Silicotungstate, a Potential Electron Transporting Layer for Low

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Silicotungstate, a Potential Electron Transporting Layer for Low-Temperature Perovskite Solar Cells Yoon Ho Choi, Hyun Bin Kim, In Seok Yang, Sang Do Sung, Young Sik Choi, Jeongho Kim, and Wan In Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05146 • Publication Date (Web): 12 Jul 2017 Downloaded from http://pubs.acs.org on July 13, 2017

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ACS Applied Materials & Interfaces

Silicotungstate, a Potential Electron Transporting Layer for LowTemperature Perovskite Solar Cells Yoon Ho Choi, Hyun Bin Kim, In Seok Yang, Sang Do Sung, Young Sik Choi, Jeongho Kim, and Wan In Lee*

Department of Chemistry and Chemical Engineering, Inha University, Incheon 22212, Korea

ABSTRACT Thin films of a heteropolytungstate, lithium silicotungstate (Li4SiW12O40, termed Li-ST), prepared by a solution process at low temperature, were successfully applied as electron transporting layer (ETL) of planar-type perovskite solar cells (PSCs). Dense and uniform Li-ST films were prepared on FTO glass by depositing a thin Li-ST buffer layer, followed by coating of a main Li-ST layer. The film thickness was controlled by varying the number of coating cycles, consisting of spin-coating and thermal treatment at 150oC. In particular, by employing 60 nmthick Li-ST layer obtained by two cycles of coating, the fabricated CH3NH3PbI3 PSC device demonstrates the photovoltaic conversion efficiency (PCE) of 14.26% with JSC of 22.16 mA cm-2, VOC of 0.993 mV and FF of 64.81%. The obtained PCE is significantly higher than that of the PSC employing a TiO2 layer processed at the same temperature (PCE=12.27%). Spectroscopic analyses by time-resolved photoluminescence and pulsed light-induced transient measurement of photocurrent indicate that the Li-ST layer collects electrons from CH3NH3PbI3 more efficiently and also exhibits longer electron lifetime than the TiO2 layer thermally treated at 150oC. Thus LiST is considered to be a promising ETL material that can be applied for the fabrication of flexible PSC devices.

Keywords: perovskite solar cell; electron transporting layer (ETL); silicotungstate; heteropolytungstate; low temperature process 1

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■ INTRODUCTION Perovskite solar cells (PSCs) have recently attracted extensive interest due to their outstanding photovoltaic performance as well as low cost in device fabrication.1–7 Typically, CH3NH3PbI3 with a bandgap of 1.55 eV is used as light absorber for PSCs. To collect electrons and holes from the perovskite light absorber, mesoporous TiO2 is mainly used as electron transporting layer (ETL), while spiro-OMETAD, PTAA, or other organic hole conductors are applied as hole transporting layer (HTL).8–11 Although most of highly efficient PSC devices with photovoltaic conversion efficiencies (PCEs) over 20% employ mesoporous TiO2 layers,12–14 TiO2 is suspected to be one of the main components deteriorating the stability of PSC devices, because the organometallic CH3NH3PbI3 in contact with the TiO2 layer can be decomposed by light irradiation due to strong photocatalytic reactivity of TiO2.15,16 As alternatives, ZnO,17,18 Zn2SnO419–21 and SnO222–24 can also be used as ETL of PSCs. In particular, Sn-based oxides have been successfully used as n-type compact layer for the planar-type PSCs.22–24 Recently, flexible PSC devices have been developed as an effort to diversify commercial application of PSC devices.25–27 In order to fabricate flexible PSC devices, it is required to develop low temperature process at a temperature lower than ~150oC, because the flexible devices are generally prepared on plastic substrates. As a result, the limiting step of determining the process temperature is the formation of inorganic ETL layer because most of inorganic materials used for ETLs are not sufficiently crystallized at a temperature as low as ~150oC and consequently electron transport becomes inefficient with poor crystallization. For instance, the TiO2 layer heat-treated at 150oC shows significantly poorer electron transport property than the films processed at a high temperature such as 500oC. Due to this limitation, most of the flexible PSCs employing TiO2 layer processed at low temperatures shows PCEs in the range of only 12– 14%.28–30 Recently, efficient flexible PSCs employing Zn2SnO4- and SnO2-based ETLs have also been reported.23,31,32 In addition, a mixture of WOx and TiOx prepared at a low temperature was applied as ETL for flexible PSC showing PCE of ~13%.33 Heteropolytungstates, belonging to the polyoxometallate family, are a polyanionic nanocluster and have the structure of a cage-like framework composed of tungsten oxide 2


