Design and Synthesis of Conjugated Polymers with Efficient Light Absorption for High Performance Polymer Solar Cells

Abstract

Until now regioregular poly(3-hexylthiophene) (P3HT) has been studied as an electron donor material for organic photovoltaics, and bulk heterojunction photovoltaic devices based on P3HT and [6,6]-phenyl C61 butyric acid methyl ester (PC61BM) have been reported to have power conversion efficiency (PCE) as high as 5%. However for devices utilizing, the main problem of P3HT is its quite large bandgap (1.92.0 eV) which limits the light absorption from the solar spectrum. To overcome the absorption limitation, extensive effort has focused on developing donor–acceptor (D–A) conjugated polymers. One unique feature of these polymers is that their HOMO and LUMO energy levels are governed by the HOMO energy level of the donor and the LUMO energy level of the acceptor, respectively. Therefore, the energy levels of polymers can be tuned by choosing appropriate combination of D and A unit. Among various semiconducting polymers for polymer solar cells (PSCs), a copolymer consisting of fluorene and dithienyl benzothiadiazole (PFDTBT) exhibits a high VOC of around 1 V because of its deep HOMO energy level, but it shows relatively low PCE with moderate JSC due to a large bandgap of 1.8 eV, which is the same problem of P3HT. For the purpose of reducing the bandgap of PFDTBT, we introduced a strong electron-donating oxygen atom into fluorene to synthesize benzochromene (BC), which was then polymerized with electron deficient dithienyl benzothiadiazole (DTBT) to afford two low-bandgap polymers, PBCDTBT. The PBCDTBT-based solar cell exhibits a high power conversion efficiency (PCE) of 5.74%, which is much higher than that of the reference fluorene-based polymer (PFDTBT) (2.56%). The higher PCEs of PBCDTBT is mainly attributed to higher JSC, which arises from stronger and broader absorption, higher exciton generation rate, more effective charge separation and higher hole mobility as compared to PFDTBT, although the VOC is sacrificed to some extent. Then, to enhance the JSC without VOC loss, we introduce ternary blend strategy. Ternary blends incorporating multiple donor absorbers with complementary absorption into active layers provide an opportunity to enhance the PCE of organic solar cells. In addition to complementary absorption between the donors, ternary blends exhibit improved internal morphology by choosing appropriate third component. For this purpose, in this study we design a ternary blend solar cell with two donors composed of diketopyrrolopyrrole-based polymer (PTDPP2T) and small molecule ((TDPP)2Ph), and one acceptor (PC71BM). Consequently, the PCE of ternary blend solar cell is increased from 6.60% (PTDPP2T:PC71BM) to 7.48% (PTDPP2T:(TDPP)2Ph:PC71BM), which is attributed to complementary absorption behavior between PTDPP2T and (TDPP)2Ph. Also, when the small amount of (TDPP)2Ph (<10 wt%) is added to PTDPP2T:PC71BM, a narrower fibril size with face-on orientation in qz direction on the substrate is obtained, which facilitates charge transport. Another effective strategy to further enhance JSC of the PSCs is to synthesize random copolymers composed of one electron donating unit and two different electron accepting units, if the absorptions of two electron accepting units are complementary to each other. To this end, we synthesized a new series of conjugated random copolymer composed of bithiophene (electron donating unit) with TDPP and pyridine-capped diketopyrrolopyrrole (PyDPP) (co-electron accepting units). The random copolymers show broad light absorption and face-on orientation on the substrate, which is beneficial to achieving high short circuit current. The VOC of the random copolymer can also be controlled systematically by varying the ratio of PyDPP to TDPP in the copolymer, since the HOMO energy level becomes deeper as the PyDPP content in the random copolymer is increased. Consequently, the solar cell device made of the random copolymer with the ratio of 3:1 (TDPP:PyDPP) shows higher PCE (8.11%) than those made of corresponding homopolymers, PTDPP2T (6.70%) and PPyDPP2T (4.14%). From these results, it can be concluded that the absorption properties of the active layers are influenced by chemical modification of polymer and third component in ternary blend. Also these absorption properties are important factors to design high performance PSCs.