Energy Selective Contacts Based on Quantum Well Structures of Al2O3 and Group IV Materials for Hot Carrier Solar Cells

Abstract

This study aims to realize effective energy selective contacts (ESCs) for hot carrier solar cells (HCSCs) by using quantum well structures based on silicon or germanium wells. HCSCs require highly selective ESCs to extract optimal hot carriers only, in order to achieve high efficiency. ESCs rely on resonant tunnelling phenomena in ultrathin quantum well devices such as double-barrier resonant tunnelling diodes (DB RTD).

To find suitable barrier materials for Si and Ge, an in-depth review is carried out to sort previous DBRTDs according to their materials and to compare their performance, by which Al2O3 is identified as the most potential barrier material.

Two experimental approaches are carried out to construct DBRTDs with amorphous Al2O3 (a-Al2O3) and crystalline Al2O3 (c-Al2O3) barriers, respectively. For a-Al2O3 barriers, two types of wells, thermally annealed nanocrystalline Ge (nc-Ge) films and Si quantum dots (QDs) deposited by Langmuir-Blodgett (LB) method, are employed. For the c-Al2O3 barriers, sputtering and pulsed laser deposition (PLD) are used to grow hetero-epitaxial gamma-Al2O3 films on Si substrates.

The DBRTD based on a-Al2O3/nc-Ge achieves room temperature resonant tunnelling with peak to valley ratio (PVCR) 2.43, full width at half maximum (FWHM) 0.05V, and peak current density (PCD) 2.1A/cm2, applicable for ESCs. High selection with a quality factor (PVCR 20.5/FWHM 0.012V) 1708/V has been achieved by the DBRTD of a-Al2O3/Si QD at room temperature, very promising for the ESC purpose.

A 3nm strained layer of epitaxial gamma-Al2O3 is achieved by sputtering on Si (100) substrates, with ideal interfaces for RTD purpose. The novel e-beam annealed epitaxy of gamma-Al2O3 on Si (111) substrates is observed, with thick strained layers up to 14nm. PLD achieves 56nm thick relaxed hetero-epitaxial gamma-Al2O3 films on Si (111) substrates, potential for RTD and the epitaxy of III-V materials on Si.

To integrate ESCs with HCSCs, calculations based on conservation model reveal ESCs of ultralow interface thermal conductance could enable HCSCs to achieve high efficiency, with relatively short carrier thermal lifetime.

This work has successfully achieved two novel devices, developed two new approaches, and designed a new way to construct and integrate ESCs into HCSCs.