Photoelectrochemical characterization of squaraine-sensitized nickel oxide cathodes deposited via screen-printing for p-type dye-sensitized solar cells

In the present paper we report on the employment of the screen-printing method for the deposition of nickel oxide (NiOx) layers when preformed nanoparticles of the metal oxide (diameter <50 nm) constitute the precursors in the paste. The applicative purpose of this study is the deposition of mesoporous NiOx, electrodes in the configuration of thin films (thickness, l <= 4 mu m) for the realization of p-type dyesensitized solar cells (p-DSCs). Three different squaraine-based dyes (here indicated with VG1C8,VG10C8 and DS2/35), have been used for the first time as sensitizers of a p-type DSC electrode. VG1C8 and VG10C8 present two carboxylic groups as anchoring moieties, whereas DS2135 sensitizer possesses four acidic anchoring groups. All three squaraines are symmetrical and differ mainly for the extent of electronic conjugation. The colorant erythrosine b (ERY B) was taken as commercial benchmark for the comparison of the different dye-sensitizers. The influence of the conditions of NiOx, sensitization and of NiOx surface pre-treatment on the photoelectrochemical performance of the corresponding p-DSCs have been analyzed. The highest overall conversion efficiency achieved within this set of organic colorants was 0.025% with VG1C8 and VG10C8 as colorants when NiOx surface was pre-treated with alkali. The alkaline pretreatment of the NiOx surface led to the highest efficiencies of the incident photon-to-current conversion (IPCEMAX =11% at about 390 nm) for the three squaraines here considered in correspondence of the wavelength of their maximum absorption. The prolongation of the sensitization time of NiOx cathodes up to 16 h leads to a general increase of the open circuit voltage in the corresponding p-DSCs.