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Diluted-N Multiple Quantum Well Solar Cells with Flat Conduction Band Structure for Solar Cell Applications
W. Yanwachirakul, N. Miyashita (Okada Lab.), H. Sodabanlu, K. Watanabe, M. Sugiyama Y. Okada (Okada Lab.) and Y. Nakano
Today, the multijunction solar cells could provide the highest world record of conversion efficiency. In order to construct the optimized 3- and 4-junction solar cells with a Ge bottom cell, a material with an energy gap in range of 1.0-1.2 eV and lattice-matching to Ge and GaAs was required for the middle-cell layer, and the diluted N was potentially expected to be one of this candidate materials.
Because of the special characteristic of the diluted-N material, the N-localized state would hybrid to a conduction band edge, then split the band into E+ and E-. The calculation, regarding the band-anti-crossing (BAC) model, showed that a small amount of N-incorporation could drastically decrease the energy gap to be below 1.2 eV, and a lattice constant was adjustable by additionally incorporating In. However, this material naturally has a short carrier lifetime due to a perturbation of carrier transport by N-localized states.
We propose an InGaAs/GaNAs multiple quantum well (MQW) structure with strain balancing. Periodic insertion of InGaAs layers without N is expected to mitigate non-radiative recombination of electrons via the N-localized states. By adjusting the contents of In and N, the flat conduction band edge can be obtained, which is beneficial for delocalizing electrons in the MQW and improving carrier collection to p- and n- terminals of a solar cell.
Fig. 1 Modulation of conduction-band-edge energy by the addition of dilute nitrogen
Fig. 2 Change of the band-edge energies by the addition of In and N, respectively.
Fig. 3 An InGaAs/GaAsN strain-balanced quantum well structure with flat conduction band edge