Microcrystalline Silicon Tunnel Junction for Monolithic Tandem Solar Cells Using Silicon Heterojunction Technology

In this article, we developed a microcrystalline silicon tunnel junction to be used as a tunnel recombination junction between a large-gap top cell and a silicon heterojunction bottom cell, in a monolithic tandem integration. This junction is composed of a p-type layer on the top of an n-type layer, deposited by plasma-enhanced chemical vapor deposition at low temperature (200 °C). Microcrystalline phase percentage was controlled with Raman spectroscopy and ellipsometry measurements. The total stack has a thickness of 40 nm and an average conductivity around 10 S/cm. Minority carrier lifetime measurements showed an improvement of the field effect and the passivation with the addition of this junction on top of silicon heterojunction solar cells. Moreover, implementation of microcrystalline layer on top of reference rear emitter silicon heterojunction solar cells improved the fill factor and did not induce parasitic absorption above 700 nm. Simple test structures were fabricated in order to characterize the tunnel junction and optimize it. Then, we carried out dark temperature-dependent I-V measurements on those test structures and observed peaks and valleys, characteristic of the junction tunnel behavior. The developed tunnel junction shows low contact resistivity and activation energies; these are promising results for the complete integration on the tandem device.