Efficiency Improvement of Sb2Se3 Solar Cells via Grain Boundary

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Efficiency Improvement of SbSe Solar Cell via Grain Boundary Inversion Chao Chen, Kanghua Li, Shiyou Chen, Liang Wang, Shuaicheng Lu, Yuhao Liu, Dengbing Li, Haisheng Song, and Jiang Tang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01456 • Publication Date (Web): 07 Sep 2018 Downloaded from http://pubs.acs.org on September 9, 2018

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ACS Energy Letters

Efficiency Improvement of Sb2Se3 Solar Cell via Grain Boundary Inversion Chao Chen,†#∆ Kanghua Li,†#∆ Shiyou Chen,// Liang Wang,#∆ Shuaicheng Lu,#∆ Yuhao Liu#∆, Dengbing Li, #∆ Haisheng Song,#∆ and Jiang Tang*#∆ #

Wuhan National Laboratory for Optoelectronics (WNLO), Huazhong University of Science and

Technology, 1037 Luoyu Road, Wuhan, 430074, Hubei, P. R. China. ∆

Shenzhen R&D Center of Huazhong University of Science and Technology, Shenzhen, 518000,

P. R. China. //

Key Laboratory of Polar Materials and Devices (MOE), East China Normal University,

Shanghai 200241, China

Corresponding Author *E-mail: [email protected]

ABSTRACT: Sb2Se3 is a promising low-cost and low-toxicity photovoltaic material. Recent researches revealed recombination losses in the absorber limited the efficiency of Sb2Se3 solar cells. Herein we demonstrated a strategy of grain boundary (GB) inversion to alleviate such

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recombination loss. Owning to its one-dimensional crystal structure, we successfully inverted the GBs of Sb2Se3 films by introducing n-type Cu interstitial doping at GBs via a low-temperature CuCl2 treatment. A built-in electric field is established between p-type grain interiors and n-type GBs, which spatially separates phototogerated carriers, suppresses recombination and enhances carrier collection. Finally, we obtained an efficiency of 7.04%, which is the highest efficiency of Sb2Se3 solar cells based on rapid thermal evaporation technology. We envision this GB inversion strategy is generally applicable to Sb2Se3 solar cells with different device configuration or produced from other methods, and is extendable to other emerging low-dimensional solar cells.

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Traditional chalcogenide-based thin-film absorbers such as Cu(In,Ga)(S,Se)2 (CIGSSe) and CdTe have achieved impressive power-conversion efficiencies (PCE) of 22.1% and 22.6%, respectively, yet concerns remain regarding the scarcity of Te, In and Ga and the toxicity of Cd. Thus, Earth-abundant, low-cost and low-toxic photovoltaic materials have drawn great attention in the last few decades. Based on these requirements, Cu2ZnSn(Se,S)4

is the most promising

material with a certified efficiency of 12.6%,1 but its complex constituents and defects may


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hinder further efficiency improvement. These inspire us to explore new alternative materials with less complex, non-toxic and earth-abundant absorbers, such as SnS,2 GeSe,3 CuSbS2,4 CuSbSe25 and Sb2Se3.6-8 Recently, Sb2Se3 has emerged as a promising absorber material because of its excellent photovoltaic properties, such as an appropriate band gap (1.17 eV direct and 1.03 eV indirect) for single junction terrestrial solar cell,9 a high absorption coefficient (>105 cm-1 at visible light),9-10 and a low melting point of 611 °C,11 enabling deposition of polycrystalline film at relatively low temperature. Apart from these, Sb2Se3 possesses Earth-abundant, low-toxic and low-cost constituents,12 and low structural complexity with only one crystallographic phase.13 A great progress of Sb2Se3 solar cells has been made since 2014. Sb2Se3 solar cells with sensitized configuration by solution process exhibited an efficiency of 3.21%.14 Simultaneously, our group fabricated the first Sb2Se3 thin-film solar cells using hydrazine solution method with a PCE of 2.26%.12 After two years, our group reported superstrate Sb2Se3 thin-film solar cells with a certified efficiency of 5.6% based on rapid thermal evaporation (RTE) technology.6 Subsequently, the substrate Sb2Se3 thin-film solar cells were also rapidly developed with PCE over 4%.15-17 In 2017, the superstrate Sb2Se3 solar cells demonstrated a certified efficiency of 6.5% employing 1,2-ethanedithiol (EDT) treated PbS colloidal quantum dots as hole-transporting layer (HTL).8 For the HTL-free planar Sb2Se3 solar cells, the certified efficiency of 5.93% was achieved in 2017 using ZnO as buffer layer,7 and this device demonstrated excellent stability, nearly fully passing the IEC61646 standard.7 We further developed vapour transport deposition to improve the crystallinity of Sb2Se3 thin films, obtaining a certified efficiency of 7.6%,18 the current efficiency record of Sb2Se3 solar cells.


