Light Soaking and Gas Effect on Nanocrystalline TiO2/Sb2S3/CuSCN Photovoltaic Cellsfollowing Extremely Thin Absorber Concept

Shinji Nezu,* Gerardo Larramona, Christophe Chone, Alain Jacob, Bruno Delatouche,Daniel Pere, and Camille MoisanIMRA Europe S.A.S., 220 rue Albert Caquot, B.P.213, 06904 Sophia Antipolis Cedex, France

ReceiVed: January 15, 2010; ReVised Manuscript ReceiVed: March 3, 2010

Photovoltaic cells with the structure of nanocrystalline TiO2/Sb2S3/(LiSCN)CuSCN were prepared followingthe extremely thin absorber cell concept and characterized under different gas atmospheres. The cells showedan important light soaking effect in which the cell performance typically evolved from an initial lower efficiencyinto a higher one in contact with ambient air. Oxygen was shown to be involved with the effect. The best cellefficiency was 3.7% at ∼1 sun, with a photocurrent Jsc normalized at 1 sun of 11.6 mA/cm2, an open-circuitvoltage Voc of 0.56 V, and a filling factor, ff, of 0.58. The cells are shown to run more than 300 days, keepingthe efficiency higher than 1.5% at ∼1 sun. The need for taking into account the influence of oxygen andhumidity during photovoltaic measurement is emphasized for further investigation of this type of cell.

Introduction

Inorganic absorber-sensitized nanoporous solar cell compris-ing a porous heterojunction of “n-a-p” type (“a” meaningabsorber, also known as n-i-p type, “i” meaning intrinsic) hasbeen of interest because of the expectation for overcoming thedrawback of instability originated from use of organic dye andliquid electrolyte in a conventional dye-sensitized nanoporoussolar cell (DSC).1 In such a cell known as extremely thinabsorber cells (ETA cells), a thin layer of an inorganic absorberis deposited over the internal surface of a porous film made ofa transparent (nonabsorbing or wide bandgap) n-type inorganicsemiconductor, the rest of the pore volume being filled with atransparent p-type inorganic semiconductor.2-9 Photons are onlyabsorbed in the absorber layer, and the generated electrons andholes are injected into the two separated phases, n-type andp-type, respectively. The expected advantage for this kind ofcell is that a fast charge separation will occur due to the lowthickness of the absorber, and charge recombination of oppositecarriers traveling in separate phases will be highly reduced.Consequently, the absorber material will need lower restrictionsconcerning the number of defects, allowing a low fabricationcost.

TiO2 or ZnO as n-type materials and CuSCN as p-typematerials have been commonly used. Levy-Clement’s team5 hasreported 2.3% at 0.36 sun with a ZnO/CdSe/CuSCN cell.Dittrich’s team6 has reported 3.4% at 1 sun and cell area of0.03 cm2 with a ZnO/In2S3/CuSCN cell. We7 have also reported1.3% at 1 sun with a TiO2/CdS/CuSCN cell of 0.54 cm2 (usinga high internal surface nanocrystalline TiO2 and a CdS coating∼5-10 nm thick). Subsequently, we have patented TiO2/Sb2S3/CuSCN cells8 which can provide 3.4% efficiency with the bandgap of ∼1.65 eV (∼750 nm). The best external quantumefficiency (EQE) attained to a maximum of 80%,8 which isapproximately the maximum value that can be reached due toat least ∼15% loss by absorption/reflection by the TCO glasssubstrate. Recently, Hode’s team9 also reported 3.37% forsimilar Sb2S3 nanoporous solar cells in the configuration TiO2/

In-OH-S/Sb2S3/(KSCN)CuSCN with a cell area of 0.15 cm2.In their report, they9 also confirmed the beneficial effect ofthiocyanate pretreatment before CuSCN impregnation shownin our previous report.7 Following these results with CdS andSb2S3 cells, we have been intensively studying them in orderto optimize and to confirm the stability.

