(508b) Molecular Engineering Tailored Interface for Efficient and Stable Perovskite Solar Cells with Conducting Polymer

The long-term stability of perovskite solar cells (PSCs) still remains a challenge. In perovskite devices, both the perovskite layer and the charge transporting layer have to endure the long-time operation. One of the persistent obstacles is the stability of charge transporting layer and the interfaces between perovskite and charge transporting layer. Poly(triarylamine) (PTAA) is considered as one of the most commonly used polymeric hole transporting layer (HTL) for long-term thermal stability. However, the devices fabricated with PTAA are limited by their efficiencies, especially in devices with n-i-p architecture, which hinders them being recognized by a broader field. The strong hydrophobic property of PTAA impedes the adhesion to the perovskite surface, and therefore, inhibits efficient hole extraction.

Here we demonstrate that bulky conjugated ligands, specifically oligothiophene, which can be used to synthesize two-dimensional (2D) perovskites, are necessary to increase the adhesion at the interface between perovskite and PTAA. Our design results in a power conversion efficiency of over 23% on n-i-p structured PSCs. The fine-tuned energy level alignment for efficient and gradient hole extraction, significantly enhanced hole mobility and reduced interface defect density are responsible for the high efficiency. These effects subsequently leads to the possibility of benefitting from the extraordinary stability of PTAA without sacrificing the device performance. This strategy is generalized on other polymeric hole transporting layer as a solution for interface adhesion issue. Furthermore, the growth of surface 2D perovskite layers with these conjugated ligands can be further tailored through molecular engineering. We show that the solubility of the conjugated ligands and the structures of 2D perovskites can be managed via molecular configuration, which lead to the control over ligand coverage and the ligand packing on surface. We also discovered that the rational design of conjugated ligands enables control of the 2D perovskite growth kinetics, allowing preferential in-plane growth over out-of-plane growth which enhances the uniformity of 2D layers. The properly-tuned conjugated ligand endow the devices with a significantly enhanced thermal stability at 85°C and operational stability under one-sun illumination. This design strategy should guide the future work on application of polymeric HTL in perovskite devices via adhesive interface control.