Chemical and morphological stability of Perovskite and organic solar cells

Abstract

The stability of perovskite and organic solar cells remains a major challenge to realise their potential. To overcome the numerous degradation mechanisms, it is important to understand the underlying causes. This thesis will present device and material characterisation on two areas: firstly, elucidating perovskite environmental stability and, secondly, the effects of PCBM dimerisation in organic solar cells.

The first results chapter investigates the environmental stability of methylammonium lead iodide. In this thesis, degradation under light and oxygen is found to be more detrimental than humidity. Injected charge (not photogenerated) also induce the degradation with oxygen; thus, all optoelectronic applications are vulnerable. Efficient electron extraction minimises superoxide generation and subsequent degradation. These results determine the dominant environmental instability and present pathways to curtail it.

The second results chapter explores halide tuning of methylammonium lead iodide-bromide (MAPI-Br) to optimise environmental stability. The stability of thin films and solar cells for all halide ratios is investigated under light and oxygen stress and humid nitrogen stress. Partial bromide substitution does not improve stability. However, for MAPBr, the crystal structure and optical properties are unchanged after 120 h of light and oxygen stress. It is concluded that iodide limits mixed halide stability and MAPBr has excellent environmental stability for high voltage solar cell applications.

The third results chapter investigates the morphological stability of polystyrene:PCBM blends under simultaneous light and thermal stress. Neutron reflectivity and atomic force microscopy are used to probe the vertical stratification and topography, respectively. It is found that PCBM dimerisation inhibits both surface relaxation and PCBM enrichment to the bottom PEDOT:PSS interface. Modelling the diffusion shows PCBM dimers to be effectively immobile over the timescales investigated. These results present a framework to understand polymer:fullerene morphological stability under operating conditions.

The final results chapter develops a complete model to describe PCBM dimerisation. Predicting the photo-dimerisation and thermal decomposition within any polymer matrix, under any light and temperature profile. During operating conditions, the PCBM dimer will dominate the reaction equilibrium; therefore, clarifying the role of dimers is critical. Evidence of distinct dimer populations is observed with differing chemical and electronic properties. DFT simulations conclude this is due to PCBM dimer conformations. These results can rationalise the double-edged effects of PCBM dimerisation previously reported.