Conference Paper

n-Type silicon - Enabling efficiencies > 20% in industrial production

DOI: 10.1109/PVSC.2010.5614203 Conference: Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE
Source: IEEE Xplore

ABSTRACT In the first part of this paper we estimate the efficiency potential of crystalline silicon solar cells on conventionally pulled p-type boron-doped Czochralski-grown silicon with typical oxygen concentrations. Taking into account an industrial high-efficiency cell structure featuring fine-line metallization, shallow and well-passivated emitter and a rear surface structure with dielectric passivation and local laser-fired point contacts, the maximum achievable efficiency is around 20%. The main limitation of such a cell is due to the rather low bulk lifetime after light-induced degradation. Even when avoiding the metastable boron-oxygen defect by using Gallium-doped or magnetic Cz-silicon, it has to be kept in mind that the detrimental impact of metal contaminations on p-type silicon is greater than on n-type silicon. A potential strategy to reduce this loss is the use of n-type silicon. Therefore, the second part of the paper discusses different architectures for solar cells on n-type silicon substrates and shows the latest results achieved at Fraunhofer ISE in this field.

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Available from: Stefan W. Glunz, Aug 17, 2015
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    • "Currently, the industrial crystalline silicon solar cell production is still dominated by p-type silicon solar cells. However, as the silicon PV industry tends to introduce advanced high-efficiency solar cell concepts, the quality of the base material is becoming more and more important [1]. n-type Cz silicon does not suffer from light induced degradation, which is known to occur for p-type Cz silicon due to boron-oxygen pairs [2] [3] [4]. "
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    ABSTRACT: Recently, we presented an industrially feasible passivation and contacting scheme for the front side boron emitter of n-type silicon solar cells based on firing processes. On these cells, efficiencies up to 20.8% have been achieved on small areas. These cells feature a fully-metalized BSF on the rear side, which limits the VOC to about 655mV. When changing to a PERT cell design with a passivated BSF, both the VOC as well as the JSC can be improved due to a reduced recombination at the rear and an improved optical confinement. In this work we studied different POCl3 diffusions for their applicability to n-type PERT solar cells with respect to passivation and metallization. The achieved results have been used to fabricate a first batch of n-type PERT solar cells, on which VOC values up to 671mV have been measured. The improved internal quantum efficiency above 900nm confirms the improvement of the rear side of the cell. The boron emitter of this cell was passivated with a stack of 5Å ALD Al2O3 (four ALD cycles) and 70nm PECVD SiNx. Thus the VOC of 671mV demonstrates furthermore, that the Al2O3 thickness of fired Al2O3/SiNx stacks for the passivation of boron emitters can be drastically reduced to four atomic layers of Al2O3.
    Energy Procedia 12/2011; 8:479-486. DOI:10.1016/j.egypro.2011.06.169
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    • "Although laboratory-scale cells (4 cm 2 ) with Al-alloyed rear emitters have reached 20.0% with amorphous Si passivation of the p + emitter surface [15] and 20.1% with Al 2 O 3 passivation [16], it is believed that the cell in Fig. 2 represents the highest efficiency achieved for production-worthy cells (239 cm 2 ), where the alloyed Al remains in contact with the p + emitter and where the front contacts are screen printed. Similar results for large-area cells have been reported recently with somewhat different processing conditions (plated contacts, planarized back) [17], [18]. Furthermore, detailed modeling of this cell structure suggests efficiencies in the neighborhood of 19.5% are possible with advanced metallization [19]. "
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    ABSTRACT: This paper describes ion-implanted, screen-printed, high efficiency, stable, n-base silicon cells fabricated from readily available 156 mm n-Cz wafers, along with prototype modules assembled from such cells. Two approaches are described. The first approach, which involves a single phosphorus implant, has been used to produce cells (239 cm2) having a tight distribution of Jsc, Voc, and FF over a wide range of wafer resistivity (factor of 10), with Fraunhofer-certified efficiencies up to 18.5%. In spite of the full screen-printed and alloyed Al back, a method has been developed for soldering such cells in a module. The second approach, which involves implanting both phosphorus for BSF and boron for front emitter, has been used to produce n-base cells having local back contacts and dielectric (SiNx/SiO2) surface passivation. Efficiencies up to 19.1%, certified by Fraunhofer, have been realized on 239 cm2 cells.
    IEEE Journal of Photovoltaics 10/2011; 1(2):003337-003337. DOI:10.1109/PVSC.2011.6186657 · 3.00 Impact Factor
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    • "In spite of this technological improvement, the solar cell efficiency will be still limited by the recombination mechanism inherent in boron-doped p-type material (oxygen-boron-complex traps). This limitation is estimated to be around 20 % [1]. "
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    ABSTRACT: Several aspects of the suitability of PECVD Al 2 O 3 deposition process for solar cell mass production are reported. Among them the throughput, the firing stability, the homogeneity and the repeatability of the process quality have been investigated. Additionally a carrier lifetime higher than 1 ms has been obtained after firing for a layer of only 10 nm combined with a SiNx capping layer. The latter allows an interesting compromise between a low consumption of expensive precursors and a high passivation quality. PECVD Al 2 O 3 has also been studied, in order to offer an industrially suitable solution for the passivation of boron-doped emitters of n-type solar cells. An emitter saturation current density as low as 36 fA cm -2 has been obtained on a 90 Ω/sq emitter with a surface concentration of 7 1019 cm-3, which correspondents to a surface recombination velocity of only 350 cm s -1 .
    Proceedings of the 26th European Photovoltaic Solar Energy Conference and ExhibitionProceedings of the 26th European Photovoltaic Solar Energy Conference and Exhibition; 01/2011
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