This paper presents examples of recent process developments at ECN in silicon solar cells on n-type monocrystalline base material. For all PV manufacturers, the challenge is to increase module efficiencies while maintaining low production cost. An effective way to move to higher and more stable efficiencies, using low-cost industrial-type processing, is n-type solar cell technology. The solar cell considered in this paper is the n-pasha cell – a bifacial solar cell with homogeneous diffusions and screen-printed metallization. The n-pasha cell is currently produced on an industrial scale by Yingli Solar; in 2011 a maximum solar cell conversion efficiency of 19.97% was obtained using this cell concept on 239cm2 n-type Cz at the ECN laboratory. The focus of the paper will be increasing efficiency by optimization of the cell process, in particular the front-side metallization, and by improvements to the rear-surface passivation. These two steps have contributed an increase in efficiency of 0.8%, allowing cell efficiencies of 20% to be reached.
Coupled device and process simulation tools, collectively known as technology computer-aided design (TCAD), have been used in the integrated circuit industry for over 30 years. These tools allow researchers to quickly home in on optimized device designs and manufacturing processes with minimal experimental expenditures.
The PV industry has been slower to adopt these tools, but is quickly developing competency in using them. This paper introduces a predictive defect engineering paradigm and simulation tool, while demonstrating its effectiveness at increasing the performance and throughput of current industrial processes. The impurity-to-efficiency (I2E) simulator is a coupled process and device simulation tool that links wafer material purity, processing parameters and cell design to device performance. The tool has been validated with experimental data and used successfully with partners in industry. The simulator has also been deployed in a free web-accessible applet, which is available for use by the industrial and academic communities.
In the photovoltaics industry, contacts to crystalline silicon are typically formed by the firing of screen-printed metal pastes. However, the stability of dielectric surface passivation layers during the high-temperature contact formation has turned out to be a major challenge for some of the best passivating layers, such as intrinsic amorphous silicon. Capping of well-passivating dielectric layers by hydrogen-rich silicon nitride (SiNx), however, has been demonstrated to improve the thermal stability, an effect which can be attributed to the atomic hydrogen (H) diffusing out of the interface during firing, and passivating dangling bonds. This paper presents the results of investigations into the influence of two different dielectric passivation stacks on the firing stability, namely SiNy/SiNx (y < x) and Al2O3/SiNx stacks. Excellent firing stability was demonstrated for both stack systems. Effective surface recombination velocities of < 10cm/s were measured after a conventional co firing process on 1.5Ωcm p-type float-zone silicon wafers for both passivation schemes. On the solar cell level, however, better results were obtained using the Al2O3/SiNx stack, where an efficiency of 19.5% was achieved for a large-area screen-printed solar cell fabricated on conventional Czochralski-grown silicon.
In the photovoltaic industry, laser edge isolation (LEI) is a well-established process at the end of the process chain. However, because the cell properties vary from one cell producer to the next, no systematic approach is defined in industry for establishing an efficient isolation groove. Nevertheless, a general approach has to be defined for analyzing the LEI process for silicon solar cells. Besides the material aspects and laser parameters, atmospheric boundary conditions must be considered. This paper presents investigations into the ablation of a specific type of mc-silicon solar cell, and the most suitable laser, as well as the ambient parameters, is determined based on the results of the experiments.
A new production process for crystalline silicon (c-Si) solar cells, specifically p-type back-contact solar cells, is proposed. In contrast to the conventional c-Si solar cell manufacturing method, this new technology eliminates the etching process and reduces the industrial three-step electrode printing to only one step, greatly improving the technological process. Furthermore, the proposed process is also largely compatible with a traditional c-Si solar cell production line. Oxidation technology for producing the SiO2 film on a c-Si wafer, together with corrosive window technology, such as through HF corrosive paste screen printing, for creating the patterning on the wafer covered with SiO2 film, are used in the fabrication of the p-type back-contact solar cells.
Crystalline silicon wafer technology currently dominates industrial solar cell production. Common devices feature opposing electrodes situated at the front and the rear surface of the wafer, and subsequent front-to-rear interconnection is used for module assembly. This paper describes the status and perspectives of the emitter wrap-through (EWT) cell concept, which is a fully back-contacted solar cell. The functions which have to be fulfilled for this concept, as well as the corresponding challenges and advances, are discussed.
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.
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.
To make solar energy cost effective, the photovoltaic (PV) industry has to reduce its manufacturing costs well below 1€/Wp. To reach this cost target, roadmaps for c-Si technology foresee a drastic reduction in the amount of high-purity Si used and an increase in solar cell efficiencies beyond 20%. But this requires advanced cell concepts that put more stringent requirements on process steps such as doping, cleaning and surface passivation. Several processes in the technology and analysis toolbox of microelectronics offer opportunities to meet these stringent requirements. In this paper, we give examples of recent progress in solar cell development that has been achieved by implementing CMOS-like process steps, and we discuss how these processes can be attuned to the needs and benefits of the solar industry.
The need for higher efficiency solar cells is becoming more and more urgent nowadays in the photovoltaic industry. In this paper, a new method of increasing efficiency is described whereby SiN is coated by a special commercial chemical after the final step of manufacturing, which is screen printing. No mask is required for this method, but a drying temperature of 200–400°C is mandatory to activate the SiN layer. It is shown that the efficiency of a crystalline solar cell can be increased by at least 0.16% (absolute value) on average. At the same time, modules made from these solar cells do not degrade after sun exposure, and have the potential to pass the stringent standards of a potential-induced degradation (PID) test. The total cost for all the equipment and the chemical is around US$300,000 for retrofitting two (30MW each) production lines.