Modulation of energy levels by donor groups: an effective approach for optimizing the efficiency of zinc-porphyrin based solar cells

In the design of efficient sensitizers for dye-sensitized solar cells (DSSCs), it is vital to modulate the HOMO and LUMO orbitals by introducing suitable donor and acceptor groups. In this respect, triphenylamine has been extensively used as the donor group. Porphyrin has strong absorption in the UV-Vis range, which makes it a promising dye candidate. In addition, a porphyrin molecule contains eight β positions and four meso-positions, which may be conveniently utilized for introducing multiple donor and acceptor moieties. In this work, various numbers of triphenylamine and trimethoxyphenyl groups are introduced to the porphyrin meso-positions as electron donors, with the aim to systematically modulate the energy levels and investigate the effect on the efficiency of the DSSCs. The HOMO–LUMO energy gaps remain almost constant irrespective of the donor groups, thus, the variation of the donors leads to a contradictory effect on the processes of electron injection and dye regeneration. In the case of the zinc porphyrin with two triphenylamine units and one trimethoxyphenyl group as the electron donors (M3T2P), moderate and well-matched HOMO and LUMO energy levels are observed, thus affording high efficiencies for both the electron injection and the dye regeneration processes. The overall conversion efficiencies (η) of the DSSCs based on these dyes lie in the range of 2.70–5.45%, with the optimized performance achieved by M3T2P, which can be rationalized by the efficient electron injection and dye regeneration processes, and the long electron lifetime, as demonstrated by the electrochemical impedance spectroscopy measurements. These results provide further insights into the strategy for elevating the efficiencies of DSSCs, especially those based on porphyrins, simply by modulating the electron donor groups.