the ii - vi semiconductor zno is one of the most important multifunctional materials due to its various exotic properties such as direct wide band gap ( 3.37 ev ) and high optical gain of 300 cm ( 100 cm for gan ) at room temperature , large saturation velocity ( 3.2  10 cm / s ) , high breakdown voltage , large exciton binding energy ( 60 mev ) , piezoelectric , biocompatibility , and so on [ 1 - 12 ] .
zno can be used in variety of high - technological practical applications such as ultraviolet ( uv ) lasers , light - emitting diodes , photodetectors , piezoelectric transducers and actuators , hydrogen storage , chemical and biosensors , surface acoustic wave guides , solar cells , photocatalysts , etc .
the use of zno nanomaterials as photoelectrodes for the fabrication of dye - sensitized solar cells ( dsscs ) has received a great attention due to its compatibility and higher electronic mobility with tio2 nanomaterials and similar electron affinity and band gap ( 3.37 ev at 298 k ) .
therefore , some zno nanostructures have been used as photoelectrode materials for the fabrication of dsscs and reported in the literature [ 15 - 21 ] .
hsu et al .   reported the zno nanorods - based dssc with the electricity conversion efficiency ( ece ) of 0.22% .
branched zno nanowires based dsscs , grown by thermal evaporation process at 8001,000 c , with an ece of ~0.46% have been reported by suh et al . .
in another report , by using branched zno nanowires grown by mocvd process , the fabricated dsscs exhibited an ece of ~0.5% .
cheng et al .   also demonstrated the thermally grown zno nanorods - based dssc with the ece of 0.6% . in this paper , we report the direct synthesis of well - crystallized zno nanocombs on fto substrates and their dsscs application .
to fabricate the dsscs , the thin films of as - grown zno nanocombs on fto substrates were used as photoanode materials , which exhibited an overall light to electricity conversion efficiency of 0.68% . to the best of our knowledge ,
the use of zno nanocombs for the fabrication of dsscs is not reported yet in the literature .
the high purity metallic zinc powder ( 99.999% ) and oxygen gas were used as source materials . in a typical reaction process ,
about 1.5 g of metallic zinc powder was put into a ceramic boat and placed at the center of the quartz tube . the furnace temperature was raised up to the desired temperature , and oxygen and nitrogen were fed continuously into the quartz tube furnace with the flow rates of 60 and 240 sccm , respectively .
the temperature of the substrate , placed 8-cm away from the source boat , was 570 c .
the metallic zinc was vaporized and oxidized with o2 , and finally deposited onto the fto substrate .
for dssc fabrication , the prepared zno nanocomb thin - film electrodes was immersed in the ethanolic solution of 0.3 mm cis - bis ( isothiocyanato ) bis(2,2-bipyridyl-4,4-dicarboxylato)-ruthenium ( ii ) bis - tetrabutylammonium ( n719 , solaronix ) at room temperature for 6 h. the dye - adsorbed zno nanocombs thin - film electrodes were then rinsed with acetonitrile and dried under a nitrogen stream .
pt counter electrode was prepared by electron beam deposition of a thin layer of pt ( ~ 60 nm ) on the top of ito glass .
the pt electrode was placed over the dye - adsorbed zno nanocombs electrode , and the edges of the cell were sealed with 60-m thick sealing sheet ( sx 1170 - 60 , solaronix ) .
sealing was accomplished by pressing the two electrodes together on a double hot - plate at a temperature of about 70 c .
the electrolyte , consisting of 0.5 m lii , 0.05 mm i2 , and 0.2 m tert - butyl pyridine in acetonitrile , was introduced into the cell through one of two small holes drilled in the counter - electrode .
the holes were then covered and sealed with a small square of sealing sheet and microscope objective glass .
the light source was 1000-w metal halide lamp , and its radiant power was adjusted with respect to si reference solar cell to about one - sun - light intensity ( 100 mw / cm ) .
figure 1a shows the low - magnification fesem image of the zno nanocombs and reveals that the nanocombs are densely grown and uniformly distributed over the large area of the substrate surface . from the high - magnification images
, it is seen that the nanocombs are made by two components , i.e. nanorodlike branches and wide ribbonlike stems .
the branches ( teeth ) of the nanocombs are uniform and nicely attached along one side of the ribbonlike stem .
the width of the stem is ~1.2  0.3 m , and the stem is several micrometers long .
the diameter and length of each tooth is ~300  100 nm and ~3  0.5 m respectively .
