Two fullerene-terthiophene dyads without hexyl chains (3T-C₆₀) and with hexyl chains (3TH-C₆₀) on the terthiophene substituent are synthesized by 1,3-dipolar cycloaddition of corresponding azomethine ylides to C₆₀. The cyclic voltammetry studies indicate no apparent electronic communication between the terthiophene pendent group and the fulleropyrrolidine core in the ground state. However, a significant florescence quenching is observed for 3T-C₆₀ and 3TH-C₆₀, compared to their fluorescent terthiophene (3T) and 3TH precursors, respectively, suggesting the occurrence of strong intramolecular electron/energy transfers in the photoexcited state. Furthermore, these new fulleropyrrolidine derivatives are applied as electron acceptors to fabricate poly(3-hexylthiophene) (P3HT) based bulk heterojunction solar cells. The incident photon-to-current efficiency (IPCE) value of the P3HT/3T-C₆₀ device is significantly higher than that of the P3HT/PCBM cell in wavelengths of 350-420 nm. This finding provides direct evidence for the contribution of 3T excitons to the photocurrent. Replacing 3T-C₆₀ with 3TH-C₆₀ effectively improves the morphology of the photoactive layer and widens the window of optimal D/A ratios, raising the power conversion efficiency (PCE) from 2.14% to 2.54%. Importantly, these devices exhibit superior stability of PCE against high-temperature aging.
[6,6]‐Phenyl‐C61‐butyric acid methyl ester (PC61BM and PC71BM) is an archetypal electron acceptor in organic photovoltaic devices. However, it nucleates and grows to form aggregates or crystallites on a length scale of tens of nanometres to micrometres under thermal aging, which often results in a significant decrease in device efficiency and stability. To overcome this thermally induced performance degradation, many methods have been reported to date such as the introduction of hydrogen, halogen bonding, thermally or photochemically crosslinkable groups onto the fullerene, and the suppression of nucleation and growth of fullerene crystallites under thermal aging has been successfully demonstrated. Even though those methods are highly useful for the suppression of aggregation, we successfully demonstrated another one simple method for the same: Introduction of bulkier groups onto the fullerene, which can act as anchoring group to suppress the aggregation. In an extension of our previous work, quinoxaline (TQT), benzothiadiazole (TBTT) and benzoselenadiazole (TBST) based bulkier groups are linked to the fullerene, denoted as TQT‐C60, TBTT‐C60 and TBST‐C60, respectively, through the 1,3‐dipolar cycloaddition of corresponding azomethine ylides with fullerene. Single junction bulk heterojunction polymer solar cells were fabricated with the configuration ITO/PEDOT:PSS/P3HT:dyad/Ca/Al. The morphological stability of the active layer was monitored by transmission electron microscopy and optical microscopy. Independent of heteroatoms, all the dyads show excellent morphological stability under thermal aging compared to the archetypal acceptor PCBM due to the anchoring of substituent groups. © 2018 Society of Chemical Industry
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