This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE JOURNAL OF PHOTOVOLTAICS 1 Air Gaps as Intermediate Selective Reflectors to Reach Theoretical Efficiency

Abstract—Efficient external luminescence is a prerequisite for high-voltage solar cells. To approach the Shockley–Queisser limit, a highly reflective rear mirror is required. This mirror enhances the voltage of the solar cell by providing internally luminescent photons with multiple opportunities for escaping out the front sur-face. Likewise, intermediate reflectors in a multibandgap solar cell can assist external luminescence to enhance the voltage for each cell in a stack. These intermediate reflectors must also transmit the subbandgap photons to the next cell in the stack. A practi-cal implementation of an intermediate selective reflector is an air gap sandwiched by antireflection coatings. The air gap provides perfect reflection for angles outside the escape cone, and the an-tireflection coating transmits angles inside the escape cone. As the incoming sunlight is within the escape cone, it is transmitted on to the next cell, while most of the internally trapped luminescence is reflected. We calculate that air gap intermediate reflectors, along with a rear mirror, can provide an absolute efficiency increase of 5 % in multibandgap cells. Index Terms—Detailed balance limit, intermediate reflector, mirror, multijunction solar cell, quasi-equilibrium. I