PHYSICAL REVIEW APPLIED 10, 064012 (2018) Self-Organized Large-Scale Integration of Mesoscale-Ordered Heterojunctions for Process-Intensified Photovoltaics Siddharth Thakur, 1 Saptak Rarotra, 1 Mitradip Bhattacharjee, 2 Shirsendu Mitra, 1 Gayatri Natu, 2, * Tapas Kumar Mandal, 1,2 Ashok Kumar Dasmahapatra, 1,2, † and Dipankar Bandyopadhyay 1,2, ‡ 1 Department of Chemical Engineering, Indian Institute of Technology Guwahati, Assam 781039, India 2 Centre for Nanotechnology, Indian Institute of Technology Guwahati, Assam 781039, India (Received 19 March 2018; revised manuscript received 1 August 2018; published 5 December 2018) 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, cap- illary, 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 forma- tion 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 collec- tion 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. DOI: 10.1103/PhysRevApplied.10.064012 I. INTRODUCTION In recent times, the expansive optical, electronic, or mechanical properties of the dense and discrete nanos- tructures of diverse materials emerging from the quan- tum realm have been staging a paradigm shift in the performance of a variety of cutting-edge applications, which include portable memory devices, photovoltaic cells, photocatalysts, lab-on-a-chip instruments, point-of- care-testing devices, and fuel cells [1–6]. For example, * gayatri.natu@iitg.ac.in † akdm@iitg.ac.in ‡ dipban@iitg.ac.in the discretization of a single unit of traditional thin-film solar cells into an array of miniaturized cells can harvest solar energy more efficiently owing to the availability of a higher surface-to-volume ratio for the superior pho- ton absorption, electron-hole pair dissociation, and charge transport [7,8]. The merit of this proposition arises from the experimental evidence obtained for the energy har- vesters with micro- or nanoscale foot-print areas, which have routinely shown improved efficiency when com- pared with their macroscopic counterparts. Large-scale fabrication of such mesoscale units can be one of the alternative pathways to develop the next-generation high- performance solar cell technologies [8–10]. However, one of the major limiting factors in the feasibility of such 2331-7019/18/10(6)/064012(15) 064012-1 © 2018 American Physical Society