Materials for ultra-thin silicon solar cells
|German title:||Materialien für ultradünne Silizium-Solarzellen|
|Duration:||1st April 2011 - 31st December 2013|
|Description:||The quality of silicon used for electronic components is very high and generally exceeds the requirements placed on the material quality of solar silicon. For this reason, there are many efforts to use more cost-effective processes to produce silicon for photovoltaics whose impurity levels are higher than those of "electronic grade" silicon. Since very different processes are used here (Siemens and fluidized bed processes, direct purification processes of the metallurgical silicon) and in many cases are still under development, the materials are very different in terms of the degree of impurity. Nevertheless, the concentrations of the mainly very harmful metallic impurities are still so low that the detection limits of most measuring methods are not sufficient to perform a rapid incoming inspection of the silicon materials used. In individual cases, foreign atom contents can only be determined more precisely by complex methods, such as radioactive measurement procedures. Solar cell manufacturers are therefore still dependent on assessing their material on the basis of the solar cells produced from it. However, the solar cell parameters depend very much on the solar cell process, so that a qualitative assessment of the starting material is difficult, especially when comparing different solar cell manufacturers. Standardization of the material parameters is therefore an important technical challenge for the future development of the solar industry.
The following scientific and technical goals should therefore be achieved in the xµ-Material project:
- Development of more sensitive and faster measurement methods for the identification of (mainly metallic) impurity atoms in the silicon - starting material.
- Identification of the most harmful impurities and their influence on the efficiency of mono- and multicrystalline solar cells produced according to industrial standard processes
- Development of standard requirements for the silicon feedstock for different industrial solar cell processes and establishment of a certification laboratory and measurement standards for the solar industry
- Investigation of the suitability and specification of silicon starting material for the production of ultra-thin wafers and solar cells
Within the framework of the joint project xµ-Material, extensive investigations were carried out at CiS to develop methods for the quality determination of silicon, the quality of silicon and its influence on solar cell efficiency. In three key experiments, the materials were characterized by means of charge carrier lifetime measurements, infrared and photoluminescence spectroscopy and processed into solar cells. The influence of the solar cell process on the properties of the silicon was analyzed. Furthermore, method development regarding lifetime measurements and the measurement of the interstitial dissolved iron content was carried out.
In the key experiment BoschMono the influence of the solar cell process and especially of the thermal budget on the properties of the silicon was analyzed. For this purpose, a p-type solar cell process was applied at CiS. Samples were taken and characterized after different process steps in each case. Positive and negative effects of the individual process steps on the silicon quality could be identified and explained. For example, the diffusion of the phosphorus into the silicon traps harmful impurities, e.g. iron, and thus improves the silicon. The deposition of the silicon nitride layer onto the phosphorus-doped layer resulted in a reduction of the silicon quality. This is probably due to vacancies caused by stresses at the silicon nitride-silicon interface, which diffuse into the silicon and create defects there. The results were presented in two publications in peer-reviewed journals and at the SiliconPV 2013 conference in Hamelin.
In the key experiment MiniMono, analysis methods were developed using model materials to characterize them. The validity of these methods was then tested by fabricating and analyzing solar cells. Within this key experiment, CiS performed the basic characterization of the electrical properties and fabricated and characterized the solar cells. The results of this experiment will be published as a paper in a peer-reviewed journal and were presented in a talk at the 2nd Silicon Materials Workshop, Rome, 7-8 October 2013.
The key experiment QMulti investigated the properties of multicrystalline silicon. At CiS, the effect of the solar cell process on a new grade of silicon (high-performance multicrystalline (HPM) silicon) with a smaller grain size was analyzed. It was found that the carrier lifetime of this grade of silicon does not increase significantly after phosphorus diffusion. This effect is observed for standard multicrystalline silicon. On mc silicon samples exhibiting different breakdown behavior in the solar cell (defect type A and B), significantly different interstitial oxygen and substitutional carbon concentrations were detected by FTIR measurements. In addition, a defect type dependent change of the concentrations during the process was detected.
