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33rd European Photovoltaic Solar Energy Conference and Exhibition; 740-744, The i2-module concept is one of the approaches for processing very thin (< 50 μm) silicon foils into highly-efficient back-contacted devices. In this work, we process kerfless epitaxial foils, that are lifted-off and layertransferred onto glass, into heterojunction interdigitated back-contacted (HJ-IBC) devices. Ethylene vinyl acetate (EVA) is used as the bonding agent. As reference, thick and thin float zone wafers are used. With a carefully-selected and optimized process sequence that is compatible with EVA-bonded silicon, we obtain extraordinary VOC values of 738 mV and 751 mV for the thick and thin FZ cells, respectively, which demonstrates the high quality rear-side passivation achieved with our process. However, epifoil cells have a much lower VOC compared to the FZ cells and this is attributed to a lower bulk lifetime (30 μs). The best efficiency for an epifoil cell is 16.1% and that for a thick FZ cell bonded using EVA is 21.1%. Improving the epifoil lifetime to a level comparable to that of FZ material could increase the epifoil efficiency to above 20%.

33rd European Photovoltaic Solar Energy Conference and Exhibition; 765-767, We demonstrate solar cell fabrication on large area (125mm 125mm) epitaxial wafers of 50μm thickness. During handling and transport no mechanical support was used for the wafers, and the process was identical to the process that was used for 50μm and 160μm Czochralski wafers. Due to the wafer preparation procedure, thin epitaxial wafers were found to be mechanically weaker than Czochralski wafers. This resulted in wafer breakage especially during wet processing. The efficiency of epitaxial cells reached 17%, which was lower than the 19% and 20% efficiencies obtained with 50μm and 160μm Czochralski wafers. The cell results were thus found to be a good metric for substrate quality.

33rd European Photovoltaic Solar Energy Conference and Exhibition; 431-436, As PV production ramps up and numerous installed solar panels reach their end of life, potentially huge waste sources of silicon become available. Such silicon can be recovered and recycled back into the PV industry, for making solar cells directly on the recycled Si. In cases where the recycled Si is of very low purity and cannot be used directly as active material, they can instead be used to make low-cost Si supporting carriers that are highly-conductive. These supporting substrates have been made using low-cost techniques such as by sintering of low-cost Si powders. These conductive substrates can act as mechanical carriers of thin silicon (< 50 μm) and as rearcontacts in what is called a wafer-equivalent concept. In this work, thin epitaxial silicon foils from the porous siliconbased lift-off process are used. For conductive bonding, low-temperature Ag paste from INKRON (IPC-114) has been preferably used due to its robust bonding properties (low temperature curing, high conductivity and minimal volume change). As a first demonstrative step, thin (

32nd European Photovoltaic Solar Energy Conference and Exhibition; 328-330, In imec’s i2-module concept, silicon heterojunction interdigitated back-contacted (SHJ-IBC) solar cells are fabricated on monocrystalline foils bonded to glass. The proposed technology allows for cell processing on thin wafers mechanically supported by the glass, increasing the yield of processing such thin wafers. A process sequence for SHJ-IBC cell fabrication that can be applied to bonded thin foils is described. We investigated and optimized individual process steps on thick wafers. Then the developed steps were integrated into a process flow to fabricate solar cells on wafers with different thicknesses and bonding agents. On wafers with a thickness of 190 μm, functional cells with efficiencies of 22.6% and 21.7% were made on freestanding and silicone bonded wafers, respectively. On thin wafers of 57 μm, our best SHJ-IBC cell on an EVA bonded wafer exhibits excellent Voc of 740 mV and efficiency of 20.0%, which demonstrates the high potential of the i2-module concept.

32nd European Photovoltaic Solar Energy Conference and Exhibition; 313-316, Layer transfer process based on a porous silicon template is one of the promising replacements for the conventional wafer sawing technologies. For the process to be cost-effective, one challenge is the multiple recycling of the Si substrate used as a template for epitaxial growth. The Si substrate after detachment has different types of defects. Therefore, reusing the substrate without surface reconditioning induces low detachment yields and generates low quality epitaxial Si-foils. The current work presents the first steps taken into the realization of successful substrate re-use. Firstly, possible defects and undesirable features on the surface after the detachment process are identified. Secondly, a wet-chemical reconditioning process is developed. Finally, the effectiveness of this reconditioning process is evaluated by monitoring the evolution of the foil quality through different generations. Until now, 5 generations of epitaxial foils have been demonstrated at imec.

