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Title: | On the critical competition between singlet exciton decay and free charge generation in non-fullerene based organic solar cells with low energetic offsets | Authors: | Pranav, Manasi Shukla, Atul Moser, David Rumeney, Julia Liu, Wenlan Wang , Rong Sun, Bowen SMEETS, Sander Tokmoldin, Nurlan Cao, Yonglin He, Guorui Beitz, Thorben Jaiser, Frank Hultzsch, Thomas Shoaee, Safa MAES, Wouter Lueer, Larry Brabec, Christoph VANDEWAL, Koen Andrienko, Denis Ludwigs, Sabine Neher, Dieter |
Issue Date: | 2024 | Publisher: | ROYAL SOC CHEMISTRY | Source: | Energy & Environmental Science (print), | Status: | Early view | Abstract: | Reducing voltage losses while maintaining high photocurrents is the holy grail of current research on non-fullerene acceptor (NFA) based organic solar cell. Recent focus lies in understanding the various fundamental mechanisms in organic blends with minimal energy offsets - particularly the relationship between ionization energy offset (Delta IE) and free charge generation. Here, we quantitatively probe this relationship in multiple NFA-based blends by mixing Y-series NFAs with PM6 of different molecular weights, covering a broad power conversion efficiency (PCE) range: from 15% down to 1%. Spectroelectrochemistry reveals that a Delta IE of more than 0.3 eV is necessary for efficient photocurrent generation. Bias-dependent time-delayed collection experiments reveal a very pronounced field-dependence of free charge generation for small Delta IE blends, which is mirrored by a strong and simultaneous field-dependence of the quantified photoluminescence from the NFA local singlet exciton (LE). We find that the decay of singlet excitons is the primary competition to free charge generation in low-offset NFA-based organic solar cells, with neither noticeable losses from charge-transfer (CT) decay nor evidence for LE-CT hybridization. In agreement with this conclusion, transient absorption spectroscopy consistently reveals that a smaller Delta IE slows the NFA exciton dissociation into free charges, albeit restorable by an electric field. Our experimental data align with Marcus theory calculations, supported by density functional theory simulations, for zero-field free charge generation and exciton decay efficiencies. We conclude that efficient photocurrent generation generally requires that the CT state is located below the LE, but that this restriction is lifted in systems with a small reorganization energy for charge transfer. A quantitative study, supported by Marcus theory and DFT, showing why the fate of singlet excitons is the pivot to free charge generation in low-energy offset organic solar cells. | Notes: | Neher, D (corresponding author), Univ Potsdam, Inst Phys & Astron, Karl Liebknecht Str 24-25, D-14476 Potsdam, Germany. neher@uni-potsdam.de |
Document URI: | http://hdl.handle.net/1942/43689 | ISSN: | 1754-5692 | e-ISSN: | 1754-5706 | DOI: | 10.1039/d4ee01409j | ISI #: | 001290353900001 | Rights: | The Royal Society of Chemistry 2024. Open Access Article. Published on 30 July 2024. Downloaded on 9/6/2024 1:33:54 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence | Category: | A1 | Type: | Journal Contribution |
Appears in Collections: | Research publications |
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On the critical competition between singlet exciton decay.pdf | Early view | 4.64 MB | Adobe PDF | View/Open |
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