Interfacial defect mitigation in bilayer organic photovoltaics via low-temperature annealing for enhanced indoor light performance

초록

Organic photovoltaics (OPVs) are promising for indoor energy harvesting due to their lightweight, and tunable absorption. However, bulk heterojunction (BHJ) structures suffer from severe recombination losses under lowintensity illumination, limiting device efficiency. In contrast, planar bilayer OPVs mitigate these losses by providing well-defined donor-acceptor interfaces and direct charge transport pathways. Here, bilayer OPVs were fabricated using a water-assisted lamination method, enabling the transfer of the donor polymer PTB7 onto a PC71BM acceptor layer without intermixing. To mitigate interfacial trap states caused by oxygen adsorption, we applied low-temperature annealing at 40 degrees C. Bilayer OPVs annealed for 1 min achieved the highest power conversion efficiency of 2.36 % under 1000 lux compact fluorescent lamp (CFL) illumination. Angle-resolved Xray photoelectron spectroscopy confirmed the removal of surface-bound oxygen, while photoluminescence and contact angle measurements revealed suppressed trap states and preserved pi-conjugation. Optical absorption and XPS depth-profile analyses further verified that the bilayer morphology remained intact without donor-acceptor interdiffusion. In addition, light-intensity-dependent open-circuit voltage analysis indicated reduced trap-assisted recombination in optimally annealed devices. These findings demonstrate that lowtemperature annealing is an effective strategy to enhance the efficiency of bilayer OPVs by eliminating interfacial adsorbed traps while maintaining structural stability under indoor illumination.

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