THIN-FILM SILICON SOLAR CELLS

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1 ENGINEERING SCIENCES Micro- and Nanotechnology THIN-FILM SILICON SOLAR CELLS Arvind Shah, Editor The main authors of Thin-Film Silicon Solar Cells are Christophe Ballif, Wolfhard Beyer, Friedhelm Finger, Horst Schade, Arvind Shah, and Nicolas Wyrsch, with additional contributions by Jean-Eric Bouree, Corinne Droz, Luc Feitknecht, Daniel Oppizzi, Martin Python, Julian Randall, Ricardo Rüther, Michael Stückelberger, and Reto Tscharner. EPFL Press A Swiss academic publisher distributed by CRC Press

Nicolas Wyrsch, with additional contributions by Jean-Eric Bouree, Corinne Droz, Luc Feitknecht, Daniel Oppizzi, Martin

2 CONTENTS 1 INTRODUCTION A strong market growth from 1999 to A technology coming to maturity: crystalline silicon High-efficiency crystalline silicon solar cells The silicon feed-stock issue: a trigger for thin-film deployment Thin-film silicon: a unique thin-film technology with a "long" history Amorphous silicon, microcrystalline silicon and "micromorph" devices Synergy with the display sector and emergence of a large PV sector Perspectives and challenges for thin-film silicon technology References 15 2 BASIC PROPERTIES OF HYDROGENATED AMORPHOUS SILICON (a-si:h) Introduction Structure of amorphous silicon "Free" and "trapped" carriers (electrons and holes); mobility gap Gap states Bandtail states Midgap states: dangling bonds Light-induced degradation (Staebler-Wronski effect) Optical absorption: optical gap and sub-bandgap absorption Absorption coefficient plot Link between density of states and absorption coefficient Exponential density of states in bandtails and Urbach energy in plot of absorption coefficient Determination of the optical gap Relationship between sub-bandgap absorption and defect density Measurement of sub-bandgap absorption Transport, conductivity and recombination Transport model 47

6 Amorphous silicon, microcrystalline silicon and "micromorph" devices 9 1.7 Synergy with the display sector and emergence of a large PV sector 11 1.

3 xiv Thin-Film Silicon Solar Cells Measurement of conductivity in a co-planar configuration Dark conductivity a dark Recombination Photoconductivity Doping of amorphous silicon layers Hydrogen in a-si:h Introduction Hydrogen incorporation Hydrogen dilution during deposition Hydrogen effusion and hydrogen surface desorption Hydrogen diffusion Hydrogen solubility effects Hydrogen effects on optoelectronic properties Effect of hydrogen incorporation on the bandgap ofa-si:h Stability of dangling bond passivation Hydrogen and material microstracture Role of hydrogen in light-induced degradation Amorphous silicon-germanium and silcon-carbon Alloys Introduction Fabrication Structure of a-si:ge:h and a-si:c:h alloys Hydrogen incorporation, effusion, surface desorption and diffusion Microstructural effects (voids) Dangling bonds, density of defect states Hydrogen stability versus alloy composition Doping effects Light-induced degradation Optical absorption Electronic transport properties Slope of the valence bandtail; Urbach energy Strategies for obtaining good quality alloys Conclusions References 89 3 BASIC PROPERTIES OF HYDROGENATED MICROCRYSTALLINE SILICON History Structural properties of ux;-si:h Structure Defects and gap states 107

.. 67 2.6.5 Hydrogen diffusion 70 2.6.6 Hydrogen solubility effects 70 2.6.7 Hydrogen effects on optoelectronic properties 73 2.6.8 Effect of hydrogen incorporation on the bandgap ofa-si:h 73 2.6.9 Stability of dangling bond passivation 74 2.

4 Contents xv Hydrogen, defect passivation, impurities and doping Schematic picture for the structure of ue-si:h Relationships between structural and other properties of ue-si:h material Optical properties Electronic properties and transport Metastability - instability Alloys Summary References THEORY OF SOLAR CELL DEVICES (SEMI-CONDUCTOR DIODES) 145 PART I: INTRODUCTION AND "/jm-type" DIODES Conversion of light into electrical carriers by a semi-conductor diode First step: generation of electron-hole pairs Second step: separation of electrons and holes The "ри-type" or "classical" diode: dark characteristics The "pra-type" or "classical" diode: Properties under illumination Photo-generation and superposition principle (ideal case) Limitations of a "real" diode (under illumination) Maximum power point (MPP) and fill factor (FF) of a solar cell Basic solar cell parameters J sc, V oc, FF Limits on solar cell efficiency Limits at standard test conditions (STC) Variation in light intensity Variation in operating temperature Variation in the spectrum of the incoming light THEORY OF SOLAR CELL DEVICES (SEMI-CONDUCTOR DIODES) 176 PART II: >«-TYPE" SOLAR CELLS Introduction to "рш-type" solar cells Basic structure and properties Formation of the internal electric field Carrier profiles in the intrinsic layer: free carriers p f andn { Trapped charge carriers p t and n t in bandtails 186

8 References 135 4 THEORY OF SOLAR CELL DEVICES (SEMI-CONDUCTOR DIODES) 145 PART I: INTRODUCTION AND "/jm-type" DIODES 145 4.

