MICROCRYSTALLINE SILICON SOLAR CELLS:THEORY, DIAGNOSIS AND STABILITY著名的瑞士IMT关于微晶硅薄膜电池的博士论文
de beuchat FACULTE DES SCIENCES Secretariat- Decanat de la faculte IMPRIMATUR POUR LA THESE Microcrystalline silicon solar cells theory, diagnosis and stability Fanny SCULATI-MEILLAUD UNIVERSITE DE NEUCHATEL FACULTE DES SCIENCES La faculte des sciences de i' Universite de Neuchatel le rapport des membres du Mmes E. Vallat-Sauvain, M.C. Lux-Steiner(Berlin, D) MM. A Shah(directeur de these), C. Ballif A.N. Tiwari(ETH Zurich)et M. Vanecek(Prague CZ autorise Impression de la presente these Neuchatel, le 27 novembre 2006 Le doyen J -P. Derendi UNIVERSITE DE NEUCHATEL UL ES SCIENCES cretariat decant de la faculte H-2009 Nucha atelePhone:+41327182100Fax:+41327182103e-mail:secretariat.sciencesounine.chBwww.unine.ch'sciences Table of contents 1 Introduction ..mms msommsmmsmmmsmsm 1 1.1 Motivation 1.2 Amorphous silicon(a-Si:H)…… 1●。。。鲁●自布春鲁s。.鲁鲁·4 1,3 Microcrystalline silicon(μc-Si;H)…… 1267 1.4 Outline of the thesis 2. Characterization techniques 2.1 Current density -voltage J(V)measurement. 9 2.2 External quantum efficiency measurement 23 Micro- Raman spectroscopy………… 2.4 Fourier- Transform Photocurrent Spectroscopy(FTPS)………13 3. Theoretical limits for the main parameters of single- unction and tandem solar cells as a function of bandgap energy…·6 3.1Introductiona17 3.2 Pn junction….....….18 3.2. I Ideal pn junction..................................18 3.2.2 Theoretical limits for the pn junction 21 3.2.3 Semi-experimental limits for pn junction 26 3.3 Pin junction 3.4. Single-junction solar cell 6 3.4.I Limit for short-circuit current density Jsc..... 27 3.4.2 Limits for open-circuit voltage Voc, fill factor FF and efficiency n 28 3.5 Tandem solar cell. 3.6 Conclusions…39 3.6. I Single-junction solar cells 39 3.6.2 Tandem solar cells...................................39 4. Diagnosis of thin-film microcrystalline silicon solar cells.. 41 4.1 Introduction 41 4.2 Variable Illumination Measurements (VIM) 42 4.2. I Theory(pn/pin junctions)............42 4.2.2 Effects of recombination current density (rec), shunt resistance (Rsh)and defect-related absorption(a(0. 8 eV))on fill factor(FF losses 48 4,2 Dark j(V)…....,,.,……54 4.2.1.a-Si:H 中··+· 54 4.2.2.c-Si:H...........14 4.2.3 Dark J(V)measurements 54 4.3 Effect of i-layer thickness on the utproduct of solar cells and individual i-layers 55 4.3.1 Introduction.… 55 4.3.2 a-Si: H and uc-Si: H layers. 画自,画B指 56 4.3.3 a-Si: H and uc-Si: H cells................58 4.4 Variable gas flow series………62 4.4.1 Samples 62 4.4.2 VIM measurements.……163 4.5 Pin and nip dilution series : light-soaking…… 66 4.5.1 Samples……,,,,,,,…,,…,…,,,………6 4.5.2 VIM measurements ..........................................67 4.5.3 FTPS measurements 4.5.4 Dark J(V)measurements 69 4.6 Solar cells with low shunt resistance . 46.1 Samples…… 2 4.6.2VIM measurements 72 4.7 Conclusions…n73 5. Light-induced degradation of thin-film microcrystalline silicon (uc-Si: H) 5.1 Introduction 52 Light-induced degradation: observations and models………78 5.2. 1 Observations 78 5.2.2 Models(from a-Si: H) 。非。非D 80 5.3 Samples 5.4 Light-soaking and annealing conditions, characterization techniques 83 5.5 Initial parameters of the solar cells .ooooooooooooooo...o000000000000000080884 5.6 Light-induced degradation kinetics 5.7 Light-induced degradation as a function of crystallinity .ooooeoooeeooooo &8 5.7.1 Electrical parameters……………… 5.7.2 Shunt resistance and collection voltage……………..….…..9y4 5.7.3 Defect-related absorption....….….….…...97 5.8 Defect annealing…...,….100 58.1 Electrical parameters………………… 10 5.8.2 Defect-related absorption ....................100 5.9 Annealing kinetics.oo.o ..00.0000.0.00.000.00.0 0. 00...0. 0. o.. 102 5.10 Model for light-induced defect creation and annealing in uc-Si: H solar cells…106 6. Proton-induced degradation of thin-film microcrystalline silicon (uc-Si: H) 6.1 Introduction.mn......mmmm.111 6.2 Proton irradiation: observations and models mmm..112 6.2.1. Observations .112 6.2.2 Models 115 6.3 Samples…616 6. 