Properties of dilute-N GaAs quantum structures for application in Intermediate Band Solar Cell research

Martin Elborg

ICYS-Sengen Researcher

Dilute-N III-V semiconductors are a relatively new and intriguing family of materials both from a viewpoint of fundamental study as well as for possible applications in optoelectronic devices. When N is incorporated into GaAs, an extremely large bowing factor induces a drastic change in band gap energy which greatly lowers the conduction band edge while leaving the valance band edge practically unchanged. The band gap of Ga1-xNxAs can be tuned from 1.42 to 1.0 eV for already low N concentrations of only 0-5%, which makes the material particularly interesting for application in solar cells, such as multi-junction solar cells and Intermediate Band Solar Cells (IBSCs). The unique properties of GaNAs allows the fabrication of deeply confined quantum structures in a larger band gap material to generate an additional photocurrent in an IBSC from the absorption of sub-band gap energy photons. The maximum theoretical conversion efficiency of such a cell is calculated to be ~60% which requires specific absorption energies of 1.9 eV, 1.2 eV, and 0.7 eV from conduction band (CB) to valance band (VB), VB to IB, and IB to CB, respectively. To achieve such band alignment, in this work I embed GaNAs quantum structures in AlGaAs and analyze their optical transitions by photoluminescence and photocurrent spectrum measurements as dependence on N concentration. Besides the fundamental QW transition, N-induced localized states both below and above the QW transition as well as a large Stokes-shift characteristic for the material is found. Close-to-ideal band alignment is achieved at a N concentration of 3.1%. Furthermore, the possibility of forming intermediate energy states exclusively by a CB offset while holes can smoothly separate in a zero VB offset band alignment is explored using GaNAs embedded in GaAs. Smooth hole transport is demonstrated using current-voltage characteristics under front-side excitation of the solar cells.