All-solution-processed transparent front contact for monograin layer kesterite solar cells

The paper presents a fully solution-processed transparent front contact for kesterite monograin layer solar cells, which is composed of a silver nanowire network above a ZnO-based window bilayer. This window bilayer comprises a layer of ZnO nanoparticles and a second layer of chemical-bath-deposited doped ZnO. The optimized front contact can be processed within minutes and performs equally to a reference front contact with a sputtered (ZnO)-Zn-i/ZnO:Al window bilayer, both on solar cell and module level with sizes up to 20 x 20 cm(2). To achieve this, it was necessary to obtain a compact morphology of the chemical-bath-deposited, doped ZnO film even for very short deposition times. It is found that, firstly, this depends on the homogeneity of the seed layer, which is the prerequisite for thin film growth by chemical bath deposition (CBD) on the substrate. It is shown that the ZnO nanoparticle layer covers the rough monograin substrate in a continuous and unperturbed manner. The second aspect of importance was the dopant type used in the CBD process (Al, Ga, or In). Optimal combination of photovoltaic performance and process speed was obtained for Ga-doped ZnO, yielding compact films for deposition times as low as 1 minute. The solution-processed window layers are susceptible to a light-soaking effect, which impairs the cell performance upon storage in dark conditions. While this effect can be easily overcome in the laboratory, the effect in a real application scenario was investigated under outdoor conditions for a period of 4 weeks.