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Energy Procedia 44 ( 2014 ) 209 – 215

1876-6102 © 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and peer-review under responsibility of The European Materials Research Society (E-MRS)

doi: 10.1016/j.egypro.2013.12.029

ScienceDirect

E-MRS Spring Meeting 2013 Symposium D - Advanced Inorganic Materials and Structures for

Photovoltaics, 27-31 May 2013, Strasbourg, France

Microcrystalline SiGe Absorber Layers in Thin-Film Silicon Solar

Cells

K. v. Maydell, K. Grunewald, M. Kellermann, O. Sergeev, P. Klement, N. Reininghaus,

T. Kilper

NEXT ENERGY · EWE Research Centre for Energy Technology at the University of Oldenburg, Carl-von-Ossietzky-Str. 15, 26129 Oldenburg, Germany

Abstract

We report on physical properties of microcrystalline silicon-germanium (μc-SiGe:H) absorber layers for the use as a bottom structure in silicon based multijunction thin-film solar cells. Due to incorporation of Ge the absorption of the film is enhanced compared to pure μc-Si:H films. This provides the opportunity to significantly reduce the absorber layer thickness. The experiments were carried out in a 13.56MHz PECVD reactor using germane, silane and hydrogen as process gases. Single layers were characterized for their optical and electrical properties. Results from single and multijunction solar cells using a μc-SiGe:H absorbers will be shown. In tandem solar cells a reduction of about 60% of the absorber layer thickness could be reached by using SiGe alloys compared to pristine silicon tandem cells.

© 2013 The Authors. Published by Elsevier Ltd.

Selection and peer-review under responsibility of The European Materials Research Society (E-MRS).

Keywords: Silicon thin-film solar cell; silicon germanium alloys, μc-Si:H solar cells, tandem solar cells

Available online at www.sciencedirect.com

© 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license.

Selection and peer-review under responsibility of The European Materials Research Society (E-MRS)

Corresponding author: karsten.von.maydell@next-energy.de

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1. Introduction

Beside low production costs and the usage of abundant raw materials silicon based thin-film solar cells have the advantage to be built up as multijunction devices like tandem or triple junction solar cells. In a tandem configuration based on the micromorph technology (amorphous silicon and microcrystalline silicon) the stabilized efficiencies exceed 12% [1]. Recently, Kim et al. published a new record for silicon based thin-film solar cells. The solar cell exhibits a stabilized efficiency of 13.4% and is based on triple junction technology consisting of amorphous and microcrystalline silicon absorber layers [2]. One drawback of the technology is the relatively large bottom cell absorber thicknesses. In tandem configuration the absorber layer thickness is in the range of 1.3-1.7μm; in triple junction configuration the bottom cell absorber layer has to be increased to about 2.5μm due to the poor absorption of the material [3]. This in fact has the disadvantage of long tact times in production and thus high production costs. One solution to overcome this problem is to optimize light trapping by using high scattering transparent conductive oxide front contacts and novel back scattering structures. On the other hand it is also possible to increase the deposition rate. An alternative and attractive way is to decrease the absorber layer thicknesses. However, in best case all of the above mentioned points have to be optimized. One way to decrease the film thickness is to increase the absorption coefficient of the materials. Matsui introduced the usage of hydrogenated microcrystalline silicon/germanium alloys (μc-SiGe:H) which might be favorable due to higher absorption of incident light [4, 5].

In this contribution we report on the development of μc-SiGe:H absorber layers for the usage as ultrathin bottom absorbers in silicon based multijunction solar cells. It has been reported by several authors, that a Ge concentration in the film larger than 20% leads to a high defect formation and thus the Ge variation is limited in this contribution to maximum about 22% [4, 5]. The experiments were compared to samples and solar cells without Ge in the films.

2. Experimental

Microcrystalline silicon and silicon-germanium films were prepared using plasma enhanced chemical vapor deposition (PECVD) working at 13.56MHz which is implemented in a VonArdenne PECVD/PVD cluster tool. For solar cell preparation the precursor gases, SiH4, GeH4, H2, PH3 and B2H6 were used. The substrate temperature during the absorber layer deposition was 200°C, the pressure was 9mbar and the power density 0.14 W/cm². The

Fig. 1: Absorption coefficient as a function of photon energy for μc-SiGe:H films with different Ge concentrations X. The dotted lines denote the pristine materials.

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SiH4 flow was fixed to 5sccm, whereas the GeH4 flow was varied between 0.2 and 0.5sccm. Single layers were deposited on glass substrates, solar cell structures on sputtered and HCl-etched ZnO:Al substrates on glass with structure height of about 80-100nm. The germanium gas-phase concentration GC is defined as:

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Whereas X is the germanium concentration in the resulting SiGe film given in at.% measured by energy dispersive X-ray spectroscopy. The absorption coefficient was determined using a Varian UV-VIS spectrometer. Raman measurements were performed by using a Bruker Senterra system with an excitation wavelength of 488nm. To determine the Ge concentration in the films energy dispersive X-ray spectroscopy (EDX by Oxford) was used which is implemented in a Zeiss scanning electron microscopy system. Solar cell efficiencies were measured by a 2-lamp system from WACOM and the external quantum efficiency by a system from RERA.