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At present, the main issue of Sb2Se3 solar cells is the low PCE because of the recombination loss caused by the deep defect centres such as vacancy defects (VSe) and substitutional defects (SeSb or SbSe).18-19 Spatially separating the electron-hole pairs is one way for reducing the carrier recombination at these deep defect centres by building electric field between grain boundaries (GBs) and grain interiors (GIs). As-produced Sb2Se3 film is weakly p-type, so we could inverse the GBs into n-type by external doping. Then the Fermi level difference between GBs and GIs can results in an electric field from GBs to GIs, thus electrons and holes are driven to GBs and GIs, respectively, restraining the electron-hole recombination. N-type doping in Sb2Se3 can be obtained by two approaches: metal cation interstitial doping (such as Cui) or substitutional doping by halogens (such as ISe). Sb2Se3 consists of one-dimensional (Sb4Se6)n chains stacked through van der Waals forces.6, 20 The distance between chains is 3.29 Å (Figure 1a),21 which is spacious enough to host most single atom cations. We thus choose the interstitial doping strategy for n-type GIs by introducing metal cations into Sb2Se3 film. For easy implementation, cations with low diffusion barrier and large diffusion coefficient such as Cu2+, Ag+, Na+ etc. are preferred. Na+ has been recognized as an inert dopant in Sb2Se3 because of no activation,22 and serious hysteresis was observed in Ag+ treated Sb2Se3 solar cells (Figure S1), indicating Ag+ is highly mobile within Sb2Se3 film. We thus focus on Cu2+ in this study, but other cations such as In3+ and Cs+ are also effective based on our preliminary study. Herein we report the efficiency improvement of Sb2Se3 thin film solar cells through grain boundary inversion via Cu doping. Specifically, we soak the ITO (indium tin oxide)/CdS/Sb2Se3 films into aqueous ammonia CuCl2 solution, and the Cu2+ ions diffuse into Sb2Se3 films along the GBs, change the GBs of Sb2Se3 film to n-type by forming interstitial defect (Cui), which causes spatial separation of photogenerated electrons and holes, suppresses the recombination


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loss and improves the open-circuit voltage (VOC) and short-circuit current density (JSC) of the devices. In the end, combining a further (NH4)2S treatment to remove the detrimental n-type layer at the back surface, we obtained a device efficiency of 7.04%, which is the highest efficiency of RTE-based Sb2Se3 solar cells. This method takes the full advantage of easy doping in low-dimensional materials, and avoids the high-temperature process required for doping in three dimensional CdTe and CIGSSe solar cells.18,19 Furthermore, this method can be also extendable to other emerging low-dimensional solar cells such as one-dimensional Sb2S323 and Bi2S324 as well as two-dimensional SnS,2 GeSe,3 CuSbS24 and CuSbSe25 solar cells.

Figure 1. (a) The minimum distance of two neighboring Sb2Se3 chains is 3.29 Å. (b) Representative J-V curves of devices without and with 0.01 and 0.05 mol L-1 CuCl2 treatment. (c) The relative resistivity ( /) as a function of the square root of annealing time (t0.5) at different annealing temperature. (d) The temperature dependence of DCu


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(diffusion coefficient of Cu2+ ion) in Sb2Se3. (e) The experimental and fitted diffusion coefficient as a function of temperature. (f) Element distribution SIMS depth profiles of CuCl2 treated CdS/Sb2Se3 device. Sb, Se, Cd, S, Cu and Cl elements were measured. (g) The schematic diagram of Cu2+ ion diffusion. The shaded area schematically marks the Cu2+ ion diffusion zone.

