Bromine-Substituted Indolo[3,2-b]carbazole-Based Small Organic Molecules as Dopant-Free Hole-Transporting Layers for Inverted Perovskite Solar Cells

In this paper, three indolo[3,2-b]carbazole-based small organic molecules (ICZ, ICZ-2Br, and ICZ-4Br) are rapidly synthesized and used as dopant-free hole transport layers (HTLs) for inverted perovskite solar cells. To investigate the electronic structure and dipolar moment of HTLs on the performance of devices, various numbers of bromine atoms on the termination of HTLs are substituted. It is found that the introduction of bromine atoms can improve the hole mobility of the indolo[3,2-b]carbazole derivatives compared with that of the molecules without bromine substitution. Among the three molecules, ICZ-2Br, which has two bromine atoms, exhibited the highest hole mobility due to its strong electronegativity and large dipolar moment. Using ICZ-2Br as the HTL, the inverted device achieved a power conversion efficiency (PCE) of 20.35%. In addition, the stability of the devices is evaluated by continuous light soaking for 1000 h without encapsulation. After this prolonged exposure, the devices can maintain over 90% of their original PCE, indicating excellent stability. Overall, this study demonstrates the effectiveness of bromine-substituted indolo[3,2-b]carbazole derivatives as HTLs for inverted perovskite solar cells. These findings contribute to the development of the rapid synthesis of HTLs for efficient and stable perovskite solar cells.