Brian Rounsaville

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.

The Study of Silane-Free SiC x N y Film for Crystalline Silicon Solar Cells

We deposited plasma-enhanced chemical vapor deposition silicon carbon nitride (SiC x N y ) antireflection coating and passivation layers using a silane-free process. We used a solid polymer source developed at SiXtron Advanced Materials to eliminate the storage and handling of dangerous pyrophoric silane gas. We used ammonia flow rate as a control for the chemical and optical properties in the silane-free process. As NH 3 flow rate increases, the carbon content, refractive index, extinction coefficient, and surface charge density of the film decrease. At an ammonia flow rate of 3000 sccm, which is similar to the conventional SiN x , the extinction coefficients for the two films were similar. This led to an emitter dark saturation current density (J oe ) of 404 fA/cm 2 for the two films on 45 Ω/□ emitters. However, a stack passivation of SiO 2 /SiC x N y on an 80 Ω/□ emitter resulted in an emitter dark saturation current density of 95 fA/cm 2 , which is enough to provide a good surface passivation for high efficiency solar cells. An energy conversion efficiency of 17.4% was obtained for a 149 cm 2 textured Czochralski screen-printed solar cell with this stack passivation. For a 156 cm 2 nontextured multicrystalline silicon, with only SiC x N y and a 45 Ω/□ emitter, we obtained 14.9% efficiency.

Capitalizing on the Glass-Etching Effect of Silver Plating Chemistry to Contact Si Solar Cells With Homogeneous 100–110

Homogeneous high-sheet-resistance emitter (HHSE), excellent surface passivation, and high-quality contacts, along with narrow gridlines, are needed for high-efficiency solar cells. However, HHSE in conjunction with screen-printed (SP) contacts often gives low fill factor (FF) because of high contact resistance. We capitalized on the glass-etching property of light-induced plating of silver to decrease the contact resistance and formed high-quality contacts to 100-110 Ω/sq HHSE. This led to the achievement of 78.5% FF, 38.3 mA/cm2 short-circuit current density (JSC) due to narrow line widths (65 μm), and efficiency of 18.7%.

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Two-dimensional numerical simulations were performed to derive design rules for low-cost, high-efficiency interdigitated back contact (IBC) solar cells on a low-cost substrate. The IBC solar cells were designed to be fabricated using either the conventional screen printing or photolithography metallization processes. Bulk lifetime, bulk resistivity, contact spacing (pitch), contact opening width, recombination in the gap between the p+ BSF and n+ emitter, and the ratio of emitter width to pitch have been used as key variables in the simulations. It is found that short circuit current density (Jsc) is not only a strong function of the bulk lifetime but also the emitter coverage of the rear surface. Fill factor (FF) decreases as the emitter coverage increases because the majority carriers need to travel a longer distance through the substrate for longer emitter width. The simulated IBC results were compared with those for conventional screen printed solar cells. It was found that the IBC solar cell outperforms the screen printed (SP) solar cell when the bulk lifetime is above 50 mus due to higher Voc and Jsc , which suggests that higher performance can be realized on low-cost substrates with the IBC structure

Emitters

In this paper we report on the design, fabrication and modeling of 49 cm2, 200-mum thick, 1-5 Omega-cm, n- and p-type lang111rang and lang100rang screen-printed silicon solar cells. A simple process involving RTP front surface phosphorus diffusion, low frequency PECVD silicon nitride deposition, screen-printing of Al metal and Ag front grid followed by co-firing of front and back contacts produced cell efficiencies of 15.4% on n-type lang111rang Si, 15.1% on n-type lang100rang Si, 15.8% on p-type lang111rang Si and 16.1% on p-type lang100rang Si. Open circuit voltage was comparable for n and p type cells and was also independent of wafer orientation. High fill factor values (0.771-0.783) for all the devices ruled out appreciable shunting which has been a problem for the development of co-fired n-type lang100rang silicon solar cells with Al back junction. Model calculations were performed using PC1D to support the experimental results and provide guidelines for achieving >17% n-type silicon solar cells by rapid firing of Al back junction

2D-Modeling and Development of Interdigitated Back Contact Solar Cells on Low-Cost Substrates

In this paper we report on the formation of high quality contacts to HHSE with a sheet resistance variation of ≥30 Ω/sq using a commercial front-side Ag paste, PV16A from DuPont. We fabricated and characterized solar cells with emitter sheet resistances of 65, 75, 85, 95 and 105 Ω/sq. We found that emitter sheet resistances in the range of 65–95 Ω/sq can be contacted with low average series resistance of 0.63–0.77 Ω-cm² and high fill factor (FF) of 77.4–78.8%. The 75 Ω/sq emitter gave the best average efficiency of 18.3%, followed by 18.2% for the 65 Ω/sq, 18.0% for the 85 Ω/sq, and finally 17.7% for the 95 Ω/sq. The 105 Ω/sq emitter gave a low FF due to high series resistance, but the shunt resistance and ideality factor were excellent, which suggests the paste was not encroaching the shallow emitter junction.

Rapid Thermal Processing of High Efficiency N-Type Silicon Solar Cells with Al Back Junction

Fine and tall grid lines on high sheet resistance emitter can increase the efficiency of silicon solar cells by reducing the shadow loss and improving the blue response. However, current screen printing pastes are neither able to provide the adequate line geometry nor make good ohmic contact to high sheet resistance emitter. This paper uses a combination of inkjet printing and light induced plating of silver to solve both problems. The ink jetting of grid lines is used to write ∼38 µm wide and ∼4 µm tall seed lines. These grid lines grow to 65 µm wide and 20–25 µm tall after light induced plating of silver. The silver plating solution penetrates the entire contact region underneath the grid lines to reduce the PbO in the glass layer to Pb metal in addition to plating the silver crystallites underneath. This lowers the contact resistance. Prolonged plating builds the line height to reduce the line resistance. A combination of low contact and line resistances produced fill factor of 79.2% on 95-Ω/sq emitter. This resulted in 239-cm2 Cz silicon solar cell efficiencies up to 18.7% with an average of 18.4%.

Formation of high quality screen-printed contacts to homogeneous high sheet resistance emitters (HHSE)

In this letter, we report on the ink-jet full gridline cells with three different Ag dosages (70, 90, and 130 mg) on 75-Ω/sq emitters. We observed that the open-circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF ) were highest for the 130-mg dose and decrease thereafter as the Ag dosage decreases. Because of the decreased FF with decreasing Ag dosage, the efficiency followed the same trend. However, we demonstrated, for the first time, fully ink-jetted front Ag gridlines with an FF of 0.765 and efficiency of 18.4% on 239-cm2 commercial CZ wafers with a sheet resistance of 75 Ω/sq . Noteworthy is the achievement of 17.8% efficiency with only 70 mg of Ag, which is ~ 68% less Ag than that used on cells with screen-printed gridlines. This represents a Ag cost saving of ~

High efficiency silicon solar cells with ink jetted seed and plated grid on high sheet resistance emitter

0.245 per 6-inch wafer compared to the screen-printed counterpart with 220 mg of Ag.