Hemant P. Mungekar

Flame Radiation and Soot Emission From Partially Premixed Methane Counterflow Flames

Motivated by heat transfer and environmental concerns, a study of flame radiation and soot particulate emission is reported for partial premixing in low strain-rate (<20 s -1 ) methane counterflow flames. Temperature, OH concentration, and soot volume fraction distributions were measured along the stagnation streamline for progressive addition of oxygen to methane. These measurements along with an optically thin model for soot and gas radiation were used to study the effect of partial premixing on flame radiation and soot emission. It was found that with progressive partial premixing, the peak soot loading and the thickness of the soot zone first decreased and then increased, and while the gas radiation was enhanced, the gas radiative fraction (gas radiation per unit chemical energy release) showed a systematic decrease. The net radiative fraction (soot+gas), however, first decreased and then increased. A configuration with the soot zone spatially entrapped between the premixed and non-premixed reaction zones was experimentally found. This flame configuration has the potential to enhance radiative heat transfer while simultaneously reducing soot and NO x emissions.

Optimization of Rear Local Contacts on High Efficiency PERC Solar Cells Structures

A local contact formation process and integration scheme have been developed for the fabrication of rear passivated point contact solar cells. Conversion efficiency of 19.6% was achieved using  mm, pseudo square, p-type single crystalline silicon wafers. This is a significant improvement when compared to unpassivated, full area aluminum back surface field solar cells, which exhibit only 18.9% conversion efficiency on the same wafer type. The effect of rear contact formation on cell efficiency was studied as a function of contact area and contact pitch, hence the metallization fraction. Contact shape and the thickness of Al-BSF layer were found to be heavily dependent on the laser ablation pattern and contact area. Simulated cell parameters as a function of metallization showed that there is a tradeoff between open circuit voltage and fill factor gains as the metallization fraction varies. The rear surface was passivated with an Al2O3 layer and a capping layer. The rear surface contact pattern was created by laser ablation and the contact geometry was optimized to obtain voids free contact filling, resulting in a uniform back surface field. The efficiency gain in rear passivated cells over the reference cells is mainly due to improved short circuit current and open circuit voltage.

Development of high efficiency mono-crystalline silicon solar cells: Optimization of rear local contacts formation on dielectrically passivated surfaces

An integration process was developed for the fabrication of rear passivated point contact solar cells achieving 19.36% conversion efficiency by using 156×156mm, pseudo square, p-type single crystalline silicon wafers. This is a significant improvement when compared to unpassivated, full area aluminum back surface field solar cells, which exhibit only 18.64% conversion efficiency on the same wafer type. The rear surface was passivated with a Al2O3 layer and a SiNX capping layer. The thicknesses of individual films were optimized to obtain maximum minority carrier lifetimes. The rear surface contact pattern was created by laser ablation and the contact geometry was optimized to obtain voids free contact filling resulting in a uniform back surface field. Internal quantum efficiency and reflectance measurement show significant improvement in rear passivated cells in the infrared wavelength region in comparison to reference cells. The rear surface internal reflectivity for the passivated cell was 93% while that for the reference cell was only about 73%. The rear surface recombination velocity for the rear passivated cell was about 52 cm/s while that for the reference cell was about 300 cm/s. The efficiency gain in rear passivated cells over the reference cells is mainly due to improved short circuit current and open circuit voltage. However, rear passivated solar cells show lower fill factors due to increased series resistance.

Development of no-rinse screen printable etch paste for contact via in dielectric films

Screen printable etch paste was developed for low cost patterning of dielectric films. This technique offers a simple print and bake approach to pattern without the need for a rinse step. Residue analysis of the etched area showed no sign of any organic residue left behind by the etch process. High carrier lifetime indicates no significant contamination or silicon surface damage. The etched via diameter of 150 µm was demonstrated in thermal oxide while smaller via size of 100 µm was achieved in silicon nitride (SiNx) or a combination of silicon oxide and nitride. Contact resistance measurement through the etched vias using TLM method showed values less than 3mΩ.cm2. The etch paste and process technique can be easily used in PERC cell as well as advanced architecture Interdigitated Back Contact (IBC) cell for low cost patterning.

Progress in production-worthy point contact solar cells process

This work reports on the integration process of rear point contact solar cells with reduced recombination and better light trapping than the conventional cells. Al₂O₃/SiN_x passivation stacks were used to ensure the backside passivation and the effective lifetime of minority carrier is found to be >;100 μs (estimated surface recombination velocity ~50 cm/s) on solar p-type Cz wafers. The estimated fixed charge associated with the Al₂O₃ is in the range of -5e12 C/cm². Laser ablation of Al₂O₃/SiN_x stack and aluminum alloying are studied in detail to understand the process window for clean ablation and back surface field. It is found that laser energy plays an important role cleanly ablating the Al₂O₃/SiN_x passivation stack. The solar cell is fabricated using standard processes based on screen-printed aluminum paste onto laser ablated passivation layer consisting of Al₂O₃/SiN_x. Aluminum alloying is dependent on the firing profile and amount of Al melted into Si. Back point contact cells show improvements in J_sc by 1 mA/cm² and V_oc by 10 mV due to better response in infrared spectrum. The best conversion efficiency of back point contact solar cells fabricated with standard industrial emitter and backside passivation is 19.35%.

Feature Evolution During Sub 100NM Gap-Fill and Etch

Inductively coupled plasma (ICP) reactors are being used at low gas pressure ( 1013 /cm2 ) processes in semiconductor fabrication. In these reactors plasma is generated by inductively coupled electric field while positive ions are accelerated anisotropically by applying a negative bias RF to the substrate. Semiconductor manufacturers face many challenges as wafer size increases while device geometries decrease. Two key challenges for both process design and electronics processing equipment design are (a) scale up of process from 200mm to 300mm diameter substrate, and (b) deposition and etching features with high aspect ratios. A unified phenomenological model to explain profile evolution trend as a function of aspect ratio for deposition (gap fill) and trench etch using ICP reactors is presented. Trends for feature evolution as a function of pressure for gap fill and trench etch are reviewed and explained. The article emphasizes importance of low pressure for sub-100nm gap-fill and trench-etch applications in ICP processing reactors.Copyright