J. Jung, Feng Liu, T. P. Russell
Jun 1, 2015
Citations
2
Influential Citations
55
Citations
Journal
Advanced Energy Materials
Abstract
DOI: 10.1002/aenm.201500065 characteristics. [ 8 ] Moreover, the molecular frontier orbitals of (D–A)-type copolymer can be readily tuned by rational choice of D and A units. More specifi cally, it has been recognized that the electron-donating and electron-withdrawing powers of D and A units should be weak for obtaining medium bandgap copoly mer. [ 9 ] Particularly, weak electron-donating characteristic of D unit ensures a low-lying HOMO energy level of the resulting conjugated polymer, which is benefi cial to achievement of high V OC of PSCs. Among many building blocks for D unit, anthracene has recently been used as the electron-donating moiety to construct (D–A)-type copolymers for PSCs. [ 10 ] Owing to its weak electrondonating characteristic, anthracene-based polymers possess deep HOMO energy level, which is favorable for high V OC in PCSs. Moreover, the planar and rigid nature of anthracene unit affords the corresponding copolymer to exhibit high charge transporting properties. [ 11 ] In addition, the functionalization of anthracene through its 9,10-positions offers a viable way to fi netune the optoelectronic properties with high solution processability of the corresponding polymer. [ 12 ] Therefore, anthracene derivatives are very promising candidate as relatively weak D unit for construction of medium bandgap copolymer for high performance PSCs. In this communication, we designed and synthesized medium bandgap (D−A)-type copolymers, comprising 9,10-thienylanthracene (TA) and benzothiadiazole (BT) as D and A unit, respectively. To optimize the molecular structure of the copolymer for high photovoltaic performance, four BT derivatives with different substituents were utilized for constructing the D–A copolymers. All four copolymers displayed low-lying HOMO energy levels and medium bandgaps in the range of (1.8–2.1) eV. Among them, fl uorine-substituted polymers exhibited high crystallinity, which facilitates effi cient chargecarrier transport and thus high J SC . Also, the fl uorinated polymers showed superior photo-induced charge generation with suppressed bimolecular recombination, which is benefi cial to achievement of high J SC . More importantly, the fl uorinated polymers formed nanoscale phase-separated bulk heterojunction morphology without addition of processing additive and post treatment, leading to a promising PCE as high as 8.05%, which is among the best value reported for medium bandgap polymers. This work clearly demonstrates that TA and fl uorinated BT units are very promising D and A units for medium bandgap copolymer for high photovoltaic performance. The copolymers composed of TA and BT units are synthesized by Stille cross-coupling polymerization in toluene with a Pd catalyst, as shown in Scheme 1 . When the number average molecular weights ( M n ) and polydispersity indexes (PDI) of Rapid development of polymer solar cells (PSCs) based on bulk heterojunction structure composed of a conjugated polymer and a fullerene derivative as an electron-donor and acceptor, respectively, has led to dramatic advance in the solar cell performance. [ 1 ] The power conversion effciency (PCE) of PSCs has signifi cantly been increased over the last few years exceeding 10% mainly due to remarkable development of novel p-type conjugated polymers. [ 2 ] Recent researches on the development of conjugated polymers have extensively focused on design and synthesis of low bandgap polymers in order to extend the light absorption range over 800 nm for enhanced photon absorption and thereby improved short-circuit current density ( J SC ) of PSCs. [ 3 ] However, low bandgap polymers are generally reported to exhibit high-lying highest occupied molecular orbital (HOMO) energy level due to reduced bandgap ( E g ), which imposes a limitation on PSCs to achieve high open-circuit voltage ( V OC ) and thus high PCE. [ 4 ] Furthermore, the intrinsic narrow absorption range of low bandgap polymer, which signifi cantly limits the use of full solar spectrum, is far from the ideal broad photon absorption. [ 5 ]