Gallium Doped ZnO for Thin Film Solar Cells

Abstract: Gallium doped ZnO films with resistivities as low as 0.410 -3 cm were grown by rf-magnetron sputtering. For a given thickness the sputter time to deposit gal-lium doped ZnO was in average only two thirds of that needed to deposit aluminium doped ZnO. Despite the higher sputter rates for ZnO:Ga the electrical transport proper-ties were almost the same for both type of films with mobilities around 10cm 2 /Vs and net carrier concentrations of n = 410 20 cm -3 . The band gap was determined to be E g = 3.24 eV. Solar cells prepared with Cu(In,Ga)(S,Se) 2 absorber layers exhibited ef-ficiencies of up to 13.6%. 1. Introduction ZnO transparent conductive oxide (TCO) films are standard window layers for Cu(In,Ga)(S,Se) 2 (CIGS) thin film solar cells [1]. In addition, a growing interest to develop ZnO as a replacement for the highly priced ITO in displays could be observed during the last few years [2]. For these applications the ZnO acts as window material and the requirements for that purpose are high transmittance and low reflectivity as well as a low resistivity in or-der to minimize the series resistance and enhance the fill factor. Recently it was also report-ed, that CIGS solar cells with gallium doped ZnO have a better stability against environmen-tal influences. This led to a smaller decrease of the efficiency in not encapsulated cells after a few weeks of outside testing [3]. The presented investigations were performed to study whether rf-sputtered Ga