these teeth are arranged in a proper manner with a distance of ~200  50 nm between each other [ figure ( b ) and inset ( b ) ] . the x - ray diffraction ( xrd )
pattern exhibits that the as - grown nanocombs are single - crystalline with the wurtzite hexagonal - phase pure zno ( jcpds # 361451 ) ( fig .
except zno , no characteristic peaks for other impurities such as zinc and substrate were observed in the spectrum , which confirms that the obtained products are single - crystalline wurtzite hexagonal - phase zno grown in highdensity on the fto substrate .
in addition to this , the energy dispersive spectroscopy ( eds ) confirmed that the as - grown nanocombs are made with almost 1:1 stoichiometry of zinc and oxygen ( fig .
. further structural characterization of the grown products was made using the transmission electron microscope ( tem ) and high - resolution tem combined with the selected area electron diffraction ( saed ) pattern .
figure 2a shows the low - magnification tem image of the nanocombs , which reveals the full consistency with the fesem observation in terms of morphology and dimensionality .
clearly , it is seen in the tem image that the branches of the nanocombs are attached along one side of the ribbonlike stem .
the hrtem image of one tooth of comblike structure circled in figure(a ) demonstrated a well - defined lattice fringes with the lattice spacing of 0.52 nm , corresponds to the d - spacing of the   crystal plane of the wurtzite hexagonal zno , confirmed that the branches of the comb structures are grown along the   direction ( fig .
the corresponding saed pattern of a branch of comb projected to the [ 20 ] zone axis is also consistent with hrtem observation ( fig .
figure 2c shows the room - temperature photoluminescence ( pl ) spectrum measured using a he  cd laser line with an exciton wavelength of 325 nm .
the obtained pl spectrum exhibited a narrow peak at ~385 nm in the uv region , also called near band edge emission , and a broad emission peak at ~570 nm in the visible region , also known as deep - level emission .
it is well known that the uv emission has been realized to the exciton emission , while the deep - level emission is generally explained as the radial recombination of photo - generated hole with a singly ionized charged state of the oxygen vacancy .
typical ( a ) low- and ( b ) high - magnification fesem images ; ( c ) xrd pattern and ( d ) eds spectrum of high density - grown zno nanocombs on fto substrate typical ( a ) low- and ( b ) high - magnification tem image and their corresponding saed pattern [ inset(b ) ] ; and ( c ) room - temperature pl spectrum of as - grown zno nanocombs used for the fabrication of dssc as a wurtzite hexagonal - phase zno possesses a positively charged zn-(0001 ) surfaces that are catalytically active , the negatively charged o-(0001 ) surfaces are chemically inert .
the comb stem grows along the [ 2 0 ] direction , while the top and bottom surfaces are zinc and oxygen terminated ( 0001 ) respectively .
it is reported that the catalytically active zn - terminated ( 0001 ) surfaces tend to have tiny zn clusters and other zn particles at the growth front , which could provide an active site for the further growth process , and hence comb teeth can grow in front of zinc - terminated ( 0001 ) surfaces . due to higher growth velocity in   direction of zno crystals ,
voltage ( i  v ) characteristics for dsscs fabricated with zno nanocombs thin - film electrodes and measured under a simulated illumination with a light intensity of 100 mw / cm ( am = 1.5 ) .
a maximum electricity conversion efficiency of 0.68% was achieved by highly branched zno nanocombs thin - film dsscs .
the fabricated dsscs also obtained a maximum short - circuit current density ( jsc ) of 3.14 ma / cm with low voc of 0.671 v and low ff of 34% .
the low jsc and conversion efficiency reveal that low dye absorption on the surface of zno thin film and result in the low - light harvesting and fast interface recombination rate of electron and holes .
it is reported that the interface recombination loss in zno - based dsscs is mostly due to the uncovered oxide surface ( with no dye molecule anchored on ) , where oxide contacts with electrolyte closely and thus increases the probability of charge recombination between the electrons in oxide and the holes in the electrolyte .
the low voc can be explained by the gapping between the spikes of zno nanocombs , which also cause direct contact of electrolyte to the fto glass ,   results the low ff .
the low ff and photocurrent may be explained by the fast recombination rate between the photoexcited carriers at the nanocombs and the electrolyte interfaces , which is related to series resistance rs = ( dv / di)i=0 .
