Vertically Oriented BiI₃ Template Featured BiI₃/Polymer Heterojunction for High Photocurrent and Long-Term Stable Solar Cells

Nontoxic and stable materials are one of the necessities for commercialization of solar devices. However, most lead-free absorbers have limited light absorption range as well as poor morphology. In this work, a vertically oriented BiI3 template induced by Li-bis(trifluoromethanesulfonyl)imide dopant is intimately integrated with a light-absorbing polymer to form an organic–inorganic BiI3/polymer heterojunction absorber in solar cells. Compared with the dopant-free BiI3/polymer, the broadened light absorption of the doped BiI3/polymer enhances the external quantum efficiency (EQE) of the device beyond 500 nm as well as extends the EQE edge from 650 to 750 nm, which significantly increases the short-circuit current (Jsc) of the device from 1.3 to 3.7 mA cm–2. The polymer top layer is further optimized to improve charge extraction, which achieved the highest Jsc recorded (7.8 mA cm–2) for BiI3-based solar cells and an efficiency of 1.03%. Moreover, the encapsulated device shows no degradation after storing in ambient conditions for nearly 2 years.