Influence of humidity for preparing sol-gel ZnO layer: Characterization and optimization for optoelectronic device applications

Sol-gel zinc oxide (ZnO) is very useful for constructing various optoelectronic devices including different types of sensors and solar cells at low cost and fast processing time. Here, we adopted a zinc acetate dehydrate, with low processing temperature and suitable for flexible electronics, as a precursor to produce ZnO thin film. Then, we explored the morphological and optical properties of various sol-gel ZnO layers depending on the humidity of the sol-gel process because the humidity affects the morphological and optical characteristics, dramatically changing the performance of optoelectronic devices. As a result, extremely high humidity produced a ZnO layer with rough surface, high dispersive transmittance, and high trap density because rapid hydrolysis does not give atoms enough time to move to thermodynamically-stable sites. On the other hand, extremely low humidity produced a ZnO layer with smooth surface, low dispersive transmittance and oxygen-deficient trap sites owing to insufficient hydrolysis. Therefore, a ZnO layer prepared at humidity of 25% had low surface roughness, low trap density, and desirable transmittance characteristics, achieving the best device performance when applied to organic solar cells. This study will be helpful in determining the optimum processing conditions to obtain optoelectronic devices with the best performance.