Harnessing the open-circuit voltage via a new series of Ru(II) sensitizers bearing (iso-)quinolinyl pyrazolate ancillaries

A novel class of Ru(II) sensitizers (TFRS-51-TFRS-54) with a 4,4′-dicarboxy-2,2′-bipyridine anchoring ligand and two trans-oriented isoquinolinyl (or quinolinyl) pyrazolate ancillaries were designed, characterized and used to fabricate dye sensitized solar cell (DSC) devices. In sharp contrast to Ru(II) sensitizers that employ functionalized thiophene appendages in their ancillary bipyridal ligands, the extended π-conjugation introduced by the isoquinolinyl or quinolinyl groups improves the optical absorptivity, particularly for the absorption located at 500 nm when compared with the parent sensitizer TFRS-1 possessing less conjugated 5-pyrid-2-yl pyrazolate ancillaries. As a result, DSCs incorporated with these dyes show much improved J_SC compared with the reference device. Moreover, the use of bulky t-butyl substituents on the ancillary ligands improves the cell performance with excellent V_OC of up to 830 mV recorded. Also, the addition of tetra-n-butyl ammonium deoxycholate [TBA][DOC] as co-adsorbent to the dye solution further improves the power conversion efficiency (η). The best solar cell parameters recorded were JSC = 16.3 mA cm^-2, VOC = 860 mV, FF = 0.72, and η = 10.1% for a device sensitized with TFRS-52. The markedly high open-circuit voltage is confirmed by the longer electron lifetime revealed in transient photovoltage (TPV) measurement versus the TFRS-1 sensitizer, and is probably derived from a combination of the higher conduction band edge of TiO₂ induced by the in situ metathesis of carboxylate anchors and the reduced recombination contributed by the bulky sensitizer.