Stability improvement of photovoltaic performance in antimony sulfide-based hybrid solar cells

Sb 2 S 3 -based hybrid solar cells were prepared in the combination with electron transporting layer of TiO 2 or ZnO nanoparticles in addition to poly(3-hexylthiophene)-2,5-diyl/(3, 4-ethylenedioxythiophene): poly(styrene sulfonate), zinc phthalocyanine (ZnPc), or MoO 3 for hole transporting layer. Photovoltaic performance and durability of the hybrid solar cells were compared each other with or without encapsulation by using glass and UV cutoff ﬁlm. Among these hybrid solar cells, it was found that a combination of glass-ITO/TiO 2 /Sb 2 S 3 /ZnPC/Au encapsulated with glass and UV cut ﬁlter has the highest durability with keeping the relative power conversion efﬁciency of 90% through the stability test under 1 sun at 63 ◦ C at a relative humidity of 50% for 1,500 h. © The Author(s) 2017. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (


Experimental
Preparation of TiO 2 nanoparticle.-A14.7 g of TiO 2 powder P-90 (Degussa, Evonic Japan Co., Ltd.) was dispersed in 62.3 g of 95% ethanol.After addition of 439 g of zirconia beads (φ 50 μm), the TiO 2 powder was ground with beads mill at 2000 rpm for 2 h.After the removal of the zirconia beads by filtration, ca.60 g of TiO 2 slurry was obtained.Addition of adjusted amount of ethanol into the above TiO 2 sludge resulted in 6 wt% TiO 2 -dispersed solution.
Preparation of ZnO nanoparticle.-ZnOnanoparticles were prepared according to the reported procedures 24 with slight modifications.Zinc acetate dihydrate (0.298 g) was dissolved in methanol (10 g) and stirred for 3 min with heating up to 65 • C. 5 g of 3 wt% KOH in methanol was poured into the above solution and stirred for 2.5 h.After centrifugation at 20,000 rpm for 15 min and the removal of the supernatant, the crude precipitates were dispersed in 150 g of methanol through sonication.Thereafter, following another centrifugation at 20,000 rpm for 30 min and the removal of the supernatant, the purified precipitates were homogenously dispersed again in 0.5 g of CHCl 3 as ZnO nanoparticles with an average diameter of ca. 5 nm, as confirmed by TEM observation.Methanol was added to the above ZnO-dispersed solution and adjusted to 2 wt%.

Preparation of Sb 2 S 3 -based hybrid solar cells.-In the prepara-
tion of solar cell devices, indium-tin oxide (ITO)-coated glass substrates (Geomatec, 10 cm −2 ) were spin coated with the above dispersed solution of TiO 2 or ZnO at 4,000 rpm followed by annealing at 300 • C for 10 min with a hot-plate.A ∼100-nm-thick Sb 2 S 3 was deposited by thermal evaporation of the Sb 2 S 3 -powder as the source under vacuum (5 × 10 −2 Pa) onto the metal oxide layer.It was annealed at 260 • C for 10 min ramping from 220 • C under the reduced pressure (1 × 10 2 Pa).After cooling down to 220 • C, HTL was subsequently coated onto the Sb 2 S 3 layer as follows.0.5 wt% P3HT in chlorobenzene was spin-coated at 4,000 rpm and 3 times diluted PE-DOT:PSS solution (Clevios P) with methanol was then coated at 2,500 rpm followed by annealing at 110 • C for 10 min.While ZnPc or MoO 3 was deposited by thermal evaporation resulting in a 20-nm-thick layer.Thereafter Au electrode was deposited by thermal evaporation.
The solar cells obtained as described above were encapsulated with glass and epoxy resin as sealer.A cut filter (> 420 nm, Asahi Spectra Co.,Ltd) was utilized for cut off the UV region of the irradiation light.
Evaluations of photovoltaic performance.-Photovoltaiccharacteristics were measured under AM1.5 (100 mW cm −2 ) with solar simulator of HAL-320 (Asahi Spectra Co., Ltd.) in the combination with COSMO CIV-1000F applying a bias from −0.2 to 1.0 V in steps of 0.05 V and the current density was measured just after 0.5 s under biased conditions.

Photovoltaic performance of the Sb 2 S 3 -based hybrid solar cells for 1 d.-
The photovoltaic performance of the Sb 2 S 3based hybrid solar cells with three different HTLs consisting of glass-ITO/TiO 2 /Sb 2 S 3 /HTL (HTL = P3HT/PEDOT: PSS, ZnPc, or MoO 3 )/Au were prepared and compared with each other.The current density-voltage (J -V) curves of the initial (viz., freshly prepared) photovoltaic performance (solid) and the curves after 1 d at 63 • C and 50% RH in the dark (broken) are shown in Figure 1.PCEs of ca.3% were attained with P3HT/PEDOT: PSS and ZnPc.While PCE of 0.25% was obtained when MoO 3 was applied.Since relatively higher series resistance was observed in the case of MoO 3 , it might be estimated that the mobility in MoO 3 is lower than other HTLs and resulted in low PCE.With regard to the stability, the PCE of P3HT/PEDOT: PSS decreased slightly after 1 d at 63 • C and 50% RH even when there was  no irradiation of light.This result is probably ascribed to the acidic property of the diluted PEDOT:PSS solution, which tends to lower the cell performance. 25,26On the contrary, there was no lowering the PCEs after 1 d at 63 • C and 50% RH in the dark both in the cases of ZnPc and MoO 3 .

