Fabrication and Characterization of photoelectrochemical Solar Cells Using Chemically Synthesized Zinc Oxide Thin Films

Abstract

Mesoporous nanostructures of zinc oxide (ZnO) were successfully synthesized by chemical bath deposition (CBD) technique and used to fabricate photoelectrochemical (PEC) solar cells. The synthesis proceeded in an alkaline bath of an aqueous solution of 0.1 M Zn(NO3)2.6H2O at a bath temperature of 353 K and pH of 11.5 on microscope glass slide and stainless steel slide substrates. The ZnO was doped with Al and Cu at varying concentrations from 1-5 at. %. As-deposited films were annealed at 673 K for 2 h. The synthesized ZnO thin films had thickness in the range of 2.03-10.43 µm. Crystal structure studies revealed that all the ZnO thin films were polycrystalline with hexagonal wurtzite structure and preferential growth in the 002 crystal plane. Crystallite sizes in the range of 8-29 nm were obtained along the 002 crystal plane and the crystallinity of the doped samples was strongly affected by the concentrations of the dopants. Surface morphological studies indicated that the synthesized ZnO thin films had nanoflakes, nanodendrites and nanorods morphologies which confirmed the effects of surfactant, dopants and annealing on ZnO. Optical studies revealed that all the films had low absorbance in the visible region of the solar spectrum with transmittance ranging from 42-90 %. Energy band gaps of the undoped and doped ZnO were found to decrease from 3.03 eV to 2.70 eV. All the measured optical properties of the ZnO thin films showed strong dependence on the concentration of the dopants. Surface wettability studies confirmed that all the synthesized ZnO thin films were porous (hydrophilic), giving water contact angles in the range of 0o to 71.3o. For the first time in literature, this synthesized ZnO thin films were further sensitized with Indigofera arrecta plant dye and also Rhodamine 6G to develop/fabricate dye-sensitized solar cells (DSSCs). The fabricated PEC solar cells produced short circuit current (ISC) of 12.34 μA/cm2 and open circuit voltage (VOC) of 388 V for unsensitized undoped ZnO electrode giving power conversion efficiencies (η) of 0.003 and fill factor (FF) of 0.43. Unsensitized aluminum doped zinc oxide (AZO) electrode yielded ISC in the range of 21 μA/cm2 to 29 μA/cm2, VOC in the range of 333 mV to 641 mV, η in the range 0.0037 % to 0.01 % and FF in the range of 0.34 to 0.43. While unsensitized copper doped zinc oxide (CZO) electrodes produced ISC in the range of 16 μA/cm2 to 98 μA/cm2 and VOC in the rage of 774 mV to 796 mV, giving η in the range of 0.0009 % to 0.062 % and FF in the range of 0.06 to 0.63. Further upon dye-sensitization of the ZnO electrodes with Rhodamine 6G and Indigofera arrecta plant dye respectively, the PEC solar cells of undoped ZnO produced ISC of 0.24 mA/cm2 and VOC of 360mV for Rhodamine 6G and ISC of 0.29 mA/cm2 and VOC of 595 mV for Indigofera arrecta plant dye. These produced η of 0.11 % and FF of 0.45 for Rhodamine 6G and η of 0.16 % and FF of 0.44 for Indigofera arrecta plant dye. AZO electrodes produced ISC in the ranges of 0.3 mA/cm2 to 0.4 mA/cm2 and VOC in the ranges 376 mV to 515 mV using Rhodamine 6G and ISC in the range of 0.84 mA/cm2 to 1.35 mA/cm2 and VOC in the range of 596 mV to 664 mV for Indigofera arrecta plant dye. These yielded η in the range of 0.16 % to 0.22 % and FF in the range of 0.40 to 0.49 for Rhodamine 6G and η in the range of 0.29 % to 0.51 % and FF in the range of 0.42 to 0.47 using Indigofera arrecta plant dye. On the other hand, CZO electrodes produced ISC in the range of 0.49 mA/cm2 to 0.97 mA/cm2 and VOC in the range of 355 mV to 473 mV for Rhodamine 6G and ISC in the range of 0.91 mA/cm2 to 6.8 mA/cm2 and VOC in the range of 656 mV to 914 mV using Indigofera arrecta plant dye. These yielded η in the range of 0.18 % to 0.39 % and FF in the range of 0.39 to 0.46 using Rhodamine 6G and η in the range of 0.40 % to 4.16 % and FF in the range of 0.42 to 0.54 for Indigofera arrecta plant dye. The η of 4.16 % obtained in this work using Indigofera arrecta plant dye is the highest ever obtained using natural dyes from plants as found in the available literature. Electrochemical impedance spectra of all the PEC solar cells shows impedance variations that agrees with the ISC and VOC results for all the cells as stated above.