The Effects of Different Technologies on Synthesis of CuInSe 2 Prepared by Coreduction

Abstract

Solution thermal method was used to synthesize CuInSe₂ powders from metal chlorides and SeO₂ at 160∼200°C. The phases of product samples were analyzed by x-ray diffraction (XRD), the size and morphology of the products were observed by JSM6380LA scanning electron microscope (SEM). The aim is to explore the best technology to obtain the pure CuInSe₂ powders. Experimental results show that, CuInSe₂ phase can be formed with metal chlorides by reacting for 20 h at 160°C, 180°C and 200°C and has XRD peaks corresponding to (112), (204) and (312) crystal planes, respectively. It consists of irregular spherical grains with about 0.2∼0.5 μm in diameter, which agglomerate larger particles with average particle size about 1∼3 μm. CuInSe₂ phase was formed more easily at the temperature higher than 180°C and shows better crystallinity.

Introduction

The CuInSe₂ materials with suitable band gap and excellent absorption coefficient and reliable stability have become one of the most interesting issues in photovoltaic materials research (1). The conventional methods for preparing CuInSe₂ mostly require vacuum environment, which leads to complex processes in preparation and equipment with high cost. As the result, it is hard to produce CuInSe₂ materials on a large scale (2). It is also a hot spot for preparing absorbing layer CuInSe₂ film from CuInSe₂ powder in a certain way (3-4-5). Preparation methods for CuInSe₂ powders are mainly divided into solid phase synthesis methods such as mechanical grinding and thermal sintering, and liquid phase synthesis methods such as hydrothermal method or solvent thermal method (6, 7), thermal injection method, microwave assisted synthesis method, etc. (8-9-10). In this work CuInSe₂ powders were prepared by solution thermal coreduction method under the conditions of different reaction temperatures, reaction time and raw materials. The effects of different conditions on the phase, crystallinity, size and morphology of CuInSe₂ powders were investigated. Cu₂ZnSnSe₄ and ZnSe were successfully synthesized by our group, which provided a good reference for this work (11, 12).

Experimental details

For preparing CuInSe₂ product powder, CuCl₂·2H₂O, InCl₃ and SeO₂ with a stoichiometric ratio of 1:1:2 were added into a stainless steel autoclave with a teflon liner of 20 mL capacity. The autoclave was filled with absolute ethyl alcohol up to 80% of the total volume. After ultrasonic agitation for about 30 min, 2 mL hydrazine hydrate (N₂H₄·H₂O) was poured into the reactants. The autoclave was sealed and heated at different temperatures (140∼200°C) for 10∼48 h in an electric furnace. After heating, it was cooled down to room temperature naturally. The black products were collected by filtration, washed with deionized water and absolute ethanol, and then dried at 70°C.

The phases of product powder samples were analyzed by x-ray diffraction (XRD) on a model of Bruker D8 Advance XRD system with Ni-filtered Cu-Kα (λ = 1.5059 Å). The composition, sizes and morphology of the products were characterized by JSM6380LA scanning electron microscope (SEM).

Results and discussion

The effects of different reaction temperatures on CuInSe₂ films

Figure 1 shows the XRD patterns of product powders prepared by reacting for 20 h at 160°C, 180°C and 200°C. According to the standard PDF card No.65-2740, it can be known that CuInSe₂ phase can be formed with XRD peaks corresponding to (112), (204) and (312) crystal planes, respectively. When the reacting time is 20 h, impurity phases of Cu₂Se and In(OH)₃ appeared in the products at 160°C and 180°C, respectively. SEM image and EDS results of product powders prepared by reacting for 20 h at 160°C are shown in Figure 2. It indicates that the product powder consists of irregular spherical grains with about 0.2 μm in diameter, which agglomerate larger particles with average particle size about 1∼3 μm. The EDS results can prove that the product powder contains three elements of Cu, In, and Se which may be reduced from raw materials in experiments.

Fig. 1

XRD patterns of product powders prepared by reacting for 20 h.

Fig. 2

SEM and EDS results of product powders prepared by reacting for 20 h at 160°C

The CuInSe₂ films prepared with longer reaction time

Figure 3 shows XRD patterns of product powders prepared by reacting for 48 h at 140°C, 160°C, 180°C and 200°C. It can be seen that, when the reaction temperatures are 180°C and 200°C, the phase of the product powders obtained was CuInSe₂ without obvious impurities. Its strong XRD peaks also correspond to (112), (204) and (312) crystal planes, repectively. When the temperature is below 160°C, the XRD peaks intensity for CuInSe₂ become very low, the impurities such as CuSe and Se began to appear in the product powders obtained at 140°C and 160°C. So it can be considered that CuInSe₂ phase was formed more easily at the temperature higher than 180°C.

Fig. 3

XRD patterns of product powders prepared by reacting for 48 h.

Comparing Figure 1 with Figure 3, it can be seen that there was impurity phase CuSe in the product powder reacted for 20 h, while the sample obtained with longer time 48 h shows better crystallinity with less impurity phase.

Figure 4 shows the SEM images and EDS results of product powders prepared by reacting for 48 h 180°C and 200°C. Figure 4a shows the sample of 180°C has very serious agglomeration phenomenon and larger particle size about 1.5∼2.0 μm. However, Figure 4b indicates that the product sample obtained at 200°C with good dispersion and is composed of spherical particles with diameters about 0.3∼0.5 μm. The EDS results proved that for the product powders prepared by reacting for 48 h contains Cu, In, and Se elements which can be reduced in experiments.

Fig. 4

SEM images of product powders prepared by reacting for 48 h. SEM images: (a) 180°C +48 h, (b) 200°C +48 h; EDS results: (C) 180°C +48 h, (d) 200°C +48 h.

Discussion for the reaction mechanism

The reaction mechanism is proposed as follows: When the CuCl₂·2H₂O, InCl₃, SeO₂ and the reducing agent are put into the autoclave and heated, the reactive Cu, In, and Se atoms are easily reduced from raw materials by reducing agent N₂H₄·H₂O, and can easily combine to be CuInSe₂ molecules. The possible reacting processes are as follows:

CuCl₂·2H₂O + N₂H₄·H₂O → Cu + 2HCl + N₂↑ + 3H₂O + H₂↑

SeO₂ + H₂O → H₂SeO₃

H₂SeO₃ + N₂H₄·H₂O → Se + N₂↑ + 4H₂O

2InCl₃·4H₂O + 2N₂H₄·H₂O → 2In + 6HCl + 2N₂↑ + 10H₂O + H₂↑

Cu + 2Se + In → CuInSe₂

Conclusions

The CuInSe₂ powders were synthesized by solution thermal coreduction method from metal chlorides and SeO₂ with hydrazine hydrate as reducing agent at 160∼200°C. CuInSe₂ phase can be formed with metal chlorides by reacting for 20 h at 160°C, 180°C and 200°C. The XRD peaks correspond to (112), (204) and (312) crystal planes, respectively. The product powders consist of irregular spherical grains with about 0.2∼0.5 μm in diameter, which agglomerate larger particles with average particle size about 1∼3 μm. CuInSe₂ phase was formed more easily at the temperature higher than 180°C and shows better crystallinity with less impurity phase.