High open-circuit voltage under the bulk photovoltaic effect for the chiral smectic crystal phase of a double chiral ferroelectric liquid crystal doped with a fullerene derivative

Abstract

A bulk photovoltaic effect was studied in a chiral smectic crystal phase of a bisfluorophenylterthiophene derivative bearing two lactate moieties doped with a fullerene derivative. The chiral smectic crystal phase was a ferroelectric and macroscopically polarized state that could be formed by cooling the sample from a chiral smectic A phase under a DC bias application. The fullerene derivatives aggregated to form microcrystals which promoted hole–electron dissociation in the polarized smectic phase. The open circuit voltage exceeded 1.2 V and the external quantum efficiency was around 80∼50% under near UV and blue light illumination. This open circuit voltage exceeds those of the organic solar cells using a fullerene-based acceptor. The polarity of the photovoltaic effect could be inverted by the change of the poling bias during the phase transition into the smectic crystal phase. Unlike conventional organic thin film solar cells, the photovoltaic effect was observed in a symmetrical cell consisting of an ITO cathode and anode. Moreover, thick films of around 1–2 μm can be used for the bulk photovoltaic devices. The concentration of the fullerene derivative was only 8 mol%. However, the dissociation of excitons into hole and electrons was promoted because the relative permittivity exceeded 30.