The direct band gaps of the group III-nitride alloys can be continuously tuned in a very large energy range from 0.65 eV in InN to 6.2 eV in AlN.  In addition, changing the composition from InN to AlN allows for varying the electron affinity of the alloys from 5.8 to 2.1 eV.  These unique properties offer a possibility of using group III-nitride alloys for high efficiency tandem cells.  A successful implementation of group III-nitrides for multijunction PV devices requires p- and n-type doping in the whole alloy composition range. As grown, undoped InGaN films are always n-type.  Reproducible p-type doping with Mg acceptors in the whole composition range has been reported by several groups.  Any design of the multijunction cells based on InGaN or InAlN alloys must consider the n-type surface inversion layer that is experimentally found for p- In_xGa_1-xN with x > 0.4.  A number of possible cell designs will be presented, including structures with graded band gaps. In addition results on a hybrid InGaN/Si double junction (2J) cell will be discussed in detail. Our experimental work has established that for x=0.5 the conduction band of In_xGa_1-xN aligns with the valence band of Si. Thus, a low resistance ohmic contact is expected at a junction between n-type In_0.5Ga_0.5N and p-Si, eliminating the requirement of heavily doped layers needed for the tunnel junctions in the current multijunction cells. This greatly simplifies the design of a 2J hybrid cell. Efficiencies exceeding 30% are predicted for an optimized p-InGaN/n-InGaN/p-Si/n-Si cell based on realistic absorption and transport parameters for InGaN.  It should be noted that similar considerations apply to the In_yAl_1-yN/Si system with y=0.7. Latest results on the performance of the InGaN based tandem cells will be presented and the remaining obstacles in achieving higher power conversion efficiencies will be discussed.

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1. Irradiation-induced defects in InN and GaN studied with positron annihilation
2. Compositional modulation in the InxGa1-xN layers; relation to their optical properties
3. Band Structure and Properties of InN and In-rich In1-xGaxN Alloys