n-Type Doping of Sb2S3 Light-Harvesting Films Enabling High

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Energy, Environmental, and Catalysis Applications

N-type Doping of Sb2S3 Light Harvesting Film Enabling High Efficiency Planar Heterojunction Solar Cells Rongfeng Tang, Xiaomin Wang, Chenhui Jiang, Shiang Li, Weifeng Liu, Huanxin Ju, Shangfeng Yang, Chang-Fei Zhu, and Tao Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08965 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 23, 2018

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ACS Applied Materials & Interfaces

N-type Doping of Sb2S3 Light Harvesting Film Enabling High Efficiency Planar Heterojunction Solar Cells Rongfeng Tang, † Xiaomin Wang, † Chenhui Jiang, † Shiang Li, † Weifeng Liu, † Huanxin Ju, ‡ Shangfeng Yang, † Changfei Zhu,*,† and Tao Chen*,† †

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 ‡

National Synchrotron Radiation Laboratory, University of Science and Technology of China,

Hefei, Anhui 230029, P. R. China

ACS Paragon Plus Environment

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ABSTRACT Sb2S3 is a kind of new light absorption materials possessing high stability in ambient environment, high absorption coefficient in visible range and abundant elemental storage. To improve the power conversion efficiency of Sb2S3 based solar cells, here we control the defect in Sb2S3 absorber films. It is found that the increase of sulfur vacancy is able to upgrade photovoltaic properties. With the increase of sulfur vacancy, the carrier concentrations are increased. This n-type doping gives rise to upshift of Fermi level of Sb2S3 so that the charge transport from Sb2S3 to electron selection material becomes dynamically favorable. The introdution of ZnCl2 in film fabrication is also found to regulate the film growth for enhanced crystallinity. Finally, the photovoltaic parameters, short-circuit current density, open-circuit voltage as well as the fill factor of the device based on Sb2S3 film are all considerably enhanced, boosting the final power conversion efficiency from 5.15% to 6.35%. This efficiency is the highest value in planar heterojunction Sb2S3 solar cells and among the top values in all kinds of Sb2S3 solar cells. This research provides fundamental understanding regarding the properties of Sb2S3 and a convenient approach for enhancing the performance of Sb2S3 solar cells.

KEYWORDS: solar cell, photovoltaics, Sb2S3, doping, planar heterojunction


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INTRODUCTION Metal chalcogenides, such as Cu(In,Ga)Se2, Cu2ZnSn(S,Se)4 and CdTe, have been manifested as efficient light absorption materials for solar cell applications.1,2 As a newcomer to the metal chalcogenides based solar cells, Sb2S3 has attracted more and more attentions for absorber material in solar cells.3-7 Sb2S3 exhibits band gap of 1.7-1.9 eV, enabling applications as either light harvesting material in a single junction solar cells or wide band gap subcell coupling with low band gap silicon or Cu(In,Ga)Se2 for high performance tandem solar cells.8 It also has high absorption coefficient (α >104 cm-1) in visible range, high stability in ambient environment, environmental benign compositional elements.8-10 Compared with other photovoltaic metal chalcogenides, the melting temperature of Sb2S3 is as low as 550 oC, indicating that the formation of high crystallinity film can be achieved at low temperature (around 300 oC) which in turn reduces energy input in device fabrication. In addition, the steady-state photocurrent grating measurement shows that the carrier diffusion length is in hundreds nanometers scale,11,12 offering flexibility in device construction in terms of sensitized mesoporous structure or planar heterojunction device configuration. These superior properties afford great promise for practical solar cells. In device application, Sb2S3 was initially applied as sensitizer in mesoscopic solar cells. Through a chemical bath deposition (CBD) method, an ultrathin Sb2S3 was deposited onto TiO2 nanoparticle network, forming extremely thin absorber (ETA) solar cells.13 A power conversion efficiency (PCE) of 3.37% was obtained. In this research, the excellent stability with regard to light soaking under load was also demonstrated. By engineering the surface defect of CBD derived Sb2S3 absorber layer, Seok et al. obtained 7.5% PCE in mesoporous solar cell.14 Afterwards, many efforts have been put into the exploration of synthetic methods of Sb2S3


