Photoelectrochemical characterization of dye-modified ZnO hybrid thin films prepared by electrochemical deposition

摘要

Dye-sensitized electrodeposited ZnO thin films were studied in their photoelectrochemical characteristics. Such electrodes can be applied for dye-sensitized solar cells. The main analysis techniques were wavelength- dependent photocurrent measurements to obtain the incident photon to current conversion efficiency (IPCE) of the films as well as time- and frequency- resolved measurements of the photocurrent (IMPS) and photovoltage (IMVS) to characterize in detail individual steps of photoelectrochemical reactions. The films were further analysed in their absorption spectrum, SEM, film thickness, dye content, porosity and surface area. 5,10,15,20-tetrakis-(4-sulfonatophenyl)porphyrin (TSTPPZn), 2,9,16,23-tetrasulfophthalocyanine (TSPcZn), Eosin Y and Coumarin 343 were used as sensitizer. Electrochemically induced deposition of ZnO from aqueous solutions can provide porous and crystalline ZnO at low temperature on a great number of conductive substrates. Sensitized ZnO can be prepared directly in one step if the sensitizers are dissolved in the deposition bath or in a multi- step procedure following deposition in the presence of specific structure-directing agents (SDA) that influence the morphology, orientation and porosity of ZnO. Films studied here were prepared in the presence of Eosin Y, Coumarin 343 or sodium dodecyl sulfate (SDS) as SDA. Sensitized photocurrents were measured for all sensitizers studied here. Even more than one sensitizer could be used in one ZnO film to widen the spectral response. The interaction of two different sensitizers in the film further decreased the recombination of the generated electrons. Films prepared in one step generally showed only small efficiency because the sensitizers tended to aggregate and hindered the accessibility of ZnO pores. The photoelectrochemical efficiency of electrodeposited ZnO was clearly improved by removing the SDA from the surface after preparation and then adsorb the sensitizer in a separate step (“re-adsorption”). Eosin Y, e.g., is an efficient SDA to obtain a porous and highly crystalline ZnO and the efficiency of such re-ad TSPcZn / ZnO and re-ad TSTPPZn / ZnO have been improved considerably to IPCE values of 31 % (680 nm) and 15 % (420 nm). Intensity modulated analysis showed that the electron transit time of such efficient electrodes is approximately one order faster than the electron lifetime speaking for efficient harvesting of photogenerated electrons and widely suppressed recombination. A typical electron diffusion coefficient of about 1 x 10-5 cm2 s-1 at a photocurrent of 100 ìA and a diffusion length above 5 µm, larger than the film thickness of 2-3 µm were found. The use of Coumarin 343 as SDA led to a rotation of the ZnO growth direction and thereby further improved the electron diffusion coefficient and also the diffusion length in ZnO. The results of the photoelectrochemical electrode kinetics confirm the good photoelectrochemical properties of these electrodeposited ZnO electrodes and show their perspective to be used as electrodes in dye- sensitized solar cells.