SUPPLEMENTARY INFORMATION

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1 Thin-film Sb2Se3 photovoltaics with oriented one-dimensional ribbons and benign grain boundaries Ying Zhou 1,2, Liang Wang 1,2, Shiyou Chen 3, Sikai Qin 1,2, Xinsheng Liu 1,2, Jie Chen 1,2, Ding-Jiang Xue 1,2, Miao Luo 1,2, Yuanzhi Cao 1, Yibing Cheng 1, Edward H. Sargent 4, Jiang Tang 1,2* 1 Wuhan National Laboratory for Optoelectronics (WNLO), and 2 School of Optical and Electronic Information, Huazhong University of Science and Technology (HUST), Wuhan, , China 3. Key Laboratory for Polar Materials and Devices (MOE), East China Normal University, Shanghai , China 4 Department of Electrical and Computer Engineering, University of Toronto, Toronto, Ontario, M5S 3G4, Canada Correspondence and requests for materials should be addressed to J. T. ( jtang@mail.hust.edu.cn) NATURE PHOTONICS 1

2 1. The RTE setup and process Figure S1 The schematic diagram of RTE equipment for Sb 2Se 3 film deposition. Sb 2Se 3 powder (dark-grey) was placed on top of the AlN flake inside the graphite crucible, and FTO/CdS substrate was suspended on top of the graphite crucible with CdS side facing down. The distance between the substrate and the powder is 0.8 cm. Vacuum was maintained at about 8 mtorr through simple mechanical pump, and temperature was controlled through the infrared heaters surrounding the quartz tube and monitored by the thermal couple (TC). 2. Confirmation of the RTE deposited film to be pure Sb 2Se 3 Figure S2 X-ray Photoelectron Spectrum (XPS) of the Sb 2Se 3 thin film deposited by RTE process: (a) Sb 3d and (b) Se 3d. The binding energy for the Sb 3d 3/2 and 3d 5/2 is ev and ev, and the binding energy for Se 3d 3/2 and 3d 5/2 is ev and ev, respectively. These values, combined with the perfect Gaussian Lorentzian fitting of the XPS spectra indicated that our RTE deposited film is pure Sb 2Se 3 without any detectable impurities. 2 NATURE PHOTONICS

3 SUPPLEMENTARY INFORMATION 3. Confirmation of single-crystalline Sb 2Se 3 grains. Figure S3 TEM and Selected area electron diffraction (SAED) analysis of our Sb 2Se 3/CdS device. (a) Cross-sectional TEM image of a full Sb 2Se 3 photovoltaic device (scale bar 100 nm). The Sb 2Se 3 film is pinhole free with large grains extending from the bottom CdS layer to the top electrode. The darker spots with irregular shapes are caused by damage during the FIB process. The SAED patterns of three different grains (I, II and III) in panel (a) were shown in panel (b), (c), (d), respectively, demonstrating our Sb 2Se 3 thin film was composed of single crystalline Sb 2Se 3 grain s. 4. Composition analysis of Sb 2Se 3 film deposited onto substrate temperature of 300 and 350. We carried out energy-dispersive X-ray (EDX) spectroscopy analysis on our Sb 2Se 3 samples. To assure the results are credible, we kept the measurement parameters identical throughout the whole experiments; we also calibrated our EDX setup using commercially available Sb 2Se 3 powder (99.999% purity from Alfa Aesar) (Table 1). Then we measured Sb 2Se 3 films deposited with substrate temperature of 300 (Class A) and 350 (Class B). For both samples, films produced from two different batches and for each film three randomly selected spots were measured and hence data from a total of six spots were averaged. The measured molar ratio between Se and Sb was exactly the same, 1.515, for Class A and Class B sample (Table 2, 3), indicating the films were actually slightly Se rich. Such observation indicated that Sb 2Se 3 decomposition (Sb 2Se 3 (s) = 0.25 Sb 4(s) + SbSe (g) + Se 2 (g) ) occurred during the evaporation because the evaporating source experienced a high temperature, however the decomposition of the Sb 2Se 3 film should be infinitesimal since the temperature is low (< 350 ) and the duration is short (~10 min including the cooling process). Se with slightly larger value than the stoichiometric ratio was evaporated from the source and condensed into the film, leading to Se-rich film. NATURE PHOTONICS 3

4 Figure S4 Representative EDX spectra for Sb 2Se 3 films deposited at substrate temperature of 300 and 350. For EDX measurement, the working distance is 5 mm and the beam voltage is 10kV. Table 1. Calibration of the EDX setup using commercial Sb 2Se 3 powder (Alfa Aesar, %) No. Se(At%) Sb(At%) Average Correction factor x Se/Sb 1.50 Table 2. EDX result of Class A Sb 2Se 3 film (film deposited at substrate temperature of 300 ) No. Se(At%) Se(At%) Average Correction result Se/Sb NATURE PHOTONICS