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absorber material.15-17 For instance, thermal decomposition of organometallics,11,12 rapid thermal deposition and atomic layer deposition have been developed for the fabrication of Sb2S3 films.16,18,19 Inspired by the investigations on solid-state dye-sensitized solar cells, a series of hole transporting materials such as P3HT (poly(3-hexylthiophene),7,21 PCPDTBT (poly(2,6-(4,4bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b´]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)),22,23 Spiro-OMeTAD (2,2´,7,7´-tetrakis(N,N-di-p-methoxyphenylamine)-9,9´-spirobifluorene)24,25 and CuSCN13,26,27 have also been examined for device performance optimization. Although Sb2S3-sensitized device structure has gained great progress in energy conversion efficiency, planar heterojunction solar cell is advantageous in terms of practical applications. This device structure allows convenient consecutive film deposition by either solution processing or vapor deposition. To date, the highest PCE of 5.77% was obtained in planar heterojunction Sb2S3 solar cell employing atomic layer deposition derived Sb2S3 film.8,18 In planar heterojunction solar cells, both the surface defect and bulk defect play critical roles in dictating the device performance. Previous studies have pointed out that the high resistivity of Sb2S3 is one of the major reasons for low short-circuit current density (JSC) and fill factor (FF).28 To improve the charge transport property in Sb2S3 films, one can either manipulate intrinsic defect by tuning the Sb/S atomic ratio or introduce extrinsic defect. In this research, we present the defect engineering by manipulating Sb/S atomic ratio in the Sb2S3 film. Our results show that small amount of ZnCl2 in the precursor materials could induce the generation of sulfur vacancies and thus increase the electron concentration. Consequently, the series resistance of device is considerably reduced while the recombination resistance is greatly increased. These changes lead to increments on the open-circuit voltage (VOC), JSC, and FF of the devices. The final PCE is thus increased from 5.15% to 6.05% on average, among which the best device delivers a PCE of


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6.35%. RESULTS AND DISCUSSION For the synthesis of Sb2S3 film, SbCl3 and thiourea (TU) with different molar ratio are dissolved in 2-methoxyethanol to form a transparent solution (Figure 1a, inset). The mixture solution is stable for several weeks. ZnCl2 is also able to be dissolved in the solution, which allows the incorporation of ZnCl2 into the precursor solution with tunable ZnCl2/SbCl3 molar ratios. The formation of Sb2S3 film is conducted by spin coating of the solution onto a FTO/TiO2 substrate, followed by thermal decomposition in high vacuum condition. The optimization on the ratios of SbCl3/TU is provided in Supporting Information. As a consequence, SbCl3/TU molar ratio of 1:3 is able to generate surface morphology with high uniformity (Figure S1), this ratio is thus chosen for the following investigations. In order to find suitable annealing condition for the synthesis of high quality Sb2S3 films, we conduct thermogravimetric analysis (TGA) of the precursor solution (Figure 1a). The first significant weight loss of molecular precursor occurs at ~84

o

C, this process involves

evaporation of 2-chloroethylmethylether (b.p. 89-90 oC) generated by the reaction between Cland 2-methoxyethanol. The second weight loss is found at 200 oC, this process is associated with the decomposition of molecular precursor, accompanied by generation of hydrogen sulfide from TU decomposition. However, no crystalline peaks of Sb2S3 appeared in the X-ray diffraction (XRD) pattern (Figure S2a). When the temperature reaches to 290 ºC, negligible weight loss is observed, indicating the crystallization of Sb2S3. To examine the detailed crystallization process and morphology change, we conduct X-ray diffraction (XRD) and SEM characterization. It can be seen that, the diffraction peaks are weak


