Bulk Heterojunction Solar Cells with Large Open-Circuit Voltage: Electron Transfer with Small Donor-Acceptor Energy Offset
✍ Scribed by Xiong Gong; Minghong Tong; Fulvio G. Brunetti; Junghwa Seo; Yanming Sun; Daniel Moses; Fred Wudl; Alan J. Heeger
- Publisher
- John Wiley and Sons
- Year
- 2011
- Tongue
- English
- Weight
- 457 KB
- Volume
- 23
- Category
- Article
- ISSN
- 0935-9648
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✦ Synopsis
Power conversion effi ciencies (PCEs) (in response to solar AM1.5 radiation) as high as 6-8% have been reported for bulk heterojunction (BHJ) polymer solar cells. [ 1 , 2 ] In order to attain PCEs over 10%, BHJ materials capable of generating larger open circuit voltage (V oc ) are required. [ 3 , 4 ] One approach to increase V oc is to develop low-bandgap semiconducting polymers with deeper highest occupied molecular orbital (HOMO) energies. An alternative approach is to develop new electron acceptors with higher lowest unoccupied molecular orbital (LUMO) energies. The pathway to low-bandgap semiconducting polymers with deeper HOMOs is well understood, and BHJ solar cells fabricated by semiconducting polymers with deeper HOMOs have successfully exhibited larger V oc . However, the discovery of new (non-fullerene) electron acceptors with higher LUMOs remains undeveloped. In all BHJ materials studied to date, a signifi cant fraction of the photon energy (typically more than half) is given up in the charge transfer process because of the large energy offset between the LUMO of the donor polymer and the LUMO of the acceptor molecule. The factors that limit the V oc in BHJ solar cells remain controversial. The commonly accepted reason for this loss is that in order to obtain the required charge transfer, the LUMO offset must be larger than the exciton binding energy in the donor polymer. Here we demonstrate ultrafast charge transfer and the generation of photocurrent observed from BHJ PSCs even though the LUMO energy offset between the donor polymer and the acceptor molecule is only 0.12 eV. As a result, V oc = 1.20 V. The results clarify the path toward higher PCE by demonstrating that charge transfer and charge separation can occur in polymer BHJ systems with small donor-acceptor LUMO offset. Thus, V oc values close to the theoretical maximum, the band gap of the semiconducting polymer, should be possible for BHJ solar cells just as for inorganic solar cells.
Electron acceptors, by defi nition, are mild-to-moderate oxidizing agents. A working hypothesis is that the large asymmetry in forward to back charge transfer rates shown by polymer:fullerene systems originates from strain-relief