Synthesis and characterization of ZnO and ZnO:Ga films and their application in dye-sensitized solar cells

Abstract

Highly crystalline ZnO and Ga-modified zinc oxide (ZnO:Ga) nanoparticles containing 1, 3 and 5 atom% of Ga 3+ were prepared by precipitation method at low temperature. The films were characterized by XRD, BET, XPS and SEM. No evidence of zinc gallate formation (ZnGa 2 O 4 ), even in the samples containing 5 atom% of gallium, was detected by XRD. XPS data revealed that Ga is present into the ZnO matrix as Ga 3+ , according to the characteristic binding energies. The particle size decreased as the gallium level was increased as observed by SEM, which might be related to a faster hydrolysis reaction rate. The smaller particle size provided films with higher porosity and surface area, enabling a higher dye loading. When these films were applied to dye-sensitized solar cells (DSSCs) as photoelectrodes, the device based on ZnO:Ga 5 atom% presented an overall conversion efficiency of 6% (at 10 mW cm −2 ), a three-fold increase compared to the ZnO-based DSSCs under the same conditions. To our knowledge, this is one of the highest efficiencies reported so far for ZnO-based DSSCs. Transient absorption (TAS) study of the photoinduced dynamics of dye-sensitized ZnO:Ga films showed that the higher the gallium content, the higher the amount of dye cation formed, while no significant change on the recombination dynamics was observed. The study indicates that Ga-modification of nanocrystalline ZnO leads to an improvement of photocurrent and overall efficiency in the corresponding device.

Stepped light-induced transient measurements of photocurrent and voltage in dye-sensitized solar cells based on ZnO and ZnO:Ga

“…27 The increase in roughness factor, that is, a higher actual area per projected area ratio, provided a higher dye loading upon increasing gallium content, which is reflected in a higher short-circuit photocurrent density ͑J sc ͒, as observed in Table I. However, the increase in roughness factor is not enough to explain the increase in J sc values.…”

“…We have recently shown, by transient absorption spectroscopy ͑TAS͒ measurements, some evidence for a higher density of dye cations generated for the case of ZnO:Ga electrodes compared to pristine ZnO. 27 While it suggests a higher charge injection efficiency in agreement with the higher J sc , the longer electron diffusion length in the ZnO:Ga electrodes could be one of the main reasons for the enhanced J sc .…”

“…31 Besides, for the unmatched electron density condition, the open-circuit photovoltage ͑V oc ͒ from DSCs based on ZnO:Ga electrodes is smaller than that observed for DSCs based on ZnO electrodes, corroborating our previous work. 27 The electron diffusion coefficients in DSCs with and without Ga-modified ZnO electrodes are shown in Fig. 3.…”

“…32 Additional data from the XPS spectra of ZnO and ZnO:Ga photoelectrodes showed the presence of gallium as Ga 3+ in ZnO:Ga powder samples by the characteristic binding energies. 27 Therefore, it is believed that some of the Ga 3+ ions seem to substitute for Zn 2+ ions in the wurtzite hexagonal structure of ZnO ͑Ref. 27͒ or occupy the position of zinc vacancies.…”

“…Recently we reported the enhancement of the photocurrent and overall efficiencies in DSCs based on GZO porous electrodes compared to ZnO. 27 The aim of this work is to elucidate the higher energy conversion efficiencies of DSCs based on ZnO:Ga by means of stepped light-induced transient measurements of photocurrent and voltage ͑SLIM-PCV͒.…”