generally , rs is ascribed to the bulk resistance of semiconductor oxide films , tco electrode , metallic contacts , and electrolyte . from the i  v curve , rs of zno nanocombs - based dssc is relatively high ( ~213  cm ) , which increased the charge recombination between the photoexcited carriers at the nanocombs and the redox electrolyte . the high rs results in the low ff and photocurrent .
figure 3b shows the uv  vis absorption spectrum of desorbed dye obtained from the zno electrodes by dipping the zno nanocombs electrode in 0.1 mm naoh solution for 10 min .
it was observed that low dye absorption ( ~3.23  10 mol / cm ) by zno nanocombs film surface electrode was probably due to the nonporous morphologies of the nanocombs .
it is well known that the high dye absorption by porous thin film leads to high light - harvesting efficiency .
therefore , low jsc and  are related to less absorption of dye molecules and insufficient light harvesting from the zno nanocombs thin - film electrodes .
3b demonstrates the general morphologies of zno nanocombs after the dye absorption and , interestingly , there was no distinct change observed in the general morphologies of the nanocombs after dye absorption , hence the nanocombs retain their morphologies after dye absorption .
v ) characteristics of zno nanocombs - based dssc and ( b ) typical uv  vis absorption spectra of the desorbed dye ( n719 ) from the zno nanocombs electrode thin films .
inset of ( b ) exhibits the surface morphology of the comblike structures after the desorption of dye in order to elucidate the charge transfer properties of as - grown zno nanocombs substrates , an electrochemical impedance spectroscopy ( eis ) measurement was used .
eis measurements were taken out under the illumination of 100 mw / cm ( am = 1.5 ) by applying a 10 mv ac signal over the frequency range of 10 hz100 khz using a potentiostat with lock in amplifier , as shown in fig .
equivalent circuit is composed of the resistance of redox electrolyte solution ( rs ) , the charge transfer resistance at the interface of electrolyte and zno nanocombs ( rct ) , the charge transfer resistance at the interface of zno nanocombs and tco ( rzno / tco ) , the capacitance of accumulation ( of e- ) layer of the zno nanocombs ( cacc ) and space charge capacitance ( csc ) . the value of real impedance ( zre ) at high and medium frequencies represents the rzno / tco and rct .
figure 4 exhibits the ac impedance curve of dssc fabricated with thermally grown zno nanocombs electrode .
a very high rzno / tco ( 90  ) and rct ( 29.6  ) were obtained for zno nanocombs thin - film electrodes , which are lesser than that of tio2 thin - film electrodes .
it is reported that a small rct suggests fast electron transfer , whereas a large rct indicates slow electron transfer .
the high rct ( 29.6  ) of zno nanocombs thin - film electrode explains the slow electron transfer , which results in the low photocurrent density and conversion efficiency .
therefore , the high charge transfer resistance at zno / electrolyte interface reveals a slow electron transfer through the zno nanocombs thin - film electrode , which results in the low isc , ff , and conversion efficiency of the fabricated dssc .
inset shows the equivalent circuit model of the dsscs , wherersis the resistance of redox electrolyte solution , rctthe charge transfer resistance at the interface of electrolyte and zno nanocombs , [ rct ] is the charge transfer resistance at the interface of zno nanocombs and tco [ rzno / tco ] , is the capacitance of accumulation ( of e- ) layer of the zno nanocombs [ cacc ] andcscspace charge capacitance
figure 1a shows the low - magnification fesem image of the zno nanocombs and reveals that the nanocombs are densely grown and uniformly distributed over the large area of the substrate surface . from the high - magnification images
, it is seen that the nanocombs are made by two components , i.e. nanorodlike branches and wide ribbonlike stems .
the branches ( teeth ) of the nanocombs are uniform and nicely attached along one side of the ribbonlike stem .
the width of the stem is ~1.2  0.3 m , and the stem is several micrometers long .
the diameter and length of each tooth is ~300  100 nm and ~3  0.5 m respectively .
these teeth are arranged in a proper manner with a distance of ~200  50 nm between each other [ figure ( b ) and inset ( b ) ] . the x - ray diffraction ( xrd )
pattern exhibits that the as - grown nanocombs are single - crystalline with the wurtzite hexagonal - phase pure zno ( jcpds # 361451 ) ( fig .
except zno , no characteristic peaks for other impurities such as zinc and substrate were observed in the spectrum , which confirms that the obtained products are single - crystalline wurtzite hexagonal - phase zno grown in highdensity on the fto substrate .
in addition to this , the energy dispersive spectroscopy ( eds ) confirmed that the as - grown nanocombs are made with almost 1:1 stoichiometry of zinc and oxygen ( fig .
