Vinodh Shanmugam

Electrical and Microstructural Analysis of Contact Formation on Lightly Doped Phosphorus Emitters Using Thick-Film Ag Screen Printing Pastes

Screen printing of the metallization of phosphorus diffused emitters is a well-established process for industrial silicon wafer-based solar cells. Previously, screen printed silver pastes typically required a very high phosphorus surface doping concentration to ensure a low-resistance ohmic contact. Recently, paste manufacturers have focused on the development of silver pastes capable of contacting phosphorus emitters with progressively lower surface concentrations, to minimize surface recombination losses and enable higher cell conversion efficiencies. In this paper, we report on the progress of contacting inline-diffused phosphorus emitters, of which the surface concentrations have been reduced by an etch-back process, using two different pastes. Solar cells with emitter surface concentrations ranging from 4.0 × 1020 to 1.7 × 1020 phosphorus atoms/cm 3 were made using two different silver pastes. We present a microstructural analysis of the contact formation, which indicates the possible dominant current transport mechanisms for the two pastes. A high density of silver crystallites formed with a very narrow interfacial glass layer makes the Sol 9600 paste suitable for contacting lowly doped phosphorus emitters. Efficiency gains of 0.2%-0.3% (absolute) were achieved, reaching a maximum efficiency of 18.6% on 156 mm × 156 mm p-type pseudo-square Cz mono-crystalline silicon solar cells.

Analysis of Fine-Line Screen and Stencil-Printed Metal Contacts for Silicon Wafer Solar Cells

Primary challenges to fine-line silver printing for solar cells are achieving high aspect ratios and uniform lines with a low level of striations. This paper compares two high-throughput printing technologies, namely, printing by screens versus stencils. A statistical method is introduced to evaluate the quality of the printed front grid based on the distributions of printed metal line profiles, line segment conductance, overall electroluminescence (EL) pattern, and solar cell light current-voltage (I-V) characteristics. The model distribution, combined with finite-element modeling to predict realistic cell-level voltage variations, adequately describes all four kinds of characteristics. It predicts well the diverging performance of screen- and stencil-printed solar cells as the line width becomes less than 50 μm. Experimentally, the highest batch average efficiency of 18.8% was achieved on 156 mm × 156 mm p-type monocrystalline silicon solar cells printed with stencils having 30-μm line openings, using only 78 mg of silver paste per cell.

Heavy phosphorous tube-diffusion and non-acidic deep chemical etch-back assisted efficiency enhancement of industrial multicrystalline silicon wafer solar cells

Improvement in emitter and bulk regions of multicrystalline silicon (multi-Si) cells by phosphorus (P) gettering is a well-known technique. Earlier researchers exploited P gettering using a combination of deep emitter formation, complete emitter etching and re-diffusion, or, the use of sacrificial dielectric layers. In this work, our approach consists of heavy P diffusion in a tube diffusion furnace, followed by chemical etch-back of the P diffused layer. The novelty of our work is three-fold. Firstly, for the first time a low-cost, non-acidic emitter etch-back process – the ‘SERIS etch’ is applied on the tube-diffused emitter. Earlier the ‘SERIS etch’ was reported only for the inline-diffused cells. Secondly, a deep etch-back (change in sheet resistance by ∼40 Ω sq−1) is performed to get the advantage of P gettering on heavily diffused emitter without affecting its surface reflectance and doping uniformity. Thirdly, unlike previously reported works, our process does not required additional diffusion or dielectric deposition processes; hence it is cost-effective and industry competitive. For the screen-printed full-area aluminium back surface field multi-Si solar cells, an average cell efficiency gain of 0.5% (absolute) is observed for etched-back cells as compared to reference cells with as-diffused emitter (no etch-back). As both groups of cells are of same sheet resistance, the efficiency gain reflects the positive effect phosphorous diffusion gettering for the etch-back cells using our modified process.

Quantifying Edge and Peripheral Recombination Losses in Industrial Silicon Solar Cells

A finite-element model is constructed to represent a silicon solar cell as a vast network of diodes with different saturation current densities, with focus on the definition of three recombination parameters to describe the vicinity of the wafer edges. By simulating the voltage distribution across the cell plane, as well as the cell current-voltage characteristics at different illumination intensities, these peripheral and edge recombination parameters are extracted for various cell types and processes by comparison with corresponding measurement data. It is noted that the monocrystalline silicon PERC cells studied have significantly lower peripheral and second diode edge recombination compared with Al-BSF cells. For the Al-BSF cells studied, there can be ~0.25%-0.6% absolute efficiency gain if the peripheral and edge recombination sources are eliminated.

Impact of femtosecond laser processing on dielectric layers for solar cell applications

Laser ablation using ultrashort pulses is becoming more relevant in the fabrication of solar cells due to their ability to produce well-defined grooves. In this work, laser grooving of dielectric layers using laser pulses of 480  femtosecond and 515 nm (green) wavelength at various pulse fluences and pulse overlaps is presented. The dielectric is either a single-layer film (SiNX) or a double-layer stack (AlOX/SiNX or SiO2/SiNX). An analytical model that enables one to determine the laser groove depth and width by calculating the equivalent fluence along the midpoint of the groove and by extracting the single-pulse ablation properties (threshold fluence, spot radius, and energy penetration depth) of the dielectrics is presented. The modeled groove dimensions match with the experimentally measured values, thereby allowing precise patterning of the dielectrics. Micromachining considerations such as the ablation rate and ablation efficiency are calculated from the modeled groove depth which enables one to optimize the ablation process. The optimum fluence zone, where the process yields >90% of its maximum ablation rate and efficiency, is identified to be just above the threshold fluence. Moreover, the results indicate that the optimum fluence zone remains practically unaffected irrespective of the chosen laser average power. Hence, the precise calculation of groove dimensions and processing under the optimum fluence zone provides a well-defined and efficient laser grooving process that is essential for high-efficiency solar cell architectures.Laser ablation using ultrashort pulses is becoming more relevant in the fabrication of solar cells due to their ability to produce well-defined grooves. In this work, laser grooving of dielectric layers using laser pulses of 480  femtosecond and 515 nm (green) wavelength at various pulse fluences and pulse overlaps is presented. The dielectric is either a single-layer film (SiNX) or a double-layer stack (AlOX/SiNX or SiO2/SiNX). An analytical model that enables one to determine the laser groove depth and width by calculating the equivalent fluence along the midpoint of the groove and by extracting the single-pulse ablation properties (threshold fluence, spot radius, and energy penetration depth) of the dielectrics is presented. The modeled groove dimensions match with the experimentally measured values, thereby allowing precise patterning of the dielectrics. Micromachining considerations such as the ablation rate and ablation efficiency are calculated from the modeled groove depth which enables one to opt...

Novel method for determining front-side metallisation-induced recombination parameters in silicon solar cells

Metallisation of phosphorus-doped silicon surfaces using screen-printed silver pastes is a key process in the production of silicon wafer solar cells. The metal-silicon interface in a solar cell is a highly recombination-active region that impacts the device voltage. In order to optimize screen printing for silicon wafer solar cells it is necessary to reliably determine recombination parameters at the metal-silicon interface. A novel method is presented in this paper to determine such parameters for multicrystalline silicon solar cells by applying photoluminescesnce (PL) imaging at various illumination intensities on finished cells and test structures with a varying front metallisation fraction.