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FABRICATION OF Cu(In, Ga)Se2 THIN FILMS BY SELENEZATION PROCESS FOR SOLAR CELLS
V. F. Gremenok, E. P. Zaretskaya, V. B. Zalesski*, K. Bente**, W. Schmitz**
Institute of Solid State and Semiconductor Physics, National Academy of Science of Belarus
Minsk, Belarus
* Institute of Electronics, National Academy of Science of Belarus, Minsk, Belarus
** Institute für Mineralogie, Kristallographie und Materialwissenschaft, Universität Leipzig, Germany
Date of birth: September 15, 1954.
Education: Belorusian State University, Minsk, Republic of Belarus, 1976. Title/Position: Doctor of Sciences (Ph.D); Head of Group, Leading Researcher.
Affiliation: Institute of Solid State and Semiconductor Physics, National Academy of Sciences of Belarus.
Professional Experience: physics and technology of thin film materials for opto- and microelectronic applications. Thin film solar cells based on complex semiconductors Expert of INTAS Project proposals since 2000 and the Belorusian Republican Foundation for Fundamental Research since 1999. Publications: 126 papers in refereed editions and 12 patents.
Cu(In, Ga)Se2 thin films have been prepared by a two-step growth process in which metallic precursors were sputter-deposited onto glass substrates followed by an annealing under N2 flow with solid Se source close to samples. An advantage of the process is that it is pollution free, as it avoids any toxic gases. Characterisations included studies of morphological features, formation of crystalline phases and the depth compositional uniformity of the final thin films. From these studies optimum growth parameters were determined for the preparation of high-quality chalcopyrite thin films with good structural and compositional properties. The formation and optoelectronic properties of glass/Mo/Cu(In.Ga)Se2/CdS/ ZnO/Al-Ni thin film solar cells are also reported. The better conversion efficiencies were around 9-11 %.
Gremenok Valery Feliksovich
Introduction
Thin film technology provides excellent prospects for low cost solar cell and high efficiency modules that can be fabricated on large-area substrates using high-throughput processing techniques. These devices are typically fabricated on polycrystalline absorber layers with thicknesses in the 1-10 |im range. Among the polycrystalline compounds developed as absorber layers for high efficiency thin film solar cells are the chalcopyrite Cu(In, Ga)Se2 (CIGS) compounds. These semiconductors are especially attractive for solar cell applications because of their high (>104 cm-1) optical absorption coefficient, stability and their versatile optical and electrical characteristics which can be manipulated and tuned for the specific need in a given device structure [1, 2].
The record high efficiency of CIGS based solar cells (19.2 %) is by far the highest compared with those achieved by other thin film technologies such as CdTe or amorphous a-Si [3]. This result was reported for laboratory scale solar cells prepared by the co-evaporation of all elements from individual sources. However, this technique is complex and requires accurate temperature control to ensure uniform elemental fluxes and a high degree of stoichiometric reproducibility of CIGS thin films.
In terms of large-scale applications, one of the promising approach is two-stage processing of thin chalcopyrite films, involving deposition of Cu-In-Ga precursors with industrial growth processes in the first step followed by their seleniza-tion using H2Se gas or Se vapour [2, 4].
The object of the present research is to prepare Cu(In, Ga)Se2 absorber films by a two-step growth
process in which metallic precursors were sputter-deposited onto glass substrates followed by an annealing under N2 flow with solid Se source close to samples. An advantage of the process is that it is pollution free, as it avoids any toxic gases. Characterisations included studies of morphological features, formation of crystalline phases and the depth compositional uniformity of the final thin films. From these studies optimum growth parameters were determined for the preparation of high-quality chalcopyrite thin films with good structural and compositional properties. The formation and optoelectronic properties of ZnO/CdS/CIGS/Mo/glass solar sell structures are also reported.
