Filip Duerinckx

Chirped porous silicon reflectors for thin-film epitaxial silicon solar cells

The studies of porous silicon as a one-dimensional photonic crystal have led to solutions allowing the fabrication of broad photonic band gaps as large as several hundreds nanometers for various types of applications. In this work we demonstrate the use of the chirping process, i.e., the gradual increase in the spatial period of the structure, as it is used in image processing, to design porous silicon broad-band reflectors for thin-film silicon solar cells. Modeling of those layers is done using linear design method and a simulation software. Such chirped structures are fabricated by an anodization process. Samples are prepared on mono and multicrystalline silicon substrates with 15, 40, 60, and 80 layers. The reflectance spectra of such prepared porous reflectors are evaluated and the results show an increase in bandwidth of over 50% of the total width in comparison with the conventional, unchirped reflectors. We demonstrate clear advantages of introducing chirped multilayer structures over conventional...

Large-Area Thin-Film Free-Standing Monocrystalline Si Solar Cells by Layer Transfer

This paper describes our progress achieved in thin-film free-standing monocrystalline silicon (FMS) solar cells by layer transfer process using porous silicon. The highest cell efficiency achieved for a 1times1 cm2 FMS solar cell is 13.5% with an open circuit voltage Voc=612 mV, current density Jsc =28.9 mA/cm2 and fill factor of FF=76.7%. In the effort of scaling-up of the cell area, the lift-off process was adapted to 10times10 cm2 substrates. The large-area epilayers from APCVD and LPCVD, respectively, were successfully processed into FMS solar cells, giving the best efficiency of 11.4% for the APCVD-FMS cell with an area of 11.1 cm2. Detailed study by quantum efficiency measurements, combined with SEM, revealed that the enhanced internal quantum efficiency and reflectance in the spectral range of lambda>825 nm for the APCVD-FMS cell can be attributed to the internal light reflection from the well-defined interface between the epilayer and the sintered porous, or quasi-monocrystalline silicon (QMS) layer

Hydrogen passivation of newly developed EMC-multi-crystalline silicon

Abstract Continuous casting with an electromagnetic cold crucible is a promising way of producing multi-crystalline (mc) silicon on an industrial basis for solar cell production. Casting equipment, capable of producing ingots of 130×130 mm2 cross-section and 600 mm length has been developed and installed. Thorough characterisation of the produced material revealed a relatively high defect density (grain boundaries, dislocations) and small grain sizes of 1–4 mm in diameter. Although the effective minority carrier diffusion length is high on as-cut wafers (>150 μm) it decreases during solar cell processing to values of 50 μm. This can be attributed to standard high temperature processing steps that lead to redistribution and agglomeration of residual impurities (e.g. metals, carbon) at extended crystallographic defects which then act as strong recombination centres. In order to passivate these recombination centres, a PECVD SiNx-layer is deposited which acts as a source of hydrogen and also as an anti-reflective coating. During the firing of the screen-printed metal contacts through the SiNx-layer, atomic hydrogen is released from this layer and diffuses into the bulk of the wafers where it saturates dangling bonds and passivates impurities at crystal defects. After this treatment the minority carrier diffusion length can be restored to values of around 100 μm on finished solar cells.

The impact of silicon solar cell architecture and cell interconnection on energy yield in hot & sunny climates

Extensive knowledge of the dependence of solar cell and module performance on temperature and irradiance is essential for their optimal application in the field. Here we study such dependencies in the most common high-efficiency silicon solar cell architectures, including so-called Aluminum back-surface-field (BSF), passivated emitter and rear cell (PERC), passivated emitter rear totally diffused (PERT), and silicon heterojunction (SHJ) solar cells. We compare measured temperature coefficients (TC) of the different electrical parameters with values collected from commercial module data sheets. While similar TC values of the open-circuit voltage and the short circuit current density are obtained for cells and modules of a given technology, we systematically find that the TC under maximum power-point (MPP) conditions is lower in the modules. We attribute this discrepancy to additional series resistance in the modules from solar cell interconnections. This detrimental effect can be reduced by using a cell design that exhibits a high characteristic load resistance (defined by its voltage-over-current ratio at MPP), such as the SHJ architecture. We calculate the energy yield for moderate and hot climate conditions for each cell architecture, taking into account ohmic cell-to-module losses caused by cell interconnections. Our calculations allow us to conclude that maximizing energy production in hot and sunny environments requires not only a high open-circuit voltage, but also a minimal series-to-load-resistance ratio.

Large-Area n-Type PERT Solar Cells Featuring Rear p + Emitter Passivated by ALD Al 2 O 3

We present large-area n-type PERT solar cells featuring a rear boron emitter passivated by a stack of ALD Al₂O₃ and PECVD SiO_x. After illustrating the technological and fundamental advantages of such a device architecture, we show that the Al₂O₃/SiO_x stack employed to passivate the boron emitter is unaffected by the rear metallization processes and can suppress the Shockley-Read-Hall surface recombination current to values below 2 fA/cm², provided that the Al₂O₃ thickness is larger than 7 nm. Efficiencies of 21.5% on 156-mm commercial-grade Cz-Si substrates are demonstrated in this study, when the rear Al₂O₃ /SiO_x passivation is applied in combination with a homogeneous front-surface field (FSF). The passivation stack developed herein can sustain cell efficiencies in excess of 22% and V_oc above 685 mV when a selective FSF is implemented, despite the absence of passivated contacts. Finally, we demonstrate that such cells do not suffer from light-induced degradation.

