Interfacial Charge-selective Extraction Materials for Efficient and Robust Perovskite and Organic Solar Cells

Abstract

Perovskite solar cells (PSCs) and organic solar cells (OSCs) have attracted significant research attention over the past few decades due to their advantages in lightweight design, solution-processability, and tunable band gaps. Despite impressive progress in photoactive materials, the photovoltaic parameters and long-term operational stability depend strongly on interfacial materials, which play a crucial role in the selective extraction of charge carriers. In applying each PSC and OSC, simultaneous consideration of modulation of the work function, energy-level alignment, and adhesion to each photoactive layer and electrode is strongly recommended to achieve appropriate extraction of selective charge carriers at the interfacial contact. In this work, we approach the design of interfacial hole- and electron-selective extraction materials, focusing on simultaneously improving both efficiency and operational stability for device applications. First, a series of heteroatoms(O, S, and Se) was applied to the head group of self-assembled monolayer (SAM) molecules to promote enhanced interaction with the perovskite layer. The dipole orientation of all SAM series modifies the WF of the transparent anode, thereby enabling proper energy-level alignment for hole-selective extraction. Among the series of SAM molecules, Se substitution leads to improved hole transfer and suppression of trap-mediated recombination via strong passivating perovskite defect sites at the interfacial contact, compared with other heteroatoms (O and S), resulting in enhanced photovoltaic parameters and operational stability in p-i-n PSCs. Furthermore, the series of SAMs was extended to OSCs, with enhanced photovoltaic parameters, and the Se-substituted SAM molecule was confirmed to suppress trap-site generation and improve operational stability, demonstrating its universal applicability as a hole-selective interfacial layer. This confirms the extended applicability of hole-selective heteroatom-substituted SAMs for both PSCs and OSCs. As a second aspect, the perylene diimide-based polymer was designed to enhance operational stability in OSC applications, especially against thermal stress. Through hydrogen-bond-driven chain entanglement in the amorphous morphology of the new polymer electron extraction layer (EEL), the elevated glass transition temperature promotes the fixation of its morphology. Moreover, intrachain electron transport was controlled by the vinyl or ethynyl linker groups and molecular weight. Overall, a polymer EEL with a vinyl group linker and moderate molecular weight exhibited an incomparable enhancement among the series of EELs in photovoltaic parameters and thermal stability of OSCs, supported by a moderately smooth surface, well-aligned energy levels for electron-selective extraction, and suppression of thermal movement. Systematic studies on thermal stability reveal the portion of thermal degradation attributed solely to the EEL in the overall thermal degradation of devices. These findings can be integrated into the design of highly efficient, thermally stable EELs for universal application in optoelectronic devices. Collectively, this research primarily focused on the critical features of interfacial materials for selective charge-carrier extraction and operational stability in PSCs and OSCs. These results provide a step toward designing new charge-selective interfacial materials for PSCs and OSCs and toward further advancing these technologies as next-generation sustainable energy sources.