Experimental Technique
All thin films were deposited onto 7 x 2.5 cm2 soda lime glass substrates with and without 0.51.0 |im thick molybdenum layers, which were dc sputtered. CIGS films with thicknesses from 0.8 to 2.9 |im were grown using a two-stage technological process: deposition of Cu, In and Ga layers and subsequent selenization. Metallic precursors were prepared by ion-plasma sputtering with a deposition rate of 10 nm/min. A specially designed target was used to deposit films of required composition. The target of 120 mm diameter and 3 mm thickness had a design, which includes a base, presenting an In-Ga alloy, on which Cu plates of 5 x 5 mm2 size were placed uniformly. The In-Ga base was formed by alloying of In-Ga of required composition in a specially fabricated Teflon mould in air with subsequent re-melting in vacuum. The Ga content in the Cu-In-Ga film was controlled by choise of In/Ga ratio in the initial alloy. Proceeding from the fact that for composite targets a given component content in film depends on a material sputtering coefficient value and its sputtered surface area, the Cu/(In, Ga) ratio in a film was set by the ratio of Cu and (In, Ga) areas taking into account their sputtering coefficients. Preliminary estimation of sputtering coefficients of corresponding materials was performed using sputtering of Cu and (In, Ga) targets. Based on these estimations, the initial target composition was chosen, and subsequently corrected using elemental composition data analysis for the formed films. The substrate temperature during growth was maintained at 100 °C and the base pressure of the system was about 2 • 10-4 Torr.
The Cu-In-Ga precursors have been selenized by diffusion of elemental selenium from solid state sources into the alloy films in nitrogen flow using two-step annealing temperature profile. The process was performed in an electrical furnace which allowed fast heating of the samples. The selenization system consisted of a special container for precursors and pots to accommodate Se material. Spacing between samples was maintained at 4 mm to ensure energetic over-pressure of selenium during formation of the CIGS films. The
furnace was heated at two adjacent working zones up to temperature values corresponding to seleni-zation stages. The first selenization stage was made at temperatures from 240 to 270 °C for 10 to 30 min, the second one — at 460 to 540 °C for 10 to 50 min. Prior to heating, the system was purged for at least 40 min with nitrogen. i
The bulk composition and surface morpholo- ®
a
gy of thin films before and after selenization were |
investigated by electron probe microanalysis ^
(EPMA) using a CAMECA SX-100 and by ener- J
gy-dispersive X-ray (EDX) analysis using a JEOL £
6400 SEM apparatus. The depth profiling was |
done by Auger electron spectroscopy (AES) using $
a Perkin Elmer Physical Electronics model 590. 3
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An Ar ion beam was used for sputter etching. ™ The crystal structure and crystalline phases of the materials were studied by X-ray diffraction (XRD) using a Siemens D-5000 diffractometer with CuKa (X = 1.5418 A) radiation. The 29-range for the diffractometer was set from 10° to 100° with a step size of 0.02°. The observed phases were determined by comparing the d-spacing with the Joint Committee on Powder Diffraction Standard (JCPDS) data files.
Thin film photovoltaic cells were prepared by deposition of Al-Ni/ZnO/CdS/CIGS stacked layers on Mo-coated soda lime glass substrates by the standard process. The different steps of this process were the chemical bath deposition of a thin (appoximately 40 nm) CdS buffer layer on the CIGS film absorber followed by transparent-conductivity RF sputtered ZnO window layer, and vacuum deposition of (Ni-Al) grid as the front contact.
J-V measurements of thin film solar cells (area 0.5 cm2) were performed under standard test conditions (AM0: 1353 W/m2; 25 °C). The average "total area" cell parameters (VOS — open-circuit voltage; JSC — short-circuit current density; FF — fill factor; n — efficiency) of the best cells have been used for comparisons and for data interpretation.
Results and discussion
The ultimate aim of these investigations was to develop a relatively simple and reproducible deposition process for the preparation of device quality Cu(In, Ga)Se2 thin films. Because H2Se gas is toxic, CIGS films were grown by two-step annealing of evaporated Cu-In-Ga alloy precursors in N2 flow with solod Se source close to samples at a wide range of reaction temperatures. The structural and compositional features of the thin films were evaluated before and after the respective steps of selenization.
