Novel p-dopant toward highly efficient and stable perovskite solar cells†
Li-TFSI is the most common p-dopant for the hole conductor spiro-MeOTAD in the normal structure (n–i–p) of perovskite solar cells (PSCs), which consistently yield the highest power conversion efficiency (PCE) albeit at the risk of lower long-term operational stability. Here we successfully replace conventional Li-TFSI with Zn-TFSI2, which not only acts as a highly effective p-dopant but also enhances considerably both the photovoltaic performance and long-term stability. The incorporation of Zn-TFSI2 as a dopant for spiro-MeOTAD leads to an increase by one order in the hole mobility compared to Li-TFSI from 3.78 × 10−3 cm2 V−1 s−1 to 3.83 × 10−2 cm2 V−1 s−1. Furthermore, the device with Zn-TFSI2 showed an 80 mV higher built-in voltage and a bigger recombination resistance than the one with Li-TFSI, which were responsible for the striking increase in both the open-circuit voltage and fill factor, leading to a stabilized PCE of 22.0% for the best cells. Remarkably, the device employing Zn-TFSI2 demonstrated superb photo-stability, showing even a 2% increase in the PCE after 600 h light soaking at the maximum power point (mpp) under full sun, while the PCE of the device with Li-TFSI decreased by 20% under the same conditions. Similarly, the device with Zn-TFSI2 showed better operational stability at 50 °C resulting in a 21% decrease in the PCE after 100 h aging at the mpp under full sun while the Li-TFSI based one showed a 55% decrease. Moreover, the Zn-TFSI2 based device was capable of effectively resisting humidity compared to the one based on Li-TFSI from shelf stability monitoring (R.H. ≥ 40%) in the dark.