Effect of post deposition annealing on the structure, morphology, optical and electrical properties of CuInGaSe2 thin films
Introduction
Copper indium selenide (CIS) and its alloy system Cu(In,Ga,Al)(Se,S)2 belong to the semiconducting I-III-VI2 materials family that crystallise in the tetragonal chalcopyrite structure. The alloy system Cu(In,Ga,Al)(S,Se)2 provides the possibility of building alloys with a wide range of bandgap energies, i. e Eg between 1.04 eV for CuInSe2 and 3.45 eV for CuAlS2. The other similar materials in I-III-VI2 compound semiconductors are Silver indium selenide (AIS) AgGaS2(Eg = 2.69 eV), AgGaSe2(Eg = 1.83 eV) and AgGaTe2(Eg = 1.1–1.36 eV) and quaternary I2-II-IV-VI4(Ag2ZnSnS4) [1] are promising materials for solar cell applications. Introduction of Ga in AIS structure results in AIGS quaternary semiconductor and band gap value can be altered similar to Cu based chalcopyrite (CIGS). But Cu based chalcopyrite absorber show the best performance among the chalcopyrite materials. CIGS based absorbers are polycrystalline films with wider grain size so electronic activity of grain boundary and extended defects are low. Also CIGS semiconductors have better temperature coefficient and they are stable under radiation which make them suitable for space applications. In spite of this, CIGS films have different chemical elements and this complicate the structural and electronic properties by different intrinsic defects. The occurrence of several Cu poor phases such as CuIn3Se5 and CuIn5Se5 results as wider band gap energy aswell [2]. Until now, copper indium gallium diselenide (CIGS) with band gap energy (1.14 eV) exhibits the highest efficiency among the CIS alloy system [3]. Reports suggest that, partial replacement of In with 20–30% Ga increases the band gap of the absorber layer matching well with the solar spectrum thereby exhibiting better electronic quality of CIGS when compared to CIS. Further, as an absorber layer in solar cells, CIGS thin films exhibit direct band gap and high absorption coefficient in a wide spectral range [4]. CIGS thin films have been prepared by various methods, such as co-evaporation [5], [6], [7], [8], [9], DC sputtering [10], [11], [12], [13], RF sputtering [14], [15], [16], [17], electro deposition [18],electron-beam gun evaporation [19], [20], Molecular beam epitaxy [21], co sputtering [22], and effusion [23], [24]. The highest efficiency of CIGS based solar cells obtained by co-evaporation has been more than 20% in the laboratory [25]. Co evaporation followed by selenisation remains to be one of the important methods in laboratory for synthesis of high efficient CIGS thin film solar cells. Co evaporation delivers precise control on Cu, In, Ga, Se elements to form a desired stoichiometry and uniformity which provides the highest conversion efficiency. Yet, multiple sources co-evaporation has issues in scaling up in industry scale production because of the challenges for equipment and the difficulty in controlling the uniformity of the films on large area which limits the solar cells production [26]. Nevertheless, co-evaporation has many disadvantages such as low deposition rate, process complexity and requires precise temperature control over the individual evaporation sources. In fact, the stoichiometry deviations in CIGS are astonishingly large yielding a wide process window with respect to composition. Sputtering method is easily scalable using commercially available deposition equipment and can provide good uniformity over large areas with high deposition rates.
The photovoltaic grade CIGS films have a slightly In-rich composition. Cu-rich CIGS shows the segregation of a secondary CuySe phase preferentially at the surface of the absorber film. The metallic nature of this phase does not allow the formation of efficient heterojunctions. However, from previous works, it is observed that Cu rich films have larger grains than In rich films. It is also clear that the overall growth processes for high quality CIGS material have to go through a copper rich stage but have to end up with indium rich composition. In addition, if the Gallium content which replaces the Indium in CIGS increases more than 30%, the concentration of recombination active defects increases which leads to lower device efficiencies.
Our main objective was to prepare CIGS absorber layer using RF magnetron sputtering technique for thin film solar cells. The importance of the absorber layer in a thin film solar cell structure is that most of the short circuit current emerges from the absorber layer. Several techniques have been attempted to deposit the targeted CIGS composition i. e Overall absorber layer CIGS will be in rich of the thin film solar cell structure, which is reported elsewhere [27]. Therefore, it becomes inevitable to formulate a deposition sequence using an appropriate deposition technique for the preparation of device quality CIGS thin films. In order to establish the optimal order of Cu, In and Ga layers in the absorber layer, several attempts have already been done using elemental (Cu, In) and/or binary (CuIn, CuGa) metallic targets in sputtering technique followed by post-selenisation [28], [29]. In the present study, we have used ternary metallic targets and sputtered CIG films in the sequence glass substares/CuInGarich/CuInrichGa followed by post-selenisation process to achieve CIGS thin films. So the approach is unique as for as target composition and sequence is concerned.
