How to interpret absorption and fluorescence spectra of charge transfer states in an organic solar cell
| Autor: | Alexander Rudnick, Anna Köhler, Heinz Bässler, Frank-Julian Kahle |
|---|---|
| Rok vydání: | 2018 |
| Předmět: |
Photoluminescence
Materials science Organic solar cell Process Chemistry and Technology 02 engineering and technology 010402 general chemistry 021001 nanoscience & nanotechnology 01 natural sciences Spectral line 0104 chemical sciences Marcus theory Electron transfer Mechanics of Materials Atomic electron transition Ionization General Materials Science Electrical and Electronic Engineering Atomic physics 0210 nano-technology Spectroscopy |
| Zdroj: | Materials Horizons. 5:837-848 |
| ISSN: | 2051-6355 2051-6347 |
| DOI: | 10.1039/c8mh00564h |
| Popis: | The aim of the present work is to identify the appropriate framework for analyzing photoluminescence and photocurrent (EQE) spectra of charge transfer (CT) states in donor–acceptor blends used as active materials for organic solar cells. It was stimulated by the work of Vandewal et al. (J. Am. Chem. Soc., 2017, 139(4), 1699–1704) who analyzed EQE spectra of CT states of a series of blend systems in terms of Marcus theory assuming that, first, the spectral shape reflects the reorganization energy of the donor upon ionization and, second, that disorder effects are unimportant. To test this assumption we applied gated photoluminescence (PL) spectroscopy within a temperature range from 5 to 295 K combined with EQE as well as electroluminescence (EL) experiments on 1 : 1 Me-LPPP : PCBM blends by weight. We find that the PL spectra are virtually temperature independent and the temporal decay of the emission features a power law with an exponent close to −3/2 as Hong and Noolandi predicted for distributed geminately bound electron-holes pairs. The EL spectrum reveals a red-shift by 100 meV relative to the PL spectrum. The results are inconsistent with both Marcus’ electron transfer theory and the original Marcus–Levich–Jortner (MLJ) theory, and they prove that disorder effects are crucial. Both PL and EQE spectra can be rationalized in terms of the classic Franck–Condon picture of electronic transitions that couple to intra-molecular vibrations as well as low frequency modes of the donor–acceptor pair that forms the CT state. |
| Databáze: | OpenAIRE |
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