Supporting Information

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1 Supporting Information Room-Temperature Processed Nb 2 as the Electron Transporting Layer for Efficient Planar Perovskite Solar Cells Xufeng Ling, 1 Jianyu Yuan, 1 Dongyang Liu, 1 Yongjie Wang, 1 Yannan Zhang, 1 Si Chen, 1 Haihua Wu, 1 Feng Jin, 3 Fupeng Wu, 1 Guozheng Shi, 1 Xun Tang, 1 Jiawei Zheng, 1 Shengzhong (Frank) Liu, 2 Zhike Liu*,2 and Wanli Ma*,1 1 Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou , China 2 Key Laboratory of Applied Surface and Colloid Chemistry, National Ministry of Education, Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi an , China 3 Shanghai Ultra-precision Optical Manufacturing Engineering Research Center, and Key Laboratory of Micro and Nano Photonic Structures (Ministry of Education), Department of Optical Science and Engineering, Fudan University, Shanghai , China Corresponding Author zhike2015@snnu.edu.cn (Z. Liu) wlma@suda.edu.cn (W. Ma) S-1

2 Figure S1. AFM height images of bare FTO (left), a-nb 2 /FTO film (middle) and c-nb 2 /FTO film (right). Figure S2. Top-view SEM image of MAPbI 3 film deposited on a-nb 2 /FTO (left) and c-nb 2 /FTO (right) substrate. The scar bar is 500 nm. a-nb 2 Nb 3d 3/2 Nb 3d 5/2 a-nb 2 O 1s Intensity (a.u.) ev c-nb 2 Nb 3d 3/2 Nb 3d 5/ ev c-nb ev O 1s ev ev ev ev ev Binding Energy (ev) Binding Energy (ev) Figure S3. The Nb 3d and O 1s core level spectra of a-nb 2 and c-nb 2 obtained from XPS measurements. S-2

3 Figure S4. Surface potential images of (a) HOPG, (b) a-nb 2 and (c) c-nb 2 obtained from KPFM measurements. The scale bar is 1 µm. The work function of the a-nb 2 and c-nb 2 are characterized by KPFM though probing the surface potential difference (SPD) between Ti/Ir-coated tip and the samples. 1 The SPD is defined as the following Eq. (1): (1) Where e is elementary charge of electron, WF t is the work function of Ti/Ir-coated tip, and WF s is the work function of sample surface. WF t is calibrated using highly ordered pyrolytic graphite (HOPG) with a constant work function of 4.60 ev. Finally, the work function of samples can be calculated by the Eq. (2): 4.60 (2) As shown in Figure S4, the 5 5 µm 2 scan area is measured on both HOPG and the samples. The mean distribution of SPD from the KPFM images is V, 0.21 V and 0.50 V for HOPG, a-nb 2 and c-nb 2, respectively. Therefore, the work function of the a-nb 2 and c-nb 2 are 4.31 and 4.02 ev, respectively. The Fermi level is ev for a-nb 2 and ev for c-nb 2, respectively. 0.8 a-nb 2 Absorbance (a.u.) c-nb Wavelength (nm) Figure S5. UV-vis absorption spectra of glass/fto/nb 2 /MAPbI 3 films. S-3

4 Current Density (ma cm -2 ) nm 85 nm 120 nm 150 nm Voltage (V) Figure S6. J-V curves of cells with different thickness of a-nb Normalized PCE TiO 2 a-nb 2 c-nb Time (h) Figure S7. Stability data of PSCs based on different ETLs stored in air without encapsulation. S-4

5 Counts PCE (%) Figure S8. The PCE distribution histogram of the flexible PSCs. R s R tr R rec CPE tr CPE rec Figure S9. The equivalent circuit model employed to fit the Nyquist plots for PSCs. Thickness [nm] V oc [V] J sc [ma cm -2 ] FF PCE [%] Table S1. J-V parameters of cells with different thickness of Nb 2. ETL R s [Ω] R tr [Ω] R rec [Ω] CPE tr Freq Power CPE rec Freq Power a-nb c-nb Table S2. EIS parameters of the PSCs with different ETLs. S-5

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