Photoelectrochemical characterization and optimization of a liquid-junction photovoltaic cell based on electrodeposited CdSe thin films: influence of annealing and photoetching on the physical parameters determining the cell performance

The influence of annealing and etching (chemical- and photoetching) treatments on the physical parameters (grain size, d, donor contraction, ND, and hole diffusion length, Lp) determining the photoelectrochemical behaviour of electrodeposited CdSe thin films in contact with sulfide/polysulfide electrolytes has been systematically studied. Annealing in flowing He containing a few ppm of O2 produces two main effects: (1) A progressive increase of the grain size ; (2) the generation of Se vacancies (VSe) behaving as donor centers. VSe is partially compensated by Cd vacancies (acceptor centers) and neutralized by oxygen, which is able to diffuse into the CdSe lattice occupying Se vacancies. At annealing temperatures near 600°C an appreciable oxidation occurs of the electrode surface, which becomes covered by a thin, photoelectrochemically inactive CdO layer. Chemisorbed O2 is the main source of recombination at grain boundaries. This oxygen can be desorbed by electrode heating in H2 atmosphere at ∼ 200°C. Photoetching in 1M Na2SO3 of electrodes annealed at 500°C produces maximum quantum efficiencies of near 100% with monochromatic radiation of 580 nm (ND = 7 × 1017 cm−3, Lp = 8 × 10−6 cm and d = 2 × 10−5 cm). The main effect of photoetching is to remove from the electrode surface a thin layer rich in oxygen and Se vacancies where recombination is favored.