V2O5 as Hole Transporting Material for Efficient All Inorganic Sb2S3

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V2O5 as Hole Transporting Material for Efficient All Inorganic Sb2S3 Solar Cells Lijian Zhang, Chenhui Jiang, Chunyan Wu, Huanxin Ju, Guoshun Jiang, Weifeng Liu, Chang-Fei Zhu, and Tao Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 24 Jul 2018 Downloaded from http://pubs.acs.org on July 24, 2018

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ACS Applied Materials & Interfaces

V2O5 as Hole Transporting Material for Efficient All Inorganic Sb2S3 Solar Cells Lijian Zhang,a Chenhui Jiang,a Chunyan Wu,a Huanxin Ju,b,* Guoshun Jiang,a Weifeng Liu,a Changfei Zhu,a,* Tao Chena,* a. CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, No. 96 Jinzhai Road, Hefei, Anhui Province, 230026, P. R. China. b. National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, P. R. China.

KEYWORDS: Sb2S3, power conversion efficiency, V2O5, hole transporting material, thin films

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ABSTRACT: This research demonstrates that V2O5 is able to serve as hole transporting material to substitute organic transporting materials for Sb2S3 solar cells, offering all inorganic solar cells. The V2O5 thin film is prepared by thermal decomposition of spin-coated vanadium (V) triisopropoxide oxide solution. Mechanistic investigation shows that heat treatment of V2O5 layer has crucial influence on the power conversion efficiency of device. Low temperature annealing is unable to remove the organic molecules that increases the charge transfer resistance, while high temperature treatment leads to the increase of work function of V2O5 that blocks hole transporting from Sb2S3 to V2O5. Electrochemical and compositional characterizations show that the interfacial contact of V2O5/Sb2S3 can be essentially improved with appropriate annealing. The optimized power conversion efficiency of device based on Sb2S3/V2O5 heterojunction reaches 4.8%, which is the highest power conversion efficiency in full inorganic Sb2S3-based solar cells with planar heterojunction solar cells. Furthermore, the employment of V2O5 as hole transporting material leads to significant improvement in moisture stability compared with the device based organic hole transporting material. Our research provides a material choice for the development of full inorganic solar cells based on Sb2S3, Sb2(S,Se)3 and Sb2Se3.


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1. INTRODUCTION Antimony sulfide (Sb2S3) has attracted considerable attention for using as absorber layer in solar cells because of its abundant compositional elements, environmental friendliness, excellent moisture and air stability, suitable optical band gap (~1.7 eV) and high optical absorption coefficient in the visible range (~105 cm-1 at 450 nm).1-5 In general, Sb2S3 has applied in either planar heterojunction structure or sensitized device configuration.5-8 Comparing these two kinds of device configurations, planar structure is advantageous in terms of convenient and scalable device fabrications, while the sensitized device structures lead to higher power conversion efficiency (PCE) in current studies.5,

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As for a typical planar structure solar cell, a light

absorption layer is sandwiched between electron transporting layer (ETL) and hole-transporting layer (HTL). They are essential device components to accelerate the separation and transfer of photogenerated electrons and holes to the respective electrodes.5 The film properties of HTL and ETL, such as conductivity and energy levels, significantly affect device performance.10 In Sb2S3 solar cell research, many efforts have been focused on developing appropriate synthetic methods to prepare high quality Sb2S3 film, including spray pyrolysis, chemical bath deposition, atomic layer deposition, thermal evaporation and spin coating. 4-5, 11-13 Inspired by the development of dye-sensitized solar cells and perovskite solar cells, HTLs applied in Sb2S3 solar cells

are

typically

benzothiadiazole)

poly(3-hexylthiophene)

(PCPDTBT),15

spirobifluorene (Spiro-OMeTAD),4

(P3HT),14

poly(cyclopentadithiophene-alt-

2,2´,7,7´-tetrakis(N,N-di-p-methoxyphenylamine)-9,9´(poly(3,4ethylenedioxythiophene)/poly(styrenesulfonate))

(PEDOT:PSS).8 However, the high price and relatively poor stability of organic HTLs raises concerns in the future applications. 16-19


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Generally, inorganic HTLs in device is able to enhance the device stability such as NiO and NiXMg1–XO in perovskite solar cells.20-21 Recently, a few attempts have been made to explore inorganic HTL in Sb2S3 solar cells to substitute for organic HTLs. For instance, Ito et al. used CuSCN as HTL for the Sb2S3 sensitized nanoporous TiO2 solar cell and achieved a PCE of 5.7%.22 However, the planar Sb2S3 solar cell with CuSCN and CuI as HTLs only generated PCE around 1.7% and 1.18%, respectively.23-24 Graphene was also used as HTL in planar Sb2S3 solar cell and delivered a PCE of 1.65%.5 The use of Au film as both hole collector and electrode in depleted Sb2S3 solar cells generates decent PCE of 2%-4%.6,

