Ultraviolet laser ablation of fluorine-doped tin oxide thin films for dye-sensitized back-contact solar cells

Abstract

In this study, laser ablation of a fluorine-doped tin oxide (FTO) thin film on a glass substrate was conducted using a 355 nm Nd:YVO₄ ultraviolet (UV) laser to obtain a 4 × 4 mm microstructure. The microstructure contains a symmetric set of interdigitated FTO finger electrodes of a monolithic back-contact dye-sensitized solar cell (BC-DSC) on a common substrate. The effects of UV laser ablation parameters (such as laser fluence, repetition frequency, and scanning speed) on the size precision and quality of the microstructure were investigated using a 4 × 4 orthogonal design and an assistant experimental design. The incident photon-to-electron conversion efficiency and the current–voltage characteristics of the BC-DSC base of the interdigitated FTO finger electrodes were also determined. The experimental results show that an FTO film microstructure with high precision and good quality can be produced on a glass substrate via laser ablation with high scanning speed, high repetition frequency, and appropriate laser fluence.

Introduction

Dye-sensitized solar cells (DSCs) have recently attracted considerable attention due to their simple fabrication processes and low fabrication costs [1], [2], [3]. Transparent conductive substrates play an important role in achieving the high optical transmittance and low resistivity required in a functional DSC [4]. Two kinds of transparent conductive oxides (TCOs) are widely used in the DSC: indium-doped tin oxides (ITOs) and fluorine-doped tin oxides (FTOs). FTO is superior to ITO because the former has low and temperature-stable resistivity [5], [6].

In 2010, Fu et al. [7] reported the fabrication process of a monolithic, back-contact, dye-sensitized solar cell (BC-DSC) with an array of two interdigitated finger electrodes. Both the working and counter electrodes were placed onto the same substrate to efficiently extract photogenerated positive and negative charges from an overlying dye-sensitized heterojunction (Fig. 1). In such structures, the effective area of the electrodes was slightly sacrificed. However, the BC-DSC can prevent optical transmission losses because of the reduction in the conductive glass substrate. In addition, several cheap and opaque substrate materials with good conductivity can be used. Furthermore, the large-scale fabrication of interdigitated finger electrodes can be easily realized, which significantly reduces the cost of the BC-DSC and promotes the commercialization of the DSC to a large extent. Such back-contact concepts have been applied in improving the performance of silicon cells [8]. The quality of these microstructured electrodes determines the performance of the BC-DSC fabricated via conventional photolithography techniques [7]. However, these techniques have several disadvantages that are likely to limit their applications in commercial DSC production. The disadvantages include (1) the sequential use of several masks to fabricate a set of 4 × 4 mm microstructured electrodes decreases the efficiency of the system and results in quality defects; (2) it is difficult to apply these techniques in the fabrication of a large BC-DSC; and (3) the manufacturing cost is very high.

Numerous studies on laser ablation have been conducted. Different laser systems were used to process the TCO thin film layers on glass substrates. Chen et al. [9] described the pulsed ultraviolet (UV) laser ablation characteristics of ITO thin films on glass substrates. They reported that the laser repetition frequency and scanning speed affected the spot overlap rate and the ablation width. Chen et al. [10] conducted similar investigations on both glass and polycarbonate substrates. Farson et al. [11] proposed the ablation of ITO films using a 755 nm Ti:sapphire laser with a repetition rate of 2 kHz and a pulse width of 150 fs. Kim et al. [12] studied the ablation of FTO thin-film layers using a simple pulsed 1064 nm laser with a Gaussian mode generated using a pin-hole inserted within the laser resonator. These studies had expounded the characteristics and principles of laser ablation on TCO thin films at different degrees. However, the following issues exist in these studies: (1) most of the previous laser ablation studies only focused on ITO thin films in the manufacturing of flat panel displays; (2) the pulse repetition frequencies of the aforementioned laser systems were less than 10 kHz, making the laser scanning speed too low to achieve high microprocessing efficiency; and (3) these studies only focused on the investigations of a simple ablation line, in which differences in the microfabrication of FTO thin films exist.

It has been reported that the UV light absorption rate of FTO glass is bigger than that of the near-infrared region [13], and the focal spot size of UV laser is smaller than that of near-infrared laser when the parameters of optical systems are fixed. In the present work, a third-harmonic Nd:YVO₄ laser microprocessing system – which has high resolution, high precision, high flexibility without using any mask, high efficiency, and relatively low cost – was used to fabricate a set of 4 × 4 mm microstructured BC-DSC electrodes. The relationships between the laser ablation size precision, the quality of the FTO film, and the UV laser ablation parameters were systematically analyzed using a 4 × 4 orthogonal design and an assistant experimental design. The key parameters and characteristics of laser ablation were also presented in this paper. The incident photon-to-electron conversion efficiency (IPCE) and the current–voltage characteristic of the BC-DSC were also measured to determine the feasibility of the proposed techniques.