Photovoltaics International Papers

In situ, real-time and off-line polarization spectroscopies have been applied in studies of large-area spatial uniformity of the components of multilayer stacks in hydrogenated silicon (Si:H) and cadmium telluride (CdTe) thin-film photovoltaic (PV) technologies. Such reflection spectroscopies involve first the measurement of spectra in the reflected-to-incident polarization state ratio of the light wave (or the ellipsometry angles of the reflecting multilayer stack), and then the analysis of these spectra to determine the thicknesses and properties of component layers of the stack. In addition, expanded capabilities result from measurement/analysis of the irradiance ratio and the degree of polarization of the reflected beam, simultaneously with the polarization state ratio, particularly for rough surfaces with in-plane roughness scales of the order of the optical wavelength or greater that scatter and depolarize the light beam. This paper provides examples of 1) real-time monitoring of texture etching of the transparent conducting oxide ZnO:Al; 2) real-time monitoring and off-line mapping of roll-to-roll deposited hydrogenated amorphous silicon (a-Si:H); and 3) large-area mapping of coated glass panels used in low-cost CdTe PV technology. For a-Si:H and CdTe thin-film PV technologies, the focus is on the characterization of the window layers, which are p-type protocrystalline Si:H and n-type cadmium sulphide (CdS), respectively. Analysis of the thickness, phase and structure of the window layer material over the area of the PV panel is critical in order to design processes for uniformity of high performance. Descriptions are given of future directions in novel instrumentation development that will enable mapping for uniformity evaluation at the high speeds required for on-line analysis.

By definition, PV module certification is simply based on conformance to standards. The IEC norms for PV modules are considered to be adequate quality requirements for guaranteeing initial quality. However, it is commonly understood that two products A and B may meet the standard’s requirements, but overall qualty – considering long-term stability, performance and safety – can still be quite different. PV module testing should therefore be carried out more frequently and beyond IEC requirements. A factory inspection once a year – as suggested by most certification bodies to ensure continuous quality of certified crystalline modules – may not be sufficient. The need for additional control is demonstrated in this paper, with reference to our experience from PV module testing and quality assurance activities for wholesalers and project developers. We present the necessity of additional measurements under standard test conditions (STC) and advanced testing methods, which are becoming essential for reliability.

There are still a lot of “ifs” when it comes to concentrator photovoltaics, but it’s starting to look like the question of “when” the technology will start to gain serious market traction may be sooner than some think. With tens of megawatts of projects either recently finished, under construction, or in the last phases of project development — and hundreds more MWs in the longerterm pipeline — deployment of the highefficiency systems may reach triple digits by the end of 2011 or beginning of 2012. On the technology front, as many as a half-dozen cell companies are bringing 40%-efficient cells to market this year, which will help to further reduce CPV’s increasingly compelling levelized cost of energy.

Liyou Yang started in the thin-film game in 1985 with BP Solar, where he eventually ran the company’s amorphous-silicon research efforts. “Once you get into it,” he smiled, “you get hooked.” During the course of our conversation at Astronergy’s headquarters, the Rutgers-educated president/CEO would often reference his time at the old company, using his early experiences as reminders of just how far the technology and the solar industry in general have come since those pioneering days in the 1980s and ‘90s.

A growing number of thin-film photovoltaic module producers are either trying to keep up with the current cost leader or aiming to differentiate on product design. Calyxo is dedicated to both keeping the pace in the US$0.50/Wp race and introducing new product generations, therefore delivering more value to the customer. We have tried to improve the methodology and approaches for knowledge building in the individual process steps, by learning the relevant interactions between them, as well as ramping volume and lowering manufacturing cost in the first production line. Developing and building the deposition equipment suited to the high process temperatures of approximately 1000°C at atmospheric pressure took some time, but the technology itself now enables Calyxo to benefit from significant cost savings both on capital investment and operational cost – compared to some well-known vacuum deposition methods. Besides the continuous decrease in manufacturing costs, even early on in building the manufacturing capacity, the ability to design the product itself according to the needs of the customers proved itself to be a decisive factor in ensuring competitiveness. This paper aims to give an insight into some of the basic design features of a new product generation and how the so-called new CX3 product will generate more watts by improved performance: delivering better customer value by decreased voltage to save on BOS costs and ensuring further increased field durability through an optimized package design.

Phosphorus dopant pastes are an attractive alternative to the conventional phosphorus oxychloride (POCl3) dopant source for emitter processing in solar cells, as they allow the fabrication of selective emitters on an industrial scale. In this paper it is demonstrated that single-sided uniform screen-printed emitters, processed with phosphorus dopant pastes, can getter multicrystalline silicon (mc-Si) wafers more effectively than conventional double-sided uniform POCl3 emitters. This result is confirmed by minority carrier lifetime measurements with the quasi-stead-state photoconductance (QSSPC) method. Solar cells with selective emitters were processed using phosphorus dopant pastes on mc-Si wafers and were subsequently characterized. The current-voltage (I-V) results are improved compared to uniform POCl3 emitter solar cells and an increased internal quantum efficiency (IQE) for selective emitter solar cells is demonstrated.

Upgraded metallurgical-grade silicon (UMG-Si), once looked on as a cost-effective and energy-efficient alternative to Si produced via the Siemens route, has experienced a severe regression of late. This has been caused both by the market conditions and by specific physical properties of these materials. Meanwhile, the qualities and the rated influence of negative physical effects have changed partially. Hopes are again rising that these materials, which have to be compensated to meet the desired net doping specifications, might achieve an economical breakthrough instead of long-dreaded low breakdown voltages. In the following paper, we summarize a few of our results on multicrystalline UMG silicon as well as results published by other research groups in the last few years.

Technology computer-aided design (TCAD) is pervasive throughout research, development and manufacturing in the semiconductor industry. It allows very low-cost evaluation of process options and competing technologies, guides process development and transfer to production and supports more efficient process improvement with minimal down time in the factory environment. This paper reviews the use of TCAD in the semiconductor industry and shows, with some illustrative examples, its important enabling role for the PV industry.

Reduction of silicon wafer thickness without increasing the wafer’s strength can lead to a high fracture rate during subsequent handling and processing steps. The cracking of solar cells has become one of the major sources of solar module failure and rejection. Hence, it is important to evaluate the mechanical strength of silicon solar wafers and influencing factors. The purpose of this work is to understand the fracture behaviour of multicrystalline silicon wafers and to obtain information regarding the fracture of solar wafers and solar cells. The effects on silicon wafer strength of saw damage and of grain size, boundaries and triple junctions are investigated, while the effects of surface roughness and the damage layer removal process are also considered. Significant changes in fracture strength are found as a result of different silicon wafer crystallinity and surface roughness. Results indicate that fracture strength of a processed silicon wafer is mainly affected by the following factors: the saw-damage layer thickness, surface roughness, cracks/ defects at the edges and the number of grain boundaries – which all serve as possible crack initiation points. The effects of metallization paste type and firing conditions on the strength of solar cells are also considered, with findings indicating that the aluminium paste type and firing conditions influence the strength of solar cells.

Exceptional demand characterized the PV industry in 2010. Uncertainty regarding incentive schemes in a number of key markets drove global installations, and inverter shipments grew by over 160% as investors and developers rushed to complete projects, fearing that incentives would be reduced or removed altogether. IMS Research estimates that inverter shipments exceeded 20GW in 2010 and sales of small three-phase inverters, rated between 10-20kW, grew by around 200% in 2010. Inverters rated at over 500kW are estimated to have grown at a similar rate, but continue to represent a smaller share of revenues.