In-situ electrical resistance measurement of the selenization process in the CuInGa–Se system

doi:10.1016/j.tsf.2010.08.017

Thin Solid Films

Volume 519, Issue 1, 29 October 2010, Pages 244-250

https://doi.org/10.1016/j.tsf.2010.08.017 Get rights and content

Abstract

In this work the selenization reactions and reaction paths in CuIn_xGa_1-xSe₂ thin films prepared by sputtering and post-selenization process are investigated. The in-situ electrical resistance measurement technique is applied to monitor all the selenization reactions. The crystal structure is determined by X-ray diffraction (XRD) measurement. From the analysis of resistance-temperature curves and the XRD patterns, the phase evolutions of various crystalline and selenization reaction paths have been obtained. From these measurements, the reaction mechanisms and kinetics in the CuInGa–Se system are further understood.

Introduction

Chalcopyrite CuIn_xGa_1-xSe₂（CIGS）thin film is a promising absorber for solar cells [1], [2], [3]. Sputtering and post-selenization have been well verified for large scale production of uniform CIGS thin films with a high deposition rate [4], [5]. The solar cell efficiency, however, is lower than that of the co-evaporated one [6]. This promotes future study of reaction mechanisms and kinetics of CIGS thin films prepared by sputtering and post-selenization process. We have investigated in detail the phase evolution during the selenization process, which has possible reactions such as Cu-Se, In-Se, Ga–Se, CuIn-Se and CuGa–Se. The results are helpful to comprehend the reaction mechanisms in the CuInGa–Se system, which will eventually advance the performance of the CIGS thin film solar cells prepared by sputtering and post-selenization process.

In our previous work [7], the Ga–Se reaction in the selenization process by in-situ resistance measurement has been investigated. We characterized element Se diffusion as well as the Ga–Se phase transformation during the thermal process. We found that the Ga–Se reaction rate decreases with increasing Ga selenization temperature. In-situ resistance measurement was applied to investigate the selenization process of co-evaporated CuInS₂ [8], CuGaSe₂ [9] and CuInSe₂ [10] films as well as the stacked elemental layer film [11]. It is a powerful tool to reveal the reaction mechanisms of thin film formation. So in this work we use the in-situ resistance measurement to study the reaction mechanisms of selenization in CuInGa–Se system.

Section snippets

Experimental details

CuIn_1-xGa_xSe₂ thin films were prepared by the two-stage method. In the first stage the precursors were prepared by DC magnetron sputtering from In or CuGa targets, which were sputtered onto unheated Mo-coated soda lime glass substrates in Argon ambience. The detailed sputtering process [12] and the preparation of Ga precursors [7] have been described previously. Six metallic precursors, Cu, In, Ga, CuIn, CuGa and CuInGa, were used for film preparation.

In the second stage of the process, the

Cu–Se reactions

The resistance of Cu–Se reaction versus temperature is recorded in Fig. 2(a). The resistance is almost a constant with temperature below 190 °C and it started increase dramatically with a peak resistance at 219 °C. It indicated the reaction of Cu–Se between 190 °C and 230 °C. The reaction resulted in Cu_2-xSe, which was verified by XRD measurement. Furthermore, the resistance of Cu_2-xSe was found to decrease with decreasing temperature as shown in Fig. 2(b).

The Cu–Se reaction time was short as shown

Conclusion

The in-situ resistance measurement was applied to investigate the reaction temperature of binary and ternary selenization reactions of the sputtered metallic precursors in the selenization process. From the XRD and GIXRD patterns, the phase evolutions and the reaction mechanisms of the binary and ternary selenization reaction processes were analyzed. It suggested that the quaternary compound CIGS has been formed from the solid solution of CuInSe₂ and CuGaSe₂ during the sputtering and

Acknowledgements

This work is supported by the National 863 Program of China (Grant No. 2004AA513020), the Specialized Research Fund for the Doctoral Program of Higher Education (No. 200800551008) and by the Scientific Research Foundation for Young Scholars (Grant No. J02050).

