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 × 10⁴ 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.