Publication date: 8th January 2019
All-small-molecule based organic solar cells (OSC) has become an attractive research topic in the field of photovoltaics due to the significant achievements in power conversion efficiency and stability in a short period of intensive research [1]. The elementary processes that take place in the active layer of the OSC upon photon absorption, such as exciton generation and charge separation, are known to be mainly responsible for the final device performance. However, the excited state dynamics of the small-molecule based donor-acceptor system have not been fully explored. Here, we study the excited state dynamics of prototypical small-molecule p-DTS(FBTTh2)2 (donor) and NIDCS-MO (acceptor) as well as of the blend using ultrafast time-resolved pump-probe spectroscopy. We found that excitonic coupling in this class of material is small, and both neutral and charged excited states are localized on the donor or acceptor molecules, which contradicts the high internal quantum efficiency and open circuit voltage achieved in OSCs with this material [2]. By performing target analysis on the time-resolved pump-probe data, we found that the exciton dissociation rate is slow compared to that in a polymer-based OSCs. Surprisingly, the recombination processes are even slower, and which explains the high internal quantum yields. Furthermore, with the semiclassical Marcus theory, we demonstrate that energy conversion and charge separation yields in this system are almost optimum, as the activation energy for charge generation is negligible but high for recombination which justifies the high open circuit voltage obtained in OSCs with this system. Finally, we predict that low-optical bandgap small-molecule systems with less donor:acceptor electronic coupling can push the efficiency of OSCs [3].
This work was supported by the European Commission (FP7 ITN “POCAONTAS”, Nr. 316633), by the European COST Action Nanospectroscopy, MP1302. Financial support at IMDEA was provided by the Spanish Ministerio de Economía y Competitividad (MINECO-FEDER project CTQ2014-58801), by the 'Severo Ochoa' Programme for Centres of Excellence in R&D (MINECO, Grant SEV-2016-0686), by the Comunidad de Madrid (Project Mad2D, Grant No. S2013/MIT-3007) and by the Campus of International Excellence (CEI) UAM+CSIC. The work at Seoul National University was supported by the National Research Foundation of Korea (NRF) through a grant funded by the Korean Government (MSIP; No. 2009-0081571[RIAM0417-20150013]), by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the NRF under the MSIP, Korea (2012M3A6A7055540), and by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20173010012960). J. S. acknowledges a PhD grant of the Chinese Scholarship Council (CSC). M. van den Berg acknowledges the Landesgraduiertenförderungsgesetz of the University of Tübingen for financial support. The authors thank H. Bolink (Valencia) for access to the integrating sphere.
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