Investigation of factors limiting efficiency in Cu(In,Ga)Se2 thin film solar cells during rapid evaporation process
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).
References (19)
- et al.
Monolithically integrated flexible CIGS solar cells and submodules with an Al2O3 dielectric layer
Sol. Energy Mater. Sol. Cells
(2013) - et al.
Fabrication of substrate-type CuInSe2 thin film solar cells
Sol. Energy Mater. Sol. Cells
(1994) - et al.
Raman and XPS studies of CIGS/Mo interfaces under various annealing temperatures
Mater. Lett.
(2014) - et al.
Raman investigations of Cu(In,Ga)Se2 thin films with various copper contents
Thin Solid Films
(2008) - et al.
The influence of Na on metastable defect kinetics in CIGS materials
Thin Solid Films
(2009) - et al.
CIGS absorbers and processes
Prog. Photovolt. Res. Appl.
(2010) - et al.
Properties of Cu(In,Ga)Se2 solar cells with new record efficiencies up to 21.7%
Phys. Status Solidi Rapid Res. Lett.
(2015) - et al.
Cu(In,Ga)Se2 solar cells grown by a three-stage process with different evaporation rates
- et al.
Progress toward 20% efficiency in Cu(In,Ga)Se2 polycrystalline thin-film solar cells
Prog. Photovolt.
(1999)