Rigid triarylamine-based D-A-π-A structural organic sensitizers for solar cells: the significant enhancement of open-circuit photovoltage with a long alkyl group

Five new organic D-A-pi-A sensitizers DIA6-DIA10 containing benzothiadiazole (BTD) or benzotriazole (BTZ) as auxiliary electron withdrawing units have been synthesized. Their photophysical, electrochemical and photovoltaic properties have been studied. Consequently, although all of these dyes have long alkyl chains grafted on a pi-bridge, DSSCs with dyes containing a BTZ unit displayed higher open circuit voltages (757-829 mV) than those containing a BTD unit (682-712 mV). This may be because the nitrogen-based heterocyclic group of BTZ can lift the conduction band edge and reduce the charge recombination that arises from the absence of the sulfur atom. Benefiting from this and the introduction of alkyl chains on the nitrogen atom, DSSCs with one dye containing BTZ and thieno[3,2-b] thiophene (TT) as the pi-spacer (DIA7) showed the highest open circuit voltage (V-oc = 829 mV). The addition of one more thiophene unit to DIA6 broadens the absorption spectra, leading the DIA8 device to produce the largest photocurrent. Among these sensitizers, the best photovoltaic performance was obtained by the DIA8 device, with a short circuit photocurrent density (J(sc)) of 13.45 mA cm(-2), an open-circuit photovoltage (V-oc) of 757 mV, and a fill factor (FF) of 70%, corresponding to an overall conversion efficiency (eta) of 7.15% which reached 93% of the reference dye N719-based cell under standard global AM 1.5 solar light conditions.