Low-Resistivity Screen-Printed Contacts on Indium Tin Oxide Layers for Silicon Solar Cells with Passivating Contacts
The capability for contact formation on indium tin oxide layers of different low-temperature screen printing silver pastes after thermal curing and photonic sintering is evaluated in detail. The 80-nm-thick indium tin oxide layers used in this study are sputtered at three different oxygen gas flows and, consequently, differ by their electrical and optical properties. All pastes are analyzed by means of simultaneous thermogravimetry-differential scanning calorimetry. After printing and contact formation processing, the lateral resistivity of the metal contacts and their contact resistivity to the indium tin oxide layers are determined. Furthermore, the microstructure of the metal electrodes is investigated to gain a deeper understanding of the underlying contact formation mechanisms. Besides low-temperature curing at Tcuring, low = 180-220°C, as conventionally applied to silicon heterojunction solar cells, photonic sintering and thermal curing at medium temperatures Tcuring, mid = 250-350°C are utilized. The higher curing temperatures in the range of Tcuring, mid enhance the sintering and densification processes of the different pastes, and thereby, lateral resistivities of 2.4-3.5 mO·cm and contact resistivities of 1.0-2.5 mO·cm 2 are achieved. The results indicate a promising industry-relevant non-firing-through metallization approach for, e.g., poly-silicon-based structures like tunnel oxide passivating contacts, which allow comparatively high temperatures for contact formation processing. With thermal curing at Tcuring, low and photonic sintering, both compatible with temperature-sensitive silicon heterojunction solar cells, lateral finger resistivities of 6.5-9.4 mO·cm and contact resistivities of 1.2-2.5 mO·cm 2 are achieved. Thereby, photonic sintering has the potential of a significantly reduced process time from several minutes down to 1-4 ms.