Photovoltaics (PVs) will be a key pillar of our future sustainable energy system, and 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 professor 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.

Crystalline silicon will remain the dominant PV technology. Classical thin-filmhas to show lower prices or comparable efficiencies. Highly-efficient concentrated photovoltaics (CPVs) will take up a rapidly increasing niche market, competing with concentrating solar power (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 his thoughts on ‘Photovoltaics: Pillar of our Future Sustainable Energy System,’ at the recently held Intersolar Europe 2012 in Munich, Germany.

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The technology challenge in PV will be to generate innovations in efficiency and cost reduction fast enough to maintain a profitmargin; 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 lead to higher prices and less pressure for cost reduction and innovation, ultimately hurting the economies that adopt them.

Bright future for PV
Globally, the required power is 16 TW today, and it will reach at least 30 TW by 2050. PV is offering at least 10 per cent of this requirement. Optimistically, it should meet 30-40 per cent of the global energy needs. 3TW power corresponds to 12 TW or 12,000 GW of PV capacity.

PV capacity globally installed till 2009 was 20 GW. It was +17 GW in 2010 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 hundreds 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 and onshore wind but less than nuclear and fossil fuels by 2030 or earlier!

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For a 100 per cent renewable energy scenario, at least three components or technologies are required in a ratio of 1:1:1. These are solar energy (PV and solar thermal), wind and hydro, and 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 annual electricity need of Germany is about 600 TWh. 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.

If you look at the global PV production development by technology, by 2011, thin filmhad accounted for 3204 MWp, ribbon-Si 120 MWp, multi-Si 10,336 MWp and mono-Si 9114 MWp, respectively.

Efficiencies in the solar cell market
Efficienciesin the solar cell market range from 1 to 5 per cent for organic, dye and nanostructure cells. PV technologies of interest in the next one to two decades are:
1. 6-11 per cent: Thin-film cells (a-Si, microcrystalline-Si, CIS, CIGS, CdTe)
2. 14-18 per cent: mc-Si, umg-Si, simple c-Si cells
3. 20-24 per cent: High-efficiencyc-Si cells
4. 36-41.1 per cent: High-efficiencyIII/V tandem cells for concentrators with 25-30 per cent module efficienc

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

Prof. Weber cited examples: Solar cells built with 100 per cent umg-Si have a conversion efficiencyof less than 20 per cent. Silicor plans a umg-Si plant with a capacity of 16,000 tpa at a cap-ex of $600 million. The median efficiencyat CaliSolar is now 16.6 per cent. At least 11 per cent of the cells have a conversion efficiencyof above 17 per cent with the highest at 17.7 per cent. Q-cells has achieved 18.2 per cent efficiencyusing umg-Si cell with backside contacts.

The advantages and future requirements for mc-Si include a mature process and no scalability limit. Quality needs to be high enough for 20 per cent efficien solar cells. Diffusion length should be greater than 500 μm at cell thickness of 150 μm. Impurities should exhibit low activity/good gettering behaviour. 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.

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Creating market barriers will lead to higher prices and less pressure for cost reduction and innovation, ultimately hurting the economies that adopt them

Prof. Weber gave another example of a high-quality block-crystallised silicon material. There should be polarity switch in umg silicon. The umg Si is compensated: both boron (B) and phosphorous (P) are present in the feedstock. The dopant crossover is due to different segregation coefficients.The consequence: p- and n-type Si within the same brick and even single wafers. Dopant engineering is needed to avoid the p-n switch or increase the 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. High-throughput, low-cost Si deposition will be required for quick progress.

The key design data of a 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 a thousand wafers per hour. The data includes:
1. Three tracks, each with two car-riers. Each carrier holds three 156×156 mm2 wafers in height
2. Total deposition area: 5 m2
3. Maximum transport speed: 12 m/h
4. Furnace: max. 360kW, resistance-heated, 2x8m2 footprint, 2m stable zone
5. Process temperature up to 1300°C
6. Available process gases (maximum consumption/min): SiHCl3 (300 gm), SiCl4 (500 gm), SiCl3(CH3) (300 gm), H2 (4000 sl), HCl (50 sl)
7. Throughput > 30 m2/h (equivalent to 1200 wafers per hour) for 20μm layer thickness. A simple scale-up is possible.

As per the current status of ProConCVD, all hardware installations have been completed. The transport and heating system is in operation. The infrastructure is online. The firsthigh-quality epitaxial layers have been done successfully. Strategies to increase the efficiency o normal crystalline silicon solar cells include advanced metallisation, selective emitters, dielectric surface passivation, thinner wafers, process control, ultra-light trapping, material quality and back-contact cells. Estimating the efficiencypotential on boron-doped Cz-Si, with a limitation due to metastable boron-oxygen defect, there is the optimised industrial cell structure (PERL). The efficiencyis 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.

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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.

Prof. Weber talked about the efficiencies of Ni/CuSn metallisation. Solar cell properties include direct plating, lowly doped emitter (120 ohms/square) and dielectric rear passivation. It also has excellent efficiecies and fillfactors. As for the thin-filmCIS solar cell structure, the key challenge is to realise the impressive small-area lab effciency results in production-size modules and volume production.

He touched upon the benefits of multi-junction sola cells and high-efficiencyISE triple-junction solar cells obtained by MOCVD thin-filmdeposition. Advantages of high-concentration PV cells include system effiiencies of 25 per cent AC today, about 200 MW/year worldwide production capacity, no cooling water or intentional hot water, modular kW to GW scale, and one-year energy payback time.

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

The author is an executive editor at EFY