Kenta Nakayashiki

Effect of Ag Particle Size in Thick-Film Ag Paste on the Electrical and Physical Properties of Screen Printed Contacts and Silicon Solar Cells

The impact of the Ag particle (metal powder in the screen printed paste) size on the quality of Ag thick-film ohmic contacts to high-sheet-resistance emitters of Si solar cells is investigated. Spherical particle size wasvaried in the range of 0.10-10 μm (ultrafine to large). Even though ultrathin glass regions are achieved for the large particle paste, giving low specific contact resistance (ρ c ), secondary ion mass spectroscopy measurements showed a higher Ag concentration (>10 1 5 cm - 3 ) at the p-n junction that increased the junction leakage current (J o 2 ) and decreased the V o c by ∼7 mV and fill factor (FF) by ∼0.02. Pastes with ultrafine Ag particles generally produced a thick glass layer at the Ag-Si contact interface, which increased ρ c and series resistance (R s ) (≥ 1 Ω cm 2 ), and lowered the FF by ∼0.03. Small to medium size Ag particles in the paste produced thin glass regions and many regularly distributed Ag crystallites at the contact interface. This resulted in low R s (< 1 Ω cm 2 ), high shunt resistance (60,558 Ω cm 2 ), low J o 2 (∼20 nA/cm 2 ), and high FF (0.781). Cell efficiencies of ∼17.4% were achieved on untextured float zone Si with 100 Ω/□ emitter by rapid firing of screen printed contacts in a lamp-heated belt furnace.

Factors Limiting the Formation of Uniform and Thick Aluminum–Back-Surface Field and Its Potential

Theoretical calculations reveal that the quality of an aluminum-back-surface field (BSF) in a silicon solar cell can be improved by either increasing the thickness of the deposited aluminum (Al), peak alloying temperature, or both. However, this study shows that there is a critical temperature for a given screen-printed Al thickness, above which the BSF quality begins to degrade because of nonuniformity triggered by the agglomeration of Al-Si melt in combination with the bandgap narrowing resulting from the high doping effect in the agglomerated regions. It is found that this critical temperature decreases with the increase in the thickness of the deposited Al layer and, therefore, limits the quality and thickness of the Al-BSF that can be achieved before degradation sets in. This nonuniformity of Al-BSF is observed in the form of scattered Al bumps with thick and thin BSF regions. A combination of experimental results and model calculations is used to provide improved understanding and guidelines for choosing the optimal combination of Al thickness and alloying temperature.

High-efficiency solar cells on edge-defined film-fed grown (18.2%) and string ribbon (17.8%) silicon by rapid thermal processing

Solar cell efficiencies of 18.2 and 17.8% were achieved on edge-defined film-fed grown and string ribbon multicrystalline silicon, respectively. Improved understanding and hydrogenation of defects in ribbon materials contributed to the significant increase in bulk lifetime from 1–5 μs to as high as 90–100 μs during cell processing. It was found that SiNx-induced defect hydrogenation in these ribbon materials takes place within one second at 740–750 °C. The bulk lifetime decreases at annealing temperatures above 750 °C or annealing times above one second due to the enhanced dissociation of the hydrogenated defects coupled with the decrease in hydrogen supply from the SiNx film deposited by plasma enhanced chemical vapor deposition.

Boron Diffusion with Boric Acid for High Efficiency Silicon Solar Cells

A boron diffusion process using boric acid as a low cost, nontoxic spin-on source is introduced. Using dilute solutions of boric acid, sheet resistances ranging from 20 to 200 Ω/□ were achieved, along with saturation current densities as low as 85 fA/cm 2 . These results indicate that boric acid is a suitable source for forming both p + emitters and back surface fields for high efficiency n- and p-type solar cells. The degradation of the minority carrier bulk lifetime, which is a common efficiency-limiting characteristic of low cost boron sources, was also minimized through the use of a high purity boric acid source. The ability to achieve low sheet resistances, high bulk lifetimes and low saturation current densities with boric acid were exploited to achieve a 19.7% efficient screen printed solar cell exhibiting a bulk lifetime >400 μs.

Concentration and penetration depth of H introduced into crystalline Si by hydrogenation methods used to fabricate solar cells

The hydrogenation of crystalline Si by methods used to passivate defects in Si solar cells has been studied by infrared spectroscopy. For these experiments, floating-zone Si that contained Pt impurities that act as traps for H was used as a model system in which H could be directly detected. In this model system, the concentration and indiffusion depth of H were determined for different hydrogenation treatments so that their effectiveness could be compared. The postdeposition annealing of a hydrogen-rich SiNx surface layer was found to introduce H into the Si bulk with a concentration of ∼1015cm−3 under the best conditions investigated here.

