Ionization of hole-transporting materials as a method for improving the photovoltaic performance of perovskite solar cells

In the operating mechanisms of perovskite solar cells, hole-transporting materials (HTMs) facilitate directional charge transfer and electron blocking. In addition, HTMs are also important in forming the perovskite layers for inverted perovskite solar cells, improving device efficiency. We present a method for increasing efficiency by ionizing HTMs, introducing defect-passivating abilities, improved interfacial properties, and ideal surface topographies. Compared to their non-ionized counterpart, the ionic HTMs have well-matched energy levels and smooth perovskite layers, resulting in higher short-circuit current densities. These experimental findings are corroborated by atomistic first principle electronic structure calculations of model perovskite systems. Furthermore, we conducted a comparative study of different ionizing counter anions for HTMs. The iodide-based ionic HTM, PMO-I, has a maximum efficiency of 20.46%, 1.71% higher than that of the non-ionic HTM, PMO (18.75%). We present ionization as a novel method for improving perovskite solar cells performance. The ionic HTMs give higher efficiency than the non-ionic derivative due to improved interfacial properties, defect passivation, and ideal surface topographies.