Interfaces in metal halide perovskite photovoltaics
Keywords: tandem solar cells
photophysics
semiconductors
metal halide perovskites
photovoltaics
device physics
passivation

Metal halide perovskite (MHP) semiconductors are an extremely promising class of materials for photovoltaic (PV) applications. The outstanding optoelectronic properties of MHPs and their relative ease of manufacture have caused much excitement in the PV research and industrial communities alike. In turn, the solar-to-electrical power conversion efficiencies of PV devices based on MHPs have continued to soar over the past decade. Typically, and throughout this thesis, these are hybrid organic-inorganic semiconductors which combine many of the best aspects of both inorganic and organic semiconductors. Namely, their high charge-carrier mobility, long charge-carrier lifetimes and diffusion lengths are typical of inorganic semiconductors, whereas their low temperature processability and tunable optoelectronic properties are more typical of organic semiconductors.
It has become apparent that the interfaces between the MHP semiconductor and the organic semiconductors required as charge transport layers for full photovoltaic devices are the limiting factor constraining the performance of perovskite solar cells. These interfaces introduce additional, rapid non-radiative recombination pathways which are detrimental to the open-circuit voltage of PV devices. Therefore, the highest efficiency devices to-date are delivered via a process known as ‘passivation’ wherein an additional chemical treatment is applied between the perovskite and the charge transport layer to reduce non-radiative recombination at these interfaces. This process has pushed the performance of single junction perovskite solar cells towards their radiative limit. Regrettably, these passivation treatments can often introduce additional instabilities into the device structure and hence are not suitable for long-term applications.
More recently, as single-junction perovskite solar cells have neared their maximum attainable efficiency, an increasing focus has been dedicated to wide (> 1.7 eV) perovskites which are suitable for tandem solar cells. Tandem solar cells significantly raise the thermodynamic limit and so are able to deliver much higher power conversion efficiencies. However, the wide bandgap perovskites perform significantly poorly compared to their single junction counterparts. The reasons for this had been unclear and are studied in detail herein.
In this thesis, I focus on understanding and improving the interfaces present within MHP solar cells. In the first instance, a novel passivation strategy is implemented for single junction devices and its influence on the optoelectronic properties of the perovskite semiconductor and full devices is investigated in detail. Importantly, the benefit of this passivation strategy is maintained over the course of harsh (85 ◦C, full spectrum AM1.5 illumination) ageing, a major advance in the field.
Following these investigations, the technologically relevant (for stable tandem solar cells) methylammonium-free wide bandgap perovskite is investigated in detail. Compared to their lower bandgap (≤ 1.6 eV) counterparts, these types of perovskites suffer from higher levels of non-radiative losses constraining their efficiencies far below their thermodynamic potential. The energy losses in MA-free high-bromide-content wide bandgap perovskites are studied in detail. These perovskites are found to be characterised by large non-radiative recombination losses in the bulk material and especially that the interfaces with transport layers in solar cell devices strongly limit their open-circuit voltage. In particular, the interface with the hole transport layer is discovered to perform particularly poorly, in contrast to 1.6 eV bandgap MHPs which are generally limited by the interface with the electron transport layer. To overcome these losses, we incorporate and investigate the recombination mechanisms present with perovskites treated with the ionic additive 1-butyl-1-methylpiperidinium tetrafluoroborate ([BMP]+[BF4]−). We find that this additive not only improves the radiative efficiency of the bulk perovskite, but also reduces the non-radiative recombination at both the hole and electron transport layer interfaces of full photovoltaic devices. The recombination processes in full devices are studied in detail, and directions for further enhancements are suggested.
Finally, following the discovery that the hole transport layer is the limiting factor in these wide bandgap devices, I focus on improving this interface in detail. I create a novel hole transporting material developed by blending two commercially available polymers. For certain blend ratios, the PV device performance is significantly enhanced and the PV device physics of this system is investigated in detail. The structure and basic optoelectronic properties of the MHP semiconductor is found to be insensitive to the different polymer blends onto which it was deposited. The photophysical properties of these polymer blend systems themselves are investigated in detail. While substantial changes to the photophysics are observed, the reason for enhanced device performance remain elusive and ought to be the focus of future research.