As part of the subcontract to the TU Ilmenau, low-temperature photoluminescence spectroscopy measurements were performed on different material types to complement the range of methods investigated in the overall project. Silicon samples, which were provided by the TU Dresden, were to be investigated with respect to the dopant type and also the dopant concentration by means of TTPL. In one sample a boron concentration of 1.3-1014cm 3 could be determined. This is in good agreement with the resistivity determined by the 4-peak measurement methods. In the remaining samples the luminescence peak of the free exciton could not be resolved, so that a quantification was not possible. The reason for this is once a high non-radiative recombination over interfering sites in the middle of the band gap. This reduces the measured intensity, so that the signal-to-noise ratio deteriorates. The second reason is the high concentration of dopants. This leads to a reduction in the number of free excitons compared to bound excitons. Furthermore, it was investigated whether nitrogen-correlated luminescence peaks can be used to determine the concentration of nitrogen. A calibration of the nitrogen-correlated luminescence peak with respect to the nitrogen concentration was not possible. Probably, the PL intensity of the nitrogen-correlated peak is influenced by other defects besides the nitrogen concentration.
An establishment of lifetime measurement methods could be implemented through the participation of CiS within the SEMI organization. Three documents were created and published. The document SEMI AUX 017-0310E is an auxiliary document and describes the physical basics of lifetime measurement in general. In addition, two measurement standards SEMI PV13-0211 and SEMI PV09-1110 were published by the SEMI organization. The SEMI PV13-0211 standard gives a prescription for measuring carrier lifetime when using an eddy current sensor as a detector for conductivity. SEMI PV09-1110 standardizes lifetime measurement based on reflected microwaves. By applying these standards, it is possible to compare lifetime measurements performed by different parties and with different measurement devices.
Light-induced degradation (LID) reduces efficiency by 1-2% absolute in boron-doped CZ silicon solar cells. In this project, the occurrence of LID in silicon doped with indium and gallium was investigated by carrier lifetime measurements. For indium-doped silicon, it was found that LID also occurs. Using these results, a new defect model was proposed. The results and the model were published in two peer-reviewed journals and presented to the SiliconPV 2014 conference in 's-Hertogenbosch. In addition, the influence, of the increased temperature in the silicon due to illumination, on the charge carrier lifetime was investigated. It was found that the temperature change due to illumination contributes only about 5% to the observed lifetime change. The results were presented as part of a publication in a peer-reviewed journal and at the SiliconPV 2013 conference in Hamelin. In a further experiment on LID investigation, it was shown that TTFTIR measurement methodology can be used to help clarify the LID defect.
The concentration of acceptors in silicon can be determined by analyzing the iron-acceptor pair kinetics. Within this project, the methodology was used to measure boron concentration, particularly for compensated materials. The methodology for determining the interstitial dissolved iron content by lifetime measurements was compared with DLTS measurements. It was found that the DLTS method systematically determined interstitial iron contents about an order of magnitude lower. Contamination of iron at the edge of Czochralski silicon crystals was analyzed and simulated. By comparing the measured interstitial iron profile and the profile obtained by simulated solid phase diffusion of iron, it was determined that contamination of the silicon crystal must have occurred just above the silicon melt.
K. Lauer, C. Möller, K. Neckermann, M. Blech, M. Herms, T. Mchedlidze, J. Weber, and S. Meyer, Impact of a p-type Solar Cell Process on the Electrical Quality of Czochralski Silicon, Energy Procedia, vol. 38, pp. 589596, Jan. 2013.
A. L. Blum, J. S. Swirhun, R. A. Sinton, F. Yan, S. Herasimenka, T. Roth, K. Lauer, J. Haunschild, B. Lim, K. Bothe, Z. Hameiri, B. Seipel, R. Xiong, M. Dhamrin, and J. D. Murphy, Interlaboratory Study of Eddy-Current Measurement of Excess-Carrier Recombination Lifetime, IEEE Journal of Photovoltaics, vol. 4, no. 1, pp. 525531, Jan. 2014.
