Hydrogen Passivation and Laser Doping for Silicon Solar Cells

Cover
Contents
List of figures
List of tables
About the editors
1 Industrial silicon solar cells
	1.1 Climate change and the use of fossil fuels
	1.2 Solar photovoltaics for future energy generation
	1.3 The reducing cost of solar PV
	1.4 Screen-printed silicon solar cells – the workhorse of the photovoltaics industry
	1.5 Limitations to current commercial silicon solar cells
		1.5.1 Performance-limiting defects and light-induced degradation
		1.5.2 Selective emitter design
	1.6 Overview of chapter structure
	1.7 Acknowledgements
	References
2 Hydrogen passivation mechanisms
	2.1 The role of hydrogen passivation in high-efficiency silicon solar cells
	2.2 Methods of introducing hydrogen in silicon solar cells
		2.2.1 Annealing in a molecular hydrogen ambient
		2.2.2 Exposure to a hydrogen containing plasma
			2.2.2.1 Direct hydrogen plasma
			2.2.2.2 Remote plasma hydrogenation
		2.2.3 Dielectric layer deposition
		2.2.4 Unintentional incorporation of hydrogen
	2.3 Behaviour of interstitial hydrogen in silicon
		2.3.1 Donor and acceptor levels of hydrogen in silicon
		2.3.2 Neutralisation of dopant atoms
		2.3.3 Hydrogen dimer formation
		2.3.4 Hydrogen interactions with other defects
	2.4 Hydrogen migration in silicon
		2.4.1 Hydrogen diffusivity
		2.4.2 Impact of charge state on hydrogen diffusivity
		2.4.3 Uncertainties around hydrogen passivation
	2.5 Detection of hydrogen in silicon
	2.6 Fractional charge state concentrations of interstitial hydrogen in silicon in thermal equilibrium
	2.7 Increased generation of H0 using carrier injection
	2.8 Unified modelling of hydrogen and defects
	2.9 Production sequences for industrial silicon solar cells
		2.9.1 Al-BSF solar cells
		2.9.2 PERC solar cells
	2.10 Summary
	2.11 Acknowledgements
	References
3 Hydrogen passivation of silicon surfaces
	3.1 Recombination and passivation at silicon interfaces
		3.1.1 The surface as a defect
		3.1.2 Characterisation of surface passivation in silicon
		3.1.3 Dielectric coatings for surface passivation
		3.1.4 State of the art in surface passivation of silicon
	3.2 Hydrogen in silicon oxide thin films
		3.2.1 Thermal silicon dioxide
		3.2.2 Silicon oxides grown by PECVD, ALD, or chemical anodisation
	3.3 Hydrogen in silicon nitride thin films
	3.4 Hydrogen in amorphous silicon thin films
	3.5 Hydrogen in aluminium oxide thin films
	3.6 Hydrogen in tunnelling oxide passivating contact layers
	3.7 Outlook for dielectric hydrogenation processes
	References
4 Hydrogen passivation of bulk defects
	4.1 Hydrogenation of structural defects
		4.1.1 Structural defects in cast silicon wafers
		4.1.2 Passivation of grain boundaries and dislocation clusters
			4.1.2.1 Characterisation of structural defects
			4.1.2.2 Challenges of passivation
			4.1.2.3 Impact of dielectric layer composition and thermal processing
		4.1.3 Dehydrogenation during extended annealing or high-temperature processes
		4.1.4 Development of cast-mono and high-performance multi-crystalline silicon material
		4.1.5 Benefits of an additional hydrogenation processes after firing
	4.2 Hydrogen passivation of impurity-related defects
		4.2.1 Passivation of dopants
	4.3 Hydrogenation of process-induced defects
		4.3.1 Oxygen precipitates
		4.3.2 Laser-induced defects
	4.4 Hydrogenation of light- and carrier-induced defects
	4.5 Complementary nature of gettering and hydrogenation for silicon solar cells
		4.5.1 Impact of gettering and hydrogenation on lifetime
		4.5.2 Gettering and hydrogenation in Al-BSF and PERC solar cells
		4.5.3 Impact of evolving solar cell architectures and processes on gettering and hydrogen passivation
		4.5.4 Gettering and hydrogenation in high-efficiency n-type cell technologies
	4.6 Gettering and hydrogenation for heterojunction silicon solar cells
		4.6.1 Impact of gettering and hydrogenation for SHJ solar cells
	4.7 Summary
	References
5 The boron-oxygen defect system
