Energy selective contacts based on double-barrier tunnelling structures for hot carrier solar cells

Abstract

The hot carrier solar (HCSC) is an innovative concept of photovoltaics device that has the potential to surpass the Shockley-Queisser efficiency limit. To achieve high power conversion efficiency, Energy Selective Contacts (ESC) with narrow selection width are required to extract hot carriers within the optimized energy range. The aim of this project was to fabricate ESC based on double-barrier tunneling structures (DBRTD) including double-barrier quantum well (DBQW) and double-barrier quantum dots structure (DBQD).

Due to the potential slow thermalization rate in this material, cubic hafnium nitride (c-HfN) is considered as one of the candidates for HCSC absorber material. Therefore, DBRTD based on hafnium compounds as ESC provides possibility of complete HCSC device based on this material system. Hafnium oxide (HfO2) is identified as the barrier material and semiconducting HfNx with high N content as the quantum well material. The HfNx thin films transfer from metallic for x?1 to insulating or semiconducting for x>1.1. XPS and absorption measurement results show the potential existence of one semiconductor material Hf3N4 in the HfN1.38 film. Moreover, germanium (Ge) is also considered in this project as the quantum well material due to its small electron effective mass and large Bohr radius. These properties of Ge provide the possibility of engineering the tunneling positions in double-barrier tunneling structure. In other words, the extraction energy in HCSC can be adjusted by varying the Ge QW width, which extends the application of Ge DBQW structure for different absorbers.

DBQW structure based on Ge QW and HfO2 barrier were fabricated by RF reactive sputtering and atomic layer deposition (ALD) followed by post-annealing. Prominent tunneling peaks were observed at room temperature for different Ge well width. The tunneling peaks become sharper for thinner well width. However, DBQW structure based on HfN1.38/HfO2 show a typical resistor character and no tunneling effect was observed. DBQDs structure based on PbS QDs and HfO2 barrier were fabricated by Langmuir-Blodgett (LB) and ALD at low temperature. The selection width for the DBQDs structure is narrower than that for DBQW structure due to the three-dimensional energy confinement, while DBQW structure provides larger current density than DBQDs structure. Moreover, lower extraction energies require narrow selection width for optimizing the energy conversion efficiency, while higher extraction position needs larger current density to achieve optimized power output. Therefore, DBQDs and DBQW show their advantages on different occasions.