A comprehensive study of hole collection in heterojunction solar cells

We conduct a systematic investigation into the mechanism of hole collection in amorphous/crystalline silicon heterojunctions solar cells using transient-capacitance techniques. The devices are formed by depositing undoped amorphous silicon (i layer) followed by p-type amorphous silicon on n-type crystalline silicon wafers. For i layers varying from 3.2 to 96 nm, we find only a factor of four change in hole collection-rate at low temperature where thermal emission over the valence band offset is precluded. We conclude that holes traverse the i layer by hopping through defects rather than direct tunneling through the entire i layer. This process is weakly thermally activated with a rate above 1 x 10(4) s(-1) at room temperature. Near room temperature and with thick i layers, we observe hole collection with a high activation energy that depends on measurement conditions. We demonstrate that hopping through defects is the dominant mode of hole collection for solar cell operation at room temperature and above. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4764031]