In the course of such study, we noticed significant effects ofgas in contact with the cells. In this report, the newest resultsfollowing our patent disclosure regarding Sb2S3 nanocrystallinephotovoltaic cells are presented first in order to outline thephotovoltaic behavior such as the light soaking effect. Then thegas effect is described in detail to show the difference in lightsoaking behavior between air and a N2 atmosphere. We alsostudied cells which underwent heat treatment under differentatmospheres. The latest results of cell stability tests based onthe acquired knowledge are presented. Finally, morphology onthe cell nanostructure using scanning electron microscopy (SEM)and transmission electron microscopy (TEM) analysis is given.

Experimental Section

Figure 1 shows the solar cell structure. Commercial F-dopedSnO2 transparent conductive oxide (TCO) glasses from AsahiGlass were used as front contact. A hole-barrier compact TiO2

layer of ∼20 nm thickness (confirmed by SEM and TEM) wasdeposited by spray pyrolysis.10 Porous nanocrystalline TiO2 filmsof ∼3 µm thickness, ∼40-50 nm average particle size, and

* To whom correspondence should be addressed, nezu@ai-i.aisin.co.jp.

Figure 1. Schematic of the nanostructured solar cell.

J. Phys. Chem. C 2010, 114, 6854–68596854

10.1021/jp100401e 2010 American Chemical SocietyPublished on Web 03/18/2010

150-300 roughness factor were fabricated as described in ourprevious publication.7 After the antimony sulfide depositiondescribed below, the samples were dipped into 0.5 M aqueoussolution of LiSCN salt. Non-salt-treatment samples were alsoprepared for comparison. Then they were dried by N2 gasblowing to the surface for several seconds. CuSCN filling wasmade by impregnation and evaporation of a CuSCN solutionin propyl sulfide,7 a method similar to that reported for solidDSC cells of the type TiO2/dye/CuSCN.11,12 Just after theCuSCN filling, a back contact of a gold layer of ∼40 nmthickness was deposited by thermal evaporation using anEdwards 306 evaporator. The completed cells were stored in aN2 glovebox until measurement (typically 1 night). Some cellswere heat pretreated under N2 or synthetic dry air at 80 °C for15 h before measurement.

Deposition of antimony sulfide on the nanocrystalline TiO2

films was done by chemical bath deposition (CBD). Severalmethods have been reported for depositing thin films of Sb2S3

on flat substrates, due to its interesting properties for differentapplications. Such methods included wet chemistry processes(such as CBD13-18 and electrodeposition19), thermal evaporation,radio frequency magnetron sputtering, or MO-CVD. Fordeposition inside nanocrystalline porous TiO2 films, we used aCBD method based on thiosulfate precursor, similar to someprocedures reported for deposition of thin films on a flatsubstrate.16-18 The CBD bath was done by preparing an initial∼1 M solution of SbCl3 in acetone and by diluting this onewith ∼1 M Na2S2O3 cold aqueous solution and cold water, soas to have final concentrations of Sb(III) and S2O3

2- of ∼0.025and ∼0.25 M, respectively. The thiosulfate anion acts ascomplexing agent and sulfide source. Samples were immersedvertically in the bath and placed in a refrigerator at 5-10 °C.The CBD reaction was left for a typical period of 2 h. Thensamples were rinsed with water and dried by flowing nitrogen.Afterward, samples were annealed in a N2 glovebox at 300-320°C for about 15-30 min. Among the different methods that wetried, this one allowed the deposit of a coating of antimonysulfide inside the porous TiO2 films without producing an outer-layer or blocking the internal pores.

