# Quantitative insights into the phase behaviour and miscibility of organic
photovoltaic active layers from the perspective of neutron spectroscopy

Mohamed Zbiri  Institut Laue-Langevin, 71 avenue des Martyrs, Grenoble Cedex
9, 38042, France<EMAIL_ADDRESS>Peter A. Finn  Materials Research Institute and
School of Biological and Chemical Sciences, Queen Mary University of London,
Mile End Road, London E1 4NS, United Kingdom  Christian B. Nielsen  Materials
Research Institute and School of Biological and Chemical Sciences, Queen Mary
University of London, Mile End Road, London E1 4NS, United Kingdom  Anne A.Y.
Guilbert  Department of Physics and Centre for Plastic Electronics, Imperial
College London, Prince Consort Road, London SW7 2AZ, United Kingdom
<EMAIL_ADDRESS>
###### Abstract

We present a neutron spectroscopy based method to study quantitatively the
partial miscibility and phase behaviour of an organic photovoltaic active
layer made of conjugated polymer:small molecule blends, presently illustrated
with the regio-random poly(3-hexylthiophene-2,5-diyl) and fullerene
[6,6]-Phenyl C61 butyric acid methyl ester (RRa-P3HT:PCBM) system. We perform
both inelastic neutron scattering and quasi-elastic neutron scattering
measurements to study the structural dynamics of blends of different
compositions enabling us to resolve the phase behaviour. The difference of
neutron cross sections between RRa-P3HT and PCBM, and the use of deuteration
technique, offer a unique opportunity to probe the miscibility limit of
fullerene in the amorphous polymer-rich phase and to tune the contrast between
the polymer and the fullerene phases, respectively. Therefore, the proposed
approach should be universal and relevant to study new non-fullerene acceptors
that are closely related - in terms of chemical structures - to the polymer,
where other conventional imaging and spectroscopic techniques present a poor
contrast between the blend components.

##  1 Introduction

Excitons, generated upon light absorption in conjugated polymers, are known to
dissociate into free charges in the presence of an electron acceptor material.
Bulk heterojunctions made of polymer donor and small-molecule acceptor
materials constitute the active layer of organic solar cells. The
microstructure of such blends is complex with most likely three phases, a
small-molecule rich phase, an amorphous polymer-rich phase and if the polymer
is semi-crystalline, a pure crystalline polymer phase.1 Only few polymers such
as poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT)2 are
known to co-crystallise with fullerene acceptors. Because of the asymmetry of
the donor and acceptor molecular weights, the small molecule rich phase is
nearly pure. The amorphous polymer-rich phase is beneficial for charge
separation3 while the presence of nearly pure percolated donor and acceptor
domains are beneficial for the transport of charges generated at the
donor:acceptor heterojunction to the electrodes.
If crystallinity is relatively simple to monitor by methods such as X-Ray
diffraction, the composition of the amorphous mixture of the blends4, 5 as
well as changes in conformation with respect to the neat materials is more
difficult to access.6, 7 Although crystallinity has been shown to improve
charge transport8 and potentially lead to extra driving force for charge
separation by lowering the electronic energy levels,2 a spinodal-type
decomposition emerged as a new picture for phase separation at length scales
directly relevant to the operation of the devices,9, 10, 11 with the
coarsening of this phase separation directly linked to burn in degradation
mechanisms.10 The crucial role of the Flory-Huggins interaction parameter
($\chi$) in controlling phase behaviour, i.e. miscibility in the amorphous
phase has been emphasised and related to solar cell efficiency.11, 12 $\chi$
is both composition- and temperature-dependent, and is related to the
thermodynamical stability of the blend. However, the formation of the bulk
heterojunction proceeds through solution processing.13 Thus, the final
microstructure is not thermodynamically stable but kinetically trapped.
Crystal seeds of small molecules and more or less large crystals of the
polymer may form in the solution depending on the quality of the solvent for
each component of the blend.14, 15, 16 Moreover, liquid-liquid demixing may
occur during solvent evaporation which could contribute to enhance phase
separation.

