Chlorophyll-Derived, Cyclic Tetrapyrrole-Based Purpurins as Efficient Near-Infrared-Absorption Donor Materials for Dye-Sensitized and Organic Solar Cells

Abstract

Near-infrared-absorbing chlorophyll derivatives, namely purpurin-18 methyl esters, with (c-H2P and c-ZnP) and without (H2P and ZnP) carboxyl group at the C3 position of chlorin macrocycle have been studied in either dye-sensitized solar cells (DSSCs) or organic-heterojunction solar cells (OSCs). The presence of carboxyl groups gives negligible effect to either the energy levels or the electronic distribution of the purpurin molecules. These purpurins readily form aggregation not only on semiconductor surface through self-assembled adsorption but also on solid surface through strong π–π interaction during spin-casting. In carboxyl purpurin-based DSSCs, the energy gap between the lowest unoccupied molecular orbital (LUMO) level of purpurin and conduction band edge of TiO2 is insufficient for the charge separation, and results in low photocurrent generation, except for a case that using both c-ZnP as sensitizer and 4-tert-butylpyridine-free electrolyte gives the best solar power conversion efficiency (η) of up to 5.1 %. The replacement of TiO2 by SnO2 with a deeper conduction band edge increases the photocurrent, but reduces the photovoltage, and as the results, gives negligible improvement at η values. In purpurin-based OSCs, in contrast, the energy gap between the LUMO levels of purpurin molecules and fullerene acceptors is sufficient for efficient charge separation. In C70-based planar heterojunction solar cells, a thin active layer of 5 nm gave the best η values of up to 1.3 % and 1.7 % for H2P and ZnP, respectively. To improve the photovoltaic performance, bulk heterojunction (BHJ) solar cells with the purpurin: PC70BM blends were fabricated. The optimal blend ratio was 1:4 for both purpurins, due to the most efficiently balanced charge transport toward both electrodes. ZnO as the exciton blocking layer improved both the photocurrent and voltage for the ZnP-based BHJ solar cells but not for the H2P-based BHJ solar cells. This difference has been attributed to an inefficient charge separation at the H2P/ZnO interface that competes with the normal charge separation at the H2P/PC70BM interface. The best η value of up to 2 % has been achieved for ZnP-based BHJ solar cells at the elevated temperature.