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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 ones. Although some understanding of chirping exists mainly in the field of image processing and photonics, detailed study of chirped porous structures as broad-band reflectors is needed for their implementation in thin-film epitaxial silicon solar cells. A record efficiency of almost 14% was reached for those types of reflectors on a large area 71 cm2 epitaxial Si solar cell, giving a boost in efficiency of 0.6% absolute in comparison with the solar cells containing conventional reflectors. [ABSTRACT FROM AUTHOR]

Lithium fluoride (LiF) is currently a very popular dielectric material used as a passivation or transport layer in a variety of applications, especially in high-efficiency solar cells. Despite this, its conduction properties and interface behavior with silicon remain largely unexplored. In this work, a LiF metal–insulator–semiconductor (MIS) structure is fabricated and characterized, and its properties are compared to the well-understood aluminum oxide (Al2O3) MIS structure. First, a higher current density in LiF compared to Al2O3 is highlighted, as well as its PN junction-like behavior with n-type silicon (n-Si), being rather unconventional for a dielectric layer. C–V measurements showcase the likely presence of an interface defect, causing an increase in the apparent doping and a shift in the flatband voltage VFB by +70 meV. This defect is found to be of the acceptor type, which renders the interface fixed charge more negative and improves the field-effect passivation in the case of a negative Qf. Finally, a density of interface states D i t ≈ 2 × 10 11 cm − 2 eV − 1 was found for LiF/n-Si, which is a low value showing appropriate chemical passivation at the interface. Overall, this work enables us to shed more light on the interface properties of LiF on n-Si, which is an essential step toward its wider use in state-of-the-art solar cells and other silicon-based devices. [ABSTRACT FROM AUTHOR]

Abstract: In this paper the tradeoff between process control and cost-effectiveness for PERL-type process flows is investigated. Three aspects are considered: (1) whether a surface pre-treatment (for example special cleans and/or oxidation) prior to the deposition of the Al2O3-based rear-side passivation stack has to be added to the flow; (2) the quantification of the impact on cell parameters of skipping the rear-side polishing and/or the Al2O3 deposition; and (3) the possibility of replacing Al paste with PVD Al for rear side metallization. It is found that: (1) SPM-like cleans or thermal oxidation prior to Al2O3 deposition are useful steps to guarantee a stable Al2O3-based passivation process. (2) The efficiency drops by more than 1% by skipping the polishing, but only ≈0.15% by removing the Al2O3 from the passivation stack. Therefore, while rear-side polishing appears to be a necessary step for PERC/PERL technologies, the insertion of Al2O3 in the passivation stack could be traded off for a reduced production cost, depending on the (company-specific) details of cost-calculations. (3) Replacing Al paste with PVD Al for rear side metallization results in a deteriorated reflectance. The effect is larger for thinner passivation stacks. Although less advantageous in term of material costs with respect to PVD Al, screen-printed Al would therefore be the preferred choice for PERL cells featuring thin passivation stacks. [Copyright &y& Elsevier]

[Display omitted] • Active P concentration of 1.3∙1020 cm−3 achieved in in situ doped LPCVD poly-Si. • Thorough electrical and structural characterization of in situ doped films reported. • J 0 down to 1.5 fA/cm2 and ρ c down to 1.8 mΩ∙cm2 achieved with the developed films. • At the rear of solar cells, 0.41 mA/cm2 J sc loss expected for use of 100 nm poly-Si. In situ phosphorus (P)-doped polycrystalline silicon (poly-Si) films by low pressure chemical vapor deposition (LPCVD) were studied in this work for the fabrication of poly-Si passivating contacts. In situ doping was targeted for enabling the full potential of the high-throughput LPCVD technique, as it could allow leaner fabrication of industrial solar cells featuring poly-Si passivating contacts than the more common ex situ doping routes. By careful optimization of the deposition temperature and the flows of the carrier gas (H 2) and the dopant precursor (PH 3), high doping in the poly-Si layers was achieved with active P concentrations up to 1.3⋅1020 cm−3. While reduction in the deposition rate (r dep) and thus in the throughput is a known problem when growing in situ P-doped films by LPCVD, this reduction could be limited, and the resulting r dep was equal to 0.078 nm/s. The developed poly-Si films were characterized both structurally and in terms of their passivation potential in poly-Si contacts. The latter yielded recombination current densities down to 1.5 fA/cm2 in passivated (J 0,p) and 25.6 fA/cm2 in screen-printing metallized (J 0,m) regions on saw-damage removed (SDR) Cz-Si surfaces, accompanied by a contact resistivity (ρ c,m) of 4.9 mΩ⋅cm2. On textured Cz-Si surfaces, the corresponding values were J 0,p = 3.5 fA/cm2, J 0,m = 56.7 fA/cm2, and ρ c,m = 1.8 mΩ⋅cm2. Optical impact of the developed poly-Si films was also assessed and a short circuit density loss of 0.41 mA/cm2 is predicted per each 100 nm of poly-Si applied at the rear side of solar cells. [ABSTRACT FROM AUTHOR]

