Hydrogen Passivation and Laser Doping for Silicon Solar Cells
2: Australian Centre for Advanced Photovoltaics, The University of New South Wales, New South Wales Sydney, Sydney, Australia
Photovoltaic electricity generation is a rapidly growing industry, and a key pillar of a decarbonised energy system. In modern solar cells, laser technology is used to form localised structures such as a selective emitter through doping or to locally ablate dielectric layers for contact definition. A critical factor is the ability to passivate the laser-induced defects to prevent premature charge carrier recombination reducing the cell efficiency. Hydrogenation is such a passivation technique. The exact mechanisms have until recently been poorly understood, so this timely reference covers the recent breakthroughs in the understanding of hydrogen passivation. The book addresses key technologies for improving the efficiency of solar cells, including the industry-dominating PERC concept with an added rear passivation layer to reduce recombination. Coverage includes hydrogen passivation mechanisms, bulk and surface defect passivation, hydrogenation of light-induced defects, potential negative impacts of hydrogen, and laser doping for rapid diffusion and for selective emitter formation. This work also provides brand new results that enable low-quality silicon to be used for heterojunction applications and could pave the way for future low-cost, high-efficiency silicon solar cell technologies featuring passivated contacts to be fabricated on p-type wafers. This work is indispensable for researchers in the field of photovoltaic energy, in academia as well as industry.
Inspec keywords: passivation; elemental semiconductors; laser ablation; hydrogenation; hydrogen; diffusion; semiconductor heterojunctions; solar cells; silicon
Other keywords: localised structures; rapid diffusion; photovoltaic energy; surface defect passivation; Si; contact definition; p-type wafers; selective emitter formation; exact mechanisms; low-quality silicon; decarbonised energy system; low-cost high-efficiency silicon solar cell technologies; photovoltaic electricity generation; hydrogen passivation mechanisms; laser technology; light-induced defect hydrogeneration; premature charge carrier recombination; dielectric layer ablation; bulk defect passivation; H; laser-induced defect passivation; laser doping; industry-dominating PERC concept; rear passivation layer; heterojunction applications
Subjects: Impurity and defect levels in elemental semiconductors; Monographs, and collections; Surface treatment (semiconductor technology); Solar cells and arrays; Handbooks and dictionaries; Elemental semiconductors; Laser materials processing; Surface treatment and degradation in semiconductor technology; Laser materials processing; Photoelectric conversion; solar cells and arrays; Textbooks; General electrical engineering topics
- Book DOI: 10.1049/PBPO134E
- Chapter DOI: 10.1049/PBPO134E
- ISBN: 9781785616235
- e-ISBN: 9781785616242
- Page count: 518
- Format: PDF
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Front Matter
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1 Industrial silicon solar cells
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Climate change is the biggest threat facing humanity. In order to curtail the emission of greenhouse gases into the atmosphere to address climate change, it is essential to rapidly shift global energy systems away from a reliance on fossil fuels. The abundance of the solar resource and the recent reductions in the cost of solar panels makes solar photovoltaics (PV) an ideal candidate for this purpose. The performance of industrial silicon solar cells has improved significantly over the past ten years, with multiple manufacturers mass-producing solar cells with efficiencies exceeding 23%. This is related to both incremental improvements of the industrial processes on the production line and improved device architectures. Passivated emitter and rear cell (PERC) solar cells are currently dominating the market of commercial silicon solar cells because of the high-efficiency design compared with conventional cell structures. Two key factors will help further drive this technology's success: the passivation of bulk defects in silicon using hydrogen to improve the cell reliability and the use of lasers to effectively produce a 'selective emitter', which is crucial to help improve the cell efficiency. Suntech used these technologies to achieve the world's first commercial p-type solar cell with an efficiency above 20% and the same technologies were used by LONGi Solar to recently reach over 23.8% efficiency. The purpose of this textbook is to provide a comprehensive overview of the current state of the art for both hydrogen passivation and laser doping of silicon solar cells.
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2 Hydrogen passivation mechanisms
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This chapter attempts to give an introduction to the behaviour of hydrogen in silicon, and how this relates to its ability to passivate defects. It is shown that the use of hydrogen passivation can result in remarkable improvements in carrier lifetimes, and hence performance of silicon photovoltaic devices. The key considerations for effective hydrogen passivation have also been outlined as follows: 1. Introduction of atomic hydrogen to the device 2. Diffusion of atomic hydrogen within the device to reach defects 3. Reaction of atomic hydrogen with defects.
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3 Hydrogen passivation of silicon surfaces
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This chapter discusses the role that hydrogen takes in the passivation of silicon surfaces. The basic concepts of surface recombination and passivation are first introduced, to then examine how hydrogen can influence the dynamics of charge carriers at the surface of silicon. The different materials and synthesis techniques used to achieve optimal surface passivation are covered. In particular, the role of hydrogen in all materials and detection techniques are reviewed in detail, drawing on the latest findings in the field.
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4 Hydrogen passivation of bulk defects
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The dielectric layers used in the fabrication of commercial silicon solar cells such as silicon nitride (SiNx) antireflection coating and the rear dielectric stack of aluminium oxide (AlOx) and SiNx in passivated emitter and rear cell (PERC) solar cells provide an opportunity for both bulk and surface defect passivation by hydrogen without applying additional hydrogen source, such as forming gas.