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In the previous strategies, a selenization or crystallization process has been implemented for obtaining a CIGS semiconductor with electronic properties to be used as an absorbing material. The selenization process is perhaps as diverse as the electrodeposition process, some studies have been published about: (i) the influence of the conditions of selenization in the structure and composition of the films [36], (ii) the reaction mechanisms among the precursors [37], and (iii) the structure of the precursor layers [38]. In the studies where the electrolytic cell configuration is mentioned, it consists of electrodes suspended vertically from the top of the cell [16,39–41].
Cu(In,Ga)Se₂ (CIGS) is a semiconductor which has been extensively studied for photovoltaic applications. With this semiconductor, efficiencies of solar cell greater than 20% have been achieved by physical vapor deposition. There are different techniques to make the CIGS absorber layer. However, electrodeposition is the most adequate technique to make it for different technical and economic reasons. In this paper, the performance of CIGS solar cells are compared using different strategies to synthesize the CIGS films on Mo-coated glass substrates by one-step electrodeposition employing an electrolytic cell with horizontal electrodes. For the above, two strategies are presented, these are: (i) DC electrodeposition and (ii) DC electrodeposition plus mechanical perturbations to the working electrode during the CIGS film growth. In the as-electrodeposited CIGS film a grain growth in selenium atmosphere process was applied and photovoltaic devices were completed with CdS, ZnO and ZnO:Al layers. The photovoltaic devices that were made with the CIGS films obtained by DC electrodeposition plus mechanical perturbations exhibit better absorber morphology with respect to that where only DC electrodeposition was used. The short circuit current density obtained with the absorber synthetized only by DC electrodeposition was of 7.35 mA/cm² while for the absorber synthetized by DC electrodeposition plus mechanical perturbations was of 28.38 mA/cm², both under 1000 W/m² (AM 1.5).
Leaching and re-synthesis of CIGS nanocrystallites from spent CIGS targets
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In the popular CIGS sputtering process a CuInGa metallic precursor layer is first deposited onto a Mo/glass substrate using CuGa or In metal as the sputtering targets. The material is then selenized using Se or H2Se gases [4–6]. This process presents both cost and environmental problems because of the complexity involved using highly toxic H2Se gas [7–9].
The original target utilization is only about 30% in the CIGS (Cu(In, Ga)Se₂) thin film solar cell sputtering process. The remaining 70% of the target must be recycled to achieve sustainable use of the rare metals, In, Ga, and Se. It is therefore very important to establish spent CIGS target recycling technology to reduce environmental damage.
High CIGS recovery was obtained in this study by autoclave leaching CIGS powder in a sulfate solution using H₂O₂ as the strong oxidant. The CIGS leaching mechanism was studied using XRD and SEM. H₂O₂ was first adsorbed onto the CIGS surface. Then copper–selenium compound was formed on the CIGS surface. The copper–selenium compound decomposition is the rate-controlling step in the CIGS leaching process. Nearly complete CIGS dissolution can be obtained by autoclave leaching at 140 °C for 4 h using 3 M H₂SO₄ as the leaching agent and H₂O₂ as the oxidant. In this study, a direct recycling process combining leaching process and re-synthesis of CIGS nanoparticles was developed to recover CIGS materials for reinsertion into the CIGS supply chain. Mono-dispersed CIGS nano-particles with the mean particle size of 9 nm can be obtained using spent CIGS target as raw materials.
The influence of cracked selenium flux on CIGS thin film growth and device performance prepared by two-step selenization processes
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Several methods have been employed to develop CIGS solar cells such as co-evaporation and selenization. Highest-efficiency of 21.7% has been achieved through the co-evaporation process [3], while the selenization process is a relatively more effective technique to produce large-area and high-throughput CIGS modules [4]. It is known that selenium plays an important role in CIGS films growth in the selenization process [5–7].
Thermal-cracking system has been adopted to produce cracked selenium and the influence of cracked selenium flux on the structure and reaction pathway during the first-step selenization is investigated. High Cracked-Selenium (HC-Se) may facilitate the Cu–Se and In–Se reaction at lower temperatures. The “Polygon grains” observed in the HC-Se samples play a key role in further selenium diffusion into the film since they make the film more incompact. In addition, activation energy analysis indicates that Cu_2−xSe and β-In₂Se₃ formed in the samples prepared in HC-Se atmosphere may result in different growth pathway of Cu(In_1−xGa_x)Se₂ (CIGS) thin film compared with that prepared in Low Cracked-Selenium (LC-Se) atmosphere during the first-step selenization, which restrains lamination in CIGS films effectively so that the distribution of Ga is more uniform throughout CIGS films and no small grains of CuGaSe₂ (CGS) accumulate near the Mo back-contact. As a result, the shunt conductance in this CIGS thin film device prepared in HC-Se is reduced, and the fill factor, open-circuit voltage, as well as the cell efficiency are improved.
Ga gradient behavior of CIGS thin films prepared through selenization of CuGa/In stacked elemental layers