Figure 1: (a) Schematic illustration of the quantities extracted from the
neutron spectroscopic measurements. From the measured dynamical structure
function, S(Q, E) (color coded map), at 360 K using IN6, the diffraction
pattern (left), QENS spectrum, and low-energy INS spectrum (bottom) are
obtained. The mid-to-high energy vibrational spectrum (bottom right) was
measured at 10 K using IN1-Lagrange. (b) Total, incoherent and coherent
macroscopic neutron cross-sections ($\Sigma$) as a function of h-PCBM
concentration in the presently studied samples. The samples represented by
scatter points are h-RRa-P3HT:h-PCBM at 296 K (0 wt%, 20 wt%, 75 wt% and 100
wt% h-PCBM) and d-RRa-P3HT:h-PCBM at 360 K (0 wt%, 35 wt%, 50 wt%, 100 wt%
h-PCBM). The macroscopic neutron cross-sections are extracted from the QENS
data as explained in Supporting Information.

Previously, we used a combination of quasi-elastic neutron scattering (QENS)
measurements17 and molecular dynamics (MD) simulations18 to investigate the
impact of each component of a blend of regio-regular
poly(3-hexylthiophene-2,5-diyl) (RR-P3HT) and fullerene [6,6]-Phenyl C61
butyric acid methyl ester (PCBM) on their respective dynamics. We observed
that, upon blending with PCBM, the QENS signal of P3HT is narrowing, while
upon blending with RR-P3HT, the QENS signal of PCBM is broadening. We did
interpret these observations as a signature of the frustration of RR-P3HT and
plasticization of PCBM upon blending, respectively. The frustration of RR-P3HT
was also observed by other groups on a different time scale. 19, 20, 21 Our MD
simulations suggested that these changes were due to the partial miscibility
of P3HT:PCBM, in particular the formation of an amorphous mixture of
P3HT:PCBM. MD simulations further revealed a conformational change of P3HT
chain to accommodate PCBM with enhanced cofaciality between the polymer
thiophene rings and the PCBM cage. This has further been supported by Zheng et
al., whose MD simulations pointed towards the same cofaciality between P3HT
and PCBM. They calculated the transfer integrals between P3HT and PCBM in such
arrangement, 22 concluding that the enhanced cofaciality was beneficial for
the charge separation processes in organic solar cells.
Presently, we study blends of regio-random P3HT and PCBM (RRa-P3HT:PCBM), with
various compositions of PCBM, to clarify the impact of partial miscibility of
conjugated polymer:small molecule systems. With respect to previous
studies,19, 20, 17, 21 we go a step further by using both inelastic neutron
scattering (INS) and quasi-elastic neutron scattering (QENS) (Figure 1 a) to
resolve simultaneously changes in microstructure, morphology and dynamics of
both the polymer and fullerene upon blending as a function of temperature. The
observed morphological changes are further rationalised by quantum chemical
calculations. To gain deeper insights, we use the deuteration technique to
vary the contrast between the polymer and the fullerene. In the following,
hydrogenated and deuterated materials will be labeled with the prefix h- and
d-, respectively. The different sample compositions and their neutron cross
sections are presented in Figure 1 b. We propose to take advantage of the
difference of neutron cross sections between conjugated polymer and fullerene
to evaluate the miscibility limit of fullerene within the amorphous polymer-
rich phase. The neutron spectroscopy based method presently described should
be universal and relevant to study blends with new non-fullerene acceptors
that are closely related in terms of chemical structures to the polymer, which
otherwise lead to a poor contrast between the blend components when using
conventional imaging and optical spectroscopy techniques.23

##  2 Results and discussion

###  2.1 Evaluating the phase composition of the blends

Figures 2 a and b show the Q-averaged QENS spectra of neat h-RRa-P3HT, blends
of h-RRa-P3HT:h-PCBM of compositions of 20 wt%, 45 wt% and 75 wt% h-PCBM, neat
h-PCBM at 296K, and neat h-PCBM, blends of d-RRa-P3HT:h-PCBM of compositions
of 35 wt% and 50 wt% h-PCBM, neat d-RRa-P3HT at 360K, respectively. As the
concentration of h-PCBM increases in the h-RRa-P3HT:h-PCBM blends, the QENS
spectra are narrowing while as the concentration of d-RRa-P3HT increases in
the blends of d-RRa-P3HT:h-PCBM, the QENS spectra are broadening. The
incoherent contribution of h-PCBM is too small compared to that of h-RRa-P3HT,
thus, the QENS signal is largely dominated by the h-RRa-P3HT incoherent
contribution. However, as the concentration of h-PCBM increases, the
associated contribution is expected to become more significant. We selected
concentrations where the overall incoherent contribution to the signal is
higher than the coherent counterpart,21 although it is the difference which is
reduced in the case of d-RRa-P3HT (Figure 1 b). It is therefore reasonable to
state that adding h-PCBM to h-RRa-P3HT frustrates the P3HT dynamics, and
adding d-RRa-P3HT to h-PCBM plasticizes PCBM, in agreement with previous
findings.17

Figure 2: Q-averaged QENS spectra of (a) neat h-RRa-P3HT, blends of
h-RRa-P3HT:h-PCBM of 20 wt%, 45 wt% and 75 wt% h-PCBM concentration and neat
h-PCBM at 296K and (b) neat d-RRa-P3HT, blends of d-RRa-P3HT:h-PCBM of 35 wt%
and 50 wt% h-PCBM concentration and neat h-PCBM at 360K.

