Hexagonal Array Patterned PMMA Buffer Layer for Efficient Hole Transport and Tailored Interfacial Properties of FTO-Based Organic Solar Cells

Organic photovoltaics (OPVs) have attracted substantial attention due to their solution-based low-cost processability. However, challenges remain with the OPV module in terms of reducing the process cost and enhancing the stability. For example, more than half of the process cost can be consumed by the indium tin oxide (ITO) substrate. Further, the indium dopant can be easily diffused out of ITO, thereby deteriorating the device stability. Here, a hexagonal array patterned poly(methyl methacrylate) (PMMA) buffer layer is introduced between the fluorine doped tin oxide (FTO) substrate and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) so as to achieve ITO-free-OPVs with enhanced power conversion efficiency and long-term stability. The rough surface property of FTO is amended by the patterned PMMA layer via nanoimprinting lithography using a rigiflex imprinting mold with the hexagonal pillar pattern array. The surface morphology and film properties of the PMMA layer are tailored by the optimized O₂-plasma treatment. Consequently, the patterned PMMA/FTO ameliorates the morphology and interfacial properties of the PEDOT:PSS layer, which contributes to enhancing the device performance. Over 8% higher power conversion efficiency is achieved in comparison to OPVs with bare ITO. In addition, the patterned PMMA/FTO prevents the diffusion of heavy metal components, thereby higher stability is achieved in comparison to OPVs with bare ITO. [Figure not available: see fulltext.].