Dr. Suhas Mahesh

Abstract:

Hybrid perovskite semiconductors have recently emerged as ideal candidates for multijunction photovoltaics due to their tunable bandgap. This thesis examines some key issues limiting the development of such next-generation photovoltaic technology.

A key problem is that mixed-halide devices fail to deliver the expected increase in quasi-fermi level splitting when the bromide fraction is increased. Here, spectroscopy along with detailed balance calculations is used to quantify the voltage loss resulting from halide segregation. Our results indicate that, contrary to popular belief, the primary loss mechanism in wide-bandgap cells is high trap state densities in the bulk and at the interfaces. We thus identify that focussing on maximising the initial radiative efficiency of the mixed halide films and devices is more important than attempting to suppress phase segregation.

The thesis also investigates loss mechanisms in wide-bandgap double perovskite Cs2AgBiBr6, which has shown promising stability and optoelectronic properties. Experiment and modelling are combined to reveal that short electron diffusion length underpins the poor short-circuit current seen in such devices. Our findings point to an important materials issue that must be attended to with priority to bring Cs2AgBiBr6 solar cells on track towards maturity.

Finally, optical modelling is used to analyse what is arguably the largest optical loss mechanism in all-perovskite tandems today: a large refractive index mismatch at the recombination layer. This loss is quantified, and the thesis demonstrates that replacing ITO with an intermediate refractive index material, such as Nb doped TiO2, can significantly enhance light incoupling into the low bandgap cell, boosting the PCE by upto 1% absolute. Interestingly, index matching also has the unintended consequence of making cell performance less sensitive to thickness variation during manufacturing.