The fabrication procedure of CdS/Sb2Se3 devices is described in Experimental Methods. CuCl2 solution was prepared by dissolving 13.4 mg (or 67.0 mg) CuCl2 powder into 10 mL ammonia aqueous solution (28% wt.) with the Cu ion concentration of 0.01 (or 0.05) mol L-1. The devices were soaked in this solution for 10 min at room temperature in ambient air, subsequently washed by deionized water and dried by N2 gas. We abandoned the simpler CuCl2 aqueous solution because the CdS layer was dissolved by the acidic aqueous CuCl2 solution based on our experience. In addition, NH3·H2O can slow down the release rate of Cu2+ ion via forming copper ammonia complex ([Cu(NH3)4]2+), so that the treatment can be performed uniformly and mildly. The performances of untreated and CuCl2 treated device are summarized in Table 1. Figure 1b shows the corresponding current density-voltage (J-V) curves under simulated AM 1.5G (100 mW cm-2) irradiation. The device treated with 0.01 mol L-1 CuCl2 yielded the highest efficiency of 6.32% (0.66% higher than control device) with the improvements of JSC from 25.8 to 28.1 mA cm-2 and VOC from 0.391 to 0.413 V. No hysteresis was observed between forward (from VOC to JSC) and backward (from JSC to VOC, Figure S2). The device treated with more concentrated CuCl2 (0.05 mol L-1) also exhibited boosted JSC, but depressed FF. We will give the explanation following. To exclude the influence of Cl-, NH4+ and NH3·H2O in our precursor, we treated the device with NH4Cl ammonia solution (0.01 mol L-1), and the device performance exhibited


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negligible difference from control device (Table 1). Thus, we can conclude that Cu2+ ion is responsible for the improvement of device performance. Table 1. The performance of control devices as well as NH4Cl and CuCl2 treated devices. The values in parentheses are the average performance of 10 devices Sample W/O treatment 0.01 mol L-1 NH4Cl 0.01 mol L-1 CuCl2 0.05 mol L-1 CuCl2

VOC (V) 0.391 (0.392) 0.392 (0.389) 0.413 (0.411) 0.390 (0.387)

JSC (mA cm-2) 25.8 (25.5) 26.0 (25.7) 28.1 (27.9) 28.9 (28.6)

FF (%) 56.1 (55.7) 55.6 (56.2) 54.4 (53.8) 48.3 (47.9)

PCE (%) 5.66 (5.56) 5.67 (5.62) 6.32 (6.17) 5.44 (5.30)

The diffusion ability of Cu2+ in Sb2Se3 is determined by the diffusion coefficient of Cu2+ ion (DCu). We attempted to estimate DCu from the relationship of annealing time and resistivity as described by eq 1,25-26 which is a general method to measure ion diffusion coefficient in CdTe and CIGSSe films:25 / =

.

.

(1)

where ρ0 and ρ are the initial and time-dependent resistivity, t is annealing time, and h is the film thickness. Eq 1 assumes that the resistance of Cu-doped Sb2Se3 is much lower than that of undoped Sb2Se3. This assumption is met in our experiment because the conductivity of Cu-doped Sb2Se3 is an order of magnitude greater than that of undoped Sb2Se3 (Figure S3). The device structure is ITO/Sb2Se3/Au with Sb2Se3 treated by 0.01 mol L-1 CuCl2 solution following the same procedure as described above. The current-voltage (I-V) curves at different annealing temperature and time are presented in Figure S4, and the resistivity is derived by the least square


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fitting method. The relative resistivity ( /) as a function of the square root of annealing time (t0.5) at different annealing temperature is shown in Figure 1c. By linear fitting using eq 1, the DCu at different temperatures can be extracted as the black dots in Figure 1d. The temperature dependency of DCu is given by Arrhenius equation (eq 2),27 = /

(2)

where Ea is the thermal activation energy, D0 is the prefactor, k is the Boltzmann constant, and T is the absolute temperature. Ea and D0 is calculated from the slope and y-axis intercept in Figure 1e as 0.188 eV and 4.68 × 10 cm# s % , respectively. The Ea of Cu2+ diffusion in Sb2Se3 is much lower than that of CdTe (0.33 eV).25 Consequently the DCu of Sb2Se3 at 298 K (3.4 × 10%# cm# s %, calculated from eq 2) is larger than that of CdTe (2.2 × 10%# cm# s %).25 It is well-known that Cu diffuses very fast in CdTe, but it diffuses faster in Sb2Se3, explaining why we can effectively treat Sb2Se3 using CuCl2 solution by a low-temperature solution process. The low Ea and large DCu are consistent with the foregoing description about the large distance (3.29 Å) between the two neighbouring (Sb4Se6)n chains, which enables fast Cu diffusion even within the GIs of Sb2Se3 film. When the concentration of CuCl2 solution is increased to 0.05 mol L-1, more than optimum Cu ions diffuse into CdS/Sb2Se3 interface, then degrades device performance, especially FF. Next, we investigated the distribution of Cu element in Sb2Se3 through secondary ion mass spectroscopy (SIMS) measurement. SIMS is widely used to study local atomic distributions in CIGSSe and CdTe solar cells because of its low detection limit.28-29 The sample was identical to 0.01 mol L-1 CuCl2 treated solar cell but without Au electrode. Figure 1f shows the SIMS signals of Sb, Se, Cd, S, Cu and Cl. Sb, Se, Cd and S are the commonplace result as the previous