5 SUPPLEMENTARY INFORMATION Table 3. EDX result of Class B Sb 2Se 3 film (film deposited at substrate temperature of 350 ) No. Se(At%) Se(At%) Average Correction result Se/Sb Calculation of doping density for Class A (deposited onto 300 substrate) and Class B devices (deposited onto 350 substrate). Capacitance-voltage measurements were applied to our devices FTO/CdS/Sb 2Se 3/Au. An ac frequency of 10 Hz was applied for the measurement. For the CdS film, the relative dielectric constant (ε r,n) of 9 and donor concentration of cm -3 were applied for calculation. The relative dielectric constant (ε r,p) of Sb 2Se 3 was measured as 19 based on ellipsometry measurement. Based on these values and the C -2 -V curve, the doping density (holes) of Sb 2Se 3 film was calculated using equation 1 to be approximately 7.4x10 16 cm -3 and 8.0x10 16 cm -3 for Class A and Class B device, respectively. Please note C-V measurements give substantially higher doping density than values obtained from Hall effect measurement (trend is the same), and this discrepancy is possibly originated from the use of total capacitance instead of junction capacitance for C-V analysis, and from the different mechanisms of these two techniques. N A = d 2 (1 / C ) dv 2 N rn, D 2 qa N 2 0 r, n r, p D r, p (1) NATURE PHOTONICS 5

6 Figure S5 C-V fitting of Sb 2Se 3 solar cell deposited at 300 and 350. Abrupt heterojunction was assumed for the calculation. 6. Calculation of texture coefficient of device A and B. Calculation of texture coefficient (TC) for a given crystal planes for our Sb 2Se 3 film is based on the following equation 1 TC I 1 I /( ) N ( hkl ) ( hi kili ) hkl I N 0( hkl ) i 1 I 0( hi kili ) Where I (hkl) is the diffraction peak intensity of (hkl) plane in the measured XRD pattern, while I 0(hkl) is the diffraction peak intensity of (hkl) plane in the standard XRD pattern (for our Sb 2Se 3 film, JCPDS ). By definition TC ranges from 1 for the film with completely no texture to N for the film with singly oriented crystals. 6 NATURE PHOTONICS

7 SUPPLEMENTARY INFORMATION Figure S6 The texture coefficients of all presented diffraction peaks in device A and B. Larger TC value means enhanced orientation in this direction. Clearly, the TC values of the (hk0) planes were closed to zero, and the TC values of (hkl, where l 0) were larger than 1 in device A. This indicated that [hk0] orientated grains (grains where (Sb 4Se 6)n ribbons stacking in parallel with the substrates and having poor carrier transport through hopping from one ribbon to adjacent ribbons) are suppressed in the Sb 2Se 3 film in device A. In contrast, diffraction peaks for the [hk0] orientated grains with poor carrier transport were strengthened in device B, suggesting very poor carrier transport and hence reduced device performance in this Sb 2Se 3 solar cell. 7. Comparison of measured device performance in-house with the certified result. Figure S7 The IV curve tested in our laboratory and by Newport. (a) Comparison of the same device performance tested in our lab (Voc=0.39 V Jsc=26.05 ma/cm 2, FF=52.4% and η=5.36%) with the result certified by Newport (Voc= 0.40 V, Jsc =25.14 ma/cm 2, FF = 55.7% and η=5.6%). The good agreement in efficiency confirmed that solar cell efficiency measurement in our lab is credible. (b) Forward and reversed I-V scans of our Sb 2Se 3 thin film solar cells. Unlike CH 3NH 3PbI 3 solar cells where a huge difference was observed 2, our Sb 2Se 3 solar cells showed perfect overlapping and completely free of hysteresis. NATURE PHOTONICS 7

8 8. Validation of device efficiency measurement in the lab To minimize possible Jsc overestimation from current collecting outside the device area, device masking and isolation was applied for our efficiency measurement. Moreover, we also built a device with 1.08 cm 2 size and measured 5.35% device efficiency. Figure S8 Digital photos of device efficiency measurement employing mechanical scribing and mask shadowing. Photos in panel a and d are taken from the Au contact side so the devices look orange. Light is illuminated from the metal mask side. Figure S9 Validation of device efficiency measurement. I-V characteristics of small (0.095 cm 2 ) (a) and large (1.08 cm 2 ) (b) Sb 2Se 3 solar cells measured under 100 mw/cm 2 simulated class 3A AM1.5G solar illumination. Mechanical scribing and mask were applied for these measurements. Reference: 1. Prabhakar, T. et al. Effects of growth process on the optical and electrical properties in Al-doped ZnO thin films. J. Appl. Phys. 115, (2014). 2. Jeon, N. J. et al. Solvent engineering for high-performance inorganic organic hybrid perovskite solar cells. Nature Mater. 13, (2014). 8 NATURE PHOTONICS