Abstract
The impact of fullerene side chain functionalization with thiophene and carbazole groups on the device properties of bulk-heterojunction polymer:fullerene solar cells is discussed through a systematic investigation of material blends consisting of the conjugated polymer poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3‴-di(2-octyldodecyl)-2,2′;5′,2″;5″,2‴-quaterthiophen-5,5‴-diyl)] (PffBT4T-2OD) as donor and C60 or C70 fulleropyrrolidines as acceptors. The photovoltaic performance clearly depended on the molecular structure of the fulleropyrrolidine substituents although no direct correlation with the surface morphology of the photoactive layer, as determined by atomic force microscopy, could be established. Although some fulleropyrrolidines possess favorable lowest unoccupied molecular orbital levels, when compared to the standard PC71BM, they originated OPV cells with inferior efficiencies than PC71BM-based reference cells. Fulleropyrrolidines based on C60 produced, in general, better devices than those based on C70, and we attribute this observation to the detrimental effect of the structural and energetic disorder that is present in the regioisomer mixtures of C70-based fullerenes, but absent in the C60-based fullerenes. These results provide new additional knowledge on the effect of the fullerene functionalization on the efficiency of organic solar cells.
Highlights
Organic photovoltaic (OPV) cells are a promising solar energy harvesting technology because of their flexibility, light weight, and compatibility with large-scale production using roll-to-roll methodsMaterials 2020, 13, 1267; doi:10.3390/ma13061267 www.mdpi.com/journal/materials (R2R) that are expected to reduce the module fabrication cost and the energy payback time [1,2,3]
PicoLE atomic force microscopy (AFM) in contact mode and several scans were imaged in flattened mode data to remove the background slope
We have demonstrated a simple approach for the chemical modification of N-methylWe have demonstrated a simple approach for the chemical modification of N-methyl fulleropyrrolidine acceptors that can be extended to thiophene and carbazole based moieties
Summary
Organic photovoltaic (OPV) cells are a promising solar energy harvesting technology because of their flexibility, light weight, and compatibility with large-scale production using roll-to-roll methodsMaterials 2020, 13, 1267; doi:10.3390/ma13061267 www.mdpi.com/journal/materials (R2R) that are expected to reduce the module fabrication cost and the energy payback time [1,2,3]. Organic photovoltaic (OPV) cells are a promising solar energy harvesting technology because of their flexibility, light weight, and compatibility with large-scale production using roll-to-roll methods. Much effort has been made in the last two decades—focusing in the material design, device engineering, and morphology optimization—aiming to increase the power conversion efficiencies (PCE) of the OPV device. The active layer in a OPV device is made of a blend of a p-type polymer and an n-type acceptor forming a bicontinuous interpenetrating network, known as bulk-heterojunction (BHJ). The n-type acceptors used in OPVs can be either fullerene derivatives or non-fullerene small molecules. The performance of OPVs using non-fullerene acceptors has already outperformed its fullerene-based counterpart, the research in polymer:fullerene solar cells using new ‘non-standard’ fullerenes remains very active [16,17,18,19,20,21,22]
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