25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain
INFLUENCE OF THE SURFACE ROUGHNESS ON CIGS-BASED SOLAR CELL PARAMETERS
Z. Jehl*1, F. Erfurth1, N. Naghavi1, L. Lombez1, I. Gerard2, M. Bouttemy2, P. TranVan2, A. Etcheberry2 , G. Voorwinden3, B. Dimmler3, W. Wischmann4, M. Powalla4, J.F. Guillemoles1, D. Lincot1 1,Institut de Recherche et Développement sur l´Energie Photovoltaïque (IRDEP -UMR 7174 EDF-CNRS-CHIMIEPARISTECH), 6 quai Watier, 78401Chatou Cedex, France 2,ILV - UMR 8180 CNRS, Université de Versailles St Quentin, 45 Av. des Etats Unis, 78035 Versailles CEDEX 3,Wuerth Elektronik Research GmbH, Industriestr. 4, 70565 Stuttgart, Germany; 4Zentrum für Sonnenenergie- und Wasserstoff-Forschung (ZSW), Industriestr. 6, 70565 Stuttgart, Germany *
[email protected] [email protected]
ABSTRACT: The influence of CIGSe surface roughness over the photovoltaic parameters is reported. The as deposited rough CIGSe surface is smoothed using a chemical etch, and the bare CIGSe surface is characterized by SEM, AFM and Reflectivity. As the roughness decreases, the total reflectivity of the CIGSe surface increases, but the ratio of diffuse reflectivity decreases as expected for a more flat surface. The sample are processed into solar cells and characterized by I(V) measurements. While the open circuit voltage and the fill factor remains constant, the short circuit current decreases with the decreased roughness, resulting in a reduction of the solar cell efficiency from 14% down to 11%. Quantum efficiency and reflectivity measurements are performed on the solar cells to study the mechanisms involved.
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Introduction
2500 nm using a Perkin-Elmer Lambda 900 UV/VIS/NIR spectrometer.
Understanding the mechanism of light absorption and carrier collection relative to the roughness of the CIGSe surface is important, and may helps to define a pathway for future improvements, especially for the case of ultrathin devices [1-5] that require to maximize those two parameters. Previous studies on the thinning of the CIGSe layer using a etching process have shown the etching leads to a flat and specular surface [6]. However, no one focused on studying the influence of the CIGSe texturation on the cells. In this study, we report the characterization of CIGSe surface and solar cells, investigating the relation between performances and the roughness of the CIGSe surface.
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Results and discussion
Figure 1 shows the evolution of the CIGSe surface from reference non etched sample (fig.1a) to a 4 minutes etched sample (fig. 1c) with cross section SEM pictures. The etching process flatten progressively the surface, without reducing significantly the CIGSe thickness which remains above 2 µm. No apparent damages are observed on the etched surfaces. The smoothing effect of the HBr/Br2 etching on CIGSe absorbers is confirmed by AFM measurements. Figure 2 shows the root mean square (RMS) parameters of the surface roughness on a scanning area of 50x50µm2. The RMS drops from 230 nm down to 90 nm after 4 minutes of etching. Reflectivity measurements performed on the bare CIGSe surface are presented figure 3a; as expected, when the surface becomes smoother, the total reflectivity is increased; in the CIGSe absorption range (400 nm – 1100 nm), the enhancement of the reflectivity for the 4 minute etched sample represents a relative loss of light penetrating the absorber of about 25% compared to the as-deposited non etched CIGSe sample. Interference fringes appear for wavelength below the bandgap of the CIGSe (λ ≥ 1100 nm). The appearance of these fringes is caused by the smoothing of the absorber surface providing a more homogeneous thickness of the absorber film, which thus acts as interference layer. The as deposited reference CIGSe sample does not exhibit any interference fringes due to its high texturation. When cells are completed by deposition of the standard CdS/i-ZnO/ ZnO:Al layer structure, the difference in reflectivity between standard and etched CIGSe is strongly reduced (figure 3b). This pseudo anti-reflecting effect occurs due to the improved adaptation of the optical index at the interfaces air/ZnO/CIGSe compared
Experimental
The investigated Cu(In,Ga)Se2 absorbers (CIGSe) were deposited on Mo covered glass by coevaporation at Wurth Solar [7]. The absorber layers with a standard thickness of 2 µm were chemically etched using an aqueous solution of HBr/Br2. This method is described in detail by Bouttemy et al.[8]. The electrical properties of cells were characterized by current voltage measurements at 25 °C under illumination (AM 1.5 global spectrum). Individual cells of 0.1 cm2 were delimited by mechanical scribing. Absolute spectral response measurements were made with a monochromator (Spectral Products CM 110) under chopped illumination and a lock-in technique. The surface morphology of the samples was investigated by scanning electron microscopy (SEM) using a Leo Supra 35 field emission gun (FEG). The AFM images were obtained with a D3100 microscope and Nanoscope IIIa controller, using contact mode with DNP-20 tips. The optical reflectivity were measured in the range of 250 to
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25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain
in the solar cell. The roughness of the CIGSe is therefore found to be a major parameter for efficient solar cells.
