In this work, fifteen polymers have been studied to test their potential as donors for polymer solar cells by density functional theory. Those polymers contained five homopolymers based on pyridazine, [1,2,thiadiazolo [3,4-pyridazine, [1,2,oxadiazole [3,4-pyridazine, isothiazolo [3,4-pyridazine and isoxazolo [3,4-pyridazine, and ten copolymers composed of the above compounds and thiophene incorporated with 1:1 and 1:2 ratios. The fifteen polymers have been examined in terms of the abilities of absorbing sunlight, stabilities in the environment, and photovoltaic properties. The results suggest that the copolymes DTHP, DTHTP, DTHOP, DTHITP, and DTHIXP are good material candidates of polymer donor for polymer solar cells.

The structural, electronic, and optical properties of PBDTTBT are comprehensively studied by density functional theory to rationalize the experimentally observed properties. Periodic boundary conditions method is employed to simulate the polymer block and calculate effective charge mass from the band structure calculation to describe charge transport properties. Moreover, both time-dependent density functional theory and a set of multidimensional visualization techniques are used to characterize the exciton dissociation ability in the PCBM: PBDTTBT interface. These theoretical methods and calculation techniques not only promote deep understanding of the connection between chemical structures and optical and electronic properties of the donor-acceptor system but also can be used to rationally design a novel donor-acceptor system. Based on the same calculated methods as PBDTTBT, four copolymers PBDTTTP, PBDTTTO, PBDTTTPD, and PBDTTFPD are designed to study their potentials as donors in polymer BHJ. The results indicate that PBDTTBT’s well conjugation benefits its good stability, and its wide and strong absorption spectra in the range of visible light, appropriate FMO levels, well charge transport, and favorable exciton dissociation lead to its photovoltaic performance. Furthermore, through comparing the four designed polymers with PBDTTBT, we conclude that the four designed polymers have stronger exciton dissociation ability and larger open-circuit voltage and external quantum efficiencies. Consequently, the four designed copolymers are promising candidates for polymer BHJ solar cells.