to recommend corrective actions. A construction plaster-producing company was used to present and evaluate the advantages of this novel approach, while the result shows the competency and eligibility of Quality 4.0 in action.

**Keywords:** statistical process control; control chart pattern; disruptions; disruption management; fault diagnosis; Industry 4.0; construction industry; plaster production; neural networks; decision support systems; expert systems; failure mode and effects analysis (FMEA); discriminant analysis

#### **1. Introduction, Background, and Problem Statement**


**1. Introduction**

A properly maintained network of forest roads provides the forest accessibility necessary for the sustainable use of forest resources, including forest maintenance, timber harvesting, hunting management, and recreational activities [1]. On the other hand, forest roads gradually lose their functionality due to erosion caused by use and exposure to rainfall after establishment, and if the degree of erosion becomes severe enough, they are damaged and become dysfunctional. The forms and factors of erosion and damage vary. Accumulation of sediment and organic matter in culverts crossing forest roads reduces the drainage capacity of culverts [2], causing gully of the road surface due to overflow of stream water [3], culvert failure, and road body failure at stream crossings. Overflow of road surface runoff water causes road body failure by significantly changing the shape of the rutted area itself. Sediment accumulation on the road surface due to soil avalanches from the cut slope or its upper slope makes the road impassable for vehicular traffic.

Soil erosion, in which forest roads are directly or indirectly involved, is an important source of sediment to streams in forested watersheds, affecting the hydrologic system of the forested watershed [4]. Sediment inputs to forested stream systems can have adverse effects on water quality, such as turbidity, increased nutrient concentrations, and reduced water clarity. Therefore, studies have examined the effects of rainfall, vegetation, and season on sediment runoff generated by the existing road network [5,6], as well as studies on the interaction between the road network and the river network [7].

Facilities for proper drainage are of paramount importance in road design [8]. Therefore, studies have been conducted on the layout and dimensioning of appropriate drainage

**Citation:** Watanabe, M.; Saito, M.; Toda, K.; Shirasawa, H. Rain-Driven Failure Risk on Forest Roads around Catchment Landforms in Mountainous Areas of Japan. *Forests* **2023**, *14*, 537. https://doi.org/ 10.3390/f14030537

Academic Editor: Chong Xu

Received: 15 February 2023 Revised: 7 March 2023 Accepted: 8 March 2023 Published: 9 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

1 United Graduate School of Agricultural Sciences, Iwate University, Morioka 020-8550, Japan

facilities, taking into account the topography and hydrological factors of the hillsides and the road structure [2,9]. From the viewpoint of minimizing the amount of road surface erosion, guidelines have been given for appropriate cross-sectional trench spacing according to the longitudinal gradient [10]. In recent years, forest road design and maintenance methods have been proposed that take into account the impact on the forest environment as well as economic efficiency; Aruga et al. [11] attempted to reduce sediment deposition to rivers and the total cost of forest road alignment planning; Dodson et al. [12] studied the environmental impact on rivers, the cost both environmental and economic to the river, the occurrence of road failures that may lead to both environmental and economic costs, and the total cost of owning a forest road, they examined a method for scheduling maintenance and improvement activities on forest road segments to minimize these three costs. Saito et al. [13] studied an automatic forest road design model that considers shallow landslides using LiDAR data. Pellegrini and Grigolato [14] examined how the integrated use of GIS and Analytical Hierarchy Process (AHP) analysis can be used to determine priorities in road network maintenance work to minimize sediment generation from road surfaces and maximize the social value of roads.

In Japan, studies have analyzed the risk of forest road failure from the perspective of topography and forest road structure. Located on the eastern edge of monsoon Asia, one of the world's rainiest regions, Japan has an average annual precipitation of 1718 mm, which is about twice the world average (880 mm) [15]. The rainy season in June and typhoons in autumn bring torrential rains. Geologically, the terrain is complex and steep due to its location in an orogenic belt where the Pacific Plate is subducting toward the continental plate, and there are many mountainous areas with active erosion. Therefore, the placement of forest roads on unstable slopes may cause slope failure. Kondo and Kamiya [16] surveyed 100 forest road failures that occurred on three forest road routes over a 13-year period from 1977 to 1989 and analyzed the characteristics of damage-prone stream crossings using Quantification II. Surface failures on hillsides occur most frequently in concave landforms (zero-order basin) where seepage water and weathered sediments are concentrated and have not yet developed in the primary valley [17]. Yoshimura and Kanzaki [18] analyzed slope failure factors by analysis of variance using only topographic factors legible from topographic maps. In the analysis, inclination, cross-section slope, turning point inclination, and catchment area were used as the factors estimating the risk of slope failure. They reported that the risk of failure is particularly high on concave slopes. Yoshimura et al. [19] also examined forest road alignment geometry in slope failure areas and reported that the risk of failure was significantly higher on inner curves where water had been concentrated, followed by a higher risk of failure at outer curve inflection points (the starting and ending points of a curve on a curve located on a ridge). Kondo and Kamiya [16] and Yoshimura et al. [19] suggest that the topography and structural configuration of forest road sections have some influence on the risk of forest road failure in Japan, although they do not consider the layout of drainage facilities. In Japanese forest road engineering textbooks, the rising water level of the stream parallel to the forest road has also been cited as a cause of damage on the valley side of forest roads [20].

