Absorption coefficients of intermediate-band media

This paper models the absorption coefficients of an intermediate-band (IB) absorbing medium. Equilibrium absorption coefficients are presented for several IB absorbers, each distinguished by their energy-wavevector dispersion and equilibrium temperature. Nonequilibrium absorption coefficients are also presented for solar cells implemented with IB absorbers. Several simplifying assumptions are made including that the energy-wavevector dispersions are parabolic. The model requires the absolute locations of three quasi-Fermi levels. This is made possible by using two balance equations. One of these, a charge-neutrality condition, necessitates the numerical computation of the carrier statistics in each band of the IB absorber. The use of the incomplete Fermi-Dirac functions makes this possible. The authors conclude that (i) if the concentration of intermediate states is greater than the concentration of carriers in the conduction band and greater than the concentration of carriers in the valence band, then the IB will be partially filled; (ii) an IB absorber may or may not absorb all photons with energies greater than the smallest bandgap in the system; (iii) an IB absorber may permit absorption overlap so that an absorbed photon would likely generate an electron-hole pair across a bandgap other than the largest bandgap less than the energy of the absorbed photon; (iv) as the temperature of the IB absorber approaches absolute zero, the absorption edges resulting from transitions at intermediate levels may blueshift.