The average sheet resistance of standard solar cells has been increased from <60 ohm/sq to over 90 ohm/sq in recent years. This has been achieved by reducing the doping concentration in the emitter, improving the absorption of blue light, and reducing the recombination of charge carriers in the emitter. The impact of two factors on the interaction between silicon and silver has now become more critical. The Schottky Hall Barrier is increased when the doping concentration of the emitter is reduced. This results in an increase in contact resistance. The second effect of the reduced doping concentration is a strong reduction of the so-called dead layer, which consists of non-conducting Phosphor, acting as recombination centers. Reducing the dead layer decreases the recombination of charge carriers in the emitter, making the emitter more transparent. In a highly transparent emitter, the recombination below the metal fingers has relative higher impact on losses in voltage and current. Improvements in the contact mechanism of silver paste are essential to fully utilize the improved emitters. Using state-of-the-art silver pastes results in a low-contact resistance (FF) and excellent passivation below the metal fingers (VOC, ISC).*
Due to the lower surface concentration, the contact resistance between the silicon and silver increases. Improvements in the contact formation keeps this contact resistance low. But a lower doping in the emitter results in a higher transparency: less recombination in the emitter, so more charge carriers make it to the surface. Without adjusting the contact formation for this, more and more charges would be lost on the silver/silicon interface. In order to make fully use of the improvements in the emitter, both contact resistance and passivation below the silver fingers is essential.
* FF= Fill Factor, VOC = Open-Circuit Voltage, ISC = Short-Circuit Current