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<JUN, Issue, 2012>
Cover Story :
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Table of
  Contents
Cell & Module

A Magic Combination

N-type silicon wafers, IBC solar cells (Zebra), and silicone module encapsulation technology (Simba) bring a breakthrough in the productivity of cells and modules.
What we see happening now in the market is an unprecedented cell production expansion and a sharp drop in price. Strong changes force cell manufacturers to take steps towards vertical integration. Silfab and ISC Konstanz are jointly working to create a new opportunity for cell and module manufacturers: the Zebra & Simba technology approach. As a combination of processes and materials already available on the market, Zebra & Simba approach offers a 39% gain in electricity generated over 40 years. With this technology, todays industry standard manufacturing lines can be set up to produce super high-efficiency cells and modules at a very competitive cost per Watt and per Watt/hour.

There can hardly be any doubt! Within the next few years, grid parity will be reached by photovoltaic systems, and the PV industry will start developing its own strength on a massive scale worldwide. We cant argue over the definition of grid parity: when PV electricity is competitive vs. conventional grid power. In some places this is already the case while in most cases it is not yet a reality.

We might disagree over the criteria to be used to establish if grid parity has, or hasnt, been reached already under specific conditions. However, for someone like me, who started a PV manufacturing business in 1981 and has worked on expanding it since then, todays perspective is nothing less than a dream that comes true. Only a few years ago, myself and other PV experts were still considered by mainstream public opinion as eccentric--if not crazy--entrepreneurs. Today, while PV cell production has reached 27.2 GW in 2010, I am proud to lead a vertically-integrated PV provider, Silfab Group, involved in the final rush towards grid parity and sustainable energy production.

We are now at a pivoting point of the PV industrys history, marked by two major factors: 

(1) On one hand, what we see happening in the market is an unprecedented cell production capacity expansion: a stunning + 118% leap to 27.2 GW in 2010, according to Photon International. This massive increase is continuing in 2011, as only the top ranking 12 manufacturers have expanded over 1 GW each, 19 manufacturers over 0.5 GW this year, and other 180 manufacturers are expanding up to 0.5 GW each. This brings the total volume to a possible 35-40 GW/year, a production capacity that could serve tens of millions of people while incentive schemes are being trimmed in major European markets such as Germany and Italy. The result is a sharp drop in prices which forces manufacturers to take steps towards vertical integration, innovation, diversification of their products, and the implementation of cost saving procedures.

(2) On the other hand, since the beginning, the photovoltaic industry has been using raw materials basically conceived for the semiconductor industry. Just recently, the developing PV markets have reached a critical mass which enables the PV industry to put pressure on raw material manufacturers. Starting in 2006, for the first time, polysilicon used in the photovoltaic industry had exceeded all other uses. Before reaching this milestone, PV raw materials and technologies had changed very little. In fact, from the standpoint of materials and technology, todays photovoltaic is basically the same as it was two and a half decades ago. Yet, improvements such as bigger cells, 3 bus bars, the selective emitter, etc., have brought the conversion efficiency from the 90s, being 12-14% to present days 15-19%. However, this technology has its limitations. Expected efficiency limits for front-contact cells are at 20%.

From these two major trends, a paradigm change is emerging and transforming the PV industry. Research on N-type silicon PV cells and on new encapsulants is the leading edge of this trend. And Silfabs R&D efforts have been focused on this path. Its Zebra/Simba new technologies are at the forefront of these developments.

 

 

Improving the Productivity of Cell

 

What is needed today to increase PV module efficiency is a vertical integration in order to create a synergy between the cell and the module innovations. For example, the recently introduced selective emitter cell absorbs a broader spectrum of solar radiation, though this advantage becomes noticeably reduced because the EVA, which makes up the module, absorbs the greater quantity of such broader band. Silfab is now working towards changing this short-sighted approach.

Silfab has succeeded in improving the productivity of the cell together with that of the module by combining innovative production processes while using materials and equipment readily available on the market. This is the pairing of Zebra, a new super-efficient back contact solar cell based on large area (156 mm x 156 mm) conventional N-type monocrystalline (Cz) silicon wafers, and Simba, a new technology that allows the module to noticeably expand its transparency to a wider solar spectrum and significantly increase its durability and productivity over time.

Silfab is thus acting as a technology provider, when it comes to Zebra/Simba technologies, and expects to broaden its already significant number of partnerships through this new initiative. Let me now briefly further illustrate how Zebra/Simba accomplishes its gains in productivity and lifespan extension, while significantly lowering the cost of production in terms of $/Wp. From a technological point of view, three main factors are involved: N-type silicon wafers, IBC solar cells (Zebra), and silicone module encapsulation technology (Simba).

 

N-Type Silicon

 

The crucial role played by P-type silicon in the semiconductor industry during the last three decades has been the main reason why traditional photovoltaic cells were built around it. However, it is worldwide agreed that N-type silicon, thanks to its greater tolerance to common impurities, is technically easier to be produced and more cost effective than P-type silicon, while N-type ingots/wafers show a much higher electrical quality, which is the key and essential element to get super high efficiency of PV cells.

Another important advantage of N-type silicon versus P-type silicon, is the zero Light Induced Degradation (LID), as opposed to the 1.5-2% of the P-type silicon cell. This means that upon its first exposure to solar light, the traditional module experiences an initial degradation that consequently reduces its performance over its entire life. This effect doesnt happen with the N-type silicon-based PV cell which maintains its full performance over its life.

