Enhanced Photoelectrochemical Performance of Nanostructured Zinc Oxide Photoelectrodes Via Morphology Control
Authors: Demirci Sankir Nurdan , Yolacan Demet , Komurcu Pelin , Sankir Mehmet 
Published: February 2020

Photoelectrochemical solar cells (PEC) provide both harvesting and storage of the solar energy by converting it to chemical energy. In other words PEC systems convert sunlight into fuels by means of electrochemical reactions [1]. Total system efficiency of the PEC is directly affected by the performance of the working photoelectrode and the counter electrode. Here we investigated the influence of the morphology of the zinc oxide (ZnO) nanostructured photoelectrodes on the efficiency. ZnO photoelectrodes with nano-film, nanowire, and urchin-like morphologies have been used as working electrode in the three-electrode in one compartment cell configuration.
Nano-films (ZnO-NF), 60 nm thick, have been deposited on the indium tin oxide (ITO) coated glass via physical vapor deposition. These films were used as the seeding layer to grow the ZnO nanowire arrays (ZnO-NW) by chemical bath deposition (CBD). Finally we fabricated the urchin-like nanostructures (ZnO-UL) on fluorine doped tin oxide coated (FTO) glass substrates by again CBD technique. In order to increase the coverage and the adhesion of the urchin-like structures we modified the surface of the FTO via hydroxylation process [2]. These electrodes were labeled as ZnO-H-UL.
As expected, the light absorption of the nanowire and urchin-like structures was superior to the nanofilms. Therefore, nanowire formation slightly increased the current density under illumination at VAg/AgCl=0V bias. On the other hand, current density increased approximately 103 times and reached to 3 mA/cm2 for the ZnO-H-UL samples compare to the nano-films. Another drastic change was observed for the photoresponse, which is a measure of photo-generated current in the system. In other words photoresponse of the electrode can be calculated from the ratio of Jill-Jdark to Jdark, where Jill is the current density under illumination and Jdark is the current density at dark. The maximum photoresponse of the ZnO-NF was 4.5 indicating almost no current change in dark and under illumination. However, this value was 55, 7.9x102, and 1.2x103 for ZnO-NW, ZnO-UL and ZnO-H-UL electrodes, respectively. Enhancement in the photo generated current density for the urchin like structure could be related with the better light absorption and longer electron lifetime. According to our best knowledge this is the highest photoresponse reported in literature. Efficiency of the ZnO electrodes was calculated using the well known applied bias photon-to-current efficiency (ABPE) definition [3]. ABPE of the ZnO-NF and NW electrodes was 0.09 and 0.32 %, respectively. Efficiency of the ZnO electrodes increased with formation of needle like structures on the spherical bases. 0.65 and 0.87 % ABPE was observed for ZnO-UL and H-UL nanostructures, respectively. One can determine the overall PEC device efficiency by the ratio between the energy of the produced hydrogen and the total energy spent to accomplish the water splitting reaction [4]. PEC solar cells produced using ZnO-H-UL photoanode had the overall device efficiency of the 2.5%. Better photoresponse, photo current density, and the device efficiency of the ZnO-H-UL electrodes compare to the other nanostructures made them very favorable in PEC systems as working electrodes.
References:
[1] M. Gratzel, Nature 414 (2001) 338-344.
[2] W. Zhang, D. Zhang, T. Fan, J. Gu, J. Ding, H. Wang, Q. Guo, H. Ogawa, Chem Mater 21 (2009) 33-40.
[3] Z. Chen, T.F. Jaramillo, T.G. Deutsch, A. Kleiman-Shwarsctein, A.J. Forman,
N. Gaillard, R. Garland, K. Takanabe, C. Heske, M. Sunkara, E.W. McFarland, K. Domen, E.L. Miller, J.A. Turner, H.N. Dinh, J.Mater Res 25 (2010) 3-16.
[4] P. Dias, T. Lopes, L. Andrade, A. Mendes, J Power Sources 272 (2014) 567-580.