Synthesis And Photovoltaic Properties Of Polymers Based On Pyrazine And Small Molecules Based On Porphyrin

With the advent of the global energy crisis, looking for new and non-polluting renewableenergy has become a worldwide issue. Organic photovoltaic technology has attractedextensive attention due to the potential advantages, including simple preparation, low cost,and light weight. And most important of all, the solar energy was limitless reserve.In order to obtain ideal active materials, scientists have been trying to optimize thematerial properties through molecular design. D-A polymers consisting of alternated donorand acceptor units had the better ability for intramolecular charge transfer and couldeffectively adjust the molecular energy levels and band gap. Therefore, the synthesis of D-Apolymers was a very important design direction for solar cell materials. In recent years, thetwo-dimensional conjugated polymers have received widespread attention in regulating theelectronic structure of polymers. The side chains of the donor unit in D-A polymers could bemodified by conjugated group, which has made certain achievements. The side chains of theacceptor unit in conjugated polymers could also be modified by different groups, but this fieldhas not carry on in-depth research. We selected quinoxaline as receptor unit, and bymodifying the side chains of quinoxaline with different conjugate groups, we synthetized aseries of quinoxaline derivatives. Then we synthetized a series of D-A (D) conjugatedpolymers with large flat structure of benzodithiophene as electron donor unit and quinoxalineas the electron acceptor unit.We systematically investigated the impacts of differentlyconjugated side chains at the acceptor unit in D-A polymers on their optical absorption,electrical energy level, hole mobility, and photovoltaic properties. The results showed that theefficiency of the photovoltaic devices based on the PBDT-TQ and PBDT-PTQ photovoltaicdevice can reach to4.37%and3.58%respectively, and the photoelectric conversionefficiency of the photovoltaic devices based on PBDT-PTTQ and PBDT-TTQ are2.51%and1.36%respectively.At the same time, we selected thiophene as electron donor unit and quinoxaline withdifferent side chains as electron acceptors unit, and through copolymerization, another seriesof two-dimensional conjugated polymers were obtained. And we also systematically investigated the impacts of differently conjugated side chains at the acceptor unit of D-Apolymers on their optical absorption, electrical energy level, and photovoltaic properties. Theresults showed that the efficiency of the photovoltaic devices based on the PTH–TTQ was3.66%.Both studies showed that different conjugate side chains in electron acceptors unit couldchange the electron density distribution of the backbone in polymer, affect the π-π*transitionand intramolecular charge transfer, and change the molecular self-assembly ability, thus thepolymer photovoltaic performance was affected.In line with the fused ring systems, the enhanced co-planarity between the aromaticblocks is also beneficial for the π-conjugation and charge transport. We selected ethynylene aslinkage to enhance the co-planarity, the increasing open-circuit voltage could also be achievedby introducing ethynylene unit into D-A polymer main chains because the oxidation potentialmight be increased by the presence of electron-withdrawing triple bond spacers. Therefore,two new low bandgap copolymers PPE-DTTP and PP-DTTP both consisting of5,7-dithien-2,3-di(4-dodecyloxy-phenyl) thieno[3-4-b]pyrazine (DTTP) and2,5-bis-(n-dodecyloxy)benzene linked directly in PP-DTTP but with ethynylene linkages inPPE-DTTP as the electron donors were designed and synthesized.We investigated the effectsof ethynylene linkages on the performances of the organic solar cells (OSCs). The resultsshowed that the efficiency of the photovoltaic devices based on the PPE-DTTP can beimproved to1.20%after optimization, while the photoelectric conversion efficiency of thelower band gap polymer PP-DTTP was only0.30%.Porphyrins have extensively π-conjugated systems which were contributed to electrontransfer from donor unit to acceptor unit, and have high molar absorption coefficients in lightabsorption, and their properties can be easily tuned via synthetic modifications for theperiphery or by metal insertion into the cavity of porphyrin. These physical and chemicalproperties make porphyrins to be widely concerned. So far the highest PCE of BHJ organicsolar cells based on the solution-processed porphyrin small molecules is approached to5%.Therefore, we selected porphyrin as the donor unit, ethynylene and2,5-bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione(DPP) asacceptor groups, and synthesized linear type porphyrin small molecule H2PDPP. And we also obtained the metal porphyrin molecules based on zinc, copper and platinum withcharacteristics of triplet as the center of porphyrin (CuPDPP, ZnPDPP and PtPDPP).Theexcited state with the characteristics of triplet could be formed between platinum ions with d8electronic structure and porphyrin complex in platinum porphyrin PtPDPP. This feature couldincrease the lifetime of the exciton.Thereby, the transmission distance of exciton couldincrease, and the probability of electron and hole being collected by the electrodes beforerecombination could enhance, both of which could help to improve molecular photovoltaicperformance. We also systematically investigate the optical properties and electrochemicalproperties of the different nature of the metal porphyrin molecules. At the same time, we wereprepared photovoltaic device based on ZnPDPP, whose photoelectric conversion efficiencywas4.93%. The efficiency of this device could be promoted. The optimization of the deviceand other small molecule materials characterization will be done in the further research.