Technical Papers

The aim of this work is to study the effects of dark lines on the face of polycrystalline silicon solar cells. The formative processes of dark lines were observed by laser scanning microscopy. Following the initial appearance of a few etch pits on the surface of the cells, extending the etching time saw these etch pits increase in size, eventually merging to form a single line, known as a ‘dark line’. Dark lines are lines that are linked together by a series of contiguous dislocation outcrops and have the potential to reduce silicon wafer lifetime, adversely affect both the electroluminescence and the quantum efficiency of a solar cell, and have resulting negative effects on the cell’s electrical properties.

Savvy solar panel manufacturers understand that wringing excess costs from every stage of the value chain is simply the price of admission to today’s crowded market. They also know that reliability and quality are not only critical for delivering on a 25-year warranty promise, but also drive the true cost of energy over the lifetime of the system. This factor is becoming increasingly apparent, especially in industrial- and utility-scale solar projects, as they age and the power output of many lower quality systems begins to degrade to unexpected levels. Many of those systems used UL or IEC certifications as a proxy for good reliability. Unfortunately, UL certification is primarily concerned with user safety, and even the IEC requirements are not rigorous enough to ensure trouble-free operation throughout the system lifetime. High reliability and quality require testing and manufacturing methods that go far beyond the certification tests.

This paper focuses on the latest developments from research on MWT (metal wrap-through) solar cells at Fraunhofer ISE. An overview of the current cell results for mc-Si and Cz-Si material with both Al-BSF and passivated rear side is presented. Recent progress in cell technology and challenges to reaching efficiencies of 20% for industrially processed large-area MWT solar cells are also discussed. Up to recently, MWT cell efficiencies of up to 19% for Cz-Si and up to 17.5% for mc-Si have been reached with industrially feasible processing. Improvements to the design of the MWT cell to increase cell efficiency and to allow an easy module assembly are also presented in this paper, as are first calibrated IV measurements of MWT solar cells.

As PV power generation adoption becomes more widely adopted globally, the grid-connected inverter market looks set to take its rightful role as a critically important element of solar installations. The grid-connected inverter market will deliver power quality and the stability of the electricity networks in order to ensure a stable and reliable grid operation. In order to keep up with these developments, network operators will release new grid codes to monitor the increased uptake, to which manufacturers must adhere. An additional obstacle for the inverter manufacturers is the wide range of requirements and norms that vary from country to country and, in many cases, even from utility to utility. This article presents a review of the new challenges facing grid-connected PV inverters in the light of these new developments.

As yet, procedures for long-term tests of photovoltaic modules in outdoor conditions have not been considered by international standardization committees. Although many laboratories perform long-term PV outdoor tests, a commonly agreed and standardized procedure has so far not been adopted. The European Distributed Energy Resources Laboratories’ (DERlab) approach to filling the gap of international standardization has led to the development of a basic protocol that complies with European and international standards, while providing specific common guidelines and procedures for measuring the energy yield of PV modules for at least one year in outdoor conditions. The DERlab procedures for long-term PV module testing are described in this paper, and the range of analyses that can be derived from the data, such as module degradation, are discussed. The paper also presents the DERlab approach to measuring module performance in outdoor conditions, which can be used to complement energy-rating methods suggested in international standards. DERlab has created consistent measuring procedures that allow the direct comparison of the energy yield of solar modules taking into account the site-specific factors of different locations and varying climatic conditions, as well as a maintenance guideline.

One of the busiest of the couple-dozen solar manufacturing factory floors I’ve seen this year belonged to ECD Uni-Solar, at its Auburn Hills 2 (AH2) facility just up the road from the Palace where the NBA’s Detroit Pistons play hoops. When I toured the plant in late July, the three production areas – cell deposition, cell finishing, and module stringing/lamination/final assembly – were humming, as the 1.5-milelong rolls of flexible stainless-steel starting material were transformed into triplejunction amorphous-silicon thin-film PV laminates. The company’s latest quarterly results confirm those observations at the factory, as production output grew some 58% over the previous period – from 21.2MW to 33.6MW – pushing capacity utilization to about 90%.

As it makes its way towards a non-subsidised market, the photovoltaic sector has to deal with decreasing margins. To ensure investment goals are met in spite of this, it is imperative that PV power plants generate optimal yields. Comprehensive quality assurance for PV power plants covers all phases of the completion process from the planning to system operation. This article explains the extent of standard quality assurance measures that include yield assessments, module measurements, system testing and yield monitoring. It outlines the potential of linking these quality assurance measures and stresses the importance of the measures themselves being of high quality. Up-to-date scientific findings from Fraunhofer ISE are presented in order to further optimise quality assurance measures.

A hydrogenated amorphous Si (a-Si:H) film, combined with a silicon nitride (SiNx:H) capping layer and a post-deposition anneal, can hugely enhance the surface passivation on crystalline silicon wafers. In this work, the influence of various deposition temperatures of a-Si:H films on the thermal stability of a-Si:H/SiNx:H stacks and a possible mechanism are discussed. Both minority carrier lifetime measurement and grazing-angle XRD were employed to study the thermal stability of a-Si:H/SiNx:H stacks, and the results are interpreted in terms of dihydrides concentration and epitaxial crystallization. With an appropriate thermal treatment, the a-Si:H film deposited at 130°C and capped by SiNx:H showed better passivation performance than 200°C-deposited a-Si:H/SiNx:H stacks, but under an excessive thermal budget the former showed more severe degradation of carrier lifetime. The more dihydride-rich composition within 130°C-deposited a-Si:H/SiNx:H stacks could be regarded as providing more effective intermediates for hydrogen interchanges, but on the other hand, it is also more susceptible to epitaxial crystallization.

A selective emitter is a doping layer that is heavily doped beneath the electrode and lightly doped in between the electrode grids. One of the disadvantages of conventional selective-emitter techniques is the need for a high phosphorus surface concentration to obtain low contact resistance and limit the shunts in the emitter. Effective emitter passivation below the contact is difficult because of the use of emitters with low sheet resistances and high doping concentrations. In this study, the selective emitter in the optimized light/light sheet-resistance combination was formed to reduce recombination under the metal contact. The fabrication of optimized light/light doped emitters was performed using a single-step diffusion process. Besides the benefit of low surface recombination for light/light combination, this approach also removes the need for a very precise alignment between the opened emitter pattern and the front screen-printed silver fingers. This work illustrates the achievement of an efficiency improvement of more than 0.4% absolute in large-scale production for selective emitter solar cells.

The workhorse of the photovoltaic industry, crystalline-silicon solar cells, continues to have additional headroom for conversion efficiency improvement as well as decreased production costs. As some companies have already demonstrated, clear pathways exist to bring about the achievement of >20%-efficient monocrystalline cells through the use of existing and novel production techniques. A newcomer to the solar cell and module sector, Suniva, has rapidly become a volume manufacturer using innovations originally developed at the University Center of Excellence in Photovoltaics (UCEP) at the Georgia Institute of Technology. This paper discusses the company’s first- and second-generation production technologies, including the implementation of ion implantation as a high-volume process, as well as details of cell-making approaches in the development stage.