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Impact of the rear surface roughness on Industrial-Type PERC Solar Cells

C.Kranz1, S. Wyczanowski1, S. Dorn1, K. Weise3, C. Klein3, K. Bothe1, T. Dullweber1, R. Brendel1,2 1 Institute for Solar Energy Research Hamelin (ISFH), Am Ohrberg 1, D-31860 Emmerthal, Germany 2 Department of Solar Energy, Institute of Solid-State Physics,Leibniz University of Hanover, Appelstrasse 2, D-30167 Hanover, Germany 3 RENA GmbH, Hans-Bunte-Strasse 19, D-79108 Freiburg, Germany

ABSTRACT: Screen-Printed PERC solar cells are a promising candidate for next-generation industrial-type solar cells. Industrial PERC cell process flows typically involve wet chemical polishing of the rear side in order to reduce the surface roughness and to improve the conversion efficiency. In this paper, we show that Al2O3/SiNx passivation stacks achieve an excellent surface recombination velocity below 10 cm/s even for small polishing removal of 5 μm corresponding to rough surfaces. In contrast, SiOx/SiNy passivation stacks exhibit a strong increase of the surface recombination velocity up to 30 cm/s for rough surfaces. Accordingly, we find that the efficiency of PERC solar cells with Al2O3/SiNx rear passivation is almost independent of the polishing etch depth with best efficiencies up to 19.7% for 7.5 μm polishing removal whereas the efficiency of SiOx/SiNy rear passivated PERC cells strongly decreases for rougher surfaces.

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Impact of the rear surface roughness on Industrial-Type PERC Solar Cells

How much rear side polishing is required?

E. Cornagliotti 1, A Uruena1, J. Horzel1, J. John1, L. Tous1, D. Hendrickx1, V. Prajapati1, S. Singh1, R. Hoyer2, F. Delahaye 2, K. Weise2, D. Queisser2, H. Nussbaumer2 and J. Poortmans1
1 IMEC vzw., Kapeldreef 75, 3001 Leuven, Belgium
2 RENA GmbH, Hohenweg 1, 78148 Gütenbach, Germany

In this work we study the impact of chemical polishing on the performance of mono-crystalline PERC cells. Starting from random pyramid textured surfaces, the rear side polishing step is performed by means of an inline single-side wet-etch tool. The influence of the pyramid smoothening on light trapping, surface recombination and contact formation is here studied separately. We conclude from our analysis that a moderate smoothening, corresponding to around 5-6 μm Si etch, is required in order to maximize the cell performance and that, on the other hand, a complete planarization of the rear surface can be detrimental, besides being cost un-effective. While the surface recombination generally benefits from a flat rear surface, light absorption and contact formation are more effective when roughness Ra is in the range between 300-500 nm. Contact formation is also influenced by the roughness because mass exchange between Al and Si takes place all over the rear side surface.

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How much rear side polishing is required?

Thermal stability of spatial ALD deposited Al2O3 capped by PECVD SiNx

Richter, Schuldis, Benick, Hermle, Glunz; Fraunhofer ISE, Souren, Gortzen; SoLayTec

Al2O3 deposited by atomic layer deposition (ALD) is known to provide an excellent passivation on lowly- and highly-doped p-type silicon surfaces even after contact firing processes. Therefore, Al2O3 layers are of great interest for the passivation of the rear side of p-type solar cells as well as the boron-doped emitter of n-type solar cells. In this work we studied the passivation quality of fired Al2O3/SiNx layer stacks on lowly- and highly-doped p‑type silicon surfaces applying an in-line capable, spatial ALD tool for the Al2O3
deposition. On both surfaces we observed a very high level of surface passivation for stacks with 4 nm Al2O3 after firing. For lowly-doped p-type wafers, we measured effective lifetimes up to ~2.8 ms, which correspond to an upper limit for the surface recombination velocity of ~3 cm/s.

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Thermal stability of spatial ALD deposited Al2O3 capped by PECVD SiNx

Ultrafast ALD compared to other technologies

Roger Görtzen, SoLayTec
Dillenburgstraat 9G, 5652 AM Eindhoven, The Netherlands

What will be the standard for Al2O3? At this moment there are a few options for deposition of Al2O3. Spatial ALD, Batch ALD and, of course, PECVD, currently the most common. This article cannot go into too much detail about every solution but the obvious question to ask is “what will be the standard for Al2O3 for the next three years?”
In summary, there is still a lot of investigation needed to get this into mass production and this is why SoLayTec has a lab-to-fab solution available for Al2O3. Let’s plan another publication in 2015, to find out which technology has become the new standard for Al2O3.

Source: Power & Energy Solutions PES

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Ultrafast ALD compared to other technologies

High Throughput, Low Cost Deposition Of Alumina

Vermeer, Ad; SoLayTec, TechnologyRoozeboom, Fred; TNO, Science and Industry; Technical UniversityEindhoven, Applied PhysicsPoodt, Paul; TNO, thin film technologiesGortzen, Roger; SoLayTec, Marketing and Sales

Atomic Layer Deposition (ALD) is a gas phase deposition technique for depositing veryhigh quality thin films with an unsurpassed conformality. The main drawback of ALD howeveris the very low deposition rate (~ 1 nm/min). Recently, record deposition rates for alumina of upto 1 nm/s were reached using spatial ALD, while maintaining the typical assets regarding filmquality as obtained by conventional, slow ALD [1]. This allows for ALD at high throughputnumbers.One interesting application is passivation of crystalline silicon solar cells. Applying a thinalumina layer is reported to increase solar cell efficiency and enables the use of thinner wafers,thus reducing the main cost factor [2]. In this paper we report on the latest progress made bySoLayTec that delivered a working prototype of a system realizing full area single sideddeposition of alumina on 156 x 156 mm2, mono- and multi crystalline silicon wafers for solarcell applications. The alumina layers showed excellent passivation. Based on this concept, ahigh-throughput ALD deposition tool is being developed targeting throughput numbers of up to3000 wafers/hr, making ALD ready for mass production. This will bring on new opportunities inother applications.