	5.1 B-O-related degradation in p-type Cz silicon
		5.1.1 Identification of the defect and its concentration
		5.1.2 Correlation between boron doping, the interstitial oxygen concentration and both the NDD and minority carrier lifetime
		5.1.3 Degradation characteristics
			5.1.3.1 Two-stage degradation
			5.1.3.2 Impact of the doping concentration and illumination intensity on the degradation rates
			5.1.3.3 Inducing B-O degradation with carrier injection
		5.1.4 Temporary recovery from B-O defects through dark annealing
		5.1.5 Defect theories
		5.1.6 Generalised model for the B-O defect system
	5.2 Alternative silicon wafers to avoid B-O-related degradation
		5.2.1 Methods to decrease the interstitial oxygen concentration
		5.2.2 Casted multi-crystalline and quasi-mono-crystalline silicon
		5.2.3 Decreasing the boron-doping concentration
		5.2.4 Germanium and carbon co-doping
		5.2.5 Summary of wafer alternatives
	5.3 Multiple pathways of B-O LID mitigation during cell processing
		5.3.1 Thermal processes to modify the B-O precursor concentration – A four-state model
			5.3.1.1 High-temperature tube-furnace processes
			5.3.1.2 Annealing at intermediate temperatures
			5.3.1.3 Rapid thermal processing such as metallisation firing
			5.3.1.4 Unknown nature and kinetics of precursor annihilation and reformation
		5.3.2 Annealing with illumination or carrier injection for stable passivation of B-O defects – A three-state model
			5.3.2.1 Determination of reaction rates for kinetic modelling
			5.3.3.2 Basic kinetic modelling using ordinary differential equations
	5.4 Fundamentals of illuminated annealing for B-O LID mitigation
		5.4.1 The role of hydrogen
			5.4.1.1 The importance of firing
			5.4.1.2 The complicated impact of firing on B-O passivation
			5.4.1.3 Impact of firing process parameters on passivation kinetics
			5.4.1.4 Impact of film and dielectric stack composition
			5.4.1.5 Modulating kinetics with prior annealing
			5.4.1.6 Passivation with unintentionally introduced hydrogen
			5.4.1.7 Summary of the role of hydrogen for B-O defect passivation
		5.4.2 Role of hydrogen charge states
		5.4.3 The critical role of defect formation
		5.4.4 Development of rapid laser-based processes
		5.4.5 Long-term stability of passivation
	5.5 Eliminating B-O LID in commercial solar cells
		5.5.1 Commercial B-O LID mitigation solutions for cells
		5.5.2 B-O LID mitigation during module lamination
		5.5.3 Self-recovery in the field
	5.6 B-O LID in p-type silicon heterojunction solar cells
	5.7 Summary
	References
6 Negative impacts of hydrogen in silicon
	6.1 Hydrogen complexes with other species
		6.1.1 Carbon-hydrogen (CH) complexes in silicon
		6.1.2 Oxygen-hydrogen (OH) complexes in silicon
		6.1.3 Carbon-oxygen-hydrogen (COH) complexes in silicon
		6.1.4 Transition metal-hydrogen complexes in silicon
		6.1.5 Vacancy-hydrogen complexes in silicon
	6.2 Light- and elevated temperature-induced degradation
		6.2.1 The impact of LeTID – cells, modules and systems
		6.2.2 The key behaviours of LeTID
			6.2.2.1 Dependence on firing on degradation extent
			6.2.2.2 A universal defect in silicon
			6.2.2.3 LeTID dependence on dielectrics
			6.2.2.4 LeTID characterisation
		6.2.3 The search for the root cause of LeTID
			6.2.3.1 Metallic impurities
			6.2.3.2 Hydrogen: a growing consensus
		6.2.4 The role of hydrogen in LeTID
			6.2.4.1 The impact of hydrogenated dielectric films
			6.2.4.2 Direct correlation between hydrogen and LeTID
			6.2.4.3 Analysis using DLTS
		6.2.5 LeTID mitigation
		6.2.6 Models for LeTID
		6.2.7 LeTID in p-type silicon heterojunction solar cells
	6.3 Negative effects due to hydrogen behaviour
		6.3.1 Hydrogen-induced contact resistance
		6.3.2 Neutralisation of charge and dopants
		6.3.3 Formation of hydrogen-induced platelets
	6.4 Summary
	Acknowledgment
	References
7 Laser doping for rapid diffusion in silicon
	7.1 The origin of laser doping for silicon solar cell manufacturing
	7.2 Dopant diffusion in crystalline silicon