Effect of hole transporting layers (HTLs) on stability.-The durability test of the cells with UV cut filter was performed at 63
• C and 50% RH under 1 sun as shown in Figure 2. PCE of P3HT/PEDOT:PSS decreased to the relative value of 10% after 100 h photo-irradiation.On the other hand, in the cases of ZnPc and MoO 3 , PCEs kept constant until 300 h irradiation.However, that of MoO 3 started to decrease gradually after 300 h, while ZnPc indicated stable and 90% of PCE was retained after 500 h.TEM-EDX observations of the cross-sectional views of glass-ITO/TiO 2 /Sb 2 S 3 / MoO 3 /Au before and after the durability test for 576 h revealed that Sb and Mo diffused opposite directions one another (Figure 3).These diffusions during the durability test might affect the lowering of PCEs.

Effect of electron transporting layers (ETLs) on stability.-Two
ETLs were utilized and compared in terms of durability for the    4) indicate that almost constant stability of TiO 2 -based cell and remarkable lowering of the PCE and lability of ZnO-based one.In particular, the fill factor (FF) and the series resistance (R s ) of glass-ITO/TiO 2 /Sb 2 S 3 /ZnPC/Au had been improved as FF from 0.415 to 0.471, and R s from 33 ohm cm 2 to 20 ohm cm 2 after 7 d, respectively.Therefore, using TiO 2 is effective to adjust the interface of the device under the conditions with UV cut filter at 63 • C and 50% RH under 1 sun.On the other hand, it was found by TEM-EDX observations of the cross-sectional views of glass-ITO/ZnO/Sb 2 S 3 /ZnPC/Au (Figure 5) that initial distribution of S was well-corresponding with that of Sb, however, S diffused wider than Sb and migrated toward Zn after the durability test and presumably substitution of S of Sb 2 S 3 for O of ZnO might occur and result in the formation of ZnS.
Effect of UV light irradiation on stability.-Inorder to investigate the effect of UV light irradiation, another durability test of glass-ITO/TiO 2 /Sb 2 S 3 /ZnPC/Au without UV cut filter was performed at 63 • C and 50% RH under 1 sun.The J -V curves of the initial photovoltaic performance (solid) and after 3 d (broken) were indicated in Figure 6.The relative PCE of 90% was retained for 1,500 h with UV cut filter, however the photovoltaic ability has almost been lost without the cut filter after 72 h.Decolorization was observed after the irradiation without the UV cut filter though the decolorization did not occur with the filter.Once the TiO 2 layer was removed from the cell, described as glass-ITO/Sb 2 S 3 /ZnPC/Au, there was no decolorization even when without using the UV cut filter.In this context, anatase TiO 2 nanoparticles in this case act as the photocatalyst during the UV light irradiation 27 to oxidize and decompose Sb 2 S 3 , which resulted in the above decolorization of the cells.TEM-EDX observations of the cross-sectional views of colored (being not decolorized) site and decolorized site (Figure 7) indicated that the existence of S and O was confirmed at the colored site though only O was confirmed (viz.S was not detected) at the decolorized site.This is also supporting the oxidative decomposition of Sb 2 S 3 by photoactivated TiO 2 .

Summary
We prepared Sb 2 S 3 -based solar cells with ETL of TiO 2 or ZnO in addition to HTL of P3HT/PEDOT: PSS, ZnPc, or MoO 3 and compared their photovoltaic performance in terms of durability with encapsulation by using glass and UV cutoff films.Combination with ZnPc and TiO 2 has the highest durability with retaining the relative PCE of 90% under the conditions of 1 sun at 63 • C at a relative humidity of 50% for 1,500 h.Effective suppression of the photoactivation of TiO 2 through UV light irradiation by using cut filter is essential to realize the long term stability and promising further extension for optimization of the photovoltaic performance.

Figure 3 .
Figure 3. Cross sectional TEM images (right) and EDX images of S, Sb, and Mo elemental maps (left 3) of Sb 2 S 3 /MoO 3 layers of the hybrid solar cells composed of glass-ITO/TiO 2 /Sb 2 S 3 / MoO 3 /Au fresh after preparation (above), and after 576 h under 1 sun at 63 • C and 50% RH (below).The thin film samples for the observations were prepared by FIB method after carbon coating and TEM and EDX have been operated with JEOL JEM-2010FEF at 200 kV.

Figure 5 .
Figure 5. Cross sectional TEM images (right) and EDX images of S, Sb, and Zn elemental maps (left 3) of ZnO/Sb 2 S 3 layers of the hybrid solar cells composed of glass-ITO/ZnO/Sb 2 S 3 /ZnPc/Au fresh after preparation (above), and after 7 d at 63 • C and 50% RH under 1 sun (below).The thin film samples for the observations were prepared by FIB method after carbon coating and TEM and EDX have been operated with JEOL JEM-2010FEF at 200 kV.

Figure 7 .
Figure 7. Cross sectional EDX images of S (red), Sb (green), and Ti (blue) elemental maps of colored site (left) and decolorized site (right) of ITO/TiO 2 / Sb 2 S 3 layers of the hybrid solar cells composed of glass-ITO/TiO 2 / Sb 2 S 3 /ZnPc/Au without UV cut filter after 1 d at 63 • C and 50% RH under 1 sun.