Reducing Regioisomers of Fullerene-Bisadducts by Tether-Directed Remote Functionalization: Investigation of Electronically and Sterically Isomeric Effects on Bulk-Heterojunction Solar Cells

Abstract

C60 bis-adduct containing a mixture of regio-isomers with different LUMO energy levels and steric geometries could greatly affect the morphological and bulk properties. To investigate the regio-isomer effect on solar cell performance, we have successfully designed and synthesized a regio-selective 4-acetatephenyl-4'-methylphenylmethano C60 bis-adduct (S-APM-CBA) by "tether-directed remote functionalization" strategy and a random 4-acetatephenyl-4'-methylphenylmethano C60 bis-adduct denoted as R-APM-CBA by traditional cyclopropanation. The dramatic reduction in the number of regio-isomers in S-APM-CBA is confirmed by the (1)H NMR and HPLC measurements and theoretical calculation. Compared to the R-APM-CBA-based device with a Jsc of 6.63 mA/cm(2), an FF of 44.3% and a PCE of 2.46%, the device using S-APM-CBA yielded a much lower Jsc of 1.48 mA/cm(2), an FF of 32.2%, and a PCE of 0.38%. Consistently, the electron-only device using S-AMP-CBA exhibited lower electron mobility than the R-AMP-CBA-based device. These results imply that the electronic shallow-trap effect ascribed to the LUMO energy variations turned out to be insignificant in the AMP-CBA system. The lower efficiency and mobility of S-AMP-CBA might due to the assumption that the most probable trans-4-III isomer in S-AMP-CBA prevents the intermolecular facial contact of fullerenes, thereby hindering the electron transporting. Furthermore, the nanomorphology of S-AMP-CBA and R-AMP-CBA active layers could be different because of their different three-dimensional structures. This research demonstrated that steric effect of regio-isomers in a given C60 bis-adduct is more crucial than electronic shallow-trap effect.

Separation and identification of indene–C70 bisadduct isomers

“…With the observations in this study, it can be argued that the success of IC 70 BA (and in extension, IC 60 BA) as the electron acceptor component in BHJ solar cells is serendipitous and surprising given the large number of chemical structures involved. It is noteworthy that there have been several studies on reducing the number of isomers in fullerene bisadduct materials using synthetic strategies and about the successful application in solar cell devices [ 18 – 22 ]. While high material purity and composition is generally considered an advantage for organic electronic materials, the material criteria for bulk heterojunction organic solar cell applications is less clear.…”

Towards the synthesis of poly(azafulleroid)s: main chain fullerene oligomers for organic photovoltaic devices

“…Further evidence that PAFs are based on [5,6]‐open azafulleroid repeat units can be found on closer inspection of the UV–visible curves. Notably, the characteristic absorption for [6,6]‐closed fullerene derivatives at 420 nm is not observed, whereas the shoulder at ca 416 nm, characteristic of an open fulleroid structure, is seen (detailed in Table ). Therefore this isomer is the most likely for the repeat units in the polymer chain, and has been used for Scheme .…”

“…However, SACAP results in a regio‐irregular polymer with a mixture of trans ‐3 and trans ‐2 in‐chain isomers. These intra‐chain variations can result in different C 60 lowest unoccupied molecular orbitals (LUMOs) along the chain, which may result in electronic traps . Therefore a challenge is to find a facile polymerization of C 60 that results in regio‐regular polymers.…”

Recent progress in the synthesis of regio‐isomerically pure bis‐adducts of empty and endohedral fullerenes

“…PCEs up to 10.6% have been achieved using isomeric mixtures of fullerene bis ‐adducts, such as indene‐C 61 ‐ bis ‐adducts (IC 61 BA) in a solution processed tandem solar cell . Although not all isomerically pure bis ‐regioisomers have been tested in solar cell devices, most of the reported cases have demonstrated that pure regioisomers many times perform better than the corresponding isomeric mixtures, although some exceptions have also been noted . These observations justify the necessity to synthesize pure bis ‐adducts, and this in turn has led to a renaissance of the methods and reagents available to decrease the number of fullerene multiadduct derivative isomers.…”

“…Recent activity in this area has resulted in new and regioisomerically pure bis ‐adducts that have performed reasonably well in OPV devices . In some cases, even when using the tether‐directed method, the number of isomers obtained is still rather large, for example, in the case of the synthesis of tethered bis ‐PC 61 BM (phenyl butyric acid methyl ester‐C 61 ) and tethered bis ‐DPM‐C 60 (diphenyl methano) for which the number of isomers obtained was 7 and 12, respectively. The main reasons for these observations are the lack of symmetry of the addend, in which case diastereomers can also be observed and the extreme flexibility of the tether used .…”