I. Kuzma-Filipek

Reorganized Porous Silicon Bragg Reflectors for Thin-Film Silicon Solar Cells

Stacks of porous silicon layers have been successfully applied to maximize internal reflection at the interface between a silicon substrate and an epitaxially grown layer. The stack is consist of alternating porous layers of high and low porosity, defined by the quarter-wavelength rule. During the hydrogen bake prior to epitaxial growth of the epitaxial layer, the porous silicon stack crystallizes in the form of thin quasi-monocrystalline silicon layers incorporating large voids. Experimental data of the measured external reflectance have been linked to the internal reflectance. An optical-path-length enhancement factor of seven was calculated in the wavelength range of 900-1200 nm. Application on thin-film epitaxial solar cells showed a 12% increase in short-circuit current and efficiency

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...

Short-Circuit Current Densities Exceeding 30

We demonstrate the use of chirped porous-silicon broadband optical reflectors for thin-film epitaxial silicon solar cells. The benefits of chirped multilayer structures over conventional Bragg reflectors on the cell level are presented. By combining these chirped reflectors with shallow emitters, we show that both low- and high-energy photons are more effectively absorbed in the thin (20-mum ) epitaxial active layer of the cell. This resulted in a high conversion efficiency of 14.2% on screen-printed epitaxial solar cells made on highly doped multicrystalline silicon substrate. The corresponding photogenerated current densities are well above 30 mA/cm2 .

\hbox{mA/cm}^{2}

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.

by Use of Chirped Porous-Silicon Reflectors and Shallow Emitters in Thin-Film (20-

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.

\mu\hbox{m}

Epitaxial thin film silicon solar cell technology, consisting of a thin high quality epitaxial layer on top of a highly doped low-cost silicon substrate, is one of the most promising midterm alternatives for cost effective industrial solar cell manufacturing. Currently, CVD is used to grow the active layer. However, besides the growth of the base, also the emitter can be grown by CVD, which allows the growth of almost any doping profile, designed as desired to optimize the emitter and base properties. A two step emitter with a highly doped top layer acting as a front surface field and a moderate doped bulk layer was designed and tested. Very sharp transitions between the different epi-layers boost the open-circuit voltage drastically up to values around 650 mV. However, the Jsc of those cells were limited to values around 29 mA/cm2. To enhance the Jsc, light trapping methods are introduced to enhance the optical thickness of the 20 ¿m thick active base. In this paper, solar cell processes are established integrating both a CVD grown emitter, and state-of-the-art concepts for optical light trapping in epitaxial cells. In this way, the significant increase in Voc is combined with an improved short-circuit currents and leads to a record efficiency of 16.1% with a current density of 33.2 mA/cm2, approaching the Jsc of bulk silicon solar cells.

) Epitaxial 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.