ZnO nanorods on undoped and indium-doped ZnO thin films as a TCO layer on nonconductive glass for dye-sensitized solar cells

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Abstract

We present the growth of ZnO nanostructures on indium-doped ZnO film on a non-conductive glass substrate. The indium-doped ZnO film was used as the transparent conductive layer replaces the ITO layer. Various indium doping concentrations can change the electrical properties of ZnO film. The reduced electrical resistivity was investigated from 16.60 × 10−2 to 10 × 10−2 Ω cm. after doping with the optimal concentration of 2 wt% indium. It is found that the characteristic of ZnO nanostructures was strongly affected with indium doping concentration in ZnO films. The overall structural characteristics of ZnO ranged from 100–500 nm in size and 7–10 μm in length and the branch-like structures can be revealed from the 2 wt% indium-doped ZnO film. The room-temperature photoluminescence spectra show a sharp ultraviolet band of 353 nm, indicated to the ZnO nanorods structure. The branch-like structures on the 2 wt% indium-doped film can be yielded the photovoltaic properties with a short-circuit current density of 3.96 mA/cm2, an open-circuit voltage of 0.72 V, a fill factor of 20% and an overall power conversion efficiency of 0.56% under irradiance of 100 mW/cm2 (AM 1.5 G).

Highlights

► Undoped/indium-doped ZnO film as a TCO layer can be represented FTO and ITO layer. ► ZnO nanorods with branch-like structure obtained from 2 wt% indium doping. ► The immersion time of 30 min for photoelectrode, yielded the highest efficiency. ► An η of 0.56%, yielded by the optimal dye loading and indium doping conditions.

Introduction

Since the pioneering work on solar cells of Grätzel et al. [1], dye-sensitized solar cells (DSSCs) have been attracted to be an extensively promising low-cost source for solar-energy, ease of fabrication compared with silicon based photovoltaic device and their solar conversion efficiency achieving 11.3% under full sunlight intensity in 2005 [2]. Recently, DSSCs made from porphyrin dye as a sensitizer and introduced with cobalt (I/II) based redox electrolyte can be improved efficiency exceeding 13% under half sunlight intensity in 2011 [3]. A typical DSSCs are fabricated by consisting of a coating layer of dye molecules onto a TiO2 nanostructured film deposited on transparent conducting oxide glass (TCO-typically fluorine-doped tin oxide-FTO). The TiO2 nanoparticles are sintered to form an interpenetrating network for improved electron conduction. The dye molecules are sensitive when illuminated by light and electrons are injected into the conduction band of the TiO2 nanoparticles. The dye-coated TiO2 nanoparticles as a photoelectrode sandwiched together with counterelectrode made from platinum-coated conductive glass. A transferring photoelectron with reduction and oxidation reaction occurs at electrolyte region which is injected and filled between photoelectrode and counterelectrode, mostly used iodide and triiodide ions as a redox couple [4], [5]. The injected photoelectrons are conducted through an interconnecting TiO2 network to the current-collecting TCO anode, passing the external circuit to reach the counterelectrode. The three-dimensional nanoparticulate network greatly increases the surface area for dye absorption, resulting in enhanced light absorption and enhanced conversion efficiency [6]. However, there are numerous grain boundaries between the nanoparticles in the sintered TiO2 network, which form scattering centers for photoelectron conduction, leading to a decreased photocurrent [7]. To solve this problem, researchers have developed new DSSC structures by replacing the nanoporous TiO2 photoelectrode with single-crystalline nanowire electrodes. Electron conduction in the crystalline DSSCs has been found to be markedly enhanced over that in the nanoparticulate DSSCs [8], [9]. Besides TiO2, another one promising semiconductor oxide has also been explored of low toxicity for use as photoelectrodes in DSSCs is ZnO because it has a wide energy gap of 3.37 eV at room temperature [10], nearly identical to that of TiO2. ZnO has a free-exciton binding energy of 60 meV [11] and it is advantageous for many applications such as gas sensors [12], [13], solar cells [13], [14], [15], varistors [16], [17], photocatalyst [18] and ultraviolet (UV) light emitting diodes (LEDs) [19] due to its good electrical and optoelectronics. Much effort has been devoted to investigate the photovoltaic properties of ZnO nanorod-based DSSCs [20], [21]. Crystalline ZnO nanostructures, i.e., nanorods or nanowires have been synthesized by vapor–liquid–solid (VLS) epitaxy [22], chemical vapor deposition (CVD) [23], pulsed laser deposition (PLD) [24], and hydrothermal processes [25].

In this work we develop a new structure for ZnO nanostructures-based DSSCs. ZnO nanostructures and an n-type dopant in ZnO film were grown on a substrate in a two-step aqueous-solution process. In order to improve the electrical properties of ZnO film for solar cells application, group III element such as Al, Ga and In can be used as cation dopants in ZnO film. When it occupies for Zn2+ cation or in the position of ZnO lattice, leading to the change of electronegativity and ionic radius [26] and ZnO film also changed from insulator through n-type semiconductor to conductor by controlling the doping level and this challenging issues devoted by many groups that have been successfully incorporated cation dopants in ZnO film [27], [28], [29], [30], [31]. Among of these cation dopants, much of works have been done by using Al as a dopant due to its ionic radius is smaller than that of In and Ga, but since Al formed with oxygen, it has very high reactivity with oxygen due to the free energy of formation of Al2O3 is lower than that of ZnO. A large reactivity and low resistivity to oxidize with oxygen of metal dopants were observed and affected to reduce the quality of the film. In this sense, indium is an attractive dopant for ZnO film, as it does not react with oxygen at room temperature and large resistivity to oxidation relative to Al and Ga. We interested to synthesize an indium-doped ZnO film as a transparent layer in the visible region and it can be used to replace FTO as the TCO layer in DSSCs. The new structure eliminates the heterogeneous interface between the TCO and the nanostructures, which leads to enhanced electrical conduction and higher efficiency. We present the results of the growth of nanostructures, characterization, effects of indium doping and photovoltaic properties of the ZnO-based DSSCs.

Section snippets

Synthesis of ZnO nanorods and indium-doped ZnO thin films

The ZnO nanorod arrays and ZnO thin films were grown in an aqueous solution using a two-step method of chemical bath deposition (CBD). In the first step, a ZnO film was grown on a glass substrate; in the second step, ZnO nanorod arrays were synthesized on top of the ZnO film. Prior to growth, the Corning 1737 glass substrates were cleaned with acetone, methanol and distilled water in an ultrasonic cleaner 5 min for each steps, then dried using high pressure N2 gas blowing. The precursors for the

Characterization of ZnO nanorods on undoped and various concentrations indium-doped ZnO films

Fig. 1 shows FE-SEM images in top view of ZnO nanorods on undoped and various concentrations indium-doped ZnO films. Fig. 1a shows the nanorods were grown on undoped ZnO film and oriented perpendicular to the substrate. The diameters and length ranged ∼500 nm and 8 μm, respectively. The nanorods on the 1 wt% indium-doped sample (shown in Fig. 1b), some of nanorods are not oriented perpendicular to the substrate. The sizes of nanorods are in the range of 500 nm–1.5 μm, the length ranged of 7–10 μm.

Conclusions

ZnO nanorods were prepared by a simple low-temperature aqueous solution technique on undoped and indium-doped ZnO films. The undoped film consists of nascent nanorods, but doping changes the surface morphology of the film. Nanorods grown on the undoped film are single-crystalline and vertically aligned to the substrate and become branch-like structures after 2 wt% doped in the film. PL spectra reveal the good optical properties of the nanorods and indicate that being ZnO. Doping with 2 wt% indium

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