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Obviously, compared with

organic HTLs, the inorganic HTLs usually generates relatively low device performance especially in planar heterojunction solar cells. Hence, it is of great significance to explore new inorganic HTLs for Sb2S3 solar cell applications with enhanced device performance. Among various inorganic semiconductors, V2O5 has received particular attention for using as a HTL because of its outstanding physical properties including suitable optical and electrical properties and ambient stability.26-28 It has been widely used as hole transporter in organic photovoltaic devices,27-30 while not in Sb2S3 solar cells. The conduction band (CB) and valence band (VB) of V2O5 are -2.6 and -5.4 eV,31 while those of Sb2S3 are -4.1 and -5.9 eV, respectively.13 Therefore, efficient hole-transportation is expected to occur at the Sb2S3/V2O5 interface and the V2O5 layer can also prevents electron leakage efficiently because of the sufficiently high conduction band offset between V2O5 (-2.6 eV) and Sb2S3 (-4.1 eV). These properties point out that the replacement of organic HTL by V2O5 in Sb2S3 solar cell is feasible. However, to the best of our knowledge, the application of V2O5 for Sb2S3 solar cells as HTL has not yet been reported. Herein, we successfully applied V2O5 as HTL in Sb2S3-based absorber to construct efficient planar heterojunction solar cell. By spin-coating the vanadium(V) triisopropoxide oxide (VTIPO)


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solution on the Sb2S3 layer followed by annealing at elevated temperature, uniform and compact layer of V2O5 thin film is fabricated. Our investigation shows that, with proper annealing, this in situ synthesis does not generate detrimental effect on the Sb2S3 films, efficient hole extraction is thus achieved for photovoltaic energy conversion. The interfacial charge transport is ameliorated by optimizing the thickness and annealing temperature with regard to the V2O5 film preparation. Finally, the device based on V2O5 HTL delivers an impressive PCE of 4.8%. 2. RESULTS AND DISCUSSION The solution deposition of V2O5 thin film is conducted via a hydrolytic polycondensation process.32 To set suitable annealing procedure, weight loss behavior of VTIPO solution was examined by thermal gravimetric analysis (TGA). The first stage of weight loss from about 20 °C to 90 °C is mainly owing to the evaporation of water and isopropanol (Figure 1a). When the temperature increased to ~104 °C, 2.3% weight loss is observed because of VTIPO decomposing into metal oxide. There is no weight loss in the range of 104 °C to 200 °C. Presumably, the transformation to solid V2O5 thin film is occurred at this stage, suggesting that the preparation of V2O5 film for HTL should be annealed in this temperature range or even higher temperature. The crystallinity of V2O5 thin film was characterized by X-ray diffraction (XRD). The sample was deposited on glass, treated at 150 °C, 300 °C and 400 °C for 60 min in glove box. As shown in Figure 1b, the XRD patterns show very broad and diffuse patterns, indicating amorphous nature when the temperature are 150 °C and 300 °C. As the temperature increases to 400 °C, two diffraction peaks belonging to V2O5 (JCPDS No. 53-0538) is found, implying that the amorphous film begins to crystallization. The diffraction peaks at 18.7 and 27.8 o are indexed as (110) and (101) crystal planes of V2O5. This result suggests that the deposited amorphous film at


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150 °C is V2O5. It is worthy of noting that the high temperature annealing is not feasible for device investigation. We thus study the properties of V2O5 films which are annealed at 100-180 o

C. To examine the chemical composition and valence state of the as-synthesized film, X-ray

photoelectron spectroscopy (XPS) characterization was carried out where a thin layer of V2O5 film was deposited on Sb2S3 surface treated at 100 °C, 150 °C and 180 °C for 10min, respectively. The high-resolution XPS plots of these three samples subjected to LorentzianGaussian fitting are shown in Figure 1c and the binding energy values of the main peaks in the XPS spectra are listed in Table S1. According to the XPS plots, the V2O5 thin films prepared with different annealing temperatures exhibit similar XPS results. The V 2p spectra demonstrate a typical two-peak patterns of V (2p3/2 and 2p1/2) because of spin-orbit splitting. Two different oxidation states of V (V5+ and V4+) in V2O5 are extracted from the fitted V 2p3/2 and V 2p1/2 spectra. In order to simplify the discussion, the result of V2O5 annealed at 150 oC is discussed here. As shown in Figure 1c and Table S1, the characteristic peaks at 517.58 eV and 525.18 eV are corresponding to V5+.27 The fitted shoulders at 516.43 eV and 524.03 eV are attributed to V4+.33 The appearance of V4+ state is an indication of oxygen vacancies in V2O5, which can enhance the electrical properties of transition metal oxide when they are employed as HTLs.30-31, 33

Because of high oxidizing power of V5+ in V2O5, V5+ can be reduced to V4+ in alkoxides or

aqueous solution.34-36 In addition, the main peak observed at 530.64 eV corresponds to O1s originating from O2- in V2O5. It should be noted that sulfate species in the O1s spectra are detected (~530.8 eV) with the annealing temperature of 150 oC and 180 oC.37 The formation of sulfate may be attributed to the reaction between Sb2S3 and H2O in the VTIPO solution. The formation of sulfate (Sb2(SO4)3) at high annealing temperature is also evidenced by XPS spectra