5 xvi Thin-Film Silicon Solar Cells 4.6 Effect of trapped charge in valence and conduction bandtails on electric field and carrier transport Deformation of electric field in Mayer by trapped carriers: Concept Deformation of electric field in Mayer by trapped carriers: numerical simulations for amorphous silicon Mobilities in amorphous and microcrystalline silicon Dangling bonds and their role in field deformation Dangling bond charge states Field deformation by charged dangling bonds within the Mayer: Concept Field deformation by charged dangling bonds within the Mayer: numerical simulation for an amorphous silicon solar cell with d x = 300 nm Field deformation within the Mayer: summary of situation for different Mayer thicknesses Recombination and Collection pli and iln interfaces Recombination Collection and drift lengths Electrical description of the pin-solar cell Equivalent circuit and extended "superposition principle" Shunts Variable illumination measurements (VIM) Reverse saturation current J 0 and open circuit voltage V oc Fill factor in pin-type thin-film silicon solar cells Limits for the short-circuit current / sc in pin-type thin-film silicon solar cells Light-induced degradation or "Staebler-Wronski effect" in thin-film silicon solar cells Spectral response, light trapping and efficiency limits Spectral response (SR) and external quantum efficiency (EQE) measurements Light trapping in thin-film silicon solar cells Limits for the efficiency r\ in pin-type thin-film silicon solar cells Summary and conclusions References TANDEM AND MULTI-JUNCTION SOLAR CELLS Introduction, general concept Principle of the two-terminal tandem cell Construction of basic J-V diagram: Rules for finding tandem J sc, V oc, FF. 240

7 Dangling bonds and their role in field deformation 193 4.7.1 Dangling bond charge states 193 4.7.2 Field deformation by charged dangling bonds within the Mayer: Concept 196 4.7.3 Field deformation by charged dangling bonds within the Mayer: numerical simulation for an amorphous silicon solar cell with d x = 300 nm 198 4.

6 Contents xvii Recombination (tunnel) junction Efficiency limits for tandems Practical problems of two-terminal tandem cells Light trapping Efficiency variation due to changes in the solar spectrum Temperature coefficients Pinholes and Shunts Cracks Typical tandem and multi-junction cells Amorphous tandem cells a-si:h/a-si:h Triple-junction amorphous cells with germanium Micromorph (a-si:h/ue-si:h) tandem cells Triple-junctions with microcrystalline silicon Spectral response (SR) and External Quantum Efficiency (EQE) measurements General principles Use of "colored" bias light beams for SR/EQEmeasurements on tandems and triple-junction cells SR/EQE measurements for a-si:h/a-si:h tandem cells Shunt detection in sub-cells by SR/EQE measurements SR/EQE measurements for triple-junction cells SR/EQE measurements for "micromorph" tandem cells Necessity for voltage correction (with bias voltage) Conclusions References MODULE FABRICATION AND PERFORMANCE Plasma-enhanced chemical vapor deposition (PECVD) Electrical plasma properties VHF plasma excitation Device-grade material Deposition parameters Deposition rate Deposition regimes for a-si:h and [xc-si:h Upscaling Deposition systems Roll-to-roll depositions Novel deposition systems Hot-wire chemical vapor deposition (HWCVD) Introduction Description of the HWCVD technique Filament materials 307

4.3 Micromorph (a-si:h/ue-si:h) tandem cells 253 5.4.4 Triple-junctions with microcrystalline silicon 254 5.5 Spectral response (SR) and External Quantum Efficiency (EQE) measurements 255 5.5.1 General principles 255 5.

7 xviii Thin-Film Silicon Solar Cells Types of materials deposited by HWCVD Mechanisms of the deposition process Filament aging Amorphous and microcrystalline silicon films, and microcrystalline silicon carbide alloys Silicon nitride and silicon oxynitride films Doped layers ^-layers Doped microcrystalline layers Transparent conductive oxides (TCO) as contact materials Glass substrates and specific TCO materials Qualification of TCO materials Surface texture of TCO Cell optics Light management in cells Optical losses Laser scribing and series connection of cells Cell interconnection scheme Power losses due to the series connection of cells Module performance Efficiencies Energy yield Partial shading Shunting Module Finishing Encapsulation Module certification Long-term stability Conclusions References EXAMPLES OF SOLAR MODULE APPLICATIONS Building-integrated photovoltaics (BIPV): aspects and examples PV Facade in Munich (Germany) Alpine roof integrated PV PV Roof at Auvernier, Switzerland (by Reto Tscharner) PV installation in Brazil Stillwell Avenue Station, New York City Stand-alone and portable applications Indoor applications of amorphous silicon solar cells Why is amorphous silicon well suited for indoor applications? Design guidelines for solar powering of indoor applications 387

4.1 Glass substrates and specific TCO materials 316 6.4.2 Qualification of TCO materials 317 6.4.3 Surface texture of TCO 319 6.4.4 Cell optics 324 6.4.5 Light management in cells 326 6.4.6 Optical losses 328 6.

8 Contents xix 7.4 Space applications Introduction Satellite power generators and specific power density Radiation resistance of a-si:h and other PV technologies a-si:h based cells for space Space applications of a-si:h modules Conclusions References THIN-FILM ELECTRONICS Thin-film transistors and display technology Introduction TFTs and flat panel displays TFT configurations and basic characteristics a-si:h TFT operation ue-si:h and poly-si TFT performance and other issues Large-area imagers Introduction and device configuration Performance and limitations Thin-film sensors on CMOS Chips Introduction a-si:h sensor integration Performance and limitations Conclusions References 421 INDEX 425

1.3 TFT configurations and basic characteristics 405 8.1.4 a-si:h TFT operation 407 8.1.5 ue-si:h and poly-si TFT performance and other issues 413 8.2 Large-area imagers 413 8.2.1 Introduction and device configuration 413 8.

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