Solution-processed high-haze ZnO pyramidal textures directly grown on a TCO substrate and the light-trapping effect in Cu2O solar cells

Abstract

We have prepared pyramidally textured ZnO (PT-ZnO) films with a high light-scattering effect using a simple low temperature (75 °C) solution process that involved no annealing and vacuum processes. The PT-ZnO films, which were composed of dense ZnO columnar grains with a (101)-faceted pyramidal tip, were formed directly on a TCO substrate by electrodeposition. The electrodeposition was performed within 10 min from Zn(NO₃)₂ aqueous solutions that contained NaCl; during deposition, a two-step electrolytic current was applied to separately control the nucleation density and grain size of ZnO grains. Light-scattering effects from the pyramid structure were examined by measuring haze values in the visible region and comparing the results with those from planar and nanorod structures. The PT-ZnO films showed a haze value of ∼52%, which is 3.7 and 2.2 times higher than those of planar and nanorod ZnO films, respectively. On the basis of total transmittance and haze, the optimal thickness of the pyramidal textures was determined to be 0.7–1.2 μm. Theoretical simulation of light scattering on the PT-ZnO was performed using a simple geometric model which indicated that the refraction angle, path length and intensity of transmitted light vary significantly depending on the outer refractive index of ZnO. The light-trapping effect derived from the PT-ZnO was demonstrated by fabricating ZnO/Cu₂O thin-film solar cells. The Cu₂O solar cell using the PT-ZnO showed efficient light absorption, achieving a 1.3 times higher short circuit current density than that using the planar ZnO.