This paper discusses the evolution of our 2.05 eV AlGaInP top sub-cell. Although we were not successful in introducing a more optimum 1.94 eV AlGaInP alloy into the top sub-cell of our 3 or 4J-IMM (inverted metamorphic multijunction) structure because of insufficient current density capacity, we did successfully develop a 2.05 eV AlGaInP top subcell structure for 5 or 6J-IMM application. To achieve the desired current capacity of 11.3mA/cm2, we fabricated 2.05 eV hetero-junctions composed of a 1.91 eV GaInP emitter and a 2.05 eV AlGaInP base. Such a hetero-junction top sub-cell incorporated in a 3J-IMM iso-type structure exhibited sufficient J sc but reduced Voc =1.466 V, being only 43 mV greater than the 1.91 eV GaInP homo-junction subcell. Minor modification to the emitter of our AlGaInP homojunction permitted the fabrication of a 2.05 eV sub-cell in a 5J-IMM iso-type structure with sufficient current density and improved Voc=1.537 V. 5 and 6JIMM cells incorporating these top sub-cells demonstrated maximum Vocs= 4.839 and 5.195 V, respectively.