Influence of annealing temperature on the phase composition and optical properties of CuInS₂ films

Abstract

We studied the growth of CuInS₂ thin films by single-source evaporation of CuInS₂ powder in a high-vacuum system with a base pressure of 10⁻³ Pa. After evaporation, the films were annealed in a sulfur atmosphere at temperatures from 200 to 500 °C for 1 h. XRD curves and Raman spectra of the films demonstrated that chalcopyrite CuInS₂ was the major crystalline phase. The morphology of Cu_xS exhibited a star-like structure, which we report for the first time. The phase composition and optical properties of our polycrystalline thin films were effectively modified by annealing in S. For films annealed at 200 and 350 °C, a secondary CuIn₁₁S₁₇ phase appeared, which may be related to solid-state reaction in the S atmosphere. This secondary CuIn₁₁S₁₇ phase has not been widely reported in previous studies. After annealing at 500 °C, only a chalcopyrite phase was detected, with bandgap energy of 1.46 eV, which is nearly identical to the optimal bandgap energy (1.5 eV) of single-crystal CuInS₂. This indicates that the composition of the CuInS₂ film annealed at 500 °C was nearly stoichiometric. The bandgap of the samples first increased and then decreased with increasing annealing temperature, which may be attributed to an increase in grain size, the secondary CuIn₁₁S₁₇ phase, and deviation from stoichiometry.

Introduction

Chalcopyrite thin-film solar cells are already being used in the rapidly growing photovoltaics market. In particular, the ternary compound CuInS₂ has potential for high conversion efficiency owing to its high absorption coefficient of approximately 10⁵ cm⁻¹ and its direct bandgap of 1.5 eV [1], which matches well to the solar spectrum. Furthermore, no high toxic component was included in this compound semiconductor. However, the efficiency of CuInS₂ solar cells has only reached 12.2% [2], which is much lower than that of other analogous chalcopyrite thin-film solar cells (CuInSe₂). To improve the efficiency of CuInS₂ solar cells, a variety of methods have been used to deposit CuInS₂ thin films, including wet chemical routes [3], rapid thermal processes [4], co-evaporation of elemental sources [5], sulfurization of metallic precursor films [6], chemical vapor deposition [7], sputtering [8], electro-deposition [9], and spray pyrolysis [10], [11]. Some groups have used thermal evaporation of a single source (CuInS₂ powder) to prepare thin films. Akaki et al. prepared Sb-doped CuInS₂ thin films by single-source thermal evaporation and studied their structural and electrical–optical properties [12]. They obtained polycrystalline CuInS₂ thin films after annealing at >200 °C. The Sb-doped CuInS₂ thin films were close to stoichiometry compared to nondoped CuInS₂ thin films. Abaab et al. deposited CuInS₂ thin films by single-source vacuum thermal evaporation of a substrate submitted to a longitudinal thermal gradient [13]. Some of the films were annealed in a sulfur atmosphere and converted to homogeneous CuInS₂ layers. The optical bandgap of the films after sulfurization was 1.5 eV, which is close to the optimal value for photovoltaic energy conversion. Rabeh et al. prepared CuInS₂ films by single-source thermal evaporation and post-growth treatment at 100–350 °C in air [14]. For annealing temperatures >200 °C, the n-type conductivity was stable and the resistance correlated well with the annealing temperature. Zribi et al. studied the optical properties of sodium-doped CuInS₂ thin films grown by single-source evaporation [15]. They observed that the optical bandgap decreased with increasing sodium concentration. The refractive index and extinction coefficient also depended on the sodium concentration. The same group grew Na-doped and Sn-doped CuInS₂ thin films by double source thermal evaporation using CuInS₂ powder and the doping element [16], [17], [18]. The doped films were annealed at 250–500 °C under vacuum. The structure, morphological, and optical properties of the films were studied by XRD and UV-Vis-NIR spectroscopy. The change in bandgap for doped samples annealed at 250–500 °C was discussed in detail.

For device applications it is important to know the structure and optical properties of CuInS₂. As our source material we used a bulk CuInS₂ polycrystalline ingot grown by the vertical Bridgman method. CuInS₂ thin films were prepared by the single-source thermal evaporation method. The films were then annealed at 200–500 °C for 60 min in a sulfur atmosphere. The effects of annealing temperature on the phase composition and optical properties of the films are discussed.