Band gap‐voltage offset and energy production in next‐generation multijunction solar cells

Abstract

The potential for new 4‐, 5‐, and 6‐junction solar cell architectures to reach 50% efficiency is highly leveraging for the economics of concentrator photovoltaic (CPV) systems.The theoretical performance of such next‐generation cells, and experimental results for 3‐ and 4‐junction CPV cells, are examined here to evaluate their impact for real‐world solar electricity generation. Semiconductor device physics equations are formulated in terms of the band gap‐voltage offset Woc  (Eg/q) − Voc, to give a clearer physical understanding and more general analysis of the multiple subcell band gaps in multijunction cells. Band gap‐voltage offset is shown experimentally to be largely independent of band gap E g for a wide range of metamorphic and lattice‐matched semiconductors from 0.67 to 2.1 eV. Its theoretical E g dependence is calculated from that of the radiative recombination coefficient, and at a more fundamental level using the Shockley‐Queisser detailed balance model, bearing out experimental observations. Energy production of 4‐, 5‐, and 6‐junction CPV cells, calculated for changing air mass and spectrum over the course of the day, is found to be significantly greater than for conventional 3‐junction cells. The spectral sensitivity of these next‐generation cell designs is fairly low, and is outweighed by their higher efficiency. Lattice‐matched GaInP/GaInAs/Ge cells have reached an independently confirmed efficiency of 41.6%, the highest efficiency yet demonstrated for any type of solar cell. Light I‐V measurements of this record 41.6% cell, of next‐generation upright metamorphic 3‐junction cells with 40% target production efficiency, and of experimental 4‐junction CPV cells are presented. Copyright © 2010 John Wiley & Sons, Ltd.