Influence of the Selenophene Auxiliary Acceptor in Porphyrin Sensitizers for High-Performance Dye-Sensitized Solar Cells

Porphyrin-based dyes are widely used as sensitizers for dye-sensitized solar cells (DSSCs), but their power conversion efficiencies (PCEs) are limited mainly due to charge recombination and aggregation tendency. Here, we report two D-pi-A porphyrin-based sensitizers, LG 28 and LG 29, which comprise a 3-ethynylphenothiazine as an electron donor, meso-substituted porphyrin as a pi-spacer, and selenophene and cyanoacrylic acid as an auxiliary acceptor and anchoring group, respectively. When compared to the LG5 dye, we have replaced the thiophene (LG5) unit with 2-phenylselenophene (LG 28) and 2-(selenophenye-2yl)thiophene (LG 29) auxiliary acceptors. Upon successful substitution with the selenophene unit, the molar extinction coefficient was increased without noticeable change in absorption maxima. Density functional theory studies suggest that the intramolecular interactions between donor phenothiazine and acceptor anchoring groups in both sensitizers play a pivotal role. Excited state properties are found to be quenched for the sensitizers upon the formation of self-assembled monolayers on nanocrystalline TiO2. Finally, we have evaluated the device performances of both sensitizers in DSSCs using I-/I-3(-) liquid redox electrolyte and device efficiency of 8.83 and 10.31% using LG 28 and LG 29-based sensitizers, respectively. To understand the observed photovoltaic trends, we have performed ultrafast transient absorption studies on bare LG 28 and LG 29 sensitizers in the presence of TiO2 in solution phase, respectively. The described results confirm multilevel electron injection from different higher excited singlet states of the LG 28 sensitizer to the TiO2 conduction band with an overall rate of electron injection of 3.84 x 10(11) s(-1), contrary to electron injection from the lowest excited state of the LG 29 sensitizer to the TiO2 conduction band with a two-order slower rate of electron injection with a higher device efficiency than that of LG 28, and it is attributed to onset incident-photon-to-currents efficiency, which extends to 900 nm in LG 29 dye.