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octahedra and a hetero-metal ion located at the center of the framework. Due to their diverse electrochemical properties, high charge mobility, and tunability of their crystallographic structures, heteropolytungstates have been utilized for various applications, for example, photocatalysts,34,35 catalysts for various organic reactions,36,37 counter electrode materials or modifying agents of photoanode for sensitized solar cells,38–44 supercapacitors,45,46 and others. In this work, we introduce a heteropolytungstate as ETL material for PSC. Among various heteropolytungstates, silicotungstate (SiW12O404-) was chosen because it possesses high electron mobility and can be prepared in film easily by a solution process at low temperatures. The crystallographic structure of SiW12O404-, as described in Figure 1a, belongs to the Keggin type heteropolytungstate. Four of W3O102- clusters, which are composed of three tungsten oxide octahedra sharing the edges, are mutually interconnected by sharing their corners to form a spherical cage-like framework, while a Si4+ ion is located at the center of the framework. It was reported that the conduction band (CB) of SiW12O404- is located at -4.55 V and its band gap is 3.4 eV.47–51 Therefore, when SiW12O404- is used as ETL for PSC, the photoexcited electrons in the perovskite light absorber can be easily injected into SiW12O404- and then transported to the FTO (-4.8 V),52,53 as shown in Figure 1b. In addition, due to the large band gap, SiW12O404--based ETLs provide high transparency in the entire visible region. Silicotungstates are soluble in polar solvents such as water and alcohols. In the present work we selected the lithium salt of silicotungstate (Li4SiW12O40, termed Li-ST) to increase its solubility in polar solvent and enhance the electron mobility through the prepared ETL layer. Thin films of Li-ST can be prepared by a simple solution process such as spin-coating method, while the coated film can be crystallized at a temperature lower than 150oC. Hence, the Li-ST is expected to be a promising candidate for ETL of flexible PSCs. Herein, we developed a lowtemperature solution process to deposit thin Li-ST compact layers, and found that the planar-type PSCs employing the thin Li-ST layer exhibits promising photovoltaic properties. The electron injection and transport behaviors of the thin Li-ST layers were also analyzed by employing two different spectroscopic techniques.

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■ EXPERIMENTAL METHODS Deposition of Li-ST Layers. One mmol silicotungstic acid (H4SiW12O40, 99%, Aldrich) was dissolved in 20 mL water in a flask. In a separate flask, 2 mmol lithium carbonate (Li2CO3, 98%, Aldrich) was dissolved in 100 mL water, and this solution was then added dropwise to the silicotungstic acid solution while the mixture was stirred vigorously. The mixture was stirred and heated at 60oC for 2 h to obtain lithium silicotungstate (Li4SiW12O40, Li-ST). To collect Li-ST powder, water in the solution was removed using a vacuum evaporator. Finally, the collected LiST was dried in a vacuum oven for 2 h. The patterned FTO glass (Pilkington, TEC8) was dipped into 1 mM HCl solution for 5 min and washed by deionized water. In order to modify the surface of FTO, the prepared FTO glass was then dipped into 30 mM L-lysine aqueous solution for 1 h and rinsed thoroughly by deionized water. 15 mM Li-ST solution in ethanol was dropped on the surface-modified FTO glass and then spun at 3,000 rpm for 30 s, followed by heat-treatment at 150oC for 30 min to obtain the Li-ST buffer layer. To increase the thickness of the Li-ST layers, the 30 mM Li-ST aqueous solution was dropped and then spun at 3,000 rpm for 30 s over the Li-ST buffer layer by several coating cycles, each of which consists of spin-coating at 3,000 rpm and subsequent thermal treatment at 150oC for 30 min. Fabrication of PSCs. CH3NH3PbI3 layer was deposited by a so-called one-step method. 461 mg PbI2 (99.9985%, Alfa) and 159 mg methylammonium iodide (MAI, Aldrich) were added to the mixture of 600 mg dimethylformamide (99.8%, Aldrich) and 78 mg dimethyl sulfoxide (99.9%, Aldrich). Fifty µL CH3NH3PbI3 solution was dropped on the ETL/FTO electrode with a size of 2 cm × 2 cm, and then spun at 4000 rpm for 20 s. After 10 s of spinning, 50 µL diethyl ether was dropped on the coated CH3NH3PbI3 layer, followed by annealing on a hot plate at 60oC for 1 min and then 100oC for 5 min. As HTL for PSC devices, spiro-OMETAD layer was prepared by spin coating method, and the Au back contact layer was then deposited using a thermal evaporator. Detailed conditions and procedures are described elsewhere.54

