Development of a Low Temperature Silver Paste for High Efficiency Screen-Printed Solar Cells
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The screen-printing technology provides a low cost high-throughput approach to good contacts for silicon solar cells. However, currently screen-printed contacts are formed at the expense of slight performance and fill factor loss. The front grid contact is particularly important and requires low contact resistance, high shunt resistance, and low junction recombination for high fill factor. Often contacts are fired in the moderate to high temperature range (750-800 degrees C) to achieve low series resistance. However, high temperature firing can lead to junction shunting and recombination, which degrades fill factor. Moreover shallow or higher sheet resistance emitters (50-100 Ω/sq) are desirable for high performance, which makes devices even more vulnerable to high temperature firing. Therefore, in this study, we modify the paste composition by adding some dopants and additives to lower the peak firing temperature for good ohmic contacts. This also reduces the wafer bowing and enhances SiN-induced defect hydrogenation in multicrystalline silicon substrates. The results show that increasing the additives concentration lowered the optimum firing temperature from 780 to 720 degrees C. In addition, the ideality factor is reduced significantly at the lower firing temperature. Thus additives used in this study were able to lower the peak firing temperature and increase the fill factor without hurting the series resistance. Fill factor of 0.774 on textured CZ was obtained at ~720 degrees C peak firing temperature for paste G (SOL9807). These pastes were formulated at Heraeus. Heraeus paste formulation differs in the nature and the amount of additives in the pastes.