State-of-the-art mid-frequency sputtered ZnO films for thin film silicon solar cells and modules

This paper reports on the use of ZnO-films, prepared by reactive mid-frequency (mf) sputtering on large area (60 x 100 cm²) glass substrates, in silicon thin film p-i-n solar cells and modules. It summarizes the status in the final phase of a joint R&D project that aims at the development of cost-effective high-quality large area ZnO substrates feasible for an industrial solar module production. The mf sputter technique using low-cost metallic Zn:Al targets yields films with resisitivities down to 2.6 x 10-4 Omegacm and high optical transmittance in the visible wavelength region. The variation of the sputter pressure in a wide range between 300 and 1200 mPa leads to films with significantly differing etching behavior in diluted acids, while good optical and electrical properties can be maintained. In general, films prepared in the high pressure regime develop strongly textured light scattering surfaces after etching, which is a necessary prerequisite to obtain highly efficient solar cells. Initial efficiencies of small area a-Si cells on texturetched ZnO-films prepared by mf-sputtering on 60 x 100 cm² standard float glass (3 mm) range from 8 to 9 % (highest efficiency 9.2 %, i-layer thickness 350 nm) First 0.6 m² modules on ZnO prove the principal applicability of the films for an industrial manufacturing process.