Having determined the macroscopic densities (see Supporting Information) of
the neat polymer and fullerene phases, $\Sigma_{RRa-P3HT}$ and $\Sigma_{PCBM}$
(Figure 1 b), respectively, we can proceed with modelling the QENS data to
gain quantitative insights into the concentration-dependent phase behaviour,
as shown in Figure 3 a and b. RRa-P3HT is fully amorphous and thus, it is
reasonable to assume that the studied samples with an overall PCBM
concentration higher than the miscibility limit, $\mu$, exhibit two phases; a
nearly pure h-PCBM phase and an amorphous RRa-P3HT rich phase. The QENS signal
can, therefore, be expressed for an overall h-PCBM concentration $c_{0}$
larger than $\mu$ as follow:

$(c_{0}\times\Sigma_{PCBM}+(1-c_{0})\times\Sigma_{RRa-P3HT})\times
S(c_{0},E,Q)=\\\ \frac{c_{0}-\mu}{1-\mu}\times\Sigma_{PCBM}\times
S^{PCBM}(E,Q)+\\\
\frac{1-c_{0}}{1-\mu}\times(\mu\times\Sigma_{PCBM}+(1-\mu)\times\Sigma_{RRa-P3HT})\times
S^{mix}(\mu,E,Q)$ (1)

where $S(c_{0},E,Q)$ is the concentration-dependent total dynamical scattering
function, $S^{PCBM}(E,Q)$ is the dynamical scattering function of the PCBM
phase and $S^{mix}(\mu,E,Q)$ is the mixed-phase dynamical scattering function
at the miscibility limit. Below the miscibility limit $\mu$, it is reasonable
to assume that we have a solid solution (amorphous mixture) and we assume
that:

$\frac{\mu}{S^{mix}(c_{0},E,Q)}=\frac{c_{0}}{S^{mix}(\mu,E,Q)}+\frac{\mu-
c_{0}}{S^{RRa-P3HT}(E,Q)}$ (2)

where $S^{mix}(c_{0},E,Q)$ is the concentration-dependent dynamical scattering
function of the mixed-phase below the miscibility limit and
$S^{RRa-P3HT}(E,Q)$ is the dynamical scattering function of the RRa-P3HT
phase. In order to describe continuously the change and evolution in phase
behaviour, we use logistic functions with a large $k$ parameter to approximate
step functions, so the previous equation becomes:

$S(c_{0},E,Q)=\frac{1}{1+e^{k(c_{0}-\mu)}}\times{\frac{\mu}{\frac{c_{0}}{S^{mix}(\mu,E,Q)}+\frac{\mu-
c_{0}}{S^{RRa-P3HT}(E,Q)}}}+\\\
\frac{1}{1+e^{-k(c_{0}-\mu)}}\times\\{\frac{c_{0}-\mu}{1-\mu}\times\frac{\Sigma_{PCBM}}{c_{0}\times\Sigma_{PCBM}+(1-c_{0})\times\Sigma_{RRa-P3HT}}\times
S^{PCBM}(E,Q)\\\
+\frac{1-c_{0}}{1-\mu}\times\frac{\mu\times\Sigma_{PCBM}+(1-\mu)\times\Sigma_{RRa-P3HT}}{c_{0}\times\Sigma_{PCBM}+(1-c_{0})\times\Sigma_{RRa-P3HT}}\times
S^{mix}(\mu,E,Q)\\}$ (3)

