The effect of band offsets in quantum dots

• Band offsets in quantum dots are very dependent on strain.
• Band offsets are entirely determined for the measured quantum efficiency.
• They are non-destructively determined in the finished device.

The insertion of quantum dots in a host material produces band offsets which are greatly dependent on the field of strains brought about by this insertion. Based on the Empiric KP Hamiltonian model, the energy spectrum of the quantum dot/host system is easily calculated and a relationship between the conduction and valence band offsets is determined by the energy at which the lowest peak of the sub-bandgap quantum efficiency of an intermediate band solar cell is situated; therefore knowledge of the valence band offset leads to knowledge of both offsets. The calculated sub-bandgap quantum efficiency due to the quantum dot is rather insensitive to the value of the valence band offset. However, the calculated quantum efficiency of the wetting layer, modeled as a quantum well, is sensitive to the valence band offset and a fitting with the measured value is possible resulting in a determination of both offsets in the finished solar cell with its final field of strains. The method is applied to an intermediate-band solar cell prototype made with InAs quantum dots in GaAs.

The internal quantum efficiency vs the energy of the incident radiation for different VB offsets potentials. The value of the band gap of the host material and the value of the transition VB121 to CB111 in the quantum dot remain constant. The fitting at the wetting layer gives the valence band offset.Figure optionsDownload as PowerPoint slide