3. Results and discussion

Fig. 1 shows the absorption coefficient as a function of the incident photon energy for samples with different Ge concentrations, X, in the films. The dashed lines represent the pristine materials (μc-Ge:H and μc-Si:H). The absorption coefficient of Ge is about 2 magnitudes higher than of Si at 1.5 eV. It can be seen clearly, that the

] [ ] [ ] [ 4 4 4 GeH SiH GeH GC 

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absorption coefficient in the interesting energy region is increasing when X is increased starting from pristine μc-Si:H films as expected. The E04 band gap value decreases from 2.15eV for X=0% to 1.88eV for X=17%. The incorporation of germanium in the film has also influence on the lattice parameters. In Fig. 2 Raman spectra are

shown for the same samples as in Fig. 1. The microcrystalline growth, indicated by the LO-TO phonon line of crystalline silicon located around 520cm-1 dominates the spectrum for X = 0 which is broadened by a resonance located around 505cm-1 and the a-Si:H resonance at around 480cm-1. With increasing germanium content in the film the LO-TO phonon resonance shifts to smaller wave numbers. Additionally, a new resonance emerges when X

Fig. 3: Position of the LO-TO phonon mode as a function of the Ge concentration in the film (black triangles). The red line is a guide to the eye.

Fig.4: Ge concentration X in the SiGe film as a function of the gas-phase concentration determined by EDX (black triangles) and by the Raman shift as described in the text (open triangles).

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increases located at around 400cm-1 which is a symmetrical SiGe resonance. The shift of the Si LO-TO phonon mode is shown in more detail in Fig. 3. As already mentioned the LO-TO phonon mode shifts from about 521cm-1

to about 507cm-1 when X is creased from 0% to 17% (Fig. 3, black triangles). This can be attributed to internal stress due to the incorporation of Ge in the silicon network. This is accompanied by a reduction of the Raman

crystallinity from values larger than 88% to about 86% (in the range of 0<X<13 at.% - not shown here). The Raman crystallinity was calculated by only using the resonance of the silicon lattice after Droz et al [6]. The SiGe or Ge resonances were not taken into account for the calculation of the Raman crystallinity. It should be noted that the Raman crystallinity for the use in tandem solar cells should be in the range of 50-60% [7]. The approximately linear decrease of the LO-TO phonon resonance with increasing Ge concentration can be used to estimate the Ge content in the microcrystalline SiGe film from the gas phase concentration. Alonso et al. [8] proposed a linear dependency as

X

Si

Si

520



70

Z

Fig. 5: External quantum efficiency for different μc-SiGe:H single junction solar cells with varying Ge concentration.

Fig. 6: Open circuit voltage (left) and efficiency (right) for μc-SiGe solar cells as a function the of Ge concentration in the absorber.

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In this equation ZSi-Si denotes the LO-TO resonance and X the Ge-content; 520 is the value of the silicon LO-TO resonance. Equation 2 was evaluated for X and the Peak position for different germane gas-phase concentration was used as input parameter. These values are plotted in Fig. 4 for different germane gas-phase concentrations. For comparison values for X determined by EDX are also shown in the same figure. It can be seen in Fig. 4 that the evaluation of ZSi-Si is a fast and useful method for the estimation of the Ge concentration in the film.

The microcrystalline absorber layers were implemented in single junction solar cells consisting of TCO/μc-Si(p):H/μc-SiGe(i):H/a-Si(n):H/Ag. Fig. 5 summarizes the EQE data for the solar cells with different Ge concentrations. As expected from the absorption measurements in Fig. 1 the EQE in the red part of the spectrum increases with increasing X. This also affects the estimated short circuit current which increases from 20.7 mA/cm² to values larger than 22 mA/cm². This result shows that, to gain the same short circuit current, the film thickness of the absorber by using SiGe alloys can be reduced compared to μc-Si:H absorbers. Fig. 6 shows Voc and efficiency of the same solar cell series but with extended range of X. The open circuit voltage, VOC, stays nearly constant with increasing X for X < 18%. When the Ge concentration increases to values larger 18% the VOC drops down to about 350mV for X=22%. The relatively low values for VOC (best μc-Si:H single cells achieved show values of VOC up to 550mV) can be explained by the high Raman crystallinity of the films (about 80%). The efficiency increases from about 5% to 5.5% at about X=12-16% due to enhanced short circuit current and then drops down to 3% when X increases to values larger 20%. This decrease in VOC and efficiency can be attributed to enhanced defect creation in the absorber due to the incorporation of Ge in the Si-network and has been also seen by other authors [9].