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of S 2p in V2O5/Sb2S3 thin films (Figure S1). By the further analyzing the XPS results, the binding energy of the highest intensity peak of V 2p3/2 in the raw data shifts from 517.6 eV to 517.1 eV as the temperature increasing from 100 oC to 180 oC. According to above analysis, the binding energy of V4+ locates relative low energy position. This shift may be due to the increase of V4+ content. Therefore, the ratio of V4+ to total V can be calculated according to the XPS results. It is found that the ratios of V4+ to total V of V2O5 thin films annealed at 100 °C, 150 °C and 180 °C is 30%, 32% and 43%, respectively. The reason why the amount of V4+ increases with the temperature increasing maybe because the redox reaction rate in the as deposited film gets faster as the temperature increases.27, 34-35, 38 Therefore, it can be concluded that the main phase of the thin film is nonstoichiometric V2O5 along with V4+ impurity.27, 30, 33 The absorption properties of V2O5 annealed at 100 °C, 150 °C and 180 °C for 10 min are also characterized and the results are depicted in Figure S2. The absorption drops moderately with the temperature increasing, which is similar to previous report and may come from the property of thermochromism of V2O5.39 According to the Tauc formula, (αhν)2 = c(hν-Eg) (inset of Figure 1d), the derived band gaps are about 2.64 eV, 2.65 eV and 2.71 eV for the V2O5 films annealed at 100 °C, 150 °C and 180 °C, respectively. The slight increase in band gap is probably due to the change of V2O5 stoichiometry, which is agreement to above XPS discussion.36 In this study, the formation of V2O5 thin film for device application is conducted onto the pre-formed Sb2S3 surface. To examine whether the in situ synthesis of V2O5 influences the light absorption property of Sb2S3 absorber layer, UV–visible absorption of Glass/FTO/compact-TiO2 (cTiO2)/Sb2S3 and Glass/FTO/c-TiO2/Sb2S3/V2O5 films annealed at 100 °C, 150 °C, 180 °C for 10 min were examined (Figure 1d). The absorption onset of Glass/FTO/c-TiO2/Sb2S3 is about 750 nm, corresponding to an optical bandgap of 1.65 eV. After spin-coating the V2O5 layer on


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Glass/FTO/c-TiO2/Sb2S3 treated at different temperatures, the light absorption of Glass/FTO/cTiO2/Sb2S3/V2O5 film increases slightly in the UV region of 300−400 nm owing to the absorption of V2O5. In 400−900 nm, light absorption of the Glass/FTO/c-TiO2/Sb2S3/V2O5 film keeps almost the same as that of pristine Glass/FTO/c-TiO2/Sb2S3 film, implying that V2O5 does not alter light absorption properties of Sb2S3 film. It is known that the surface morphology of HTL surface such as the coverage and uniformity strongly affects the final device performance.40 Therefore, the surface morphology V2O5 film is analyzed through scanning electronic microscopy (SEM). A typical surface image of V2O5 thin film treated at 150 °C on top of uniform Sb2S3 film is shown in Figure 2a and Figure S3, exhibiting quite uniform, compact and pin hole free characteristics. Such morphology is able to prevent the contact between Au electrode and Sb2S3 film, suppressing charge carrier recombination. A cross sectional image of a complete device (FTO/c-TiO2/Sb2S3/V2O5/Au) is shown in Figure 2b and the corresponding device structure diagram is displayed in Figure 2c. The thicknesses of TiO2 compact layer, Sb2S3 film, optimal thickness V2O5 and Au are measured to be 35, 87, 47, 133 nm, respectively. Considering the photovoltaic stability of Sb2S3 films, the Sb2S3/V2O5 films are annealed at 100 °C (device 1), 150 °C (device 2) and 180 °C (device 3) for device performance optimization. The current density-voltage (J-V) behavior was measured under 1 Sun (AM 1.5G) illumination (Figure 2d). The device based on Spiro-OMeTAD HTL was also fabricated for comparative study. The photovoltaic parameters are summarized in Table 1. When V2O5 is annealed at 100 oC, the best device delivers short-circuit current density (JSC) of 14.49 mA cm-2, open-circuit voltage (VOC) of 0.56 V, fill factor (FF) of 43.05%, with a final PCE of 3.49% (Table 1). Once the annealing temperature is raised to 150 oC, the PCE is increased to 4.8%, with JSC, VOC and FF of


