| 1. |
- Jasiunas, Rokas, et al.
(author)
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Dead Ends Limit Charge Carrier Extraction from All-Polymer Bulk Heterojunction Solar Cells
- 2018
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In: Advanced Electronic Materials. - : WILEY. - 2199-160X. ; 4:8
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Journal article (peer-reviewed)abstract
- Extraction of photocreated charge carriers from a prototypical all-polymer organic solar cell is investigated by combining transient photocurrent and time-delayed collection field experiments with numerical simulations. It is found that extraction is significantly hampered by charges getting trapped in spatial traps that are tentatively attributed to dead ends in the intermixed polymer networkin photovoltaic devices based on the same donor polymer and a fullerene acceptor this effect is much weaker. The slow-down in charge extraction leads to enhanced recombination and associated performance losses. These effects are observed in addition to the dispersive behavior that is characteristic of charge motion in energetically disordered media. Upon annealing the effects of spatial traps diminish, rationalizing the doubling in device power conversion efficiency after annealing.
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| 2. |
- Jasiunas, Rokas, et al.
(author)
-
From Generation to Extraction: A Time-Resolved Investigation of Photophysical Processes in Non-fullerene Organic Solar Cells
- 2020
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In: The Journal of Physical Chemistry C. - : AMER CHEMICAL SOC. - 1932-7447 .- 1932-7455. ; 124:39, s. 21283-21292
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Journal article (peer-reviewed)abstract
- Non-fullerene organic solar cells (NFOSCs) demonstrate record efficiencies exceeding 16%. In comparison with cells with the fullerene-based acceptor, the NFOSCs benefit from a longer wavelength absorption, which leads to a small highest occupied molecular orbital (HOMO) level offset. Here, we use several advanced transient investigation techniques, covering timescale from sub-ps to mu s, to address all sequence of processes starting from photoexcitation of donors or acceptors to carrier extraction in several NFOSCs and cells with phenyl-C71-butyric acid methyl ester (PCBM). Though small offsets result in higher open-circuit voltage, we show that at the same time, it limits cell performance because of inefficient hole transfer from excited acceptors to donors and enhanced geminate recombination. We demonstrate that 100 meV HOMO level offset and proper acceptor molecule structures are sufficient for efficient hole transfer (>70%) and for reduction of the geminate recombination losses down to about 20%. Subsequent extraction of unbound charge carriers in all NFOSCs is slightly faster than in cells with PCBM, while non-geminate carrier recombination causing additional losses is slightly slower in the best performing NFOSCs than in cells with PCBM.
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| 3. |
- Jasiunas, Rokas, et al.
(author)
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Interplay between charge separation and hole back transfer determines the efficiency of non-fullerene organic solar cells with low energy level offset
- 2022
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In: Organic electronics. - : ELSEVIER. - 1566-1199 .- 1878-5530. ; 108
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Journal article (peer-reviewed)abstract
- Organic bulk heterojunction solar cells with electron acceptors based on small donor-acceptor type molecules show record efficiencies mainly due to their long wavelength absorption, which enables efficient harvesting of solar light and, thus, causes high current density. Meanwhile, relative positions of HOMO and LUMO levels of donor and acceptor materials determine the open circuit voltage. Here, we apply ultrafast transient absorption and transient luminescence techniques together with specially-designed modelling technique to address charge carrier generation and recombination dynamics in detail. We demonstrate the importance of careful adjustment of the HOMO and LUMO levels, as their positions determine formation and recombination rates of interfacial charge transfer (CT) states. An insufficient donor and acceptor LUMO level offset, lower than -300 meV, leads to slow and inefficient CT state formation, while an offset of the HOMO level below -100 meV leads to fast CT state recombination, which we attribute to the back transfer of a hole from the donor to the acceptor.
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| 4. |
- Jasiunas, Rokas, et al.
(author)
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Thermally Activated Reverse Electron Transfer Limits Carrier Generation Efficiency in PM6:Y6 Non-Fullerene Organic Solar Cells
- 2022
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In: Solar RRL. - : WILEY-V C H VERLAG GMBH. - 2367-198X. ; 6:6
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Journal article (peer-reviewed)abstract
- Transient absorption and time-resolved fluorescence measurements in a wide temperature range are used to investigate the mechanism of charge carrier generation in efficient organic solar cells based on a PM6:Y6 donor-acceptor blend. The generation mechanisms differ significantly under excitation of a donor or acceptor. The investigations reveal a temperature-dependent interplay between the formation of interfacial charge transfer (CT) states and intra-moiety CT states of the acceptor, their separation into free charge carriers and carrier recombination. The efficient charge carrier generation is ensured by the carrier separation over a small energy barrier, which is easily surmountable at room temperature. However, the overall yield of charge carrier generation at room temperature is reduced by the recombination of charge carriers due to the thermally activated back transfer of electrons from the acceptor to the donor via the highest occupied molecular orbit (HOMO) levels, which is enabled by the small energy offset between HOMO levels of the donor and the acceptor.
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