Hybrid concentrated solar power/photovoltaic systems (CSP/PV) combine the advantages of the two separate systems while reducing their drawbacks. The design of such a hybrid system is challenging due to the various trade-offs between the thermal and electrical performance, and the overall system complexity. A reliable simulation model that includes all relevant optical, thermal, and photovoltaic aspects, can therefore be extremely useful to analyse the combined system performance and fine-tune the various design parameters. While multi-physics modelling of photovoltaic systems is well established, this is not the case for hybrid CSP/PV. In this paper, a novel multi-physics framework is presented for a hybrid system consisting of a parabolic trough with integrated PV cells covered by a dichroic coating, focusing incident sunlight towards a thermal receiver. Instead of monofacial PV cells, bifacial cells are considered for harvesting also the diffuse and ground reflected light at the back of the trough. The presented framework relies on an existing simulation tool for PV modules, that is combined with a ray-tracer that includes the spectral beam splitting functionality of the coating, and a novel 1.5D thermal model for the receiver tube. Long term outdoor monitoring results are used to predict the averaged, time-resolved annular thermal and electric energy yield of the system. This energy production is compared for three different multilayer coating designs with an increasing amount of (TiO 2 , SiO 2) layers. These results show that the amount of reflected sunlight towards the thermal receiver can be enhanced at the expense of the transmitted sunlight towards the PV cells, when a higher number of layers are used. The reduction of the incident power on the PV cells is however almost fully compensated by the enhanced spectral match of the transmitted light with the spectral response of the considered bifacial cells, in addition to the enhanced cell efficiency due to the lower thermalization losses. This results in superior system efficiency, for the application scenario where the generated thermal energy is also converted in electrical energy, and geographical locations with sufficient direct sunlight; a conclusion that is drawn from comparing the total electrical energy yield in Spain and Belgium, as a function of the thermal to electrical conversion fraction. [Display omitted] • Demonstration of a novel multi-physics simulation framework for hybrid CSP/PV systems. • Integration of long-term monitoring results allows predictive energy yield modelling. • Usage of bifacial PV cells enhances CSP/PV system efficiency for diffuse sunlight. • Number of layers in spectral beam splitter has crucial impact on system performance. • Geographical location determines potential application scenarios of hybrid CSP/PV. [ABSTRACT FROM AUTHOR]

Abstract In this work we investigated the optical and electrical performance of p-type epitaxial layers as the rear emitter of bifacial n-type PERT solar cells. In the first part of this paper, the surface morphology of epitaxial layers grown on textured surfaces is studied. Because of the epitaxial growth, a pyramids-rounding effect is observed as a result of {311} and {911} facet propagation. The growth pattern was quantified and modelled. In the second part of this paper, the optical performance of semi-device test structures is evaluated. The trend of the optical results in bifacial solar cell structures indicates that a final pyramid angle at the rear side around 20° gives the maximum light absorption in the wafer substrate. In this work we demonstrate that the epitaxial growth of the emitter on the textured rear side of these devices can already give a pyramid angle of 25° without having to introduce any additional polishing steps to modify the morphology of the textured surface. In the last part of this paper, we present the electrical results for semi-device structures created to quantify the recombination losses in the passivated and metallized regions of those p-type epitaxial emitters. These results indicate that by introducing a rear epitaxial emitter in the bifacial n-type PERT cell structure, we can increase the implied V oc up to 17 mV compared to a diffused emitter with the same sheet resistance. Highlights • The pyramids-rounding effect is simultaneously implemented during the epitaxial emitter growth step. • The trend of the optical results in bifacial solar cell structures indicates that a final pyramid angle at the rear side around 20° gives the maximum light absorption in the wafer substrate. In this work we demonstrate that the epitaxial growth of the emitter on the textured rear side of these devices can already give a pyramid angle of 25 ° without having to introduce any additional polishing steps to modify the morphology of the textured surface. • Thick but lowly doped epitaxial emitters reduce recombination loss compared to diffused counterpart. [ABSTRACT FROM AUTHOR]