C. Möller and K. Lauer, Charge Carrier Lifetime Shift Induced by Temperature Variation during a MWPCD Measurement, Energy Procedia, vol. 38, pp. 153160, 2013.
C. Möller and K. Lauer, Light-induced degradation in indium-doped silicon, physica status solidi (RRL) - Rapid Research Letters, vol. 7, no. 7, pp. 461464, Jul. 2013.
K. Lauer, Comparison of RF-PCD and MW-PCD Lifetime Measurements on Silicon Wafers and Bricks, Workshop on Test Methods for Silicon Feedstock Materials, Bricks and Wafers, Intersolar Europe (2012)
M. Herms, V. Osinniy, M. Kirpo, F. Dreckschmidt, J. Neusel, O. Gybin, A. Grochocki, C. Möller, and K. Lauer, Windmill-like Structure in Cz-Si, Energy Procedia, vol. 38, pp. 8085, Jan. 2013.
T. Mchedlidze, L. Scheffler, J. Weber, M. Herms, J. Neusel, V. Osinniy, C. Möller, and K. Lauer, Local detection of deep carrier traps in the pn-junction of silicon solar cells, Applied Physics Letters, vol. 103, no. 1, p. 013901, Jul. 2013.
S. Meyer, S. Wahl, A. Molchanov, K. Neckermann, C. Möller, K. Lauer, C. Hagendorf, Influence of the feedstock purity on the solar cell efficiency, accepted to Solar Energy Materials & Solar Cells
C. Möller and K. Lauer, ASi-Sii defect model of light induced degradation in silicon submitted to Energy Procedia
C. Möller, K. Lauer, F. Gibaja, T. Bartel, and F. Kirscht, Iron acceptor association in compensated multicrystalline silicon, DPG Frühjahrstagung, Regensburg (2013)
C. Möller, ASi-Sii-Defektmodell zur Erklärung der lichtinduzierten Degradation (LID) in Silizium, Silicon Forest, Falkau (2014)
C. Möller and K. Lauer, ASi-Sii-defect as possible origin for light-induced degradation, 49. Punktdefekttreffen, Dresden (2014)
T. Mchedlidze, C. Möller, K. Lauer and J. Weber, Formation of near junction and bulk traps in crystalline silicon solar cells, DPG Frühjahrstagung, Dresden (2014)
T. Mchedlidze, C. Möller, K. Lauer and J. Weber, Formation of near-to-junction and bulk traps during crystalline silicon solar cell fabrication process using iron contaminated feedstock, submitted to EMRS, Strasbourg (2014)
S. Meyer, S. Wahl, C. Hagendorf, K. Neckermann, C. Möller, K. Lauer, Feedstock trace element load vs. cell efficiency, Silicon Materials Workshop, Rom (2013)
S. Meyer, S. Wahl, A. Molchanov, K. Neckermann, C. Möller, K. Lauer, C. Hagendorf, The influence of feedstock quality on the cell efficiency, Deutsche Kristallzüchtungstagung DKT, Halle (2014)
|Partners:||- CiS Forschungsinstitut für Mikrosensorik und Photovoltaik GmbH, 99099 Erfurt
- Bosch Solar Wafers GmbH, 99310 Arnstadt
- PVA Vakuum Anlagenbau Jena GmbH, 07751 Jena
- PV Silicon Forschungs und Produktions GmbH, 99099 Erfurt
- Q-Cells AG, 06766 Bitterfeld - Wolfen
- Schott Solar Wafer GmbH, 07745 Jena
- Bundesanstalt für Materialforschung und - prüfung, 12489 Berlin
- Fraunhofer Center für Silizium Photovoltaik, 06120 Halle/Saale
- Fraunhofer Technologiezentrum für Halbleitermaterialien, 09599 Freiberg
- TU Bergakademie Freiberg Institut für Experimentelle Physik, 09599 Freiberg
|Funded by:||Federal Ministry of Education & Research|
|Funding code:||03SF0398 A|
|Contact:||Contact us about this project via our former business unit Silicon Detectors|
« back to project overview