Pushing the FF of Non-Fullerene Acceptors Based Solar Cells Above 80%: Relating Order to Reduced Recombination to Device Performance

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Recent advances in organic bulk heterojunction solar cells, enabled in particular by the recent development of non-fullerene acceptors (NFAs), have led to large improvements in the power conversion efficiency, with current leading certified efficiencies of 17.6% for single-junction devices. As an important photovoltaic parameter, the fill factor (FF) of OSCs stands for the electric field dependence of free charge generation and extraction in competition to geminate and non-geminate losses. However, the FF values of such high efficiency cells still lie well below the Shockley-Queisser limit. Therefore, the overreaching goal of this project is to push the FF of highly-efficient organic solar cells by investigating and understanding the particular roles that crystallinity and miscibility of the donor and the acceptor in the photoactive layer have on the processes determining the JV-characteristics. Hereby, we will focus on the detailed investigation of the transport and recombination of free photogenerated carriers in relation to structural and energetic properties of the multicomponent blend layer. To this end, we will employ various techniques established at Potsdam University such as T-dependent space charge limited currents, resistive photovoltage, time-delayed collection field and photoinduced absorption to arrive at a conclusive picture of the charge carrier dynamics. These studies will be performed on a series of blends based on Y-type NFAs, which are chosen for their particular chemical and structural characteristics. Results from these experimental investigations will serve as input to numerical simulations to confirm the role of bulk and surface recombination and how they determine primarily the FF but also the VOC. By doing so, we aim at identifying the criteria which needs to be met to realize, simultaneously, efficient charge extraction and highly suppressed (non-Langevin) recombination, and to push the FF of these devices reliably above 80%.