Opportunities for Silicon Epitaxy in Bulk Crystalline Silicon Photovoltaics

This work presents an overview of the opportunities in bulk crystalline silicon photovoltaics that have been explored using silicon epitaxy as doping technology. Epitaxy demonstrates to be an elegant and versatile technology which brings a lot of new opportunities to further simplify and improve the design and performance of bulk solar cells. Advantages are the doping profile flexibility, the reduced thermal budget, the absence of additional steps to remove glassy layers or activate dopants, the simplified integration of local doping by means of selective epitaxy, and the possibility of single-side deposition. The results presented herein demonstrate the potential of epitaxy by applying the process in three cell structures to grow a boron-doped layer. First, epitaxy is used to grow blanket doped layers as emitters on the full rear side of n-type PERT cells. Second, selective epitaxy is applied to locally grow the interdigitated emitter in n-type IBC cells. Third, selective epitaxy is applied to form the local BSF in p-type PERL cells. For each of these cell concepts, silicon epitaxy helped to simplify the reference BBr3 diffusion-based process, while keeping high efficiencies: 20.5 % for n-type PERT (226 cm cell), 22.8 % for IBC (4 cm cell) and at least +0.5 mA/cm and +10 % escape reflectance for p-type PERL cells compared to the standard PERC.

Kerfless Epitaxial Mono Crystalline Si Wafers With Built-In Junction and From Reused Substrates for High-Efficiency PERx Cells

This paper proposes a kerfless wafer structure with built-in p-n junctions in n-type silicon wafers grown using Crystal Solars high throughput epitaxy technology. Compared with a conventional p-type emitter by boron diffusion, ion implantation, or epitaxy, the built-in p-type emitter has a reduced and uniform doping concentration and increased thickness. The epitaxially grown wafers and conventional Czochralski (CZ) n-type wafers were processed into solar cells. A best efficiency of 22.5% with epitaxially grown wafers was achieved, with a 6 mV gain in open-circuit voltage, suggesting a high wafer quality and superiority of the deep epitaxial emitter over a standard boron-diffused emitter. Substrate reuse associated with the kerfless epitaxy technology is studied as well, with respect to its impact on solar cell efficiency. The data suggest no degradation in cell efficiency due to substrate reuse.

Optical Modeling of Capped Multi-Layer Porous Silicon as a Back Reflector in Thin-Film Solar Cells

A model for the optical properties of closed-porosity silicon is developed as a tool for design and optimization of porous silicon stacks made of layers of alternate (high/low) porosity which are possible candidates for use as back reflectors in thin-film silicon solar cells on cheap substrates. The model takes into account the change in pore morphology during the epitaxial growth process of the silicon film on the stack, with the pores taking the form of relatively large closed cavities where retardation and interference effects play an important role. The calculated front-surface reflectance of structures consisting of a silicon epitaxial layer with a plasma-textured front surface grown over a reflecting porous silicon stack are compared with the corresponding experimental measurements, showing a good agreement between theory and experiment

IIb-2 – Low Cost Industrial Technologies of Crystalline Silicon Solar Cells

Publisher Summary nThis chapter discusses industrial technologies for manufacturing crystalline silicon solar cells at low costs. Typical efficiency of commercially produced crystalline silicon solar cells lies in the range of 14%–17%. Because the efficiency of the cell influences the production cost at any production stage, considerable efforts are directed toward efficiency improvement of solar cells. The required near future efficiency goals for industrial cells are 18–20% on monocrystalline, and 16–18% on multicrystalline silicon. Based on laboratory scale achievements, one can consider that production type cells able to fulfill the efficiency goal should have features including front surface texturing, optimized emitter surface concentration and doping profile, effective front surface passivation, fine line front electrode, and front electrode passivation. This chapter discusses the concepts of cell processing, substrates, etching, texturing, optical confinement, and junction formation in detail. It then explains front surface passivation and functions of antireflection coating. Techniques of gettering by phosphorous diffusion and gettering by aluminum treatment are also discussed. Concepts related to screen-printed solar cells, buried contact solar cells, solar cells on silicon ribbons, and back-contacted solar cells are explained in detail in the chapter.

Plasma Texturing and Porous Silicon Mirrors for Epitaxial Thin Film Crystalline Silicon Solar Cells

Thin film silicon solar cells, consisting of an epitaxially grown active layer on a low quality highly doped silicon substrate, incorporate many attractive features usually associated with their sister cells based on bulk silicon. However, the efficiency of the current epitaxial semiindustrial screen printed cells is limited to 11-12% mainly due to optical shortcomings. This paper will give an overview of our work aimed at tackling the 2 most important problems: (i) Finding and implementing an adequate front surface texture and (ii) the simulation, fabrication and incorporation of an intermediate reflector. The former issue has been addressed by the development of plasma texturing based on halogen species. This method allows us to fulfil the sometimes contradictory requirements for the textured surface, i.e. a uniform and reduced reflection, a strong lambertian character to scatter the light and a limited removal of silicon. It will be shown that the scattering efficiency is dependent on both the wavelength of the impinging light and on the silicon removal during the texturing process. The second and main issue of this work is the limited absorption volume of the epitaxial layer. To resolve this drawback, an intermediate reflector is placed at the epi/substrate interface to enhance the path length of the low energy photons through the epi-layer. In practice, a multilayer porous silicon stack is created by electrochemical anodization of the substrate. The reflection at the epi/reflector/substrate interface is a combination of several different effects including a Bragg mirror and Total Internal Reflection (TIR). Measurements of the external reflectance as well as extraction of the internal reflection parameters are used to clarify the issue. Advanced structures, including chirped porous silicon stacks, are introduced. Finally, the benefits of the reflector on the level of the epitaxial silicon solar cell are analysed. Efficiencies close to 14% are obtained for epitaxial cells incorporating an advanced porous Si reflector.