Understanding the Efficiency Limits of Perovskite Solar Cells

Abstract

This thesis focuses on the characterisation and modelling of perovskite solar cells to understand the origins of performance limitations and demonstrate potential pathways to higher efficiency devices. Perovskite photovoltaic technology demonstrated rapid efficiency increases from 3.8% to 25.2% in just one decade, making it one of the most promising photovoltaic technologies. Understanding the efficiency limits through optoelectrical characterisation and numerical simulation plays a vital role in unlocking the full potential of perovskite solar cells. This thesis begins with the relationship between device energy band alignment and key solar cell performance parameters, particularly open circuit voltage and fill factor. By combining device modeling with luminescence and cell JV measurements, carrier transport bottlenecks formed due to band misalignment at the perovskite-transport layer interface were studied in detail. The suns-photoluminescence technique, first applied to the characterisation of silicon cells, was investigated and applied to perovskite cells in this work. This technique allows for the reconstruction of sample / cell JV curves without any transport limitations. Such a technique can be an efficient and effective method to identify performance limitations in the device. Perovskite solar cells also feature mobile ionic species in the perovskite absorber, and the impact of these ions on cell steady state performance -in particular, to what extent and under what circumstances they may limit device performance - is not well understood yet. By employing an advanced numerical drift and diffusion model, the influence of these mobile ionic species in perovskite solar cells was studied in some detail. Amongst other findings, it is shown that the presence of a single mobile ionic species can lead to effective doping of the perovskite bulk, which is detrimental to cell performance by lowering the conductivity of one type of carrier. The presence of two mobile ionic species can effectively screen the device internal electric field at steady state, which transforms the carrier transport from a combination of drift and diffusion to a situation where transport occurs via diffusion only. The presence of mobile ions, therefore, has little impact in high quality perovskite films with high mobilities and low rates of non-radiative recombination. For devices with lower carrier mobilities or operating significantly below the radiative limit, the loss of the drift field can result in significant carrier transport and device efficiency losses. The results also demonstrate that a high concentration of dopants is required in perovskite devices for such doping to have a measurable impact on device performance, especially in the presence of mobile ions. The energy alignment of the transport layers, on the other hand, has a substantial impact on device performance and can in principle result in a doping-like effect, with generally adverse consequences for cell efficiency. The final section of this thesis presents a method of fabricating high quality perovskite films with large grains and high luminescence efficiency through a hot embossing technique. This technique allows devices to be fabricated in two separate half stacks and joined at the perovskite / perovskite interface seamlessly. The impact of processing temperature, pressure, duration, and precursor composition was investigated for MAPbI3 films. Temperature was found to be the primary factor dictating final film grain size and quality, while pressure and duration are less critical. Under conditions of high precursor concentration and excess PbI2, a transparent phase was formed during the lamination process, which demonstrates superior passivating properties. A complete device is also presented to demonstrate the feasibility of the hot embossing technique for device fabrication.