Surface passivation of silicon solar cells using industrially relevant Al2O3 deposition techniques (original) (raw)
Related papers
Surface passivation of high-efficiency silicon solar cells by atomic-layer-deposited Al 2 O 3
Progress in Photovoltaics: Research and Applications, 2008
Atomic-layer-deposited aluminium oxide (Al 2 O 3 ) is applied as rear-surface-passivating dielectric layer to passivated emitter and rear cell (PERC)-type crystalline silicon (c-Si) solar cells. The excellent passivation of low-resistivity p-type silicon by the negative-charge-dielectric Al 2 O 3 is confirmed on the device level by an independently confirmed energy conversion efficiency of 20Á6%. The best results are obtained for a stack consisting of a 30 nm Al 2 O 3 film covered by a 200 nm plasma-enhanced-chemical-vapour-deposited silicon oxide (SiO x ) layer, resulting in a rear surface recombination velocity (SRV) of 70 cm/s. Comparable results are obtained for a 130 nm single-layer of Al 2 O 3 , resulting in a rear SRV of 90 cm/s.
Advances in the Surface Passivation of Silicon Solar Cells
Energy Procedia, 2012
The surface passivation properties of aluminium oxide (Al 2 O 3) on crystalline Si are compared with the traditional passivation system of silicon nitride (SiN x). It is shown that Al 2 O 3 has fundamental advantages over SiN x when applied to the rear of p-type silicon solar cells as well as to the p + emitter of n-type silicon solar cells. Special emphasis is paid to the transfer of Al 2 O 3 into industrial solar cell production. We compare different Al 2 O 3 deposition techniques suitable for mass production such as ultrafast spatial atomic layer deposition, inline plasma-enhanced chemical vapour deposition and reactive sputtering. Finally, we review the most recent cell results with Al 2 O 3 passivation and give a brief outlook on the future prospects of Al 2 O 3 in silicon solar cell production.
2008 33rd IEEE Photovolatic Specialists Conference, 2008
We present independently confirmed efficiencies above 20% for PERC-type solar cells with the pointcontacted rear being either passivated by atomic-layerdeposited Al2O3 or by stacks consisting of an ultrathin Al2O3 film and a thicker PECVD-SiOx layer. Internal quantum efficiency measurements reveal that the effective rear surface recombination velocities of the single-layer Al2O3passivated cells are comparable to those measured on reference cells passivated by an aluminum-annealed thermal SiO2, while those of the Al2O3/SiOx-passivated cells are even lower. Very low effective rear surface recombination velocities of only 70 cm/s are reported for the Al2O3/SiOx stacks, including metalized areas on the cell rear.
Sputtered Aluminum Oxide for Rear Side Passivation of P-Type Silicon Solar Cells
Aluminum oxide is an excellent candidate for the surface passivation of silicon wafers. Due the incorporation of a high density of negative charges near the interface surface and a low defect density a very good passivation can be achieved. Today, aluminum oxide layers are predominantly deposited by atomic layer deposition and plasma-enhanced chemical vapor deposition. Reactive sputtering is an alternative not requiring trimethylaluminum. Nevertheless, there are doubts concerning the passivation quality of sputtered aluminum oxide. In this contribution we analyse the influence of deposition parameters on the properties of the sputtered layers. Measurements of interface defects density and the density of fixed charges at the interface can explain a good passivation quality after firing. Additionally, results for LFC-PERC solar cells are presented showing a statistically significant improve in efficiency compared to standard BSF solar cells. This can be explained by a lower recombination rate and a higher reflectivity at the rear side of the solar cell.
Progress in Photovoltaics: Research and Applications, 2011
A next generation material for surface passivation of crystalline Si is Al 2 O 3 . It has been shown that both thermal and plasma-assisted (PA) atomic layer deposition (ALD) Al 2 O 3 provide an adequate level of surface passivation for both p-and n-type Si substrates. 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 and Al 2 O 3 deposition rates up to 1.2 nm/s have been demonstrated. In this work, the passivation quality and uniformity of the experimental spatially separated ALD Al 2 O 3 films are evaluated and compared to conventional temporal ALD Al 2 O 3 , by use of quasi-steady-state photo-conductance (QSSPC) and carrier density imaging (CDI). It is shown that spatially separated Al 2 O 3 films of increasing thickness provide an increasing surface passivation level. Moreover, on p-type CZ Si, 10 and 30 nm spatial ALD Al 2 O 3 layers can achieve the same level of surface passivation as equivalent temporal ALD Al 2 O 3 layers. In contrast, on n-type FZ Si, spatially separated ALD Al 2 O 3 samples generally do not reach the same optimal passivation quality as equivalent conventional temporal ALD Al 2 O 3 samples. Nevertheless, after ''firing'', 30 nm of spatially separated ALD Al 2 O 3 on 250 mm thick n-type (2.4 V 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 Al 2 O 3 .
PROGRESS IN THE SURFACE PASSIVATION OF SILICON SOLAR CELLS
In order to increase the efficiency of silicon-wafer-based solar cells in production well above 20%, it is indispensable to improve the currently applied level of surface passivation at the front as well as at the rear of the cells. This paper focuses on two main challenges: (i) the low-temperature passivation of lowly doped p-type silicon surfaces at the cell rear and (ii) the passivation of highly boron-doped p + emitter surfaces as used at the front of solar cells on high-lifetime n-type silicon wafers. In the past, low surface recombination velocities (< 20 cm/s) have been achieved on low-resistivity (~1 Ωcm) p-type silicon using plasma-enhanced chemical-vapour-deposited (PECVD) silicon nitride (SiN x ) as well as amorphous silicon (a-Si). However, the high density of fixed positive charges within the PECVD-SiN x layer induces an inversion layer at the rear of p-type Si cells, producing a detrimental parasitic shunting, which reduces the short-circuit current density by up to 3 mA/cm 2 . The passivation quality of a-Si on the other hand is very temperature sensitive. More recently it has been shown that atomic-layer-deposited (ALD) aluminium oxide (Al 2 O 3 ) provides an outstanding level of surface passivation, which can be attributed to its extremely high negative fixed charge density in combination with the very gentle deposition technique ALD, leading to low interface state densities. The application of these ALD-Al 2 O 3 layers to the rear of p-type solar cells shows that this new passivation scheme is indeed suitable for high efficiencies and that due to the large negative fixed charge density no parasitic shunting occurs. We also demonstrate that ALD-Al 2 O 3 seems to be the ideal passivation layer for borondoped p + emitter surfaces. In a direct comparison with other passivation schemes, it is found that Al 2 O 3 even outperforms optimized thermally grown SiO 2 and opens the possibility of achieving very large open-circuit voltages up to V oc = 740 mV.
Applied Physics Letters, 2013
High-quality surface passivation of crystalline Si is achieved using 10 nm thick Al2O3 films fabricated by thermal atomic layer deposition at 100 °C. After a 5 min post deposition annealing at 200 °C, the effective carrier lifetime is 1 ms, indicating a functional level of surface passivation. The interplay between the chemical and the field effect passivation is investigated monitoring the density of interface traps and the amount of fixed charges with conductance-voltage and capacitance-voltage techniques. The physical mechanisms underlying the surface passivation are described. The combination of low processing temperatures, thin layers, and good passivation properties facilitate a technology for low-temperature solar cells.