Crystalline Silicon To Remain Dominant PV Tech

Crystalline Silicon To Remain Dominant PV Technology

GERMANY: Harvesting solar energy through photovoltaics will be a key pillar of our future, sustainable energy system; 1:1:1 for wind, solar, and others (hydro, biomass, geothermal) is a reasonable expectation, according to Prof. Eicke R. Weber, director of the Fraunhofer Institute for Solar Energy Systems ISE and Prof. for Physics/Solar Energy at the Faculty of Mathematics and Physics and at the Faculty of Engineering at the Albert-Ludwigs-University of Freiburg, Germany,

Prof. Eicke Weber

Crystalline silicon will remain the dominant PV technology; classical thin film has to show lower prices or comparable efficiencies; highly-efficient CPV will take up a rapidly increasing niche market, competing with CSP.

Prof. Weber, along with S. Janz and S. Glunz from the Fraunhofer-Institute for Solar Energy Systems ISE and Albert-Ludwigs, University, Freiburg, Germany, presented on Photovoltaics: Pillar of our Future Sustainable Energy System, at the recently held Intersolar Europe 2012 at Munich, Germany.

The technology challenge in PV will be to generate innovations in efficiency and cost reduction fast enough to maintain a profit margin; only players in the XGW-range will survive in the long term. State support for investments in XGW plants (e.g., credit guarantees) might be necessary to maintain globally a level playing field for PV production.

Creating market barriers will ultimately hurt the economies that adopt them by higher prices and less pressure for cost reduction and innovation!

Bright future for PV
There is a bright future for PV. The globally required power is 16 TW today, and at least 30 TW by 2050. PV is offering at least 10 per cent. Optimistically, it should meet 30-40 per cent of the global energy needs. 3 TW power corresponds to 12 TW PV, 12,000 GW.

PV globally installed till 2009 has been 20 GW. In 2010, it was +17 GW and increased to +25 GW in 2011. To reach 12.000 GW, we would need almost 500 years at this rate! The PV market will move from a $ 50 billion market to 100s of billion dollars in a few years! This will be accompanied by a drastic cost reduction, making PV one of the most inexpensive ways to produce energy, in the range of 5 cts/kWh, comparable with hydro, onshore wind, less than nuclear, fossil fuels by 2030 or earlier!

At least three components or technologies are required at about 1:1:1 for a 100 per cent renewable energy scenario. These are: solar energy (PV and solar thermal), wind and hydro, geothermal and biomass.

The maximal sum of PV and wind production was 7.6 TWh in January 2012. The minimal sum was 5.6 TWh in February 2012. The total electricity need of Germany is about 600 TWh/yr. The global market forecast is said to be 30 GW in 2014 and 110 GW in 2020. The annual growth rate should be in the range of 20-30 per cent.

Giving a round-up of the global PV production development by technology by 2011, thin film had accounted for 3,204 MWp, ribbon-Si 120 MWp, multi-Si 10,336 MWp and mono-Si 9,114 MWp, respectively.

Efficiencies in solar cell market
According to Prof. Weber, the segmentation of the efficiencies in the solar cell market include 1-5 per cent for organic, dye, nanostructure cells. Technologies of interest for the future of PV in the next one to two decades are as follows:

* 6-11 per cent: Thin film cells (a-Si, microcryst.-Si, CIS, CIGS, CdTe).
* 14-18 per cent: mc-Si, umg-Si, simple c-Si cells.
* 20-24 per cent: High efficiency, mainly c-Si cells.
* 36-41.1 per cent: High-efficiency III/V tandem cells for concentrators with 25-30 per cent module efficiency.

Price learning curve for all c-Si PV technologies indicated that with each doubling of cumulative production, price went down by 20 per cent. Thin film technologies must ramp up fast enough to maintain a clear cost advantage at lower efficiencies!

Examples of solar cell conversion efficiency
Prof. Weber cited examples. For instance, solar cell conversion efficiency with 100 per cent umg-Si. Silicor plans: umg-Si plant for 16,000t/a at a cap-ex of $600 million. The median efficiency at CaliSolar is now 16.6 per cent. At least 11 per cent of the cells are above 17 per cent with the highest at 17.7 per cent. Also, Q-cells achieved 18.2 per cent efficiency using umg-Si cell with backside contacts.