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15.29 mA cm-2, 0.59 V, 52.92%, respectively. This performance is comparable to the Sb2S3 solar cell using Spiro-OMeTAD as HTL, with VOC of 0.61 V, JSC of 15.1 mA cm-2, FF of 54.57% and PCE of 5.0% (Table 1 and Figure S4a). However, further increasing the annealing temperature leads to the PCE reduced down to 3.96%. Compared with device annealed at 150 oC, the JSC, VOC show considerable decrement while FF exhibits only slight decrement. The statistical distribution of photovoltaic parameters are shown in Figure 2f - i and the detail parameters of each solar cell can be found in Table S3. The average photovoltaic parameters summarized in Table 1. As shown in Figure 2f-i, the difference in VOC, JSC, FF and PCE among devices 1, 2 and 3 are conspicuously beyond their associated standard deviations, suggesting that the differences are results of annealing process for V2O5, rather than batch-to-batch variation. Furthermore, the reproducibility of device 2 is comparable to that of the device with Spiro-OMeTAD as HTL, further indicating that V2O5 is promising to replace Spiro-OMeTAD as HTL for Sb2S3-based solar cells. In the device fabrication, low temperature treatment V2O5 precursor thin film may not be able to eliminate all the organic solvent (i.e. at 100 oC). The embedded organic molecules would increase the series resistance (RS). To verify our assumption, we characterize the residue carbon of the sample by Elemental Analyzer (CHN Mode). The weight ratio of C to total weight is list in Table S2. It can be obvious found that the carbon content decrease upon the increase of annealing temperature, which are 1.07%, 0.37% and 0.32% for the V2O5 films prepared with annealing temperature at 100, 150 and 180 oC, respectively. On the other hand, RS of device 1 is 17.89 Ω•cm2, which is the highest among the three devices. When the temperature increases to 150 °C, RS is reduced to 11.64 Ω•cm2 (device 2), and an approximately 37% PCE enhancement is thus achieved when compared with that of 100 °C. The improved PCE is mainly originated


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from the significantly enhanced JSC and FF, indicating the charge collection efficiency is improved due to the elimination of organic solvent and the reduction of RS to 11.64 Ω•cm2. In addition, with the annealing temperature increasing from 100 oC to 150 oC, the shunt resistance (RSH) of the device increases from 191.07 Ω•cm2 to 314.94 Ω•cm2. The enlarged RSH means that the V2O5 HTL treated at 150 °C can suppressed recombination probability so that the charge collection efficiency is enhanced, resulting in a high JSC and FF.41 Further increasing the annealing temperature shifts the valence band of V2O5 towards unfavorable alignment (vide infra). In this case, the device efficiency shows decrement to 3.96% (device 3), though the RS is further reduced to 11.24 Ω•cm2 (Table 1). The incident photon-to-current conversion efficiency (IPCE) spectra of device 1, 2 and 3 are shown in Figure 2e and the device based on Spiro-OMeTAD display in Figure S4b. The onset photocurrent generation is located at 750 nm, which is corresponding to the light absorption properties of the as-synthesized Sb2S3 films with a band gap of 1.65 eV. More importantly, it can be clearly seen that device 2 possesses higher photocurrent conversion efficiency compared to other devices. This result is consistent with increased JSC values from J-V characteristics. What’s more, no obvious differences in IPCE at around 500 nm among 3 samples. The short penetration depth of short wavelength light and similar carrier extraction efficiency at TiO2/Sb2S3 interface are attributed to this observation.42-45 Strikingly, the photocurrent generation of device 2 exceeds 70% in longer wavelength and 80% in short wavelength, indicating the effectiveness of using V2O5 for Sb2S3 based solar cells. In order to study how thermal annealing influences the electric properties of V2O5 films and in turn device performance, ultraviolet photoelectron spectroscopy (UPS) measurement is conducted to study the energy levels (Figure 3a and b). The work function (WF) values are


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obtained from photon energy (40 eV)-binding energy of the secondary edge (high binding energy cutoff) in the UPS spectra (Figure 3a). As a result, the WF of bare Sb2S3 deposited on the compact TiO2 layer is -4.71 eV, which is in agreement with reported results.13 As for V2O5 layer deposited on Sb2S3 layer, the WFs are -4.4 eV, -4.3 eV and -5.0 eV associated with annealing treatment at 100 °C, 150 °C and 180 °C, respectively. Actually, the reported value of work function of V2O5 range from 4.7 eV to 7.0 eV,30,

46-48

which is sensitive to the materials

processing. In the current study, the WF changes essentially when V2O5 thin film is annealed at temperature 180 °C, the amount of V4+ in V2O5 may be ascribed to this notable change.27, 30 According to above XPS analysis, the ratios of V4+ to total V of V2O5 films annealed at 100 °C, 150 °C are 30% and 32%, while it is increased to 43% when the annealing temperature increases to 180 °C. This significantly increased V4+ in the V2O5 would alter electronic structure of V2O5 as well as the WF. From Figure 3a and the valance band (VB) spectra (Figure 3b), the VB for Sb2S3 and V2O5 treated at 100 °C, 150 °C and 180 °C are -5.9 eV, -5.84 eV, -5.72 eV and -6.4 eV, respectively. The energy levels are summarized in Figure 3c. It is found that the VB values of V2O5 treated at 100 °C and 150 °C are similar (-5.84 eV vs. -5.72 eV). However, as the treatment temperature increasing to 180 °C, the value increases to -6.4 eV. The energy levels are summarized in Figure 3c. In this regard, a potential barrier forms between the Sb2S3 and V2O5 interface, rendering it difficult for hole transporting from Sb2S3 to V2O5.49-51 This is the critical reason for low PCE of V2O5 films annealed at 180 °C. To further understand the effect of annealing temperature on PCE of the devices, charge transfer and recombination processes between Sb2S3/V2O5 interfaces is studied by electrochemical impedance spectra (EIS) characterization, which was measured in the dark at applied bias voltage of 0.5 V with frequency ranging from 1 Hz to 500K Hz and AC amplitude