		7.2.1 Conventional doping methods
		7.2.2 Laser doping
	7.3 Approaches for laser doping
	7.4 Impact of laser parameters on doping profiles
		7.4.1 Characterisation of laser-doped regions
			7.4.1.1 Sheet resistance
			7.4.1.2 ECV/SIMS
			7.4.1.3 SEM/EBIC
		7.4.2 Type of laser
		7.4.3 Diffusion profile in laser-doped regions
		7.4.4 Impact of laser speed
		7.4.5 Choice of wavelength
		7.4.6 Beam optics
	7.5 Chapter summary
	References
8 Laser-doped selective emitter formation and the passivation of laser-induced defects
	8.1 The selective emitter concept
		8.1.1 Trade-offs leading to the use of a selective emitter
		8.1.2 Emitter recombination, surface recombination velocity and quantum efficiency
		8.1.3 Contact resistivity and metal/Si interface recombination
		8.1.4 Lateral emitter resistance
	8.2 Techniques for selective emitter formation
		8.2.1 Dielectric patterning using photolithography
		8.2.2 Dielectric patterning through laser ablation
		8.2.3 Dielectric patterning through inkjet or aerosol jet printing
		8.2.4 Use of doped silicon inks
		8.2.5 Ion implantation
		8.2.6 Etch-back technique
	8.3 Laser doping for selective emitter formation
		8.3.1 Industrial LDSE PERC solar cells with aligned screen-printed contacts
		8.3.2 LDSE solar cells with self-aligned plated contacts
	8.4 Laser-induced defect generation
		8.4.1 Characterisation of laser-induced defects
			8.4.1.1 Electron beam microscopy-based characterisation
			8.4.1.2 Combined QSSPC and photoluminescence-based characterisation
			8.4.1.3 Spectral photoluminescence-based characterisation
			8.4.1.4 DLTS-based characterisation
	8.5 Techniques to minimise the impact of laser-induced defects
		8.5.1 Reduction in laser-doping coverage and metal/Si interface area
		8.5.2 Laser doping prior to dielectric deposition
			8.5.2.1 Simultaneous laser doping and grooving
		8.5.3 Localised passivated contact structures on laser-doped regions
		8.5.4 Performing laser doping through dielectric layers
		8.5.5 Deep-junction laser doping
		8.5.6 Laser- and thermal-annealing for localised repair of laser damage
	8.6 Hydrogen passivation of laser-induced defects
		8.6.1 Performing laser doping prior to firing
		8.6.2 Additional low-temperature belt furnace-annealing processes
		8.6.3 Simultaneous laser-induced defect passivation and light-induced degradation mitigation
	8.7 Conclusion
	References
9 Applications of laser doping
	9.1 Local back surface field formation for PERC-type cell structures
		9.1.1 Laser doping as an alternative to the Al/Si alloying process for PERC
		9.1.2 Aluminium oxide as a dopant source for p++ silicon formation
	9.2 Semiconductor finger solar cells
		9.2.1 Laser-doped semiconductor finger solar cells
		9.2.2 Advanced laser-doped semiconductor finger cells
		9.2.3 Future potential of the semiconductor finger technology
	9.3 Interdigitated back-contact solar cells
		9.3.1 Manufacturing challenges of IBC cells
		9.3.2 Laser-doped IBC cells
	9.4 Laser doping through doped surface layers of opposite polarity
		9.4.1 Simplified interdigitated back-contact cells with laser-doped compensated contact
		9.4.2 Isolation of shunts or high recombination areas such as edges
	9.5 Laser doping for enhanced silicon gettering
	9.6 Summary
	References
10 Conclusion and future outlook
	10.1 Importance of hydrogenation and laser doping for silicon solar cells
		10.1.1 Hydrogen passivation
		10.1.2 Laser doping
	10.2 Future outlook
		10.2.1 Future issues for industrial PERC solar cells
			10.2.1.1 Next-generation laser-doped selective emitters for PERC
			10.2.1.2 Surface-related degradation
			10.2.1.3 A shift to gallium-doped silicon wafers
		10.2.2 N-type passivated contact solar cells
			10.2.2.1 Selective emitter for TOPCon solar cells
			10.2.2.2 Instability in n-type passivated contact solar cells
			10.2.2.3 Post-cell treatments for SHJ solar cells
		10.2.3 Silicon-based tandem solar cells
	References
Index
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