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Measurements and Characterizations. Photocurrent density˗voltage (J˗V) curves of PSC devices were measured at 25oC in air under an irradiation of AM 1.5G one sun light.55,56 The PSC devices were masked with a non-reflective black metal aperture to define the active area, typically 0.122 cm2, while the active area of each device was measured using an optical microscope. For measurement of J˗V curves, the applied voltages were scanned in the reverse direction with the scan rate of 100 mV/s, and the dwelling time before the voltage scan was 20 s. Incident photon-to-current efficiency (IPCE) spectra were obtained in the wavelength range of 300 – 850 nm using an IPCE measurement system (PV Measurements, Inc.) 55,56. Time-resolved photoluminescence (TR-PL) spectra were obtained by a time-correlated single photon counting spectrometer (FluoTime 200, PicoQuant),57,58 and the electron life time (τe) was determined by a home-made equipment of pulsed light-induced transient measurement (PLITM) of photocurrent.59,60 Details are described in our previous report.56

■ RESULTS AND DISCUSSION Surface of the FTO substrate was modified by a short-chain amino acid, L-lysine, in order to improve wettability toward Li-ST. Initially, positive charges were generated on the FTO surface by treating with dilute hydrochloric acid, and the substrate was then dipped into an aqueous solution of L-lysine. By this process, the carboxyl group of L-lysine is anchored onto the FTO substrate, whereas the positively charged amine group is directing upward from the FTO surface. As a result, rather hydrophobic surface of the FTO glass became highly hydrophilic. As shown in Figure 2a, the Li-ST solution in ethanol forms a large droplet when a few drops of the Li-ST solution are dropped on the bare FTO glass. However, it is spread completely on the L-lysinetreated FTO glass, clearly indicating the highly hydrophilic surface. A dilute Li-ST ethanol solution was then spin-coated on the L-lysine-treated FTO glass to form a thin Li-ST layer (denoted as Li-ST buffer layer). It is deduced that SiW12O404- polyanion can be attached to the NH3+ terminal group of L-lysine by electrostatic attraction, as described in Figure 2b. Thus, it is expected that the prepared thin Li-ST buffer layer uniformly covers the entire surface of rough FTO layer. To increase the thickness of the Li-ST layers, the Li-ST 5



