Preparation Of D-A₁-D-A₂ Conjugated Polymers For Polymer Solar Cell Applications

Until now, the D-A type narrow bandgap conjugated polymers which can absorb the sunlight in the range 350 nm-750 nm were typically choosed as the donor material of high-efficiency solar cells ?PSCs?. In this paper, the D-A₁-D-A₂ type narrow bandgap conjugated polymers with an electron-rich unit and two electron-deficient units were synthesized to broaden the range of visible light absorption. These novel polymers can broaden the range of visible light absorption for the complementary "A" unit and contribute to charge transfer for the easy crystallization in the film state.The conventional Suzuki polymerization method and Stille polymerization process generally have complex purification procedure and toxic organic Sn. In this paper, the D-A₁-D-A₂ type narrow bandgap conjugated polymers were synthesized through economic and environmental direct arylation poly-condensation method which was rarely used for the synthesis of monomers. In order to preferably explore the synthesis and optical performance of D-A₁-D-A₂ type narrow bandgap conjugated polymers, the works were made as follows:1. The four D-A₁-D-A₂ type narrow bandgap conjugated polymer P1, P2, P3 and P4 of BT series were synthesized through direct arylation. DPP was choosed as the "A₁" unit of the four polymers, BT and its derivatives were taken as "A₂" unit and the bithiophene and terthienyl worked as "D" units. The thermal and optical properties of the four polymers were studied and the results indicated that all of the four polymers had good thermal stability and broad sunlight absorption range even in the infrared region. Electrochemical tests indicated that the energy level of four polymers matched the PC₇₁BM energy level in order to be the donor materials of polymer solar cell.2. The four D-A₁-D-A₂ type narrow bandgap conjugated polymers P5, P6, P7 and P8 were also synthesized through direct arylation. DPP was choosed as the "A₁" unit, TPD or BseT were taken as "A₂" unit and the bithiophene or bithiophene with hexyl chain worked as "D" units. The thermal stability, optical performance and electrochemical properties of the four polymers were all studied. The results showed that polymers P5-P8 had good thermal stability, broad sunlight absorption range and the same energy level as PC₇₁BM.3. The devices made of polymers P1 and P2 were optimized and the effect of different treatments on the photoelectric conversion efficiency of the devices was investigated. The photovoltaic properties of all the eight polymers was tested and verified through AFM, EQE and electron and hole mobility. Finally, the devices of all polymers were prepared and the power conversion efficiency of P1-P4 was 3.43%,3.66%,0.50% and 0.45%, respectively. The phase diagrams of P3 and P4 showed that the blend film surfaces of polymers were too rough to get higher device efficiency than PI and P2. The power conversion efficiency of P5-P8 was 0.92%,2.79%,0.28% and 1.93%, respectively. Device results revealed that the introduction of hexyl chain in the "D" units of P6 and P8 improved the solubility of the polymers to form good morphology and obtained higher device efficiency than P5 and P7.