Today, crystalline-Si photovoltaics (PV) dominate the market, accounting for more than 85% of market share in 2010. A large scientific community made up of academic as well as industrial stakeholders strives to find solutions to improve device efficiencies and to drive down costs. One of the important cost elements of a module is the c-Si wafer itself. This paper discusses the fabrication of a carpet of c-Si foils on glass, either by layer transfer of an epitaxially-grown layer or by bonding of a very thin wafer, and processing this c-Si thin-foil device into a photovoltaic module. This could constitute an advantageous meet-in-the-middle strategy that benefits not only from c-Si material quality but also from thin-film processing developments.
Advances in nanofabrication for enhancing the efficiency of optical devices, such as solar cells and photo-detectors, via nanostructuring have attracted a great deal of interest. A photoconversion strategy employing nanorods (NRs) has emerged as a powerful way of overcoming the limitations of planar wafer-based or thin-film solar cells. But there is also a broad spectrum of challenges to be tackled when it comes to putting into practice cost-effective NR solar cell concepts. ROD-SOL is a 10-partner, ‘nanotechnology for energy’ project with end-users, equipment manufacturers and institutes from six countries forming the consortium. The aim of the project is to provide the photovoltaic market with a highly efficient (> 10%), potentially low-cost, thin-film solar cell concept on glass, based on silicon nanorods. This paper presents the project’s achievements and discusses what the future might hold for nanotech-based solar energy production.
Advances in nanofabrication for enhancing the efficiency of optical devices, such as solar cells and photo-detectors, via nanostructuring have attracted a great deal of interest. A photoconversion strategy employing nanorods (NRs) has emerged as a powerful way of overcoming the limitations of planar wafer-based or thin-film solar cells. But there is also a broad spectrum of challenges to be tackled when it comes to putting into practice cost-effective NR solar cell concepts. ROD-SOL is a 10-partner, ‘nanotechnology for energy’ project with end-users, equipment manufacturers and institutes from six countries forming the consortium. The aim of the project is to provide the photovoltaic market with a highly efficient (> 10%), potentially low-cost, thin-film solar cell concept on glass, based on silicon nanorods. This paper presents the project’s achievements and discusses what the future might hold for nanotech-based solar energy production.
In terms of material properties, plasma-enhanced chemical vapour deposition (PECVD) of ZnO has advantages over sputtering techniques, due to the variety of available precursors, different dopants for achieving certain levels of n-type and, even controversially discussed, p-type transparent conductive oxides (TCOs) on various substrate materials. This paper considers the deposition of boron-doped zinc oxide for n-type TCO-application on substrates of dimensions up to 50×50cm2 and at a temperature range of 50 to 450°C using a PECVD reactor with a plasma frequency of 13.56MHz. The materials’ characteristics such as transparency, carrier concentration and structural properties are discussed as a function of the deposition parameters. The deposition temperature strongly affects the crystallographic and morphological appearance of the deposited thin films, which was investigated using field emission scanning electron microscope (FESEM) and X-ray diffraction (XRD) methods. The electronic band structure-dependent characteristics were studied using ultraviolet-visible (UV-vis) spectroscopy and Hall measurements. Second ion mass spectroscopy (SIMS) measurements complete the characterization methods for qualitatively verifying the incorporation of dopants and impurities. Results are reported for columnar-grown boron-doped ZnO with optical transparency greater than 80% in the visible range and a maximum carrier concentration of 1020cm-3.
Sales of critical subsystems used in thin-film PV manufacturing equipment are expected to reach $325M in 2011, and the outlook is for this figure to grow by 4% in 2012 to $335M. This expectation is going against the trend for the industry as a whole, which is predicted to decline next year as revenues from cell and module manufacturing weaken. The reason for this countermovement is the opportunities available to manufacturers who are willing to invest in the latest thin-film PV equipment to drive down costs and force unprofitable competitors out of business. While the same opportunities exist for crystalline silicon manufacturing, the number of well-resourced companies signalling their intention to invest in thin-film technologies should ensure a positive year for suppliers of equipment and critical subsystems to this segment of the industry.
Because of its attractive electronic and optical properties, zinc oxide (ZnO) has found widespread use as a front and back electrode in commercial solar cells. ZnO can be deposited on glass using a variety of different methods, of which vacuum-based techniques are the most commonly used in industrial applications. Aluminium-doped sputtered ZnO:Al (AZO) has been studied intensively for use as a front contact in a-Si/µc-Si tandem cells. The implementation of AZO in series production has been hindered by reproducibility issues stemming from the combination of deposition and subsequent etching steps that are necessary to tune the ‘haze’ of the layers for optimal light scattering. Boron-doped ZnO:B (BZO), deposited by low-pressure chemical vapour deposition (LPCVD), has become a cost-effective option for module manufacturers, since the desired layer morphology can be produced as grown without the need of post-growth chemical etching. This paper addresses the different aspects of using AZO and BZO layers as front contacts for a-Si/µc-Si tandem modules fabricated in series production. The properties of the underlying ZnO layers put restrictions on the layer properties and process parameters that are used in the deposition of a-Si and µc-Si.
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 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.
The low material cost and proven manufacturability of thin-film silicon has made this material very attractive for low-cost photovoltaics (PV). It is widely recognized that increasing the light-to-electricity conversion efficiency will play a critical role in expanding the acceptance of these products. The first commercial thin-film silicon solar cell consisted of a singlejunction structure using amorphous silicon; multijunction cells incorporating amorphous silicon and silicon germanium were later used to further improve efficiency. An even later development was the incorporation of nanocrystalline silicon as an active layer. This very interesting material, which consists of nanocrystallites embedded in an amorphous tissue, has already given rise to a significant increase in the performance of these multijunction cells. Most recently, some very innovative light-trapping concepts have been suggested that can improve the efficiency further. Both these topics, however, have required expertise not readily available within one organization. A thin-film silicon team has been established under a US Department of Energy’s Solar America Initiative programme to address the material, device and manufacturability issues for this technology. United Solar Ovonic is the team leader, with Colorado School of Mines, University of Oregon, Syracuse University and the National Renewable Energy Laboratory (NREL) as members. The collaborative effort has resulted in a new understanding of the material and devices; innovative light trapping ideas were developed, and worldrecord initial efficiencies of 16.3% for small-area cells and 12% for large-area encapsulated cells were reached. Of equal importance is United Solar’s decision to introduce this technology into production. This paper presents the important technical results obtained under this programme and will discuss future directions.
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.