Copper yttrium selenide: A potential photovoltaic absorption material for solar cells

Accepted Manuscript Copper yttrium selenide: A potential photovoltaic absorption material for solar cells

Shina Li, Ruixin Ma, Xiaoyong Zhang, Xiang Li, Weishuang Zhao, Zhu Hongmin PII: DOI: Reference:

S0264-1275(17)30047-3 doi: 10.1016/j.matdes.2017.01.037 JMADE 2678

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Materials & Design

Received date: Revised date: Accepted date:

31 July 2016 11 January 2017 12 January 2017

Please cite this article as: Shina Li, Ruixin Ma, Xiaoyong Zhang, Xiang Li, Weishuang Zhao, Zhu Hongmin , Copper yttrium selenide: A potential photovoltaic absorption material for solar cells. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Jmade(2017), doi: 10.1016/ j.matdes.2017.01.037

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ACCEPTED MANUSCRIPT Copper yttrium selenide: a potential photovoltaic absorption material for solar cells Shina Li1 , Ruixin Ma1,2 *, Xiaoyong Zhang，Xiang Li1, Weishuang Zhao1, Zhu Hongmin1 1 School of Metallurgical and Ecological Engineering, University of Science and Technology

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Beijing, Beijing 100083, P R China

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2 Beijing Key Laboratory of Special Melting and Preparation of High-End Metal Materials

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E-mail address: [email protected]

Abstract: A new kind of photovoltaic material, copper yttrium selenide (CuYSe2)

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was synthesized using a self propagating high-temperature synthesis method. Here the

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reaction process of the SHS process of CuYSe2 has been illustrated. The crystalline structure and morphology of the CuYSe2 powders were characterized using X-ray

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diffraction (XRD) and field emission scanning electron microscopy (FESEM). The

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band gap of the CuYSe2 material was estimated to be 1.53 eV based on the UV-vis spectrum of the material. Thin films of CuYSe2 were prepared using a simple,

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low-cost non-vacuum method. The current-voltage (I-V) behavior of CuYSe2

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photovoltaic device was determined. The Ilight/Idark ratio of the CuYSe2 device was found to be 2.81, implying that CuYSe2 is a very promising candidate for application as the absorption materials in thin film solar cells. Keywords: Copper yttrium selenide (CuYSe2); Absorption materials; Solar cell; Self propagating high- temperature 1. Introduction The copper based ternary and quaternary semiconductor compounds are the most 1

ACCEPTED MANUSCRIPT widely used absorption materials in thin film solar cells, due to their unique structural and photoelectric properties [1, 2]. At present, research on this class of materials has focused on IIIA compounds (IIIA=B, Al, Ga, In), such as CuInSe2, Cu (In, Ga) Se2, Cu(In, Al)Se2 and so on[3-6]. Among these semiconductor compounds, Cu (In, Ga) (S,

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Se) (CIGSS) has the highest efficiency of 22.3% to date [7]. However, using of

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indium (In) and gallium (Ga) in the absorption materials has been shown to increase

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the cost of solar cells and limit their wide-scale application in the CIGSS solar cell, because the scarcity of these elements increases their cost [8]. In addition, it is

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difficult to control the composition and structure of the CIGSS [9]. Significant efforts

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have been expended to increase the conversion efficiency and decrease use of In and Ga in the CIGSS [10-13]. In contrast to the IIIA materials, the IIIB group elements

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(such as Sc, Y, La et al.), have a unique electronic structure, and a high quantum

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absorption at shorter wavelengths and subsequently emit light at longer wavelengths [14-17]. In addition, the ionic radius of Y3+ (0.89 nm) is very close to that of In3+

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(0.81nm), but yttrium (Y) is cheaper and more abundant than In or Ga. In recent years,

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the merits of doping semiconductors such as CuInSe(S)2 with rare earth elements have attracted some research attention. Yakushev et al.[18] studied the optical characteristics of Er-CuInSe2. Guo et al. [19] successfully prepared CuIn0.9Ce0.1Se2 (CICS) powders that exhibited a desirable energy band gap of 1.4 eV. Nevertheless, the Voc of a CICS cell was only 73 mV. The structure of the CuScS2 semiconductor and its electronic properties were reported by Scanlon et al. [20,21]. In addition, Brik studied the structural, electronic, optical and elastic properties of ternary 2

