Synthesis and characterization of perylene–bithiophene–triphenylamine triads: studies on the effect of alkyl-substitution in p-type NiO based photocathodes

Abstract

We report the synthesis of new donor–π–acceptor (D–π–A) dyes and their application in dye-sensitized solar cells (DSCs) with nickel(II) oxide (NiO)-based photocathodes. These D–π–A sensitizers incorporate a triphenylamine donor, a bithiophene π-bridge, and a perylenemonoimide (PMI) acceptor group. Two carboxylate groups attached to the triphenylamine afford strong anchoring to the NiO surface. The dyes in this series were varied firstly by the inclusion of an ethynyl linker between bithiophene and the triphenylamine moieties (1vs. 2), thereby increasing the length of the conjugated bridge. Despite very similar optoelectronic properties, the ethynyl-containing dye 2 showed a ∼25% improvement in power conversion efficiency in p-DSCs compared to 1, mostly attributed to the increased current density. Contrary to initial expectations, there was no major influence of the distance between the PMI unit of the dye and the NiO surface on the photoinduced dye anion lifetime, as measured by nanosecond transient absorption spectroscopy (TAS). Furthermore, altering the position of the alkyl chains on the bridging bithiophene in 3 and 4 resulted in a modest red shift in the dye absorption on account of increased charge delocalisation between the PMI and the π-bridge, owing to a reduced torsion angle between the PMI and the adjacent thiophene unit. Quantum-chemical DFT calculations were performed in order to evaluate these torsion angles and to study their influence on the electron density distribution in the relevant molecular orbitals. These changes of the molecular structure of the isomeric dyes 3 and 4 did not translate into improved photovoltaic performance, which is primarily attributed to lower charge photogeneration rates probed by transient absorption spectroscopy. While for p-type DSCs impressive overall solar-to-electric conversion efficiency of 0.04–0.10% under full sun illumination (simulated AM1.5G sunlight, 100 mW cm−2) and a broad incident photon to current efficiency (IPCE) response (350–700 nm) is demonstrated for these new dyes, the study clearly shows the need for judicious design rules for p-type sensitizers for application in photocathodic DSCs.