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aqueous solution was then deposited over the Li-ST buffer layer by several cycles of coating, each of which consists of spin-coating and subsequent thermal treatment at 150oC. Instead of water-based sol, an ethanol-based dilute Li-ST sol was used to prepare the Li-ST buffer layer. Compared with water, ethanol can infiltrate into small pores more efficiently due to its significantly lower viscosity and surface tension. Hence, the ethanol-based Li-ST sol will show better wettability toward the highly rough FTO substrate. For the formation of the main Li-ST layer, however, we used water-based sol, to suppress the damage to the initially deposited Li-ST layer during the repeated spin-coating processes. Figure 3 shows plan-view and cross-sectional SEM images of various Li-ST films deposited on the FTO substrate. It can be seen in Figure 3a that the surface of highly rough FTO glass (see Figure S1a and b) is uniformly covered by the Li-ST buffer layer, which was obtained by spin coating with dilute Li-ST sol solution. With the increase of regular coating cycles over the Li-ST buffer layer, more Li-ST seems to be deposited over the highly rough FTO surface. Surfaces of the Li-ST buffer layer and the Li-ST layer prepared by two cycles of coating were examined by a high-resolution SEM, as shown in Figure 3b. Irrespective of Li-ST layer thickness, the Li-ST films exhibit quite similar morphologies and the FTO surface seems to be fully covered by tiny Li-ST grains. Even the protruded parts of the large FTO grains were completely coated with LiST, providing the evidence for the uniform coverage of Li-ST layer over the rough FTO substrate. From the cross-sectional SEM images in Figure 3c, the thickness of the Li-ST buffer layer is estimated to be approximately 15 nm, though its thickness is not uniform. Then, over the Li-ST buffer layer, the Li-ST aqueous solution was deposited by several coating cycles. As the cycles of coating were increased to 1, 2 and 3, the thicknesses of Li-ST layer were increased to ~40, ~60 and ~80 nm, respectively, suggesting that the thickness of approximately 20 nm is added per each coating cycle. Figure 4 shows X-ray diffraction patterns of the Li-ST films heat-treated at various temperatures. All of the Li-ST films were prepared by two cycles of coating on the FTO glass. The characteristic (110) peak of the cubic silicotungstate phase at ~9o was observed for the films 6


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heat-treated at 100–450oC. However, the (110) peak, the main peak of the silicotungstate phase, was no longer observed for the films thermally treated at 500oC, suggesting that the Li-ST structure begins to decompose at temperatures higher than 450oC. It was reported that the decomposed products of Li4SiW12O40 at elevated temperatures are a mixture of tetragonal WO3 and Li2W5O16.61 Thus, the XRD data indicate that amorphous WO3 and other impurity phases begin to be formed at about 500oC. Figure 3d shows the cross-sectional SEM image of a typical perovskite solar cell consisting of 60 nm-thick Li-ST layer on the FTO substrate, 350 nm-thick CH3NH3PbI3 layer, ~200 nm-thick spiro-OMETAD layer, and ~60 nm-thick Au counter electrode. The fabricated PSC is considered to be a planar-type device, because the Li-ST layer is a dense film without internal pore structures. Figure 5a shows J-V curves of the PSC devices employing Li-ST layers of various thicknesses, all of which were heat-treated at 150oC. The PSC device with the bare Li-ST buffer layer showed a high short circuit current (JSC), but the open circuit voltage (VOC) value was very low, suggesting the occurrence of considerable charge recombination in this device. Presumably, in this device, the Li-ST buffer layer with 15 nm thickness is not suitable to block the charge recombination between electrons in FTO and holes in the valence band (VB) of CH3NH3PbI3, because pinholes are usually included in the ultra-thin films prepared by solution process. The optimized PCE was obtained from the Li-ST layer prepared by two cycles of coating, implying that the optimum Li-ST thickness is 60 nm. On the other hand, thicker Li-ST layers obtained by additional coating cycles lead to significantly lower PCE, mainly due to the decrease in VOC and fill factor (FF). The optimized PSC device demonstrates the PCE of 14.26% with JSC of 22.16 mA cm-2, VOC of 0.993 mV and FF of 64.81%. Detailed photovoltaic (PV) parameters for the PSCs with Li-ST layers of various thicknesses are listed in Table 1. For all cases, PCE and other PV parameters were acquired from the PSC devices ranked top 20% in efficiency among the fabricated devices, while the PCE distributions are illustrated in Figure S2. Figure S3 shows J-V curves of the PSC devices employing Li-ST layers prepared under three different conditions. PSC-1 and PSC-2 denotes the PSCs with and without introducing the L7



