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Geminate recombination in organic photovoltaic blend PCDTBT/PC <sub/>71</sub> BM studied by out-of-phase electron spin echo spectroscopy
摘要： The key process in organic solar cell operation is charge separation under light illumination. Due to the low dielectric constant of organic materials, the Coulomb attraction energy within the interfacial charge-transfer state (CTS) is larger than the thermal energy. Understanding the mechanism of charge separation at the organic donor/acceptor interface still remains a challenge and requires knowledge of the CTS temporal evolution. To address this problem, the CTS in the benchmark photovoltaic blend PCDTBT/PC71BM was studied by the out-of-phase Electron Spin Echo (ESE). The protocol for determining the CTS geminate recombination rate for certain electron-hole distances was developed. Simulating the out-of-phase ESE trace for the CTS in the PCDTBT/PC71BM blend allows precise determination of the electron-hole distance distribution function and its evolution with the increase in the delay after the laser flash. Distances of charge separation up to 6 nm were detected upon thermalization at a temperature of 20 K. Assuming the exponential decay of the recombination rate, the attenuation factor β = 0.08 ??1 is estimated for the PCDTBT/PC71BM blend. Such a low attenuation factor is probably caused by a high degree of hole delocalization along the PCDTBT chain.
关键词： PCDTBT/PC71BM blend,electron spin echo,geminate recombination,organic solar cells,charge transfer state
更新于2025-09-23 15:19:57
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Reversible intermolecular-coupled-intramolecular (RICI) proton transfer occurring on the reaction-radius <i>a</i> of 2-naphthol-6,8-disulfonate photoacid
摘要： Steady-state and time-resolved fluorescence techniques were employed to study the excited-state proton transfer (ESPT) from a reversibly dissociating photoacid, 2-naphthol-6,8-disulfonate (2N68DS). The reaction was carried out in water and in acetonitrile–water solutions. We find by carefully analyzing the geminate recombination dynamics of the photobase–proton pair that follows the ESPT reaction that there are ? side group at the 8 position which two targets for the proton back-recombination reaction: the original O? dissociation site and the SO3 is closest to the proton OH dissociation site. This observation is corroborated in acetonitrile-water mixtures of χwater < 0.14, where a slow ? group via H-bonding water. The proton-transferred R?O? fluorescence band in mixtures of χwater < 0.14 where only intramolecular ESPT occurs is red shifted by about 2000 cm?1 from the free R?O? band in neat water. As the water content in the mixture increases above χwater = 0.14, the R?O? fluorescence band shifts noticeably to the blue region. For χwater > 0.23 the band resembles the free anion band observed in pure water. Concomitantly, the ESPT rate increases when χwater increases because the intermolecular ESPT to the solvent (bulk water) gradually prevails over the much slower intramolecular via the water-bridges ESPT process.
关键词： 2-naphthol-6,8-disulfonate,photoacid,intermolecular ESPT,geminate recombination,intramolecular ESPT,excited-state proton transfer
更新于2025-09-23 15:19:57
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Delocalization boosts charge separation in organic solar cells
摘要： Organic solar cells (OSCs) utilizing π-conjugated polymers have attracted widespread interest over the past three decades because of their potential advantages, including low weight, thin film flexibility, and low-cost manufacturing. However, their power conversion efficiency (PCE) has been far below that of inorganic analogs. Geminate recombination of charge transfer excitons is a major loss process in OSCs. This paper reviews our recent progress in using transient absorption spectroscopy to understand geminate recombination in bulk heterojunction OSCs, including the impact of polymer crystallinity on charge generation and dissociation mechanisms in nonfullerene acceptor-based OSCs. The first example of a high PCE with a small photon energy loss is also presented. The importance of delocalization of the charge wave function to suppress geminate recombination is highlighted by this focus review.
关键词： Polymer crystallinity,Power conversion efficiency,Organic solar cells,Transient absorption spectroscopy,Dissociation mechanisms,Photon energy loss,π-conjugated polymers,Charge generation,Nonfullerene acceptor,Geminate recombination
更新于2025-09-23 15:19:57