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literature.30 The SIMS signals of Cu and Cl elements are relatively noisier due to their lower concentration. Cl mainly gathered at the Sb2Se3 back surface, while Cu could diffuse from the back surface into the bulk even close to the CdS/Sb2Se3 junction. The concentration of Cu element exponentially decayed from the back surface to the junction, following the Fick’s diffusion law.31 The indiffusible nature of Cl ions, probably because of their large ion radius (1.81 Å) or electrostatic repulsive force, can account for its negligible effect on device performance. We also analysed Cu and Cl distribution within the CuCl2 treated Sb2Se3 films by X-ray photoelectron spectroscopy (XPS) study. By comparing the XPS signal intensity of Cu and Cl elements on samples without and with etching, we conclude again Cu diffuse into the interior of Sb2Se3 film while Cl remains on the surface. Further careful binding energy analysis confirmed the valence of Cu as +2 (Figure S5), the same valent states as in the precursor solution. Cu ions mainly diffuse along the GBs in Sb2Se3 films, as schematically shown in Figure 1g, because GBs have smaller compactness than GIs and serve as the express way for fast diffusion, as frequently observed in other thin film solar cells.32 Because of the low Ea and large DCu, we believe Cu diffusion within the GIs is also not negligible, a consequence enabled by the unique one dimensional crystal structure of Sb2Se3. Due to its low concentration, we failed to obtain the microscopic surface and section element distribution of Cu by energy dispersive spectroscopy or electron energy loss spectroscopy.


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Figure 2. KPFM measurement on Sb2Se3 with and without CuCl2 treatment. (a) and (c) are the surface morphology of Sb2Se3 without and with CuCl2 treatment, respectively. (b) and (d) are the surface potential of Sb2Se3 without and with CuCl2 treatment, respectively. (e) and (f) are the corresponding height and electric potential profiling plots along the red dash lines in (a)-(d). (g) The calculated formation energies of the Cu dopants on different doping sites, CuSb and Cui, under the Se-rich (blue) and Se-poor (red) conditions. Cui is a n-type shallow defect with 0.11 eV depth below the conduction band.

Kelvin probe force microscopy (KPFM),33 an effective tool to detect the surface potential distribution of films, was performed on control and CuCl2 treated Sb2Se3 films. Topography and the local contact potential difference (LCPD) were simultaneously measured. LCPD was calculated using eq 3,34 ()*+ = (,-./01 2 (34/

(3)


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where Vtip and Vsample are surface potentials of the probe tip and sample, respectively. Figure 2a,c are the surface morphology of untreated and treated devices, which showed similar results. Figure 2b,d are the surface potential. Apparently, the potential contrast between GBs and GIs are larger for CuCl2 treated device. To exclude the influence of topography on the surface potential, we selected similar topography profiling (Figure 2e, marked using red dash lines in Figure 2a-d) to obtain the surface potential profiling (Figure 2f). With the similar average height difference (62 nm and 58 nm for untreated and treated device, respectively), the surface potential of GBs in CuCl2 treated sample has a larger average drop (104 mV) comparing to the control sample (64 mV). The increased potential difference at GBs indicated Cu treatment can shift up the Fermi level of Sb2Se3, confirming the n-type inversion at GBs. Ultraviolet photoelectron spectroscopy (UPS) measurement further verified that the Fermi lever was closer to conduction band (Figure S6). We carried out density functional theory simulation to explore the origin of the n-type inversion after CuCl2 treatment.35 The Cu dopant in Sb2Se3 can form substitutional CuSb (an acceptor, increasing p-type conductivity) and interstitial Cui (a donor, increasing n-type conductivity). Under the Se-rich condition, the formation energies for the two doping sites are both high (>1 eV) as shown by the blue lines in Figure 2g. When Se is poor, Cui has much lower formation energy (0.75 eV) than CuSb (1.46 eV) (Figure 2g), indicating that Cui is easier to form than CuSb under the Se-poor condition. In our Sb2Se3 films, Sb2Se3 was prone to be Se deficient because of the large vapour pressure of Se leading to Se loss during the cooling stage.19, 36 Thus, we can own the n-type doping to Cui after CuCl2 treatment. In addition, as shown in Figure 2g, this is a shallow defects (0.11 eV below the conduction band), indicating it just causes band bending instead of forming a recombination centre.


