Review: Fullerene based acceptors for efficient bulk heterojunction organic solar cell applications

Bulk hetro-junction organic solar cells (BHJ-OSCs) which made by coating the blend of highly electron rich, poly-3-hexylthiophene (P3HT) as a donor and extremely electron deficient, soluble C-60 derivative, [6,6]-phenyl C61 -butyric acid methyl ester (PC61BM) as an acceptor between the high work function positive electrode, ITO and low work function metal negative electrode Al, showed exciting results than the bilayer counterpart in the last two decades, thus the scientific circle and solar cell manufacturing groups are focusing on this treasured field. Fullerene derivative based BHJ-OSCs showed astonishing efficiencies due the high donor-acceptor (D-A) interface, high open-circuit voltage (V-oe), easy processing, and thermal stability. Hence, the design of the fullerene derivative plays a vital role in the efficiency of the OSCs. Numerous papers were published on the PC61/71BM and various functionally modified fullerene acceptors in the past several years with higher efficiency, higher LUMO level, and improved solubility. The symmetrical structure of PC61BM showed high aggregation properties, insufficient absorption in the visible region, but unsymmetrical PC71BM had a good optical absorption in the visible region, hence it was the most desirable choice for best performing solar cells. From the comparison, it was clear that the modification in the PC61/71BM core with aryl group, alkyl chain length, and end group modification didn't show any significant difference in the PCE with the P3HT polymer donor in BHJOSCs device structure. Fullerenes multi-adducts performed better than the PC61/71EM due to the development in the V-oc,. In multi-functionalization, mainly bis-fonctionalization showed better performance than mono adduct. C-60 and C-70 based hetro bis-adducts namely, C-60(CH2)(Ind), and OQMF70 showed the highest PCE of 5.90%, and 6.61% respectively, which was comparable with the indene based bis-adducts. Commonly, trisadduct fullerenes showed the lowest PCE due to the isomer effect and electron trapping, but OQBMF was a hetro tris-adduct of C-60-methanofullerenes, which showed a excellent PCE of 6.43% due to improved V. e. Similarly, indene substituted C-60 and C-70 fullerenes based bis-adducts showed supreme PCE, of 6.5% (IC(60)BA) and-6.7% (IC(70)BA) respectively, peak PCE was attributed to the high electron donating indene group, which enhanced the LUMO level due to the shrinkage in the pi-system. The dihydronapthyl based fullerene bis-adduct derivatives such as, NC(60)BA, and NC(70)BA showed the best performance of 5.37%, and 5.95% respectively, than the mono adduct. Even though abundant fulleropyrrolidines were reported, the performance of the fulleropyrrolidines based device was very deprived than the PC61/71BM based devices. Though the OSCs are comparatively efficient for commercial application due the insufficient efficiency and lower environmental stability factors controls the early arrival to the market. Therefore, it is essential to go through the recent fullerene derivatives based literatures, including mono-, bis-, tris-and multi-adducts of various methanofullerene derivative, indene based fullerene derivatives, dihydronaptyl substituted fullerenes, fulleropyrolidines, 1,2 and 1,4-adducts and fullerene derivatives for OSCs applications. In this regard, we discussed the present development in fullerene derivative and summarized the efficient fullerene derivatives for the BHJ-OSCs application and suggest a possible scenario for the future development in the fullerene cage.