Electrodeposition of zinc oxide on transparent conducting metal oxide nanofibers and its performance in dye sensitized solar cells

Abstract

Transparent conducting nanofibers of indium tin oxide (ITO) and antimony tin oxide were prepared on fluorine doped tin oxide glass substrates by electrospinning. Onto the obtained nanofiber mats first a dense zinc oxide layer followed by a nanoporous ZnO layer were electrochemically deposited. Transmission electron microscopy shows that only ITO nanofibers were covered with dense ZnO layers. For application in dye-sensitized solar cells (DSSCs) the dense layer is needed in order to suppress the back reaction of photogenerated electrons from the ZnO to the electrolyte. Therefore only films with ITO nanofibers were tested as porous electron collection layers in DSSC in view of electron transport and electron collection efficiency, and compared to ZnO layers electrodeposited under identical conditions but without nanofibers. Contrary to the expectation the conductive nanofibers do not improve the electron transport in the photoelectrodes and the solar to electrical conversion efficiency is limited to about 2.4%. It is discussed why the presence of nanofibers mats, which was found to be advantageous for TiO₂-based DSSCs before, is not favorable for ZnO-based DSSCs.

Introduction

Since the first publication on dye-sensitized solar cells (DSSCs) with a high solar to electrical energy conversion efficiency of 7.1% by O’Regan and Grätzel [1] in 1991, this kind of solar cell has attracted much interest. One of the reasons is the quite high efficiency in relation to their low production cost. Typically a nanocrystalline dye-sensitized semiconductor film of TiO₂ with non-ordered nanopores is used as light absorbing layer, but it is also possible to use nanoporous ZnO [2], due to its almost similar band gap energy and band positions [3]. In case of sintered nanocrystallineTiO₂ on glass electrodes Yella et al. just recently published a new record efficiency of 12.3% employing a complex Zn-based porphyrin dye as sensitizer and a cobalt (II/III)-based redox electrolyte instead of the typical iodide/triiodide redox couple [4]. Electrochemically deposited, nanoporous ZnO films have the advantage that they can be prepared without applying high temperatures of about 400 °C, as necessary for the preparation of nanocrystalline TiO₂. ZnO crystallizes already at about 70 °C allowing the use of conducting flexible polymer substrates as electrodes [5]. ZnO-based DSSCs typically employ easy accessible metal-free dyes, for example the indoline dye D149, as sensitizer. The current champion efficiencies for ZnO-based DSSCs fabricated at temperatures below 100 °C are 5.6% on glass electrodes [3] and 3.8% on flexible polyethylenterephthalate/indium tin oxide (PET/ITO) conducting polymer electrodes [5].

The electron transport in the photoanodes strongly affects the efficiency of the solar cells, since electron diffusion through a nanoporous semiconductor layer with a thickness of typically 10–20 μm is in general a slow process, during which the electrons can recombine with the redox electrolyte or an oxidized dye molecule. Haller et al. expanded the surface of the ZnO by electrochemically depositing a porous ZnO layer on as well electrochemically grown ZnO nanorods [6]. In the dense ZnO nanorods the tendency for recombination is lowered, but the electron diffusion is still much slower than in transparent conducting oxide (TCO) materials.

Therefore, another approach to decrease the electron transport time and, thus, to improve the collection efficiency is to use TCO films with a high surface area or even porosity. The TCO surface is then coated with a thin layer of a dye-sensitized semiconductor film drastically limiting the transport length through the semiconductor. This so-called core-shell concept was first realized by Zaban et al., using films of TCO nanoparticles [7]. Later Fattakhova-Rohling et al. electrodeposited ZnO into TCO, i.e. ITO films with a continuous ordered 3-dimensional mesopores network prepared by a sol–gel method [8].

A probably easier approach is the coating of networks of interconnected TCO nanofibers prepared by electrospinning with a porous semiconductor material. Ostermann et al. deposited anodically a 500 nm thick porous TiO₂ layer onto antimony tin oxide (ATO) nanofibers, which led, in comparison to the plain substrate without nanofibers, to an increase of the short-circuit photocurrent density from 0.9 mA cm⁻² to 3.9 mA cm⁻² and reached a solar to electrical power conversion efficiency of 1% [9]. The porosity was achieved by addition of surfactants. Iskandar et al. coated an ITO nanofiber film with nanocrystalline TiO₂ by the doctor blade technique, which led to an increase of the photocurrent density from 8.8 mA cm⁻² without nanofibers to 10.2 mA cm⁻², resulting in an efficiency of about 4% [10]. In both studies no blocking layer was used between the nanofibers and the nanoporous layer, which is typically recommended to suppress the back reaction between the highly conductive nanofibers and the redox electrolyte [11].

In this study we investigated electrospun TCO nanofiber substrates covered with ZnO instead of TiO₂ toward their use in dye-sensitized solar cells. Since fully crystalline ZnO can be easily electrodeposited at 70 °C from O₂-saturated ZnCl₂ aqueous solutions alternatively as compact or porous layer, this method allows the consecutive deposition of a blocking layer and a porous electron collecting layer in the same setup. The electrodeposition of the porous ZnO is simply achieved by addition of the pore-creating additive eosin Y (EY) to the electrodeposition bath after completion of the blocking layer. The eosin Y can be desorbed easily by soaking the film in aqueous KOH (pH = 10.5) [12], resulting in fully crystalline and highly porous ZnO films on top of the compact ZnO layer without further calcination [13], [14]. We compare the solar to electrical power conversion efficiencies and electron transport properties of thus prepared ZnO/ITO nanofiber DSSCs to DSSCs based on porous ZnO electrodeposited under identical conditions on flat FTO substrates without nanofibers.