Vijay Yelundur

Superhydrophobic and low light reflectivity silicon surfaces fabricated by hierarchical etching.

Silicon is employed in a variety of electronic and optical devices such as integrated circuits, photovoltaics, sensors, and detectors. In this paper, Au-assisted etching of silicon has been used to prepare superhydrophobic surfaces that may add unique properties to such devices. Surfaces were characterized by contact angle and contact angle hysteresis. Superhydrophobic surfaces with reduced hysteresis were prepared by Au-assisted etching of pyramid-structured silicon surfaces to generate hierarchical surfaces. Consideration of the Laplace pressure on hydrophobized hierarchical surfaces gives insight into the manner by which contact is established at the liquid/composite surface interface. Light reflectivity from the etched surfaces was also investigated to assess application of these structures to photovoltaic devices.

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.

Enhanced silicon solar cell performance by rapid thermal firing of screen-printed metals

Rapid thermal processing (RTP) of screen-printed (SP) Al on the back and silver (Ag) grid on the front produced significant improvement in back surface field (BSF) of n/sup +/-p-p/sup +/ float-zone (FZ) Si solar cells. Two-step firing was found to form more effective BSF than co-firing, resulting in 0.6-1.0% increase in absolute cell efficiency. In addition, RTP was found to be more effective than the beltline processing (BLP), resulting in 0.5-1.0% increase in absolute cell efficiency. Although the Al-BSF formed by the BLP was inferior to the RTP, the difference between the two is virtually eliminated during the subsequent RTP contact firing. Internal quantum efficiency (IQE) analysis of the solar cells gave effective back surface recombination velocities (S/sub eff/) of >5000 cm/s and /spl sim/1500 cm/s for co-firing in the BLP and the RTP, respectively. Two-step firing produced S/sub eff/ of /spl sim/1500 cm/s and /spl sim/700 cm/s in the BLP and the RTP, respectively. However, S/sub eff/ for the two-step firing, involving BLP BSF formation followed by RTP contact firing, was found to be /spl sim/700 cm/s, which indicates that RTP contact firing with a faster ramp-up (100/spl deg/C/s) restores the poor-quality BLP BSF. On the other hand, BLP contact firing with a slow ramp-up (<10/spl deg/C/s) degrades the high-quality RTP BSF, increasing S/sub eff/ from /spl sim/700 cm/s to /spl sim/1500 cm/s.

High efficiency selective emitter enabled through patterned ion implantation

Selective emitter cell architectures offer the opportunity of improved cell efficiency over standard cell architectures through improved blue response, reduced saturation current and lower contact resistance. However, few selective emitter cell concepts have been successfully adopted into high volume manufacturing, often due to the associated increase in process complexity and cost. This paper demonstrates that patterned ion implantation provides a roadmap to lower PV module and system

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

/Wp costs through improved cell efficiency and reduced manufacturing cost. Ion implanted cell efficiency improvements, which can be up to +1% absolute, are a result of not only the selective emitter cell architecture, but also improved emitter quality, oxide passivation and increased light collection area through the elimination of laser edge isolation. Manufacturing cost reductions result from reduced processing steps and improved process uniformity and cell binning.

Effective interfaces in silicon heterojunction 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.

Fundamental understanding and implementation of Al-enhanced PECVD SiNx hydrogenation in silicon ribbons

Thin hydrogenated amorphous silicon (a-Si:H) layers deposited by hot-wire chemical vapor deposition (HWCVD) are investigated for use in silicon heterojunction (SHJ) solar cells on p-type crystalline silicon wafers. A requirement for excellent emitter quality is minimization of interface recombination. Best results necessitate immediate a-Si:H deposition and an abrupt and flat interface to the c-Si substrate. We obtain a record planar HJ efficiency of 16.9% with a high V/sub oc/ of 652 mV on p-type float-zone (FZ) silicon substrates with HWCVD a-Si:H(n) emitters and screen-printed Al-BSF contacts. H pretreatment by HWCVD is beneficial when limited to a very short period prior to emitter deposition.

Improved string ribbon silicon solar cell performance by rapid thermal firing of screen-printed contacts

Presented at the 12th International Photovoltaic Science and Engineering Conference; Jeju Island, Korea; June 11-15, 2001.

Silver Contact Grid: Inferred Contact Resistivity and Cost Minimization in 19% Silicon Solar Cells

Al-enhanced SiN/sub x/-induced hydrogenation is implemented to improve the minority carrier lifetime in string ribbon Si. Rapid cooling after the hydrogenation anneal is found to increase the spatially averaged relative lifetime enhancement by over 160% for string ribbon Si samples with a spatially averaged as-grown lifetime of 2.9 /spl mu/s. Partial coverage of back surface by Al eliminates wafer bowing in 100 /spl mu/m thick substrates, but reduces the spatially averaged lifetime enhancement to below 100% because vacancy generation at the back surface is decreased. Rapid thermal Firing (RTF) of screen-printed contacts, with high heating and cooling rates, is found to improve string ribbon solar cell efficiency by an average of 1.2% absolute over lamp heated belt furnace contact firing. Light beam-induced current (LBIC) mapping and light biased or differential internal quantum efficiency (IQE) analysis show that the enhancement in cell performance is primarily due to an improved effective diffusion length and diffusion length uniformity, which are both a result of the improved retention of hydrogen at defects achieved during rapid cooling after contact firing. Screen-printed string ribbon cells with independently confirmed efficiencies as high as 14.7% are achieved through an understanding and implementation of hydrogen passivation of defects.

Rapid thermal processing of next generation silicon solar cells

The analysis of silicon solar cell contacts having an H-bar front grid pattern is extended by enabling the contact resistivity to be inferred from the measurement of total series resistance and the determination of six individual components of series resistance. Analysis of the contact system was completed for a representative 19% cell fabricated from a 156-mm pseudosquare p-Cz wafer using standard production processes, including phosphorus ion implantation, thermal oxide surface passivation, silicon nitride deposition, and screen-printing and firing of front silver gridlines and busbars, back silver soldering pads, and back aluminum contact. Gridline width was measured to be 80 μm after firing, with an average thickness of 7.4 μm and an effective resistivity of 4.7 μΩ·cm. Contact resistivity to the uniform 91 Ω/□ emitter was inferred to be 5.3 mΩ·cm2 from the total series resistance and its components. Using these values, gridline spacing was optimized for maximum efficiency (1.7 mm, 91 lines, 19.1%,