Interface Recombination in TiO2/Silicon Heterojunctions for Silicon Photovoltaic Applications

Abstract

Solar photovoltaics (PV), the technology that converts sunlight into electricity, has

immense potential to become a significant electricity source. Nevertheless, the laws

of economics dictate that to grow from the current 2% of U.S. electricity generation

and to achieve large scale adoption of solar PV, the cost needs to be reduced to the

point where it achieves grid parity. For silicon solar cells, which form 90% of the

PV market, a significant and slowly declining component of the cost is due to the

high-temperature (> 900 °C) processing required to form p-n junctions. In this thesis,

the replacement of the high-temperature p-n junction with a low-temperature amorphous

titanium dioxide (TiO2)/silicon heterojunction is investigated. The TiO2/Si

heterojunction forms an electron-selective, hole-blocking contact. A chemical vapor

deposition method using only one precursor is utilized, leading to a maximum deposition

condition of 100 °C. High-quality passivation of the TiO2/Si interface is achieved,

with a minimum surface recombination velocity of 28 cm/s. This passivated TiO2 is

used in a double-sided PEDOT/n-Si/TiO2 solar cell, demonstrating an open-circuit

voltage increase of 45 mV. Further, a heterojunction bipolar transistor (HBT) method

is developed to investigate the current mechanisms across the TiO2/p-Si heterojunction,

leading to the determination that 4nm of TiO2 provides the optimal thickness.

And finally, an analytical model is developed to explain the current mechanisms observed

across the TiO2/Si interface. From this model, it is determined that once

ΔEV (TiO2/Si) is large enough (400 meV), the two key parameters are the Schottky

barrier height (resulting in band-bending in silicon) and the recombination velocity

at the TiO2/Si interface. Data corroborates this, indicating the hole-blocking mechanism

is due to band-bending induced by the unpinning of the Al/Si interface and

TiO2 charge, as opposed to due to the TiO2 valence band edge.