Nonequilibrium site distribution governs charge-transfer electroluminescence at disordered organic heterointerfaces
The interface between electron-donating (D) and electron-accepting (A) materials in organic photovoltaic (OPV) devices is commonly probed by charge-transfer (CT) electroluminescence (EL) measurements to estimate the CT energy, which critically relates to device open-circuit voltage. It is generally...
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| Main Authors: | , , , , , , |
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| Format: | Article (Journal) |
| Language: | English |
| Published: |
November 19, 2019
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| In: |
Proceedings of the National Academy of Sciences of the United States of America
Year: 2019, Volume: 116, Issue: 47, Pages: 23416-23425 |
| ISSN: | 1091-6490 |
| DOI: | 10.1073/pnas.1908776116 |
| Online Access: | Verlag, Volltext: https://doi.org/10.1073/pnas.1908776116 Verlag, Volltext: https://www.pnas.org/content/116/47/23416 
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| Author Notes: | Armantas Melianas, Nikolaos Felekidis, Yuttapoom Puttisong, Stefan C.J. Meskers, Olle Inganäs, Weimin M. Chen, Martijn Kemerink |
| Summary: | The interface between electron-donating (D) and electron-accepting (A) materials in organic photovoltaic (OPV) devices is commonly probed by charge-transfer (CT) electroluminescence (EL) measurements to estimate the CT energy, which critically relates to device open-circuit voltage. It is generally assumed that during CT-EL injected charges recombine at close-to-equilibrium energies in their respective density of states (DOS). Here, we explicitly quantify that CT-EL instead originates from higher-energy DOS site distributions significantly above DOS equilibrium energies. To demonstrate this, we have developed a quantitative and experimentally calibrated model for CT-EL at organic D/A heterointerfaces, which simultaneously accounts for the charge transport physics in an energetically disordered DOS and the Franck-Condon broadening. The 0-0 CT-EL transition lineshape is numerically calculated using measured energetic disorder values as input to 3-dimensional kinetic Monte Carlo simulations. We account for vibrational CT-EL overtones by selectively measuring the dominant vibrational phonon-mode energy governing CT luminescence at the D/A interface using fluorescence line-narrowing spectroscopy. Our model numerically reproduces the measured CT-EL spectra and their bias dependence and reveals the higher-lying manifold of DOS sites responsible for CT-EL. Lowest-energy CT states are situated ∼180 to 570 meV below the 0-0 CT-EL transition, enabling photogenerated carrier thermalization to these low-lying DOS sites when the OPV device is operated as a solar cell rather than as a light-emitting diode. Nonequilibrium site distribution rationalizes the experimentally observed weak current-density dependence of CT-EL and poses fundamental questions on reciprocity relations relating light emission to photovoltaic action and regarding minimal attainable photovoltaic energy conversion losses in OPV devices. |
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| Item Description: | Gesehen am 25.11.2019 |
| Physical Description: | Online Resource |
| ISSN: | 1091-6490 |
| DOI: | 10.1073/pnas.1908776116 |