Journal
ENERGY & ENVIRONMENTAL SCIENCE
Volume 11, Issue 1, Pages 211-220Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c7ee02967e
Keywords
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Funding
- European Commission (ITN POCAONTAS'') [316633]
- European COST Action Nanospectroscopy [MP1302]
- Spanish Ministerio de Economia y Competitividad (MINECO-FEDER) [CTQ2014-58801]
- 'Severo Ochoa' Programme for Centres of Excellence in RD (MINECO) [SEV-2016-0686]
- Comunidad de Madrid [S2013/MIT-3007]
- Campus of International Excellence (CEI) UAM+CSIC
- National Research Foundation of Korea (NRF) - Korean Government (MSIP) [2009-0081571[RIAM0417-20150013]]
- Global Frontier R&D Program on Center for Multiscale Energy System - NRF under the MSIP, Korea [2012M3A6A7055540]
- Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea [20173010012960]
- Chinese Scholarship Council (CSC)
- Landesgraduiertenforderung of the University of Tubingen
- Korea Institute of Energy Technology Evaluation and Planning (KETEP)
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Solution-processable all-small-molecule organic solar cells (OSCs) have shown dramatic progress in improving stability and photovoltaic efficiency. However, knowledge of photoexcitation dynamics in this novel class of materials is very limited. To fully exploit the design capacities inherent in small molecule chemistry, the elementary processes and branching yields must be known in detail. Here, we present a combined computational-experimental study of photoexcitation dynamics of a prototypical all-small-molecule photovoltaic blend, p-DTS(FBTTh2)(2) as a donor and NIDCS-MO as an acceptor. Femtosecond spectroscopy data show that excitonic coupling is small and that the charge transfer states are localized, at first glance contradicting the high internal quantum efficiency (IQE) and open circuit voltage (V-OC) of this material. A target analysis of the femtosecond spectra yields exciton dissociation rates of 1/(25 ps) and 1/(100 ps) for the as-deposited and annealed blend, respectively. These rates are far slower than in typical polymer based organic solar cells. Still, internal quantum yields are high because parasitic quenching processes are found to be even slower. In the framework of semiclassical Marcus theory, we demonstrate that our system shows near-optimum energy conversion and charge separation yields, due to negligible activation energy for charge generation but high activation energy for charge recombination, allowing enough time to separate localized charge transfer states. We thus justify both the high internal quantum yields and the high open circuit voltage found in this system. Finally, we predict that highly efficient and stable low-optical bandgap systems can be realized by reducing the electronic coupling between the donor and acceptor.
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