The following apparatus and setups were used for thecharacterization of components and cells. Current-voltage I-Vcharacterization and quantum efficiency (QE, also known asIPCE) were recorded with in-house setups.7 I-V curve measure-ments were made in ambient air (room air) or in controlledatmospheres of synthetic dry air flow or N2 gas flow. Anunsealed cell was placed in a sample holder which had a maskto have an exact and fixed illuminated surface of 0.54 cm2 (0.6× 0.9 cm). For measurements in controlled atmospheres, it wasput in a cell container made of a metal box with a glass windowfor light illumination and gas inlet and outlet to allow controlledgas environment. I-V plots were recorded typically at irradianceof ∼1 sun ()1000 W/m2). Series resistance of solar cells wasestimated from a single I-V plot under illumination. Thatconsists in making the derivative of the I-V curve (at a singleirradiance) of a scan up to sufficiently high voltage forwardbias, so as to achieve a plateau on the plot of (-dV/dI) versusV, which directly gives the value of Rs. SEM images wereacquired with a field-emission scanning electron microscope,Hitachi S-4700, to which an EDX (energy-dispersive X-raymicroanalysis) Noran System SIX was coupled. TEM analysiswas carried out with a JEOL 2100F FEG-200 kV microscope,having a STEM accessory, and an integrated JEOL JED-2300TEDX analyzer.

Results and Discussion

Cell Performance with Light Soaking in Ambient Air.Experiments were carried out using cells treated by LiSCN forthis purpose, since they provides higher performance as pointedout for CuSCN containing cells by our previous report.7

Unsealed cells exposed to ambient air (room air) were used forphotovoltaic measurement. Cells were stored overnight in a N2

glovebox before measurement. Higher cell efficiencies wereobtained after the cells were held under light soaking at 1 sunin ambient air and open circuit condition for ∼20-200 min.Before light soaking, cells showed an efficiency typically <1%in their first I-V measurement. Figure 2 shows the evolutionof I-V curves of one of the best cells during light soaking. Asshown in the figure, the performance improved with time, thehighest cell efficiency η at ∼1 sun (1006 W/m2) and t ) 100min after starting illumination being 3.7% with a photocurrentJsc normalized at 1 sun of 11.6 mA/cm2

, an open-circuit voltageVoc of 0.56 V, and a filling factor (ff) of 0.58, while the initialefficiency before starting illumination (t ) 0) was only 0.8%.The time to reach maximum efficiency varied from cell to cell.In a systematic experiment consisting of ∼36 cells made andmeasured in similar conditions with LiSCN treatment, cellefficiency was in the range 1.5-3.7% (average 2.9%), Jsc inthe range 6.1-13.2 mA/cm2 (average 10.5 mA/cm2), Voc in therange 0.43-0.58 V (average 0.53 V), and ff in the range0.44-0.60 (average 0.52). The first I-V curve, at t ) 0 (seeFigure 2), shows an inclined or sigmoid shape characteristic ofcells exposed to ambient air. However, the shape of the firstI-V curves taken in many measurements was not reproducible,showing different values for initial Jsc with different fillingfactors. During such light soaking the cell temperature reacheda temperature of ∼45-60 °C, since the cell holder was madewith a plastic body and the cooler of the I-V setup was notable to cool the cell down to 25 °C. Light soaking at 25-30 °C(using a metal cell holder) was less rapid, which means thatheating was accelerating the performance increase.

Although we introduced the use of LiSCN treatment for cellshaving TiO2/Sb2S3/CuSCN configuration because of their higherperformance, we have found that non-salt-treated cells also showlight soaking effect from an initial efficiency at 1 sun of <0.5%to a final efficiency of 1-1.5% (Jsc ∼ 6-9 mA/cm2, Voc ∼ 0.4,ff ∼ 0.4). Their evolution of performance during light soakingis slower than that for LiSCN cells, the time to reach maximumperformance being ∼10-20 h typically. Higher series resistanceand lower Voc are characteristic of non-salt-treated cells. Wehave also investigated for different salt-treated cells using

Figure 2. I-V curves of a TiO2/Sb2S3/(LiSCN)CuSCN cell at 1 sun(1006 W/m2) measured at different time t after starting light soakingunder ambient air at open circuit (cell active area of 0.54 cm2): (a) t )0; (b) t ) 10 min; (c) t ) 20 min; (d) t ) 100 min.