The two quantities to fit are the miscibility $\mu$ and the scattering
intensity at the miscibility limit $S^{mix}(\mu,E,Q)$. We fit successfully the
QENS spectra using this model (Figures 3 a and b). We found miscibility limits
of about 20 wt% PCBM for h-RRa-P3HT:h-PCBM at 296 K, 27 wt% PCBM and 30 wt%
PCBM for d-RRa-P3HT:h-PCBM at 296 K and 360 K, respectively. These values are
within the range found by means of other techniques.24, 4, 1, 25 Furthermore,
we found that as expected and supported by other techniques, the miscibility
is increasing slightly with temperature.1 The observed difference in the
miscibility limits obtained at 296K for h-RRa-P3HT:h-PCBM and
d-RRa-P3HT:h-PCBM can be attributed to factors like deuteration, difference in
molecular weight of the two polymers and the difference in regioregularity
(Table 1). Interestingly, this simple model assuming two phases above the
miscibility limit captures well the microstructure of the blends, hence
allowing us to extract the QENS spectra at the miscibility limit for each
blend at different temperatures (Figures 3 c and d). The further narrowing of
the QENS spectra of h-P3HT:h-PCBM for overall PCBM concentrations larger than
the miscibility limit is not due to an extra frustration of the RRa-P3HT but
to the presence of the almost neat crystalline PCBM phase.

Figure 3:  (a) and (b) Concentration-dependent integrals (scatter points) of
the elastic region of the Q-averaged QENS spectra of h-RRa-P3HT:h-PCBM at
296K, and d-RRa-P3HT:h-PCBM at 296K and 360K, respectively. The integration
was done between -0.05 and 0.05 meV. Lines are fits using logistic functions-
based Equation 3. (c) Q-averaged QENS spectra of h-RRa-P3HT, h-RRa-P3HT:h-PCBM
of 20 wt%, $\mu$ 20 wt%, and 75 wt% h-PCBM concentration and h-PCBM at 296K.
(d) Q-averaged QENS spectra of d-RRa-P3HT, blends of d-RRa-P3HT:h-PCBM of
$\mu$ 30 wt% ,35 wt% and 50 wt% h-PCBM concentration and h-PCBM at 360K. $\mu$
is the miscibility concentration for each system at the studied temperature.

###  2.2 Monitoring simultaneously miscibility and microstructure

The phase separation can be enhanced by the crystallisation of one of the
blend components; here, the PCBM phase. By averaging the $S(Q,E)$ signal in
energy, we can extract the neutron diffractograms of the samples, therefore
allowing us to study miscibility and crystallisation of PCBM simultaneously
(Figure 4). The background in the diffractograms presented in Figures 4 b, c
and d is due to the incoherent contribution to the signal, and it follows the
calculated trend (Figure 1). RRa-P3HT is indeed mainly amorphous with a broad
Bragg peak around Q 1.4 Å-1, which in terms of distance is linked with the
$\pi-\pi$ interaction and stacking in the material. As the PCBM concentration
increases, peaks signature of PCBM crystallisation can be observed, where for
instance the h-RRa-P3HT:h-PCBM 75wt% h-PCBM sample is clearly crystalline.
Although h-PCBM is highly crystalline at 360K, it is not easy to distinguish
the crystalline signal in the d-RRa-P3HT:h-PCBM blends due to the small
difference in the coherent cross sections of d-RRa-P3HT and h-PCBM. However,
the presence of small peaks related to h-PCBM confirms that crystals of PCBM
are present in the d-RRa-P3HT:h-PCBM blend with 50 wt% h-PCBM.

Figure 4: (a) Q-averaged QENS spectra at the miscibility limit. (b)
concentration-dependent neutron diffraction patterns of h-RRa-P3HT:h-PCBM at
296K. (c) and (d ) concentration-dependent neutron diffraction patterns of
d-RRa-P3HT:h-PCBM at 296K and 360K, respectively.

###  2.3 Probing changes in morphology by means of inelastic neutron
scattering

In a recent work,26 we used synergistically various neutron scattering
techniques, including inelastic neutron scattering (INS), to map out the
structural dynamics of RR-P3HT and RRa-P3HT up to the nanosecond time scale.
Here, we use INS to probe the changes in morphology in the two phases present
in the different blends of RRa-P3HT:PCBM: (i) an amorphous mixture of
RRa-P3HT:PCBM with a concentration of about 20 wt% PCBM for the blend
h-RRa-P3HT:h-PCBM and about 30 wt% for the blend d-RRa-P3HT:h-PCBM, and (ii) a
PCBM-rich phase. So far, given that we considered mainly the incoherent
scattering of the samples, we assumed that the PCBM-rich phase in those blends
were similar to the neat PCBM samples. However, RRa-P3HT is likely to be also
miscible to a lesser extent with PCBM and thus, the morphology of PCBM is
likely to be different in the PCBM-rich phase compared to the neat PCBM phase.

