Highly-efficient Cd-free CuInS₂ thin-film solar cells and mini-modules with Zn(S,O) buffer layers prepared by an alternative chemical bath process

Recent progress in fabricating Cd- and Se-free wide-gap chalcopyrite thin-film solar devices with Zn(S,O) buffer layers prepared by an alternative chemical bath process (CED) using thiourea as complexing agent is discussed. Zn(S,O) has a larger band gap (Eg = 3.6-3.8 eV) than the conventional buffer material CdS (E_g = 2.4eV) currently used in chalcopyrite-based thin films solar cells. Thus, Zn(S,O) is a potential alternative buffer material, which already results in Cd-free solar cell devices with increased spectral response in the blue wavelength region if low-gap chalcopyrites are used. Suitable conditions for reproducible deposition of good-quality Zn(S,O) thin films on wide-gap CuInS₂ ('CIS') absorbers have been identified for an alternative, low-temperature chemical route. The thickness of the different Zn(S,O) buffers and the coverage of the CIS absorber by those layers as well as their surface composition were controlled by scanning electron microscopy, X-ray photoelectron spectroscopy, and X-ray excited Auger electron spectroscopy. The minimum thickness required for a complete coverage of the rough CIS absorber by a Zn(S,O) layer deposited by this CBD process was estimated to ∼15 nm. The high transparency of this Zn(S,O) buffer layer in the short-wavelength region leads to an increase of ∼1mA/cm² in the short-circuit current density of corresponding CIS-based solar cells. Active area efficiencies exceeding 11.0% (total area: 10.4%) have been achieved for the first time, with an open circuit voltage of 700.4mV, a fill factor of 65-8% and a short-circuit current density of 24.5mA/cm² (total area: 22.5mA/cm²). These results are comparable to the performance of CdS buffered reference cells. First integrated series interconnected mini-modules on 5 Times; 5 cm² substrates have been prepared and already reach an efficiency (active area: 17.2 cm²) of above 8%.