Photovoltaics International Papers

Despite the drop in price of silicon wafers, they are still one of the main factors influencing the cost and performance of Si-based solar cells. These two consequences have initiated a growing commercial interest in mono-cast (castmono, mono-like or quasi-mono) Si wafers, supported by R&D in the areas of material characterization, correlation with cell efficiencies, and mono-cast material use in advanced cell technologies. This paper gives a broad overview and comparison of commercially available grades of mono-cast material from different suppliers. The performance of the material from production in high-throughput screen-printing lines, as well as an analysis of the main material characteristics influencing these results, is presented. A characterization using a lifetime tester and a photoluminescence (PL) imaging tool has shown that not only grain boundaries but also dislocations could cause a drop in cell Voc of more than 15mV. Wafers with large surface areas of Si lattice planes, when processed with anisotropic texturing, could yield an increase in Isc greater than 400mA for 6" substrates, as compared to the isotropic-textured equivalents. Furthermore, when a high-grade mono-cast material processed in anisotropic texturing was compared with CZ mono material from the same supplier and of the same resistivity, light-induced degradation (LID), presented as combined Voc and Isc degradation, was only one-third of that in CZ material. However, although mono-cast material has the potential to increase cell line performance to the same level as that gained by important process and technological improvements, it imposes very high requirements for better material sorting in order to achieve stable cell electrical performance and module aesthetics acceptable to the market.

Because most of the costs of developing a PV power plant are paid before any energy is generated, optimizing the energy production from the plant is critical during plant design. Lost energy and increased operations costs due to non-optimal site characterization, technology choice, plant design, installation and other factors result in lower energy production and a higher levelized cost of energy (LCOE). Many design decisions are based on results from PV performance models. Current PV performance models can represent only some of the differences between sites, technologies, designs and operations choices. This paper provides a description of what is currently known about some of the performance tradeoffs faced by PV plant designers and operators. It presents a vision for improving PV performance models so that in the near future a full optimization can be carried out to improve the performance and lower the costs of PV plants. This will hasten the adoption of clean energy production from the sun.

Economics will always play a crucial role in the way PV technology advances. However, the current generation of products is facing substantial business challenges in the attempt to scale the product technologies. This paper is the fifth in a series covering business analysis for PV processes. The methods applied in these papers fall into two categories: cost of ownership (COO) modelling and cost and resource modelling. Both methods examine the business considerations associated with the adoption of new processes, tools or materials. This is more critical than ever. Nearterm issues – in some cases the survival of the business – heavily influence today's decision processes. This paper tries to identify the areas that it is thought will produce the largest near-term paybacks. The areas identified are n-type wafers, Al2O3 passivation and copper metallization.

To achieve project cash flow expectations, it is necessary to operate, maintain and optimize the performance of a PV power asset to meet or exceed the pro forma operating assumptions. To assume as given the achievement of these model assumptions is both naive and risky. Experience in operating the largest fleet of solar PV power plants in the world has demonstrated that project financial hurdle rates can be missed by as much as 25% if the plant is not well maintained and its performance is not optimized. Conversely, an optimized PV asset can generate cash flows 2–10% higher than expected if the optimization approach described in this paper is implemented.

Because potential-induced degradation (PID) can cause power losses of more than 30% for modules out in the field, there has already been an extensive effort placed on avoiding this adverse phenomenon. A key feature at the cell level is the silicon nitride (SiNx) anti-reflective coating (ARC). Apart from the known dependency of PID susceptibility on the refractive index, the impact of the deposition parameters has also been under investigation. This paper illustrates the influence of different silicon nitrite layers and their ability to prevent PID. A large number of cells and modules were therefore manufactured, differing only in the type of ARC. The modules were subsequently PID tested under three different climatic conditions, and acceleration factors and activation energies were determined from these tests. In addition this paper presents the results of addressing the weak-light performance and the hot-spot risk of panels after PID exposure. Finally, the reversibility of PID was also investigated in relation to the state of degradation of these samples.

Since the demonstration of the first CuInSe2 solar cell in 1974 by scientists at Bell Laboratories, a lot of effort has been put into the development of cost-effective processes for highly efficient Cu(In,Ga)(Se,S)2 – or CIGS – solar cell devices. In 2012 these efforts led to the first gigawatt CIGS solar module production facility operated by Solar Frontier, a company that has a long history in R&D and originates from ARCO Solar, who developed the first commercial CIGS solar modules at the beginning of the 1990s. However, several start-up companies employing CIGS technology are presently struggling in the currently harsh market environment. Even though world-record laboratory solar cells now demonstrate 20.3% efficiency using a three-stage co-evaporation process, and full-size modules achieve 14.6% employing a similar method, efforts in research and development are more important than ever in order to increase cell efficiency, to bridge the gap between cell and module efficiencies, and to develop cost-effective and robust manufacturing processes. This paper gives an overview of current research topics under investigation by research institutes and industry, with a main focus on CIGS absorber formation. Along with other research results published by groups all over the world, this paper covers recent research results obtained at the Helmholtz-Zentrum Berlin für Materialien und Energie (HZB) and briefly mentions the work of the Photovoltaic Competence Center Berlin (PVcomB), a joint initiative of the Technical University of Berlin (TU Berlin) and HZB.