Electrical measurements have shown a direct impact of reflection of acoustic waves back into a transducer. Impedance measurements illustrate in specific cases the existence of multiple resonance peaks when reflected acoustic waves are present. Current and voltage measurements have confirmed this result. From these results, one can already conclude that acoustic reflections have a large impact on the operation of a transducer. Furthermore, it is shown that for megasonic cleaning tools with a face-to-face configuration of transducer and wafer, a precise control over the distance (control over the reflections) between the transducer and wafer is very important. Particle Removal Efficiency (PRE) measurements immediately show a major dependence on the position of the wafer. The PRE dependence is directly linked to the forward power consumed by the transducer, which is largely influenced by the position of the wafer or, in other words, by the reflection of acoustic waves.

31st European Photovoltaic Solar Energy Conference and Exhibition; 365-368, We describe the process for fabricating interdigitated back-contacted cells compatible with thin (40μm) monocrystalline silicon foils. Throughout the process, the foils are supported by either the silicon parent substrate, or by a host (glass) superstrate using adhesive bonding. For a number of process steps, we describe possible issues, boundary conditions and performance. Using the integrated process which is fully compatible with bonded foils, cells were fabricated on a freestanding, non-textured wafer. An efficiency of 18.0% was achieved, the main limitations being the absence of surface texturing and a non-optimized amorphous silicon emitter.

29th European Photovoltaic Solar Energy Conference and Exhibition; 1967-1970, ITRPV roadmaps predict that the thickness of wafers will continuously decrease. For thicknesses below 100 μm, alternatives for wafer sawing need to be developed. IMEC proposes a foil creation technique by lifting off a thin epitaxially grown layer as a substitute for wafer sawing to create substrates with a thickness around 40 μm. To enable integration of those foils into solar cells, with sufficiently high yield, an ‘i2-module’ concept is proposed. This is an interdigitated back-contact solar module process that can be applied on very thin foils, and where those foils are supported by either the parent substrate or the module glass at all times. To define a process flow adapted to the requirements and limitations of the epitaxial foils, the stability of the foils is tested during a wide range of front-side processes. It is found that epitaxial foils are not sufficiently stable during high temperature processes. Therefore, a process sequence based on low-temperature processes is defined for front-side processing. The yield of each step in the front-side process flow is evaluated. The constrains for front-side processing, together with the constrains defined by the silicone bond, lead to the choice of a back-contact heterojunction (HJ) cell with a PECVD a-Si:H emitter.

29th European Photovoltaic Solar Energy Conference and Exhibition; 1024-1027, In this paper, the electrical and optical characteristics of the Indium-Doped Tin Oxide (ITO) are tuned in order to design an ITO-based spacer between the a-Si:H and the metal contact suited for Amorphous / Crystalline Heterojunction interdigitated Back Contact Solar Cells (a-Si:H / c-Si HJ i-BC). ITO carrier concentration, mobility, resistivity, extinction coefficient, and work function, as well as the ITO / a-Si:H specific contact resistances are evaluated as a function of O2 relative flow rate during ITO deposition. An ITO having carrier concentration of 4.5x1019 cm-3, mobility of 20.2 cm2/(V.s) and resistivity of 6.24 mΩ.cm is found to be an optimum leading to ITO / a-Si:H(p/i) and ITO / a-Si:H(n/i) specific contact resistances values of 0.6 Ω.cm2 and 0.1 Ω.cm2 respectively and a RS reflectance of 58% at 1200nm when integrated in Silver / ITO / a-Si:H / c-Si(textured) test structures.

28th European Photovoltaic Solar Energy Conference and Exhibition; 1351-1354, In imec's i2Module concept, rear-contacted solar cells are fabricated after bonding monocrystalline silicon foils to module glass. The specific structure of the substrate after bonding imposes a temperature budget limitation of 300°C. This presents a challenge to device integration, amongst others for emitter passivation. It is shown that state-of-the-art emitter passivation by thermal oxide can be degraded by plasma processes and metal sputtering. Hydrogenated amorphous silicon presents an alternative that is compatible with low process temperatures and is not affected by subsequent processing. J0 values of approximately 50fA/cm2 have been obtained, both on freestanding wafers and on bonded thin silicon 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.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 1118-1120, Thermal atomic layer deposit ion (ALD) of Al2O3 provides an adequate level of surface passivat ion for both p-type and n-type Si solar cells. To obtain the most qualitat ive and uniform surface passivat ion adv anced cleaning development is required. The studied pre-deposit ion t reatments include an HF (Si-H) or oxidizing (Si-OH) last step and finish with simple hot -air drying or more sophist icated Marangoni drying. To examine the quality and uniformity of surface passivat ion - after cleaning and Al2O3 deposit ion - carrier density imaging (CDI) and quasisteady- state photo-conductance (QSSPC) are applied. A hydrophilic surface clean that leads to improved surface passivat ion level is found. Si-H start ing surfaces lead to equivalent passivat ion quality but worse passivat ion uniformity. The hydrophilic surface clean is preferred because it is thermodynamically stable, enables higher and more uniform ALD growth and consequent ly exhibits bet ter surface passivat ion uniformity.