Forest road failures in Japan are more likely to occur in forest road segments with topography, such as stream crossings, stream sides, and zero-order basins [16–20]. However, all of the existing studies in Japan are only the results of a few lines of investigation in some areas. Thus, for example, Kondo and Kamiya [16] surveyed 100 forest road failures that occurred on three forest road routes over a 13-year period from 1977 to 1989. They reported that failures at stream crossings accounted for 72% of the total in the research site and reported a characteristic form of damage, but it is not known to what extent this trend applies in Japan. Wemple et al. [21] reported that of 103 sediment transport events that occurred near forest roads in the western Cascade Range region of Oregon due to heavy rainfall in February 1996, three-quarters were mass movements, debris flows, cut slope slides, fill slope slides, and Phillips [22] investigated 116 landslides that occurred during two heavy rainfall events on the east coast of the North Island and found 17 landslides

associated with forest roads. Sidle et al. [23] studied mass wasting along a road in Yunnan, China, to determine the extent to which landslide erosion was caused by cut-and-fill slope failure or dry ravel. The results show that the landslide erosion was caused by a cut slope, embankment slope failure, and dry ravel. However, only one or two heavy rainfall events were reported in each case.

Factors and countermeasures for forest road failure differ depending on the type of failure that occurs, but the statistical understanding of the type of damage events is not sufficient because of the limited study area in the existing studies. Since we do not know the macroscopic occurrence trends of failures, it is also impossible to determine whether there are sufficient guidelines and research on the important countermeasures for failures that occur in the target area. In addition to stream crossings, other failure hazards have been identified in concave terrain, but it is not known to what extent the susceptibility to failure differs. If there is a difference in the likelihood of failure even in the same category of damage-prone areas, the weight should be changed when selecting routes and scheduling maintenance and management. If there is a pattern of failure that tends to require high repair costs, then the most important issue from the viewpoint of reducing repair costs is to prevent the occurrence of such a pattern of failure. Since the sensitivity to rainfall intensity may differ depending on the damage pattern, it is important to understand the relationship between the failure pattern and rainfall intensity in order to consider the maintenance of forest roads under climate change. By understanding the overall trend of forest road failure, more strategic maintenance may be possible. In Japan, forest road failures occur frequently due to heavy rains caused by typhoons and rainy season fronts, which predominantly occur from July to October. The total number of damaged segments of forest roads and the total amount spent on repair in Japan in the last 3 years has been 8181 (19.3 billion yen) in the 2017 fiscal year, 13,241 (39.8 billion yen) in the 2018 fiscal year, and 12,448 (34.1 billion yen) in the 2019 fiscal year [24]. However, the latest Basic Plan for Forestry and Forest Products states that the desired length for forest roads is 250,000 km, compared to the current 190,000 km [25]. If the frequency of heavy rainfall and the length of forest roads increase in the future, the number of failures caused by heavy rainfall and the cost of repair is very likely to increase [26]. In recent years, there has been a shortage of forest engineers who can formulate repair plans [24]. If the number of failures is not reduced, it may not be possible to provide the labor required for failure repair in the future. If the number of failures increases, the time required for failure recovery may become longer, with forest roads remaining impassable for a longer period and forestry activities potentially being severely hampered. As the intensity and frequency of rainfall events increase, it is important to avoid planning routes in unstable terrain and to take appropriate precautionary measures in hazardous areas on existing routes. This is particularly important to promote appropriate forest road maintenance in the context of limited human and economic resources. To achieve this, it is necessary to identify what types of forest road failure are important to address.

Therefore, in order to provide statistical information on forest road failures, this study collected data on a large number of forest road failures that occurred in mountainous areas in Japan and statistically analyzed the characteristics of each topographic type at the points where failures occurred. Specifically, forest road segments were classified into four categories: stream crossings, streamside, zero-order basin, and others, and comparisons were made regarding the length of forest road failure, the relative probability of occurrence, repair costs, and induced rainfall intensity in each category.

The terms "erosion" and "failure" to forest roads are not strictly defined. In this study, the difference between the two terms is the degree of erosion, and the term "failure" is used when a forest road is so eroded that it cannot fulfill its design purpose and becomes dysfunctional (Figure 1). In this study, "failure" includes shallow landslides, slope failures, road surface erosion, and road body failure.

**Figure 1.** Image of forest road failure. A culvert at a stream crossing was destroyed by heavy rain, and the road body was eroded by stream water, making it inaccessible to vehicles.

#### **2. Materials and Methods**