Origin of charge carrier recombination losses in perovskite-based solar cells revealed by interfacial energetics and surface photovoltage

Perovskite solar cells with power conversion efficiencies (PCE) above 25% have a great potential to deliver the solution for humanity’s need of cheap, renewable energy. To reach closer to the theoretical single-junction solar cell efficiency limit (~33%) further understanding of loss mechanisms present in such devices is essential. In this thesis, the origin of charge carrier recombination losses at different interfaces of perovskite-based devices are investigated by means of surface photovoltage (SPV), energy level measurements and other complementary characterisation techniques. Firstly, the origin of open-circuit voltage (Voc) in perovskite solar cells is studied. Strong correlation between thickness dependent Voc and SPV reveals the presence of interfacial hole accumulation in both conventional and inverted methylammonium lead iodide (MAPbI3) solar cells and its effect on device performance is discussed. The findings suggest application of low doping level hole transport layers with reduced interfacial recombination enabling a significantly enhanced Voc and PCE. Next, a light soaking effect which leads to increased Voc is investigated. Temperature-dependent SPV and drift-diffusion simulations are applied to show that slow ion migration is linked to perovskite layers with low defect density allowing high PCE and device stability. Charge carrier recombination losses are also studied in integrated perovskite/organic bulk heterojunction (BHJ) solar cells. The experimental results provide guidelines in terms of energetics for selection of photoactive materials to be applied in future high efficiency integrated cells. Finally, strategies to shift the energy levels of perovskite photoactive layers by compositional modification are presented. It is found that the incorporation of ~10% bromide into MAPbI3 results in shifting its energy levels shallower, leading to significantly enhanced PCE. Overall, the findings of this thesis provide important insights into the mechanism of charge carrier recombination losses in perovskite-based solar cells that can be used to further improve their device efficiencies.Open Acces