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when the annealing temperature (AT) is 240 and 255 °C (Figure S3). Once the AT reaches to 270 °C, diffraction peaks at 15.64°, 17.52°, 24.88°, 29.24° and 32.35° can be clearly identified (Figure 1b), which are indexed as (020), (120), (130), (211) and (221) crystal planes of stibnite Sb2S3.29 Meanwhile, no significant change is observed on the surface morphologies of Sb2S3 films with AT at 240, 255 and 270 oC (Figure S4). Further increasing the temperature to 285 °C and 300 °C, the XRD patterns remain nearly unchanged (Figure S3). However, the morphologies of the thin films are deteriorated with much more pin holes appeared (Figure S4). Based on these results, a suitable annealing at 270 oC is identified for the generation of both uniform surface morphology and good crystallinity. Furthermore, crystal structures of the films fabricated with ZnCl2/SbCl3 molar ratios of 1%, 3%, 5%, and 7% are characterized to examine whether the addition of ZnCl2 influences the crystallization of Sb2S3 film. For all of the concentration ratios, the diffraction patterns do not show noticeable shift as compared with the pristine Sb2S3 film (Figure 1b). The close ionic radius of Zn2+ (74 pm) and Sb3+ (76 pm) results in the resemble diffraction patterns. A closer inspection shows that the intensity of the diffraction peaks displays slightly enhancement, indicating that the crystallinity is enhanced upon the ZnCl2 addition. In general, the enhanced crystallinity would facilitate charge transport in the devices. In order to probe the chemical state of the doped Sb2S3 films, we conduct X-ray photoelectron spectroscopy (XPS) analysis (Figure 2a-c). The binding energy at 538.8 and 529.5 eV are ascribed to the Sb 3d3/2 and Sb 3d5/2 of Sb2S3. The binding energy at 540.0 eV and 530.7 eV are assigned to Sb 3d3/2 and Sb 3d5/2 of Sb2O3, which are impurities possibly stemming from the synthesis or post oxidation in air.14,30-32 The binding energy at 1021.8 eV is ascribed to the Zn 3p 3/2 (Figure 2b).33 Furthermore, the Zn-S bond is detected at 161.9 eV (Figure 2c). Due to the


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fact that there is no crystal plane of ZnS observed from the XRD characterization, even at elevated doping concentrations of 7%, the formation of either amorphous ZnS or Zn-S bond at the grain boundaries is proposed. A detailed calculation shows that the atomic ratio of S/Sb is 1.345 in pristine Sb2S3. In addition, although the atomic ratios of Zn/(Zn+Sb) in the precursor materials are determined, the final product may not necessarily maintain their initial ratios. The exact concentration of Zn/(Zn+Sb) in the final products are estimated to be 1.5%, 3.7%, 4.8% and 8.6% and corresponding S/(Sb+Zn) atomic ratios are 1:1.340, 1:1.290, 1:1.290 and 1:1.250 for doping concentration of 1%, 3%, 5% and 7%, respectively. Suppose that all Zn ions in the film forms Zn-S bond, with the increase of Zn in the Sb2S3 film, the S/Sb atomic ratio reduced to 1:1.300, 1:1.301, 1:1.304 and 1:1.274 for doping concentrations of 1.5%, 3.7%, 4.8% and 8.6%, respectively. It can be seen that in this synthesis, the final product is always sulfur deficient. The decreased S/Sb atomic ratio in the film indicate more sulfur vacancies are generated. It is worth noting that even if we increase the thiourea in the precursor solutions, the final products are still sulfur deficient. Furthermore, the coordination between zinc and sulfur generate more sulfur vacancies in the Sb2S3 films. Furthermore, we conduct UV-Vis light absorption characterizations of the as-synthesized films (Figure 2d). The pristine Sb2S3 exhibits strong absorption from UV to visible region and the onset absorption is located at 720 nm. The onset absorption slightly shifts to longer wavelength ending at 724, 727, 732, and 741 nm with S/Sb ratios of 1:1.300, 1:1.301, 1:1.304 and 1:1.274, respectively. Tauc plots of the films are shown in the inset in Figure 2d. As a result, the band gap of the films are determined as 1.72, 1.71, 1.70, 1.69 and 1.67 eV for pristine Sb2S3 and Sb2S3 with doping concentrations of 1.5%, 3.7%, 4.8% and 8.6%, respectively. Since the


