John S. Renshaw

Understanding and Use of IR Belt Furnace for Rapid Thermal Firing of Screen-Printed Contacts to Si Solar Cells

We have simulated the rapid thermal firing process using a high-throughput conveyor belt furnace to study the physics of solar cell contact formation in mass production. We show that as sinter dwell time decreases, a lower Ag finger contact resistance is observed. Scanning electron micrographs reveal a correlation between glass thickness at the Ag/Si finger interface and Ag finger contact resistance. Secondary ion mass spectrometry shows that glass-frit and Ag emitter penetration are controlled by sinter dwell time. The observed trends in contact formation lead to lower series resistance, higher fill factors, and greater efficiencies with rapid firing.

Investigation of the Mechanism Resulting in low Resistance Ag Thick-Film Contact to Si Solar Cells in the Context of Emitter Doping Density and Contact Firing for Current-Generation Ag Paste

Screen-printed thick-film Ag metallization has become highly successful in crystalline Si (c-Si) photovoltaics. However, a complete understanding of the mechanism resulting in low resistance contact is still lacking. In order to shed light on this mechanism for current-generation Ag paste, Si solar cells were fabricated using a range of emitter doping densities and contact firing conditions. Low resistance contact was found to vary as a function of emitter surface P concentration ( [Psurface]) and peak firing temperature. Scanning electron microscope (SEM) analysis revealed thin interfacial glass films (IGF) under the bulk Ag gridline. SEM analysis also showed increasing Ag crystallite density as both emitter [Psurface] and peak firing temperature increased. Two mechanisms are proposed in forming low resistance contact to highly doped emitters: 1) formation of ultrathin IGF and/or nano-Ag colloids at low firing temperature, and 2) formation of Ag crystallites at high firing temperature. However, on lightly doped emitters, low resistance contact was achieved only at higher firing temperatures, concomitant with increasing Ag crystallite density, and suggests that thin IGF decorated with nano-Ag colloids may not be sufficient for low resistance contact to lightly doped emitters.

3D-modeling of a back point contact solar cell structure with a selective emitter

Three dimensional numerical simulations were performed to investigate a novel high efficiency back contact solar cell design with a selective emitter. The effect of several physical parameters (bulk lifetime, substrate doping, emitter fraction and surface recombination velocity in the gap between the emitter and BSF) on solar cell performance is explored using the SENTAURUS DEVICE™ program (formerly DESSIS™). It is found that efficiencies in excess of 22 and 20.8 percent can be achieved on p and n type substrates respectively with a bulk lifetime of 300 microseconds.

Device optimization for screen printed interdigitated back contact solar cells

Two dimensional simulations were performed to asses the potential for screen printed interdigitated back contact solar cells. In this work we optimized the design of the rear back surface field and emitter for screen printed contacts in conjunction with the design of the front surface field for best performance. With these optimized diffusion profiles we then explored the best cell design by varying the pitch, the gap between the n+ and p+ regions and the base resistivity. Model calculations provide guidelines for designing screen printed IBC solar cells given certain limitations on base resistivity or ability to create a small gap between the n+ and p+ diffusion. In these simulations care was taken to assign realistic parameters to cell design, wafer quality, and cell dimensions that are achievable for screen printing technology. Through these simulations we show the potential for a 22% efficient solar cell with screen printed contacts. Higher emitter fraction, smaller gap, and opaque diffused regions play an important role in attaining high efficiency screen printed interdigitated back contact solar cells.

Development and Understanding of High-Efficiency Screen-Printed Concentrator Silicon Solar Cells

Concentrator cells have the potential to reduce the usage of semiconductor material while producing high efficiency and more power density in a cell. Silicon solar cells now provide a unique opportunity for low-cost concentrator systems that are suitable for low to medium (2-20X) concentration because cell technology and screen-printed contacts have improved considerably. In this paper, we report on the understanding and development of low-cost manufacturable screen-printed concentrator solar cells. Computer modeling was performed first to show that, under 10% metal coverage, it is possible for screen-printed cells to have series resistance that is low enough (<;0.29 Ω·cm2) to maintain high efficiency at low to medium concentrations. This was validated by design and fabrication of 40.56-cm2 screen-printed cells using an industrial feasible process that achieved 18.8% peak efficiency at ~6 suns and 17.2% efficiency at 20 suns. Dicing a 9.9-cm2 cell, which reduces the line resistance, raised the peak efficiency to 18.9% at 10 suns and 18.5% at 20 suns. Model calculations are performed to quantitatively establish the requirements for ~20% screen-printed 2-20X concentrator cells.

Crystalline silicon solar cells with segmented selective emitter by ultraviolet laser doping

A solar cell design with a UV laser doped segmented selective emitter is reported. Several different laser settings are explored to determine the optimum power for this process and it is found that the pitch between the segmented n⁺⁺ regions is critical to the short circuit current (J_sc) of the cell. An increase of 0.4 mA/cm² is seen in the J_sc when the pitch is increased from 50 μm to 200 μm while maintaining the fill factor of 79%.