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.
Approximately 80% of today’s silicon solar cells industrially manufactured worldwide apply screen printing for the metallization of the silver front and aluminium rear contacts. In production, conversion efficiencies of ~18–18.5% are achieved using monocrystalline silicon wafers. A baseline process has been implemented at ISFH that is very similar to the industry-standard process, displaying conversion efficiencies of up to 18.5%. An analysis of the solar cells reveals that the conversion efficiency is limited in particular by the shadowing loss due to the silver front-side metallization, as well as infrared light being absorbed in the aluminium rear-side metallization. This paper summarizes recent developments at ISFH that resulted in a 19.4% efficient large-area screen-printed solar cell, when applying a print-on-print silver front-side metallization and an SiO2/SiNx rear-surface passivation.
Competition in the premium sector is becoming more and more fierce. This is forcing PV module manufacturers to differentiate themselves through product benefits and better performance in terms of efficiency. While attention has previously been focused on cell technology, it is likely that, in the future, all module components will become part of this competition – a competition in which premium front glasses present an especially promising element. Antireflective coating (ARC) is only the beginning of this evolution. Not only do deeply textured front glasses promise significant increases in output – up to 7% – but their specific product characteristics also make them suitable for niche applications, such as airplane entry lanes and airport buildings. EuPD Research has issued a white paper devoted to solar glass, of which a synopsis is presented here.
Transparent conductive oxides (TCOs), such as aluminium-doped zinc oxide (ZnO:Al), play an important role in thin-film photovoltaics. As a material for front contacts, ZnO:Al is standard in industrial-scale production, especially in the field of Cu(In,Ga)Se2 solar cells. Over the last few years, there has been a strong push to use ZnO:Al films on glass as substrates for amorphous or amorphous/microcrystalline silicon solar cells, and these films have now been introduced as an alternative to the typically used fluorine-doped tin oxide (SnO2:F) films in production. Sputtering coaters for large area deposition of ZnO:Al are widely available, and ZnO:Al films are produced in these coaters by sputtering of ceramic targets. This technology offers high process stability and is therefore favoured over reactive sputtering of metallic targets. With respect to cost and quality, however, the reactive process is an interesting alternative. In this paper we will give an overview of the process of reactive sputtering of ZnO:Al and discuss the most important insights.
In a multicrystalline silicon (mc-Si) cell production process, acid texturing is the most popular way of carrying out surface texturing. In general, the surface reflectivity and etch depth are the criteria used for quantifying the texture quality. In this study, four groups of cells were created with different etch depths of 2.82μm, 3.83μm, 4.41μm and 5.92μm. It was found that the etch depth had a notable effect on the efficiency of a cell. Also, the best texture was obtained with an etch depth of 4.41μm, at which there was a balance between a low reflectance and the removal of the saw-damage layer. As the etch depth increased, the film deposition thickness and the front bus-bar tensile strength were seen to increase. However, no linear relationship was found to exist between the diffusion sheet resistance and the etch depth.
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.
Quality assurance and process control are becoming increasingly important in the industrial production chain to the manufacturing of silicon solar cells. There are a number of relevant wet chemical processes for the fabrication of standard screen-printed industrial solar cells, mainly for texturization and cleaning purposes. While one-component systems like pure HF for oxide-removal are easy to monitor, i.e., by conductivity measurements, typical texturization processes are much more complex due to the number of constituents. For acidic texturization of multicrystalline silicon wafers, typical mixtures involve amounts of hydrofluoric acid (HF), nitric acid (HNO3) and water. It has also been documented that mixtures can be found where additional additives like phosphoric acid (H3PO4), acetic acid (HOAc) and sulphuric acid (H2SO4) have been used [1, 2]. In alkaline random pyramid texturization for monocrystalline wafers, a base like potassium hydroxide (KOH) or sodium hydroxide (NaOH) and organic additives like 2-propanol (IPA) are used [3]. In addition to these processes, recently developed high-efficiency cell concepts require several additional wet chemical process steps like advanced cleaning processes, chemical edge isolation or single side oxide removal processes [4].
In order to obtain continuously stable and reproducible process results and to overcome process operations based on operator experience, a reliable monitoring of the bath concentrations is essential. Such quality control has the potential for significant cost reductions due to optimized durations between replacements of bath mixtures or shortening of processing times. In this context, the application of on-line analytical methods, either by means of chemical, optical or electrical measurement techniques, is of particular interest.
Optical probes based on polarized light spectroscopy, including spectroscopic ellipsometry (SE) and polarimetry, have been applied in research and process development for the three major thin-film photovoltaics technologies, including thin-film hydrogenated silicon (Si:H), cadmium telluride (CdTe), and copper indium-gallium diselenide (CuIn1−xGaxSe2). Real-time SE during materials fabrication has provided insights into the nucleation, coalescence, and structural evolution of these thin films. These insights have led, in turn, to guiding principles for PV performance optimization, as well as future directions for real-time process control. The optical properties deduced simultaneously with the layer thicknesses using real-time SE have been applied to characterize the phase composition of materials (amorphous versus crystalline), the mean free path and grain size, and the relative free carrier concentration. As a result, analytical formulae for the optical properties of PV materials have been developed with free parameters that are linked to basic materials properties. This paper shows how the formulae and associated parameter-property relationships can serve as a database for analyzing complete PV stacks, with future prospects for mapping layer thicknesses and basic materials properties in on-line monitoring applications for large-area PV plates and modules.
