Abstract
Novel C60 and C70 N-methyl-fulleropyrrolidine derivatives, containing both electron withdrawing and electron donating substituent groups, were synthesized by the well-known Prato reaction. The corresponding highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels were determined by cyclic voltammetry, from the onset oxidation and reduction potentials, respectively. Some of the novel fullerenes have higher LUMO levels than the standards PC61BM and PC71BM. When tested in PffBT4T-2OD based polymer solar cells, with the standard architecture ITO/PEDOT:PSS/Active-Layer/Ca/Al, these fullerenes do not bring about any efficiency improvements compared to the standard PC71BM system, however they show how the electronic nature of the different substituents strongly affects the efficiency of the corresponding organic photovoltaic (OPV) devices. The functionalization of C70 yields a mixture of regioisomers and density functional theory (DFT) calculations show that these have systematically different electronic properties. This electronic inhomogeneity is likely responsible for the lower performance observed in devices containing C70 derivatives. These results help to understand how new fullerene acceptors can affect the performance of OPV devices.
Highlights
Standard inorganic solar cells can achieve high efficiencies, but they possess some disadvantages including an elaborated and costly production
Novel C60 and C70 fullerene derivatives were synthesized by the well-known Prato reaction and the corresponding highest occupied molecular orbital (HOMO)/lowest unoccupied unoccupied molecular orbital (LUMO) levels measured by voltammetry
Despite the favourable LUMO levels of some of these novel fullerenes, compared to PC71 BM, all of them originated PffBT4T-2OD-based devices with poorer performances than reference devices based on the standard PC71 BM
Summary
Standard inorganic solar cells can achieve high efficiencies, but they possess some disadvantages including an elaborated and costly production. To overcome these drawbacks many efforts were made to develop several third-generation thin-film solar technologies. Organic photovoltaic cells (OPVs) [1,2,3,4,5] can be low-cost production since they can be manufactured in larger areas on flexible. The OPVs have experienced significant developments in power conversion efficiency (PCE), attaining recently over 16% for single-junction devices [7,8,9] and PCE over 17% for tandem cells [10]. Its high crystallinity and relatively high SCLC hole mobility
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