Development of Solar Cells Based on Synthetic Near-Infrared Absorbing Purpurins: Observation of Multiple Electron Injection Pathways at Cyclic Tetrapyrrole–Semiconductor Interface

Purpurin sensitizers with and without the central zinc, ZnP and H2P, have been synthesized and used in dye-sensitized solar cells. Both the sensitizers readily formed aggregates on the semiconductor surface. The DFT and TD-DFT calculations suggest that the major difference between the two sensitizers is ascribable to the energy levels of their four molecular orbitals. With biased potential in the solid-state photovoltaic diodes, the photoresponse of ZnP and H2P started from −2 and −2.5 V, respectively, and the observed difference is in agreement of the difference of calculated LUMO energy level for the two sensitizers. ZnP gave much better photovoltaic performance than H2P, when TiO2 electrode and 4-tert-butylpyridine (TBP)-free electrolyte were employed. The decrease of photocurrent of ZnP-based solar cell in TBP-containing electrolyte is attributed to the change in energy level of the electron acceptor, while that of H2P-based solar cell in TBP-containing electrolyte is ascribed to the change of electro...