Insights into the reliability of Ni/Cu plated p-PERC silicon solar cells

Selective laser ablation of dielectric layers in combination with plated Ni/Cu/Ag contacts have been investigated by many photovoltaic researchers. Despite that there has been quite some practical progress on improved processing, the reliability of plated Ni/Cu/Ag cells still needs further insight and understanding. In this paper, the impact of laser induced defects that result from a ps-laser (wavelength 355nm) ablation on the performance of p-type PERC cells has been studied. A thermal stress experiment at 235 degrees C is applied. It is shown that the defects formed during the laser ablation process do indeed decrease the cell performance. A higher laser fluence results in lower fill factor and therefore lower efficiency. Moreover, the cells with higher laser fluence ablation degrade faster compared to the cells which had lower laser fluence to open the dielectric layer. The second part of the paper focuses on characterization of the p-n junction of the laser ablated cells by Deep Level Transient Spectroscopy (DLTS) before and after thermal ageing. A hole trap around 80K was found for all samples, which is related to point defects induced during the cell processing. A broad peak around 200K observed for the ablated cells with high laser fluence could correspond to dislocations induced by the laser ablation. This peak is shifted to higher energy (closer to the silicon mid-gap) after annealing, which may be due to impurity decoration during the annealing

Comparative study of the interface passivation properties of LiF & Al2O3 using silicon MIS capacitor

Lithium fluoride (LiF) is currently a very popular dielectric material, used as passivation or transport layer in a variety of applications and especially in high-efficiency solar cells. Despite this, its conduction properties and interface behaviour with silicon remain largely unexplored. In this work, a LiF metal-insulator-semiconductor (MIS) structure is fabricated, characterized, and its properties are compared to the well-understood aluminium oxide (Al2O3) MIS structure. First, a higher current density in LiF compared to Al2O3 is highlighted, as well as its PN junction-like behaviour with n-type silicon (n-Si), being rather unconventional for a dielectric layer. CV measurements showcase the likely presence of an interface defect, causing an increase in the apparent doping and a shift in the flat-band voltage VF B by +70 meV. This defect is found to be of the acceptor type, which renders the interface fixed charge more negative and improve the field-effect passivation in case of a negative Qf . Finally, a density of interface states Dit ≈ 2 × 1011 cm−2eV−1 was found for LiF/n-Si, which is a low value showing appropriate chemical passivation at the interface. Overall, this work enables to shine more light on the interface properties of LiF on n-Si, which is an essential step towards its wider use in state-of-the art solar cells and other silicon-based devices

Optical and electrical performance of rear side epitaxial emitters for bifacial silicon solar cell application

© 2019 Elsevier B.V. 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.status: publishe

Efficiency gain in plated bifacial n-type PERT cells by means of a selective emitter approach using selective epitaxy

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Magneto-Hydrodynamic Mixing: a New Technique for Preparing Carbomer Hydrogels

Magnetohydrodynamic mixing was evaluated as an alternative to conventional high shear mixing in the preparation of carbomer hydrogels containing 1.22 wt.% Carbopol® 980 NF. Neutralization of the carbomer dispersion (pH = 2.74) with triethanolamine (TEA) enabled to adjust the pH of the mixture and tune the viscosity of the hydrogel. Using high shear mixing, this approach was limited to 0.2 wt.% TEA (pH = 3.83) as the gel became too viscous and the recirculation flow dropped from 12 to 0.3 m3/h. Magnetohydrodynamic mixing enabled to reach TEA concentrations up to 1.0 wt.% (pH = 5.31). Apparent viscosity measurements on samples having 0.2 wt.% TEA revealed lower viscosities for carbomer hydrogels prepared with high shear mixing, i.e. 6,800 mPa·s versus 8,800 mPa for magneto-hydrodynamic mixing. Based on 1H NMR evidence, this decrease in apparent viscosity was attributed to structural damage to the carbomer backbone in combination with mechanochemical degradation of the added TEA

Large-area n-Type PERT solar cells featuring rear p+ emitter passivated by ALD Al2O3

We present large-area n-type PERT solar cells featuring a rear boron emitter passivated by a stack of ALD Al2O3 and PECVD SiOx. After illustrating the technological and fundamental advantages of such a device architecture, we show that the Al2O3/SiOx 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/SiOx 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 Voc 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

Deposition of a SiOx Film Showing Enhanced Surface Passivation

A new process for deposition of silicon oxide films with excellent passivation properties was developed using an atmospheric pressure plasma reactor. This process consists of fast deposition at room temperature of a SiCxOyHz film followed by a rapid thermal anneal in air (similar treatment to a contact firing step) to convert it to a dense inorganic SiOx material. The material formed using this process shows improved passivation compared to low pressure PECVD films. The firing process and more particularly the firing temperature appears to play a critical role in passivation performance, and an optimum temperature was identified. Capacitance-Voltage measurements on a MOS structure show that the oxide layer has a very low Dit value with fixed negative charges, which has not been reported before for thick silicon oxide. This uniqueness is attributed to measured over-stoichiometry in oxygen in the dense film owing to the presence of bulk silanols. These films were successfully incorporated in PERC solar cells with cells showing efficiencies up to 19.7%