Figure 5: INS spectra measured using (a) IN6 at 296K, and (b) IN1-Lagrange at
10 K of neat h-RRa-P3HT and blend h-RRa-P3HT:h-PCBM, with 20wt% h-PCBM
composition.

Figure 5 compares the INS spectra of the blend h-RRa-P3HT:h-PCBM, with 20 wt%
of h-PCBM (at the miscibility limit), and the neat sample h-RRa-P3HT. The full
energy range is captured thanks to the combination of both the measurements
shown in Figures 5 a and b. No significant differences are observed between
the INS spectra of neat h-RRa-P3HT and the blend. Either no noticeable changes
in morphology of h-RRa-P3HT occurs upon blending with h-PCBM, or the strong
signal from hydrogens dominates the spectra and could therefore mask potential
differences. Hydrogens are mainly located on the side-chains and therefore,
their strong incoherent signal does not reflect strongly potential
conformational changes or dynamics of the polymer backbones.

Figure 6: (a) Composition-dependent INS spectra of the blend h-RRa-P3HT:h-PCBM
measured using IN6 at 296K, and IN1-Lagrange at 10K, with variable h-PCBM
concentrations of 0 wt% (neat h-RRa-P3HT), 20 wt% h-PCBM (at the miscibility
limit), 75 wt% h-PCBM and 100 wt% (neat h-PCBM). (b) Comparison between
measured and calculated INS spectra of h-PCBM. The measured INS spectra
consist of neat h-PCBM phase and rich h-PCBM phase. The calculated INS spectra
are from single molecule quantum chemical calculation and solid-state periodic
calculation. The grey-shaded areas highlight regions with marked differences
both in peak intensity and energy shift.

As expected from our two-phase model, at a PCBM concentration (75 wt% PCBM)
much higher than the miscibility limit, $\mu$, the INS spectrum of the blend
h-RRa-P3HT:h-PCBM exhibits clear PCBM-related features (Figure 6 a). However,
the INS spectrum of the 75 wt% PCBM blend cannot be reconstructed by neutron
weighting and combining the INS spectrum of h-RRa-P3HT:h-PCBM with a
concentration close to the miscibility limit (20 wt% PCBM), and the INS
spectrum of neat h-PCBM. Therefore, we assign the differences between the
neutron weighted average of the INS spectra and the measured spectrum to
changes in morphology between the neat h-PCBM and the PCBM-rich phase. Thus,
the INS spectrum of the PCBM-rich phase is extracted from the difference
between the INS spectra of the h-RRa-P3HT:h-PCBM blends of compositions above
the miscibility limit and the blend at the miscibility limit taking into
account the neutron weights (Figure 6 b). The main noticeable differences
between the neat PCBM and the PCBM-rich phase are highlighted in the grey-
shaded areas in Figure 6 b. To get a deeper insight into these changes upon
blending, we carried out density functional theory (DFT) calculations. Both
the gas-phase single/isolated molecule and solid-state periodic approaches
were adopted. This allows assessing (i) the relative strength of the intra-
molecular and inter-molecular vibrational aspects and (ii) the importance of
external (lattice) degrees-of-freedom. The single molecule approximation
neglects the interaction with the environment, while the periodic approach
accounts for the lattice dynamical modes and can point towards possible mode
coupling between the molecular vibrations (internal modes) and the lattice
(external) modes or between molecular modes. The INS spectrum of the neat
h-PCBM is well simulated using the solid-state periodic approach, while the
INS spectrum of the h-PCBM rich phase is found to be better approximated by
the single molecule calculation. We conclude then that the crystallinity
aspect of the neat PCBM is well described by the periodic calculations while
the more disordered behaviour of the PCBM rich phase makes it reasonable to
use the single molecule approximation. Since the focus is on an energy range
where intra-molecular vibrations are dominating, we suggest that the coupling
between intra-molecular vibrations and the environment decreases in the h-PCBM
rich phase and any possible effects are potentially averaged out.

Figure 7: (a) Composition-dependent INS spectra of the d-RRa-P3HT:h-PCBM
blends measured using IN6 at 360K, and IN1-Lagrange at 10K, with variable
h-PCBM concentrations of 0 wt% (neat d-RRa-P3HT), 35 wt% (close to the
miscibility limit), 50 wt% and 100 wt% (neat h-PCBM). (b) Comparison between
measured and calculated INS spectra of neat and rich d-RRa-P3HT phase. The
measured INS spectra consist of neat d-RRa-P3HT phase and the reconstructed
rich d-RRa-P3HT phase. The calculated INS spectra are from single-molecule
quantum chemical calculations of a deuterated P3HT molecule and a deuterated
P3HT molecule with a PCBM molecule.

