Band alignment engineering of p-Ge/n-Si heterojunction for low cost tandem solar cell applications

The current research work deals with synthesis and optimization of polycrystalline germanium thin films to develop Ge/Si heterojunction with appropriate band alignment for sound directional carrier transportation. The heterostructure (p-Ge/n-Si) device performance was monitored by controlling the thickness of each hetero-partner that is primarily reliant on the annealing temperature. Herein, pristine Ge-samples were annealed at various annealing temperatures 100, 150, and 200 °C to obtain high quality defect-free hetero-interface. Annealing-induced morphology evolution results for the surface passivation to adeptly reduce the recombination losses for superior electrical and optoelectronic properties of heterojunction diode. The conduction mechanism was tailored through energy band diagram by calculating the series resistance, ideality factor, forward-to-reverse current ratio, turn on voltage as well as conduction and valence band offsets in p-Ge/n-Si staggered (type-II) heterojunction. The proposed heterojunction diode explicitly displayed a highest rectification ratio of ∼21.68 at ±1.5V with a knee voltage of 1.03V. It is revealed that the proper band alignment plays an imperative role to enhance the carrier-injection. This heterostructure PN-junction may be incorporated as low cost bottom subcell in multijunction tandem solar cells to absorb infrared spectrum of incident light.