Comparative investigation of fullerene and non-fullerene acceptors blended with polymer donor P3HT for solar cell applications

Polymer solar cells based on the bulk heterojunction structure have recorded remarkable improvements in their performance over the past few years. They have elicited considerable research interest in recent years because they have a higher potential to promote low-cost, environmentally friendly, non-complex, and flexible large-area devices compared to their inorganic counterparts. In this study, different films of the organic polymer P3HT (poly-(3-hexylthiophene-2,5-diyl)) mixed with fullerene acceptors PC71BM ([6,6]-phenyl C71 butyric acid methyl ester, mixture of isomers) and non-fullerene acceptors (NFAs) IEICO-4F (2,2′-[[4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl]bis[[4-[(2-ethylhexyl)oxy]-5,2-thiophenediyl]methylidyne(5,6-difluoro-3-oxo-1H-indene-2,1(3H)-diylidene)]]bis[propanedinitrile]), and COi8DFIC (2,2-[[4,4,11,11-tetrakis(4-hexylphenyl)-4,11-dihydrothieno [2,3:4,5]thieno [2,3d]thieno[2,3:4,5]-thieno[2,3:4,5] pyrano[2,3:4,5]thieno[2,3:4,5] thieno[3,2-b]pyran-2,9-diyl]bis[methylidyne(5,6-difluoro)) were fabricated on glass substrates. Ultraviolet–visible–near-infrared (UV–VIS–NIR), and photoluminescence (PL) characterizations were performed to determine the optical properties and eventual analysis of the structural (H- and J-aggregation) differences in the blends, while atomic force microscopy was used to analyze the surface morphologies of the polymer blends. The results of ultraviolet–visible–near infrared spectroscopy depicted better light harvesting capabilities in non-fullerene acceptor based polymer blends of P3HT:COi8DFIC and P3HT:IEICO-4F, which had wider absorption spectra, indicating a larger absorption window and higher absorption intensities extending from NIR (near infrared) to the ultraviolet (UV) region compared to P3HT:PC71BM blends, which had narrower absorption windows and lower absorption intensities. P3HT-fullerene-based blends depicted a blue-shifted spectra, a characteristic of H-aggregates, while the NFA-based blends depicted red-shifted vibronic structures, a characteristic of J-aggregates. There was lower PL intensities of the non-fullerene based blends compared to the fullerene-based polymer blends, depicting more charge carrier recombination in fullerene-based polymer blends. These implied that the fullerene-based blend had lower power conversion efficiency compared to non-fullerene-based blends.