Synthesis and Isolation of cis-2 Regiospecific Ethylene-Tethered Indene Dimer–[70]Fullerene Adduct for Polymer Solar Cell Applications

Abstract

Although the utilization of [70]fullerene bis-adducts can enhance the power conversion efficiencies of polymer solar cells (PSCs) owing to their strong absorption intensities and high-lying lowest unoccupied molecular orbital energy levels, this synthetic strategy typically yields a mixture of regioisomers that would mask the intrinsic device performances depending on the substituent pattern on the [70]fullerene derivatives. In this study, a single cis-2 regioisomer of C70 bis-adduct (cis-2-[70]BIEC) has been prepared for the first time by the same strategy that had been applied to [60]fullerene to obtain a regioisomerically pure C60 bis-adduct (cis-2-[60]BIEC). Diels-Alder reaction was conducted between a rationally designed ethylene-tethered indene dimer and [70]fullerene, followed by isolation using high-performance liquid chromatography suitable for the separation of fullerene derivatives. A series of structural analysis techniques including NMR spectroscopies and X-ray crystallography were used to identify the absolute configuration of the bis-adduct. A systematic study on the optical, electrochemical, and photovoltaic properties of cis-2-[70]BIEC as well as the corresponding regioisomer mixture (bis-[70]BIEC) and the monoadduct (α-mono-[70]BIEC) has been performed to examine the effect of the pure cis-2 regioisomer. More importantly, their properties are compared with those of cis-2-[60]BIEC to address the effect of fullerene cage structures, that is, C60 versus C70. The PSC based on cis-2-[70]BIEC and poly(3-hexylthiophene) showed a remarkable power conversion efficiency of 4.2%, which is higher than those with bis-[70]BIEC (2.2%), α-mono-[70]BIEC (2.2%), cis-2-[60]BIEC (2.8%), and even a prevalent high-performance C70 monoadduct ([70]PCBM, 3.8%). Our synthetic strategy will pave the way for further development on the rational design and isolation of single fullerene bis-adduct regioisomers exhibiting high device performances.

cis-1 Isomers of tethered bismethano[70]fullerene as electron acceptors in organic photovoltaics

“…1b), 40 in which two indene units were attached at a-bonds of hexagons next to each other. 38 Note here that steric hindrance between the addends would inhibit the formation of such a close substitution pattern if the tether-directed effect was absent. 23,27 The OPV based on BIE- [70]fullerene bis-adduct isomer with cis-2 conguration, cis-2- [70]BIEC and poly(3-hexylthiophene) showed a remarkable PCE of 4.2%, which was higher than those with the regioisomer mixture of BIE- [70]fullerene bis-adduct (2.2%), BIE- [70] fullerene mono-adduct (2.2%), BIE-[60]fullerene bis-adduct isomer with cis-2 conguration (2.8%), and even [70]PCBM (3.8%).…”

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

“…[32][33][34][35][36][37][38] 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. [33] 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, [39][40][41][42][43] although some exceptions have also been noted. [21,41,44] 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. [37,40,42] 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 ) [45] and tethered bis-DPM-C 60 (diphenyl methano) [44] 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 Figure 2.…”

“…It is known that the use of shorter tethers and rigid addends decreases the number of observed isomers. To the best of our knowledge, the only pure bis ‐adducts synthesized using the tether‐directed method and applied to OPV solar cells are the recently reported IC 61 BA cis ‐2 isomer, IC 71 BA α‐2‐α isomer, and the N ‐methyl‐phenyl‐C 61 ‐propyl‐2‐fulleropyrrolidine cis ‐1a isomer with 1% and 2% higher PCEs than the corresponding isomeric mixtures using P3HT as the donor (refer to Fig. for the structures).…”

“…Pure bis ‐adducts synthesized using the tether‐directed method (a) IC 61 BA cis ‐2 isomer; (b) IC 61 BA α‐2‐α isomer; (c) N ‐methyl‐phenyl‐C 61 ‐propyl‐2‐fulleropyrrolidine cis ‐1a isomer…”