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 μm) for the realization of p-type dye-sensitized 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 DS2/35 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 pre-treatment of the NiO 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.