Development of Multifunctional Redox Mediators to Mitigate Halide Segregation in Wide-Bandgap Perovskites for Perovskite Based Tandem Solar Cells

Description

The emergence of metal halide perovskite-based solar cells (PSCs) shows great promise in ushering in the next generation PV technology due to their low material cost, streamlined manufacturing processes, and impressive power conversion efficiency (PCE) > 26%. They also exhibit good bandgap tunability, enabling them to be used as the front sub-cells in tandem solar cells (TSCs) to overcome the Shockley-Queisser PCE limit (~33%) on single-junction solar cells. Tuning perovskite bandgap for TSCs can be achieved by halide alloying among I-, Br-and Cl-. However, pronounced halide segregation in wide-bandgap (Eg>1.68eV) perovskites under illumination significantly deteriorates the long-term stability of derived devices. Various approaches, encompassing crystallization control, stress modulation, and interface engineering, have been explored to mitigate this issue. Nevertheless, the operational stability for these PSCs and their derived TSCs are still unsatisfactory. To enhance device stability, it is imperative to suppress iodide oxidation during device operation, which is the primary driving force induces halide segregation. Additionally, the presence of metallic lead (Pb0) in illuminated perovskites will introduce deep trap states to affect device performance. Therefore, it is critical to develop suitable redox mediators that can simultaneously reduce iodine and oxidize Pb0. Moreover, tailored passivation groups can also be incorporated into these redox mediators for defect passivation. This integrated approach will greatly enhance the efficiency and stability of wide-bandgap PSCs and derived TSCs. In this project, we will conduct the following tasks: (1) Develop several classes of novel organic redox mediators possessing suitable redox potentials for selective reducing iodine and oxidizing Pb0. (2) Functionalize these mediators with suitable counter ions that can help regulate energy alignment, interfacial binding, and defect passivation for both organic-inorganic hybrid and all-inorganic wide-bandgap perovskites. (3) Investigate the effectiveness of these multifunctional mediators in alleviating perovskite instability, including their structure-property relationships, half-redox reactions in perovskites, and halide segregation under various external stimuli. (4) Fabricate wide-bandgap PSCs (~1.80 eV) with high PCE (>21%) and superior long-term stability at the maximum power point (MPP) for 2000 h. (5) Integrate the developed high-performance wide-bandgap PSCs to fabricate perovskite-organic TSCs targeting PCE >27% for project demonstration. The outcomes generated from this project will help advance wide-bandgap perovskite based TSCs to achieve exceptional efficiency and stability, and train students to be the next generation leaders in advanced solar technologies. It will also help position Hong Kong as the frontier research hub in printable PSCs for affordable and sustainable clean energy.