In solar cell production, metallization is the manufacturing of metal contacts at the surfaces of solar cells in order to collect the photo-generated current for use. Being one of the most expensive steps in solar cell fabrication, it plays both an electrical and an optical role, because the contacts contribute to shading, and to the series resistance of solar cells. In addition, metal contacts may reduce the solar cells voltage due to charge carrier recombination at the metal / silicon interface. Addressing these challenges could increase solar cell conversion efficiency while cutting their production costs. This work presents state of the art methods for the metallization of crystalline Si solar cells for industrial production as well as for research and development. Different metallization technologies are compared, and ongoing R&D activities for the most relevant silicon solar cell metallization technologies are described in detail. Chapters cover fundamentals of metallization and metallization approaches, evaporated, plated and screen-printed contacts, alternative printing technologies, metallization of specific solar cell types, module interconnection technologies, and also address module technology. Written by a selection of world-renowned experts, the book provides researchers in academia and industry, solar cell manufacturing experts and advanced students with a thorough and systematic guide to advanced metallization of solar cells.
Inspec keywords: metals; solar cells; silicon
Other keywords: ink jet printing; transition metal alloys; semiconductor device testing; aluminium; integrated circuit interconnections; module technology; metallisation; silicon solar cell metallization; silicon
Subjects: Solar cells and arrays; Photoelectric conversion; solar cells and arrays
In this chapter, we aimed to provide a detailed overview of the state of the art of different metallization and interconnection technologies for silicon solar cells.
This chapter describes the general design considerations and requirements of a photovoltaic (PV) solar cell for efficiently converting radiant energy from the sun into electrical energy. The ideal converter is first considered in light of the solar spectrum and the detailed balance limit. Causes for departure from ideal behaviour are then presented and form the background for solar cell analysis. For better understanding of the requirements of solar cells, basic device principles are introduced through the course of this chapter. In addition to the physical and technological requirements, the economic and environmental impacts are finally considered, especially in light of solar cell metallization.
Every electronic device requires the metallic contacts to the two surfaces of the semiconductor bulk to transfer carriers from semiconductor to the metal contacts. The interfaces between the semiconductor and the metal contacts can pose a bar-rier to the flow of carriers into and out of the metal contacts. Since the minority and majority carriers are present at both interfaces, the minority carrier should be blocked at each interface for effective transport of the majority carrier to the respective contacts. In this chapter, therefore, the fundamentals of metallization of a silicon solar cell are explored starting with: (i) the barrier heights, (ii) current transports, (iii) contact types- selective and passivated and (iv) characterization of the contacts thereof.
In the subsequent sections, we introduce the main metallization approaches for industrial-type c-Si cells, which are based on the use of either of evaporated contacts, screen-printed contacts, alternative printing techniques or plated contacts. Importantly, the continuous progress of metallization approaches based on screen-printed contacts has led to certain metallization techniques or approaches falling out of favour and others being 're-discovered' as we will explain below. For each of those main approaches, we refer the readers to the other chapters of this book where significant further insights are given. In Section 4.8, we briefly compare the different metallization approaches based on general techno-economic aspects (typical finger widths, throughputs, costs, etc.) and finish with an outlook on metallization trends.
Metal contacts to semiconductor (SC) surfaces prepared by means of physical vapour deposition (PVD) are one of the most classical metallization techniques for SC devices, which were intensively investigated especially for microelectronic devices. This led to a huge variety of different metallization schemes, which can be adapted to almost any application to form a high-quality contact, especially also in the field of Si photovoltaic (PV) devices. That is why PVD metal contacts are frequently used especially for R&D silicon solar cells to realize, for instance, high efficiency and record devices. The realization of fine contact geometries, for instance, for a front side metal grid, requires structuring, e.g. by photolithography. Due to the relatively high process complexity and the high material waste, standard PVD tools are costly and not suited for the mass pro-duction of terrestrial silicon solar cells. However, for space application or concentrating systems, PVD metal contacts are still used also in production. There are also special PVD systems, which are suited for silicon solar cell mass production.
Industrially fabricated solar cells are primarily metallized by printed contacts. The reason for this is based on the simplicity and process speed of printing systems. The time frame for the contact formation of a solar cell is currently <1 s per wafer with the trend to even shorter processing times. Therefore, metallization processes using photolithography, as it is commonly used in microelectronics, is too time-consuming and cannot be used to process solar cells. Screen printing of thick film pastes has been and still is the leading technology in solar cell production.
With respect to the solar cell application, homogeneous, fine-line printed front contacts allowed for efficiency increases of around Δη/η = 1% compared to respective screen-printed reference over the years and the absence of mesh marks reduced the silver (Ag) laydown by 20% without reducing grid conductivity with commercially available screen-printing pastes
The solar cell metallization approach discussed in this chapter differs from the ones discussed otherwise in this book in its property that the metal contact is composed of different layers, all of which are designed to ideally fulfil their role in the solar cell contact. These roles range from creating the electrical contact with the semiconductor substrate (seed layer, either plated or deposited otherwise), over the function of current conduction (highly conductive plated layer), to the compatibility with the module integration (plated capping layer). The contact system is thus more flexible, better performing and can be adjusted more precisely to the technological needs of the solar cell, but on the other hand the process complexity is typically higher. This chapter focuses on the contacting of silicon in cell concepts where a silicon wafer is coated with a dielectric passivation and antireflection coating. Other cell concepts (silicon heterojunction (SHJ) solar cells, tandem solar cells, etc.) may also be metallized with plating.
This chapter introduces monofacial passivated emitter and rear cells (PERC) and bifacial PERC+ solar cells which are the mainstream solar cell technology in the photovoltaic (PV) industry today. The authors describe the PERC solar cell design as well as major technology development steps in the past decades such as the bifacial PERC+ design. The process technology to manufacture PERC solar cells is covered, whereas the specific aspects of the screen-printed Ag front and Al rear metal contacts are introduced in detail in later sections. The busbar design of PERC metal grids for module interconnection is also discussed. Finally, an outlook on the future improvement opportunities of PERC and PERC+ solar cells is given, in particular regarding the metal contact design and its impact on conversion efficiency.
To manufacture PV modules for solar power generation, the solar cells have to be interconnected in series to solar cell strings to enable the desired voltage output. The encapsulation serves as protection of the fragile solar cells and the interconnection against environmental impacts. In PV industry, the solar cell interconnection is realized by soldering, with a market share of around 95% as stated by the ITRPV. Besides soldering, solar cells can also be interconnected by the use of electrically conductive adhesives (ECAs). This alternative interconnection technology is discussed in detail. Additionally, welding is also investigated as an alternative interconnection technology for solar cells.
Photovoltaic (PV) module design is the process of determining the electrical connection configuration of solar cells to deliver a desired current and voltage with the supporting structures of the module being configured for maximal light absorption, environmental protection and mechanical strength. It directly impacts the performance, lifetime and levelized cost of PV-generated electricity. Over recent years, PV module design has assumed a growing focus, as the challenges for modules are becoming as critical as they are for cell design. Solar cell technology has matured and reduced massively in cost during the last decade, a new phase of innovations has occurred in module manufacturing resulting in advances such as half-cut cells for high power modules, multi-busbar (MBB) interconnection and bifacial modules. Mechanical designs are also critical to ensure the long-term performance of a module. It is necessary to understand the properties of encapsulants, thermomechanical stress in module-making processes, supporting structures and possible failure modes. Based on a physical understanding of these processes, the module design, material choices and fabrication processes can be optimised for longer operating lifetimes.