Structure-Property Relationships in Organic-Inorganic Halide Perovskite Solar Cells

Abstract

Over the past 100 years, there has been a complete disregard for the environmental impact of unregulated burning of fossil fuels. This has led to an increase in the greenhouse gas emissions, atmospheric CO2, global temperature, and sea levels, leading to an unprecedented challenge of climate change and air pollution for our planet. To counteract these changes and achieve net-zero emissions target set by the Paris climate agreement, decarbonizing electricity generation is of utmost importance. Solar energy is one of the most abundant sources of energy on Earth and harnessing it contributes to less than 1/10th the CO2 per kWh compared to fossil fuels. Therefore, solar energy will have to play a critical role in meeting the increasing energy demands. Promisingly, the cost of solar has drastically reduced by 10x over the last decade. To accelerate the solar deployment at a terra-watt scale (required to meet the growing energy demands); efficient, cheap and scalable solar energy is important. Halide perovskite solar cells serve as a complementary technology to existing commercial solar cell technology such as Si, CdTe, CIGS, etc. to meet the growing energy demands. Their cheap and scalable deposition along with high power conversion efficiencies make them an ideal candidate for wide scale deployment. Over the past 12 years, perovskite solar cells have demonstrated rapid progress in their power conversion efficiencies (PCE) with current record PCE at 25.5%. However, the PCEs are still further away from theoretically achievable efficiencies. This thesis investigates the reasons behind the current performance limitations. We identify that the microstructure has a significant effect on the performance of halide perovskite solar cells. Specifically, we identify non-radiative recombination occurring at the grain boundaries, within grains, and at the surfaces and interfaces as the primary performance limiters. Firstly, we investigate the conflicting literature on the nature of grain boundaries and their impact on the performance of the halide perovskite solar cells. This has primarily emerged due to misidentification of grains and grain boundaries in halide perovskites. Prior to our work, the most common method for identifying grains and grain boundaries in halide perovskite literature was scanning electron microscopy (SEM). Using electron back scatter diffraction (EBSD), we demonstrate that using SEM leads to misidentification and overestimation of grain boundaries. Using EBSD, we also reveal grain structure and internal misorientation that is otherwise hidden. We report the presence of local crystal misorientation which is consistent with the presence of local strain that varies from one grain to the next. Using co-aligned confocal optical photoluminescence (PL) microscopy images on the same halide perovskite samples used for EBSD, we show that PL is anticorrelated with the local grain orientation spread. This suggests that grains with higher degrees of crystalline orientational heterogeneity (local strain) exhibit more non-radiative recombination. We also find that larger grains tend to have larger grain orientation spread, consistent with higher degrees of strain and non-radiative recombination. Next, we investigate the chemical impact of large A-site cation, such as dimethyl ammonium (DMA), on the metal halide perovskites. Large A-site cations have demonstrated significant improvements in the performance and long-term operational stability of the resulting perovskite solar cells. However, the chemical impact of adding large A-site cations is largely unknown. We use time-of-flight secondary-ion mass spectrometry (TOF-SIMS) to demonstrate that DMA is indeed incorporated in the film. We also demonstrate that there is a higher concentration of DMA at the surface compared to bulk of the film. Furthermore, using a combination of PL microscopy and photo-induced force microscopy (PiFM), we also demonstrate that incorporating DMA in the film leads to higher local spectral and chemical heterogeneity, compared to the control films. Lastly, we show that the spatial chemical composition variations arise primarily due to DMA incorporation and not due to higher Cs concentration in the DMA films. These results illustrate that while DMA incorporation is beneficial to operational stability and device performance, there is still room for process optimization to achieve local compositional homogeneity and further improve the device performance and operational stability. Lastly, we control the surface recombination in the mixed-cation, mixed-halide perovskites, by passivating non-radiative defects with the polymerizable Lewis base (3-aminopropyl)trimethoxysilane (APTMS). We demonstrate average minority carrier lifetimes > 4 μs and high external photoluminescence quantum efficiencies (>20%, corresponding to ~97% of the maximum theoretical quasi-Fermi-level splitting) at low excitation fluence, a record for non-methylammonium based mixed-cation, mixed-halide perovskites. We extend the APTMS surface passivation to higher bandgap double cation (Formamidinium, Cesium) compositions (1.7 eV, 1.75 eV and 1.8 eV) relevant for multijunction solar cells. Lastly, we demonstrate that the average surface recombination velocity (SRV) decreases from ~1000 cm/s to ~10 cm/s post APTMS passivation. These results demonstrate that surface-mediated recombination is the primary non-radiative loss pathway in many MA-free mixed-cation mixed-halide films with a range of different bandgaps, which is a problem observed for a wide range of perovskite active layers and reactive electrical contacts. Our study also provides insights to develop passivating molecules that help reduce surface recombination in methylammonium-free mixed-cation mixed-halide films and indicates that surface passivation and contact engineering will enable near-theoretical device efficiencies with these materials.