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of 0.01 V. Figure 3d compares the Nyquist plots of devices 1, 2, and 3. The inset of Figure 3d depicts the corresponding fitted curves using an equivalent circuit based on transmission line model.52 As shown in Figure 3d, only one semi-circle is observed in the Nyquist plot for all devices, which indicates that photoinduced charge carrier transfer is mainly determined by diffusion-recombination at the interface between Sb2S3 and V2O5 as well as the interface between metal electrode and V2O5.41, 53 For device 1, charge-transfer resistance (RCT) is 1138.3 Ω•cm2, it decreases to 437.0 Ω•cm2 in device 2, indicating that the elimination of organic solvent improves the interfacial contact between Sb2S3 and V2O5 layers. This improvement consequently promotes holes transport from Sb2S3 to Au through V2O5 layer and suppresses electron-hole recombination.52 Thus, higher JSC and PCE are obtained. However, as the temperature increasing further, the value of RCT increases again, indicating that this annealing temperature would destroy the interfacial contact between Sb2S3 and V2O5 and thus impede hole transporting. The results of EIS measurement are in agreement with PCE evolution from J-V curves. We finally examine the device stability under extremely high humidity of 95% at 20 °C. The devices are in absence of any encapsulation. The normalized VOC, JSC, FF, and PCE are shown in Figure 4. The VOC decreases slightly in both devices (Figure 4a), suggesting that this damp examination did not cause severe damage on the interface between absorber layer and electrodes (HTL or ETL).54 For the device with Spiro-OMeTAD, the serious reduction in JSC and FF (Figure 4b and c), this change may be due to the deliquescent behavior of the doped SpiroOMeTAD in the humidity environment,55 which would increase the series resistance. The overall PCE of the device with Spiro-OMeTAD as HTL decreased to 28% of its initial efficiency after 100 hours in this humidity. Whereas all inorganic device shows excellent stability and retain 71% of the initial efficiency. It should be noted that the stability examination is conducted in very


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high relative humidity (95%), it is rational that device would show excellent stability in moderate humid environment since Sb2S3 is insensitive to moisture and air (Figure S6). In all, the substitution of Spiro-OMeTAD for V2O5 layers leads to a substantially improved device lifetime. 3. CONCLUSIONS In summary, we have demonstrated that a facile solution method derived V2O5 layers can be inorganic hole extraction layer for Sb2S3 based solar cells. The energy levels of the assynthesized V2O5 films were examined, revealing that the work function considerably increased as the annealing temperature increases to 180 °C, resulting in a potential barrier forms between the Sb2S3 and V2O5 interface. On the other hand, low temperature annealing (100 oC) is unable to dissociate the organic molecules which increase the charge transport resistance. The interfacial contact is thus optimized at annealing temperature of 150 oC, the device achieves a power conversion efficiency of 4.8%. With this approach, this research achieves the highest efficiency in all-inorganic Sb2S3-based planar solar cells. According to the stability test at extremely high humidity (95%), the device based on V2O5 displays essentially enhanced performance when compared with that of Spiro-OMeTAD based device. This study provides a novel hole transporting material system for high performance Sb2S3 devices, it also enlightens the development of all inorganic solar cells based on Sb2(S,Se)3 and Sb2Se3. 4. EXPERIMENT Materials. Antimony(III) oxide (Sb2O3, 4N) and ethanol (CH3CH2OH, AR) were purchased from Sinopharm. Ultra dry carbon disulfide (CS2, 99%), ultra dry dimethyl sulfoxide (DMSO, 99%), n-butylamine (CH3(CH2)3NH2, 99.5%), lithium bis(trifluoromethylsulfonyl) imide (LiTFSI, 98%), 4-tert-butylpyridine (tBP, 96%), isopropanol (IPA, 99%)

and chlorobenzene

(99.8%) were purchased from J&K. FTO-coated glasses substrate with a sheet resistance of