In order to resolve any possible changes of conformation or coupling between
P3HT and PCBM in the mixed phase, we measured the partially deuterated blends
d-RRa-P3HT:h-PCBM with 35 wt% and 50 wt% of h-PCBM (Figure 7 a). Note that 35
wt% h-PCBM is close to the miscibility limit of 30 wt% h-PCBM, determined
above. Clear differences are observed between INS spectra of neat d-RRa-P3HT
and d-RRa-P3HT:h-PCBM with 35 wt% h-PCBM, and are mainly assigned to features
related to h-PCBM. These differences become marked by increasing the h-PCBM
concentration. We went a step further by using the INS spectrum of the h-PCBM
rich phase to reconstruct the d-RRa-P3HT rich phase spectrum (Figure 7 b). The
INS spectrum of the d-RRa-P3HT-rich phase is extracted as the difference
between the INS spectra of d-RRa-P3HT:h-PCBM blends of compositions above the
miscibility limit and the h-PCBM rich phase taking into account the neutron
weights. The DFT-based single oligomer calculation of d-RRa-P3HT reproduced
well the INS spectrum of neat d-RRa-P3HT, which is mainly amorphous. Note that
d-RRa-P3HT has hydrogenated defects along the backbones and the strong peaks
at 815 cm-1 and 1185 cm-1 can only be reproduced by adding hydrogen defects
along the backbone in the model calculations. The observed differences between
the measured INS spectra of the neat d-RRa-P3HT and rich d-RRa-P3HT phase are
captured by our reconstruction method consisting of neutron weighting and
combining the single molecule calculation for d-RRa-P3HT and the single
molecule calculation for h-PCBM. No noticeable changes in morphology of
RRa-P3HT or coupling of modes between RRa-P3HT and PCBM are observed.

##  3 Experimental

h-RRa-P3HT was obtained from Sigma-Aldrich. d-RRa-P3HT was synthesized by an
iron(III) chloride mediated oxidative polymerization of
4-d1-3-d13-hexylthiophene in chloroform at room temperature. The molecular
weights, polydispersities, and regioregularities are summarized in Table 1.
The polymers in this study come from the same batches as our previous study26.
PCBM $>$ 99% grade was obtained from Solenne BV. The as-received materials
were dissolved in chloroform (40 mg/mL) and drop-cast on a glass slide on a
hot plate at 60 °C for 1 h. The drop-cast films were then scratched from the
glass substrates and stacked in aluminum foil. Each measured sample was about
400 mg. Further details related to the materials and their characterisation
can be found in references. 17, 26.

Table 1: Molecular weight (Mw) and polydispersity (PDI) as measured by gel permeation chromatography, regioregularity (RR) as measured by NMR and scattering length density (SLD) calculated assuming a density of 1 g/cm3. | Mw (in kDa) | PDI | RR (%) | SLD (in 10-6 Å-2)
---|---|---|---|---
h-RRa-P3HT | 304 | 3.2 | 56 | 0.61417
d-RRa-P3HT | 53 | 1.95 | 73 | 5.4053

The neutron spectroscopy measurements were performed using the direct
geometry, cold neutron, time-of-flight, time-focusing spectrometer IN6, and
the hot-neutron, inverted geometry spectrometer IN1-Lagrange at the Institut
Laue-Langevin (ILL, Grenoble, France). Data were reduced, treated and analysed
in a similar way as was done in our previous related works 17, 26.
DFT-based quantum chemical isolated molecule and solid-state periodic
calculations were performed using Gaussian 16 27 and Castep 28, respectively.
For the isolated molecules, the functional/basis-set b3lyp/6-311g(d,p) was
chosen 29. For the solid-state periodic calculations, the functional PBE 30
with van der Waals corrections 31 were used. Full computational details can be
found in reference 26.

