Self-Organized Large-Scale Integration of Mesoscale-Ordered Heterojunctions for Process-Intensified Photovoltaics

Self-organization of large-area nanoscale patterns employing a single-step inexpensive process can be crucial in the fabrication of low-cost but high-performance devices. In the present study, we employ the spin dewetting of a conductive polymer to fabricate an array of micro-to-nanoscale ordered-heterojunctions (OHJ) to demonstrate the improvements in the key performance indicators of organic photovoltaic (OPV) devices in ambient conditions. For this purpose, the surface of a hole-collector polymer film [e.g., (poly-(2,3-dihydrothieno-1, 4-dioxin):poly-(styrene sulfonate) (PEDOT:PSS)], coated on a transparent conducting substrate, is decorated with physicochemical patterns of a self-assembled monolayer. Afterward, the electron donor polymer [e.g., poly (3-hexylthiophene-2,5-diyl) (P3HT)] is spin-dewetted into a large collection of digitized micro- and nanodroplets. A theoretical analysis of the governing equations with appropriate boundary conditions uncovers that the imbalance of centripetal, capillary, and van der Waals forces plays a major role in deciding the droplet spacing of the spin-dewetted morphologies. Further, simulations are performed to understand the effect of size and periodicity of the donor droplets inside the device architecture, which could lead to an enhanced current flow when compared with a planar heterojunction (PHJ) device composed of thin films. Subsequently, a detailed experimental analysis is performed to uncover the role of spin speed and the initial loading of the electron donor polymer into the solvent during spin casting on the size, periodicity, and density of the electron donor droplets on the hole-collector surface. Capping the optimally discretized P3HT droplet arrays with the electron-acceptor layer [e.g., ([6,6]-phenyl-C-61 butyric acid methyl ester (PCBM)] led to the formation of a highly corrugated donor-acceptor interface suitable for higher photon absorption, facile exciton generation, and improved exciton separation. The self-organized-large-scale-integration (SOLSI) of the spin-dewetted droplets at the charge-carrier donor-acceptor interface of the OPV-OHJ assemblage enables the enhancement by approximately 40% as compared to similar OPV-PHJ configurations. The enhanced photoconversion efficiency takes place via optimal separation of photon absorption and carrier collection pathways. The study uncovers the importance of developing high-density and large-area nanopatterns employing spin dewetting to develop process-intensified OPV-OHJ cells with improved performance at a lower fabrication cost.