High Speed Atmospheric Pressure ALD For Industrial Scale Solar Cell Passivation
B. Vermang, A. Rothschild, A. Racz, J. John, J. Poortmans, R. Mertens, P. Poodt, V. Tiba and F. Roozeboom
A next generation material for surface passivation is atomic layer deposition (ALD) Al2O3. However, conventional time-resolved ALD is limited by its low deposition rate. Therefore, an experimental high-deposition-rate prototype ALD reactor based on the spatially-separated ALD principle has been developed. This reactor leads to deposition rates up to 1.2 nm Al2O3/s. In this work, the passivation quality and uniformity of the experimental spatially-separated ALD Al2O3 films are evaluated and compared to conventional temporal ALD Al2O3, by use of quasi-steady-state photo-conductance (QSSPC) and carrier density imaging (CDI). It is shown that spatially-separated Al2O3 films of increasing thickness provide an increasing surface passivation level.
Moreover, on p-type CZ Si, 10 and 30 nm spatial ALD Al2O3 layers can achieve the same level of surface passivation as equivalent temporal ALD Al2O3 layers. In contrast, on n-type FZ Si, spatially-separated ALD Al2O3 samples generally do not reach the same optimal passivation quality as equivalent conventional temporal ALD Al2O3 samples. Nevertheless, after “firing”, 30 nm of spatially-separated ALD Al2O3 on 250 µm thick n-type (1-5 O.cm) FZ Si wafers can lead to effective surface recombination velocities as low as 2.9 cm/s, compared to 1.9 cm/s in the case of 30 nm of temporal ALD Al2O3.
Proceeding Imec PVSEC 2010 2AO115860 Downloads
Industrially Relevant Al2O3 Deposition Techniques For The Surface Passivation Solar Cells
Jan Schmidt, Florian Werner, Boris Veith, Dimitri Zielke, Robert Bock, Veronica Tiba, Paul Poodt, Fred Roozeboom, Andrew Li, Andres Cuevas and Rolf Brendel
We present independently confirmed efficiencies of 21.4% for PERC cells with plasma-assisted atom-ic-layer-deposited (plasma ALD) Al2O3 rear passivation and 20.7% for cells with thermal ALD-Al2O3. Additionally, we evaluate three different industrially relevant techniques for the deposition of surface-passivating Al2O3 layers on 1-cm p-type silicon wafers, namely high-rate spatial ALD (spatial ALD), plasma-enhanced chemical vapour deposi-tion (PECVD) and reactive sputtering. Using spatial ALD and PECVD, surface recombination velocities (SRVs) be-low 10 cm/s are obtained. Sputtered Al2O3 layers still provide an SRV of 35 – 70 cm/s. Despite their lower passiva-tion quality compared to the Al2O3 films deposited by spatial ALD and by PECVD, we demonstrate that the sputtered Al2O3 layers are still suitable for the fabrication of 20.1% efficient PERC cells. After firing at ~800°C in a conveyor-belt furnace the SRV provided by the Al2O3 films deposited by spatial ALD is still below 20 cm/s, indicating an ex-cellent firing stability. Both PECVD and sputtered Al2O3 passivation layers degrade to SRVs larger than 100 cm/s af-ter firing. Hence, the firing stability of PECVD and – in particular – sputtered Al2O3 needs further optimisation.
Proceeding ISFH PVSEC 2010 2AO.1.64551 Downloads
High-Speed Spatial Atomic-Layer Deposition of Aluminum Oxide Layers for Solar Cell Passivation
By Paul Poodt, Adriaan Lankhorst, Fred Roozeboom, Karel Spee, Diederik Maas and Ad Vermeer
Atomic-layer deposition (ALD) is a deposition technique capable of producing very thin conformal films with control of the thickness and composition of the films at the atomic level.
In this Communication we show that with spatially separated ALD of Al2O3 , growth rates of
1.2 nm/s can be achieved, showing excellent surface passivation (surface recombination velocities of < 2 cm/s ).
This implies a revolutionary breakthrough in industrial throughput ALD of Al2O3 passivation of silicon solar cells.
First published on www.materialsviews.com