Theoretical Study Of D-A Copolymer Donor Materials With High Short-circuit Current Characteristics

One of the important aspects of high efficiency bulk heterojunction(BHJ)solar cell research is the reasonable design of D-A copolymer donor materials.In this paper,10 donor units and 10 receptor units were selected respectively.The structure and band gap of D,A units and their corresponding copolymers were studied by using the first principles density functional theory(DFT).On the basis of this,the electronic structure characteristics of three kinds of high short-circuit current donor materials P1(PDTGPT),P2(PNTz4T)and P3(DPPTT-T)were studied.By analyzing its molecular chain conformation,the Bader charge transfer and the binding energy,it is ideal for the D-A donor materials for high short-circuit current organic solar cells.The results are stated below:(1)In this paper,we first discuss the effect of the number of fused thiophene rings(1 to 6 thiophene rings)on the band gap value.The results show that the band gap value of dithiophene is the smallest,and the band gap value increases with the increase of thiophene ring number.At the same time,this trend gradually decreases with the increase of the number of thiophene rings.Secondly,the number of benzene rings in the benzene ring fused thiophene ring structure also has a great influence on the band gap value.The band gap values of D7,D9 and D10 calculated in this paper are 1.34 eV,1.24 eV and 0.97 eV,respectively.It can be found that as the number of benzene rings increases,their band gap values decrease in turn.(2)The results show that the S atoms are distributed on both sides of the molecular chain,and the interchain steric hindrance is smaller and the band gap of the corresponding structure is smaller.The S atom distributed in the same side of the molecular chain,interchain steric hindrance larger,the corresponding structure of the band gap value.The band gap values of D3 and D4 are 0.89 eV and 1.94 eV,respectively.The values of D7 and D8 are 1.34 eV and 1.55 eV,respectively.In addition,the bridge atomic substitution has a large effect on the band gap value.(3)The structure containing the thiazole ring and the thiadiazole ring has a strong electron-withdrawing ability because both the thiazole ring and the thiadiazole ring contain carbon-nitrogen double bonds,and the unpaired electrons on the carbonnitrogen double bonds make these two structures more easy to get electrons.The calculated results also show that,in the same case,the structure containing the thiadiazole ring has a stronger electron-accepting ability than the structure containing the thiazole ring.In addition,the structure containing the lactam group also has a strong ability to obtain electrons.(4)In this paper,the calculation of P1,P2,P3 three kinds of materials show that: P1,P2,P3 photovoltaic devices appear high short circuit current on the one hand because the composition of the polymer itself,the donor unit or receptor unit energy gap is small.On the other hand because the material itself has excellent crystallinity and better stereoregularity.In addition,the molecular chain conformation results show that the secondary structure of P1 is a helical conformation,and the secondary structure of P2 and P3 is a linear conformation The results of differential charge density show that the charge transfer between D and A units of P1 is relatively large,and the charge transfer between D and A units of P2 and P3 is less.(5)The band gap of the simulated DPPX-T series of polymers shows that the band gap decreases gradually with the increase of the atomic radius of O,S,Se and Te.In addition,the results of the differential charge density show that the monothiophene ? bridge acts as a more receptor unit in the D-A copolymer.On the other hand,the thiophene ? bridge of the P3 structure acts as a donor unit.This study is expected to provide valuable guidance for designing high shortcircuit current D-A donor materials.