Copper(I) thiocyanate (CuSCN), an air-stable material, have been widely used as hole transport layer (HTL) in perovskite solar cells due to its wide band gap (3.7-3.9 eV), high optical transparency, high hole mobility (0.01-0.1 cm²V^-1s^-1), excellent hole transporting property, low-temperature processability, and variety of morphologies obtained by different fabrication techniques. Perhaps the most attractive aspect of CuSCN material is that it is inexpensive, highly abundant, intrinsically stable, commercially available, and easily handled, making it a suitable HTL for perovskite solar cells. For CuSCN film as an HTL, the different deposition methods such as doctor blading,[i] electrodeposition,[ii] spin coating,[iii] and spray coating[iv] have been widely reported in the literature. However, the commonly used solvents for solution-processable deposition have unpleasant smell, toxic and inadvisable in glove-box, which may restrict their possible practical applications in the upcoming future. To address this issue, herein, we report thermally deposited CuSCN thin film as an alternative and solventless approach to the conventional solution-processed methods for HTL in inverted perovskite solar cells.

In the present work, the thermal deposition of CuSCN, a solvent-free and green approach has been reported for the fabrication of CuSCN thin film as an HTL on ITO surface. The deposition rate and deposition time have been optimized to obtain different thicknesses of CuSCN films. The structural, electrochemical, optical, and morphological properties of the CuSCN films were characterized by XRD, Raman spectroscopy, FTIR, CV, UV-vis-NIR spectroscopy, AFM, and FESEM. The long-term electrochemical and optical stability of thermally deposited CuSCN films were examined. Thermally deposited CuSCN film was used as an HTL to fabricate inverted planar perovskite solar cells. The effect of CuSCN film thicknesses and the annealing temperature of CuSCN films on the photovoltaic performance were investigated. The different annealing temperatures (RT, 50, 100, 150, and 200 °C) of thermally deposited CuSCN films were used to optimize the perovskite device under similar environmental conditions. The champion device exhibits the maximum power conversion efficiency (PCE) of 15.71% with V_oc = 1.01 V, J_sc = 20.2 mA/cm², and FF = 0.77. For reference, perovskite solar cells without HTL have been also fabricated simultaneously. Importantly, the thermally deposited CuSCN based devices show excellent reproducibility with stability up to 240 h in ambient conditions and ⁓89% retention of the initial PCE. This study demonstrates a new methodology for obtaining better CuSCN film by thermal deposition technique for efficient, sustainable and scalable electronic device applications.

([i]) Qin, P.; Tanaka, S.; Ito, S.; Tetreault, N.; Manabe, K.; Nishino, H.; Nazeeruddin, M. K.; Grätzel, M. Inorganic hole conductor-based lead halide perovskite solar cells with 12.4% conversion efficiency. Nat. Commun. 2014, 5, 3834.

([ii]) Ye, S.; Sun, W.; Li, Y.; Yan, W.; Peng, H.; Bian, Z.; Liu, Z.; Huang, C. CuSCN-based inverted planar perovskite solar cell with an average PCE of 15.6%. Nano Lett. 2015, 15, 3723-3728.

([iii]) Jung, J. W.; Chueh, C. C.; Jen, A. K. Y. High‐performance semitransparent perovskite solar cells with 10% power conversion efficiency and 25% average visible transmittance based on transparent CuSCN as the hole‐transporting material. Adv. Energy Mater. 2015, 5, 1500486.

([iv]) Yang, I .S.; Sohn, M. R.; Do Sung, S.; Kim, Y. J.; Yoo, Y. J.; Kim, J.; Lee, W. I. Formation of pristine CuSCN layer by spray deposition method for efficient perovskite solar cell with extended stability. Nano Energy 2017, 32, 414-421.