Towards a Silicon Dioxide Electret for Silicon Solar Cells

An electret is a dielectric material exhibiting a quasi-permanent electrical charge. An electret of silicon dioxide on silicon offers the dual advantages of excellent chemical and field-effect passivation. It is thus ideally suited to the passivation of high-efficiency silicon solar cells. We engineer an electret of SiO2 was engineered via a simple process of corona charging and rapid thermal annealing (RTA). The initially surface-located charge is redistributed into energetically deeper states, where it is resistant to IPA rinse. Photoconductance lifetime measurements confirm this phenomenon. In addition, it is found that the passivation provided by the SiO2 electrets did not degrade any more rapidly than the passivation provided by uncharged SiO2 when exposed to (i) a room environment and (ii) a damp-heat environment over a period of months. The magnitude of charge is determined by combined Kelvin probe and capacitance-voltage measurements – it depends on charging time, and is adjustable between ~2 and 5×1012 cm-2; this positive charge density is equivalent to that regularly measured in amorphous SiNx.