We propose a novel method for the extraction of recombination characteristics of metal contacts on semiconductor substrates. We demonstrate this method for saturation current density measurements of contacts on boron doped emitters. It is easily generalized for the characterization of metallized high-low junctions, or of metal layers in direct contact with the bulk semiconductor. Our method is based on effective lifetime measurements as a function of metal contact area fraction. Under high injection conditions, saturation current densities of metallized junctions are extracted from the slope of the saturation current density as a function of metal coverage. At arbitrary injection levels, the difference between the saturation current density at metallized junctions and the saturation current density at passivated junctions is extracted from the slope of inverse effective lifetime versus metal contact fraction. If bulk recombination is negligible compared to emitter recombination, saturation current densities at the Simetal and at the passivated silicon surface can be independently determined for arbitrary injection levels. Our method is applied to saturation current density extraction for Al contacts on diffused boron emitters. Also, saturation currents at the emitter-air interface are extracted and compared to saturation current densities at the emitter-metal interface.

In order to relax the mechanical constraints of processing thin crystalline Si wafers into highly efficient solar cells, we propose a process sequence, where a significant part of the process is done on module level. The device structure is an interdigitated-back-contact cell with an amorphous silicon back surface field. The record cell reaches an independently confirmed efficiency of 18.4%. Although the device deserves further optimization, the result shows the compatibility of processing on glass with efficiencies exceeding 18%, which opens the door to a high-efficiency solar cell process where the potentially thin wafer is attached to a foreign carrier during the full processing sequence.

Thermal atomic layer deposition (ALD) of Al2O3 provides an adequate level of surface passivation for both p-type and n-type Si solar cells. To obtain the most qualitative and uniform surface passivation advanced cleaning development is required. The studied pre-deposition treatments include an HF (Si-H) or oxidizing (Si-OH) last step and finish with simple hot-air drying or more sophisticated Marangoni drying. To examine the quality and uniformity of surface passivation - after cleaning and Al2O3 deposition - carrier density imaging (CDI) and quasi-steady-state photo-conductance (QSSPC) are applied. A hydrophilic surface clean that leads to improved surface passivation level is found. Si-H starting surfaces lead to equivalent passivation quality but worse passivation uniformity. The hydrophilic surface clean is preferred because it is thermodynamically stable, enables higher and more uniform ALD growth and consequently exhibits better surface passivation uniformity.

Foil creation by lifting off a thin layer of a high quality silicon substrate is one of the promising substitutes for wafer sawing to create substrates thinner than 100 μm. The porous silicon-based layer transfer process is a well known method to obtain high quality foils. Despite a number of convincing lab-based solar cell show-cases, there is no breakthrough of this technology at (semi)-industrial level, because of the poor yield of processing free standing foils. This paper presents a method to fabricate back contacted solar cells based on epitaxial foils avoiding processes on free-standing foils. First, a porous silicon layer is electrochemically etched, acting as a weak sacrificial layer to detach the foil that is epitaxially grown on top of the porous silicon layer. Characterization of the epitaxial foils shows a good crystalline quality and an effective lifetime around 100 μs. Those results give indications that the obtained foils are well suited for solar cell fabrication. Front-side processing is done while the epitaxial foil is still attached to its parent substrate. A good yield is obtained for epitaxial foils that underwent the front-side processing sequence consisting of wet chemical texturing, FSF formation, passivation and ARC deposition. Afterwards, the front-side of the foil is bonded to a glass carrier and the foil is detached from its parent substrate. Silicone adhesives are used for this permanent bond. The rear-side of the solar cell is processed while bonded to glass. Therefore, only low temperature processes (<;200°C) can be used. So far, the rear-side processing sequence was performed on Float-zone reference wafers as a proof of concept resulting in a confirmed maximum efficiency of 18.4%. The rear-side processing sequence still needs to be applied on epitaxial foils.

The potential of contact passivation for increasing cell performance is indicated by several results reported in the literature. However, scant characterization of the tunneling layers used for that purpose has been reported. In this paper, contact passivation is investigated by insertion of an ultra-thin AlOx layer between an n-type emitter and a Ti/Pd/Ag contact. By using a 1.5nm thick layer, an increase of the minority carrier lifetime by a factor of 2.7 is achieved. Since current-voltage measurements indicate that an ohmic behavior is conserved for AlOx layers as thick as 1.5nm, a 1.5nm AlOx layer is found to be a candidate of choice for contact passivation.

ABSTRACT Crystalline Si (c‐Si) technology is dominating the photovoltaics market. These modules are nonetheless still relatively expensive, in particular because of the costly silicon wafers, which require large thickness mostly to ease handling. Thin‐film technologies, on the other hand, use much less active material, exhibit a much lower production cost per unit area, but achieve an efficiency still limited on module level, which increases the total system costs. A meet‐in‐the‐middle is possible and is the object of this paper. The development of c‐Si thin‐foil modules is presented: first, the fabrication of the active material on a glass module and then the processing of the Si foils into solar cells, directly on module level. The activity of IMEC in this area is put into perspective with regard to worldwide research results. It appears that great opportunities are offered to this cell concept, although some challenges still need to be tackled before cost‐effective and reliable industrial production can be launched. Copyright © 2012 John Wiley & Sons, Ltd.

Firing impacts on surface passivation provided by a SiO2 and SiO2/SiNx stack with evaporated Al films are studied by capacitance-based techniques on MIS capacitors. For devices with insulator layers consisting solely of as-deposited SiO2, the densities of either interface states (D-it) or fixed charges (Q(fc)) are hardly influenced by firing. Capping the SiO2 layer with a SiNx layer results in a shift of the peak activation energy of D-it toward the valence band (E-v) of Si. Firing this SiO2/SiNx stack leads to an increase of Q(fc), a reduction of D-it, and a moderate shift of peak activation energy of D-it toward E-v. Co-firing with the Al film on top significantly reduces the Q(fc), D-it, and D-it peak activation energy, which is resulting from the atomic hydrogen passivation. These results are of particular interest for the development of solar cells with rear surface passivation and local contacts. (C) 2011 American Institute of Physics. [doi:10.1063/1.3669405]

Surface passivation of Si solar cells is typically achieved by deposition of a dielectric layer. Via the investigation of Al2O3 passivation layers, we show that care must be taken when performing capacitance–voltage (C –V) measurements in order to obtain results that are meaningful at solar cell level. The passivation properties of a dielectric are not only affected by post‐deposition treatments but also by the presence and the nature of a metal covering the dielectric. Consequently, this Letter emphasizes how important it is to perform C –V measurements on a device structure that resembles as closely as possible that of the finished solar cell, using the same metal, deposition technique and thermal budget. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)