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band gap of ZnS is about 3.7 eV, the reduced band gap of Sb2S3 as a result of Zn-ion doping indicates that the formation of ZnS phase is less possible. Instead, zinc is most likely coordinated with sulfur at the grain boundaries to passivate the defect. For the device fabrication, a compact layer of TiO2 (c-TiO2) is utilized as electron extraction material and Spiro-OMeTAD is applied as hole transporting material (HTM). The device structure is described as FTO/c-TiO2/Sb2S3/Spiro-OMeTAD/Au. The cross-sectional SEM images of a typical device are shown in Figure 3a. The thicknesses of c-TiO2, absorption layer (Sb2S3 or doped Sb2S3), HTM and Au electrode are measured to be 45, 130, 60, and 40 nm, respectively. Figure 3b shows the current density-voltage (J-V) characteristics measured under 1 Sun (AM 1.5G) solar simulator irradiation. The detailed photovoltaic parameters are tabulated in Table 1. Pristine Sb2S3 based solar cell delivers an average PCE of 5.15%, with JSC of 14.14 mA cm-2, VOC of 0.653 V and FF of 55.7%. This efficiency is among the top values in planar heterojunction Sb2S3 solar cells.24,31 The doping of 1.5% Zn in the Sb2S3 does not lead to significant performance change, showing an average PCE of 5.13%. The doping concentration of 3.7% to 4.8% generates comparable device efficiency of 5.85% on average, with VOC, JSC and FF of 0.627 V, 16.4 mA cm-2, and 57.1%, respectively. In this case, the JSC and FF exhibit essential improvement, while the VOC have a slight decrease. Since the doping concentration of 3.7% and 4.8% exhibit similar device performance, we will focus on one doping concentration of 4.8% in following discussions. When the doping concentration is raised to 8.6%, the PCE decreases to 5.43%, while it is still higher than that in absence of doping. From Table 1, it is observed that the JSC is increased from 14.14 mA cm-2 for pristine Sb2S3 to 14.4, 16.4 and 16.8 mA cm-2 for doping concentration of 1.5%, 4.8%, and 8.6%, respectively. One of the reasons for this enhancement can be ascribed to the slightly reduced band gap as


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depicted in Figure 2d, which causes the improvement in light harvesting efficiency in longer wavelength. Remarkably, the external quantum efficiency (EQE) characterizations show that with the doping of Zn in the Sb2S3 film, the photocurrent generation in the wavelength (350-720 nm) is considerably enhanced (Figure 3c). Furthermore, it is noticed that the FF shows slight improvement with low doping concentrations, which are 55.7%, 55.6% and 57.1% for the devices with pristine Sb2S3 and those doped with 1.5% and 4.8% Zn ions. Abruptly, a severe decrease to 52.6% is observed when the doping concentration reaches 8.6%. To examine why the heaving doping leads to serious FF reduction, we conducted morphological analysis of the as-prepared films (Figure 4). With regard to the films fabricated in absence of Zn ion doping, 1.5% and 4.8% doping concentrations (Figure 4a, b, and c), the films display similar surface morphologies. However, when the doping concentration reaches 8.6%, there are much more pin holes generated (Figure 4d), which would cause the direct contact between HTM and TiO2 compact layer and in turn increase the recombination probability that ultimately reduces the FF. Capacitance-voltage (C-V) profiling is applied to estimate the carrier concentrations of the devices with regard to different doping concentrations. For the measurements, the devices were tested at room temperature in the dark at a frequency of 10 kHz. DC bias from 0.2 V to 1.2 V was employed due to the theoretical limit of measured built-in potential (Figure 5a).34 Thus, a parallel-plate capacitor model is applied to analyze the heterojunction capacitance. The MottSchottky

curves

are

fitted

by

the

following

equation:

݇ܶ 2(‫ ܧ‬− ‫ܧ‬௙ − ݁ ) 1 = ‫ܥ‬ଶ ‫ܣݍ‬ଶ ߝ଴ ߝ௥,௣ ܰ஺

where E is applied potential, Ef is the Fermi level potential, NA, q, ε0, εr and A represented carrier


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concentration, elementary charge, vacuum permittivity, relative permittivity, and device area, respectively. Based on above equation, the carrier concentration of pristine Sb2S3 film (S/Sb=1:1.345) based device is calculated to be 2.95×1017 cm-3. The carrier concentration is increased to 3.08×1017 cm-3, 3.71×1017 cm-3, and 4.70×1017 cm-3 for S/Sb molar ratios of 1:1.3, 1:1.301, 1:1.304 and 1:1.274, respectively. In addition, according to the calculations,35 chlorine from both ZnCl2 and SbCl3 could substitute sulphur vacancies that increase n-type doping. It is also a shallow defect that reduces deep VS defect concentration and consequently increases device performance. Since chlorine element was not detected in the film by XRF and XPS measurement, the replacement of sulfur vacancies by chlorine should be at very low level, if any. In any case, the increased carrier concentrations would elevate the Fermi energy level, which thus facilitate the charge transport from Sb2S3 to TiO2. To illustrate this point, ultraviolet photoelectron spectroscopy (UPS) characterization was conducted (Figure 5c and 5d). The higher binding energy cutoff and the lower binding energy onset (inset in Figure 5c) are found to be 25.81 (corrected for -5 V) and 1.27 eV for pristine Sb2S3 (Figure 5c) and 25.96 (corrected for -5 V) and 1.20 eV for 4.8% Zn-doped of Sb2S3. As a result, the work function which is equivalent to the possible Fermi level position with respect to the vacuum level is calculated to be 4.04 eV for 4.8% Zn-doped of Sb2S3, which is higher than that of pristine Sb2S3 (4.19 eV). The energy alignment is depicted in Figure 3d. This result suggests that charge transport from Sb2S3 to TiO2 becomes dynamically favorable. This is also the reason why the photocurrent generation (Figure 3c) in Zn-doped Sb2S3 film is essentially boosted. To further understand the effect of Zn ion doping on the PCE, charge transfer kinetics is studied by electrochemical impedance spectroscopy (EIS) characterization. Figure 5b compares the Nyquist plots of the devices. The inset of Figure 5b illustrates corresponding fitted curves