ACCEPTED MANUSCRIPT semiconductor CuYS2 by means of the first-principles methods and found the band gap to be 1.342/1.389 eV (GGA/LDA) [22]. The ternary chalcogenide, CuYSe2, is a promising absorber material for low cost solar cell due to its low toxicity, low cost and earth-abundant raw material

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composition. However, to the best of our knowledge, there is so far no report on the

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optoelectronic properties of CuYSe2 as a absorption material in solar cell. In this

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reported work, the successful synthesis of CuYSe2 using self propagating high-temperature synthesis (SHS) method was accomplished and the optoelectronic

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properties of synthesized CuYSe2 were measured. The results showed that the

2. Experimental section

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2.1 Synthesis of CuYSe2 powders

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CuYSe2 was very suitable for use as an absorption material of thin film solar cells.

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In this study, the CuYSe2 materials were synthesized using the SHS method. The starting materials were Cu (General research institute for nonferrous materials, 4N),

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Y(General research institute for nonferrous materials, 4N) and Se (General research

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institute for nonferrous materials, 4N) powders which were combined in with a molar ratio of Cu:Y:Se=1:1:2.1. The starting powders were uniformly mixed in a ball mill for 24 h and then cold pressed into a 12 mm diameter cylinder. The CuYSe2 cylinder was placed into an SHS furnace under an argon atmosphere. After ignition with a short burst of electrical energy, the thermochemical reaction between the constituent materials remain self-sustaining, and a combustion wave propagated through the pressed mass and then fine CuYSe2 powders are fabricated after crushing and grinding. 3

ACCEPTED MANUSCRIPT The time dependent temperature profile of the SHS process of CuYSe2 were measured by a thermocouple (K-type) centered in the pellet ( Figure 1a). 2.2 Preparation of CuYSe2 thin films by non-vacuum methods A back electrode of molybdenum (Mo) layer was deposited by RF magnetron

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sputtering on the soda-lime glass substrate. The as-obtained CuYSe2 powders were

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dispersed in toluene with magnetic stirring for 3 h to form a stable ink solution. Then

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the CuYSe2 inks were spin-coated on the Mo-coated glass substrate at 1000 rpm for 30 s to form the CuYSe2 film and then dried at 80 ℃ for 5 min on a hot plate. The

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spin-coating and drying processes were repeated several times to obtained a sufficient

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thickness of CuYSe2 film. A top electrode of Al on the CuYSe2 layer was also prepared by RF magnetron sputtering.

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2.3 Characterization

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The fabricated products of CuYSe2 powders were characterized using X-ray diffraction (XRD) with Cu Kalpha radiation (λ = 1.54178 Å). SEM images of the

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products were obtained using a field emission scanning electron microscopy (FESEM,

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JEOL, JSM-6340F). The adsorption UV-vis spectrum of the CuYSe2 powders was recorded using a UV-vis spectrophotometer (Shimadzu UV-2550, Japan). Current-voltage (I-V) characteristics of the devices were determined using the two-probe method employing an electrochemical station (CS310, Wuhan China) and xenon lamp (300 W) was used as a white light source at room temperature. 3. Results and Discussion Figure 1 (b) shows that the time dependent temperature profile of the SHS 4

ACCEPTED MANUSCRIPT process of CuYSe2. It is obviously seen that the process can be divided into three stages. The recorded temperature increased and reached the maximum temperature at 346.2 ℃ within 186 s in the first stage. In this stage, the ignition temperature (Tig1) of 116.2 ℃ is closed to the work of Su et al. [23]. In their work, the ignition temperature

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was found to be in the range of 131.7 ℃ to 222.9 ℃. The lower value of Tig1 may be

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caused by the adding highly active yttrium element in the Cu-Se system.

