Dilute Nitride Type II Quantum Dot Materials for Solar Cells based on GaAs
Collaboration research in energy with South Africa
EPSRC project with Nelson Mandela Metropolitan University, S. Africa
This aim of this project is to realize efficient, dilute nitride quantum dot materials for the development of novel solar cells with extended photo-response based on GaAs. We shall implement highly efficient GaSbN quantum dots (QD) within the active region of a technologically mature GaAs based solar cell architechture.
Figure 1. PL emission obtained from GaSb type II QD grown on GaAs by MBE
Figure 2. AFM of GaSb/ GaAs QDs
The addition of 1% N produces a band gap reduction of 230 meV in GaSb and 130 meV in GaAs. The type II transition energy estimated from the band offsets in the relaxed GaSb/GaAs system is ~800 meV and so we estimate that the photoresponse would be extended out to 670 meV (1.85 µm) using 1% N in the GaAs layers. Detailed calculations of InGaP/GaAs/Ge triple junction cells in which QDs are envisaged to extend the middle junction absorption spectrum have shown that the efficiency could be improved to 47% under single sun illumination.
Figure 3 (Left) Diagram of proposed QD solar cell structure. (Right) Schematic diagram of type II GaSbN /GaAs QD showing band gap reduction by addition of N to GaSb QD and to GaAs (dotted lines show effect of N on conduction band edge).