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using an equivalent circuit based on a transmission line model.36 The recombination resistance (RREC) extracted from the arcs are 20, 21, 76, and 31 kΩ for the devices with pristine Sb2S3 and those doped with 1.5%, 4.8% and 8.6% Zn ion concentrations. Obviously, the device with 4.8% doped concentration exhibits the highest RREC, which further explain the highest FF in the devices. The reproducibility of device fabrication was examined by separate preparation of 30 devices with Zn-doped Sb2S3 (4.8% doping concentration) as active layer, the statistical result is shown in the inset of Figure 6a, illustrating that the PCE is in the range from 5.6% to 6.35%. The EQE response of the champion device is depicted in Figure 6b, exhibiting >80% photocurrent generation efficiency in 350-670 nm. The integrated JSC from EQE is 16.6 mA cm-2, which is quite close to the J-V measurement (17.19 mA cm-2), indicating the reliability of device measurement. CONCLUSION In summary, we have demonstrated that the doping of Zn ion into Sb2S3 film is able to significantly enhance the power conversion efficiency in planar heterojunction solar cells. The introduction of ZnCl2 in the reaction precursor considerably reduce the S/Sb ratio in the final Sb2S3 films, which in turn increase the electron concentration in the final film. Consequently, the upshift of Fermi level is obtained that facilitates charge transport from Sb2S3 to TiO2. In addition, the doping of Zn ions slightly reduces the band gap and enhances the crystallinity. However, much high doping concentration would cause increasingly generated pin holes in the film, which induces serious recombination in the device. Extensive optimizations enable the device achieving a champion power conversion efficiency of 6.35%. Our results provide an effective method for


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improve the energy conversion efficiency of Sb2S3 solar cells. This investigation would also enlighten the efficiency improvement of Sb2S3, Sb2(S,Se)3 and Sb2Se3 based solar cells. EXPERIMENTAL SECTION Materials. Antimony(III) chloride (SbCl3, AR) was purchased from aladdin, zinc chloride (ZnCl2, 99.9%) was purchased from Acros. Thiourea (TU, AR), titanium isopropoxide (TIPT), lithium

bis(trifluoromethylsulfonyl)

chlorobenzene

were

purchased

imide from

(Li-TFSI), J&K.,

4-tert-butylpyridine

ethylene

glycol

(tBP)

monomethyl

and ether

(HOCH2CH2OCH3, AR) were purchased from Sinopharm, Spiro-OMeTAD (99.8%) was purchased from YOUXUAN TECH., Ltd. All chemicals were used as received without further purification. Synthesis of Sb-TU complex precursor solution. The preparation of Sb-TU complex precursor solution was accomplished by mixing 4.0 mmol of SbCl3 and 12 mmol of thiourea (TU) in a 10 mL vial, and 5 mL of ethylene glycol monomethyl ether (2-ME) was used as the solvent. Then, the mixed solution was stirred at room temperature for one day before use. Similar to above synthetic process, Zn2+ doping of Sb2S3 layer was prepared using ZnCl2 as the source of zinc. The doping amount (1, 3, 5, 7 at%) of Zn2+ was controlled according to the molar concentration of ZnCl2 and SbCl3. Firstly, 3.96, 3.88, 3.8 and 3.72 mmol of SbCl3 and 12 mmol of thiourea (TU) were dissolved into 5 mL of 2-ME solvent with a constant stirring of 30 min. Afterward, 0.04, 0.12, 0.2 and 0.28 mmol of ZnCl2 was added into above mixed solution. After stirring for one day, the Zn2+ doping of Sb-TU complex precursor solution was obtained for further use.


