Studies on Energy Loss in Organic Solar Cells

Abstract

Organic solar cells have gained significant attention as a leading contender among next-generation photovoltaic devices due to their lightweight, flexibility, and compatibility with solution processes. Conventional organic solar cells have been fabricated using conjugated polymers as donors and fullerene as acceptors, achieving a power conversion efficiency of approximately 11-12%. However, the power conversion efficiency of these organic solar cells has been rapidly improved since the emergence of non-fullerene acceptors. In 2018, J. Yuan et al. developed a non-fullerene acceptor named Y6, characterized by a small energy offset, robust light absorption in the near-infrared region, and high carrier mobility. Subsequent research has extensively investigated Y6-based organic solar cells, and currently, these devices demonstrate a photovoltaic conversion efficiency exceeding 19%, marking a substantial advancement in the field. According to the Shockley-Queisser (SQ) limit, the theoretically highest power conversion efficiency achievable by organic solar cells is approximately 32%. However, the reported maximum power conversion efficiency of organic solar cells to date is around 60% level, relatively lower than other types of solar cells, owing to significant energy losses during operation, resulting in a lower open-circuit voltage. Additionally, factors determining the efficiency of solar cells, such as short-circuit current density and fill factor, have already surpassed 75% of the SQ limit. Therefore, addressing the issue of energy loss is a more effective strategy for increasing power conversion efficiency. In this study, we analyze the energy loss in organic solar cells to understand the underlying causes. Furthermore, we propose solutions to overcome these issues, aiming to devise strategies that approach the SQ limit and achieve higher power conversion efficiency. In PART I, the fundamentals and background of organic solar cells are introduced. The causes of energy loss in organic solar cells need to be examined from a different perspective compared to other types of solar cells, primarily due to the construction of the bulk-heterojunction structure in the active layer of organic solar cells. Concepts related to this were covered, along with the design of devices for energy loss measurement, including Fourier-transform photocurrent spectroscopy (FTPS), and methods for characterizing organic solar cell properties. In PART II, we explore the fundamental theories and practical applications related to energy loss in organic solar cells, starting from the detailed balance limit proposed by W. Shockley and H. J. Queisser in 1961. The energy loss model, based on the principle of detailed balance and the reciprocity theorem derived from the SQ limit, was introduced by the research team of T. Kirchartz and U. Rau in 2008. This model categorizes energy loss within solar cells into losses due to the SQ limit, radiative recombination, and non-radiative recombination. To apply this energy loss model to practical cases, Fourier-transform photocurrent spectroscopy (FTPS) was employed to measure the external quantum efficiency spectrum, along with the application of the reciprocity theorem alongside electroluminescence spectra. Three representative cases applying the energy loss model were selected and analyzed in this study: i) The impact of the chemical design of the active layer material on energy loss. ii) The influence of surface treatment processes on the ZnO electron transport layer on energy loss. iii) Energy loss based on the structure of the active layer: BHJ, bilayer, and pseudo-bilayer. This research, rooted in energy loss, is expected to provide deeper insights into the factors affecting the open-circuit voltage during the fabrication of organic solar cells. It aims to offer better ideas for material design, surface treatment, and fabrication processes. In PART III, we have addressed the investigation of charge transfer (CT) states in organic solar cells. CT states are inherent to the bulk-heterojunction structure. Excitons generated upon light absorption consume energy at the donor-acceptor interface, leading to the separation of electrons and holes, ultimately forming CT states. Traditionally, energy loss due to CT states has been significant in fullerene-based organic solar cells; however, recent reports suggest it is nearly negligible in non- fullerene-based counterparts. This discrepancy arises from the challenge of observing CT states in the low-energy region of the external quantum efficiency spectrum in non-fullerene-based organic solar cells. Nonetheless, our study utilized electroluminescence deconvolution methods to demonstrate the existence of CT states in non-fullerene-based organic solar cells and revealed that, in these systems, only the behavior of holes in the highest occupied molecular orbital (HOMO) region significantly influences CT states. To precisely measure CT states, we applied the Franck-Condon principle and Marcus's inverted region to external quantum efficiency and electroluminescence spectra. This approach was experimentally validated across nine types of bulk-heterojunctions formed by three donor and three acceptor combinations. Our study is the first to clearly elucidate the presence of CT states and their impact on energy loss in non-fullerene-based organic solar cells. As CT states play a crucial role in explaining the limits of energy loss in organic solar cells, understanding and evaluating the impact of CT states can contribute to the fabrication of higher-performance organic solar cells, especially those with high efficiency using non-fullerene acceptors.