to the interface air/CIGSe, as the optical index of ZnO is about 2 and of CIGSe is about 2.9 [9]. The ZnO window layer causes additional interference fringes even in the CIGSe absorption range, which affect the effective light intensity penetrating the absorber bulk. In the nonabsorption range these fringes superpose with the interference fringes originating from the absorber layer, giving a non-periodic fringe structure. To investigate the impact of the CIGSe roughness over the photovoltaic parameters of the cells, we performed J(V) measurements under standard conditions (25°C, AM 1.5) and the obtained Open Circuit Voltage (Voc), Short Circuit Current (Jsc), Fill Factor (FF) and Efficiencies are shown figure 4. The FF and the Voc remain constant with the etching, indicating that no sub-gap defects are introduced by the process. The observed decrease of the efficiency, from 14 % for the rough non etched solar cell down to 11% for the 4 minutes etched smooth solar cells, is due to the decrease of the Jsc. The figure 5 where Jsc Vs RMS are plotted shows how strong is the relation between the Jsc and the roughness of the CIGSe surface with a similar evolution for both parameters. This decrease of the Jsc is analyzed by spectral response (figure 6) for a rough non etched CIGSe solar cell, and a smooth 4 minutes etched CIGSe solar cell and compared to an absorption curve. The absorption and the quantum efficiency for the rough non etched cell are very close (the difference is approximatively 5%), indicating an excellent carrier collection, whereas for the smooth etched sample, the gap between absorption and quantum efficiency is from 10% to 25 %, indicating a very poor carrier collection in the solar cell. The almost constant absorption is the infrared range is due to a combined effect of the CIGSe absorption and the ZnO:Al free carrier absorption. The roughness of the non etched CIGSe could influence the Jsc by a natural light trapping effect, but incomplete light absorption for the smooth samples is very unlikely because the CIGSe thickness is more than 2 µm which is more than enough to completely absorb light. The other effect involved could be an electrical effect; The carrier collection is probably enhanced by a more important contact area between the CIGSe and the CdS for a rough CIGSe solar cell. This phenomenon needs to be confirmed by further experiments. 4
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b
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Figure 1: SEM cross section of bare CIGSe absorber: (a) the as-deposited CIGSe with an important roughness; (b) CIGSe surface after 30 seconds of etching; (c) after 4 minutes of etching.
Conclusion
CIGSe surface have been chemically etched to reduce their surface roughness from a RMS of 230 nm to 90 nm, while keeping approximatively the same thickness (over 2 µm). The influence of the CIGSe roughness over the electrical and optical properties of CIGSe solar cells have been investigated. Reflectivity measurements shows that the reflectivity of the smooth solar cells(completed with CdS and ZnO) slightly increases. J(V) measurements and spectral response shows that both FF and Voc are constant with the surface roughness, whereas the Jsc and thus the efficiency decrease with the RMS, indicating a strong dependence between CIGSe roughness and short circuit current. This Jsc decrease with the surface smoothing is more important than expected from the losses an increased reflectivity. This is attributed to a more important contact area between CIGSe and CdS due to the surface roughness, enhancing the carrier collection
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25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain
RMS Vs Etching Time
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Figure 2: RMS of the CIGSe surface versus etching time 0,4 0 min
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Figure 3 : (a) Evolution of the total reflectivity of the bare CIGSe surface with different etching times ; (b) Evolution of the total reflectivity of the completed solar cells with different etching times
Average Efficiency (%)
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Figure 4: Photovoltaic parameters from J(V) characterization for CIGSe solar cells with different etching times
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25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain
[8] M. Bouttemy, P. Tran-Van, I. Gerard, A. Causier, A. Etcheberry, J.L. Pelouard, Z. Jehl, N. Naghavi, G. Voorwinden, B. Dimmler, M. Powalla, J.F. Guillemoles, D. Lincot, Thin Solid Film (submitted) [9] A. Campa, J. Krc, J. Malmström, M. Edoff, F. Smole, M. Topic, Thin Solid Films 515 (2007) 5968–5972
Short Circuit Current Vs RMS
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Figure 5: Short Circuit Current versus RMS for different etching times of the CIGSe surface External Quantum Efficiency Vs Approximated Absorption for two solar cells with different CIGS texturation absorption
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Figure 6: External quantum efficiency compared to approximated absorption in the solar cell for the non etched rough CIGSe and the 4 minutes etched smooth CIGSe
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References
[1] Z. Jehl, N. Naghavi, A. Etcheberry, M. Powalla, D. Lincot, et. al, EMRS spring meeting 2010, Thin Solid Film (submitted) [2] O. Lundberg, M. Bodegard, J. Malmstrom, L. Stolt, Prog. Photovoltaics 11 (2003)77. [3] A.E. Delahoy, L. Chen, B. Sang, Subcontractor Report NREL/SR-520-40145. (2006). [4] K. Ramanathan, J. C. Keane, B. To, R. G. Dhere, R. Noufi, Proc. 20th European Photovoltaic Sol. Energy Conf.. Barcelona. Spain. (2005) 1695. [5] K. Ramanathan, R. Noufi, B. To, D.L. Young, R. Bhattacharya, M.A. Contreras, R.G. Dhere, G. Teeter, 4th World Conference on Photovoltaic Solar Energy Conversion, Hawaii (2006) 380. [6] B. Canava, J. F. Guillemoles, J. Vigneron, D. Lincot, A. Etcheberry, J. Phys. Chem. Solids 64 (2003) 1791. [7] M. Powalla, M. Cemernjak, J. Eberhardt, F. Kessler, R. Kniese, H.D. Mohring, B. Dimmler, Sol. Energy Mater. Sol. Cells 18-19 (2006) 3158.
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