Modelling Novel Photovoltaic Devices

Abstract

Perovskite solar cells exhibit vacancy-mediated halide motion, leading to reduced performance, inaccurate characterisation and accelerated degradation. To study vacancy motion, a model is built that solves the drift-diffusion equations for vacancies, electrons and holes across the perovskite absorber layer and both charge transport layers, all fully coupled with the Poisson equation. The model agrees with the prevalent theory that vacancies in the perovskite form Debye layers next to the interfaces with the transport layers, building regions of net charge that act to screen the electric field within the bulk of the absorber. The slow charging and discharging of the Debye layers over the course of a current-voltage scan leads to hysteresis in that measurement. The extent to which hysteresis is present depends on the alignment of the timescale of the scan with that of ion motion.

The effects of device properties and experimental protocols are mapped by sweeping over ranges of input parameters using the model. Heating is found to shift ion motion to faster timescales due to its Arrhenius nature, from which activation energies may be extracted. Experimentally obtained activation energies for various perovskites compare well to density functional theory predictions. Reducing vacancy density and recombination improves performance and reduces hysteresis but slowing the ions does not. Increasing the doping and permittivity of the transport layers improves performance, though may increase hysteresis.

Separately, Förster resonance energy transport of excitons in organic solar cells is simulated using a kinetic Monte Carlo model. Modelling shows that transfer between different materials provides a key pathway for exciton dissociation in both binary and ternary devices. The need for energy band offsets that limit photovoltage may be relaxed if an efficient pathway is present. In ternary devices, efficient transfer from the standard wide-bandgap absorber P3HT to the infra-red sensitiser DIBSq allows funnelling of excitons from the bulk of the P3HT to interfacial regions of DIBSq, improving exciton dissociation efficiency further.

Date of Award	3 Apr 2019
Original language	English
Awarding Institution	University of Bath
Supervisor	Petra Cameron (Supervisor) & Alison Walker (Supervisor)