Elsevier

Solar Energy

Volume 174, 1 November 2018, Pages 83-96
Solar Energy

Improvement of CIGS solar cells with high performance transparent conducting Ti-doped GaZnO thin films

https://doi.org/10.1016/j.solener.2018.08.050Get rights and content

Highlights

  • High performance transparent conductive Ti-doped GaxZn1−xO (GTZO) films were grown on CIGS solar cell by sputtering.

  • Structural, electrical, optical and photovoltaic properties of the films were thoroughly studied.

  • High carrier concentration and mobility of 1.1 × 1021 cm−3 and 10.9 cm2/V-s were obtained in GTZO films.

  • High conversion efficiency of 9.1% for the CIGS solar cell with GTZO film was demonstrated.

Abstract

This study reports upon the enhancement of the optoelectronic characteristics and crystallite quality of a typical Ga-doped ZnO ternary alloy achieved by using titanium (Ti) doping to form novel Ti-doped GaZnO (GTZO) transparent conductive oxide (TCO) thin films with characteristics of low-cost and indium-free for employment in Cu(In,Ga)Se2 (CIGS) solar cell applications. These thin films are highly favorable for solar power systems. Photoluminescence measurement and X-ray diffraction were employed to assess the GTZO thin films. A high crystal quality was obtained through radio-frequency magnetron sputtering deposition with a sputtering power of 200 W, a thin-film thickness of 750 nm, a substrate temperature of 250 °C, and a growth pressure of 1 mTorr. The thin-film characteristics were a high transmittance of 86.8%, a low thin-film resistivity of 5.1 × 10−4 Ω-cm (carrier mobility of 10.9 cm2 V−1 s−1, and carrier concentration of 1.1 × 1021 cm−3), and a high figure of merit of 32.6 × 10−3 Ω−1; these characteristics established GTZO thin films as an alternative TCO for CIGS solar cell applications.

These devices demonstrated the superior optoelectronic properties of GTZO TCO thin films and the effectiveness of Ti-doping in GaZnO. The CIGS solar cells with high-performance GTZO TCOs exhibited a superior conversion efficiency of 9.1% and on average, a high external quantum efficiency of 87.1% in the wavelength range from 550 to 1100 nm. The performance of these devices demonstrated in this study indicated that GTZO TCO thin films can be used for developing high-performance CIGS solar cells.

Introduction

Transparent conductive oxide (TCO) thin films have been studied extensively in recent years because of their high transmittance and electrical conductivity. TCO films are widely used in numerous optoelectronic applications, such as in solar cells, light-emitting diodes, and panel displays. The most widely used material for TCO films is indium tin oxide (ITO), which has a low thin-film resistivity (1 × 10−4 Ω-cm) and a high visible-light optical transmittance (85%) (Shen et al., 2010). The ITO film is also adopted as a transparent conductive standard front contact in Cu(In,Ga)Se2 (CIGS) thin-film solar cells, which have gained increasing attractions for its promising high efficiency (Nakada, 2005). The emerging consumer demand for a variety of optoelectronic products and human–machine interfaces has generated considerable industrial demand for ITOs. Concerns regarding the availability of indium have recently arisen; therefore, the fabrication costs associated with optoelectronics based on ITO thin films have increased. In addition, ITO films present several disadvantages, including chemical instability, potential harm to humans, and inappropriateness for use with flexible substrates. Therefore, developing alternative low-cost indium-free TCO films exhibiting high-performance is crucial for use in novel optoelectronic devices. Considerable efforts have recently been made to investigate TCO films to replace ITO films.

Zinc oxide (ZnO) thin films with the advantages of low material cost, ease of doping, high stability, nontoxicity, and optoelectronic properties comparable (or potentially superior) to those of typical ITO films could be regarded as one of the most competitive TCO films (Maejima et al., 2010; Park et al., 2009; Sahu et al., 2007; Kuo et al., 2010, Liu et al., 2013a, Liu et al., 2013b).

The electrical properties of ZnO thin films are known to be controlled by the intrinsic defects of zinc interstitial and oxygen deficiencies, which serves as donors for providing electrons. Relatively few devices have applied nondoped ZnO thin films because of the low electrical conductivity caused by their low carrier concentrations (Al Asmar et al., 2005). The low electrical conductivity of a ZnO thin film can be further improved by impurity doping of Group III elements such as B3+, Ga3+, and Al3+ for providing extra electron. To improve carrier concentrations as well as thin-film conductivity, the use of the Ga-doped ZnO (GZO) alloy has been the most advantageous and the electrical conductivity is comparable with that of ITO film; GZO has been studied extensively because of its low material costs and high electron carrier concentrations. Studies have demonstrated high-performance GZO-based optoelectronic devices (Ishizuka et al., 2010, Sang et al., 2001, Park et al., 2014, Chang et al., 2014). Regarding the thin-film stability, GZO thin films with approached Ga3+ and Zn2+ ionic radius of 0.62 Å and 0.72 Å, respectively, indicate less crystal defects because of the reduced lattice strain and high electrochemical stability (Liu et al., 2013a, Liu et al., 2013b). The benefits of the closed crystal lattice length thus leading to highly reliable GZO-based optoelectronic devices.