2014, Surface and Coatings Technology
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However, during the selenization process, the incorporation of Ga into CuInSe2 (CIS) is difficult to achieve, that is, the phase-separated mixture of CIS, CIGS, or CGS is formed in the selenized films [11–13]. Although a number of CIGS formation mechanism studies through in situ or ex situ [14–16] phase analyses have been conducted, understanding the reaction behavior of each element through these references is difficult. The behavior of the phase separation and the formation of the single-phase quaternary CIGS are the key issues for the fabrication of high-quality CIGS thin films.
In this study, Cu (In, Ga)Se₂ (CIGS) film was formed using a two-stage process. Bilayer CuGa/In metallic precursors were sequentially deposited using both CuGa alloy and In targets in magnetron sputtering. Then, the Se layer was coated on the sputter-deposited CuGa/In precursors through evaporation. The diffusion behaviors of each element during the post-selenization annealing process were determined based on FESEM, XRD, and SIMS. The higher annealing temperature was proposed to have improved Ga diffusion toward the film surface, and thus, achieved the growth of more uniform, more densely packed, and larger grains. Moreover, Na and O contents were found to be correlated due to the strong affinity between Na and O. The distribution of Na seems to be similar to the Ga profile, which may be ascribed to the diffusion of Ga, which was retarded by Na and O. The CIGS films fabricated by the two-step method consisted of a single chalcopyrite phase, and the efficiency of the cell fabricated by the films was 2.56%.
A single source three-stage evaporation approach to CIGS absorber layer for thin film solar cells
2013, Solar Energy Materials and Solar Cells
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For thin films deposited at room temperature, no peaks other than that from Mo were observed, suggesting that as-deposited thin films are amorphous. For thin films deposited with IB=20 mA and TS=140 °C, several peaks were observed, which can be assigned to In2Se3 according to the reference (PDF#721469), consistent with previous reports [28]. Nonetheless a significant portion of the sample remains amorphous.
A novel single source three-stage evaporation process has been developed to deposit CuIn_1−xGa_xSe₂ absorber layers well suited for thin films solar cells. By varying the electron beam current and substrate temperature for each stage, the film structure, texture, morphology, composition and composition profile can be controlled precisely. The thin films are well crystallized with compact and faceted grains, with sizes ranging from 1 to 1.5 µm and also predominantly textured along [112] direction. The elemental compositions are well controlled with Cu:(In+Ga) of 0.85:0.87 and Ga:(In+Ga) of 0.25:0.26, which is within the range for the highest efficiency CIGS solar cells. Photoluminescence and Raman spectra verify that the as-deposited thin films have desired optical properties, and secondary ion mass spectra composition depth profiling suggests that the thin films have graded bandgaps. Thin film solar cells fabricated with such films achieved a power conversion efficiency of 10.6%. This approach avoids the complexity and large area uniformity issues of co-evaporation, and therefore is promising for in-line CIGS film fabrication using the evaporation method.
Influence of sputtering power on composition, structure and electrical properties of RF sputtered CuIn <inf>1-x</inf> Ga <inf>x</inf> Se <inf>2</inf> thin films
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Citation Excerpt :
It is well known that electrical properties of the materials are controlled by the film composition. The determined ratio Cu%/(In% + Ga%) for 50 W deposited CIGS films is 1.1; this composition presents an excess of copper and thus might result in the existence of Cu2−xSe which exhibit very low resistivity and metal electrical feature [21]. The 100 W deposited CIGS film with near stoichiometric composition shows semiconductor electrical feature.
In this study, Cu(In_1−xGa_x)Se₂ (CIGS) thin films were deposited at room temperature by one step radio frequency (RF) magnetron sputtering process. An one-stage vacuum annealing process without selenization was performed to improve properties of the films. Influences of sputtering power on composition, structure and electrical properties of the as-deposited and annealed films were investigated. As the sputtering power not exceeding a proper power of 100 W, the as deposited and annealed films show near stoichiometric composition and polycrystalline chalcopyrite structure. The annealed films exhibit almost the same composition as the as-deposited ones. All the sputtered and annealed films exhibit uniform and compact surface morphology without peeling and cracking. The electrical conductivity measured in 50–290 K range reveal that the 50 W and 100 W deposited films exhibit metal and semiconductor character, respectively. The 100 W deposited film present data consist with thermoionic emission at high temperatures of 200–290 K. However, Mott law with the variable range hopping mechanism is predominant in the low temperature region.

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In-situ electrical resistance measurement of the selenization process in the CuInGa–Se system

Abstract

Introduction

Section snippets

Experimental details

Cu–Se reactions

Conclusion

Acknowledgements

Sol. Energ. Mat. Sol. C.

Sol. Energ. Mat. Sol. C.

Thin Solid Films

Thin Solid Films

Thin Solid Films

Thin Solid Films

Sol. Energ. Mat. Sol. C.

Thin Solid Films

J. Phys Chem Solids

J. Cryst. Growth

Thin Solid Films