The advantages and future requirements for mc-Si include a mature process and no scalability limit. The future requirements include quality needs to be high enough for 20 per cent efficient solar cells. Diffusion length should be >500 Î¼m @ cell thickness 150 Î¼m. Impurities should be of low activity/good behavior. There should be monocrystals, leading to easy texturing. Some other requirements include increase of yield/less low-quality areas, processes for umg-Si feedstock, and cost below 0.30/Wp.

Prof. Weber gave another example of a high quality block crystallized silicon material. There should be polarity switch in umg silicon. The umg Si is compensated: both B and P are present in the feedstock. The Dopant crossover is due to different segregation co-efficients. The consequence: p- and n-type Si within same brick and even single wafers. Dopant engineering is needed to avoid the pn switch/increase yield.

There is another example: non-conventional c-Si material or solar cells made from crystalline silicon thin-films. All concepts have good to very good cost perspective. Also, high-throughput, low-cost Si deposition/epi will be required for quick progress.

The key design data of non-conventional c-Si material (ProConCVD) was presented. The ProConCVD is a massively scaled version of the ConCVD, intended to prove the scalability of the approach to a near-production level of more than 1000 wafers/hour. The data includes:

* 3 tracks, each 2 carriers, each carrier 3 156x156 mm2 wafers in height.
* Total deposition area: 5 m2.
* Maximum transport speed: 12 m/h.
* Furnace: max. 360 kW, resistance heated, 2x8 m footprint, 2 m stable zone.
* Process temperature up to 1300Â°C.
* Available process gases (maximum consumption/min): SiHCl3 (300 g), SiCl4 (500 g), SiCl3(CH3) (300 g), H2 (4000 sl), HCl (50 sl).
* Throughput > 30 m2/h (~1200 wafers/h) for 20 Î¼m layer thickness. A simple scale-up is possible.

The current status of ProConCVD include: all hardware installations have been finished. The transport and heating system is in operation. The infrastructure is on-line. The first high-quality epitaxial layers have been done successfully.

Strategies to increase the efficiency of normal crystalline silicon solar cells include advanced metallisation, selective emitters, di-electric surface passivation, thinner wafers, process control, ultra light trapping, material quality and back-contact cells.

Estimating the efficiency potential on boron-doped Cz-Si, with a limitation due to metastable boron-oxygen defect, there is the optimized industrial cell structure (PERL). The efficiency is limited to about 20 per cent due to boron oxygen lifetime degradation. The solution: n-type silicon, with no degradation and higher tolerance to metal contamination.

The lab results of high-efficiency n-type PERL cells were also shared. There was substitution of local phosphorus diffusion by laser doping from innovative double-function PassDop layer (passivation and doping). Excellent results were achieved with evaporated front contacts.

Efficiencies of Ni/Cu/Sn metallization
Prof. Weber also talked about the efficiencies of Ni/Cu/Sn metallization. Solar cell properties include direct plating, lowly doped emitter (120 ohm/sq) and dielectric rear passivation. It also has excellent efficiencies and fill factors. As for the thin-film CIS solar cell structure, the key challenge is to realize the impressive small-area lab efficiency results in production-size modules and volume production.

He touched upon the benefits of multi-junction solar cells and high-efficiency ISE triple-junction solar cells obtained by MOCVD thin-film deposition. He listed advantages of high concentration PV. These include system efficiencies ~ 25 per cent AC today, ~200 MW/y worldwide production capacity, no cooling water or intentional hot water, modular kW to GW scale and a 12-month energy payback time.

He gave an example of DESERTEC, the vision of an electricity super grid. DESERTEC is a mega renewable energy project that aims to setup a massive network of solar and wind farms stretching across the Middle East and North Africa (MENA) region and connect to Europe via a Euro-Mediterranean electricity network, made up of high voltage direct current (HVDC) transmission cables. The project, estimated at Euro 400 billion will provide 15 per cent of Europe's electricity by 2050.

-- Pradeep Chakraborty

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