##  4 Conclusions

We presented a neutron spectroscopy based methodology to study phase behavior
and morphology of the blend system P3HT:PCBM. We used a variable PCBM
composition approach and deuteration technique for P3HT to determine the
miscibility limits of the fullerene within the regio-random (amorphous) form
of P3HT (RRa-P3HT). Temperature-dependent and composition-dependent quasi-
elastic neutron scattering and inelastic neutron scattering measurements were
performed to evaluate the phase composition and behaviour of the blends, to
monitor simultaneously their miscibility and microstructure evolution and to
probe changes in their morphology. This approach enabled us to resolve the
evolution of the microstructure and morphology that are correlated with
changes in structural dynamics of the polymer and fullerene upon blending. Our
approach using single-molecule and solid-state periodic DFT calculations could
reproduce the differences in INS spectra between crystalline neat h-PCBM and
the more disordered h-PCBM rich phase. However, no clear evidences of P3HT
conformational changes over blending could be concluded. It should be reminded
though that neutrons probe an ensemble of conformations of both neat
d-RRa-P3HT phase and d-RRa-P3HT rich phase. Therefore, our approach might be
limited in the capture of morphological changes that will affect only chains
that are in close contact with the PCBM.

## Acknowledgements

The ILL is acknowledged for the use of the spectrometers IN1-Lagrange and IN6.
A. A. Y. G. acknowledges EPSRC for the award of an EPSRC Postdoctoral
Fellowship (EP/P00928X/1).

## Author contributions

A. A. Y. G. and M. Z. conceived and developed the neutron-based project,
prepared the samples, performed the neutron measurements, treated, analysed
and interpreted the neutron data and wrote the manuscript. P. A. F. and C. B.
N. synthesised d-RRa-P3HT.

