Modification of NiOx hole transport layers with 4-bromobenzylphosphonic acid and its influence on the performance of lead halide perovskite solar cells

Lead halide perovskites have proved to be exceptionally efficient absorber materials for photovoltaics. Besides improving the properties of the perovskite absorbers, device engineering and the optimization of interfaces will be equally important to further the advancement of this emerging solar cell technology. Herein, we report a successful modification of the interface between the NiO_x hole transport layer and the perovskite absorber layer using 4-bromobenzylphosphonic acid based self-assembled monolayers leading to an improved photovoltaic performance. The modification of the NiO_x layer is carried out by dip coating which allows sufficient time for the self-assembly. The change in the surface free energy and the non-polar nature of the resulting surface is corroborated by contact angle measurements. X-ray photoelectron spectroscopy confirms the presence of phosphor and bromine on the NiO_x surface. Furthermore, the resultant solar cells reveal increased photovoltage. For typical devices without and with modification, the photovoltage improves from 0.978 V to 1.029 V. The champion V_OC observed was 1.099 V. The increment in photovoltage leads to improved power conversion efficiencies for the modified cells. The maximum power point tracking measurements of the devices show stable power output of the solar cells.