Surface Modification of Porous Photoelectrode Using Etching Process for Efficiency Enhancement of ZnO Dye-Sensitized Solar Cells

Hindawi Publishing Corporation Journal of Nanomaterials Volume 2016, Article ID 7403019, 10 pages http://dx.doi.org/10.1155/2016/7403019 Research Article Surface Modification of Porous Photoelectrode Using Etching Process for Efficiency Enhancement of ZnO Dye-Sensitized Solar Cells Sutthipoj Sutthana,1 Duangmanee Wongratanaphisan,1,2 Atcharawon Gardchareon,1,2 Surachet Phadungdhitidhada,1,2 Pipat Ruankham,1,2 and Supab Choopun1,2 1 Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand Thailand Center of Excellence in Physics (ThEP Center), CHE, Bangkok 10400, Thailand 2 Correspondence should be addressed to Supab Choopun; Received 24 May 2016; Accepted 26 June 2016 Academic Editor: Meiyong Liao Copyright © 2016 Sutthipoj Sutthana et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Surface modification of porous ZnO photoelectrode using one- and two-step etching process is investigated for enhancing power conversion efficiency of ZnO dye-sensitized solar cells. ZnO films are modified by the diluted NH4 OH solutions for one-step etching process and used as photoelectrode of dye-sensitized solar cells. Rough porous films are observed after one-step etching process. The fabricated cells based on the optimized one-step etched films show a significant increase in short-circuit current density. The short-circuit current density is directly changed with amount of dye adsorption, which is related to specific surface area. The etched films exhibit higher specific surface area over two times than nonetched films. Thus, the large specific surface area is the key success for increasing amount of dye adsorption. Internal electrochemical property of fabricated cells is also improved, indicating that chemical surface of ZnO films is modified in the same time. The DSSCs fabricated on two-step etched films with NH4 OH and mixed acid HCl : HNO3 show the maximum power conversion efficiency of 2.26%. Moreover, fill factor is also increased due to better redox process because of the formation of fine porous structure during the etching process. Therefore, these results implied that the roles of etching processes are improving specific surface area and fine porous formation which can provide better dye adsorption and redox process for dye-sensitized solar cell application. 1. Introduction Dye-sensitized solar cell (DSSC) is one of the attractive solar cells over the past two decades due to various advantages such as relative high efficiency, easy and simple fabrication process on both rigid and flexible substrates, and nontoxicity, low cost, and environmental friendly raw materials [1, 2]. In addition, DSSCs can open huge opportunities for commercial large-scale production such as possibilities to design solar cell with shape flexibility, lightweight, color, and transparency products [3]. Typically, DSSC consists of photoelectrode (PE), counterelectrode (CE), and electrolyte (EL) [4, 5]. Each component of DSSC is intensively researched in order to enhance power conversion efficiency (PCE). Photoelectrode is considered to be an effective component of DSSC in PCE enhancement due to a key role in controlling photoconversion process such as dye adsorption, light scattering, charge separation, and electron transportation [6–8]. To improve the photoconversion process, surface modification of photoelectrode has been successfully applied for PCE enhancement by using various techniques. The surface treatment of ZnO photoelectrode by controlling temperature has been studied [9] and it was found that PCE is higher at optimum temperature due to an increase of surface area for dye adsorption. The other technique is using double-layer structure films with different particle size to improve light scattering in the photoelectrode [7]. The larger particle size is coated on the smaller particle size to form a scattering layer 2 which plays an important role in increasing light scattering and decreasing electron scattering in the photoelectrode and at the same time resulting in higher PCE. Plasma etching is also an attractive technique for surface modification. The etching process creates porous structure via reactive ion etching (RIE). The pore size, shape, and distribution can be controlled by selecting appropriate reactive gases and flow rate [10]. However, plasma etching is an expensive technique due to vacuum system requirement. Wet chemical etching is an alternative technique which gains a lot of attention due to a low cost, simple, and short-time process. The chemical reaction in chemical etching can create crater-like morphology of oxide films which increases specific surface area and provides better dye adsorption [11]. Moreover, there are a large variety of chemical etchants that can be used in the etching process such as HCl, HF, HNO3 , KOH, NaOH, and NH4 Cl [11–14]. In this work, surface modification of porous ZnO photoelectrode using one- and two-step etching process is investigated for enhancing power conversion efficiency of ZnO dyesensitized solar cells. Wet chemical etching process of diluted base (NH4 OH) solutions in distilled water is a promising simple process to improve specific surface area of ZnO photoelectrode, and mixed acid solution of HCl : HNO3 in distilled water is used to form fine porous structure. Increment of dye adsorption is expected due to the increased specific surface area after the surface modification. The amount of dye adsorption is directly correlated to the amount of generated electrons which can be observed in terms of short-circuit current density (𝐽sc ). Finally, power conversion efficiency of DSSC is expected to be enhanced by the optimized condition. 2. Experimental Details 2.1. Preparation of Photoelectrode. ZnO nanoparticle films were deposited onto fluorine-doped tin oxide (FTO) glass substrates by screen printing technique. Commercial FTO glass substrates were carefully cleaned with detergents, distilled water, acetone, and ethanol in an ultrasonic bath for 10 min each. They were then dried in air before depositing ZnO nanoparticle films. ZnO paste for screen printing was prepared by mixing ZnO nanoparticle in polyethylene glycol (PEG) solution. The PEG solution was prepared by dissolving 10% PEG by weight in distilled water, mixed by magnetic stirrer at room temperature. The ZnO paste was then screened onto FTO glass substrates and sintered at 400∘ C for 1 hr for calcinations and removal of any residual in the films. 2.2. Wet Chemical Etching Process of Photoelectrode. The calcined ZnO films were modified using a wet etching process. In the one-step etching process, a 5% concentration of NH4 OH in distilled water was used to etch the ZnO films at different etching times of 1, 2, and 3 min. Then, the films were immediate (...truncated)