Organic photovoltaic (OPV) devices have many interfaces that play a crucial role in determining their efficiency, including the semiconductor-semiconductor interface between the donor and acceptor and the semiconductor-metal interface between the active layer and electrodes. The electronic properties of OPVs are dependent on morphological characteristics, including semicrystalline size and percolation. The bulk-heterojunction approach is commonly used, but it is challenging to predict the final film morphology from the solution. An ideal arrangement of the donor and acceptor has been suggested from simulations to ensure ideal performance¹. In this structural arrangement, charge transfer dynamics at the donor-acceptor interface and energy alignment at the semiconductor-metal interface are well thought for obtaining ideal device performance. Even having ideal donor-acceptor arrangement,, it is known that the electrode-active layer interface can result in non-radiative recombination losses. Some recent studies claim that inserting a tunnel thin layer may be required for proper extraction of charges, but there are limited studies on the influence of the structural order of the acceptor interlayer on energy losses. A comprehensive understanding of the ideal arrangement and strategies to mitigate energy losses are necessary to maximize the potential of OPVs.

The study investigated the impact of inserting pure fullerene phase on the performance of indoor conjugated polymer-fullerene based organic photovoltaic (OPV) devices. Elevated temperature and pressure were used to assemble the pure electron accepting layer on the cathode with the active layer on the anode². This approach provides a unique opportunity to study the interface between the solution-processed cathode PCBM and the polymer-PCBM based OPVs for indoor applications.

PCBM as interlayer has been already used in perovskite solar cells. Efforts have been made to create uniform and pinhole-free PCBM by using polyethyleneimine (PEI) and polystyrene (PS) to reduce defects and surface recombination at the perovskite-PCBM interlayer³. The use of PCBM:PEI or PCBM:PS in laminated OPV structure eliminated energy loss at the cathode-BHJ interface, but the mechanism is different from that in perovskite films. Confirmations through UPS showed that PCBM:PEI and PCBM:PS lowered the work function of the cathode, aligning energy levels better with the active layer and improving charge extraction. Studies using Atomic Force Microscopy and Neutron Reflectivity showed that the mixture of PEI and PCBM separated into two components with PEI on top, acting as a spacer that reduced the attractive image potential across the interface and aligned the Fermi level. Inserting insulating polymer PS matrix into PCBM reduced thermalization losses, changing the electrical properties of PCBM.

Our study provides a unique opportunity to explore the interface between a solution-processed cathode PCBM layer and BHJ in indoor conjugated polymer-fullerene based OPVs. This is significant as there is limited research on voltage loss in indoor OPVs and this area of organic electronics remains largely unexplored.