Narrow-Bandgap Perovskites: Optoelectronic Properties and Applications in All-Perovskite Multi-Junction Photovoltaics

Abstract

Meeting global energy demand while simultaneously reducing carbon emissions will require a significant increase in renewable energy capacity; solar photovoltaics (PV) account for the majority of projected renewable capacity additions worldwide. Lower-cost and more efficient alternatives to conventional Si PV are being explored as a means of reaching PV growth targets. Halide perovskite solar cells (PSCs) have reached and recently even slightly surpassed the efficiencies of conventional semiconducting materials, owing to solution processability, low cost and defect tolerance. Further, perovskite-based multi-junction solar cells promise theoretical efficiencies far greater than that of single-junction solar cells. Unfortunately, since the narrow-bandgap subcells within multi-junction devices necessitate a reduction in bandgap achieved via partial substitution of Pb with Sn, this introduces additional challenges associated with Sn oxidation and defect formation. In this thesis I study the optoelectronic properties of mixed Pb-Sn perovskites, as well as strategies to improve the performance and operating lifetime of all-perovskite multijunction photovoltaics. I begin by studying bulk and interface defects in Pb-Sn perovskites, examining their compositionally-dependent effects on perovskite optoelectronic quality as well as their formation and evolution under various environmental stressors. I find a defect-tolerant compositional space in Pb-Sn perovskites, and lever insights into dynamic defect behavior towards targeted defect passivation. This results in an improvement in single-junction Pb-Sn PSC solar power conversion efficiency (PCE) from 18.4% to 21.6%. Next, I pursue the integration of narrow-bandgap Pb-Sn perovskite active layers in tandem and triple-junction photovoltaics. I present advances in the interlayer structure through procedural optimization, as well as insights into optimal layer thicknesses for subcell current-matching in the 2-terminal configuration using a combination of optical transfer matrix modelling and experimental measurements. The result is record-voltage all-perovskite tandems achieving a certified PCE of 26.3%, as well as record-performance all-perovskite triple-junction solar cells with a PCE of 25.1% (23.9% certified). Finally, I characterize energetic losses at each interface within the narrow-bandgap subcell, developing a surface passivation treatment using long-chain carboxylic acid compounds to specifically target the perovskite/electron transport layer interface. The result is a significant reduction in interface recombination, leading to a PCE of 23.0% for single-junction Pb-Sn PSCs with a record-low open-circuit voltage deficit. This strategy is then applied in all-perovskite tandem solar cells, leading to a record open-circuit voltage of 2.21 V and efficiency of > 27%, surpassing the record performance of conventional Si solar cells (26.8%). By investigating the optoelectronic properties and defect management of narrow-bandgap perovskites, I demonstrate significant improvements in solar PCE of single- and multi-junction perovskite solar cells. These advances represent an important step towards the integration of low cost, high efficiency next generation solar materials in the quest to sustainably meet global energy demands.