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different thiocyanates such as KSCN and NaSCN. Althoughthey showed slightly different behavior, the general tendencywas similar to that of LiSCN. The difference in behavior couldbe related to the difference in their hygroscopic nature or thecation type and volume.

We found that this light soaking effect also occurred withoutthe UV part of the radiation (using a UV cutoff filter cutting100% transmittance <500 nm), such a light soaking effect isnot exclusive of UV, contrary to UV effects reported using othernanocrystalline TiO2 cells.20

Following the above results, we started stability evaluationof sealed cells which was prepared under N2 atmosphere. Thenwe realized that they did not show a similar light soaking effectto the cells exposed to ambient air. This suggested that theobserved light soaking effect is related to oxygen and/orhumidity. In order to clarify these influences, experiments werecarried out under controlled atmosphere as described below.

Gas Effect in Light Soaking. We prepared three differenttypes of cell by applying different post-treatment after back-contact electrode deposition, that is, the final process for cellpreparation: a cell stored in a N2 glovebox at room temperaturefor 15 h (referred to as a non-heat-treated cell); a cell heated inN2 glovebox at 80 °C for 15 h (referred to as a N2 heat-treatedcell); a cell heated in a synthetic dry air glovebox at 80 °C for

15 h (referred to as a dry air heat-treated cells) in order to checkthe effect of cell drying (such as removal of possible trace ofpropyl sulfide solvent used in CuSCN filling, and effect ofhumidity remaining inside the cell). Then these three differenttypes of cells were subjected to light soaking under differentgas atmospheres in order to observe the effect of gas and theheat treatments, light soaking at ∼1 sun (keeping the celltemperature at 40 °C) starting under dry N2 for 500 min (∼8h), then dry air for 1000 min (∼16.5 h), finally dry N2 againfor 1200 min (∼20 h). During the light soaking at open circuitcondition, I-V curves were periodically measured. Figure 3shows the evolution of Jsc (mA/cm2), Voc (V), ff, series resistanceRs (Ω · cm2), and efficiency η (%) during light soaking. Onecan see the clear difference in all the performance parametersbetween the first dry N2 and the subsequent dry air light soakingfor all three different types of cell. It is evident that oxygenplays certain important roles for improving cell performance.

In the first dry N2 light soaking in Figure 3, it should be notedthat the N2 heat-treated cell showed the lowest η with the lowestJsc and Voc among the three types of cell. Although there wasslight evolution in the performance parameters such as Voc, ff,and Rs for the N2 heat-treated cell, the η was almost constant atsmaller values. It should also be noted that the initial seriesresistance of the N2 heat-treated cell was significantly higher

Figure 3. Evolution of short-circuit current density (Jsc), open-circuit voltage (Voc), filling factor (ff), efficiency (η), and series resistance (Rs),during light soaking at open circuit condition and ∼1 sun of TiO2/Sb2S3/(LiSCN)CuSCN cells under initial dry N2, subsequent dry air, and final dryN2 atmosphere for three different types of cells: (O) non heat treatment before light soaking (stored in N2 glovebox at room temperature for 15 h);(b) heated in a N2 glovebox at 80 °C for 15 h; (2) heated in a synthetic dry air glovebox at 80 °C for 15 h.

