Electrochemical Synthesis of Highly Oriented, Transparent, and Pinhole-Free ZnO and Al-Doped ZnO Films and Their Use in Heterojunction Solar Cells.

Electrochemical synthesis conditions using nonaqueous solutions were developed to prepare highly transparent (T > 90%) and crystalline ZnO and Al-doped ZnO (AZO) films for use in solar energy conversion devices. A focused effort was made to produce pinhole-free films in a reproducible manner by identifying a key condition to prevent the formation of cracks during deposition. The polycrystalline domains in the resulting films had a uniform orientation (i.e., the c-axis perpendicular to the substrate), which enhanced the electron transport properties of the films. Furthermore, electrochemical Al doping of ZnO using nonaqueous media, which was demonstrated for the first time in this study, effectively increased the carrier density and raised the Fermi level of ZnO. These films were coupled with an electrodeposited p-type Cu2O to construct p-n heterojunction solar cells to demonstrate the utilization of these films for solar energy conversion. The resulting n-ZnO/p-Cu2O and n-AZO/p-Cu2O cells showed excellent performance compared with previously reported n-ZnO/p-Cu2O cells prepared by electrodeposition. In particular, replacing ZnO with AZO resulted in simultaneous enhancements in short circuit current and open circuit potential, and the n-AZO/p-Cu2O cell achieved an average power conversion efficiency (η) of 0.92 ± 0.09%. The electrodeposition condition reported here will offer a practical and versatile way to produce ZnO or AZO films, which play key roles in various solar energy conversion devices, with qualities comparable to those prepared by vacuum-based techniques.