This paper presents the background and technology development of the use of ion implantation technology in today’s crystalline silicon solar cell manufacturing lines. The recent history of ion implantation development and commercialization is summarized, and an explanation is given for the cell efficiency improvements realized using the technique on p-type mono-crystalline cells. The potential economic impact on the factory is also discussed.

Reducing the cost of photovoltaic energy is the main objective of solar cell manufacturers. This is ideally realized by increasing cell efficiency and simultaneously decreasing manufacturing cost. To reduce fabrication costs, the international roadmap of photovoltaics (ITRPV) forecasts a reduction in cell thickness from 180μm to 120μm in the next six years, and even thinner cells may be desirable, as long as efficiency and yield are not negatively affected. In order to increase efficiency, the ITRPV forecasts an increase in share of back-contacted cells from 5% to 35% in the next eight years. In this paper the dependence of the efficiency of back-junction back-contact (BJBC) solar cells on cell thickness is investigated experimentally and numerically. To this end, BJBC silicon solar cells with cell thicknesses ranging from 45μm to 290μm are fabricated and simulated. Thinned float-zone material is used as well as monocrystalline epitaxial layers fabricated by the porous silicon process for 45μm-thick cells. The efficiency of the best cell is 22.6% (130μm cell thickness) and 18.9% for an epitaxial cell (45μm thickness). Loss mechanisms in the maximum power point of all cells are investigated by using a freeenergy loss analysis based on finite-element simulations. A lower generation and a lower recombination in thinner cells compete against each other, resulting in a maximum efficiency of 20% for a cell thickness of 45μm at a base lifetime of 20μs. At a base lifetime of 3000μs, the maximum efficiency is greater than 23% for a cell thickness beyond 290μm, but reducing the cell thickness from 290μm to about 90μm results in a power loss of less than 0.6% absolute.

The market price of Ag has fluctuated considerably over the past ten years and has impacted the manufacturing cost of Si solar cells and the price of Si PV. Reducing Ag consumption can decrease this cost; however, such reduction may come at the expense of cell performance. In order to address the issue of Ag cost reduction while maintaining high cell efficiency, phosphorus emitter profiles are tailored via POCl3 diffusion to create solar cell emitters displaying low saturation current density (J0e), variable electrically active surface phosphorus concentration ([Psurface]), and variable sheet resistance with the aim of reducing Ag consumption. By optimizing emitter diffusion conditions, it is possible to reduce screen-printed Ag paste consumption by 33% with no loss in cell performance. Using a screen-printable Ag conductor paste designed to contact low [Psurface] emitters, the performance of cells with screen-printed Ag paste dry masses of 200, 120 and 80mg is compared. By using a tailored low-J0e 55Ω/sq emitter, it is possible to achieve a high open-circuit voltage (Voc) and short-circuit current (Jsc) to yield average cell efficiencies of 18.64% and 18.73% for 120mg and 80mg Ag paste dry mass, respectively. This is compared with efficiencies of 18.52% for cells using state-of-the-art technology (industrial high [Psurface] 65Ω/sq emitter with 120mg Ag paste dry mass). On the basis of a Ag market price of US$32/troy oz and an 85% by weight thick-film paste Ag metal content, a Ag front-side metallization cost of US¢2.11/W can be achieved by using 80mg Ag paste dry mass, which translates to a Ag cost saving of US$5.4M per year for a 500MW production line when compared with the Ag cost for state-of-the-art technology. Further cost analysis shows a 1.2% area-related balance of system (BOS) cost reduction and a US¢0.1/kWh reduction when comparing low-J0e 55Ω/sq modules and state-of-the-art modules. Calculations show that an additional 0.5% absolute efficiency for state-of-the-art modules is required, to compensate the efficiency gains and Ag cost reduction afforded by low-J0e 55Ω/sq modules.

A record-low spot price in the wake of oversupply and the aggressive cost-reduction roadmap of the PV industry are putting polysilicon producers under pressure to bring down their manufacturing costs. With the dominant Siemens process approaching a limit for further cost cuts, technologies based on the deposition from monosilane (SiH4) have now become the focus of attention.