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Figure 3. Mechanism and device performance of CuCl2 treated device. Schematic diagram of carrier kinetics and band bending in a typical Sb2Se3 grain without any treatment (a), with CuCl2 treatment (b) and with a further (NH4)2S treatments (c). The dash area stands for the Cu2+ ion diffusion zone. The red wave arrow shows the hole transport in GIs and it is blocked by back surface n-type Sb2Se3 layer. The dark straight arrow denotes the electron transport along GBs. (d) Dark J-V curves of the devices with and without CuCl2 treatment. The quality factor, A, is calculated as the slopes of the blue fitting lines. (e) J-V and (f) EQE curves of control and CuCl2 treated device (note that both of them are treated by (NH4)2S).


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Next we will illuminate the influence of the inverted GBs on the device performance. Because of the inert nature of GBs in Sb2Se3 film,6 the energy band of untreated GB edge is the same as the GIs (Figure 3a). As a result, photogenerated electron-hole pairs could recombine in the GIs through the Shockley-Read-Hall process, particularly in the quasi-neutral region where carrier collection is performed solely by diffusion. However after CuCl2 treatment the grain was halfsurrounded by Cu2+ ions. Cu2+ diffuses into the grains and produces Cui mainly at grain edges, forming n-type GBs and p-type GIs. Then the band bending can induce a local GI-GB-GI electric field (Figure 3b). This electric field can drive holes into GIs, and electrons to GBs, respectively, resulting in the spatial separation of electrons and holes. Then the electrons transport along GBs, in addition to their drift within the depletion region produced by the CdS/Sb2Se3 heterojunction, and collected by CdS. Such an additional collection path along the GBs improves the collection efficiency of photogenerated electrons. Holes move towards back electrode. Consequently, the carrier recombination is restrained. The carrier recombination dynamics is understood by the dark J-V analysis of devices with and without CuCl2 treatment (Figure 3d). The reverse saturation current density related to the carrier recombination is two times less after CuCl2 treatment than that of control device, implying the decreasing recombination in CuCl2 treated device. Meanwhile, the quality factor (A) reduced from 1.76 to 1.62 after CuCl2 treatment, further confirming the improved diode quality. However, the n-type nature of the back surface of Cu-treated Sb2Se3 layer would block the collection of holes by Au electrode (Figure 3b). Removing the back n-type layer recovers the efficient hole collection (Figure 3c) and is likely to further improve the device performance. As in previous reports, (NH4)2S can slightly dissolve Sb2Se3,37 herein we selected (NH4)2S to remove the harmful n-type Sb2Se3 back layer. The detailed treatment process is descripted in Experimental Method. The J-V curve of the device


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with a (NH4)2S etching under AM 1.5G is shown in Figure 3e. The FF of our device was dramatically improved from 54.4% to 59.3%, resulting in an efficiency of 7.04%, which is 1.11% larger than previous record of RTE derived Sb2Se3 solar cells without HTL (5.93%) and 0.54% larger than RTE derived Sb2Se3 solar cells with a PbS HTL (6.5%). The improvement of FF is mainly attributed to the reduced series resistance from 3.61 Ω cm2 to 2.25 Ω cm2 by boosting hole collection. To reveal the enhancement of JSC, we measured the EQE spectra of the control and 0.01 mol L-1 CuCl2 treated device (Note that both of them are treated by (NH4)2S). As shown in Figure 3f, the EQE signals of the CuCl2 treated devices demonstrated the improvement over the wavelength range from 650 to 1000 nm. Because CuCl2 treatment doesn’t visibly change the light absorption of Sb2Se3 absorber, the photocurrent improvement is primarily attributed to the reduced carrier recombination. Reduced recombination can effectively improve the photon-generated carrier collection, particularly the carrier generated by red and near infrared photons at the back region because the carriers in this part are collected by diffusion.


