Post-Treatment Engineering of Vacuum-Deposited Cs2NaBiI6 Double Perovskite Film for Enhanced Photovoltaic Performance

Lead-free double perovskite materials have recently shown promising in optoelectronic application. However, the poor solubility of these materials makes them difficult to form a high-quality thin film via the solution-processible technology, which leads to the poor photovoltaic performance of a solar cell device. Herein, the vacuum-evaporated Cs2NaBiI6 double perovskite film with high quality achieved by the post-treatment engineering is demonstrated. It is found that soaking in isopropyl alcohol (IPA) followed by annealing in dimethyl formamide (DMF) atmosphere can enlarge the size of grains, lower trap-state density, and remove side product of the Cs2NaBiI6 film. The resulting solar cell devices show a power conversion efficiency (PCE) of 0.21% (V-OC = 0.70 V, J(SC) = 0.91 mA cm(-2), and fill factor (FF) = 33%), which is tenfold higher than the control device without post-treatment (PCE = 0.02%, V-OC = 0.49 V, J(SC) = 0.19 mA cm(-2), and FF = 24%). In addition, the device shows robust stability against moisture and oxygen in ambient condition. The vacuum deposition method accompanied with a post-treatment strategy, in this work, provides an important research direction in improving the quality of a double perovskite film and the PCE of corresponding perovskite solar cells (PSCs).