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13±1.5 Ω sq-1 and 2, 2′, 7, 7′-tetrakis (N, N –dipmethoxyphenylamine)-9, 9-spirobifluorene (Spiro-OMeTAD) were obtained from Yingkou Youxuan Trade Co., Ltd. Titanium (IV) isopropoxide (C12H28O4Ti, 97%) was purchased from Aldrich. Vanadium(V) triisopropoxide oxide (VTIPO, 96%) was purchased from Alfa Aesar. All chemicals were used as received without further purification. Photovoltaic cell fabrication. The structure of solar cells in this study is denoted as FTO/cTiO2/Sb2S3/HTL/Au. The c-TiO2 layer was spin-coated onto FTO with a mixture solution of 140 μL of titanium (IV) isopropoxide, 2 mL of ethanol, and 26 μL of 2 M HCl (diluted in ethanol) at 2000 rpm, 40 s, followed by annealing at 550 °C for 50 min in the muffle furnace. The following fabrications were all in N2-purged glove box unless otherwise stated. The Sb2S3 layer was deposited according our previous work with minor changes.4 In brief, 0.145 g Sb2O3 was dissolved in the mixture solution containing 375 L ethanol, 625 L DMSO, 750 L CS2 and 800 L n-butylamine. This solution was then spin-coated onto the TiO2 compact layer at a speed of 8000 rpm for 30 s, followed by annealing on a hot plate at 200 °C for 1 min and 300 °C for 2 min. And then, two types of hole transporting layers, V2O5 and Spiro-OMeTAD, were deposited on the Sb2S3/c-TiO2/FTO, respectively. For V2O5 HTL which was prepared according to previous lecture,28 0.25 mL VTIPO was dissolved in 3.8 mL isopropanol and 0.2 mL deionized water mixture solution and stirred for 30 min. It was then spin-coated on the Sb2S3 layer at 3000 rpm for 30 s. After that, the Sb2S3/V2O5 thin film was aged in the glove-box for 10 min and then transferred to the hot plate for thermal annealing at different temperature for 10 min. Three annealing temperatures were chosen for the preparation: 100 °C (denoted as device 1), 150 °C (denoted as device 2) and 180 °C (denoted as device 3), respectively. For Spiro-OMeTAD HTL, the spiro-OMeTAD solution was spin-coated on FTO/c-TiO2/Sb2S3 substrate at a speed of 3000


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rpm for 30 s and heated on a hot plate at 100 °C for 10 min in air. The spiro-OMeTAD solution was prepared by mixing 36.6 mg of spiro-OMeTAD, 14.5 μL of 4-tert-butylpyridine (tBP), and 9.5 μL of a 520 mg mL-1 lithium bis(trifluoromethanesulfonyl) imide (Li-TFSI) together in acetonitrile in 1 mL of chlorobenzene. Finally, the devices were put into thermal evaporator to deposit 130 nm thick Au layer under a pressure less than 5*10-4 Pa. The active area of the device is 0.12 cm2. Characterization. Thermogravimetric analysis (TGA) was recorded on a TGA Q5000 instrument under nitrogen atmosphere at a heating rate of 10 °C/min. The X-ray diffraction (XRD) patterns were recorded on a Rigaku SmartLab X-ray Diffractometer at a scan rate of 2° per min with Cu-Kα radiation (0.154 nm). The X-ray photoelectron spectroscopy (XPS) spectra were carried out on thermo ESCALAB 250 spectrometer. The surface and cross-section morphologies of the thin films were characterized by field emission SEM (FE-SEM siron 200). The elemental analysis was characterized by Elemental Analyzer (CHN Mode, German Elementar, Vario EL cube). The optical absorption spectrum was recorded on a UV-vis-365-type spectrophotometer in range of 300-900 nm. UPS experiments were performed at the Catalysis and Surface Science End station at the BL11U beam line in the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. The valance band spectra were measured with photon energy of 40 eV. A sample bias of -5 V was applied to observe the secondary electron cutoff (SEC). The work function (ϕ) can be determined by the difference between the photon energy and the binding energy of the secondary cutoff edge. The current density-voltage (J-V) characterizations were conducted using a Keithley 2400 source measurement unit under simulated AM 1.5 irradiation (100 mW·cm-2) with a standard xenon-lamp-based solar simulator (Oriel Sol 3A, USA), and the solar simulator illumination


15


Page 16 of 36

intensity was calibrated by a monocrystalline silicon reference cell (Oriel P/N 91150 V, with KG-5 visible color filter) calibrated by the National Renewable Energy Laboratory (NREL). The incident photon-to-electron conversion efficiency (IPCE) measurements were carried out on an ORIEL Intelligent Quantum Efficiency (IQE) 200TM Measurement system established with a tunable light source. Electrochemical impedance spectroscopic measurement (EIS) was performed using an electrochemical workstation (Autolab 320, Metrohm, Switzerland) with the frequency range from 100 mHz to 100 KHz under 500 mV in the dark. AC 10 mV perturbation was applied with a frequency from 500 KHz to 1 Hz. The obtained impedance spectra were fitted with Z-View software (v2.8b, Scribner Associates, USA).

ASSOCIATE CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Additional surface characterization of Sb2S3 film and photovoltaic performance of the device based on Spiro-OMeTAD as hole transporting material. AUTHOR INFORMATION Corresponding Author *(T. C.) E-mail: [email protected] *(C. F. Z.) E-mail: [email protected] *(H. X. J.) E-mail: [email protected] Notes


16

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The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Fundamental Research Funds for the Central Universities under

no.