In the end, N-type silicon is undoubtedly the solution for the production of super high-efficiency cell concept. In fact, the cell designs having the highest efficiency nowadays available at industrial level are both based on N-type monocrystalline silicon. These are Heterojunction with Intrinsic Thin Layer (HIT) and Interdigitated Back-Contact (IBC) solar cells, presently available on the market but produced with very high manufacturing costs. Our challenge is to produce N-type super high-efficiency cells at a production cost even much lower of the one of the actual conventional lines.

 

New Interdigitated Back-Contact (IBC) Solar Cell

 

For several reasons such as shadowing by the front silver contacts and limited surface passivation (even when built with the best quality wafers and using the most advanced processes) traditional front contact cells cannot achieve a conversion efficiency of solar irradiation at a level higher than 20%. Industrys interest in Interdigitated Back-Contact solar cells (IBC) stems from the limitations found in front-contact solar cells.

The Zebra cell concept is based on large (156 x 156 mm) N-type monocrystalline (Cz) silicon wafers and is a back-contact, back-junction cell without any metallization on the sunny side. The combination of conventional N-type large area wafers, as well as simple process and low capital investments leads to a significant reduction of production costs while getting a solar cell with unbeatable efficiency. This technological achievement was made possible by the magic combination of the long-term industrial experience of Silfabs technical team and the excellence of ISC Konstanzs Team who is expert in R&D on photovoltaics, especially N-type silicon solar cells.

The Zebra process sequence is a new and smarter combination of single steps that are already implemented in mass production by the PV industry, thus avoiding the need to adopt specific and dedicated equipments for industrial back-contact cells.  The process is based on metallization by standard screen printing and then patterning of rear diffusion area by laser-ablation. No photolitography is needed for local doping. The required equipment is the standard for a mono-crystalline silicon solar cell production line, plus a fast and precise laser for the local opening of SiNx, B-diffusion and B-emitter passivation processes. Fast industrial implementation, by retrofitting existing production lines, significantly reduces the cell production costs.

22% efficiency means a higher power output per cell (5+ Wp against 4+ Wp) which allows a 60 cell module to reach 306 Wp, as compared to todays average mono 60 cell modules nominal power of 245 Wp. The absence of Light Induced Degradation (LID) achievable by the use of N-type silicon gives Zebra an additional advantage of 1.5-2% on the field efficiency. Overall, including a lower power drop from cell to module of the IBC module vs. a standard one, the gain in terms of effective field power is 25% (306 Wp versus 245 Wp).

 

 

 

Silicone Encapsulant

 

An innovative module encapsulant, silicone based, is currently under advanced stage of validation at Silfab. Its optical transparency, particularly to Ultra-Violet wavelengths, enhances cell performance and allows excellent transfer of cell power to module, thus reducing the power drop to less than 3% (as opposed to about 5.5%, being the average value allowed by todays standard techniques using EVA encapsulant).

Furthermore, its long-term durability in outdoor application has been clearly demonstrated in the past decades. Silicone encapsulants very low degradation over time, coupled with the use of N-type, zero LID Zebra cells, leads to unmatched lifelong productivity of this new generation of PV modules. Expected annual degradation of todays best technological standard being at 0.8%/Yr leads to a residual power of a module of about 80% of the nominal power in 25 years. By comparison, Zebras expected annual degradation is only 0.15%/yr which leads to a residual power of over 95% after over 30 years, and almost 93% after 50 years of exposure to sunlight.

Besides the very efficient capturing of the light photons available from a wider solar spectrum and the extremely long durability, other features of silicone encapsulant, such as very low ionic impurities, low moisture absorption, low dielectric constant and broad temperature utility make it THE ideal solution for the years to come photovoltaic industry.

The technology being used in the Simba module is the same as that being used by the PV industry except for the lamination phase which is replaced by a silicone dispensing/baking phase.

 

Silfabs Mission as Technology Provider

 

Zebra cells and Simba module technologies will be licensed by Silfab who will act as a technology provider to selected tier 1 PV producers, helping them to differentiate and enhance their market position in the next years.

Being based on a magic combination of well-known industrial processes, and complete utilization of existing production lines, the capital investment needed to convert a conventional cell and module line will be very low, while the time to market will be extremely short. This makes Zebra and Simba technologies highly attractive.                 

Worldwide consensus to PV, the need to find ways to produce massive energy from non fossil fuels and the Fukushima event keep pushing several governments to support the exploitations of renewable sources. However, many of the incentives that have sustained these markets will soon end or strongly diminish. The world PV industry must quickly become more efficient throughout the entire value chain and find ways to get the grid parity soon. 

In the short and mid-term, crystalline silicon technology will continue to dominate the market, thanks to the upcoming very high-efficiency, low production cost, long durability products. The final breakthrough will come soon when the new generation PV components and systems will reach the grid parity.

By December 2011, Silfabs pilot lines are expected to start production of Zebra cells and Simba modules. For mid 2012, Silfab and ISC Konstanz are targeting a 21.5% cell efficiency on the pilot lines, leading to a 300 Wp module. The two pilot lines are now being set up in order to test and fully demonstrate the industrial viability of the process, as well as to showcase the technology to potential customers.

The PV industry does need forward-looking entrepreneurs who envisage new ways to produce energy and approach markets by using proven but advanced technologies.

The old dream can now come true.

 

 

For more information, please send your e-mails to pved@infothe.com.

2011 www.interpv.net All rights reserved. 

 
 

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