Frontier orbital and morphology engineering of conjugated polymers and block copolymers for potential high efficiency photovoltaics

The photoelectric power conversion efficiencies of currently reported organic/polymeric photovoltaic materials are still relatively low (typically less than 9% under AM 1.5 and one Sun intensity), and the three major losses are still severe, i.e., the ‘photon loss’ due to mismatch of materials energy gaps versus the sunlight photon energies, the ‘exciton loss’ and the ‘carrier loss’ due to poor solid state morphologies of existing polymeric donor/acceptor binary systems. Therefore, both molecular frontier orbitals (HOMOs, LUMOs) and phase morphologies of the photovoltaic polymers need to be optimized to further enhance the efficiency. In this presentation, our recent efforts on frontier orbital and energy gap engineering and terminal functionalizations of conjugated polymer blocks, and the donor-bridge-acceptor type block copolymer approaches will be reviewed. For instance, an earlier developed (DBAB)n block copolymer system (where D represents a conjugated donor block, A represents a conjugated acceptor block, and B represents a non-conjugated bridge unit) exhibited much better photovoltaic properties compared to the corresponding simple D/A blend system. Recently, a new DBA conjugated block copolymer system based on mono-functionalized monomers has also been synthesized. Additionally, a series of terminal functional and sulfone-containing conjugated polymers with evolving frontier orbital energy levels and gaps have recently been designed, synthesized, and studied. The HOMO/LUMO energy gaps of these new polymers were in a range of 1.5–2.6 eV, which are very attractive for solar energy applications. The terminal functional groups (aldehyde or phosphonate) make these polymer blocks potentially ideal candidates for the development of donor/acceptor block copolymer supra-molecular nanostructures for a variety of optoelectronic applications.