WK2060140023,

WK2060140022,

CX3430000001,

WK2060140024,

the

Major/Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology (2016FXZY003), National Natural Science Foundation of China (21503203) and the Recruitment Program of Global Experts. REFERENCES (1) Sung, S.-J.; Gil, E. K.; Lee, S.-J.; Choi, Y. C.; Yang, K.-J.; Kang, J.-K.; Cho, K. Y.; Kim, D.-H., Systematic Control of Nanostructured Interfaces of Planar Sb2S3 Solar Cells by Simple Spin-Coating Process and Its Effect on Photovoltaic Properties. J. Ind. Eng. Chem. 2017, 56, 196-202. (2) Lei, H.; Yang, G.; Guo, Y.; Xiong, L.; Qin, P.; Dai, X.; Zheng, X.; Ke, W.; Tao, H.; Chen, Z.; Li, B.; Fang, G., Efficient Planar Sb2S3 Solar Cells Using a Low-Temperature SolutionProcessed Tin Oxide Rlectron Conductor. Phys. Chem. Chem. Phys. 2016, 18, 16436-16443. (3) Chung, C.-C.; Tsai, T.-W.; Wu, H.-P.; Diau, E. W.-G., Phosphonic Acid and Lithium Salt as Effective p-Dopants to Oxidize Spiro-OMeTAD for Mesoscopic Sb2S3 Solar Cells. J. Phys. Chem. C 2017, 121, 18472-18479. (4) Wang, X.; Li, J.; Liu, W.; Yang, S.; Zhu, C.; Chen, T., A Fast Chemical Approach Towards Sb2S3 Film with a Large Grain Size for High-Performance Planar Heterojunction Solar Cells. Nanoscale 2017, 9, 3386-3390.


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(31) Peng, H.; Sun, W.; Li, Y.; Ye, S.; Rao, H.; Yan, W.; Zhou, H.; Bian, Z.; Huang, C., Solution Processed Inorganic V2Ox as Interfacial Function Materials for Inverted PlanarHeterojunction Perovskite Solar Cells with Enhanced Efficiency. Nano Res. 2016, 9, 2960-2971. (32) Özer, N., Electrochemical Properties of Sol-Gel Deposited Vanadium Pentoxide Films. Thin Solid Films 1997, 305, 80-87. (33) Jiang, Y.; Xiao, S.; Xu, B.; Zhan, C.; Mai, L.; Lu, X.; You, W., Enhancement of Photovoltaic Performance by Utilizing Readily Accessible Hole Transporting Layer of Vanadium(V) Oxide Hydrate in a Polymer-Fullerene Blend Solar Cell. ACS Appl Mater Interfaces 2016, 8, 11658-11666. (34) Livage, J., Interface Properties of Vanadium Pentoxide Gels. Mater. Res. Bull. 1991, 26, 1173-1180. (35) Livage, J., Vanadium Pentoxide Gels. Chem. Mater. 1991, 3, 578-593. (36) Hancox, I.; Rochford, L. A.; Clare, D.; Walker, M.; Mudd, J. J.; Sullivan, P.; Schumann, S.; McConville, C. F.; Jones, T. S., Optimization of a High Work Function Solution Processed Vanadium Oxide Hole-Extracting Layer for Small Molecule and Polymer Organic Photovoltaic Cells. J. Phys. Chem. C 2013, 117, 49-57. (37) Singh, T.; Öz, S.; Sasinska, A.; Frohnhoven, R.; Mathur, S.; Miyasaka, T., SulfateAssisted Interfacial Engineering for High Yield and Efficiency of Triple Cation Perovskite Solar Cells with Alkali-Doped TiO2 Electron-Transporting Layers. Adv. Funct. Mater. 2018, 28, 1706287.


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(53) Lei, Y.; Gu, L.; Zheng, L.; Yang, X.; He, W.; Gao, Y.; Zheng, Z., Wavelength-Dependent Charge Carrier Dynamics: the Case of Ag2S/Organic Thin Films Heterojunction Solar Cells. Electrochim. Acta 2017, 252, 33-40. (54) Lima, F. A. S.; Beliatis, M. J.; Roth, B.; Andersen, T. R.; Bortoti, A.; Reyna, Y.; Castro, E.; Vasconcelos, I. F.; Gevorgyan, S. A.; Krebs, F. C.; Lira-Cantu, M., Flexible ITO-Free Organic Solar Cells Applying Aqueous Solution-Processed V2O5 Hole Transport Layer: An Outdoor Stability Study. APL Mater. 2016, 4, 026104. (55) Liu, J.; Wu, Y.; Qin, C.; Yang, X.; Yasuda, T.; Islam, A.; Zhang, K.; Peng, W.; Chen, W.; Han, L., A Dopant-Free Hole-Transporting Material for Efficient and Stable Perovskite Solar Cells. Energy Environ. Sci. 2014, 7, 2963-2967.


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Table 1. Photovoltaic parameters of the devices with V2O5 hole transporting layer treated at 100 o

C, 150 oC and 180 oC and device with Spiro-OMeTAD as hole transporting layer.

