Radiative Recombination in Organic Solar Cells
Final Report Abstract
We were able to investigate some details of radiative recombination in organic solar cells, and its relation to non-radiative losses. We could confirm that the radiative ideality factor is indeed one, whereas the overall ideality factor usually exceeds one. A detailed report on how to properly measure ideality factors in organic solar cells as a function of temperature, with results for different solar cell types, gave interesting insights into recombination. However, some of these experimental results could not be completely interpreted by a specific density of states as of yet. Therefore, we were not able to pinpoint the detailed density of states and microstructure, and how they relate to the overall recombination rate, during the course of this project. Using novel non-fullerene acceptor molecules, we could show that the offset between donor and acceptor energy levels may be extremely small without fundamentally impeding charge-transfer state dissociation. This changes the original paradigm that a visible offset between the energy levels of donor and acceptor and the CT state must exist. This lower offset allows higher energies of the CT state at a given main absorption onset and therefore a higher product of open-circuit voltage and short-circuit current compared to what was previously possible using fullerene based acceptor molecules. Our fundamental understanding of how non-fullerene acceptors enable CT state dissociation at negligible (not detectable) energy offsets is currently still developing and object of ongoing work. What is clear is that higher CT state energies are beneficial to suppress non-radiative recombination by increasing the number of vibrational modes that have to be excited in order to dissipate the energy of the CT state. However, the higher oscillator strength of non-fullerene acceptors (relative to fullerene based acceptors) also makes radiative decay faster. While an increased radiative transition dipole moment also accelerates non-radiative recombination according to the Generalized Mulliken Hush theory this relation is not linearly proportional and depends on the dipole moment of the CT state. Acceleration of decay dynamics is not detrimental at open circuit because here the ratio of radiative and non-radiative decay processes and the shape of the absorptance matters most strongly. However, at the maximum power point charge collection has to compete with recombination and faster recombination kinetics can impose stricter requirements on charge-carrier mobilities. Future work has to identify whether this acceleration of decay dynamics with increased oscillator strengths can be largely suppressed (e.g. by choice of molecules with appropriate CT state dipole moments) in order to benefit from the increased oscillator strength of non-fullerene acceptors without suffering from faster decay dynamics. In addition to the proposed work on organic solar cells, we also investigated recombination in metalhalide perovskite solar cells. Here, the heavier elements lead to much smaller phonon energies, which we predict based on the established theory of multiphonon recombination should lead to intrinsically slow recombination and a substantial sensitivity of lifetime on the position of the defect levels. This is the opposite situation as compared to molecular semiconductors used for organic solar cells where the energetic position of defects should be less important. Given the experimentally observed slow recombination of metal-halide perovskite films, we also investigated aspects such as photon recycling which are important close the radiative limit. We could show how photon recycling has to be accounted for in the analysis of transient photoluminescence data and in the estimation of limits to the efficiency and open-circuit voltage.
Publications
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Encounter-Limited Charge Carrier Recombination in Phase Separated Organic Semiconductor Blends. Phys. Rev. Lett. 2015, 114, 136602
M. C. Heiber, C. Baumbach, V. Dyakonov, and C. Deibel
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Beyond Bulk Lifetimes: Insights into Lead Halide Perovskite Films from Time-Resolved Photoluminescence. Phys. Rev. Applied 2016, 6 (4), 044017
Staub, F.; Hempel, H.; Hebig, J. C.; Mock, J.; Paetzold, U. W.; Rau, U.; Unold, T.; Kirchartz, T.
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Reduced voltage losses yield 10% efficient fullerene free organic solar cells with >1 V open circuit voltages. Energ. Environ. Sci. 2016, 9 (12), 3783-3793
Baran, D.; Kirchartz, T.; Wheeler, S.; Dimitrov, S.; Abdelsamie, M.; Gorman, J.; Ashraf, R. S.; Holliday, S.; Wadsworth, A.; Gasparini, N.; Kaienburg, P.; Yan, H.; Amassian, A.; Brabec, C. J.; Durrant, J. R.; McCulloch, I.
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Temperature Dependence of Ideality Factors in Organic Solar Cells and the Relation to Radiative Efficiency. Adv. Energy Mater 2016, 6, 1502230
K. Tvingstedt, C. Deibel
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Efficiency Potential of Photovoltaic Materials and Devices Unveiled by Detailed-Balance Analysis. Phys. Rev. Applied 2017, 7 (4), 044016
Rau, U.; Blank, B.; Müller, T. C. M.; Kirchartz, T.
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Impact of Tortuosity on Charge-Carrier Transport in Organic Bulk Heterojunction Blends. Phys. Rev. Applied 2017, 8, 054043
M. C. Heiber, K. Kister, A. Baumann, V. Dyakonov, C. Deibel, and T.-Q. Nguyen
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Figure of Merit Guiding Research on Organic Solar Cells. J. Phys. Chem. C 2018, 122 (11), 5829
Kirchartz, T.; Kaienburg, P.; Baran, D.
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Impact of Small Phonon Energies on the Charge-Carrier Lifetimes in Metal-Halide Perovskites, J. Phys. Chem. Lett. 2018, 9 (5), 939
Kirchartz, T.; Markvart, T.; Rau, U.; Egger, D.A.
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Quantitative Analysis of the Transient Photoluminescence of CH3NH3PbI3/PC61BM Heterojunctions by Numerical Simulations. Sust. Energy & Fuels, 2018
Krogmeier, B.; Staub, F.; Grabowski, D.; Rau, U.; Kirchartz, T.