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lysine monolayer, respectively, while the Li-ST layer is composed of the buffer and the main layer prepared by two cycles of coating at 150oC for the both samples. PSC-3 denotes the PSC employing the Li-ST layer prepared by two cycles of coating without introducing the buffer layer, while the L-lysine monolayer was introduced on the FTO surface. It was found that PSC-2 and PSC-3 showed much lower VOC than PSC-1, although their JSC values were not significantly different from one another. Considerably lower VOC values of PSC-2 and PSC-3 seem to arise from their high recombination rate, presumably caused by incomplete coverage of Li-ST layer over the FTO glass. Thus, the obtained result clearly indicates that the interfacial L-lysine monolayer plays a crucial role in improving the wettability of FTO glass to the Li-ST and also the initial Li-ST buffer layer is essential to induce uniform coverage over the FTO glass. Figure S4 shows J-V curves of the PSC devices employing the Li-ST layers that were heattreated at various temperatures. Compared to the PSC device with the Li-ST layer heat-treated at 100oC, the PSC device with the Li-ST layer at 150oC exhibited higher cell performance, but in the temperature range from 150oC to 450oC, the PCE does not vary significantly depending on the heat-treatment temperature for the Li-ST layer formation. However, the Li-ST layer heattreated at 500oC showed relatively lower PCE, probably because the cubic silicotungstate phase is decomposed at this high temperature, as indicated in the XRD patterns (Figure 4). It is also suggested that, among the tungstate-based oxide phases, Li-ST is a crystallographic phase suitable for application to ETL of PSCs. Photovoltaic properties of the PSCs employing the optimized Li-ST layer prepared at 150oC (PSC-Li-ST-150) were compared with those of the planar PSCs employing conventional TiO2 layers: one is the 50 nm-thick compact TiO2 layer annealed at 500oC (PSC-TiO2-500) and the other is the TiO2 layer thermally treated at 150oC (PSC-TiO2-150). Detailed preparation procedures are described in the caption of Figure S1. Cross-sectional SEM images of PSC-TiO2500 and PSC-TiO2-150, as shown in Figure S1c and d, indicate that the thicknesses of TiO2-500 and TiO2-150 are about 50 and 40 nm, respectively. As seen in J-V curves in Figure 5b, PSCTiO2-500 shows the PCE of 16.22%, which is 14% higher than that of PSC-Li-ST-150, with JSC of 21.98 mA cm-2, VOC of 1,013 mV and FF of 72.83%. The obtained PV parameters are also 8


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listed in Table 1. In contrast, the PCE of PSC-TiO2-150 employing the TiO2 layer thermally treated at 150oC was only 12.27%, which is comparable with the PCE values reported by other research groups.28–30 Considering that the Li-ST layer used as ETL in this work was prepared at 150oC, the achieved PCE of 14.26% is quite a remarkable result. In this regard we believe that Li-ST is a promising new ETL material which can be used for the fabrication of PSCs at low temperatures. Also, IPCE spectra were obtained for the above PSCs, as shown in Figure 5c. The external quantum efficiency (EQE) maxima of PSC-Li-ST-150 and PSC-TiO2-500 were quite similar to each other with ~91% at 500–550 nm wavelengths, which are significantly higher than that of PSC-TiO2-150. Overall, the JSC values acquired from the integration of the IPCE spectra agree with the values acquired from the J-V curves (Figure 5b). In comparison with the photovoltaic properties of PSC-TiO2-500, PSC-Li-ST-150 shows comparable JSC and VOC values but has significantly lower FF value, suggesting that the Li-ST layer is relatively more resistive in transporting electrons than the TiO2 layer heat-treated at 500oC. Plan-view SEM images of the CH3NH3PbI3 films prepared on the Li-ST-150, TiO2-500 and TiO2-150 layers are shown in Figure S5. Regardless of the ETL layers employed, the grain structures of the CH3NH3PbI3 films were basically the same, suggesting that growth of the CH3NH3PbI3 grains was not initiated from the substrate surface. Electron mobility of ETLs used for the PSC devices was analyzed by using the Hall effect measurement system (Ecopia HMS-3000, Bridge Tech.) at 25oC. The TiO2-500 showed the highest electron mobility with a value of 260±50 cm2V-1s-1, which agrees to the reported values.62 It was also found that the electron mobility of Li-ST-150 (80±20 cm2V-1s-1) was significantly higher than that of TiO2-150 (20±10 cm2V-1s-1). Overall, the electron mobility of the ETLs seems to be correlated with the performance of PSC devices. J-V curves of PSC-Li-ST-150, PSC-TiO2-150 and PSC-TiO2-500, obtained by forward and backward scans, are shown in Figure S6. PSC-Li-ST-150 exhibits a relatively higher hysteresis in the J-V measurement than PSC-TiO2-500, but shows significantly lower hysteresis than PSCTiO2-150. Considering that the electron mobility decreases in the order of TiO2-500 > Li-ST-150 9