A typical surface scanning electron micrograph (SEM) of the metallic Cu-In-Ga precursors deposited by sputtering is shown in Fig. 1a. It can be seen that co-sputtered films have a flat, uniform and dense grain structure over large areas. Similarity in morphology was observed in all films with exception of those very rich in Cu which showed
smother surface morphology. The mean roughness (RMS) measured by scanning over a 10-20 mm projected area was less than 200 nm. The AES depth profiles of Cu-InGa alloy films reveal relatively uniform distribution of components in the bulk of the films (Fig. 1b). The gallium concentration was uniform over the depth of the film.
The copper concentration remained constant through most of film thickness and decreased to the top of the film. Indium concentration was constant through most of film thickness and increased to the surface. These features reflect the formation of single phase material in co-sputtered precursors.
The XRD investigation of Cu-In-Ga precursor films with different Cu/(In + Ga) composition rations showed the presence of highly oriented Cu„In9 phase and very little elemental or alloy
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20 min showed disordered sphalerite Cu(In, Ga)Se2 and In2Se3 extra phases (Fig. 2). At temperatures of 380 °C and above, all the secondary phases appeared to have been completely reacted to form single phase CIGS layers. All films exhibited (112) preferred orientation and had chalcopyrite struc-
CU11In9
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phases. Theoretically, a near exclusive phase is to be observed in Cu-rich film; however, this phase was near exclusive for films ranging in composition from In-rich to Cu-rich. These results may be explained by assuming that any elemental Cu or In is mixed so thoroughly (i. e. in-terstitially) that it is not possible to identify them by XRD. It is known that a good mixture of the alloy during the deposition with a dominant Cu11In9 phase is necessary for the formation of stoichio-metric CuInSe2 on the selenization stage [5]. The formation of the stable Cu11In9 stable phase simplifies reaction during selenization and enables the improved incorporation of the precursor into high-quality Cu(In, Ga)Se2 thin films.
As expected, the structural features (morphological features and formation of crystalline phases) of the final compound films were critically influenced by post-growth annealing. In order to fully comprehend the mechanism of reaction the CIGS films undergo during the selenization process, a study on the growth process was done under different temperature conditions ranging from 250 to 540 °C. XRD measurements showed that phase formation in Cu(In, Ga)Se2 films depends both on the composition ratio in the precursors and on the processing regimes [6,7]. The dominant effect of substrate temperature and the time of crystallization (at the second step of the process) on phase formation were revealed. The CIGS films of near stoichiometric composition grown at TS < 300 °C with selenization time below
Fig. 2. XRD pattern of Cu(In, Ga)Se2 film selenized at TS < 300 °C showing disordering sphalerite CIGS and In„Se„ phases
Fig. 3. XRD pattern of near stoichiometric Cu(In, Ga)Se2 film selenized at T > 500 °C
Fig. 4. Typical SEM micrographs of a near stoichiometric Cu(In, Ga)Se2 films selenized at different temperatures: a — 300 °C; b — 400 °C; c — 540 °C; d — cross-section view of thin films grown at 540 °C
ture. When increasing the selenization temperature to 500 °C, an obvious improvement in the crystal structure was noticed. Under optimal conditions of annealing pure chalcopyrite phase was obtained. The characteristic superlattice reflections and (116/321), (008/400) and (228/424) doublets splitting confirm the formation of cation-ordered chalcopyrite phase (Fig. 3).
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Fig. 5. Typical AES depth profile of near stoichiometric Cu(In, Ga)Se2 thin film
Figures 4a-d depict the typical surface morphologies of precursor films selenized at different temperatures for 30 min. Thus, a clear evidence of the effect of recrystallization temperature on the film morphologies can be observed. The films selenized at T2 = 300 °C show a rough surface structure which suggested that reaction process has not been fully completed (Fig. 4a) and the presence of the binary selenides.
As the annealing temperature increased up to 400 °C these binaries have disappeared in order to form compound chalcopyrite material and the films exhibit a smother surface and a more uniform grain-size distribution (Fig. 4b). XRD-analysis confirms formation of single-phase chalcopyrite Cu(In, Ga)Se2 material. It is important to note, that higher recrystallization temperature (500-540 °C) obviously leads to the complete recrystallization of the film, improvement of film microstructure with the formation of uniform and densely packed faceted grains with sizes of 2-5 nm (Fig. 4c).
