Akash Dasgupta

Abstract:

Improved understanding of heterojunction interfaces has enabled multijunction photovoltaic devices to achieve power conversion efficiencies that exceed the detailed-balance limit for single-junctions. For wide-bandgap perovskites, however, the pronounced energy loss across the heterojunctions of the active and charge transport layers impedes multijunction devices from reaching their full efficiency potential. Here we find that for polycrystalline perovskite films with mixed-halide compositions, the crystal termination—a factor influencing the reactivity and density of surface sites—plays a crucial role in interfacial passivation for wide-bandgap perovskites. We demonstrate that by templating the growth of polycrystalline perovskite films toward a preferred (100) facet, we can reduce the density of deep-level trap states and enhance the binding of modification ligands. This leads to a much-improved heterojunction interface, resulting in open-circuit voltages of 1.38 V for 1.77-eV single-junction perovskite solar cells. In addition, monolithic all-perovskite double-junction solar cells achieve open-circuit voltage values of up to 2.22 V, with maximum power point tracking efficiencies reaching 28.6% and 27.7% at 0.25 and 1.0 cm2 cell areas, respectively, along with improved operational and thermal stability at 85 °C. This work provides universally applicable insights into the crystalline facet-favourable surface modification of perovskite films, advancing their performance in optoelectronic applications.

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.