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> TiO2-150, the extent of hysteresis in PSC devices seems to be inversely proportional to the electron mobility of ETL. Based on this observation, we infer that the electron mobility of ETL is one of major factors that govern the hysteresis of PSC devices and therefore an ETL with high electron mobility is required to decrease hysteresis of PSC devices. To investigate the effects of ETL materials on the dynamics of charge injection and transport as well as the photovoltaic performance of PSCs, we used two types of time-resolved spectroscopic techniques. Firstly, to analyze the efficiency of charge injection from CH3NH3PbI3 layer to the ETL, we measured TR-PL of PSC devices (without introducing HTL) that employ the Li-ST layer processed at 150oC (Li-ST-150) and the TiO2 layers processed at 500oC (TiO2500) and 150oC (TiO2-150).57,58 As shown in Table 2, the average time constant (τinterface) of PL decay was determined to be 34.40 ns. With ETLs present in the devices, the TR-PL decay is accelerated due to electron injection from the perovskite to the ETL. In particular, τinterface for the TR-PL decay of the devices decreases with the change of ETL in the order of TiO2-150 (15.81 ns), Li-ST-150 (2.60 ns), and TiO2-500 (1.90 ns), as shown in Figure 6a and Table 2. According to the result of the TR-PL measurement, electron injection from CH3NH3PbI3 to ETL is more efficient in Li-ST-150 than in TiO2-150, while TiO2-500 exhibits the most efficient charge injection. Secondly, we measured the lifetime (τe) of photo-injected electrons in the thin Li-ST-150, TiO2-500, and TiO2-150 layers by pulsed light-induced transient measurement (PLITM) of photocurrent.59,60 The τe value represents the lifetime of photogenerated electrons that survive the charge recombination between the CB of ETL and the VB of perovskite or the HOMO of HTM. Specifically, τe was determined from a single-exponential fit of the temporal decay of transient photocurrent at the open-circuit condition. Figure 6b shows τe measured as a function of JSC for the PSCs employing Li-ST-150, TiO2-150 or TiO2-500 layers. While PSC-TiO2-500 exhibited the highest τe value, PSC-Li-ST-150 still showed significantly higher τe value than PSC-TiO2150, which is considered to have a high level of defects in the prepared TiO2 layer. The obtained result is consistent with the trend of photovoltaic properties of PSCs employing Li-ST-150, TiO2-150 and TiO2-500 layers. Consequently, Li-ST is considered to be a potential candidate for 10


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ETL materials that can replace TiO2 in fabricating the flexible PSCs, although it is less efficient than TiO2-500 in transporting electrons.