Recently, large area hybrid silicon heterojunction cells (SHJ) with nickel/copper (Ni/Cu) plated front contacts were shown to benefit from a simpler processing sequence than n-PERT cells featuring a boron diffused rear emitter. Hybrid refers here to the combination of a diffused front surface field (FSF) and a SHJ rear emitter. In this work, further process simplifications for such hybrid SHJ cells are evaluated. Based on reflectance and adhesion considerations, stacks of tin-doped indium oxide and copper (ITO/Cu) and chromium/silver (Cr/Ag) are tested as alternative to the previously chosen ITO/Ag rear contact stack. Using ITO/Cu/Al as rear contact stack, average efficiencies of 20.8±0.2% are presented on 6-inch wafers. We also demonstrate that the excimer laser annealing step previously employed to form nickel silicides at the front side can be skipped without comprising solder tab adhesion quality. [ABSTRACT FROM AUTHOR]

Poly-Si/SiO x passivating contacts enable the manufacturing of highly-efficient Si solar cells, but their fabrication commonly relies on an extra high-temperature process such as dopant diffusion or thermal annealing for achieving excellent passivation and contacting properties. This extra process is eliminated in the fired passivating contact (FPC) approach used for simplified fabrication of poly-Si/SiO x passivating contacts. Instead, FPCs rely on the thermal budget of the fast/short and high-temperature firing process used for metallization of solar cells to achieve similar final properties. Despite this, compatibility of FPCs with industrially viable metallization techniques has not been demonstrated yet, which is studied in this work for fire-through Ag screen-printing and Ni/Ag plating. With screen-printing, low recombination current density (J 0) down to 4.9 fA/cm2, low contact resistivity between the Ag contacts and the FPC (ρ c,m) down to 7.2 mΩ⋅cm2, and Ohmic transport through the FPC including the SiO x film were achieved using wet-chemically grown SiO x. Nevertheless, J 0 of metallized regions (J 0,m) exceeded 1000 fA/cm2. Reducing J 0,m was attempted by mitigating the blistering observed in FPCs, but J 0,m remained high. With Ni/Ag plating, excellent surface passivation with J 0 down to 2.7 fA/cm2 and very low J 0,m < 50 fA/cm2 were achieved, but no Ohmic contacts could be obtained. Integration of screen-printed FPCs in large-area n-TOPCon solar cells was also demonstrated, yielding average efficiencies of 18.4%, limited mainly by the high J 0,m and series resistance of the FPCs. The results presented reveal the challenges for the industrialization of FPCs and provide valuable insights for tackling these. [Display omitted] • Fired passivating contacts (FPCs) developed relying only on the thermal budget of firing and not on conventional annealing. • Compatibility of FPCs with fire-through Ag screen-printing and Ni/Ag plating tested. • Screen-printing: J 0,pass ≈ 5 fA/cm2 & ρ c < 10 mΩ∙cm2 achieved, but J 0,metal was high. • Plating: J 0,pass < 3 fA/cm2 & J 0,metal < 50 fA/cm2 achieved, but contacts were non-Ohmic. • Integration of FPCs in screen-printed large-area n-TOPCon cells led to 18.4% efficiency. [ABSTRACT FROM AUTHOR]

