This issue of Photovoltaics International, our 23rd, offers key insights into some of the technologies that are ready to move from lab to fab in support of these goals. ISC Konstanz offer a glimpse of what the low-cost, high-efficiency solar cells of the future might look like. On page 35 the institute’s authors give an overview of what they call Konstanz’ “technology zooâ€, encompassing their so-called BiSoN, PELICAN and ZEBRA cell concepts, all of which are aimed at increasing energy yield at the lowest possible cost.
The n-Pasha n-type silicon solar cell currently achieves an average conversion efficiency of 20.2% using a relatively simple process flow. This bifacial cell concept developed by ECN is based on homogeneously doped p+ front and n+ back surfaces. To enhance the cell efficiency, it is important to reduce the carrier recombination within the boron-diffused p+ region and at its surface. This paper addresses a novel way to tune the boron-doping profile and presents advanced surface passivation schemes. In particular, it is demonstrated that a very thin (2nm) Al2O3 interlayer improves the passivation of the boron-doped surface; the Al2O3 films were deposited in industrial atomic layer deposition (ALD) reactors (batch or spatial). Moreover, it is shown that the boron-doping profile can be improved by etching back the boron diffusion. On the basis of the results presented, it is expect that n-Pasha solar cells with 21% efficiency will soon be within reach.
This paper presents the minimum aspects to consider for the commissioning of large-scale PV plants. This methodology has been successfully implemented in the commissioning of more than 40 PV facilities worldwide and it represents a very useful tool to assure the good performance of the PV project.
The crystalline silicon (c-Si) module price has been fluctuating slightly around the US$0.72/Wp level for the last 18 months. This pricing, at an estimated cumulative PV module shipment volume of 149GWp, indicates a trend change for the PV industry. C-Si module pricing appears to be currently above the production cost and should therefore yield a profit margin. However, there is still a mismatch between manufacturing capacity and future market demand. A closer look at the pricing figures reveals that there is no indication to give the allclear during the ongoing consolidation process in the PV industry. C-Si module pricing is not reflecting the
increase in polysilicon and wafer prices, and therefore the pressure to reduce the cell and module conversion costs remains a looming fact. This paper describes state-of-the-art c-Si cell manufacturing solutions that are in line with identified trends in materials, processes and products recently published in the 5th edition of the International Technology Roadmap for Photovoltaic (ITRPV). Currently available c-Si cell technologies offering higher efficiencies as well as materials savings will be discussed. The need for implementing these technologies in mass production without significantly increasing the cost per piece and in the face of more complex manufacturing processes will be established. The findings of the ITRPV regarding the reduction in levelized cost of electricity (LCOE) will be discussed, leading to the conclusion that contemporary cell technology supports the long-term competitiveness of PV-based power generation.
Recent advances at the cell level and in tabber-stringer equipment have led to the development of the next generation of cell interconnection architecture, resulting in an increase in cell and module performance. The multi-busbar (MBB) concept discussed in this paper delivers the benefits of a saving in material costs, a reduction in total series resistance and an improved light utilization for higher performance at lower cost. The combination of the cell and module concept and the stringer equipment works for a wide variety of cell types and enables an appreciable decrease in cost per watt and module size per watt.
The pioneers of utility-scale PV construction have drawn on methods used in other industries to make power plants more efficient and more competitive. This paper investigates how cutting-edge techniques in modularndesign are being used to drive down plant costs. The evolution of modular design and its attractiveness to theinvestor community are discussed.
Metal wrap-through (MWT) module technology is an attractive approach for increasing module efficiency. This paper shares the results of MWT module fabrication using a silicone electrically conductive adhesive (ECA), a conductive backsheet (CBS) with a thin organic layer surface finish, and an automated module assembly line. Very low cell-to-module (CTM) power losses are observed, leading to a multicrystalline Si module power of 266W and a full-area efficiency of 16.8%. The modules are very stable in damp-heat conditions and thermal cycling, demonstrating minimal degradation after 1.5 x IEC requirements in terms of damp heat and thermal cycling, and well below 2% degradation after 2 x IEC requirements. These MWT modules have received IEC 61215 and IEC 61730 certification.
This paper describes the degradation of sputtered aluminium-doped zinc oxide (ZnO:Al) layers which were exposed to damp heat (85°C/85% relative humidity). The ZnO:Al samples were characterized by electrical, compositional and optical measurements before, during and after damp heat exposure. Hall measurements showed that the carrier concentration stayed constant, while the mobility decreased and the overall resistivity thus increased. This mobility decrease can be explained by the enhancement of the potential barriers at the grain boundaries because of the occurrence of additional electron-trapping sites. X-ray diffraction (XRD) and optical measurements demonstrated that the crystal structure and transmission in the range 300 –1100nm did not change, thereby confirming that the bulk structure stayed constant. Depth profiling showed that the increase of the potential barriers was caused by the diffusion of H2O/OH- through the grain boundaries, leading to adsorption of these species or to the formation of Zn(OH)2 or similar species. Depth profiling also revealed the presence of carbon, chloride and sulphide in the top layer, which indicates the possible presence of Zn5(CO3)2(OH)6, Zn5(OH)8Cl2•H2O and Zn4SO4(OH)6•nH2O. Furthermore, white spots appeared on the ZnO:Al surface during damp heat exposure. The spots contained elements, such as silicon and calcium, which might have migrated from the glass and which reacted with species from the environment, including oxygen, carbon and chlorine.
The SPF solar glass certification was developed in 2002 to guarantee the quality of glazing for use as a transparent cover for solar thermal collectors. More than 200 glass types from leading manufacturers have been measured and certified to date. Despite the certification having been explicitly developed for solar thermal applications, it became widely used in the PV module industry, even though the results are not transferable and may lead to erroneous conclusions in some cases. In 2012 the certification was therefore adapted to the needs of the PV industry, and a dedicated PV solar glass certification has since been available. This paper explains the fundamentals of the certification process, which consists of three performance characterizations: 1) transmissivity, 2) incident angle modifier (IAM), and 3) UV degradation. Results are discussed for different representative glass types, including float glass, anti-reflective-coated glass and rolled glass with different structures. Furthermore, the performance of these glass types when used as covers of crystalline silicon PV
modules is compared. The examples presented also highlight the advantages of the adapted characterization methods compared with standard glass measurements.
Although considerable progress has been made in reducing the amount of Ag required per wafer in the classic screen-printing metallization of Si solar cells, the total cost of ownership of the metallization process today accounts for more than 50% of the total cell-process-related cost. There has been pressure on cell and module manufacturers to further reduce this cost, by either improving the metallization process or applying alternative contacting technologies. In this paper, the classic screen printing of standard Si-based solar cells, which has been the main metallization technique for many years, is described in detail. The required paste volume for providing the contacts in a state-of-the-art cell production process is calculated
on the basis of the contact dimensions (fingers and busbars on the front, Al layer and Al/Ag pads on the back). Taking into account today's paste prices, equipment investment, screen cost, energy, maintenance, yield, material utilization and necessary labour, the total cost of ownership of the cell metallization is also determined. The main cost drivers are discussed in detail. The cost reduction is estimated when improved printing processes – such as double, dual or stencil printing – are employed. Other promising alternative front-contact metallization technologies are listed and their potential is briefly discussed. To evaluate the competitiveness of these technologies, the limit of today's screen-printing method and its further cost reduction potential are estimated on the basis of the physical properties of cells and printing pastes.
