L.J. Geerligs

Nitric acid pretreatment for the passivation of boron emitters for n-type base silicon solar cells

We have developed a simple method to passivate industrially produced boron-doped emitters for n-type base silicon solar cells using an ultrathin (∼1.5nm) silicon dioxide layer between the silicon emitter and the silicon nitride antireflection coating film. This ultrathin oxide is grown at room temperature by soaking the silicon wafers in a solution of nitric acid prior to the deposition of the silicon nitride antireflection coating film. The n-type solar cells processed in such a way demonstrate a conversion efficiency enhancement of more than 2% absolute over the solar cells passivated without the silicon dioxide layer.

Development towards 20% efficient Si MWT solar cells for low-cost industrial production

Abstract Low “Euros per Watt-peak” and ease of industrialization are the main drivers towards successful introduction on the market. In this regard, back-contact solar cells on n-type silicon offer significant benefits. The efficiency of back contact cells, such as Metal Wrap Through (MWT) cells, compared to the traditional H-pattern cells is higher at cell level, thanks to the reduced shading losses, and is higher at module level, thanks to the reduced interconnection resistance losses. N-type silicon benefits from improved electrical properties of n-type silicon compared to p-type (higher minority carrier diffusion lengths, lower sensitivity to many impurities). Furthermore, the availability of an industrial cell process designed by ECN, resulting in bifacial cells (good rear surface passivation and light trapping), makes n-type silicon a perfect candidate for high efficiency solar cells and requires only modest changes to the current wafer and cell production processes. In order to reduce processing costs and increase module efficiencies, we have started two years ago the development of the Metal-Wrap-Through (MWT) solar cell technology on n-type mono-crystalline silicon wafers. Within the last year, efficiency of our MWT silicon solar cells manufactured from ntype Cz silicon wafers has been improved by 1% absolute. Based on common industrial cell processing steps such as diffusion, screen-printing metallization and firing through, we have obtained efficiencies up to 19.70% (in-house measurements) on large area wafers (239xa0cm2, 5xa0Ωcm), with clear potential for further improvement. In this article, we present a first direct comparison experiment between n-type bifacial MWT and “conventional” n-type bifacial Hpattern technologies, in which an efficiency gain of 0.30% absolute for MWT is demonstrated. At the moment, series resistance and, as a result, fill factor are still sub-optimal. Nevertheless, with current density (Jsc) values approaching 40xa0mA/cm2 and open circuit voltage of 644xa0mV, n-type MWT solar cells already outperform n-type H-pattern cells manufactured with a comparable process.

High Efficiency n-Type Metal-Wrap-Through Cells and Modules Using Industrial Processes

We report on our high efficiency n-type metal-wrap-through (MWT) cell and module technology. In this work, bifacial n-type MWT cells are produced by industrial processes in industrial full-scale and pilot-scale process equipment. N-type cells benefit from high recombination lifetime in the wafer and bifaciality. Also low-cost screen printed cells can yield over 20% efficiency. When combined with MWT technology, high-power back-contact modules result, which can employ very thin cells. We report a cell conversion efficiency of 20.5% (in-house measurement, certification pending), a significant gain compared to our earlier work. We will discuss performance of thin cells relative to thicker cells, comparing experimental results to modeling. Recently, two aspects of (mainly p-type) MWT technology have received increased attention: paste consumption and performance under reverse bias. We will discuss MWT paste consumption, showing how MWT technology, like multi-busbar technology, can support very low paste consumption. We also report on behavior of cells and modules under reverse bias. We also discuss the robustness of MWT technology to dissipation in hot spots under reverse bias. Finally, full-size modules have been made and cell-to-module ratios of the different I-V parameters were analysed. Modules from cells with average efficiency over 20% are pending. This work shows that low-cost n-type bifacial cells are suitable for industrial high efficiency back-contact technology.

Excellent rear side passivation on multi-crystalline silicon solar cells with 20 nm uncapped Al 2 O 3 layer: Industrialization of ALD for solar cell applications

Current bottlenecks for industrialization of Al2O3 deposited by Atomic Layer Deposition (ALD) for crystalline silicon solar cell applications are low growth rate and stability of thin and uncapped layers during co-firing. First results on the performance of a high throughput ALD proto-type, the Levitrack, are presented. Excellent passivation properties have been obtained after firing, for 12 nm thick films deposited on p-Cz (2.3 Ω.cm) with Seff <15cm/s (Δn=3×1015 cm−3). These layers are compatible with solar cells that operate at a maximum open-circuit voltage of 720mV. Furthermore, we report on the passivation of 20nm uncapped aluminum oxide layers on the rear of p-type mc-Si bifacial cells. LBIC measurements unveiled excellent passivation properties on areas covered by 20nm of Al2O3 characterized by an IQE of 91% at 980nm. Remarkably, these lifetime and cell results were obtained without lengthy post-treatments like forming gas anneal.

ECN n-type silicon solar cell technology: An industrial process that yields 18.5%

There is currently much interest in n-type solar cells because of the advantages of this material. N-type material is expected to be more favourable for obtaining high efficiencies than p-type doped substrates. We have developed a process for n-type solar cells for large area multicrystalline and monocrystalline silicon wafers. The production process is based on industrial processing steps such as screen-printed metallization and firing through. The surfaces of these cells are passivated with a layer stack consisting of SiO 2 and SiN x where the former is created by a wet chemical process and the latter by inline PECVD. We demonstrate that the surface passivation can be improved with an alternative wet chemical process for creating the SiO 2 layer. This new process results in an enhancement of the implied V oc of unmetallized cells as measured by quasi-steady-state photoconductance (QSSPC) and the V oc of completed cells. The process Improvements have yielded a new record efficiency of 18.5 % (for a particular rear reflection surface) that was independently confirmed by Fraunhofer ISE CalLab.

Remote linear radio frequency PECVD deposited high quality a-Si:H(P) layers and their application in SI heterojunction structures

In this paper, we report on deposition and properties of high quality boron doped p-type amorphous Si (a-Si:H(p)) layers on n-type float zone Si(100) wafers by remote linear radio frequency plasma-enhanced CVD. The a-Si:H(p) layers show excellent surface passivation that is comparable to the one of a-Si:H intrinsic layers (a-Si:H(i)), and high stability of the passivation when stored in the dark . Additionally, the measured dark conductivity of deposited a-Si(p) is increased up to ≫7×10−6 S/cm by annealing. In a Si heterojunction cell structure, the a-Si:H(p) layer will be the emitter on an n-type base wafer. The effective lifetime of test structures of a-Si(p)/c-Si(n)/a-Si(n) has approached 1 ms, and a high pseudo fill factor and open circuit voltage have been obtained from a SunsVoc measurement. We conclude that these a-Si:H(p) layers are very promising for the application in high performance silicon heterojunction solar cells without using an intermediate a-Si:H(i) layer.