Processing-directed optimization of gold nanoparticles—poly(3-hexylthiophene) composites for high-performance hole transport layers in organic photovoltaics

Organic photovoltaics (OPVs) are attractive for their flexibility, light weight, and scalability. Poly(3-hexylthiophene) (P3HT) is widely used as a hole transport layer due to its stability and device compatibility, but its weak absorption and limited charge transfer reduce efficiency. Embedding gold nanoparticles (Au NPs) into P3HT can address these issues by exploiting localized surface plasmon resonance (LSPR) to enhance light–matter interactions. In this study, P3HT:Au NP thin films were spin coated at 1000–4000 rpm for 30 s, systematically varying P3HT and Au NP concentrations. Optimal films were formed at 1000 and 2000 rpm, where lower speeds yielded thicker, more absorptive films. The 75% Au NP in 25% P3HT showed the highest absorption; at 1000 rpm, it displayed a 42% increase over pristine P3HT due to plasmonic enhancement and greater thickness. The optical bandgap varied with composition and spin speed, increasing at higher speeds as thinner films reduced intermolecular interactions. Extinction coefficients indicated enhanced photon harvesting from NP-driven LSPR. Scanning electron microscopy revealed uniform Au NP dispersion at 50% and 75% loadings, with limited aggregation that may promote plasmonic coupling. Surface roughness rose with NP concentration, from 6 nm for pristine P3HT to 37 nm for pristine Au NPs, reflecting the granular morphology of the metallic phase and its role in light scattering. These findings demonstrate that Au NP embedding and spin-speed control serve as complementary levers for tuning optical and morphological properties, enabling scalable strategies for more efficient OPVs.