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than those of the other cells. For the non-heat-treated cell, therewas clear evolution of the η in contrast to the N2 heat-treatedcell as shown in the figure. Its initial series resistance was thelowest among the three types of cells. For the dry air heat-treated cell, one can see the highest Jsc, Voc, and η among thecells and much lower Rs than the N2 heat-treated cell at thebeginning. Then, the Jsc, Voc, and η of the dry air heat-treatedcell became smaller than those of the non-heat-treated cells in30 min after starting light soaking. We interpret such behaviorsin the first dry N2 light soaking as follows. The non-heat-treatedcell contains the most residual water among the cells, havingthe smallest Rs before light soaking. It also contains adsorbedoxygen taken in during cell fabrication, giving rise to lightsoaking effect related to oxygen. The residual water shouldoriginate from hygroscopic LiSCN associated with wet chemicalprocess for cell preparation. The relation between seriesresistance and water content is based on results obtained inseparate experiments, in which series resistance is significantlydecreased by humidity in ambient gas. The N2 heat-treated cellcontains the least water and adsorbed oxygen inside the cell,resulting in the highest Rs and almost no light soaking effectassociated with oxygen in a sense of performance improvement.However, one may note remarkable decrease in Rs for this cellduring light soaking in the figure. This is interpreted as aconsequence that the cell still contains a small amount of oxygento cause this change with illumination. The following possiblereactions involving oxygen and illumination can be considered:photocatalytic oxidation of organic impurities such as the propylsulfide solvent and/or doping of CuSCN with (SCN)2 formedby photocatalytic oxidation of SCN- or with active oxygenspecies such as OH radicals. The dry air heat-treated cell isthought to contain less water than the non-heat-treated cell andmore adsorbed oxygen than the N2 heat-treated cell. Thus, thishigher oxygen content gives rise to lower initial Rs even justafter starting light soaking despite its lower water content beforelight soaking. Another possible interpretation is that light soakingcould take place in N2 even in the absence of any adsorbedoxygen, but photocurrent increase would take place in a muchlonger time scale (several days). In such a case, the processmight be due to the removal or photodegradation of organicimpurities under gas flowing, perhaps assisted by photoactiveantimony sulfide. However, as explained below, the beneficialeffect of oxygen on Voc and series resistance would not beattained under N2, and the final efficiency in N2 atmosphereafter continuous illumination for several days would be lowerthan in dry air atmosphere.

In the subsequent dry air light soaking, rapid evolution ofJsc, Voc, and η can be seen in Figure 3. One should note initialdecrease in ff for all three types of cells just after changing tothe dry air light soaking in the figure. Such decrease correspondsto the increase in Jsc as shown in the figure. We interpret suchbehavior as follows: upon starting of the dry air light soaking,the photocurrent is increased by a certain mechanism; however,the series resistance is not low enough to keep higher ff withincreased photocurrent at the initial stage of the dry air lightsoaking, resulting in decreasing ff. However, the decrease inRs is also accompanied by the process slowly, recovering ff afterreaching its minimum. This can be seen clearly especially forthe dry N2 heat-treated cell. From the foregoing discussion, weconclude that oxygen has two independent roles: the first oneis to decrease Rs and the second one is to increase Jsc and Voc

independently of Rs.In the last dry N2 light soaking following the dry air one, Jsc

of all the three types of cells increased for initial several hours

and then slowly decreased, the Voc starting decrease just afterchanging to dry N2 as shown in Figure 3. For the N2 heat-treatedcell, decrease in ff and increase in Rs can be seen in the figure,but for the other cells such change is not seen in the figure. Asshown in the subsequent paragraph for cell stability tests, suchinitial decrease in cell performance after changing to N2

atmosphere tends to be stabilized with time, the efficiency beingkept more than 1.5% after light soaking for more than 300 daysunder open circuit condition. This is one of the most importantcharacteristics of LiSCN treated cells. In the case of non-salt-treated cells, which reached efficiency around 1.2% by dry airlight soaking, the efficiency dropped to less than 0.5% typicallyin ∼15 h after changing to N2 atmosphere (plots not shownhere).

