Isolation of Hole Versus Electron Current at p-Si/TiO₂ Selective Contact Using a Heterojunction Bipolar Transistor Structure

Carrier-selective contacts provide an exciting avenue for developing high-efficiency, low-cost silicon photovoltaics (PV). However, evaluating and understanding the different current mechanisms across a carrier-selective contact is difficult as the current measured represents the sum of both electron and hole current components. In this paper, we develop a heterojunction bipolar transistor (HBT) structure with the electron-selective p-type Si/titanium dioxide(TiO₂)/Al contact as the base-emitter junction, which enables one to separately measure the electron and hole currents across the selective contact. An HBT with a current gain as large as ∼220 is achieved. The method is then used to evaluate the current mechanisms across a p-Si/TiO₂/Al heterojunction PV cell, where the TiO₂/p-Si replaces the n⁺-p junction. We determine that there is an optimal TiO₂ thickness of 4.1 nm for CVD-deposited TiO₂; and at the optimal thickness, the hole current is 8% of the total current, thus demonstrating that TiO₂/Si is indeed a hole-blocking electron-selective contact. The hole current ratio is corroborated with reverse-recovery experiments, confirming the validity of the HBT method.