The μc-SiGe:H solar cells were implemented in a-Si:H/μc-SiGe:H tandem solar cells and were compared to conventional a-Si:H/μc-Si:H solar cells. The aim of this evaluation is to show the possibility to reduce the bottom cell absorber by using SiGe alloys, significantly. Fig. 7 shows the EQE for three different tandem solar cells with the same short circuit current of about 11.7mA/cm². The thickness of the a-Si:H top-cell was held constant at 330nm. The Ge concentration for the μc-SiGe:H solar cells was adjusted to about 12 at.%. The thickness of the pristine μc-Si:H bottom cell absorber was 1.3μm. The Raman crystallinity of the bottom absorbers was about 80%. As shown in the figure by incorporating the μc-SiGe:H absorbers the film thickness could be reduced up to 60% exhibiting the same bottom cell current (violet EQE curve – 0.52μm). This is a remarkable result since the tact

Fig. 7: External quantum efficiency for a-Si:H/μc-SiGe:H solar cells without Ge (black) and with SiGe alloys (red and magenta) in the bottom cell.

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time of solar module production can be reduced significantly. A further reduction of the film thickness is possible due to an optimization of the Raman crystallinity of the film. An overall increase of the efficiency can be achieved by an optimized back contact and large band gap p- and n-doped layers. In the next step this concept will be implemented into triple junction solar cells.

4. Summary

In summary, the usage of thin microcrystalline silicon-germanium absorbers in thin-film silicon solar cells has been described. Due to incorporation of Ge in the Si-network the absorption coefficient increases. This is accompanied by a shift of the LO-TO Raman resonance towards smaller wave numbers. From the shift of the Raman resonance with increasing Germane concentration in the gas phase the Germanium concentration in the resulting film was estimated which stays in good agreement with EDX measurements. The incorporation of Ge causes an increase of the EQE in the red part of the spectrum and thus to an increased current while the absorber layer thickness stays constant. Optimized single μc-SiGe:H solar cells were obtained by a Ge concentration of about 12-16 at.%. Applying these findings in a-Si:H/μc-SiGe:H tandem solar cells show that the absorber thickness could be reduced by up to 60% compared to a-Si:H/μc-Si:H solar cells. This result shows that the deposition tact time in thin-film silicon solar cell production can be reduced significantly. This will be even more significant for bottom cells in triple junction concepts.

Acknowledgements

The authors would like to thank the Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit for

financial support within the project SiliziumDS12plus under contract number FKZ0325317.

References

[1] Green MA, Emery K, Hishikawa Y, Warta W, Dunlop ED. Solar Cell efficiency tables (version 41); Progress in Photovoltaics: Res and Appl. 2013; 21: 11.

[2] Kim S, Chung JW, Lee H, Park J, Heo Y, Lee HM. Remarkable progress in thin-film silicon solar cells using high-efficiency triple junction technology. Sol En Mat and Solar Cells 2013; htt://dx.doi.org/10.1016/j.solmat.2013.04.016.

[3] Berginski M, Rech B, Hüpkes J, Schöpe G, van den Donker MN, Reetz W, Kilper T, Wuttig M, ZnO film with tailored material

properties for highly efficient thin-film silicon solar modules; 21st European Photovoltaic Solar Energy Conference, 4-8 September 2006, Dresden, Germany; 2006; 1539.

[4] Matsui T, Jia H, Kondo M. Thin film solar cells incorporating microcrystalline Si1-xGex as efficient infrared absorber: an application to

double junction solar cells. Progress in Photovoltaics: Res. and Appl. 2010; 18; Nr. 1: 48-53.

[5] Matsui T, Ogata K. Isomura M, Kondo M. Microcrystalline silicon-germanium alloys for solar cell application: Growth and material properties. Journal of Non-Crystalline Solids; 2006; 352; 1255.

[6] Droz C, Vallat-Sauvain E, Bailat J, Feitknecht L, Meier J, Shah A. Relationship between Raman crystallinity and open-circuit voltage in microcrystalline silicon solar cells. Solar Energy Materials and Solar Cells 2004; 81(1), 61-71.

[7] Ellert C, Wachtendorf C., Hedler A, Klindworth M, Martinek M. Influence of Raman crystallinity on the performance of micromorph thin film silicon solar cells, Solar Energy Materials and Solar Cells 2012; 96(0), 71-76.

[8] Alonso MI, Winer K. Raman spectra of c-Si1-xGex alloys. Phys Rev B 1989; 39 (14); 10056-10062.

[9] Matsui T, Chang C, Takada T, Isomura M, Fujiwara H, Kondo M. Microcrystalline Si1−x Gex Solar Cells Exhibiting Enhanced Infrared Response with Reduced Absorber Thickness. Applied Physics Express 1; 2008; Nr. 3, 031501-1 - 031501-3. http://dx.doi.org/10.1143/APEX.1.031501.

References

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