Precursor and Interface Engineering Enabling Efficient Inverted Perovskite Solar Cells and Tandem Devices

Abstract

Solar Photovoltaic (PV) technology stands out as one of the most competitive in today's market, owing to the abundant solar radiation reaching the earth's surface, cost-effectiveness of solar panels, and the simplicity of installation. A PV cell, commonly called a solar cell, a device that directly converts sunlight into electricity through photoelectric or photochemical effects. Perovskite solar cells (PSCs) are a promising photovoltaic technology on the laboratory scale, representing one of the third-generation solar cell technologies that have emerged after the first-generation crystalline silicon solar cells and the second-generation. Although their record power conversion efficiency (PCE) of single-junction cell and all-perovskite tandem cell and have boosted beyond 26% and 29%, respectively, there is still a huge room for improvement compared to the theoretical limit. The energy loss mainly stems from the non-radiative recombination loss from the bulky absorber and non-ideal interface in the whole device. Based on the existing device structure, when photons are absorbed by the perovskite solar cells, the behavior of charge carriers can be mainly classified into the following categories: (1) Hole and electron separation; (2) Diffusion/transport throughout the bulk; (3) Carrier extraction by ETL and HTL; (4) Carrier recombination from bulk to surface. Therefore, our work mainly focuses on precursor regulation and interface engineering to regulate the crystallization process and suppress the defect formation of perovskite films, along with the effective charge transfer and collection at both interfaces.

The chapter 2 is related to perovskite precursor engineering via microcrystals. Pre-synthesized 3D methylammonium lead chloride (MAPbCl₃) and 1D 2-aminobenzothiazole lead iodide (ABTPbI₃) microcrystals into self-drying perovskite precursors, which serve as seed crystals to promote nucleation and growth of FAPbI₃-based perovskites without requiring antisolvent extraction. The combined binary microcrystals facilitate the formation of a dense and pinhole-free perovskite film with a stable perovskite lattice and defect-healed grain boundaries, enabling efficient charge carrier transfer and reduced non-radiative recombination loss. As a result, the best-performing inverted architecture device exhibits a champion power conversion efficiency of 23.27% for small-area devices (0.09 cm2) and 21.52% for large-area devices (1.0 cm²). These values are among the highest efficiencies reported for antisolvent-free PSCs.

The chapter 3 is related to aqueous synthesized perovskite microcrystals as precursor materials. We present aqueous synthesized perovskite microcrystals as precursor materials for PSCs. Our approach enables kilogram-scale mass production and synthesizes formamidinium lead iodide (FAPbI₃) microcrystals with up to 99.996% purity, with an average value of 99.994 ± 0.0015%, from inexpensive, low-purity raw materials. The reduction in calcium ions, which made up the largest impurity in the aqueous solution, led to the greatest reduction in carrier trap states, and its deliberate introduction was shown to decrease device performance. With these purified precursors, we achieved a power conversion efficiency (PCE) of 25.6% (25.3% certified) in inverted PSCs and retained 94% of the initial PCE after 1000 hours of continuous simulated solar illumination at 50 °C.

The chapter 4 is related to interfacial engineering strategy for wide-bandgap perovskites. We report an interfacial engineering approach to reduce non-radiative recombination loss of wide-bandgap perovskite solar cells. Specifically, a 2D octyl-diammonium lead iodide interlayer is adopted onto the hole-transporting layer to induce the formation of an ultrathin quasi-2D perovskite that is close to the hole-selective interface. This approach not only accelerates hole transfer and retards hole accumulation but also reduces the trap density in the perovskite layer on top, thereby efficiently suppresses non-radiative recombination pathways. Consequently, the champion wide-bandgap device (≈1.66 eV) exhibits a power conversion efficiency (PCE) of 21.05% with a V_OC of 1.23 V, where the VOC deficit of 0.43 V is among the lowest values for inverted wide-bandgap PSCs.

The chapter 5 is related to precursor engineering for Sn-Pb mixed perovskites. To achieve a highly-efficient all-perovskite tandem device, we also focus on reduce energy loss in tin-lead mixed perovskite solar cell. We proposed a multifunctional strategy to introduce cysteine hydrochloride (CSH) into perovskite precursor to effectively inhibit Sn²⁺ oxidation owing to its strong reducibility. Besides, CSH plays a pivotal role in balancing the crystallization rate and manipulating grain growth of tin-lead perovskites owing to the strong binding with tin (II) iodide. As a result, the champion device exhibited a PCE of 23.09% for Sn-Pb PSCs with remarkable improvement in both open-circuit voltage and fill factor. When paired with a wide-bandgap perovskite subcell, a 28.16%-efficient all-perovskite tandem device is further demonstrated.

Date of Award	6 Sept 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Baomin Xu (External Supervisor) & Alex JEN (Supervisor)