Prabir Kanti Basu

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.

Passivation of Boron-Doped Industrial Silicon Emitters by Thermal Atomic Layer Deposited Titanium Oxide

Passivation of p⁺ -doped silicon is demonstrated by using water (H₂O)-based thermal atomic layer-deposited titanium oxide (TiO_x) films. Emitter saturation current density (J_{0 e}) values below 30 fA/cm² are obtained on textured p⁺ -doped samples with a sheet resistance in the 80-120 Ω/sq range. This low emitter saturation current density would allow open-circuit voltages up to 720 mV when this TiO_x film is used in n-type silicon wafer solar cells with a front boron emitter. In addition, the optical properties of TiO_x make it an excellent option for use as antireflection coating on the silicon wafer solar cell after encapsulation. Thus, the results demonstrated in this paper could enable interesting new routes for future high-efficiency n-type silicon wafer solar cells.

Single-Component Damage-Etch Process for Improved Texturization of Monocrystalline Silicon Wafer Solar Cells

A new saw damage-etch process based on a hot sodium hypochlorite (NaOCl) solution is reported here. This process performs simultaneous damage removal and oxide masking of raw c-Si wafers in a single step. NaOCl is a strong oxidizing agent, and during the NaOCl damage-etch process, the oxide grown remains present even after the completion of the process. This oxide layer acts as protective mask during alkaline texture to form uniform and small (~2-4 μm height) pyramids on the 〈1 0 0〉 Si wafer surface. Unlike chemical vapor deposited silicon nitride or silicon dioxide protective masking processes reported by other researchers, this new damage-etch process is cost effective. It is also a single-component damage-etch process using only NaOCl solution. Thus, it involves easy bath preparation and performs in situ chlorine cleaning. Using the new damage-etch process, optimized texturing of the wafers is ascertained by electron microscopy and reflectivity studies of the textured surfaces. This new process is applied in the industrial R&D pilot line of the Solar Energy Research Institute of Singapore (SERIS) to fabricate screen-printed 156-mm pseudosquare p-type solar cells with tube-diffused emitters to yield efficiencies of over 18%.

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.