Minimizing Trap Charge Density towards Ideal Diode in Graphene-Silicon Schottky Solar Cell.

Photovoltaic device performance of graphene/n-Si Schottky diodes are largely affected by inhomogeneous oxide formation at the interface that suppresses tunneling current of injected and photo-excited charges. The accumulated trap charges at low current induce charge recombination at the interface and degrade the ideality factor of the diode and fill factor (FF) of the solar cell. This consequently gives rise to a nonlinear current-voltage (I-V) feature in solar cells, commonly known as S-shaped kink, which can be engineered by optimizing the interface barrier-thickness or by increasing the carrier mobility. Here we present chemical and electrochemical doping methods to increase the conductivity of graphene that transforms nonlinear kink photodiodes with low FF and solar cell efficiency towards trap free linear photovoltaic I-V. Space-charge-limited-current manifested ohmic I-V diode behavior with enhanced conductance in graphene by injecting homogeneous ionic liquid; confirming the significant reduction of trap charge density. This was further congruent with disappearance of nonlinear kink in photo-diodes with high FF and nearly ideal diodes. The solar cell efficiency obtained with our strategy is around 13.6% and suggests possibilities to reach the theoretical limit of 19% by tailoring parameters such as conductance of graphene, carrier density of Si, and oxidation of the interfaces.