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High Throughput, Low Cost Deposition Of Alumina

High-rate atomic layer deposition of Al2O3 for the surface passivation of Si solar cells

Florian Werner, Walter Stals, Roger Görtzen, Boris Veith, Rolf Brendel, Jan Schmidt, Institute for Solar Energy Research Hamelin (ISFH), Am Ohrberg 1, 31860 Emmerthal, Germany SoLayTec, Dillenburgstraat 9G, 5652 AM Eindhoven, The Netherlands

High-rate spatial atomic layer deposition (ALD) enables an industrially relevant deposition of high-quality aluminum oxide (Al2O3) films for the surface passivation of silicon solar cells. We demonstrate a homogeneous surface passivation at a deposition rate of ~ 30 nm/min on 15.6×15.6 cm2 silicon wafers of 10 nm thick Al2O3 layers deposited in a novel inline spatial ALD system. The effective surface recombination velocity on n-type Czochralski-grown (Cz) silicon wafers is shown to be virtually independent of injection level. Surface recombination velocities below 2.9 cm/s and an extremely low interface state density below 8×1010 eV−1cm−2 are achieved. We demonstrate that the novel inline spatial ALD system provides the means to integrate Al2O3 passivation layers into industrial solar cells.

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High-rate atomic layer deposition of Al2O3 for the surface passivation of Si solar cells

Dutch: Pieken in de Delta: Een snelle machine voor productie zonnecellen

Ministerie van Economische Zaken, Landbouw en Innovatie
Pieken in de Delta Zuidoost-NederlandHigh Tech Systemen en Materialen

In de wereld van zonne-energie is elke rendementsverbeteringeen kwestie van jarenlang noeste arbeid in het laboratorium.Daarom was het goed nieuws dat de Technische UniversiteitEindhoven een methode voor rendementsverbetering heeftgevonden, door een zeer dunne laag aluminiumoxide op deachterzijde van een zonnecel aan te brengen. De bestaandetechniek om de laag er op te zetten, is echter te tijdrovend vooreen industriële toepassing. Daarom is het PiD-project FAST ALDopgestart, waarin een machine wordt ontwikkeld die de snelheidvan het aanbrengen van de laag met een factor honderd verhoogt.

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Dutch: Pieken in de Delta: Een snelle machine voor productie zonnecellen

Surface passivation of si solar cells using industrially relevant deposition techniques

Jan Schmidt, Florian Werner, Boris Veith, Dimitri Zielke, Robert Bock & Rolf Brendel, Institute for Solar EnergyResearch Hamelin (ISFH), Emmerthal, Germany; Veronica Tiba, SoLayTec, Eindhoven, The Netherlands; Paul Poodt &Fred Roozeboom, TNO Science & Industry, Eindhoven, The Netherlands; Andrew Li & Andres Cuevas, The AustralianNational University (ANU), Canberra, Australia.

The next generation of industrial silicon solar cells aims at efficiencies of 20% and above. To achieve this goal usingever-thinner silicon wafers, a highly effective surface passivation of the cell front and rear is required. In the past,finding a suitable dielectric layer providing a high-quality rear passivation has been a major challenge. Aluminium oxide(Al2O3) grown by atomic layer deposition (ALD) has only recently turned out to be a nearly perfect candidate for sucha dielectric. However, conventional ALD is limited to deposition rates well below 2nm/min, which is incompatiblewith industrial solar cell production. This paper assesses the passivation quality provided by three different industriallyrelevant techniques for the deposition of Al2O3 layers, namely high-rate spatial ALD, plasma-enhanced chemicalvapour deposition (PECVD) and reactive sputtering.

Photovoltaics International 10

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Surface passivation of si solar cells using industrially relevant deposition techniques

Good things come to those who wait.... Fast ALD Al2O3

Johannes Bernreuter

The growing price pressure in the industry forces crystalline solar cell manufacturers to reach high efficiency levels in production rapidly. The photovoltaic conference in Valencia revealed that some approches which have already been under discussion for years are now increasing in popularity.

Source: Sun & Wind Energy, 11/2010

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Good things come to those who wait.... Fast ALD Al2O3

Very low surface recombination velocities on p- and n-type c-Si by ultrafast spatial ALD

Florian Werner, Boris Veith, Veronica Tiba, Paul Poodt, Fred Roozeboom, Rolf Brendel and Jan Schmidt

Using aluminum oxide Al2O3 films deposited by high-rate spatial atomic layer deposition ALD, we achieve very low surface recombination velocities of 6.5 cm/s on p-type and 8.1 cm/s on n-type crystalline silicon wafers. Using spatially separated reaction zones instead of the conventional time-sequenced precursor dosing enables growth rates up to 70 nm/min, whereas conventional ALD limits the growth rate to <2 nm/min.
The excellent passivation level is predominantly assigned to a high negative fixed charge density of Qf=-(4±1) * 1012 cm-2 in the Al2O3 films.
We demonstrate an excellent thermal stability of the passivation quality.

© 2010 American Institute of Physics: Applied Physycs Letters 97, published online 20 October 2010

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Very low surface recombination velocities on p- and n-type c-Si by ultrafast spatial ALD
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