Ankit Khanna

A Fill Factor Loss Analysis Method for Silicon Wafer Solar Cells

The fill factor of silicon wafer solar cells is strongly influenced by recombination currents and ohmic resistances. A practical upper limit for the fill factor of crystalline silicon solar cells operating under low-level injection is set by recombination in the quasi-neutral bulk and at the two cell surfaces. Series resistance, shunt resistance, and additional recombination currents further lower the fill factor. For process optimization or loss analysis of solar cells, it is important to determine the influence of both ohmic and recombination loss mechanisms on the fill factor. In this paper, a method is described to quantify the loss in fill factor due to series resistance, shunt resistance, and additional recombination currents. Only the 1-Sun J-V curve, series resistance at the maximum power point, and shunt resistance need to be determined to apply the method. Application of the method is demonstrated on an 18.4% efficient inline-diffused p-type silicon wafer solar cell and a 21.1% efficient heterojunction n-type silicon wafer solar cell. Our analysis does not require J-V curve fitting to extract diode saturation current densities or ideality factor; however, the results are shown to be consistent with curve fitting results if the cells two-diode model parameters can be unambiguously determined by curve fitting.

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.

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.

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.