. further structural characterization of the grown products was made using the transmission electron microscope ( tem ) and high - resolution tem combined with the selected area electron diffraction ( saed ) pattern .
figure 2a shows the low - magnification tem image of the nanocombs , which reveals the full consistency with the fesem observation in terms of morphology and dimensionality .
clearly , it is seen in the tem image that the branches of the nanocombs are attached along one side of the ribbonlike stem .
the hrtem image of one tooth of comblike structure circled in figure(a ) demonstrated a well - defined lattice fringes with the lattice spacing of 0.52 nm , corresponds to the d - spacing of the   crystal plane of the wurtzite hexagonal zno , confirmed that the branches of the comb structures are grown along the   direction ( fig .
the corresponding saed pattern of a branch of comb projected to the [ 20 ] zone axis is also consistent with hrtem observation ( fig .
figure 2c shows the room - temperature photoluminescence ( pl ) spectrum measured using a he  cd laser line with an exciton wavelength of 325 nm .
the obtained pl spectrum exhibited a narrow peak at ~385 nm in the uv region , also called near band edge emission , and a broad emission peak at ~570 nm in the visible region , also known as deep - level emission .
it is well known that the uv emission has been realized to the exciton emission , while the deep - level emission is generally explained as the radial recombination of photo - generated hole with a singly ionized charged state of the oxygen vacancy .
typical ( a ) low- and ( b ) high - magnification fesem images ; ( c ) xrd pattern and ( d ) eds spectrum of high density - grown zno nanocombs on fto substrate typical ( a ) low- and ( b ) high - magnification tem image and their corresponding saed pattern [ inset(b ) ] ; and ( c ) room - temperature pl spectrum of as - grown zno nanocombs used for the fabrication of dssc as a wurtzite hexagonal - phase zno possesses a positively charged zn-(0001 ) surfaces that are catalytically active , the negatively charged o-(0001 ) surfaces are chemically inert .
the comb stem grows along the [ 2 0 ] direction , while the top and bottom surfaces are zinc and oxygen terminated ( 0001 ) respectively .
it is reported that the catalytically active zn - terminated ( 0001 ) surfaces tend to have tiny zn clusters and other zn particles at the growth front , which could provide an active site for the further growth process , and hence comb teeth can grow in front of zinc - terminated ( 0001 ) surfaces . due to higher growth velocity in   direction of zno crystals ,
figure 3a shows the current density  voltage ( i  v ) characteristics for dsscs fabricated with zno nanocombs thin - film electrodes and measured under a simulated illumination with a light intensity of 100 mw / cm ( am = 1.5 ) .
a maximum electricity conversion efficiency of 0.68% was achieved by highly branched zno nanocombs thin - film dsscs .
the fabricated dsscs also obtained a maximum short - circuit current density ( jsc ) of 3.14 ma / cm with low voc of 0.671 v and low ff of 34% .
the low jsc and conversion efficiency reveal that low dye absorption on the surface of zno thin film and result in the low - light harvesting and fast interface recombination rate of electron and holes .
it is reported that the interface recombination loss in zno - based dsscs is mostly due to the uncovered oxide surface ( with no dye molecule anchored on ) , where oxide contacts with electrolyte closely and thus increases the probability of charge recombination between the electrons in oxide and the holes in the electrolyte .
the low voc can be explained by the gapping between the spikes of zno nanocombs , which also cause direct contact of electrolyte to the fto glass ,   results the low ff .
the low ff and photocurrent may be explained by the fast recombination rate between the photoexcited carriers at the nanocombs and the electrolyte interfaces , which is related to series resistance rs = ( dv / di)i=0 .
generally , rs is ascribed to the bulk resistance of semiconductor oxide films , tco electrode , metallic contacts , and electrolyte . from the i  v curve , rs of zno nanocombs - based dssc is relatively high ( ~213  cm ) , which increased the charge recombination between the photoexcited carriers at the nanocombs and the redox electrolyte . the high rs results in the low ff and photocurrent .
figure 3b shows the uv  vis absorption spectrum of desorbed dye obtained from the zno electrodes by dipping the zno nanocombs electrode in 0.1 mm naoh solution for 10 min .
it was observed that low dye absorption ( ~3.23  10 mol / cm ) by zno nanocombs film surface electrode was probably due to the nonporous morphologies of the nanocombs .
it is well known that the high dye absorption by porous thin film leads to high light - harvesting efficiency .
therefore , low jsc and  are related to less absorption of dye molecules and insufficient light harvesting from the zno nanocombs thin - film electrodes .