4 Irradiation and annealing conditions, characterization techniques 116 6.5 High-energy proton irradiation …119 6. 5.1 Proton-induced degradation..........................119 6.5.2 Delect Annealing 129 6.5.3 Model for high-energy proton defect creation and annealing in uc Si:H∴ 133 6.6 Low-energy proton irradiation 133 6.6. 1 Proton degradation as a function of crystallinity ..............133 6.6.2 Defect annealing 142 6.6. 3 Model for low-energy proton defect creation and annealing in uc Si:H∴ 144 6.7 Comparison of light-induced and proton- induced degradation in uc Si: H solar cells 145 6. 8 Conclusions.mmnmossossomomssm 147 7。 Final conclusions。 电B。 151 Referencesmmsommmmmmm 155 Acknowledgements . 159 Annexe 1 161 Keywords: Thin-film solar cells, microcrystalline silicon, characterization techniques, light induced degradation, proton-induced degradation, defect density Summary: This thesis is focused on microcrystalline silicon solar cells deposited by very High Frequency Plasma Enhanced Chemical Vapor Deposition (VHF PE-CVD) technique. Microcrystalline silicon is a mixed material, composed of an amorphous phase and of nanocrystalline grains: it exhibits a wide range of microstructures depending on both the deposition conditions and the substrate material. Various characterization techniques were used in this work as diagnostic tools for defective solar cells, in our case solar cells presenting low fill factor values(FF) A collection model, originally developed for amorphous silicon solar cells, has been adapted to microcrystalline silicon solar cells, and the empirical relationships established were compared to actual measurements on various solar cells(dilution series, gas series and individual solar cells). An excellent correspondence between the predictions and the measurements was thus shown, validating the capacity of the tools employed in this work for solar cells diagnosis. In addition, an original model has been developed for the calculation of the upper limits for the electrical parameters of pin junctions, as a function of the materials bandgap. We thus demonstrated that large gains in short-circuit current density were still possible with microcrystalline silicon solar cells Then, the stability of microcrystalline silicon solar cells under light-soaking and proton irradiation was investigated by means of electrical, as well as sub-bandgap absorption measurements. Amorphous silicon thin-film solar cells are known to suffer from the Staebler-Wronski effect, which consists of a degradation of the electrical parameters under illumination. This effect is completely reversible under thermal annealing but it represents, nevertheless, a limiting factor regarding the use of amorphous silicon in solar cells. The stability of microcrystalline silicon solar cells, that are partly composed of amorphous silicon, was, thus, of very high interest: we showed that microcrystalline silicon solar cells degrade, albeit in a softer"and slower way than amorphous silicon solar cells. We observed that the amplitude of the light-induced degradation is a function of the crystallinity of the sample: cells with a medium crystallinity (which are, with respect to conversion efficiency, the optimum cells) present a relative efficiency reduction in the order of 5 Finally, the stability of uc-Si: H solar cells under high-energy and low-energy proton irradiation was also studied: here, we showed that microcrystalline silicon solar cells degrade and recover differently depending on the energy of the proton radiation. A simple model is proposed for light-induced and proton-induced degradation of microcrystalline silicon solar cells. In the case of light-soaking, we conclude that the defects are situated at the surface of the nanocrystals, whereas, in the case of protons irradiation, the defects are directly created within the nanocrystals