The surface photovoltage of squarylium dye films

Abstract

The surface photovoltage for squarylium films prepared either by vacuum sublimation or by evaporation of an ethanol solution has been measured, and the two types of films differ both in the size of their photovoltage and in their photovoltaic relaxation times: sublimed squarylium (0.172 mV, 43.0 ms), solution grown squarylium (0.366 mV, 0.75 ms). The photovoltage spectral dependence of both films is identical to the corresponding optical absorption. The sublimed squarylium films have an absorption edge at a photon energy of 1.80 eV, which agrees with the lowest energy optical absorption observed for squarylium in solution. This is in contrast to the absorption edge at 1.24 eV observed for the solution grown squarylium films. Both the faster recombination times, and the additional optical absorption in the near IR observed for the solution grown material could originate from additional energy levels which are not present in the sublimed squarylium films. This illustrates how the opto-electronic properties of squarylium depends strongly on the method of fabrication, and the implications of this result for the potential use of squarylium in solar cells is briefly discussed.