Interface Engineering for Highly Stable Organic Photovoltaics

Abstract

Organic π-conjugated materials represent a major advance in semiconductor technology, offering distinct advantages for next-generation applications. These benefits— including solution-based fabrication for low-cost mass production, inherent lightweight characteristics for portable devices, and mechanical softness enabling flexible electronics— distinguish them significantly from conventional inorganic materials. However, the commercial viability of organic photovoltaics (OPVs) is currently impeded by several obstacles, among which achieving long-term operational stability remains the most critical challenge alongside high device performance. The transition to high-efficiency non-fullerene acceptors (NFAs) has further complicated stability issues by introducing complex chemical and morphological degradation pathways, particularly at the interfaces between the active layer and the transport layers. Therefore, in this thesis, advanced interface engineering strategies will be constructed for achieving highly stable and high-performance organic photovoltaic device applications. Chapter 1 Chapter 1 provides a comprehensive overview of the OPV field. The discussion spans the fundamentals, beginning with the chemical and electronic structure of π- conjugated semiconductors, covering basic photovoltaic principles (device architecture and operating mechanisms), and concluding with an analysis of the critical interfacial degradation challenges associated with NFA-based OPVs. Chapter 2 details novel interface engineering strategies applied to the electron transport layer (ETL) interface to address instability issues, with the added benefit of achieving enhanced device efficiency. In chapter 2-1, We first address the long-standing incompatibility of solution-processed bathocuproine (BCP) with NFA-based OPVs due to interfacial chemical interactions. By synthesizing amine-functionalized bathocuproine (BCPN), we successfully applied it as an ETL in NFA-based OPVs (PM6:Y6 and PM6:BTP- eC9), achieving high power conversion efficiencies (PCEs) of 15.5% and 16.7%, respectively. Devices using BCPN demonstrated substantially improved photostability (T80 ~140 and 100 h, respectively) compared to control devices using BCP (T80 ~1 h) under continuous photoirradiation (AM 1.5G, 100 mW·cm−2). This enhancement is attributed to the prevention of conjugated bond breakage in the NFAs and the preservation of the surface morphology of BCPN on the bulk-heterojunction (BHJ) layer. Furthermore, in chapter 2-2, we introduce a novel surface modification utilizing urea-functionalized polyethyleneimine (u-PEIs) for SnO₂ to address the energy-level mismatch and surface defects of bare SnO₂ ETLs. This bifunctional modification achieves work function modulation and surface-defect passivation, mitigating the energy barriers at the SnO₂/NFA interface. PM6:Y6-based OPVs with u-PEI- modified SnO₂ (SnO₂:u-PEI) ETLs achieved a remarkable PCE of 16%, significantly exceeding the bare device's 13.5%, along with outstanding photo- and thermal stability. In chapter 2-3, We examine the crucial role of interfacial deterioration under thermal stress, identifying that the chemical degradation of small molecule NFA (SM-NFA) on the ZnO interfacial layer is a dominant issue. The introduction of a highly polar and volatile molecule, 5-methyl-1H-benzotriazole (M-BT), into the BHJ is implemented to effectively passivate ZnO surface defects by forming a self-assembled layer. The optimized device exhibited excellent long-term thermal stability, maintaining above 85% of the initial efficiency (T85) after 1000 h at 85 °C under N2 atmosphere, vastly improving the control device's short T85 of less than 20 h. Chapter 3 describes versatile interface engineering strategies for OPVs to suppress recombination loss and enhance the device's performance, focusing on the hole transport layer (HTL) interface to achieve long-term thermal stability. Evaporated MoO₃ HTLs were found to degrade thermally via oxygen vacancy formation and species diffusion, causing performance loss. We present a solution-processable passivation using N-tert-butyl-α- phenylnitrone (PBN), whose polar nitrone (N⁺-O⁻) group forms coordinative bonds with under-coordinated Mo sites. This treatment stabilizes the MoO₃ work function, enhances oxygen vacancy formation energy by +1.67 eV, and suppresses MoO₃ diffusion. PBN- passivated PM6:L8-BO devices retained 100% efficiency (~18.2%) after 2,000 hours at 85°C, significantly outperforming control devices. The approach extends to other NFA systems, enabling robust inverted OPVs for practical use. Chapter 4 provides an overall summary of the work and the conclusions drawn in this thesis.