Akash Dasgupta

Abstract:

Despite the incredible progress made, the highest efficiency perovskite solar cells are still restricted to small areas (<1 cm2). In large part, this stems from a poor understanding of the widespread spatial heterogeneity in devices. Conventional techniques to assess heterogeneities can be time consuming, operate only at microscopic length scales, and demand specialized equipment. We overcome these limitations by using luminescence imaging to reveal large, millimeter-scale heterogeneities in the inferred electronic properties. We determine spatially resolved maps of “charge collection quality”, measured using the ratio of photoluminescence intensity at open and short circuit. We apply these methods to quantify the inhomogeneities introduced by a wide range of transport layers, thereby ranking them by suitability for upscaling. We reveal that top-contacting transport layers are the dominant source of heterogeneity in the multilayer material stack. We suggest that this methodology can be used to accelerate the development of highly efficient, large-area modules, especially through high-throughput experimentation.

Abstract:

Scaling perovskite photovoltaics to module dimensions ( 2 m²) while maintaining a high efficiency over longer periods of time is critical for commercialisation and terawatt-scale deployment. In this thesis we cover 3 different projects which work towards furthering our understanding, in service of this goal.

In the first work, we introduce a rapid luminescence imaging technique that enables spatial mapping of key device parameters across large areas. Our approach uncovers significant heterogeneities in open-circuit voltage (V_OC ) and charge collection efficiency at millimeter length scales. By correlating the photoluminescence ratio under short- and open-circuit conditions with current losses, we provide a new tool for visualising J_SC losses, offering a valuable complement to traditional V_OC mapping techniques. A detailed evaluation of transport layers reveals that top-contacting layers, such as Spiro-OMeTAD, are major sources of processing-induced hetrogenity, while bottom-contacting layers contribute minimal heterogeneity. This distinction between top and bottom layers underscores key architectural differences between n-i-p and p-i-n devices.

Using a combination of luminescence imaging, drift-diffusion simulations, and Bayesian inference, we extend our analysis to the study of degradation mechanisms in perovskite solar cells. This integrated method allows us to differentiate between bulk- and surface-driven degradation under light and heat stress. Our spatially resolved analysis shows that degradation is highly heterogeneous, with bulk degradation manifesting in more stable regions, and surface degradation occurring at defect points and spreading with aging.

Finally, we explore the potential of Si-perovskite tandem solar cells, achieving power conversion efficiencies (PCEs) approaching 36% with moderate radiative efficiencies ( 10%) and up to 38% as η_rad approaches the radiative limit. Our findings suggest that improvements in radiative efficiency and ideality factor could soon push tandem devices beyond the 36% efficiency threshold.

Abstract:

Raw data and derived dielectric constants (*n*, *k*, α) for neat Pb and mixed PbSn perovskite compositions