Interface Engineering of Transition Metal-based Hole Transport Materials for High-Performance Organic Photovoltaics

Abstract

Organic photovoltaics (OPVs) have garnered significant attention as a promising next-generation energy source owing to their lightweight, flexibility, and potential for solution processing. To realize high-performance OPVs, it is imperative to simultaneously optimize the molecular design of the photoactive layer for maximizing charge generation and engineer the device interfaces to ensure efficient charge extraction and long-term stability. This thesis presents a comprehensive study ranging from the design of non-fullerene acceptors to the interface engineering of transition metal-based hole transport materials (HTMs). First, a simplified molecular design strategy for Y6-based non-fullerene acceptors is proposed to reduce synthetic complexity while maintaining high photovoltaic performance. By removing the alkyl chains from the core and introducing octyl and fluorine substituents to the terminal units, two new acceptors, YBO-2O and YBO-FO, were synthesized. This structural modification upshifted the lowest unoccupied molecular orbital energy levels, leading to a high open-circuit voltage. Molecular dynamics simulations revealed that these derivatives preferentially form compact core-core and terminal-terminal dimeric interactions, which enhance electron mobility. Consequently, the YBO-FO-based device achieved a power conversion efficiency (PCE) of 15.01%, demonstrating that structural simplification can effectively facilitate industrial scalability without compromising efficiency. Second, the focus shifts to the interface engineering of inorganic HTMs to overcome the stability limitations of organic interlayers. A solution-based hydrogen peroxide treatment was developed to enhance the optoelectrical properties of combustion-processed NiOx. This treatment promoted the formation of Ni3+ species (NiOOH) on the surface, which effectively passivated surface defects and deepened the work function and valence band maximum. The modified NiOx film exhibited improved conductivity and favorable energy level alignment with the polymer donor. As a result, OPVs incorporating the treated NiOx achieved a maximum PCE of 15.81% with superior operational stability compared to conventional PEDOT:PSS-based devices. Finally, ruthenium chloride (RuCl3) is introduced as an efficient and robust HTM. The study systematically investigated the annealing temperature dependence, revealing that a low-temperature processed RuCl3 film possesses a deep work function and rapid charge extraction dynamics compared to its oxidized RuOx counterparts. Therefore, the RuCl3-based OPVs demonstrated an impressive PCE of 18.02%. Crucially, the device exhibited exceptional durability, maintained over 80% of its initial efficiency after prolonged light soaking and thermal stress. The successful application of RuCl3 underscored its potential as a cost-effective, stable and efficient anode interlayer for organic optoelectronic applications.