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

Subject Area Experimental Condensed Matter Physics
Term since 2021
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 450968074
 
In the domain of organic electronics, as elucidated in our initial proposal, the fill factor (FF) of organic solar cells (OSCs) has consistently fallen short of the projections delineated by the Shockley-Queisser theory. Remarkably, only a sparse selection of systems have managed to achieve FF values approaching or even exceeding 80%. Notably, the majority of such high FF cells exhibit only moderate charge carrier mobilities, around 〖10〗^(-3) (cm^2)/Vs, indicating that markedly reduced non-geminate recombination (NGR) constitutes the linchpin for high FFs. During the initial funding period, our focus primarily revolved around examining the interplay amongst structural order, energetic disorder, and NGR. Our investigations revealed that higher energetic disorder exacerbates NGR, a phenomenon explicated by the prevalence of low-lying charge-transfer (CT) states which undergo recombination at significantly accelerated rates in accordance with the energy gap law. Conversely, we observed a pronounced increase in the steady-state carrier density with declining temperature under open circuit voltage (VOC) conditions, suggestive of the uphill nature of CT reformation via free carrier recombination. The overarching objective of this renewal proposal is to furnish a comprehensive elucidation of all processes dictating the rate of free charge recombination in organic solar cells. To achieve this, we propose the utilization of a diverse array of steady-state and transient techniques, firmly established at the University of Potsdam and the Paul-Drude Institute für Festkörperforschung. These include transient and steady-state photoinduced absorption, as well as transient and steady-state photoluminescence, which will be harnessed to examine the occupancy and fate of all pertinent states. Our endeavours will encompass an exploration of the role played by the triplet state in charge recombination, the intricate dynamics governing CT reformation from free charges and their subsequent resplitting, an inquiry into the density of state distribution pertinent to photogenerated and dark-injected charges, and a discernment of the distinctive attributes characterizing high FF donor-acceptor blends. The insights gleaned from these meticulous measurements will furnish us with the requisite knowledge to formulate methodologies and recommendations conducive to the attenuation of recombination rates and the augmentation of FFs.
DFG Programme Research Grants
 
 

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