Photovoltaics International Papers

A new wafer technology, named CDS (Crystallization on Dipped Substrate), is under development and has been found to be effective in the reduction of wafer cost and silicon feedstock. CDS technology was applied to 156mm x 156mm-sized wafers, obtained via the throughput of 1825cm2/min, and the resulting cell efficiency of 14.8% was confirmed. This paper outlines the principle behind the technology and outlines the procedure.

A vast majority of silicon solar cells are manufactured using silver paste that is screen printed onto the front side of the wafer and fired to form the front-side contact. Though this method is well established within the industry, it continues to present several areas for potential efficiency improvements. The Fraunhofer Institute [1] has, among others, studied the potential of using electrodeposition of silver on top of the front side silver paste as a way to improve the front-side contact and increase cell efficiency. These results have shown cell efficiency increases of up to 0.4% absolute. This type of improvement has captured the interest of many manufacturers, but there has been a hesitancy to adopt electrodeposition as there is uncertainty as to what they can expect on their cells. Since efficiency gains are dependent upon many factors that can be unique to an individual cell, this paper provides a much-needed exploration of the potential effects of electrodeposition of silver in a way that isolates its effects from that of other factors.

Design and performance qualification testing of PV modules consists of a set of well-defined accelerated stress tests with strict pass/fail criteria. ASU-PTL is an ISO 17025-accredited testing laboratory and has been providing photovoltaic testing services since 1992. This paper presents a failure analysis on the design qualification testing of both crystalline silicon (c-Si) and thin-film technologies for two consecutive periods: 1997-2005 and 2005-2007. In the first period, the industry was growing at a slower rate with traditional manufacturers, with qualification testing of c-Si technologies being primarily conducted per Edition 1 of the IEC 61215 standard. In the second period, the industry was growing at an explosive rate with new manufacturers joining the traditional manufacturers, while qualification testing of c-Si was primarily conducted per Edition 2 of IEC 61215. Similar failure analysis according to IEC 61646 has also been carried out for thin-film technologies. The failure analysis of the test results presented in this paper indicates a large increase in the failure rates for both c-Si and thin-film technologies during the period of 2005-2007.

Each year, the photovoltaic market grows at a two-digit growth rate. However, the resulting economy-of-scale effects are not enough to achieve grid parity on their own. In order to reduce the production costs to grid parity level, new concepts and ideas must be realised as the basis for a photovoltaic factory. There are four main requirements that must be fulfilled in order to adhere to this cost reduction strategy: a highly integrated factory; automated and stable processes; a production control system (PCS) that provides the statistic data in order to continually optimise the processes; and an optimally-sized aligned production capacity.

The rapidly-growing photovoltaic market has placed a strong demand on manufacturers to decrease solar cell production costs. For thin-film solar cells, this can be achieved by increasing substrate sizes to achieve a better productivity and by adding more advanced layer stack systems to enhance the solar cell’s efficiency. Nearly all required layers of the prominent thin-film-based solar cell types (a-Si/µc-Si, CdTe and CI(G)S) can be deposited by using plasma processes. On the one hand, plasma-enhanced chemical vapor deposition (PECVD) is used for the deposition of a-Si and µc-Si layers. On the other hand, magnetron sputtering is used for coating with transparent conductive oxides as ITO (indium tin oxide) and ZAO (aluminium-doped zinc oxide), metallic back contact layers such as Ti, Al and Mo, or components of the compound semiconductor layers such as Cu and In. Magnetron sputter processes use direct current (DC) or pulsed DC, whereas radio frequency (RF) power is used for PECVD processes. Of utmost importance to get a reliable, high-efficiency solar cell is a good uniformity of the deposited layers and the need for the layer to be defect-free. Defects such as particles and splashes are created inside the plasma when an unwanted local discharge - a so-called arc - occurs. This arc can be eliminated by switching off the power supply. The faster this is done, and the less energy that is delivered into the arc, the smaller and more insignificant the defect creation will be. For this reason, as well as for precise control of electrical power, advanced, fast-reacting arc management is very important to attain high-quality solar cell coatings.

Thin-film solar cell manufacturing is poised to make a giant leap in scale with the birth of the gigawatt fab. Commercial thin-film plants are typically sized based on the capacity of the production line from the chosen equipment supplier. In most cases, initial investments have been for a single line, typically with an output capacity of no more than 60MWp. This period of initial development has allowed the industry to prove the robustness of the technology and capabilities of the equipment, as well as to understand the significance for the cost-per-watt of key cost drivers such as materials reduction, cell efficiency increases, and productivity. While large-scale manufacturing will positively impact costs, it presents a unique set of challenges for equipment and material suppliers, as well as the engineering and contracting companies tasked with designing, building, equipping and running a facility on this scale. In this paper, we present the insights of two specialty companies in the solar industry. Turner and Townsend, a design and project management consultancy, and Linde, glass manufacturer and gas and chemical company - share their views of the challenges of the gigawatt fab in three dedicated sections.

The deposition of thin films is a key technology for a large variety of technical and scientific applications. Among them is the deposition of silicon nitride (SiNx) to passivate the surface of silicon solar cells. The SiN film serves several purposes. It is a broadband anti-reflection layer, it serves to saturate dangling bonds and/or other surface states of the silicon, and last but not least, it is a protection layer to prevent alkali ions and other impurities from diffusing into the silicon causing perturbations of the performance of the solar cell. This multitude of properties to be fulfilled at the same time often causes difficulties in assessing the effect of a single process parameter, let alone the task of optimizing the SiN film in all required aspects at the same time. The aforementioned technical features of the SiN film provide the very property that largely determines the aesthetically pleasing appearance of a cell, and hence a PV module, as the colour of the module is determined by the cell composition. In order to complicate things further, there are numerous deposition techniques being applied both on a scientific level as well as in production environments.

Thin-film silicon solar cells are a potentially low-cost alternative to solar cells based on bulk silicon that are commonly used in the industry at the present time. However, a major drawback of the current epitaxial semi-industrial screen-printed cells is that they only achieve an efficiency of about 11-12%. By upgrading their efficiency, this kind of solar cell would become more attractive to the photovoltaic industry. The optimization of the front surface texture by dry texturing based on a fluorine plasma and the introduction of an intermediate porous silicon reflector at the epi/substrate interface (multiple Bragg reflector) has proven to result in an efficiency boost up to about 14%.

Climate change, oil shortage, green energy, energy security – these are some of the global `mega´-topics currently dominating the agenda in the news, in politics and in private lives. One of the industries that has most profited from the ever-growing consciousness about the need to de-carbonize current energy use is the photovoltaic industry. With this economic background, the photovoltaic industry has experienced impressive growth rates in the last decade and is expected to grow at 30% per year over at least the next couple of years. Since its upswing, it has become a multi-billion dollar industry and subject to speculation on stock exchanges worldwide.

The rapid growth of the solar energy industry owes its success to the development and production of mono- and multi-crystalline solar cells. This growth has been limited in recent years due to the lack of available supply of polysilicon, the key raw material for making the wafers that serve as the basis of the solar cell. As a result of this limitation, the price of polysilicon has increased dramatically and this has led to significant new and planned capacity expansions. These new capacity expansion announcements have been highly publicized, with little additional outside focus on other chemicals and materials.