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Then the recorded temperature quickly increased and reached the maximum temperature at 1016.2 ℃ within 33 s in the second stage. Compared with the first

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process, the second process is relatively faster and the ignition temperature (Tig2) is

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much higher than the first stage. In the last stage, the pellet cools down rapidly after reaching the maximum temperature.

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Figure 1 (a) Schematic illustration of the SHS system; (b) temperature profile of the SHS process

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The structure and phases of CuYSe2 powders were characterized by XRD as shown in Figure 2. As can be seen in this figure, the strongest three diffraction peaks

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were found at around 2ө (2-theta) =28.84 °，37.80 ° and 44.50 °, which was consistent

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with the (011), (102), and (110) planes of CuYSe2 (ICSD card No. 153955). The analytical results match well with the tetragonal structure of CuYSe2 with P-3 symmetry, which was similar to the structure of the data reported in the literature for CuYSe2 [24]. In addition to the three strongest peaks, other diffraction peaks for the product were also in very good agreement with those reported for CuYSe2, indicating that our product was composed primarily of CuYSe2 [24]. With the exception of the peaks for CuYSe2, marked by red square in Figure 2, there were four additional weak 5

ACCEPTED MANUSCRIPT peaks which are marked by blue squares located at 2θ = 25.26 °, 29.89 °, 31.88 °, and 39.88 °. This indicated that there was very little Se in the product. This was attributed to the extremely high temperatures achieved following ignition of the reactants that was caused by the combustion reaction of the SHS process. As shown in figure 1 (b),

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the maximum temperature arrive to 1016.2 ℃ in the SHS process. It is difficult to

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completely avoid the sublimation of small amounts of selenium and then coagulated

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onto the surface of CuYSe2 powder during cooling process. Because the molar ratio of raw materials, fabrication strain and anisotropy and various impurities will bring in

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departure from the stoichiometric and then affect the photoelectric property of

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CuYSe2. So the composition of regulation and control is quite important in our future work.

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Figure 2 XRD pattern of the CuYSe2 powders

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The SEM micrographs and composition of synthesized CuYSe2 powders produced by the SHS are shown in Figures 3 (a) and 2(b). The SEM images show

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that the synthesized CuYSe2 powders were composed primarily large sheet-like

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microstructures with a thickness of about 1µm and some random sized sheets of 3~5µm. The EDS chemical analysis of the synthesized CuYSe2 powders showed that the product had a Cu:Y:Se mole ratio of 1.00: 0.97: 2.42, nearly the same as the theoretical stoichiometric 1.00: 1.00: 2.00 ratio of CuYSe2. The prepared CuYSe2 powders were enriched with selenium as previously mentioned, as a result of its sublimation during the SHS high temperature process which affected the stoichiometric ratio of the main elements in the product. These results agreed with the 6

ACCEPTED MANUSCRIPT XRD results. Figure 3 (a) SEM image and (b) EDS of CuYSe2

UV-vis absorption spectroscopy was used to evaluate the optical properties of the CuYSe2 powders and the results of this analysis are shown in Figure 4. As Figure

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4 shows, the CuYSe2 sample exhibited a broad absorption in the visible region with

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absorption tails that extended into longer wavelengths. The optical band gap of the

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CuYSe2 powders was estimated from a plot of (Ahν)2 as a function of hν (A = absorbance, h = Planck’s constant, and ν = frequency). The corresponding band gaps

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of CuYSe2 powders are shown as an insert in Figure 4. The band gap of the CuYSe2

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powders was calculated to be 1.53 eV, which was larger than the CIS (1.01 eV) [25]. Substitution of 25 percent of the indium in a similar product with gallium,

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Cu(In0.75,Ga0.25)Se2 produced a band gap of 1.5 eV whereas the band gap of the

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CuYSe2 was 1.53 eV without any doping or substitution. The band gap of CuYSe2 matches the energy distribution of photons in the solar spectrum in terms of optimum

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conversion efficiency and implies that CuYSe2 is a promising material as a light

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absorption in solar cell.

