Reduced Recombination and Capacitor-like Charge Buildup in an Organic Heterojunction
| Autor: | Ardalan Armin, Bryan Kudisch, David J. Jones, Valerie D. Mitchell, Paul B. Geraghty, Kyra N. Schwarz, Nasim Zarrabi, Gregory D. Scholes, Oskar J. Sandberg, Saeed Uz Zaman Khan, Jegadesan Subbiah, Trevor A. Smith, Barry P. Rand, Kenneth P. Ghiggino |
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| Rok vydání: | 2020 |
| Předmět: |
Chemistry
business.industry Heterojunction General Chemistry 010402 general chemistry 01 natural sciences Biochemistry Electric charge Acceptor Catalysis Polymer solar cell 0104 chemical sciences Active layer Colloid and Surface Chemistry Semiconductor Chemical physics Electric field Charge carrier business |
| Zdroj: | Journal of the American Chemical Society. 142:2562-2571 |
| ISSN: | 1520-5126 0002-7863 |
| Popis: | Organic photovoltaic (OPV) efficiencies continue to rise, raising their prospects for solar energy conversion. However, researchers have long considered how to suppress the loss of free carriers by recombination-poor diffusion and significant Coulombic attraction can cause electrons and holes to encounter each other at interfaces close to where they were photogenerated. Using femtosecond transient spectroscopies, we report the nanosecond grow-in of a large transient Stark effect, caused by nanoscale electric fields of ∼487 kV/cm between photogenerated free carriers in the device active layer. We find that particular morphologies of the active layer lead to an energetic cascade for charge carriers, suppressing pathways to recombination, which is ∼2000 times less than predicted by Langevin theory. This in turn leads to the buildup of electric charge in donor and acceptor domains-away from the interface-resistant to bimolecular recombination. Interestingly, this signal is only experimentally obvious in thick films due to the different scaling of electroabsorption and photoinduced absorption signals in transient absorption spectroscopy. Rather than inhibiting device performance, we show that devices up to 600 nm thick maintain efficiencies of >8% because domains can afford much higher carrier densities. These observations suggest that with particular nanoscale morphologies the bulk heterojunction can go beyond its established role in charge photogeneration and can act as a capacitor, where adjacent free charges are held away from the interface and can be protected from bimolecular recombination. |
| Databáze: | OpenAIRE |
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