An improved understanding of multicrystalline wafer quality can explain variations in cell performance across multicrystalline silicon blocks. Infrared scanning can detect precipitates in a silicon block, while photoluminescence combined with defect etching can reveal needle-like precipitates along the grain boundaries. Such precipitates typically lead to reduced shunt resistance. Crystallographic defects that lower the current collection and the final cell efficiency can also be identified. Understanding the influence of these defects is important for the development of a crystallization technology that results in a substantially better cell efficiency. The use of the improved material quality in an innovative cell and module technology have led to the world record module efficiency of 17%. This paper will illustrate one example of how an improved understanding of multicrystalline wafer quality can explain the variations in cell performance.
This paper, the third in a series covering cost of ownership (COO) studies for photovoltaics [1], examines the need for metallization of silicon-based solar cells and how it has evolved over the past few years. The technologies and techniques that are being developed for this part of cell manufacturing in the foreseeable future are also discussed. The paper will conclude with a COO case study using the DEK Solar PV3000 as an example.
One-step screen-printing processes are still the most widely-used technique for the front-side metallization of crystalline silicon solar cells in the PV industry. This is because of the knowledge, stability and speed of the process, and despite some big disadvantages exhibited by the resulting contacts. Therefore, the metal contacts of high-efficiency laboratory cells are usually produced via advanced two-step metallization processes, which allow the application of optimized contact structures. In a first step, a narrow metal layer is applied to form the contact to the silicon wafer. Several different techniques have been developed for this first stage. In the second step, the seed layer is reinforced electrochemically with a dense layer of a metal of high conductivity, usually by light-induced plating. The transfer of such techniques into industrial scale has been pursued intensively, and may enter solar cell production lines in the near future. However, the process can still be improved based on a better process understanding, in order to benefit from the full potential of the technology.
Standard solar cell technology nowadays offers a variety of measures - some linked, some not - to continuously improve conversion efficiency. The starting point for considering the different improvement steps is a kind of standard cell as produced on most current production lines. The main elements of this cell are diffused junction, aluminium back-surface field and screen-print metallization. This type of cell suffers losses from different sources like optics, recombination and resistance that can be considerably lowered to obtain higher cell efficiency. This paper will describe improvement steps on the standard type of multi-crystalline cell before addressing cell concepts that open further potential.
The emitter or p-n junction is the core of crystalline silicon solar cells. The vast majority of silicon cells are produced using a simple process of high temperature diffusion of dopants into the crystal lattice. This paper takes a closer look at the characteristics of this diffusion and possible variations in the process, and asks whether this step can lead to optimal emitters or whether emitters should be made with different processes in order to obtain the highest possible efficiency.
Photovoltaic modules are designed to meet the reliability and safety requirements of national and international test standards. Qualification testing is a short-duration (typically, 60-90 days) accelerated testing protocol, and it may be considered as a minimum requirement to undertake reliability testing. The goal of qualification testing is to identify the initial short-term reliability issues in the field, while the qualification testing/certification is primarily driven by marketplace requirements. Safety testing, however, is a regulatory requirement where the modules are assessed for the prevention of electrical shock, fire hazards, and personal injury due to electrical, mechanical, and environmental stresses in the field. This paper examines recent reliability and safety studies conducted at TÜV Rheinland PTL’s solar module testing facility in Arizona.
Solar currently represents less than 0.5% of global electricity generation. However, as renewable electricity gains importance in the US$1 trillion global electricity market, we forecast solar photovoltaic shipments to rise at a compound annual growth rate of 50% for the next four years. We expect an increasing number of countries to promote solar energy as the cost gap between solar and fossil fuel-generated electricity closes. This paper provides an overview of what to expect from the PV market in 2010.
The key to delivering highly efficient solar cells is to absorb as much light as possible from the solar spectrum and convert it effectively into electrical energy. Anti-reflective coatings have served as agents for reducing reflective losses and improving bulk and surface passivation thus enhancing both of the parameters – short circuit current and open circuit voltage of a solar cell. Simulation studies show that an SiN/MgF dual-layer anti-reflective coating is best for a bare cell. This paper takes a closer look at how this coating can reduce the reflectance for a broad range of wavelengths and thus enhance the quantum efficiency of the cell in the blue and red region of the solar spectrum.
The sixth edition of Photovoltaics International was published in November 2009 and includes a special BIPV focus. In addition, the Thin Film section offers a comparison of different ceramic Al-doped ZnO target materials by Fraunhofer IST, and Q-Cells unveils its production technology roadmap for boosting cell efficiences in Cell Processing.
Highly conductive transparent films are of significant interest in the field of thin-film photovoltaics. ZnO-based films in particular have attracted much interest due to the low cost of materials with good film properties for CIGS and a-Si/µc-Si solar modules. Investigations have been ongoing at Fraunhofer IST into ceramic ZnO:Al2O3 targets from different manufacturers. This paper presents a comparison of target material, sputter characteristics and film properties of ZnO:Al. Sputter characteristics are in this case determined by voltage and current data showing arcing rates at different power loads and process pressures. ZnO:Al films are deposited by DC magnetron sputtering with various deposition parameters (e.g oxygen flow, total pressure, sputtering power and substrate temperature) and investigated with respect to optical and electrical properties. A correlation between film properties, sputter characteristics and target material can therefore be determined. As it appears that arcing has the biggest influence on film properties, the ceramic target material can be optimized for minimal arcing.
