Elsevier

Thin Solid Films

Volume 615, 30 September 2016, Pages 69-73
Thin Solid Films

Investigation of factors limiting efficiency in Cu(In,Ga)Se2 thin film solar cells during rapid evaporation process

https://doi.org/10.1016/j.tsf.2016.06.054Get rights and content

Highlights

  • Analyses of the origin of limits on the efficiency of the evaporated CIGS thin film

  • Raman spectroscopy clarified the residual phases deteriorating cell performance.

  • Na profiles revealed the higher evaporation rates, the less Na diffusion in the CIGS.

  • Insufficient Na diffusion during rapid evaporation mainly limits cell efficiency.

Abstract

Rapid evaporation is crucial in the low-cost manufacturing of Cu(InxGa1-x)Se2 (CIGS) thin-film solar cells. This form of evaporation deteriorates cell performance in an open-circuit voltage and fill factor. Cell performance is strongly dependent on the deposition process and properties of the thin films. With respect to evaporation rates, analyses of electrical properties, film composition, grain structure, and phase characteristics were conducted to investigate limits on the efficiency of evaporated CIGS thin-film solar cells. CIGS solar cells evaporated at different deposition rates were compared. Raman spectroscopy was used to characterize the residual phases deteriorating the cell performance of these solar cells. Sodium (Na) profiles measured using secondary-ion mass spectrometry revealed that with higher CIGS evaporation rates, Na diffusion in the CIGS layers is lower. Rapidly evaporated CIGS led to two features, residual phases of the CIGS remained and Na concentrations near the surface were insufficient. This result implies that a shortfall in the p-type carrier density during rapid evaporation is a critical factor negatively affecting the efficiency of CIGS thin-film solar cells.

Introduction

Thin-film Cu(InxGa1-x)Se2 (CIGS) is a promising material for photovoltaics because its coefficient of absorption for solar radiation is high [1]. Recently, CIGS thin-film solar cells have achieved the highest efficiencies of 21.7% among thin-film solar cells [2]. Another merit of CIGS thin film is its suitability for producing monolithically integrated structures which offer high output voltage, high productivity, and low production cost [3]. The specific feature of the evaporation method in CIGS is its flexibility in processing with regard to evaporation rate, evaporation sequence, and substrate temperature in each sequence. From the manufacturing point of view, the evaporation rate determining throughput is one of the most important issues. However, grain growth and elemental inter-diffusion in CIGS were not sufficiently improved during rapid evaporation. The solar cells deteriorated in an open-circuit voltage (Voc) and in a fill factor (FF) [4].

In evaporated CIGS, multi-graded methods such as the three-stage method [5], [6], [7] and the bi-layer method [8], [9], [10] have been generally applied to achieve high efficiency. Regarding these methods, the step involving the compositional change from the Cu-rich phase to the Cu-poor phase is reportedly the most important for film composition, grain growth structures, and phase characteristics [4]. The bi-layer method is more promising because its deposition sequence is simple. In terms of industrial mass production, this simple deposition sequence makes it possible to design compact evaporation equipment and thereby enables cost reduction of solar modules. Therefore, detailed analyses of thin-film properties concerning the CIGS evaporation rate in bi-layer method are required. In particular, the ordered vacancy compound (OVC) such as Cu(In,Ga)3Se5 and Cu2(In,Ga)4Se7 [11], and residual phases such as CuxSe, InxSe and GaxSe not fully transformed into the chalcopyrite structure are significant in determining semiconductor properties.

In this work, we analyzed defect-related factors limiting the efficiency of evaporated CIGS thin-film solar cells by varying evaporation rates. Electrical properties, film composition, grain structures, and phase characteristics [11], [12], [13] were evaluated with these CIGS solar cells. The characteristic defects of the rapid evaporation method are also discussed.

Section snippets

Device development

The thin-film solar cells were formed on soda lime glass (SLG) substrates using a sputtered molybdenum (Mo) back contact, a co-evaporated CIGS absorber, a chemical bath to deposit a cadmium sulfide (CdS) buffer, a sputtered zinc oxide (ZnO) window, and aluminum (Al) surface contacts. Mo back contacts with thickness of 600 nm were deposited by dc-magnetron sputtering. The CIGS absorber layers were grown using the bi-layer method at various evaporation rates from 30 to 100 nm/min. After depositing

Cell performance

Fig. 1 plots cell efficiencies for different CIGS evaporation rates. Cell efficiencies were inversely proportional to the evaporation rates. The J–V curves of solar cells, LR, MR, and HR, are compared in Fig. 2. The LR cell had a larger Voc and FF than the MR cell, however, the VOC × JSC value was almost the same. The main reason for the decreasing efficiency was FF deterioration. Low FF mainly resulted from low Rsh. The values of Rsh were 441 Ωcm2 for the HR cell, 2668 Ωcm2 for the MR cell, and

Conclusions

Rapidly evaporated CIGS layers have critical factors negatively affecting the CIGS thin-film solar cell efficiency. Based on Raman spectroscopy, cell performances were inversely proportional to the ratio of residual InxSe phases. The more rapid CIGS evaporation is applied, the lower the Na concentration at the surface of the CIGS layer. From the perspective of CIGS grain growth kinetics, insufficient Na diffusion during CIGS deposition was the main reason for forming donor-like defects, and

Acknowledgements

This work was supported in part by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI).

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