Stefano Nicola Granata

18% Efficiency IBC Cell With Rear-Surface Processed on Quartz

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.

High-quality epitaxial foils, obtained by a layer transfer process, for integration in back-contacted solar cells processed on glass

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.

Heterojunction Interdigitated Back-Contact Solar Cells Fabricated on Wafer Bonded to Glass

Future wafer-based silicon solar cells will be fabricated on thin (<;140 μm) wafers. However, technologies to handle thin wafers during cell processing are not yet available for industry. In this paper, a flow to handle thin wafers during rear side cell processing is developed and demonstrated on 4-in 200 μm-thick wafers. The flow involves bonding the wafers to glass after front-side processing followed by a low-temperature p-n heterojunction formation on the rear side. 2.5 × 2.5 cm2 amorphous/crystalline silicon heterojunction interdigitated back-contact solar cells are fabricated by use of lithography while bonded to glass, and they show an efficiency of up to 17.7%. Shunts, infrared light absorption, and rear side interface passivation are identified as the main efficiency losses. Dedicated experiments suggest that the passivation losses are related to the degradation of the adhesive during wafer cleaning. Hence, methods to improve the compatibility of the adhesive with the cleaning process are discussed.

Improved Surface Cleaning by In Situ Hydrogen Plasma for Amorphous/Crystalline Silicon Heterojunction Solar Cells

In future, thin wafers (< 100µm) will be employed in silicon heterojunction solar cell to decrease modules cost-per-Watt-Peak. However, in order to maintain excellent cell efficiency a higher device surface/volume ratio will demand stricter requirements on surface passivation. In this frame, the status of the crystalline surface (c-Si) prior to amorphous silicon (a-Si:H(i)) plasma deposition (PECVD) plays an important role: the c-Si chemical termination influences the quality of the interface layer a-Si:H(i)/c-Si, and affect the open circuit voltage (Voc). Previous studies have shown that smooth and fully hydrogenated c-Si surface [ lead to best quality heterojunction. These surfaces can be obtained by different wet cleaning procedures, usually terminated by an immersion in diluted HF. However, after this step, the wafer surface is highly reactive and can re-oxidize rapidly: contaminants presents in air can be adsorbed and affect wafer passivation [. For this reason, in-situ Hydrogen (H2) plasma cleaning prior to a-Si:H(i) deposition might be an interesting option to decrease the amount of contaminant on the surface. However, the experimental window is extremely narrow, since phenomena like epitaxial growth and ion-bombardment damage can easily occur [[ and worsen the surface passivation operated by a-Si:H(i) layers. In this contribution, we present an in-situ H2 plasma clean and show a decrease of Oxygen and Carbon on wafer surface after a short time (<10 sec), without detrimental effects on the subsequent passivation.

Applied PhD research in a work-based environment: an activity theory-based analysis

Activity theory is used to compare PhD undertaken at university, that is, academic PhD, with PhD performed in collaboration with industry, that is, semi-industrial PhD. The research is divided into a literature review and a case study. Semi-industrial and academic PhD are modelled as activity systems, and differences are highlighted in terms of subject, community, division of labour and instruments. Semi-industrial PhD involve interaction with people from a non-academic background, developing management skills. Furthermore, the supervision of semi-industrial PhD is more complex than that of academic PhD. If supported by frequent supervision, this complexity strengthens the PhD. If not, supervision becomes dispersive, and semi-industrial PhD students create a network of people that enables them to perform their research. However, the creation of that network is not systematic, and a lack of a network may affect the PhD research. Therefore, frequent supervision of semi-industrial PhD students should be stressed and structured.

Simplified Cleaning for a-Si:H Passivation of Wafers Bonded to Glass

Silicon photovoltaic (PV) roadmaps indicate the reduction of wafer thicknesses and the need for innovation in wafering method and cell processing. Within this framework, Imec proposes the i2-module device [1], i.e. an heterojunction interdigitated back-contact (HJ i-BC) solar module [2] processed on 40-μm thick epitaxial wafers bonded to carriers by means of silicone. In the i2-module concept, the Rear Side (RS) of the solar cell is passivated while the wafer is bonded to the module glass and the influence of the silicone on the passivation process is reduced by an O2 plasma realized in an Reactive Ion Etching (RIE) chamber [3]. In this contribution, the effect of different post-bonding cleaning sequences on the passivation of wafers/silicone/glass stacks treated with an O2 plasma is investigated and a simplified post-bonding cleaning sequence leading to state-of-the-art passivation is proposed.

2D periodic photonic nanostructures integrated in 40 _x0019_

Two-dimensional (2D) periodic photonic nanostructures are fabricated by nanoimprint lithography (NIL) and dry plasma (Dry-NIL) etching on 40 μm thick epitaxially-grown crystalline silicon (c-Si) foils, resulting in nanostructures with a parabolic profile. These nanostructures are integrated in a 40 μm thick double side contacted c-Si/aSi:H heterojunction solar cell architecture. The front side is processed when the foil is attached to the parent substrate while the back side is processed when the foil is bonded to the glass carrier. Although the efficiency of the nanopatterned cell was lower compared to the random pyramid textured cells it had a better absorption and spectral response for long wavelengths, highlighting a better light trapping behavior.