■ CONCLUSIONS Li-ST thin films were uniformly coated on the rough and rather hydrophobic FTO substrate by modifying the FTO surface with molecular L-lysine. In particular, the dense Li-ST layer of 60 nm-thickness was prepared by the initial buffer layer coating and subsequent two cycles of coating consisting of spin coating and thermal treatment at 150oC, and was proven to be the optimum ETL for the planar-type PSCs. The CH3NH3PbI3 PSC device processed at 150oC demonstrated photovoltaic conversion efficiency (PCE) of 14.26%, which is significantly higher than that of the PSC employing the TiO2 layer heat-treated at 150oC (12.27%). In comparison with PSC-TiO2-500, PSC-Li-ST-150 showed comparable JSC and VOC but significantly lower FF, suggesting that the Li-ST layer is more resistive in transporting electrons through its CB than the TiO2-500 layer. The TR-PL measurement indicates that the Li-ST layer is more efficient than the TiO2-150 in injecting electrons from CH3NH3PbI3 to ETL. From the PLITM of photocurrent measurement, it was also found that the lifetime of photoinjected electrons is significantly higher in LS-ST-150 than in TiO2-150.

■ ACKNOWLEDGMENTS This work has been supported by the Ministry of Science, ICT and Future Planning (MSIP) through the National Research Foundation of Korea (NRF) (2015M1A2A2052999) and Basic Science Research Program through NRF funded by the Ministry of Education (NRF2016R1A6A3A01008989).

■ ASOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXX. 11



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Preparation method and SEM images for the TiO2 layers heat-treated at 150oC and 500oC. J-V curves of the PSC devices employing the Li-ST layers prepared under different conditions and temperatures. PCE distribution of PSCs, SEM images of CH3NH3PbI3 films on various substrates, and J–V curves acquired by forward and reverse scans.

■ AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

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Chart.

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of

Lithium

and

Sodium

Silicotungstates,

Li4SiW12O40·26H2O

and

Na4SiW12O40·17H2O. Zh. Neorg. Khim. 1983, 28, 1192–1196. (62) Sarkar, T.; Gopinadhan, K.; Zhou, J.; Saha, S.; Coey, J. M. D.; Feng, Y. P.; Ariando; Venkatesan, T. Electron Transport at the TiO2 Surfaces of Rutile, Anatase, and Strontium Titanate: The Influence of Orbital Corrugation. ACS Appl. Mater. Interfaces 2015, 7, 24616–24621.

18


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Table 1. Photovoltaic parameters of PSCs employing the Li-ST layers with various film thicknesses and PSCs employing the TiO2-based ETLs. PCE and other photovoltaic parameters were obtained from the PSC device ranked in top 20% in efficiency among the fabricated devices. ETL for PSC

VOC (mV)

Li-ST buffer

882

JSC FF (%) (mA cm-2) 21.21 51.24

Li-ST/1 cycle

957

21.78

60.82

12.68

Li-ST/2 cycles

992

22.73

63.03

14.21

Li-ST/3 cycles

989

21.81

60.57

13.07

Li-ST/4 cycles

914

21.69

60.91

12.08

Li-ST-150

993

22.16

64.81

14.26

TiO2-150

954

20.77

61.94

12.27

TiO2-500

1013

21.98

72.83

16.22

Table 2. Parameters for the FTO/ETL/CH3NH3PbI3 devices.

TR-PL

Glass/CH3NH3PbI3

9.59

bare

CH3NH3PbI3

τinterface

τCT

CTE(%)

(ns)

(ns)

data

Devices

PCE (%)