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Figure 4. The cross-sectional SEM images of devices (a) without and (c) with CuCl2 treatment. The EBIC images of devices (b) without and (d) with CuCl2 treatment. (e) The average intensity profiling along the three dashed blue line marked in (b) and (d). Distance at 0 nm at the longitudinal coordinate denotes to the CdS/Sb2Se3 junction.

We further carried out the electron beam-induced current (EBIC) characterization to obtain the information of carrier collection.38 Figure 4a,c are the cross-sectional images of devices without and with CuCl2 treatment, respectively. Figure 4b,d are the corresponding EBIC images. In the CuCl2 treated device, the EBIC signal decreased more slowly than the control device along the direction perpendicular to CdS/Sb2Se3 heterojunction. Figure 4e shows the normalized average EBIC signals along the three blue dash lines in Figure 4b,d. Close to the CdS/Sb2Se3 interface (or within the depletion region), the treated device exhibited the same carrier collection ability as untreated device, implying the unhindered carrier collection with the assistance of the built-in electric field. However, in the back region, the EBIC signal of untreated device dramatically decreased due to the insufficient diffusion length further deteriorated by the defect-assisted recombination. When the device was treated by CuCl2, the carrier collection at the back region demonstrated a distinct improvement, indicating the carrier recombination at back region was suppressed. That is why the EQE spectrum of treated device reveals enhancement at long wavelength.

In conclusion, we have demonstrated a record efficiency of 7.04% Sb2Se3 solar cells produced by RTE process via inversing the grain boundary using n-type Cui doping. Thanks to the large


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distance between chains and small compactness at GBs, Cu2+ ions easily diffuse along the GBs and enter into the gap of chains, forming n-type Cui doping. The inversion leads to a local electric field between GBs and GIs and facilitates the spatial separation of photogenerated electrons and holes. Consequently, the carrier recombination was restrained, and carrier collection was enhanced, particularly for carriers generated by red and infrared photons, resulting in the improvement of PCE. This method takes the full advantage of easy doping in lowdimensional materials, and we hope it is also extendable to other emerging low-dimensional solar cells such as one-dimensional Sb2S3 and Bi2S3 as well as two-dimensional SnS, GeSe, CuSbS2 and CuSbSe2 solar cells.

SUPPORTING INFORMATION Experimental methods, the forward (from VOC to JSC) and backward (from JSC to VOC) current density-voltage scans of Ag+ and CuCl2 treated Sb2Se3 device, the dark current of Sb2Se3 with and without CuCl2 treatment, the time dependent current-voltage curves of ITO/Sb2Se3/Au at different annealing temperature, X-ray photoelectron spectroscopy characterization of untreated and CuCl2 treated Sb2Se3 films, ultraviolet photoelectron spectroscopy (UPS) measurement.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] ORCID Jiang Tang: 0000-0003-2574-2943


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Chao Chen: 0000-0002-8706-5346 Author Contributions †

C. C. and K. L. contributed equally to this work

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (61725401, 5171101030), the Major State Basic Research Development Program of China (2016YFA0204000 and 2016YFB0700702), and the Special Fund for Strategic New Development of Shenzhen, China (JCYJ20160414102210144) and the Fundamental Research Funds for the Central Universities (2017KFXKJC0020). The authors would like to thank the Analytical and Testing Center of HUST and the facility support of the Center for Nanoscale Characterization and Devices, WNLO. Mr. Wenjie Li from Shenzhen Institutes of Advanced Technology Chinese Academy of Sciences is accknowledged for the assistance during KPFM characterization. The calculations were finished in the computer center of ECNU.

REFERENCES (1) Wang, W.; Winkler, M. T.; Gunawan, O.; Gokmen, T.; Todorov, T. K.; Zhu, Y.; Mitzi, D. B. Device Characteristics of CZTSSe Thin-Film Solar Cells with 12.6% Efficiency. Adv. Energy Mater. 2014, 4, 1301465.