Figure 4 UV-vis absorption spectrum of CuYSe2 powders

To determine the optoelectronic properties of the CuYSe2 material, a CuYSe2 device with a Al/CuYSe2/Mo/glass substrate structure was fabricated as described in Section 2.2. The current-voltage (I-V) curves of this device were measured in the dark and under AM 1.5 illumination (100 mW cm−2) with a bias of 1.0 V range and are shown in Figure 5. The inset shows a schematic of the CuYSe2 device. As shown in 7

ACCEPTED MANUSCRIPT Figure 5, the dark current of CuYSe2 device was only 1.01×10-3 A·cm-2. However, the current output from the CuYSe2 device increased to 2.84×10-3 A·cm-2 under illumination with an incident light density of 100 mW·cm−2. The Ilight/Idark ratio of the CuYSe2 device was 2.81, which approached or exceeded the Ilight/Idark ratio of other

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semiconductor films. S. Sarkar, et al. fabricated a CuNiSnS4 film and measured a

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Ilight/Idark ratio of about 1.4 for this material [26]. Lu et al. [27] produced a 20 nm

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Cu2ZnSnS4 nano-crystalline material found that the Ilight/Idark ratio was nearly 2.0. Wang et al. [28] investigated the opto-electriconic properties of 18 nm

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nano-crystalline CuFeSe2 material and measured a Ilight/Idark ratio of nearly 7.2, which

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was considerably higher than the CuYSe2 film reported here. But the current of the CuYSe2 materials reached as much as 2.8 mA, about 3000 times greater than the

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CuFeSe2. The illumination excites electrons in the valence band which jump into the

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conduction band and then the increase in holes in the CuYSe2, generates the photocurrent in the film. The structure of CuYSe2 solar cell device, the thickness of

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each layer will play an important affects on the performance of the device. However,

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this obvious photoresponse behavior also demonstrated that the CuYSe2 semiconductor powders could be a great potential candidate for use in thin film solar cells.

Figure 5

I-V curve of the CuYSe2 film tested in the darkness (black) and under illumination (red). Inset: Schematic of I-V device

4. Conclusions In summary, a potential absorption material, CuYSe2 was prepared by the self 8

ACCEPTED MANUSCRIPT propagating high-temperature synthesis method for the first time. The process had three stages and the maximum temperature of CuYSe2 by SHS process was 1016.2℃. The synthesized CuYSe2 exhibited good absorption properties in the visible light region and the band gap was determined to be 1.53 eV, which is close to the optimum

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value for use in solar cells. Importantly, the CuYSe2 film with an Ilight/Idark ratio of

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2.81 exhibited an excellent photo-electron responsive behavior. This effort illustrates

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that CuYSe2 is a promising candidate absorption materials for use in low-cost and mass production fabrication of thin film solar cells. The success of CuYSe2

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preparation will be quite important to broaden the available species for use in low cost

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solar cells. Acknowledgment

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The work was supported by Fundamental Research Funds for the Central

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ACCEPTED MANUSCRIPT Figure Captions Figure 1 (a) Schematic illustration of the SHS system; (b) temperature profile of the SHS process

Figure 3 (a) SEM image and (b) EDS of CuYSe2

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Figure 4 UV-vis absorption spectrum of CuYSe2 powders

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Figure 2 XRD pattern of the CuYSe2 powder

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Figure 5 I-V curve of the CuYSe2 film tested in the darkness (black) and under

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ACCEPTED MANUSCRIPT Figure 1 (a) Schematic illustration of the SHS system; (b) temperature profile of the SHS process

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Figure 2 XRD pattern of the CuYSe2 powders

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Figure 3 (a) SEM image and (b) EDS of CuYSe2

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Figure 4 UV-vis absorption spectrum of CuYSe2 powders

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I-V curve of the CuYSe2 film tested in the darkness (black) and under

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights (1) A novel photovoltaic absorption material CuYSe2 has been prepared using the self propagating high-temperature synthesis method. (2) The CuYSe2 has the suitable band gap of 1.53 eV. with an Ilight/Idark ratio of 2.81 exhibited an excellent

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(3)The CuYSe2 device

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photo-electrical responsive behavior.

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