Abstract: Aiming at the development of a cost-effective and industrially up-scalable process for the production of p-Si PERL-type solar cells, this work focuses on the selection and the development of an optimal rear-contacting method of screen-printed aluminum back electrodes through AlO x -based rear passivation stacks. Laser-Fired Contacts are first optimized by developing dedicated characterization protocols to define an optimal laser process. Laser parameters are systematically varied (e.g. pulse energy, power density, focus, and contact pitch) and their effect on the contacts quality is analyzed. Besides, a new contacting method, called Extended-Laser-Ablation (ELA), is presented, allowing even higher efficiencies to be attained thanks to better quality of local back-surface fields. Both contacting routes are then implemented and compared in the PERL pilot production process flow to investigate their effects on cells performances. Top efficiencies of 19.13% (AlO x /SiN x ) and 19.5% (SiO2/AlO x /SiN x ) are reached when the ELA process is used on monocrystalline Cz Si, using a close-to-standard process (75Ω/sq homogeneous emitter, screen-printed contacts, standard cleaning procedure), without any final forming gas annealing step required. [Copyright &y& Elsevier]

The potential of passivating contacts incorporating in situ phosphorus (P)-doped polycrystalline silicon (poly-Si) films grown by low pressure chemical vapor deposition (LPCVD) is demonstrated in this work by integrating these layers at the rear side of large-area (241.3 cm2) bifacial n-type Tunnel Oxide Passivated Contact (n-TOPCon) solar cells with diffused front emitter and screen-printed contacts. In situ doped poly-Si films are studied as their use could simplify the production of industrial n-TOPCon solar cells compared to the common approach relying on ex situ doping of intrinsic LPCVD poly-Si films. The developed poly-Si passivating contacts exhibited excellent characteristics with low recombination current densities in passivated and screen-printing metallized regions down to 2.3 fA/cm2 and 65.8 fA/cm2, respectively, and a low contact resistivity of 2.0 mΩ⋅cm2. For reaching the best passivating contact characteristics and high solar cell efficiencies, a poly-Si film thickness of 150–200 nm was found to be optimal while a polished rear surface morphology was found to be beneficial. The best solar cell reached a certified power conversion efficiency of 23.01% along with a high open circuit voltage of 691.7 mV, enabled by the passivating contacts with the in situ doped poly-Si films. 1-cell glass-glass laminates were also fabricated with the developed solar cells, which showed no loss in their power output both upon 400 thermal cycles and after 1000 h of damp heat testing. Lastly, a roadmap is presented, indicating strategies to achieve efficiencies up to 25.5% with n-TOPCon solar cells incorporating the in situ P-doped LPCVD poly-Si films. [Display omitted] • In situ P-doped LPCVD poly-Si passivating contacts integrated in n-TOPCon solar cells. • 241.3 cm2-large bifacial n-TOPCon devices with 23.01% certified efficiency demonstrated. • Using ≥ 150 nm of poly-Si and polished rear side improves the solar cell efficiency. • 1-cell glass-glass laminates of the devices pass damp heat and thermal cycling tests. • Roadmap to 25.5% efficiency presented. [ABSTRACT FROM AUTHOR]

This work evaluates the potential of selective epitaxy to mitigate the recombination losses at the p-type contact regions of plated bifacial n-type PERT cells. Following the growth of a 500 nm, 2.5 1019 cm−3 boron doped epitaxial layer at the emitter contact regions defined by laser ablation of the passivating dielectrics, both an increase in V oc (+6 mV) and FF (+1% absolute) are measured in the final cells compared to the reference with a homogeneous diffused emitter. These results come with an average efficiency gain of 0.3 and 0.5% absolute for front and rear side illumination, respectively. If the diffused, blanket emitter in the passivated regions is replaced by a thicker, lowly doped epitaxial profile (3 μm, 5 1018 cm−3) to further reduce recombination, an additional rise in implied V oc after metallization of 10 mV is estimated. This increase would be the result of a reduction in J 0,pass, emitter (down to 6 fA/cm2) and J 0,plated, emitter (down to 1967 fA/cm2). Image 1 • Selective growth of doped regions by epitaxy on laser ablated textured surfaces. • Metallization by simultaneous plating of n- and p-type contacts in bifacial cells. • Reduced J 0,plated for p-type contacts by doping profile engineering with epitaxy. • Thermal SiO 2 /PECVD SiN x passivation after epitaxy at the same level than ALD Al 2 O 3. • Gain in FF and V oc for bifacial cells with a selective epitaxial emitter approach. [ABSTRACT FROM AUTHOR]