We report the use of a rapid flux calculation method using incomplete Riemann zeta functions as a replacement for the Bose–Einstein integral in detailed balance calculations to study the efficiency of tandem solar cell stacks under the terrestrial AM1.5G spectrum and under maximum concentration. The maximum limiting efficiency for unconstrained and constrained tandem stacks of up to eight solar cells, under the AM1.5G spectrum and maximum concentration, are presented. The results found agree well with previously published results with one exception highlighting the precautions necessary when calculating for devices under the AM1.5G spectrum. The band gap sensitivities of two tandem solar cell stack arrangements of current interest were also assessed. In the case of a three solar cell tandem stack the results show a large design space and illustrate that the constrained case is more sensitive to band gap variations. Finally, the effect of a non‐optimum uppermost band gap in a series constrained five solar cell tandem stack was investigated. The results indicate that a significant re‐design is only required when the uppermost band gap is greater than the optimum value with a relatively small effect on the limiting efficiency. It is concluded that this rapid flux calculation method is a powerful tool for the analysis of tandem solar cells and is particularly useful for the design of devices where optimum band gaps may not be available. Copyright © 2007 John Wiley & Sons, Ltd.

The limiting efficiency of an intermediate band (IB) solar cell under the terrestrial AM1.5 spectrum was calculated by detailed balance for various concentration levels. The results show four energy gap combinations giving similar limiting efficiencies. This is in contrast to the more studied case of an IB solar cell under a blackbody spectrum where a single optimum combination is found. A design with a subenergy gap of ∼0.57eV is found to be viable, leading to the conclusion that the design space for an IB solar cell is larger when under the AM1.5 spectrum than when under a Blackbody spectrum.

This article considers idealized solar cells whose absorbers are intermediate band ͑IB͒ media with finite bandwidths that permit both interband and intraband photoinduced electronic transitions at states within the IB. To comprehend the effect of the IB width, three classes of IB absorbers are constructed where each class is distinguished from the others by its spectral selectivity. It is shown that ͑i͒ the maximum-power efficiency tends gradually toward zero with increasing bandwidth when photoinduced interband transitions and intraband transitions are equally likely; ͑ii͒ with respect to the former, a relative efficiency enhancement may occur when photoinduced intraband transitions dominate interband transitions; and ͑iii͒ although thermodynamically consistent, efficiencies may be physically inconsistent without including photoinduced intraband transitions. Resulting from the solar surface temperature of 6000 K, the authors conclude that the largest efficiencies result when the IB width is roughly equal to or less than 800 meV.

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This article presents a thermodynamic treatment of an intermediate band solar cell that includes photoinduced electronic transitions between two distinct states of the intermediate band. The treatment also allows for two black-body sources, interband photoinduced electronic transitions, overlapping absorption coefficients, multiple electron-hole pair generation, and nonradiative processes. A schematic of the device’s thermodynamic configuration shows that the solar cell is composed of three particle engines operating in tandem. The authors present detailed-balance results where it is assumed that when there is the physical possibility of both photoinduced intra- and interband electronic transitions at intermediate levels that the latter predominates. Results indicate that as the intermediate band’s width increases, the efficiencies saturate to those of two-stack tandem solar cells while the band structures approach that of a material that should operate as a black body. The authors conclude that the assumption that interband transitions predominate over intraband transitions, which is equivalent to ignoring or excluding intraband transitions, may yield results inconsistent with physical reality. The larger the difference between the intermediate band width and the smallest band gap in the system, the more pronounced will be the inconsistency.