Unlike GZO thin films, TiO2-doped ZnO (TZO) exhibits Ti4 + (ionic radius: 0.68 Å) substitution for Zn2 + (ionic radius: 0.72 Å) ions; thus, each Ti atom acts as a donor that provides two extra free electrons and further improves thin-film conductivity (Sang et al., 2001). Moreover, studies have reported that TiO2 doping enhances the preferential c-axis orientation growth and optical transmittance of ZnO thin films (Lin et al., 2005, Liu et al., 2012, Liu et al., 2014a, Liu et al., 2014b). To develop TCOs with advantageous optoelectronic performance, Ti-doped GaZnO (GTZO) thin films were produced and studied in this paper to demonstrate remarkable preferential crystal growth, surface flatness, optical transmittance, and electron concentrations, which were less investigated previously. GTZO films can notably reduce thin-film resistivity and has high potential for use in high-performance CIGS solar cell applications.

Numerous thin-film deposition methods have been adopted to deposit TCO films. These include chemical vapor deposition, pulsed laser deposition, DC and radio-frequency (RF) magnetron sputtering, molecular beam epitaxy, and the sol–gel method (Lackner, 2006, Hirano et al., 2007, Cooray et al., 1997). Among these methods, RF magnetron sputtering has been widely employed by industry; it is regarded as an effective technique for depositing TCO films because it is relatively cost-effective, provides superior crystal growth at a high deposition rate, and produces strong thin-film adhesion with a highly uniform thickness distribution over a large substrate area (Yu et al., 2005a, Yu et al., 2005b). The sputtering technique employs argon ions to deposit a thin film of target particulates onto a substrate. Through the conversion of high potential energy into kinetic energy, the particulates on the substrate can be imbued with sputtering energy, which facilitates crystallite nucleation and thus improves the optoelectronic properties of the thin films (Wang et al., 2009). Compared with all other methods, RF magnetron sputtering cause the least sputtering damage to the thin films because relatively low power is required in an RF sputtering system with a high plasma ionization efficiency (Wang et al., 2009, Gorjanc et al., 2002).

To fabricate CIGS solar cells, TCO films can be deposited through RF magnetron sputtering as a front contact layer. Optimizing sputtering parameters such as sputtering power, and thin-film thickness is essential. In addition, the effects of substrate temperatures and growth pressures on the crystallite and optoelectronic qualities of GTZO TCO layers are also thoroughly investigated in this study. The objective of that optimization is to improve device performance and prevent the degradation of device characteristics caused by material interdiffusion through TCO/ZnO and ZnO/CdS heterointerfaces (Wang et al., 2009). In this study, the material, electrical, and optical properties of RF-sputtered GTZO thin films were fully developed and clarified. An optimized TCO film showed a high optical transmittance (86.8%) within a wide visible (VIS) wavelength range (400 to 800 nm), a high carrier concentration (2.6 × 1021 cm−3), and the lowest resistivity (5.1 × 10−4 Ω-cm); this optimal film was produced with a sputtering power of 200 W, a thin-film thickness of 750 nm, a substrate temperature of 250 °C, and a growth pressure of 1 mTorr.

The GTZO alloy material quality was assessed using room-temperature (RT) photoluminescence (PL) and X-ray diffraction (XRD) measurements. The increased ZnO near-band-edge emission (NBE) PL intensity and reduced emission of the GTZO films with respect to the PL measurement due to oxygen vacancy related defects indicate the improved crystallite quality achieved by optimizing the sputtering parameters. Additionally, the enhanced XRD (0 0 2) c-axis orientation intensity and the narrow full width at half maximum (FWHM) indicate the improved crystallinity of the optimized GTZO thin films. CIGS solar cells were fabricated with GTZO TCO films using a relatively inexpensive RF magnetron sputtering technique; the CIGS cells exhibited an advantageous short-circuit current density (Jsc) of 25.9 mA/cm2, an open-circuit voltage (Voc) of 0.5 V, a fill factor (FF) of 0.66, and a conversion efficiency of 9.1% with an average external quantum efficiency of 58.9%. The high-performance CIGS solar cells fabricated with the GTZO films demonstrated excellent performance, and GTZO films has great potential to be used as alternative indium-free TCO thin films.

Section snippets

Experimental details

GTZO TCO thin films were deposited by RF magnetron sputtering on glass substrates for the study of both material and optoelectronic characteristics. GTZO films were sputter deposited using a 3 in. thick Ga2O3/TiO2/ZnO target (3/1/96 wt%, 99.99% purity). In the fabrication of a CIGS solar cell, 1.0 μm-thick Mo back-contact layers were deposited on soda-lime glass substrates through DC-sputtering techniques. The 1.5 μm-thick CIGS absorption layer was grown using three-stage evaporation processes

Results and discussions

Because the thin-film deposition rate and the resulting crystallite quality are dependent on the sputtering power, a series of GTZO thin films deposited at different deposition rates by varying the sputtering power were considered. The correlation between the thin-film deposition rate and sputtering power is shown in Fig. 2, which presents a well-fitted linear relationship at a fixed thin-film thickness of 500 nm, as measured using SEM. The inset shows a cross-sectional SEM image of a

Conclusions

This study investigated the optimized preparation of high-quality GTZO films on glass substrates through RF magnetron sputtering with a sputtering power of 200 W, thin-film thickness of 750 nm, substrate temperatures of 250 °C, and a growth pressure of 1 mTorr for applying as the TCO front contact layers in CIGS solar cells. Experimental results revealed that the optimized GTZO TCO layers exhibited excellent material and optoelectronic properties, because of the effective electron donation by

Acknowledgements

The authors are grateful to Prof. Chyi at the National Central University for instrument support, and the Ministry of Science and Technology, Taiwan, R.O.C., for its financial support under contracts MOST106-2221-E-155-041-MY3. The provision of research equipment by the Optical Sciences Center and Center for Nano Science and Technology at National Central University is greatly appreciated.

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