In this case the E_- subband states are of highly localized character and the E₊ subband states become more extended.  This situation occurs for O in ZnTe, MnTe, MgTe. Figure 3 illustrates the formation of a narrow intermediate band by the incorporation of O into ZnTe. In addition to the O content, Mn alloying can also be used to adjust the energy level positions.  Thus, band locations corresponding to those that are optimal for a multiband solar cell can be produced in ZnTeO and ZnMnTeO.  With multiple band gaps that fall within the solar energy spectrum, Zn_1-xMn_xO_yTe_1-y is well suited for the proposed high efficiency intermediate band solar cells (IBSCs). This new II-VI oxide multi-band semiconductor was synthesized using the combination of oxygen ion implantation and pulsed laser melting.  This highly non-equilibrium technique allowed for the synthesis of ZnMnOTe alloys with up to 3% of Te replaced with O atoms.  Fig. 4 shows PR spectra from a Zn_0.88Mn_0.12Te substrate and two Zn_0.88Mn_0.12Te samples implanted with 3.3% of O followed by PLM with laser energy fluence of 0.15 and 0.3 J/cm².  Two optical transitions occurring at energies distinctly different from the fundamental band gap transition EM (=2.31 eV) of the Zn_0.88Mn_0.12Te matrix can be clearly observed at ~1.8 and 2.6 eV from the samples after PLM.

     These two optical transitions can be attributed to transitions from the valence band to the two conduction subbands, E+ (~2.6 eV) and E- (~1.8 eV) formed as a result of the hybridization of the localized O states and the extended conduction band states of ZnMnTe.
     Our results from optical transitions clearly demonstrate that three types of optical transitions are possible in this band structure; (1) the transitions from the valence band to the E+ subband with the absorption edge at E_V+=E₊(k=0)-E_V(k=0)=2.56 eV, (2) transitions from the valence band to E_- subband with the edge at E_V-=E_-(k=0)-E_V(k=0)=1.83 eV and (3) the low energy transitions from E_- to E₊ with the absorption edge at E_{_+}=E₊(k=0)-E_{_}(k=0)=0.73 eV.  The three absorption edges span much of the solar spectrum, demonstrating that these alloys are good candidates for the multi-band semiconductors envisioned for high efficiency photovoltaic devices.
     Detailed balance calculations of the power conversion efficiency for a intermediate band solar cell based on this material is shown in Fig. 5.  Even for this non-optimal band gap configuration we calculate a power conversion efficiency of 45%, which is higher than the ideal efficiency of any solar cell based on a single junction in a single-gap semiconductor and is comparable to the efficiency of double-junction cells.
The potential technological importance of the multiband semiconductors raises the question if they can also be realized in group III-N_x-V_1-x HMAs as well.  In most III-V compounds the localized N level lies above the conduction band edge.  An exception is the GaAs_1-yP_y alloy system in which N-level falls below the conduction band edge for y>0.3.  Consequently the anticrossing interaction of the N states with the extended conduction band states in these GaAsP alloys is expected to result in the formation of a narrow band of intermediate states.  Recently we have also synthesized GaN_xAs_1-yP_y with y=0 to 0.4 using N⁺-implantation followed by PLM and RTA techniques.  With an implanted N concentration of 2%, the N concentration incorporated in the As sublattice amounts to about 1% and 0.3% for films with y≤0.12 and y>0.12, respectively.  GaN_xAs_1-yP_y with y>0.2 clearly shows strong optical transitions corresponding to both the lower (E_-) and upper (E₊) conduction subbands.  GaN_xAs_1-yP_y alloys with y>0.3 have a three band structure making them suitable for testing the theoretical predictions of the highly-efficient intermediate band solar cell concept.  Theoretical ideal efficiency for IBSC using the GaN_0.02As_0.58P_0.4 is calculated to be >55%.