Cell Stability Tests. Cells in contact with ambient air andunder 1 sun illumination did not show good stability in 100 hat open circuit conditions and room temperature. Thus, wecarried out tests with a few cells placed in a metal box undercontrolled gas flow of either dry N2 or dry synthetic air. Cellswere left under a solar simulator under continuous illuminationwith irradiance close to 1 sun. Cell temperature was notcontrolled, but it was in the range ∼30-35 °C. Cells werefabricated according to the standard procedure given above withLiSCN treatment. They were initially light-soaked in drysynthetic air for a few hours. The gas atmosphere was eitherchanged to N2 or kept at dry air, and a small flow of such gaswas kept continuously. Cells were left either at open circuit orshort circuit, and performance measurement was carried outfrom time to time. Figure 4 shows the evolution of theperformance parameters for three cells in different conditionsover nearly 1 year in these stability tests. The cell under N2 atshort-circuit condition appears stable, while the other two atopen-circuit (N2 or air) decrease performance in the first100-150 days and afterward they tend to stabilize. Theefficiency after 1 year is over 1.5% for the three cells and Jsc at1 sun is kept in the range 7.5-10 mA/cm2. The up and downshifts that are observed in the plots are due to drifts on theirradiance of the old lamp used in these stability tests (even if

Figure 4. Evolution of four performance parameters normalized at 1sun (photocurrent (Jsc), open-circuit voltage (Voc), efficiency (η), andfilling factor (ff)) under continuous irradiance of ∼1 sun of three cellsof the type TiO2/Sb2S3/(LiSCN)CuSCN cells, under a controlled flowatmosphere in different conditions: ([) N2 flow, open-circuit (O.C.)condition; (4) N2 flow, short-circuit (S.C.) condition; (b) synthetic airflow, open-circuit (O.C.) condition.

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well calibrated) and possibly also in the cell temperature changesbetween points of measurements. As compared to the stabilitytests in ambient air, it seems that ambient humidity is the causeof cell degradation and should be avoided.

Structure Analysis. From the above-mentioned effect of gas,the question of why cells respond to ambient gas so quicklyarises. In order to answer it, we carried out SEM and TEManalysis for the nanostructure cells. SEM of TiO2/Sb2S3 films(before CuSCN filling) showed no absorber dots: the majorityof TiO2 nanocrystals looked free of absorber, while some otherzones of 100-200 nm size were observed in which the TiO2

nanocrystals looked totally embedded in a Sb2S3 coating. SEManalysis of cross sections of TiO2/Sb2S3/(LiSCN)CuSCN cellsshowed good filling by CuSCN (plus some outer layer), butsome slight empty volume could still be detected in some parts.Some TEM analysis was carried out in powders scratched fromTiO2/Sb2S3 films (no CuSCN filling). In one type of TEMobservations, Sb sulfide was detected by EDX/TEM in bigagglomerates containing TiO2 crystals, in agreement with theembedded zones observed by SEM. In another type of TEMobservation, we detected that the isolated TiO2 crystals werecoated with an apparently homogeneous very thin coating of∼1-3 nm thickness. EDX/TEM point analysis showed that sucha coating was composed of Sb, but no S was detected, indicatingthat the coating was very likely made of Sb oxide.

Another TEM analysis was carried out for a sample consistingin a thin slice of the cross section of a TiO2/Sb2S3/CuSCN film(LiSCN treated), obtained by lift-out FIB technique. Figure 5shows STEM/HAADF imaging and STEM/EDX mapping fora domain rich on Sb2S3 embedding the TiO2 nanocrystals. Thesize of such a domain ranged between 100 and 200 nm. In thesedomains, a good apparent contact was observed between Sb2S3,TiO2, and CuSCN crystals as shown in Figure 5, since the shapeof the CuSCN phase fitted well with the TiO2 and Sb2S3 crystalshapes, indicating that the CuSCN impregnation was efficientand good contact was achieved between CuSCN, Sb2S3, andTiO2. LiSCN could not be detected by EDX due to the lowweight of the Li atom. Apart from those domains, the rest ofthe TiO2 nanocrystals were not in direct contact with the Sb2S3

coating. CuSCN was observed inside the TiO2/Sb2S3 porosity,as well as a cap layer (of ∼500 nm thickness) at the top of thenanocrystalline TiO2 film. HR-TEM observations of the CuSCNphase inside the TiO2 film and on the top layer showed a similar

structure: the CuSCN was made of well-crystallized nan-odomains of ∼3-5 nm size. One should note clear contrast forthe CuSCN phase. This suggests that the phase could be madeof a porous structure consisting of CuSCN nanocrystals. Wehave judged that this porous structure allows gas to permeateinside the cells.

