Opto-electronic Characterisation of Thin Film Solar Cells with Confocal Microscopy

Abstract

The purpose of this work is to develop novel characterisation methods using confocal microscopy to investigate perovskite solar cell materials. It begins with a preliminary study of light management in silicon thin film solar cells for the purpose of demonstrating the capability of confocal scanning microscopy as a solar cell imaging tool. This is followed by several supporting studies to identify the stability of CH3NH3PbI3 perovskite materials when exposed to the tightly-focussed, intense laser illumination of a confocal microscope. These studies include: estimation of the laser-induced heating of CH3NH3PbI3 films during confocal measurements in order to avoid the effects of thermal-induced degradation during data collection; and measurements of films in different ambient atmospheres, showing that the film must be fully encapsulated by epoxy or kept in N2 environment to ensure stability during the measurements. PMMA coated perovskite film are shown to be protected from moisture-induced degraded, but they also exhibit an enhancement of photoluminescence (PL) signal under light exposure. During the PL measurements on CH3NH3PbI3 perovskite films, the phenomenon of light- and oxygen-induced PL enhancement was observed, which led to the exploration of trap properties and recombination kinetics of perovskite films. A combination of experimental PL measurements and numerical modelling is used to investigate the dramatic enhancement in PL following prolonged light exposure with timescales ranging from minutes to hours. The time and spatial dependence of the PL enhancement is directly observed by combining localised illumination with PL imaging, which can be explained by a combination of trap de-activation and photogenerated carrier diffusion away from the light-exposed area. Further experiments demonstrate that trap de-activation is reversible once the illumination is turned off. The observed time and spatial dependence of laser induced PL enhancement in CH3NH3PbI3 films is modelled, taking into account trap de-activation and carrier diffusion. Following this topic, a complete physical model of recombination kinetics is implemented to extract recombination coefficients and trap parameters of CH3NH3PbI3 and Cs0.07Rb0.03(FA0.85MA0.15)0.9Pb(I0.85Br0.15)3 perovskite films by fitting to excitation-dependent steady-state and transient PL measurements simultaneously. Sensitivity analysis shows that fitting a single model to the two different PL measurements provides improved accuracy in multiple-parameter fitting. A comparison of the fitted parameters of the two perovskite films suggests that the improved performance of mixed cation perovskites may result from less active trap states rather than from a lower density of traps. This analysis technique provides a simple, non-contact method to rapidly characterise the key trap properties of perovskite films. The general recombination model is further used to interpret carrier lifetimes in perovskite films. The origins of bi-exponential transient PL decay observed for many perovskite films are investigated by varying the recombination parameters in the recombination model. This analysis demonstrates that the fast and slow decays in the transient PL curve are dominated by trap-assisted recombination and radiative recombination, respectively. Simulations of the steady-state carrier lifetime as a function of carrier density over a wide excitation range demonstrate that radiative and Auger recombination coefficients could be extracted from experimental measurements at high excitation levels, and trap energy levels can be estimated at low excitation levels. The transient carrier lifetime extracted from the simulated PL decay curve matches the excitation-dependent minority carrier lifetime simulated for steady-state illumination conditions when the excitation level is sufficiently high. Therefore, the transient carrier lifetime extracted from time-resolved PL measurements can be used to estimate radiative and Auger recombination coefficients as long as the carrier density is known during the decay. This thesis contributes novel characterisation methods based on confocal microscopy for improved understanding of material properties and recombination kinetics of perovskites for photovoltaic applications. Theoretical modelling is performed to support the experimental findings and gain new insights into the detailed chemical and electronic properties of these materials. Show more