## References

  * Collins _et al._ 2011 B. A. Collins, J. R. Tumbleston and H. Ade, _The Journal of Physical Chemistry Letters_ , 2011, 2, 3135–3145
  * Jamieson _et al._ 2012 F. C. Jamieson, E. B. Domingo, T. McCarthy-Ward, M. Heeney, N. Stingelin and J. R. Durrant, _Chem. Sci._ , 2012, 3, 485–492
  * Westacott _et al._ 2013 P. Westacott, J. R. Tumbleston, S. Shoaee, S. Fearn, J. H. Bannock, J. B. Gilchrist, S. Heutz, J. DeMello, M. Heeney, H. Ade, J. Durrant, D. S. McPhail and N. Stingelin, _Energy & Environmental Science_, 2013, 6, 2756
  * Treat _et al._ 2011 N. D. Treat, M. A. Brady, G. Smith, M. F. Toney, E. J. Kramer, C. J. Hawker and M. L. Chabinyc, _Advanced Energy Materials_ , 2011, 1, 82–89
  * Collins _et al._ 2013 B. A. Collins, Z. Li, J. R. Tumbleston, E. Gann, C. R. Mcneill and H. Ade, _Advanced Energy Materials_ , 2013, 3, 65–74
  * Tsoi _et al._ 2011 W. C. Tsoi, D. T. James, J. S. Kim, P. G. Nicholson, C. E. Murphy, D. D. C. Bradley, J. Nelson and J.-S. Kim, _Journal of the American Chemical Society_ , 2011, 133, 9834–9843
  * Razzell-Hollis _et al._ 2016 J. Razzell-Hollis, S. Limbu and J.-S. Kim, _The Journal of Physical Chemistry C_ , 2016, 120, 10806–10814
  * Noriega _et al._ 2013 R. Noriega, J. Rivnay, K. Vandewal, F. P. Koch, N. Stingelin, P. Smith, M. F. Toney and A. Salleo, _Nature Materials_ , 2013, 12, 1038–1044
  * Treat and Chabinyc 2014 N. D. Treat and M. L. Chabinyc, _Annual review of physical chemistry_ , 2014, 65, 59–81
  * Li _et al._ 2017 N. Li, J. D. Perea, T. Kassar, M. Richter, T. Heumueller, G. J. Matt, Y. Hou, N. S. Güldal, H. Chen, S. Chen, S. Langner, M. Berlinghof, T. Unruh and C. J. Brabec, _Nature Communications_ , 2017, 8, 14541
  * Ye _et al._ 2018 L. Ye, B. A. Collins, X. Jiao, J. Zhao, H. Yan and H. Ade, _Advanced Energy Materials_ , 2018, 8, 1703058
  * Peng _et al._ 2018 Z. Peng, X. Jiao, L. Ye, S. Li, J. J. Rech, W. You, J. Hou and H. Ade, _Chemistry of Materials_ , 2018, 30, 3943–3951
  * Richter _et al._ 2017 L. J. Richter, D. M. DeLongchamp and A. Amassian, _Chemical Reviews_ , 2017, 117, 6332–6366
  * Troshin _et al._ 2009 P. A. Troshin, H. Hoppe, J. Renz, M. Egginger, J. Y. Mayorova, A. E. Goryachev, A. S. Peregudov, R. N. Lyubovskaya, G. Gobsch, N. S. Sariciftci and V. F. Razumov, _Advanced Functional Materials_ , 2009, 19, 779–788
  * Machui _et al._ 2011 F. Machui, S. Abbott, D. Waller, M. Koppe and C. J. Brabec, _Macromolecular Chemistry and Physics_ , 2011, 212, 2159–2165
  * Machui _et al._ 2012 F. Machui, S. Langner, X. Zhu, S. Abbott and C. J. Brabec, _Solar Energy Materials and Solar Cells_ , 2012, 100, 138–146
  * Guilbert _et al._ 2016 A. A. Y. Guilbert, M. Zbiri, M. V. C. Jenart, C. B. Nielsen and J. Nelson, _The Journal of Physical Chemistry Letters_ , 2016, 7, 2252–2257
  * Guilbert _et al._ 2017 A. A. Y. Guilbert, M. Zbiri, A. D. F. Dunbar and J. Nelson, _The Journal of Physical Chemistry B_ , 2017, 121, 9073–9080
  * Paternó _et al._ 2013 G. Paternó, F. Cacialli and V. García-Sakai, _Chemical Physics_ , 2013, 427, 142–146
  * Etampawala _et al._ 2015 T. Etampawala, D. Ratnaweera, B. Morgan, S. Diallo, E. Mamontov and M. Dadmun, _Polymer_ , 2015, 61, 155–162
  * Paternò _et al._ 2016 G. M. Paternò, J. R. Stewart, A. Wildes, F. Cacialli and V. G. Sakai, _Polymer_ , 2016, 105, 407–413
  * Zheng _et al._ 2019 Z. Zheng, N. R. Tummala, T. Wang, V. Coropceanu and J. L. Brédas, _Advanced Energy Materials_ , 2019, 9, 1803926
  * Rezasoltani _et al._ 2020 E. Rezasoltani, A. A. Y. Guilbert, J. Yan, X. Rodríguez-Martínez, M. Azzouzi, F. Eisner, S. M. Tuladhar, Z. Hamid, A. Wadsworth, I. McCulloch, M. Campoy-Quiles and J. Nelson, _Chemistry of Materials_ , 2020, 32, 8294–8305
  * Yin and Dadmun 2011 W. Yin and M. Dadmun, _ACS Nano_ , 2011, 5, 4756–4768
  * Guilbert _et al._ 2014 A. Guilbert, M. Schmidt, A. Bruno, J. Yao, S. King, S. Tuladhar, T. Kirchartz, M. Alonso, A. Goñi, N. Stingelin, S. Haque, M. Campoy-Quiles and J. Nelson, _Advanced Functional Materials_ , 2014, 24, 6972–6980
  * Guilbert _et al._ 2019 A. A. Guilbert, M. Zbiri, P. A. Finn, M. Jenart, P. Fouquet, V. Cristiglio, B. Frick, J. Nelson and C. B. Nielsen, _Chemistry of Materials_ , 2019, 31, 9635–9651
  * Frisch _et al._ 2016 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A. Petersson, H. Nakatsuji, X. Li, M. Caricato, A. V. Marenich, J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci, H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov, J. L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini, F. Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V. G. Zakrzewski, J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, K. Throssell, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M. J. Bearpark, J. J. Heyd, E. N. Brothers, K. N. Kudin, V. N. Staroverov, T. A. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. P. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, J. M. Millam, M. Klene, C. Adamo, R. Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma, O. Farkas, J. B. Foresman and D. J. Fox, _Gaussian16 Revision A.03_ , 2016
  * Clark _et al._ 2005 S. J. Clark, M. D. Segall, C. J. Pickard, P. J. Hasnip, M. I. J. Probert, K. Refson and M. C. Payne, _Zeitschrift für Kristallographie - Crystalline Materials_ , 2005, 220, 567–570
  * Stephens _et al._ 1994 P. J. Stephens, F. J. Devlin, C. F. Chabalowski and M. J. Frisch, _J. Phys. Chem._ , 1994, 98, 11623–11627
  * Perdew _et al._ 1996 J. P. Perdew, K. Burke and M. Ernzerhof, _Phys. Rev. Lett._ , 1996, 77, 3865–3868
  * Tkatchenko and Scheffler 2009 A. Tkatchenko and M. Scheffler, _Phys. Rev. Lett._ , 2009, 102, 073005