Polymer-Integrated Halide Perovskite Solar Cells and Photoelectrochemical Cells for Solar Energy Conversion

Abstract

Energy demand is skyrocketing, which has led to excessive use of non-renewable resources such as fossil fuels, which are major contributors to climate change and global warming. Therefore, to meet the increased energy demand while reducing carbon footprints, the implementation of renewable technologies for the production of clean energy is needed. In this context, the utilization of solar energy is one of the most appealing strategies for clean energy production. In recent years, halide perovskite solar cells and photoelectrochemical cells have emerged as potential candidates for harvesting solar energy into electricity or storing it directly as chemical fuels. This is mainly due to their excellent optoelectronic properties, along with simple, low-cost solution processing. Although high efficiencies have been achieved for both devices, their long-term stability remains one of the major issues that impedes their commercial deployment. In this thesis, the stability challenge of halide perovskite devices are addressed by implementing polymeric passivators to improve both extrinsic and intrinsic stability. Additionally, novel electrocatalysts have been designed and successfully integrated with these passivated perovskite devices to develop efficient and stable photoelectrochemical cells for green hydrogen production. In Part A, Chapter 2, we focus on simultaneously passivating both bulk and interfacial defects present in perovskite thin films using polystyrene as a bulk additive along with 4-fluorophenethylammonium iodide as the interfacial passivating agent. The addition of polystyrene modulates the perovskite crystallization kinetics, leading to the formation of larger grains with fewer grain boundaries. Meanwhile, 4-fluorophenethylammonium iodide passivates surface defects such as undercoordinated Pb²⁺ sites and vacancies. This combined passivation leads to perovskite solar cells with a high efficiency of 22.4%, along with significant improvement in stability, retaining 92% and 99% of their initial efficiency after 1008 h and 560 h under ISOS-D-1 and ISOS-D-2 conditions, respectively. Part A, Chapter 3 focuses on addressing thermal instability challenges in perovskite solar cells. Most high-efficiency halide perovskite compositions consist of either MA as an A-site cation or as an additive to stabilize the alpha phase of the perovskite. The addition of MA⁺ cations, even in small amounts, can lead to thermal instability due to their volatile nature. In this work, a dipolar polymeric passivator is developed by systematically modulating the electronic structure of polystyrene. A novel poly(pentafluoropolystyrene) passivator is identified, which consists of a distinct electronic distribution with highly diffused electron-rich and electron-deficient regions. These regions not only passivate both cationic and anionic defects in the bulk and at the surface of the perovskite but also exhibit strong interactions with MA⁺ species. This results in perovskite solar cells with efficiencies as high as 24%, along with significant improvement in thermal stability, retaining 95% of the initial efficiency after 2000 h under ISOS-D-2 conditions. Additionally, the storage and operational stability of the devices are greatly improved, retaining 97% and 95% of their initial efficiency after 3500 h and 500 h under ISOS-D-1 and ISOS-L-1 testing conditions, respectively. In Part B, Chapter 4, polystyrene bulk-passivated perovskite solar cells are integrated with bimetallic catalysts to fabricate perovskite photocathodes for green hydrogen production. Here, polystyrene is incorporated into the bulk of the perovskite to improve the intrinsic stability of the solar cells against ion migration. Additionally, using a facile self-assembly process of Au nanoparticles, a bimetallic Au–Pt–Ni catalyst is designed, which shows low overpotential and fast kinetics for the hydrogen evolution reaction in both acidic and basic conditions. The polymer-incorporated photocathodes show excellent performance with a half-cell solar-to-hydrogen (HC-STH) efficiency of 10.11%, along with significant improvement in stability, with T₈₀ values of 70 h (in H₂SO₄) and 78 h (in KOH), indicating improved device lifetime. In Part B, Chapter 5, highly efficient and stable bias-free water splitting is achieved by developing a polymer integrated perovskite photoelectrochemical cell. In this work, two critical challenges of PEC cells are addressed: (a) intrinsic instability of perovskite solar cells under external bias and (b) sluggish catalyst performance. Improved intrinsic stability against ion migration is achieved by implementing 4-fluoropolystyrene as both bulk and interfacial passivating agent. Meanwhile, an Au–Pt–Ni–PBA bifunctional catalyst is developed which shows low overpotential and fast kinetics for both hydrogen and oxygen evolution reactions. These combined effects lead to the fabrication of polymer-incorporated perovskite-based photocathodes with half-cell STH efficiencies of 18.6% and photoanodes with applied bias photon-to-current efficiency of 12.7%, along with excellent stability. Most impressively, the polymer-incorporated photoanodes maintain 95% of their initial performance after 270 h of operation under external bias. This improvement is primarily attributed to the incorporation of the polymer, which significantly reduces ion migration under applied bias conditions. Finally, the combined photoelectrodes enable unassisted water splitting with an STH efficiency of 13.56% and a T₉₀ of 190 h. Therefore, this work highlights the importance of improving the intrinsic stability of perovskites under external bias, along with efficient catalyst engineering.