Investigation of Modified Screen-Printing Al Pastes for Local Back Surface Field Formation

This paper reports on a low-cost screen-printing process to form a self-aligned local back surface field (LBSF) through dielectric rear surface passivation. The process involved formation of local openings through a dielectric (SiNx or stacked SiO2/SiNx) prior to full area Al screen-printing and a rapid firing. Conventional Al paste with glass frit degraded the SiNx surface passivation quality because of glass frit induced pinholes and etching of SiNx layer, and led to very thin LBSF regions. The same process with a fritless Al paste maintained the passivation quality of the SiNx, but did not provide an acceptably thick and uniform LBSF. Al pastes containing appropriate additives gave better LBSF because of the formation of a thicker and more uniform Al-BSF region. However, they exhibited somewhat lower internal back surface reflectance (<90%) compared to conventional Al paste on SiNx. More insight on these competing effects is provided by fabrication and analysis of complete solar cells

Effect of material inhomogeneity on the open-circuit voltage of string ribbon Si solar cells

The effect of material inhomogeneity on open-circuit voltage (V/sub OC/) of string ribbon Si solar cells is investigated by a combination of experimental results and a simple analytical model. Light beam-induced current (LBIC) measurements showed that a cell with no detectable defective region gave an efficiency of 15.9% with a V/sub OC/ of 616 mV. However, a neighboring cell with highly defective regions covering 38% of its area, as determined by LBIC measurements, gave an efficiency of 14.1% with V/sub OC/ of 578 mV. Another cell with 19% highly defective regions gave an efficiency of 15.0% with V/sub OC/ of 592 mV. A simple and approximate analytical model was developed to quantify the loss in V/sub OC/ on the basis of recombination intensity and the area fraction of defective regions. This model showed that the majority of the loss in V/sub OC/ is associated with the most defective region, even if its area fraction is relatively small.

Minority-carrier lifetime enhancement in edge-defined film-fed grown Si through rapid thermal processing-assisted reduction of hydrogen-defect dissociation

This paper demonstrates that a very short, 1‐s, simultaneous firing of screen-printed Al at the back and SiNx antireflection (AR) coating at the front can significantly enhance the minority-carrier lifetime in edge-defined film-fed grown (EFG) ribbon Si via SiNx-induced hydrogen passivation of defects. It was found that 1‐s firing in a rapid thermal processing system at an optimum temperature improved the average minority-carrier lifetime from 3to>80μs, resulting in ∼16% efficient 4‐cm2 screen-printed EFG Si cells. It is proposed that rapid thermal firing enhances the retention of hydrogen at defect sites by minimizing the hydrogen-defect dissociation. A combination of simulations and experiments reveals that the dissociation of hydrogen is extremely rapid at conventional firing temperatures of ∼700°C. An activation energy of 2.4–2.6eV was determined for the hydrogen-defect dissociation in EFG Si. This activation energy, in conjunction with the room-temperature photoluminescence data, suggests that the im...

Investigation of high-efficiency screen-printed textured Si solar cells with high sheet-resistance emitters

In this study it is found that the efficiency enhancement (/spl Delta//spl eta/) resulting from the use of a 100 /spl Omega//sq emitter instead of a conventional 45 /spl Omega//sq emitter is substantially enhanced further by surface texturing. This enhancement is greater for textured cells by at least /spl sim/0.4% absolute over the enhancement for planar cells, and is mainly due to the greater difference in the front-surface recombination velocity (FSRV) between the high-and low-sheet-resistance emitter textured cells. A FSRV of 60,000 cm/s resulted in a reasonably good V/sub oc/ of /spl sim/642 mV for the 100 /spl Omega//sq emitter textured cell. Our investigation of the Ag-Si contact interface shows a more regular distribution of Ag crystallite precipitation for the textured emitter (mainly at the peaks of the texture pyramids). The high contact-quality resulted in a series resistance of 0.79 /spl Omega/-cm, a junction leakage current of 18.5 nA/cm/sup 2/ yielding a FF of 0.784. This resulted in a record high-efficiency 4 cm/sup 2/ screen-printed cell of 18.8% (confirmed by NREL) on textured 0.6 /spl Omega/-cm FZ, with single-layer antireflection coating.

Defect passivation in multicrystalline silicon for solar cells

We report on the effect of hydrogen passivation in ribbon multicrystalline silicon (mc-Si) wafers from SiNx:H anti-reflecting layer using simultaneous rapid thermal annealing of Al back-contact and SiNx anti-reflection coating on the front (RTP-Al∕SiNx). Scanning room-temperature photoluminescence spectroscopy revealed a strong inhomogeneity in the increase of minority carrier lifetime caused by the hydrogen defect passivation in mc-Si. We present experimental evidence that RTP-Al∕SiNx processing leads to strong lifetime enhancement caused by hydrogen defect passivation in low-lifetime regions of mc-Si wafers. Additional details on the hydrogenation mechanism are revealed in a course of the dehydrogenation study. Hydrogen out-diffusion shows a different rate or activation energy between high and low lifetime regions of the wafers.