High efficiency multijunction II-VI photovoltaic solar cells

A monolithic multijunction photovoltaic solar cell, comprising:

• a first subcell including:
• a first base formed of a single-crystal Group IV semiconductor material to be of a first conductivity type and having a first base energy gap;
• and
• a first emitter formed of a single-crystal Group IV semiconductor material to be of a second conductivity type opposite the first conductivity type, the first emitter formed to adjoin the first base and having a first emitter energy gap;
• a first tunnel junction consisting of a first tunnel junction layer and a second tunnel junction layer, the first tunnel junction layer directly formed on the first emitter, the first tunnel junction layer formed of ZnTe or CdZnTe and degenerately doped to be of the first conductivity type, the second tunnel junction layer formed on the first tunnel junction layer, the second tunnel junction layer formed of ZnTe or CdZnTe and degenerately doped to be of the second conductivity type;
• a second subcell formed on the first tunnel junction and comprising:
• a second base formed of a single-crystal Group II-VI semiconductor material to be of the first conductivity type and to have a second base energy gap higher than the first base energy gap;
• and
• a second emitter formed of a single-crystal Group II-VI semiconductor material to be of the second conductivity type and formed to adjoin the second base;
• and
• wherein an overall ideal series efficiency of the solar cell calculated under 500 suns incident solar radiation is at least approximately 40%.