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To get a high quality film, the molar ratio of SbCl3 and TU in the mixed solution is tuned from 1:1.5 to 1:4. From the result of scanning electron microscope (SEM) images (Figure S1) we found that no evident changes are observed on the surface morphology when the SbCl3/TU ratio is higher than 1:3, while significant increase of film thickness is displayed. When SbCl3/TU ratio reaches to 1:4, the surface morphology is deteriorated seriously and much more pin-holes are appeared. In this case, no obvious changes of the thickness is observed. Considering the two factors, the SbCl3/TU ratio of 1:3 is set for further investigation. Synthesis of Zn-doped Sb2S3 film and device fabrication. To prepare Zn-doped Sb2S3 thin films, the complex solution was spin-coating on compact layer of TiO2 (c-TiO2) at a speed of 4000 rpm for 30 s, followed by pretreatment in vacuum drying oven at 125 °C for 1 min to dissocaite solvent. After cooling down to room temperature, the precursor film was annealing at 270 °C for 10 min in N2 prior to solar cell fabrication. The typical structure of solar cells in this study can be described as FTO/c-TiO2/Zn-Sb2S3/HTM/Au. Firstly, the FTO-coated glass was cleaned by deionized water, isopropanol, acetone and anhydrous alcohol for 40 min, respectively. Afterwards, c-TiO2 was deposited on pre-cleaned FTO-coated glass substrate by spin-coating a mixture solution of 140 µL of titanium isopropoxide, 2 mL of ethanol, and 20 µL of concentrated hydrochloric acid (HCl, 12M) at 3000 rpm for 30 s, followed by annealing at 550 °C for 30 min. Sb2S3 and Zn-doped Sb2S3 film were fabricated on c-TiO2 accroding to the above methods. After that, 2, 2´,7,7´-tetrakis (N, N-di-p-methoxyphenylamine)-9,9´-spirobifluorene (spiro-OMeTAD) was spin-coated on FTO/c-TiO2/Zn-Sb2S3 substrate at a speed of 3000 rpm for 30 s and heated on a hot plate at 100 °C for 5 min in air. The spiro-OMeTAD solution was prepared by mixing 36.6 mg of spiro-OMeTAD, 14.5µL of 4-tert-butylpyridine (tBP) and 9.5µL of a 520 mg·mL−1 lithium bis(trifluoromethanesulfonyl) imide (Li-TFSI) together in 1 mL of


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chlorobenzene. Finally, Au counter electrode was deposited by a thermal evaporator under pressure of 5.0×10-4 Pa. The active area of the device was defined as 0.12 cm2. Characterizations. Samples were characterized by powder X-ray diffraction (XRD) on a Bruker Advance D8 diffractometer equipped with Cu Kα radiation (λ = 1.5416 Å). The optical characteristics of the films were measured with a UV-visible spectrophotometer (SOLID 3700). The surface and cross sections morphologies of the samples were examined by SEM (FE-SEM siron 200). X-ray photoelectron spectroscopy (XPS) was conducted on a Thermo ESCALAB 250Xi systems with an Al Kα monochromatized source and a multi-detection analyzer under a 10−8 Torr. UPS experiments were performed at the Catalysis and Surface Science Endstation at the BL11U beamline in the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. The valance band (VB) spectra were measured with a photon energy of 30 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 elemental composition of Sb2S3 and Zn doped Sb2S3 thin films were determined by X-ray fluorescence spectrometer (XRF-1800). Finally, the J-V curves were recorded using a Keithley 2400 apparatus under solar-simulated AM 1.5 sunlight (100 mW/cm2) with a standard xenon-lamp-based solar simulator (Oriel Sol 3A, Japan). The solar simulator illumination 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). And the external quantum efficiency (EQE, Model SPIEQ200) was measured using a single source illumination system (halogen lamp) combined with a monochromator. The capacitance-voltage (C-V) curves were carried out using Zahner Mess Systeme PP211 electrochemical workstation at room temperature in darkness at a frequency of 10 kHz and the AC amplitude was 5mV. DC bias


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voltage was changed from 0.2 to 1.2 V. Electrochemical impedance spectroscopy (EIS) measurements were performed using Zahner Mess Systeme PP211 electrochemical workstation at a bias potential of -0.50 V in dark with the frequency ranging from 100Hz to 1MHz. ASSOCIATED CONTENT Supporting Information. SEM images of Zn-doped Sb2S3 film synthesized with SbCl3/TU ratios of 1:1.5, 1:2, 1:3 and 1:4. XRD patterns and SEM images of Zn-doped Sb2S3 precursor film and Zn-doped Sb2S3 film anealed at 240 oC, 255 oC, 270 oC, 285 oC and 300 oC. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (C. F. Z) *E-mail: [email protected] (T. C) Notes 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, U1732150, GG2060140085).


