Higher conversion efficiencies while reducing costs at the same time is the ultimate goal driving the development of solar cells. Multi-crystalline silicon has attracted considerable attention because of its high stability against light soaking. In case of solar grade multi-crystalline silicon the rigorous control of metal impurities is desirable for solar cell fabrication. It is the aim of this thesis to develop a new manufacturing process optimized for solar-grade multi-crystalline silicon solar cells. In this work the goal is to form solar cell emitters in silicon substrates by plasma immersion ion implantation of phosphine and posterior millisecond-range flash lamp annealing. These techniques were chosen as a new approach in order to decrease the production cost by reducing the amount of energy needed during fabrication. Therefore, this approach is called “Low Thermal Budget” process. After ion implantation the silicon surface is strongly disordered or amorphous up to the depth of the projected ion range. Therefore, subsequent annealing is required to remove the implantation damage and activate the doping element. Flash lamp annealing in the millisecond-range is demonstrated here as a very promising technique for the emitter formation at an overall low thermal budget. During flash lamp annealing, only the wafer surface is heated homogeneously to high temperatures at a time scales of ms. Thereby, implantation damages are annealed and phosphorous is electrically activated. The variation of pulse time allows to modify the degree of annealing of the bulk region to some extent as well. This can have an influence on the gettering behavior of metallic impurities. Ion implantation doping got in distinct consideration for doping of single-crystalline solar cells very recently. The efficient doping of multi-crystalline silicon remains the main challenge to reduce costs.
The influence of different annealing techniques on the optical and electrical properties of multi-crystalline silicon solar cells was investigated. The Raman spectroscopy showed that the silicon surface is amorphous after ion implantation. It could be demonstrated that flash lamp annealing at 1000 °C for 3 ms even without preheating is sufficient to recrystallize implanted silicon. The sheet resistance of flash lamp annealed samples is in the range of about 60 Ω/□. Without surface passivation the minority carrier diffusion length in the flash lamp annealed samples is in the range of 85 µm. This is up to one order of magnitude higher than that observed for rapid thermal or furnace annealed samples. The highest carrier concentration and efficiency as well as the lowest resistivity were obtained after annealing at 1200 °C for 20 ms for both, single- and multi-crystalline silicon wafers. Photoluminescence results point towards phosphorous cluster formation at high annealing temperatures which affects metal impurity gettering within the emitter.
Additionally, in silicon based solar cells, hydrogen plays a fundamental role due to its excellent passivation properties. The optical and electrical properties of the fabricated emitters were studied with particular interest in their dependence on the hydrogen content present in the samples. The influence of different flash lamp annealing parameters and a comparison with traditional thermal treatments such as rapid thermal and furnace annealing are presented. The samples treated by flash lamp annealing at 1200 °C for 20 ms in forming gas show sheet resistance values in the order of 60 Ω/□, and minority carrier diffusion lengths in the range of ~200 µm without the use of a capping layer for surface passivation. These results are significantly better than those obtained from rapid thermal or furnace annealed samples. The simultaneous implantation of hydrogen during the doping process, combined with optimal flash lamp annealing parameters, gave promising results for the application of this technology in replacing the conventional phosphoroxychlorid deposition and diffusion.:1 Motivation and objectives 1
2 Progress and prospects of silicon solar cells 5
3 Basics of a silicon solar cell 8
3.1 Specific characteristic of a standard silicon solar cell 12
3.2 Fundamental efficiency limits of standard silicon solar cells 14
4 Industrial process featuring low thermal budget process 17
4.1 Cleaning and etching steps 19
4.2 Emitter formation in p-type silicon 20
4.2.1 Thermal diffusion of phosphorous (industrial) 22
4.2.2 Ion beam implantation 24
4.2.3 Plasma immersion ion implantation as potential tool for the LTB process 26
4.2.4 Thermal processing of ion implanted solar cells - FLA as a novel method 28
4.3 Contact formation 30
4.3.1 Screen printing and sintering (industrial) 30
4.3.2 Gettering and BSF formation by aluminum diffusion (industrial) 32
4.3.3 Sputtering (LTB) 33
4.4 Surface passivation 33
5 Fabrication and characterization 35
5.1 Fabrication 35
5.2 Characterization of the p-n junction by ion implantation and FLA 39
5.2.1 Four-Point-Probe measurement (4-PPM) 39
5.2.2 Raman Spectroscopy (RS) 40
5.2.3 Photoluminescence Spectroscopy (PL) 41
5.2.4 Surface Photo-Voltage (SPV) 41
5.3 Analysis of hydrogen and metal impurities 46
5.3.1 Secondary Ion Mass Spectrometry (SIMS) 46
5.3.2 Elastic Recoil Detection Analyses (ERDA) and 47
Rutherford Backscattering Spectrometry (RBS) 47
5.4 Solar cell characterization 49
5.4.1 Transmission Electron Microscopy (TEM) 49
5.4.2 Auger Electron Spectroscopy (AES) 50
5.4.3 Light Beam Induced Current (LBIC) 51
5.4.4 Sun Simulator 52
6 Solar cell performance 53
6.1 Processing of the p-n junction by IBI and FLA 54
6.1.1 Variation FLA parameters 54
6.1.2 Influence of the grain size on the LD 71
6.2 Influence of the hydrogen introduced by PIII 76
6.2.1 Hydrogen profile by SIMS 76
6.2.2 H content as function of the thermal treatments 78
6.2.3 Optical properties of the silicon substrate 80
6.3 Influence of PIII and FLA on implanted iron 82
6.4 Contact formation 88
6.4.1 Antireflection layer 89
6.4.2 Back surface formation 90
6.4.3 Electrical and optical characterization 93
7 Overview of the achieved results 98
I References VIII
II Publications XVII
III Symbols index XVIII
IV Acronyms XXI
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:23014 |
| Date | 18 December 2015 |
| Creators | Krockert, Katja |
| Contributors | Möller, Hans-Joachim, Gobsch, Gerhard, TU Bergakademie Freiberg |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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