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Simultaneous Open‐Circuit Voltage Enhancement and Short‐Circuit Current Loss in Polymer: Fullerene Solar Cells Correlated by Reduced Quantum Efficiency for Photoinduced Electron Transfer
Author(s) -
Di Nuzzo Daniele,
Wetzelaer GertJan A. H.,
Bouwer Ricardo K. M.,
Gevaerts Veronique S.,
Meskers Stefan C. J.,
Hummelen Jan C.,
Blom Paul W. M.,
Janssen René A. J.
Publication year - 2013
Publication title -
advanced energy materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 10.08
H-Index - 220
eISSN - 1614-6840
pISSN - 1614-6832
DOI - 10.1002/aenm.201200426
Subject(s) - materials science , homo/lumo , fullerene , acceptor , open circuit voltage , photochemistry , quantum efficiency , energy conversion efficiency , organic solar cell , polymer solar cell , band gap , short circuit , electroluminescence , electron acceptor , photoinduced electron transfer , photoluminescence , electron transfer , optoelectronics , nanotechnology , polymer , voltage , chemistry , organic chemistry , physics , molecule , layer (electronics) , quantum mechanics , composite material , condensed matter physics
The limits of maximizing the open‐circuit voltage V oc in solar cells based on poly[2,7‐(9,9‐didecylfluorene)‐ alt ‐5,5‐(4,7‐di‐2‐thienyl‐2,1,3‐benzothiadiazole)] (PF10TBT) as a donor using different fullerene derivatives as acceptor are investigated. Bulk heterojunction solar cells with PF10TBT and [6,6]‐phenyl‐C 61 ‐butyric acid methyl ester (PCBM) give a V oc over 1 V and a power conversion efficiency of 4.2%. Devices in which PF10TBT is blended with fullerene bisadduct derivatives give an even higher V oc , but also a strong decrease in short circuit current ( J sc ). The higher V oc is attributed to the higher LUMO of the acceptors in comparison to PCBM. By investigating the photophysics of PF10TBT:fullerene blends using near‐IR photo‐ and electroluminescence, time‐resolved photoluminescence, and photoinduced absorption we find that the charge transfer (CT) state is not formed efficiently when using fullerene bisadducts. Hence, engineering acceptor materials with a LUMO level that is as high as possible can increase V oc , but will only provide a higher power conversion efficiency, when the quantum efficiency for charge transfer is preserved. To quantify this, we determine the CT energy ( E CT ) and optical band gap ( E g ), defined as the lowest first singlet state energy E S1 of either the donor or acceptor, for each of the blends and find a clear correlation between the free energy for photoinduced electron transfer and J sc . We find that E g − qV oc > 0.6 eV is a simple, but general criterion for efficient charge generation in donor‐acceptor blends.