Abstract
Fullerenes and their derivatives are widely used as electron acceptors in bulk‐heterojunction organic solar cells as they combine high electron mobility with good solubility and miscibility with relevant semiconducting polymers. However, studies on the use of fullerenes as the sole photogeneration and charge‐carrier material are scarce. Here, a new type of solution‐processed small‐molecule solar cell based on the two most commonly used methanofullerenes, namely [6,6]‐phenyl‐C61‐butyric acid methyl ester (PC60BM) and [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC70BM), as the light absorbing materials, is reported. First, it is shown that both fullerene derivatives exhibit excellent ambipolar charge transport with balanced hole and electron mobilities. When the two derivatives are spin‐coated over the wide bandgap p‐type semiconductor copper (I) thiocyanate (CuSCN), cells with power conversion efficiency (PCE) of ≈1%, are obtained. Blending the CuSCN with PC70BM is shown to increase the performance further yielding cells with an open‐circuit voltage of ≈0.93 V and a PCE of 5.4%. Microstructural analysis reveals that the key to this success is the spontaneous formation of a unique mesostructured p–n‐like heterointerface between CuSCN and PC70BM. The findings pave the way to an exciting new class of single photoactive material based solar cells.
Highlights
The μh(s) value extracted for PC60BM is the highest reported to date and exceeds that reported by Anthopoulos and co-workers[35] by more than one order of magnitude
The efficient solar cells based on methanofullerenes, namely PC60BM and PC70BM, as the sole light absorbing material and CuSCN as the transparent holeextracting material, have been demonstrated
The bilayer CuSCN/PC70BM devices were found to exhibit moderate performance with power conversion efficiency (PCE) of ≈1%, physical blending of the two components resulted in the solar cells with PCE of 5.4% and VOC in excess of 0.9 V
Summary
The performance of the organic bulk heterojunction (BHJ) solar cells has been increasing steadily over the last few years, and has reached 13% for the single-junction solar cells.[1,2,3] these high efficiencies have come at the cost of an increase in the complexity of the cells, where often finely tuned nanomorphologies are required, which renders them both less stable and reproducible, along with difficult-to-synthesize polymers and small molecules with high associated production costs.[4,5,6] In addition, the inherent trade-off between the short-circuit current (Jsc) and the open-circuit voltage (VOC), due the requirement for the lowest unoccupied molecular orbital offset between the donor and the acceptor to be more than 0.3 eV, is thought to be a limiting factor in pushing cell efficiencies to above 15%.[7]. More significant advancements have been made using the so-called Schottky junction fullerene solar cells, in which small concentrations, usually around 5%, of donor materials are incorporated into the fullerene matrix, which acts as the main absorber Such cell architectures have shown to exhibit high VOC of over 1 V, and efficiencies of up to 6%.[12,13,14,15,16,17,18,19,20,21] The importance of these cells cannot be understated; fullerenes have been the prototypical 3D semiconductor with a molecular symmetry that is unmatched by any other molecular semiconducting material, and have been synonymous with the rise of the third generation photovoltaics in the form of electron acceptors. Frenkel excitons at the CuSCN:fullerene interface due to band offset, and (iii) the existence of a unique spontaneously formed mesostructured CuSCN-nanowire:fullerene heterointerface
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