34.40

of

--

--

FTO/Li-ST-150/CH3NH3PbI3 2.60

3.11

83.6

FTO/TiO2-150/CH3NH3PbI3

15.81

29.25

54.1

FTO/TiO2-500/CH3NH3PbI3

1.90

2.01

94.5

19


and

various


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 27

FIGURE CAPTIONS

Figure 1. Crystal structure of SiW12O404- (a) and diagram for charge separation occurring in the PSC device employing SiW12O404- as electron transporting layer (b). Figure 2. Photographic image of the Li-ST coating solution dropped on the bare and the lysinetreated FTO glasses (a) and schematic diagram describing the formation of Li-ST buffer layer (b). Figure 3. Plan-view SEM images of the various Li-ST layers on the FTO glass deposited by different coating cycles (a) and magnified SEM images of the buffer Li-ST layer and the Li-ST layer obtained by two cycles of coating (b). Cross-sectional SEM images of the various Li-ST layers on the FTO glass deposited by different coating cycles (c) and the typical PSC device employing the Li-ST layer obtained by two coating cycles as ETL (d). Figure 4. X-ray diffraction patterns of the various Li-ST films heat-treated at the temperatures in the range of 100–500oC. Li-ST films were prepared on the FTO glass and ■ denotes the FTO diffraction peaks. Figure 5. J-V curves of the PSC devices employing the Li-ST layers with various film thicknesses (a). J-V curves of the several PSC devices employing Li-ST-150, TiO2-150, and TiO2-500 (b) and their corresponding IPCE spectra and integrated current densities (c). Figure 6. TR-PL decays (a) and electron lifetime (τe) as a function of JSC (b) for the several devices employing Li-ST-150, TiO2-150, and TiO2-500 as ETL.

20


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Figure 1. Crystal structure of SiW12O404- (a) and diagram for charge separation occurring in the PSC device employing SiW12O404- as electron transporting layer (b).

21



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Page 22 of 27

Figure 2. Photographic image of the Li-ST coating solution dropped on the bare and the lysinetreated FTO glasses (a) and schematic diagram describing the formation of Li-ST buffer layer (b).

22


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Plan-view SEM images of the various Li-ST layers on the FTO glass deposited by different coating cycles (a) and magnified SEM images of the buffer Li-ST layer and the Li-ST layer obtained by two cycles of coating (b). Cross-sectional SEM images of the various Li-ST layers on the FTO glass deposited by different coating cycles (c) and the typical PSC device employing the Li-ST layer obtained by two coating cycles as ETL (d). 23


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Page 24 of 27

(110)


o

100 C o

150 C o

200 C o 250 C o

300 C o 350 C o

400 C o

450 C o

500 C 5

10

15

20

25

30

35

40

Two Theta (Degree)

Figure 4. X-ray diffraction patterns of the various Li-ST films heat-treated at the temperatures in the range of 100–500oC. Li-ST films were prepared on the FTO glass and ■ denotes the FTO diffraction peaks.

24


Page 25 of 27

(a)

2

Current Density (mA/cm )

25

20

15

Li-ST buffer only 1 cycle 2 cycles 3 cycles 4 cycles

10

5

0 0

200

400

600

800

1000

Applied Voltage (mV)

(b)

2

Current Density (mA/cm )

25

20

15

PSC-Li-ST-150 PSC-TiO2-150

10

PSC-TiO2-500 5

0 0

200

400

600

800

1000

Applied Voltage (mV) 100

25

(c)

EQE (%)

80

20

PSC-Li-ST-150 PSC-TiO2-150

60

15

PSC-TiO2-500 40

10

20

5

0

400

500

600

700

800

2

0 300

Integrated Current Density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Wavelength (nm)

Figure 5. J-V curves of the PSC devices employing the Li-ST layers with various film thicknesses (a). J-V curves of the several PSC devices employing Li-ST-150, TiO2-150, and TiO2-500 (b) and their corresponding IPCE spectra and integrated current densities (c).

25


Page 26 of 27

1

(a)

MAPbI3 Li-ST-150/MAPbI3 TiO2-150/MAPbI3 TiO2-500/MAPbI3

0.1 0

5

10

15

20

Time (ns)

Electron Lifetime (s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

PL Intensity (Normalized)


1E-4

(b)

1E-5

1E-6

1E-7

Li-ST-150 TiO2-150 TiO2-500

1E-8

1E-9 0.1

2

1

JSC (mA/cm )

Figure 6. TR-PL decays (a) and electron lifetime (τe) as a function of JSC (b) for the several devices employing Li-ST-150, TiO2-150, and TiO2-500 as ETL.

26


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Table of Contents

27


Silicotungstate, a Potential Electron Transporting Layer for Low

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