Performance Limiting Factors in Ultra-Low-Bandgap PTB7-Th:COTIC-4F based Organic Solar Cells Guorui He 1, Ben Mayberry 1 , Manasi Pranav 1, 2 , Mohammad S. Shadabroo 2 , Bowen Sun 2 , Yonglin Cao 2, 3 , Safa Shoaee 2 , Martin Stolterfoht 1 , Dieter Neher 1* , Felix Lang 1* 1 Institute of Physics and Astronomy, University of Potsdam, Germany 2 Disordered Semiconductor Optoelectronics, University of Potsdam, Germany 3 University of Technology, China Understanding the performance limiting factors of organic solar cells (OSCs) with very small optical bandgaps is crucial for the development of novel tandem photovoltaics (PVs), such as OSC/OSC or perovskite/OSC tandems. For example, by combining a low-bandgap OSC ~ 1.1 eV with a high-bandgap absorber ~ 1.7 eV into a tandem solar cell, a power conversion efficiency (PCE) as high as 31.0% can be theoretically achieved, according to the detailed balance (DB) limit. In this study, we investigate the loss mechanisms of ultra-low-bandgap OSCs based on the blend of the donor polymer PTB7-Th with the non-fullerene acceptor (NFA) COTIC-4F, with a bandgap of 1.15 eV, a promising candidate for tandem PV. In a conventional device structure, we reach a record PCE value of 8.55% with an enhanced short-circuit current density ( J SC ) and fill factor (FF) compared to previous records that relied on an inverted device geometry. While setting a record for this system, our PCE metrics still fall behind higher-bandgap OSC systems. To guide further improvements, we investigate the various loss mechanisms and recombination processes using photoluminescence (PL) measurements, bias-dependent time-delayed collection field (TDCF) measurements, bias-assisted charge extraction (BACE), fluence-dependent photoinduced absorption (PIA), as well as light intensity-dependent open-circuit voltage ( V OC ). Complemented with optical and electrical device simulations, we show that J SC loss can be largely attributed to inefficient exciton dissociation in combination with geminate recombination of the charge transfer state. Further, we identify the fairly high bimolecular recombination coefficient as the main reason for the poor performance, while surface recombination is shown to mainly affect V OC . Finally, our simulations show that the simultaneous reduction of bimolecular recombination (e.g. by ternary blends), surface recombination (e.g. by self-assembled monolayers), exciton and charge transfer recombination (e.g. by vapor annealing) would enable efficiencies of >15% in the PTB7-Th:COTIC-4F system.
P20
© The Author(s), 2023
Made with FlippingBook Learn more on our blog