On the Impact of Electron Back Transfer on the Recombination Loss in Organic Photovoltaic Cells

Description

Organic photovoltaic (OPV) as one of the emerging PV technologies has drawn a lot of attentions during the last few decades. The low manufacturing cost and unique optical and mechanical properties are attractive in various applications in the PV market. However, OPVs have been suffered from large photovoltage loss (>0.7eV) which limited its efficiency. Recently, OPVs using non-fullerene (NF) molecule as acceptor have achieved remarkable progress in power conversion efficiency (PCE) over 13%. The light absorbing layer in OPV is typically a blend of organic electron-donating polymers and electron-accepting molecules. During the last two decades, numerous highly efficient polymers have been reported, while the acceptors are mainly based on fullerene derivatives. Recently, several promising NF based acceptors have been synthesized which unlocked several engineering limitations in conventional fullerene based devices. They have optical absorption window can be tailored to compromise with the polymer donors to cover a broader absorption spectrum. More importantly, their energy level can be widely tuned to align with the donor to facilitate efficient exciton dissociation and maximize the open-circuit voltage. It is believed that using NF acceptors will be a key strategic approach to tackle the large photovoltage loss issue in OPVs.Despite of the recent encouraging improvement in device efficiency, strikingly, several reported high efficiency NF based OPVs have noticeably small photovoltage loss that apparently contradicts to the previous understanding of the photovoltaic process in OPVs. One has reported that the energy offset between the polymer donor and NF acceptor only 0.1-0.2eV, compared to the general wisdom of the required excess energy of >0.3 eV for efficiency exciton dissociation. In addition, one has found the total photovoltage loss only 0.5eV, compared to the >0.7eV that previously considered the unavoidable energy and recombination losses. Consequently, the energy budget and the origin of photovoltaic loss have become hot debating topics in OPVs.In this proposed project we are aiming at: (1) Theoretically, to develop a model to investigate the impact of energy offset on charge-transfer efficiency across the organic donor/acceptor (D/A) heterointerfaces. (2) Experimentally, to use a set of comprehensive techniques to systematically probe the energetics and charge-recombination of a series of OPVs with different energy offsets. Therefore, the success of this project combining theoretical and experimental results would not only bring insight into the origin of the large photovoltage loss in OPVs, but also provide strategies for further improving the device efficiency by materials engineering.