3b demonstrates the general morphologies of zno nanocombs after the dye absorption and , interestingly , there was no distinct change observed in the general morphologies of the nanocombs after dye absorption , hence the nanocombs retain their morphologies after dye absorption .
v ) characteristics of zno nanocombs - based dssc and ( b ) typical uv  vis absorption spectra of the desorbed dye ( n719 ) from the zno nanocombs electrode thin films .
inset of ( b ) exhibits the surface morphology of the comblike structures after the desorption of dye in order to elucidate the charge transfer properties of as - grown zno nanocombs substrates , an electrochemical impedance spectroscopy ( eis ) measurement was used .
eis measurements were taken out under the illumination of 100 mw / cm ( am = 1.5 ) by applying a 10 mv ac signal over the frequency range of 10 hz100 khz using a potentiostat with lock in amplifier , as shown in fig .
4 . according to the diffusion  recombination model proposed by bisquert et al . , an equivalent circuit representing dsscs was illustrated ( inset of fig .
equivalent circuit is composed of the resistance of redox electrolyte solution ( rs ) , the charge transfer resistance at the interface of electrolyte and zno nanocombs ( rct ) , the charge transfer resistance at the interface of zno nanocombs and tco ( rzno / tco ) , the capacitance of accumulation ( of e- ) layer of the zno nanocombs ( cacc ) and space charge capacitance ( csc ) . the value of real impedance ( zre ) at high and medium frequencies represents the rzno / tco and rct .
figure 4 exhibits the ac impedance curve of dssc fabricated with thermally grown zno nanocombs electrode .
a very high rzno / tco ( 90  ) and rct ( 29.6  ) were obtained for zno nanocombs thin - film electrodes , which are lesser than that of tio2 thin - film electrodes .
it is reported that a small rct suggests fast electron transfer , whereas a large rct indicates slow electron transfer .
the high rct ( 29.6  ) of zno nanocombs thin - film electrode explains the slow electron transfer , which results in the low photocurrent density and conversion efficiency .
therefore , the high charge transfer resistance at zno / electrolyte interface reveals a slow electron transfer through the zno nanocombs thin - film electrode , which results in the low isc , ff , and conversion efficiency of the fabricated dssc .
inset shows the equivalent circuit model of the dsscs , wherersis the resistance of redox electrolyte solution , rctthe charge transfer resistance at the interface of electrolyte and zno nanocombs , [ rct ] is the charge transfer resistance at the interface of zno nanocombs and tco [ rzno / tco ] , is the capacitance of accumulation ( of e- ) layer of the zno nanocombs [ cacc ] andcscspace charge capacitance
in summary , well - crystallized zno nanocombs were directly grown onto the fto substrate via noncatalytic simple thermal evaporation process and utilized as photoanode materials to fabricate the dsscs .
the fabricated dsscs demonstrated an overall light to electricity conversion efficiency of ~0.68% with a fill factor of 34% , short - circuit current of 3.14 ma / cmand open - circuit voltage of 0.671 v. this research opens a new way to utilize various kinds of zno nanostructures as photoanode material for the fabrication of efficient dsscs .
this work has been done through the service contract between najran university , saudi arabia and chonbuk national university , south korea .
author would like to thank professor yoon - bong hahn , school of semiconductor and chemical engineering , chonbuk national university and dr .
d. h. kim , hanyang university , south korea for useful discussions and helps to carry out the experiments .
this work was partially supported by the research project funded by najran university , najran , saudi arabia .
 dye - sensitized solar cells ( dsscs ) were fabricated by using well - crystallized zno nanocombs directly grown onto the fluorine - doped tin oxide ( fto ) via noncatalytic thermal evaporation process . 
 the thin films of as - grown zno nanocombs were used as photoanode materials to fabricate the dsscs , which exhibited an overall light to electricity conversion efficiency of 0.68% with a fill factor of 34% , short - circuit current of 3.14 ma / cm2 , and open - circuit voltage of 0.671 v. to the best of our knowledge , this is first report in which thin film of zno nanocombs was used as photoanode materials to fabricate the dsscs . 
Introduction
Experimental Details
Results and Discussion
Structural and Optical Properties of As-grown ZnO Nanocombs
Photovoltaic Properties of As-grown ZnO Nanocombs
Conclusion
Acknowledgements