Electron transport processes in CuIn1−xGaxSe2 films prepared by four source co-evaporation technique

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Abstract

CuIn1−xGaxSe2 (CIGS) (0.05≤x≤0.2) films were synthesized by co-evaporation of elemental materials onto sodalime glass substrates. Electrical and optical properties of the films were studied in the temperature range of 170–380 K. The effect of grain boundary scattering on the electrical transport were examined which indicated grain boundary scattering to play an important role in describing the electron transport processes in these films at lower temperature range. Band gap, refractive index and surface roughness were determined from the analysis of the optical traces (transmittance and reflectance as a function of wavelength) recorded by a spectrophotometer. These results were correlated with the microstructural analysis by SEM and XRD. Photoluminescence (PL) studies were carried out (at 80 and 300 K) for films with different Ga content. The PL spectra were dominated by a strong emission within 1.02–1.19 eV followed by emissions at ∼0.96 and 0.87 eV.

Introduction

Thin film solar cell is a subject of intensive research since it holds the promise of producing highly efficient low cost devices. During the last decade, researchers working in the field of photovoltaics aimed primarily towards the development of inexpensive solar cells with a view to replace the silicon technology. Among different semiconducting materials, the chalcopyrite semiconductor CuInSe2 (CIS) attracted much attention for solar cell applications due to its large absorption coefficient (∼105 cm−1) and the possibility of obtaining a tunable band gap with the addition of Ga in CIS lattice. This addition of Ga increases the band gap leading to an increase in the open circuit voltage of solar cells made with CuIn1−xGaxSe2 (CIGS) as the absorber layer. Tuttle et al. [1] indicated that solar cells fabricated with homogeneous CIGS as the absorber layer should have a small amount of Ga (x∼0.27) for efficient device performance. Highly efficient devices based on CIS/CIGS were reported by several groups [1], [2], [3], [4], [5].

There are wide varieties of deposition techniques such as evaporation, sputtering spray pyrolysis, electrodeposition, etc. for the deposition of CIGS films. But, whatever be the deposition technique, the control of the composition of the deposited film is the most important criterion for device fabrication since performance of the device depends critically on the properties of the CIGS absorber layer which again is determined by the ratio Cu/(In+Ga). The chemical reaction during the formation of CIGS film by co-evaporation is generally controlled by the Se/(Cu+In+Ga) flux ratio. The substrate temperature would also influence the formation chemistry and the adatom mobility during growth. Thus, quality control is the most important consideration in obtaining good quality CIGS films [6], [7] for practical application. It was observed that Ga/(Ga+In) ratio controls the fundamental band gap and the carrier concentration. The carrier concentration generally increases with this ratio and often a saturation value is attained at x∼0.3. Chen et al. [8] indicated that although lower Ga content (x∼0.2) leads to lower open circuit voltage (Voc), there was significant improvement in the fill factor (FF). So, in this paper we preferred to restrict to small x values (0.05≤x≤0.2) of CIGS films.

Stoichiometric CIGS films are p-type which may become strongly p-type with increasing Ga content while increase in band gap of CIGS takes place through decrease in its electron affinity. Band gap grading of the absorber layer has often been used by the workers for solar cell fabrication with CIGS films. Increasing the band gap of the absorber layer towards the back contact of the device can establish a quasi-electric field, which may aid the minority carrier transport towards the junction. Grading of the absorber layer in the opposite direction by increasing the band gap near the junction would boost the open circuit voltage. Preparation of the CIGS films with the predetermined composition needs good control over the deposition process.

The CIGS films are polycrystalline with defects at the grain boundaries. The amounts of the defects depend on the deposition condition. The microstructure and, hence, the physical properties of CIGS films would also depend critically on the deposition condition. The main losses in polycrystalline solar cells are due to recombination centers existing at the grain boundaries. Large grains and less recombination centers at the grain boundaries always improve the device performance. So, the scattering effect arising out of the impurities and intrinsic defects at the grain boundaries would primarily govern the electron transport processes and, hence, the device properties of the CIGS films.

In this paper, we present our studies on the electrical and optical properties of CIGS (0.05≤x≤0.2) films deposited by four source co-evaporation technique with a view to deriving meaningful information on the effect of grain boundary scattering on the electron transport processes in CIGS films for small x values.

Section snippets

Experimental

The CIGS (0.05≤x≤0.2) films were deposited onto sodalime glass substrates with thicknesses in the range 2.0–2.2 μm. Cu, In, Ga and Se (all with purity of ∼99.999%) were co-evaporated in a conventional vacuum coating unit (20 in. diameter) at a system pressure of ∼10−5 Pa. The unit was fitted with four appropriate evaporation sources for evaporating Cu, In, Ga and Se. Alumina crucibles heated indirectly by tungsten heaters were used for evaporating Cu, Ga and In while Ta muffled boat was used for

Microstructural studies

Fig. 1a–d shows the SEM micrographs of some representative CIGS films with a predominant polycrystalline texture. The compositions of the films, determined from the energy dispersive X-ray (EDX) analysis with ZAF correction, are given in Table 1. The films were characterized by XRD analysis. The XRD traces of the films, shown in Fig. 2, indicated strong reflection from (1 1 2) plane followed by those from (2 2 0)/(2 0 4) and (3 1 2)/(1 1 6) planes. It may be seen that the relative peak intensity for the

Conclusions

The CIGS films with x values within 0.05≤x≤0.2 were deposited onto sodalime glass substrates by co-evaporating Cu, In, Ga and Se. Band gap and refractive index along with surface roughness were determined from optical transmittance/reflectance versus wavelength studies. Variation of band gap with x values (x=Ga/(In+Ga)) showed a bowing behavior Eg(x)=0.988+0.117x+0.594x2 which was in good agreement with the band gap of CGS (1.68 eV for x=1).

The effect of grain boundary scattering on the electron

Acknowledgements

The authors wish to acknowledge with thanks the financial help from the Ministry of Non-Conventional Energy Sources, Government of India, for executing this programme. One of us (S.N. Kundu) wishes to thank the Council of Scientific and Industrial Research for granting him Senior Research Fellowship.

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