A typical cross-section of the CIGS film can be seen in Fig. 4d, revealing a structure of large crystallites which develop on the top of the substrate. The SEM images of the thin films have two aspects: (1) uniform and clear columnar grains that facilitate current transport across the films, and (2) a rather densely packed microstructure free of pinholes and microcracks.
Based on the structural data, it was expected that the depth compositional uniformity of the CIGS thin films varied significantly with reaction temperature and time of selenization. This in turn was expected to critically influence the device performance of completed solar cells. In order to investigate this phenomenon, the AES depth profile was used to study the concentration of elements present in the bulk of the as-grown Cu(In, Ga)Se2 films. In the cases of films selenized at low temperatures, a large variation in the compositional uniformity as a function of the sample depth was observed. Post-growth selenization of the precursors at 500-540 °C for at least 20 min produced films with a high degree of in-depth compositional uniformity (Fig. 5). This observation supports the structural data presented in Fig. 3, indicating uniform, single-phase material after high-temperature annealing in N2/Se ambient. The Cu, In, Ga and Se concentrations and hence the Cu/(In + Ga) atomic ratio remained virtually constant through the entire depth of the film.
The composition data for CIGS films considered in this study are summarized in Table 1. The measurements were carried out at 10 kV by scanning large areas on the films. Errors are estimated to be 4.0 at. %.
Thin film Cu(In, Ga)Se2-based solar cells were fabricated according to a standard CdS/ZnO technology in our
4,0 4,5 5,0
Table 1
Data of the bulk composition properties of Cu(In, Ga)Se2 films prepared by selenization
at temperatures between 380 and 540 °C
Sample Selenization temp., °C Cu, at. % In, at. % Ga, at. % Se, at. % Cu/(In + Ga)
12 400 34.63 17.71 1.60 46.06 1.79
7.2 500 26.52 21.18 2.08 50.21 1.14
12.2 540 25.68 20.68 2.21 51.51 1.12
9 540 25.84 20.47 5.17 48.50 1.07
J-V parameters of differently processed Cu(In, Ga)Se2-based solar cells
Table 2
Sample Substrate temp., oC Vos, mV JSC, mA cm 2 FF, % n, %
12 400 186.4 26.1 25.5 1.2
7.2 500 396.9 32.6 36.7 4.7
12.2 540 415.2 42.7 51.4 9.1
9 540 430.6 47.7 52.1 10.5
laboratories. The typical J-V parameters under AM0 conditions for various glass/Mo/Cu(In, Ga)Se2/CdS/ ZnO/(Al-Ni) devices are presented in Table 2. The influence of the structural and compositional properties of the CIGS films on the conversion efficiencies of the completed cells are clearly reflected in these data. An increase in the substrate temperature and a corresponding improvement in material quality resulted in improvements in all J-V solar cell parameters.
The better conversion efficiencies were around 9-11 %. Optimization of the Ga content in Cu(In, Ga)Se2 films and CdS/ZnO technology is expected to improve FF and VOS parameters and produce solar cell devices with conversion efficiencies well above 10 %.
Conclusions
In typical two-step growth processes, the material quality of the Cu(In, Ga)Se2 absorber thin films are ultimately determined by the structural properties of the precursors and the experimental conditions followed during the selenization process. In this study, copper, indium and gallium precursors with different composition and good crystal structure were deposited by ion-plasma sputtering. Polycrystalline CIGS thin films were prepared by the selenization of metallic precursors in a controlled N2/Se environment. The phases and crystallographic structure of selenized films vary significantly with reaction temperature and time of selenization. At reaction temperatures around 500 °C, a complete recrystalli-zation of the films occurred. SEM studies revealed uniform and dense CIGS films with grain sizes of 2-5 |im. XRD studies confirmed singlephase chalcopyrite Cu(In, Ga)Se2 layers with preferential orientation in the (112) plane. AES anal-
ysis revealed a high degree of compositional uniformity through the entire depth of the film, confirming the complete interdiffusion of the binary phases. Further investigations are in progress to demonstrate the potential of this technology in the fabrication of thin film solar cells with conversion efficiencies well above 10 %.
Acknowledgements
This work was financed by the ISTC Project # B-542 and supported by DLR.
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