Loic Tous

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.

Integration of a 2D Periodic Nanopattern Into Thin Film Polycrystalline Silicon Solar Cells by Nanoimprint Lithography

The integration of 2-D periodic nanopattern defined by nanoimprint lithography and dry etching into aluminum-induced crystallization-based polycrystalline silicon thin-film solar cells is investigated experimentally. Compared with the unpatterned cell, an increase of 6% in the light absorption has been achieved thanks to the nanopattern, which, in turn, increased the short-circuit current from 20.6 to 23.8 mA/cm2. The efficiency, on the other hand, has limitedly increased from 6.4% to 6.7%. We show using the transfer length method that the surface topography modification caused by the nanopattern has increased the sheet resistance of the antireflection coating (ARC) layer as well as the contact resistance between the ARC layer and the emitter front contacts. This, in turn, resulted in increased series resistance of the nanopatterned cell, which has translated into a decreased fill factor, explaining the limited increase in the efficiency.

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.

Laser ablation and contact formation for Cu-plated large area C-silicon industrial solar cells

In this work we demonstrate the successful implementation of laser ablation of SiNx ARC to contact high ohmic emitters up to 120 Ω/sq with an advanced metallization on large area substrates. We propose Suns-Voc measurements as a fast and effective method to characterize the potential laser damage. We look at the laser ablation factors that can compromise the solar cell performance and we see how to limit the damage that can jeopardize the device performance. We define the laser ablation process window for emitters of different resistivity ranging from a typical 60 Ω/sq emitter to a emitter of 140 Ω/sq. A point-contact contacting scheme is proposed that leads to an improved Voc of the solar cell. Finally we present results of the electrical characterization of large area solar cells. Efficiencies up to 18.7% are obtained on large area, 160 µm thick CZ silicon substrates.

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.

Integration of high sheet resistance homogeneous emitters in a process flow for PERC-type solar cells with Cu contacts

This paper presents the results of the integration of a high sheet resistance (120 Ω/sq) homogeneous emitter with an industrial feasible process flow on large area p-type CZ-Si PERC solar cells with plated Ni-Si/Cu front contacts. Focus is put on the junction isolation performed by either applying a mask prior to POCl3 diffusion or by selectively removing the rear side emitter in an inline 1-sided wet-chemical etching tool after diffusion. A metallization sequence [1,2] applying Ni and Cu plating, is performed to contact the moderately doped emitters. The best experimental split in this comparison resulted in excellent average conversion efficiency of 20.2%.

Editorial to the Proceedings of the 6th Workshop on Metallization and Interconnection for Crystalline Silicon Solar Cells

For the sixth time experts and specialists from all over the world discussed the latest status, trends and new directions in the field of metallization and interconnection for crystalline silicon solar cells on May, 2 and 3, 2016, in Constance, Germany. The first Workshop on Metallization for Crystalline Silicon Solar Cells was held in 2008 in Utrecht, The Netherlands to provide a forum for metallization specialists worldwide. With workshops in Constance, Germany, 2010 and Charleroi, Belgium, 2011, and again Constance in 2013, 2014 and 2016, the Metallization Workshop is now established as a forum with valuable contributions on various metallization and interconnection topics. About 145 participants from all over the world came to discuss the results of 31 presentations. The presentations are available on www.metallizationworkshop.info as pdf documents. These proceedings contain peer-reviewed papers relating to some of the workshop contributions. During the Workshop, the participants filled in a questionnaire about their views on the future of metallization and interconnection, and the results of that survey are also given in these proceedings.