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Table 1. Photovoltaic parameters of the devices based on Sb2S3, Sb2S3 with Zn doping concentration of 1.5%, 4.8% and 8.6% as light absorbing materials, measured under one Sun AM 1.5G illumination. Device

VOC (V)

JSC (mA cm-2)

FF (%)

PCE (%)

pristine Sb2S3

0.653±0.01

14.14±0.5

55.7±2

5.15±0.3

1%

0.647±0.01

14.4±0.5

55.6±2

5.13±0.3

4.8% a)

0.627±0.02 (0.647)

16.4±0.8 (17.19)

57.1±1 (57.1)

5.85±0.5 (6.35)

8.6%

0.613±0.01

16.8±0.2

52.6±1

5.43±0.2

a)

The VOC, JSC, FF and PCE in the brackets are best device parameters.


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Figure 1. (a) Thermal gravimetric analysis (TGA) of the Sb-TU complex precursor solution (The inset is image of Sb-TU complex solution); (b) XRD patterns of the as-synthesized pristine Sb2S3 film and Sb2S3 films with Zn/(Zn+Sb) atomic ratios of 1%, 3%, 5% and 7% in the precursor solutions. The standard stibnite Sb2S3 structure (JCPDF No. 42-1393) is plotted as a blue line and F indicates the diffraction peaks from FTO substrates.


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Figure 2. High resolution core-level XPS spectra of (a) Sb 3d and O 1s; (b) Zn 2p; (c) S 2p in sample Zn-doped Sb2S3; (d) UV–Vis absorption spectra of pristine Sb2S3 (denoted as No doping) and Sb2S3 with Zn doping of 1.5%, 3%, 4.8% and 7% (the inset is Tauc plot for band gap calculation of the corresponding samples).


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Figure 3. (a) Cross sectional SEM image of a device with Zn-Sb2S3 film; (b) photocurrent density-voltage responses and (c) external quantum efficiency (EQE) of the devices based on pristine Sb2S3 and Sb2S3 with Zn-doping of 1.5%, 4.8% and 8.6%; (d) energy level diagram of the device with Sb2S3 and Zn-doped Sb2S3 as light absorption materials.


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Figure 4. SEM images of the as-synthesized pristine Sb2S3 film (a) and Sb2S3 with Zn-doping of 1.5% (b), 4.8% (c) and 8.6% (d), respectively.


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Figure 5. (a) 1/C2 versus V curves and (b) Nyquist plots of the Sb2S3-based devices without Zn doping and with Zn doping of 1.5%, 4.8% and 8.6% measured under darkness. UPS spectrum of (c) pristine Sb2S3 and (d) 4.8% Zn-doped Sb2S3 film with a higher energy cut-off. The inset in (c) and (d) is UPS spectrum of pristine Sb2S3 and 4.8% Zn-doped Sb2S3 with the lower binding energy, respectively.


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Figure 6. (a) Photocurrent density-voltage response and (b) external quantum efficiency of the best device based on 4.8% Zn-doped Sb2S3. The inset in (a) is statistics of the PCE distribution for 30 devices independently fabricated.


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Graphical Abstract N-type Doping Sb2S3 Light Harvesting Film Enabling High Efficiency Planar Heterojunction Solar Cells Rongfeng Tang, Xiaomin Wang, Chenhui Jiang, Shiang Li, Weifeng Liu, Huanxin Ju, Shangfeng Yang, Changfei Zhu* and Tao Chen*

The n-type doping of Sb2S3 film causes upshift of Fermi level and in turn facilitates charge transport from Sb2S3 to TiO2, leading to a power conversion efficiency of 6.35%, which is the highest efficiency among planar heterojunction solar cells based on Sb2S3.


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n-Type Doping of Sb2S3 Light-Harvesting Films Enabling High

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