Additives for building highly efficient lead-halide perovskite solar cells

Abstract

Organic-inorganic halide perovskite solar cells (PSCs) have emerged as a promising next-generation photovoltaic technology, thanks to their high power conversion efficiency (PCE), tunable semiconducting properties, relatively easy processing methods, and low-cost raw materials. Various approaches, including compositional engineering, interface engineering, and strain engineering, have been explored to improve the device performance. Among these methods, additive engineering remains the most common approach due to its convenience. Researchers have studied the behavior of perovskite precursor solutions and thoroughly investigated various additives to control crystallization and enhance the device efficiency and stability, resulting in many record efficiencies achieved. This thesis focuses on two new additives for inverted PSCs that demonstrate a significant impact on the device performance. Through sufficient and compelling experiments, we illustrate the mechanism of the improvements. Our research contributes to the ongoing efforts to develop more efficient and stable PSCs, which have great potential for commercialization. Triple-cation mixed perovskites have gained significant attention recently due to their exceptional optoelectronic properties and stability for perovskite solar cells. However, the introduction of cations with different sizes can cause internal strain in the perovskite lattice, which affects the quality of the resultant films. To address this, we have introduced a small amount of KBF4 as an additive to improve the quality of triple-cation mixed perovskite thin films. Our study shows that KBF4 enhances the crystallinity of the perovskite thin films and reduces internal strain. Additionally, KBF4 passivates defects in perovskite grains, resulting in longer carrier lifetimes. Consequently, the devices exhibit improved fill factor, enhanced efficiency, and better stability. Under optimum fabrication conditions, triple-cation mixed perovskite solar cells with an inverted structure demonstrate a power conversion efficiency over 23% and excellent stability under different conditions. Additive engineering has significantly contributed to the remarkable efficiency achieved by perovskite solar cells (PSCs). Numerous additives have been introduced to enhance the device performance, just like the first work, where we use KBF4 as modulator to release strain. Based on this additive, we further studied the combinational effect of two ammonium salts, PEAI (2-phenylethanamine iodide) and GlyHCl (glycine hydrochloride), to improve the performance of inverted PSCs. Our findings show that the addition of Gly+ can further enlarges the grain size and reduces defect density after the perovskite was already passivated by PEA+. This co-additive strategy helped us achieve the highest efficiency of 23.8% on inverted PSCs. In summary, we employed KBF4 as an additive to improve the performance of triple-cation mixed perovskite thin films. Our study shows that KBF4 enhances the crystallinity of the perovskite thin films and reduces internal strain, resulting in improved device efficiency and stability. In the second work, we investigated the combinational effect between PEAI and GlyHCl. These two works demonstrate methods and mechanisms to obtain high-performance inverted PSCs and triple-cation mixed perovskite thin films. The application of these additives together gave the best efficiency of 23.8%. Overall, our research provides insights into designing and selecting the additives for building efficient and stable PSCs.