Before carrying out the above structure analysis, we hadascribed the obtained exceptionally high performance to thinand uniform coating of absorber, since it is generally expectedthat thinner absorber gives better result for ETA cells. However,we now understand the charge separation of our ETA cellsthrough a scheme explained in the following. An average poresize in our nanoporous TiO2 layer has been estimated to beroughly 50 nm by BET measurement. When several continuingpores made with TiO2 nanocrystals are partially filled by a Sb2S3

domain ranging between 100 and 200 nm as shown in Figure5(b2), photoelectrons generated in the middle of each pore musttravel half the average pore size, i.e., ∼25 nm in average. Takinginto account the deviation from the average, it could be areasonable estimation that traveling distance required for themajority of photoelectrons is within twice the average pore size,i.e., ∼50 nm, which is in consistent with distance estimatedfrom the STEM/EDX mapping for TiO2 shown in Figure 5(b1).This distance should be small enough for avoiding recombina-tion loss in Sb2S3 absorber. In addition, it should be noted thatSb2S3 aggregates filling the pore of the porous TiO2 in Figure5(b2) overlaps with CuSCN (compare with image b3 in Figure5). This means that the pores partially filled with Sb2S3

aggregates may also be filled with a certain amount of CuSCNnanocrystals to fill up the pores completely. This could facilitatethe transport of photoholes from the Sb2S3 to CuSCN, possiblygiving an alternative advantage in lowering recombination loss.

Conclusions

We have characterized photovoltaic cells with the structureof nanocrystalline-TiO2/Sb2S3/(LiSCN)CuSCN. The best cellefficiency was 3.7% at ∼1 sun, with a photocurrent Jsc

normalized at 1 sun of 11.6 mA/cm2, an open-circuit voltageVoc of 0.56 V, and a filling factor (ff) of 0.58. The cells wereshown to run more than 300 days, keeping the efficiency higherthan 1.5% at ∼1 sun. The fresh cells needed to be activated bylight soaking to improve their performance. Performanceevolution during the light soaking was influenced by gasatmosphere in photovoltaic measurement, cell heat pretreatmentatmosphere, and alkali-metal thiocyanate additives inside thecells.

These results could be due to different possible causes: apossible charge screening effect due to added thiocyanates andhumidity at the TiO2/CuSCN interface; the change of theresistivity of CuSCN upon possible doping and/or upon theadsorption of chemical species such as oxygen, water, and otherions; electrochemical charge transportation in the presence ofresidual solvent, water, oxygen, and thiocyanates at the absorber/CuSCN interface; oxidation of absorber to form metal oxideresulting in lower recombination rate; or the influence of thedefect structure of CuSCN, which could change under light andthe type of gas atmosphere.

The STEM/EDX analysis revealed that Sb2S3 absorber existsas discontinuous domains ranging between 100 and 200 nm,which differed from the expected extremely thin layer (ETA)structure having continuous uniform coating of absorber onnanocrystalline TiO2. In these Sb2S3 domains the chargeseparation is assured by photoelectron transportation of amaximum distance ∼50 nm to inject into the TiO2 or by

Figure 5. TEM analysis in the middle of the cross section of a TiO2/Sb2S3/(LiSCN)CuSCN nanocrystalline cell. (a) STEM-HAADF imag-ing, and corresponding EDX mapping for: (b1) Ti, (b2) Sb, (b3) Cu,and (b4) three elements all together.

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photohole transportation to inject into the CuSCN which fillsup nanopores together with the Sb2S3. The existence of manyregions with very close distance between TiO2 and CuSCNindicates that photocurrent loss by surface recombination isnegligible in this type of cell after proper light soaking treatment.

References and Notes

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