Silicon dominates contemporary solar cell technologies. But when absorbing photons, silicon (like other semiconductors) wastes energy in excess of its bandgap. Reducing these thermalization losses and enabling better sensitivity to light is possible by sensitizing the silicon solar cell using singlet exciton fission, in which two excited states with triplet spin character (triplet excitons) are generated from a photoexcited state of higher energy with singlet spin character (a singlet exciton). Singlet exciton fission in the molecular semiconductor tetracene is known to generate triplet excitons that are energetically matched to the silicon bandgap. When the triplet excitons are transferred to silicon, they create additional electron–hole pairs, promising to increase cell efficiencies from the single-junction limit of 29 per cent to as high as 35 percent.
This thesis demonstrates a silicon solar cell employing a protective hafnium oxynitride layer with a thickness of just eight angstroms at the interface, using electric-field-effect passivation to enable efficient energy transfer of the triplet excitons formed in tetracene. The maximum combined yield of the fission in tetracene and the energy transfer to silicon is around 133 percent, establishing the potential of singlet exciton fission to increase the efficiencies of silicon solar cells and reduce the cost of the energy that they generate. Moreover, the thesis employs photoluminescent and magnetic field effect experiments to investigate the impact of different interlayer thicknesses and silicon doping levels on the electric-field-effect passivation and exciton transfer across the tetracene/silicon interface.
Thesis Supervisor: Prof. Marc A. Baldo
Committee: Prof. Vladimir Bulovic, Prof. Troy Van Voorhis