Device

Annealing temperature (°C)a

1

100

2

150

3

180

SpiroOMeTAD

-

Best

VOC

JSC

FF

(V)

(mA/cm2) (%)

PCE RSH (%)

RS

(Ωcm2) (Ωcm2)

0.56 14.49

43.05 3.49 191.1

17.9

Average 0.53 12.07

40.22 2.62 150.4

20.9

Best

0.59 15.29

52.92 4.80 315.0

11.6

Average 0.58 14.60

50.67 4.30 305.5

12.5

Best

0.55 14.13

51.21 3.96 250.4

11.2

Average 0.55 13.45

45.80 3.40 208.5

14.9

Best

0.61 15.10

54.57 5.00 364.7

10.1

Average 0.58 14.57

51.42 4.37 316.8

11.9

a. The annealing temperature for the preparation of V2O5 films.


26

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Figure 1. (a) TGA plot of triisopropoxide oxide (VTIPO) solution. (b) XRD patterns of V2O5 treated at 150 oC, 300 oC and 400 oC for 60 min, respectively. (c) XPS spectra of V2O5 thin films deposited on Sb2S3 surface with annealing temperature of 100 oC, 150 oC and 180 oC for 10 min, respectively. (d) UV–visible absorption spectra of FTO/TiO2/Sb2S3 and FTO/TiO2/Sb2S3/V2O5 annealed at 100 oC, 150 oC and 180 oC, respectively. The insert in (d) is the Tauc plots of V2O5 thin film annealed at 100 oC, 150 oC and 180 oC deduced from the light absorption spectra.


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Figure 2. (a) SEM image of V2O5 film deposited on Sb2S3 and (b) cross-sectional SEM image of a complete device, (c) A schematic illustration of the device with V2O5 as hole transporting material. (d) Current density–voltage curves, (e) incident photon-to-current conversion efficiency of the devices with V2O5 HTL treated at 100 oC, 150 oC and 180 oC. Statistical distribution of photovoltaic parameters, (f) VOC, (g) JSC, (h) FF, and (i) PCE extracted from current-voltage measurements from 20 devices for each annealing temperature under simulated AM 1.5 (100 mW cm-2) illumination.


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Figure 3. UPS spectra at (a) cut off region, (b) valence band region and (c) corresponding energy band diagram of (I) Sb2S3, (II) V2O5 treated at 100 °C, (III) V2O5 treated at 150 °C, (IV) V2O5 treated at 180 °C. (d) Nyquist plots of Sb2S3-based devices with V2O5 treated at different temperatures as hole transporting layers measured in the dark where the solid lines are corresponding to the fitting spectra. Inset in (d) showing the equivalent circuit model employed for the fitting of impedance spectra.


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Figure 4. Normalized (a) open-circuit voltage (b) short-circuit current density (c) fill factor (d) and power conversion efficiency of Sb2S3 solar cells based on V2O5 and Spiro-OMeTAD hole transporting materials as a function of storage time in air, where the relatively humidity is 95%.


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TOC


31


Figure 1. (a) TGA plot of triisopropoxide oxide (VTIPO) solution. (b) XRD patterns of V2O5 treated at 150 oC, 300 oC and 400 oC for 60 min, respectively. (c) XPS spectra of V2O5 thin films deposited on Sb2S3 surface with annealing temperature of 100 oC, 150 oC and 180 oC for 10 min, respectively. (d) UV–visible absorption spectra of FTO/TiO2/Sb2S3 and FTO/TiO2/Sb2S3/V2O5 annealed at 100 oC, 150 oC and 180 oC, respectively. The insert in (d) is the Tauc plots of V2O5 thin film annealed at 100 oC, 150 oC and 180 oC deduced from the light absorption spectra. 109x80mm (600 x 600 DPI)


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Figure 2. (a) SEM image of V2O5 film deposited on Sb2S3 and (b) cross-sectional SEM image of a complete device, (c) A schematic illustration of the device with V2O5 as hole transporting material. (d) Current density–voltage curves, (e) incident photon-to-current conversion efficiency of the devices with V2O5 HTL treated at 100 oC, 150 oC and 180 oC. Statistical distribution of photovoltaic parameters, (f) VOC, (g) JSC, (h) FF, and (i) PCE extracted from current-voltage measurements from 20 devices for each annealing temperature under simulated AM 1.5 (100 mW cm-2) illumination. 239x381mm (600 x 600 DPI)



Figure 3. UPS spectra at (a) cut off region, (b) valence band region and (c) corresponding energy band diagram of (I) Sb2S3, (II) V2O5 treated at 100 oC, (III) V2O5 treated at 150 oC, (IV) V2O5 treated at 180 oC. (d) Nyquist plots of Sb2S3-based devices with V2O5 treated at different temperatures as hole transporting layers measured in the dark where the solid lines are corresponding to the fitting spectra. Inset in (d) showing the equivalent circuit model employed for the fitting of impedance spectra. 41x11mm (600 x 600 DPI)


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Figure 4. Normalized (a) open-circuit voltage (b) short-circuit current density (c) fill factor (d) and power conversion efficiency of Sb2S3 solar cells based on V2O5 and Spiro-OMeTAD hole transporting materials as a function of storage time in air, where the relatively humidity is 95%. 108x77mm (600 x 600 DPI)



TOC 35x29mm (600 x 600 DPI)


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V2O5 as Hole Transporting Material for Efficient All Inorganic Sb2S3

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