Chemical Bath Deposited ZnO:Al Thin Films and Their Application to CuInGaSe2 Thin Film Solar Cells

Abstract

ZnO:Al thin films have been widely used as transparent conductors in variety of optoelectronic devices including CuInGaSe2 thin film solar cells. However, high cost sputtering process has been typically employed to prepare high quality ZnO:Al thin fil simultaneously having both high optical transmittance and low sheet resistance. In this study, we present the ZnO:Al thin films prepared by the chemical bath deposition using NH4OH-based aqueous solution on seeded glass substrates. We also report their properties and application to CuInGaSe2 thin film solar cells as a top transparent electrode. A seed layer on glass substrate was prepared by spin-coating of ZnO nanoparticles with size of a few tens of nanometer. For the chemical bath deposition process, ZnO powder was dissolved in NH4OH-based aqueous solution and un-dissolved ZnO powder was removed by transferring the ZnO solution through 1 um filter. Retrograde solubility of ZnO in NH4OH-based aqueous solution allows us to deposit ZnO thin films by raising temperature of the bath from room temperature to 80 oC. To obtain compact film by enhancing growth rate of crystal in lateral direction, a certain amount (~0.5-2 mM) of ammonium citrate was added into the bath. A various amount of Al(NO3)3 was added into the bath as a aluminum source to maximize electrical conductivity by forming Al substitution in Zn site. The prepared ZnO:Al films were irradiated by UV for 1 min. Optimized ZnO:Al films showed (0002) preferred orientation, optical transmittance of 90% and sheet resistance of 75 ohm/sq. Furthermore, the prepared ZnO:Al thin film was successfully implanted to CIGS thin film solar cells as a top electrode showing power conversion efficiency of 12.5% comparable to the performance (~13.7%) of CIGS solar cells having a sputter-deposited ZnO:Al film transparent electrode.