CIGS absorber layer thin films were deposited onto well cleaned glass substrates using RF magnetron sputtering (HINDHIGHVAC, India). The various sputtering parameters such as power density, substrate to target distance, base pressure, sputtering pressure and substrate temperature were maintained at 2.54 W/cm2, 4.5 cm, 10−3 Pa, 0.3 Pa and 400 °C, respectively. Prior to the sequential deposition of CIG layers, we carried out individual experiments on both CIG (Garich) and CIG (Inrich) films deposited onto glass substrates, to understand the effect of deposition time over CIG film thicknesses as shown in Fig 1. In all the experiments, the target was sputter cleaned for 10–15 min in argon atmosphere prior to the film deposition to remove contaminants from the target surface. The formation of CIGS thin films were carried out by two step process. In the first step, CIG (Garich) and CIG (Inrich) layers were deposited sequentially onto glass substrates using an optimized deposition rate of 38.5 nm/min and 91.7 nm/min, respectively. In the second step, a selenium pellet was loaded in the quartz spacer set up and then was sublimated onto CIG's coated glass substrates. During the close-spaced sublimation process, 2–3 mm distance of separation was maintained. The temperature profile during selenisation process is shown in Fig 2. Further the selenized CIGS films were subjected to rapid thermal processing (RTP) in N2 atmosphere at 600 °C for around 5 min.
The thickness of the CIGS films was measured using DEKTAK 150 surface stylus profiler and found to be 1.3 μm–1.75 μm. The structure of the CIGS films was then studied using X-ray diffractometer (Shimadzu model –XRD 6000 with CuKα radiation). The transmittance spectra were recorded using a double beam UV–vis–NIR spectrophotometer (JASCO, model–V570). The films were subjected to high resolution transmission electron microscopy (HRTEM) to study the microstructure and to establish the crystalline nature of the films. The TEM instrument used was the JEOL-2010F model equipped with a 200 kV, LaB6FEG source and selected area electron diffraction (SAED) patterns were acquired for layers of CIGS film. The surface topography of the CIGS film was studied using an NY-MDT atomic force microscope (AFM). The carrier concentration and mobility of the films were measured at room temperature (RT) by Hall effect measurements with a field strength of 0.39 T. The elemental composition of the films was measured using X-ray photoelectron spectroscopy (XPS), (SPECS GmbH make) irradiating the sample with Al Kα radiation (1486 eV).
Section snippets
Structural and morphology studies
Fig 3 shows the XRD pattern of as-deposited and annealed CIGS films. The films exhibit sharp diffraction peaks along <112>, <220>/<204>, and <312>/<116> directions substantiating the presence of CIGS phase (JCPDS card 35-1102). As expected, the as-prepared CIGS films exhibit amorphous structure as shown in Fig. 3a. After annealing at 300 °C, the crystalline nature of CIGS film is observed to improve. Fig. 3b shows the peaks along (110), (103), (211), (212), (213/105) and (205) planes pertaining
Conclusion
In the present study, we have used ternary metallic targets and sputtering of film in the sequence of CuInrichGa/CuInGarich/SLG followed by post-selenisation to deposit CIGS thin films using RF magnetron sputtering technique. From the results obtained, it is clear that the post annealed temperature at 600 °C influences the structure, surface morphology, optical and electrical properties. Depending on annealed treatment, the optical band gap of respective films is adjusted from 0.96 eV to
Acknowledgement
The authors wish to thank University Grants Commission (UGC, India) to fund the above mentioned work (UGC Major Project F .No 42-841/2013). The authors also wish to thank Mr. C. Sudarshan for his contribution in the study of CIGS film stability using Laser treatment.
References (43)
- et al.
Thin Solid Films
(2007) - et al.
Curr. Appl. Phys.
(2014) Sol. Energy Mater. Sol. C
(2011)- et al.
Thin Solid Films
(2015) - et al.
J. Phys. Chem. Solids.
(2005) - et al.
Sol. Energy
(2006) - et al.
Solid State Electron.
(2012) - et al.
Surf. Coat. Tech.
(2014) - et al.
Appl. Surf. Sci.
(2012) - et al.
Appl. Surf. Sci.
(2013)
Sol. Energy
Sol. Energy
Sol. Energy Mater. Sol. C
Sol. Energ Mater. Sol. C
Solid-State Electron.
Sol. Energy Mater. Sol. C
J. Phys. Chem. Solids
Ceram. Int.
Curr. Appl. Phys.
Appl. Surf. Sci.
J. Alloys Compd.
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