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5 The boron-oxygen defect system
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This chapter discusses the formation and elimination of B-O-related LID in p-type Cz silicon, with a particular emphasis on illuminated annealing and hydrogen passivation. Recent review papers have been published by Niewelt et al. and Hallam et al. on B-O defects and their mitigation, respectively. The chapter also includes brand new insights into the passivation of such defects in heterojunction solar cells fabricated on boron-doped p-type Cz wafers and phosphorus/boron co-doped n-type upgraded metallurgical grade (UMG) Cz silicon. It highlights the future potential for boron-doped p-type Cz wafers to be used to fabricate high-efficiency-passivated contact solar cells with stabilised open-circuit voltages over 735 mV, contrary to the widespread belief of a need to transition to gallium-doped or n-type wafers to achieve such high voltages.
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6 Negative impacts of hydrogen in silicon
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Previous chapters have discussed the power of hydrogen to passivate both the surfaces and bulk of silicon solar cells, however, the presence of hydrogen can also have unintended effects. This chapter highlights the potential negative impacts that hydrogen can have on silicon and specifically on industrial solar cells. The behaviour of hydrogen and the multitude of defects that it can form are challenging to specifically characterise for a number of reasons: hydrogen is the smallest atom and difficult to detect directly; it is highly mobile in many forms, even at room temperature, and can be impacted by measurement techniques; it can take on multiple charge states and configurations; it is difficult to eliminate in 'control' samples; hydrogen can bond with almost anything to form complexes with such similar IR signals or energy levels they can be near impossible to differentiate. Some of the common hydrogen-containing complexes that can form in silicon were discussed including carbon complexes C-H, O-H and C-O-H, TMs and vacancy hydrogen complexes.
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7 Laser doping for rapid diffusion in silicon
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This chapter will provide an introduction to laser doping, including the benefits of the significant increases in the diffusion coefficients of impurities with increased temperature, particularly when laser doping is performed in the liquid-state, and in general, the higher solubility of impurities with increasing temperature and rapid cooling. It will discuss various methods for laser doping using gas-phase dopant sources, spin-on dopant (SOD) sources, doped dielectric layers and residual dopants from emitter diffusion/phosphosilicate glass (PSG) layers. It then discusses the impact of different laser parameters on doping profiles for laser-doping purposes.
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8 Laser-doped selective emitter formation and the passivation of laser-induced defects
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This chapter will introduce the concept of selective emitters for silicon solar cells as a pathway for increased solar cell efficiencies. It will briefly discuss the record 25% p-type UNSW passivated emitter, rear locally diffused (PERL) cell, as well as several industrial approaches for selective emitter formation including the buried contact solar cell (BCSC) and inkjet masking-based methods, along with the strengths and weakness of each approach. Then the various laser-doping methods for selective emitter formation will be discussed, which can overcome many of the limitations of non-laser-doping-based methods.
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9 Applications of laser doping
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This chapter discusses further applications of laser doping for silicon solar cells. First, it will cover the application of laser doping for high-efficiency solar cell fabrication. This includes the formation of localised heavily doped p++ regions using either applied boron spin-on-dopants (SODs) or the rear surface aluminium oxide (AlOx) dielectric layer in industrial passivated emitter and rear cell (PERC) solar cells as a dopant source to form a low-cost implementation of the rear surface of the passivated emitter rear locally (PERL) diffused cell. Other high-efficiency solar cells, such as the laser-doped semiconductor finger cell and interdigitated back-contact (IBC) solar cells, will also be discussed. The ability for laser doping to form heavily diffused contact regions that can penetrate through emitter surface layers to contact the underlying bulk silicon will be reviewed, with applications in novel solar cell structures and edge junction isolation to avoid resistance-limited recombination mechanisms at low carrier concentrations. Finally, the application of laser doping for enhanced gettering of bulk impurities is discussed.
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10 Conclusion and future outlook
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The structure of industrial silicon solar cells has remained relatively unchanged. Until 2019, the market had been completely dominated by silicon solar cells manufactured using the aluminium-back surface field (Al-BSF) approach. Decades of optimisation, slight processing tweaks and the ongoing development of materials such as screen-printed pastes have led to progressive improvements in the performance of Al-BSF cells to achieve efficiencies of almost 20%. However, the efficiency is fundamentally limited by the rear surface due to the direct contact between the silicon and metal over the whole area of the device. More sophisticated cell structures, have since been developed, such as the passivated emitter, rear locally diffused (PERL) cell developed at UNSW, which held the world record for the highest efficiency single-junction silicon solar cell from 1991 until 2014, with an efficiency of 25%. In the same family as the PERL cell is the passivated emitter and rear cell (PERC). This book has primarily focused on issues regarding PERC solar cells, as they are the current dominant technology. We explore issues that will be at the forefront of future improvements in PERC solar cells. Looking further into the future, we also assess future trends in terms of two groups of silicon solar cells that are currently under investigation and are expected to make up a significant market share in the future. These include n-type passivated contact solar cells and silicon tandem solar cells.
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Back Matter
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