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(18) Wen, X.; Chen, C.; Lu, S.; Li, K.; Kondrotas, R.; Zhao, Y.; Chen, W.; Gao, L.; Wang, C.; Zhang, J. Vapor Transport Deposition of Antimony Selenide Thin Film Solar Cells with 7.6% Efficiency. Nat. Commun. 2018, 9, 2179. (19) Liu, X.; Xiao, X.; Yang, Y.; Xue, D. J.; Li, D. B.; Chen, C.; Lu, S.; Gao, L.; He, Y.; Beard, M. C. Enhanced Sb2Se3 Solar Cell Performance through Theory‐Guided Defect Control. Prog. Photovoltaics: Res. Appl. 2017, 25, 861-870. (20) Chen, C.; Bobela, D. C.; Yang, Y.; Lu, S.; Zeng, K.; Ge, C.; Yang, B.; Gao, L.; Zhao, Y.; Beard, M. C.; et al. Characterization of Basic Physical Properties of Sb2Se3 and Its Relevance for Photovoltaics. Front. Optoelectron. 2017, 10, 18-30. (21) Song, H.; Li, T.; Zhang, J.; Zhou, Y.; Luo, J.; Chen, C.; Yang, B.; Ge, C.; Wu, Y.; Tang, J. Highly Anisotropic Sb2Se3 Nanosheets: Gentle Exfoliation from the Bulk Precursors Possessing 1D Crystal Structure. Adv. Mater. 2017, 29, 1700441. (22) Li, Y.; Zhou, Y.; Luo, J.; Chen, W.; Yang, B.; Wen, X.; Lu, S.; Chen, C.; Zeng, K.; Song, H. The Effect of Sodium on Antimony Selenide Thin Tilm Solar Cells. RSC Advances 2016, 6, 87288-87293. (23) Lei, H.; Yang, G.; Guo, Y.; Xiong, L.; Qin, P.; Dai, X.; Zheng, X.; Ke, W.; Tao, H.; Chen, Z.; et al. Efficient Planar Sb2S3 Solar Cells Using a Low-Temperature Solution-Processed Tin Oxide Electron Conductor. Phys. Chem. Chem. Phys. 2016, 18, 16436-16443. (24) Song, H.; Zhan, X.; Li, D.; Zhou, Y.; Yang, B.; Zeng, K.; Zhong, J.; Miao, X.; Tang, J. Rapid Thermal Evaporation of Bi2S3 Layer for Thin Film Photovoltaics. Sol. Energy Mater. Sol. Cells 2016, 146, 1-7. (25) Dzhafarov, T.; Yesilkaya, S.; Canli, N. Y.; Caliskan, M. Diffusion and Influence of Cu on Properties of CdTe Thin Films and CdTe/CdS Cells. Sol. Energy Mater. Sol. Cells 2005, 85, 371-383. (26) Dzhafarov, T.; Cömert, H.; Altunbaş, M.; Alver, Ü.; Küçükömerog̈lu, T.; Kopya, A. Diffusion of Ag in Bi (Pb)-Sr-Ca-Cu-O Superconductors. J. Alloy Compd. 1995, 221, 264-266. (27) Arrhenius, S. Über Die Dissociationswärme Und Den Einfluss Der Temperatur Auf Den Dissociationsgrad Der Elektrolyte. Z. Phys. Chem. 1889, 4, 96-116. (28) Kranz, L.; Gretener, C.; Perrenoud, J.; Jaeger, D.; Gerstl, S. S.; Schmitt, R.; Buecheler, S.; Tiwari, A. N. Tailoring Impurity Distribution in Polycrystalline CdTe Solar Cells for Enhanced Minority Carrier Lifetime. Adv. Energy Mater. 2014, 4, 1301400. (29) Chirila, A.; Reinhard, P.; Pianezzi, F.; Bloesch, P.; Uhl, A. R.; Fella, C.; Kranz, L.; Keller, D.; Gretener, C.; Hagendorfer, H. Potassium-Induced Surface Modification of Cu(In,Ga)Se2 Thin Films for High-Efficiency Solar Cells. Nat. Mater. 2013, 12, 1107-1111. (30) Liu, X.; Chen, C.; Wang, L.; Zhong, J.; Luo, M.; Chen, J.; Xue, D.-J.; Li, D.; Zhou, Y.; Tang, J. Improving the Performance of Sb2Se3 Thin Film Solar Cells over 4% by Controlled Addition of Oxygen during Film Deposition. Prog. Photovoltaics: Res. Appl. 2015, 23, 18281836. (31) Chatterji, S. On the Applicability of Fick's Second Law to Chloride Ion Migration Through Portland Cement Concrete. Cem. Concr. Res. 1995, 25, 299-303. (32) Dharmadasa, I. Review of the CdCl2 Treatment Used in CdS/CdTe Thin Film Solar Cell Development and New Evidence towards Improved Understanding. Coatings 2014, 4, 282-307. (33) Chen, W.; Nikiforov, M. P.; Darling, S. B. Morphology Characterization in Organic and Hybrid Solar Cells. Energ. Environ. Sci. 2012, 5, 8045-8074.


