1 ENHANCEMENT OF PHOTOVOLTAIC PERFORMANCE IN P3HT:PCBM- BASED ORGANIC SOLAR CELLS Rajesh Kumar Sharma 1  , Priya Singh 2 , Anil Kumar Gupta 3 , Neha Patel 2 1 Department of Physics, Indian Institute of Technology Bombay, Mumbai, India 2 Centre for Nanoscience and Nanotechnology, University of Hyderabad, Hyderabad, India 3 Department of Materials Science and Metallurgical Engineering, Indian Institute of Technology Madras, Chennai, India  rajesh.sharma@iitb.ac.in Abstract. Organic photovoltaics (OPVs) have garnered significant attention due to their potential for low-cost, lightweight, and flexible solar energy solutions. Among various organic materials, poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blends have been widely studied due to their favorable electronic properties and compatibility. This study focuses on improving the photovoltaic parameters of P3HT:PCBM-based solar cells through various optimization techniques. By modifying the active layer morphology, enhancing the donor-acceptor interface, and optimizing processing conditions, significant improvements in power conversion efficiency (PCE), short-circuit current (Jsc), and open-circuit voltage (Voc) were achieved. The findings provide insights into the potential pathways for further enhancing the performance of organic solar cells. Keywords: Organic photovoltaics (OPVs), P3HT, PCBM, power conversion efficiency (PCE), active layer morphology, donor-acceptor interface, thermal annealing, solvent engineering, charge transport, organic solar cells 1. Introduction Organic photovoltaics (OPVs) represent an innovative and promising technology for solar energy conversion, offering numerous advantages over traditional inorganic solar cells. 1–7 These advantages include potential for low-cost production, lightweight and flexible device structures, and the ability to fabricate large-area devices using roll-to-roll processing techniques. Among the various material systems explored for OPVs, the blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) has emerged as one of the most widely studied due to its relatively high power conversion efficiency (PCE), ease of fabrication, and well-balanced charge transport properties. 8–10 The P3HT:PCBM blend serves as a model system for understanding the fundamental processes governing the performance of bulk heterojunction (BHJ) solar cells. In a typical P3HT:PCBM-based OPV, P3HT acts as the electron donor, while PCBM functions as the electron acceptor. 11 When light is absorbed by the active layer, excitons (electron-hole pairs) are generated. These excitons must then diffuse to the donor-acceptor interface where charge separation occurs, leading to the generation of free charge carriers (electrons and holes). The efficiency of this process is highly dependent on the morphology of the active layer, which influences exciton diffusion, charge separation, and charge transport to the electrodes. 12–16 Despite the significant progress made in the development of P3HT:PCBM solar cells, their performance still lags behind that of inorganic counterparts. The primary challenge lies in achieving an optimal morphology that promotes efficient exciton dissociation and charge transport while minimizing recombination losses. Several strategies have been employed to address these issues, including thermal annealing, solvent engineering, and the incorporation of interfacial layers. These approaches aim to enhance the crystallinity of P3HT, improve phase separation between P3HT and PCBM, and optimize the donor-acceptor interface. Thermal annealing is a well-established technique to improve the morphology of the P3HT:PCBM blend. By controlling the annealing temperature and duration, the crystallinity of P3HT can be enhanced, leading to improved charge carrier mobility. Additionally, thermal annealing promotes phase separation between P3HT and PCBM, forming well-defined domains that facilitate efficient exciton dissociation and charge transport. However, excessive annealing can lead to large phase-separated domains, which may increase the likelihood of charge recombination and reduce device performance. Therefore, optimizing the annealing conditions is crucial for achieving the best possible device performance.