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(34) Li, Y.; Zhou, Y.; Zhu, Y.; Chen, C.; Luo, J.; Ma, J.; Yang, B.; Wang, X.; Xia, Z.; Tang, J. Characterization of Mg and Fe Doped Sb2Se3 Thin Films for Photovoltaic Application. Appl. Phys. Lett. 2016, 109, 232104. (35) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for ab initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169-11186. (36) Leng, M.; Luo, M.; Chen, C.; Qin, S.; Chen, J.; Zhong, J.; Tang, J. Selenization of Sb2Se3 Absorber Layer: An Efficient Step to Improve Device Performance of CdS/Sb2Se3 Solar Cells. Appl. Phys. Lett. 2014, 105, 083905. (37) Xia, Z.; Zhong, J.; Leng, M.; Hu, L.; Xue, D.; Yang, B.; Zhou, Y.; Liu, X.; Qin, S.; Cheng, Y.; et al. Generalized Water-Processed Metal Chalcogenide Complexes: Synthesis and Applications. Chem. Mater. 2015, 27, 8048-8057. (38) Pala, R. A.; Leenheer, A. J.; Lichterman, M.; Atwater, H. A.; Lewis, N. S. Measurement of Minority-Carrier Diffusion Lengths Using Wedge-Shaped Semiconductor Photoelectrodes. Energ. Environ. Sci. 2014, 7, 3424-3430.


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ACS Energy Letters

Figure 1. (a) The minimum distance of two neighboring Sb2Se3 chains is 3.29 Å. (b) Representative J-V curves of devices without and with 0.01 and 0.05 mol L-1 CuCl2 treatment. (c) The relative resistivity (ρ_0/ρ) as a function of the square root of annealing time (t0.5) at different annealing temperature. (d) The temperature dependence of DCu (diffusion coefficient of Cu2+ ion) in Sb2Se3. (e) The experimental and fitted diffusion coefficient as a function of temperature. (f) Element distribution SIMS depth proﬁles of CuCl2 treated CdS/Sb2Se3 device. Sb, Se, Cd, S, Cu and Cl elements were measured. (g) The schematic diagram of Cu2+ ion diffusion. The shaded area schematically marks the Cu2+ ion diffusion zone. 313x166mm (300 x 300 DPI)



Figure 2. KPFM measurement on Sb2Se3 with and without CuCl2 treatment. (a) and (c) are the surface morphology of Sb2Se3 without and with CuCl2 treatment, respectively. (b) and (d) are the surface potential of Sb2Se3 without and with CuCl2 treatment, respectively. (e) and (f) are the corresponding height and electric potential profiling plots along the red dash lines in (a)-(d). (g) The calculated formation energies of the Cu dopants on different doping sites, CuSb and Cui, under the Se-rich (blue) and Se-poor (red) conditions. Cui is a n-type shallow defect with 0.11 eV depth below the conduction band. 467x232mm (300 x 300 DPI)


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ACS Energy Letters

Figure 3. Mechanism and device performance of CuCl2 treated device. Schematic diagram of carrier kinetics and band bending in a typical Sb2Se3 grain without any treatment (a), with CuCl2 treatment (b) and with a further (NH4)2S treatments (c). The dash area stands for the Cu2+ ion diffusion zone. The red wave arrow shows the hole transport in GIs and it is blocked by back surface n-type Sb2Se3 layer. The dark straight arrow denotes the electron transport along GBs. (d) Dark J-V curves of the devices with and without CuCl2 treatment. The quality factor, A, is calculated as the slopes of the blue fitting lines. (e) J-V and (f) EQE curves of control and CuCl2 treated device (note that both of them are treated by (NH4)2S). 56x32mm (300 x 300 DPI)



Figure 4. The cross-sectional SEM images of devices (a) without and (c) with CuCl2 treatment. The EBIC images of devices (b) without and (d) with CuCl2 treatment. (e) The average intensity proﬁling along the three dashed blue line marked in (b) and (d). Distance at 0 nm at the longitudinal coordinate denotes to the CdS/Sb2Se3 junction. 149x75mm (300 x 300 DPI)


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ToC 78x50mm (300 x 300 DPI)


Efficiency Improvement of Sb2Se3 Solar Cells via Grain Boundary

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