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  * infinite mobility : Each absorbed photon generates one single electron-hole pair. A second major limiting factor leading to a degradationof the cell efficiency in relation to the theoretical values pre-dicted by the Shockley and Queisser model is associatedwith series resistance losses, basically caused by the finitevalue of the carrier mobility. However, series resistance effects cansimply be taken into account by introducing a lumped seriesresistance parameterRs, into the initial Shockley andQueisser equation, leading to the well-known implicit equa-tion for the output current of a solar cell (sqlimitjap2015).  * no non-radiative recombination processes : electrons will not be lost due to phonons.  perfectly reflecting (minority carrier) contacts. The magnitude of these non-radiative recombination losses can be characterized by the internal fluorescence efficiency.  One major improvement in single-junction GaAs solar cellcomes from the benefits of having a thin base [9]. Comparedwith a substrate design, a thin film has the advantage of a loweroverall recombination region due to a reduction of base thicknessand a decrease in radiative recombination since the backsidemirror aids in photon recycling.Any increase in base thickness (see Fig. 11) produces a smallincreaseJSCdue to more complete absorption of sunlight.VOC,however, decreases due to a larger volume available for re-combination. As a result, the change in overall  efficiency israther small. The detailed impact of base thickness on radiativerecombination is related to the backside mirror discussion inSection IV-B. As the base thickness shrinks, emitted photonsneed to strike the backside mirror more frequently. This signif-icantly increases losses due to the backside mirror. Therefore,there is a tradeoff between more pronounced backside mirrorloss and less of a region for radiative recombination to occur—aquantitative connection that only a detailed numerical calcula-tion can precisely capture. ("Design Solar Cell Close to the QS" page 742)  * reflections from the front surface (non-ideal ARC): For ideal materials the burden of high open- circuit voltage, and thereby high efficiency, lies with optical design: The solar cell must be designed for optimal light extraction under open-circuit conditions. (Physics required to approach SQ)  * non-ideal back contact reflection (<100%): The Shockley- Queisser limit gets a significant boost from the perfect photon recycling that occurs in an ideal system. Unfortunately, for most materials, their relatively low internal fluorescence yields mean that the upper bounds on their efficiencies are much lower than the Shockley-Queisser limit. For the few material systems that are nearly ideal, such as GaAs, there is still a tremendous burden on the optical design of the solar cell. A very good rear mirror, for example, is of the utmost importance. In addition, it becomes clear that realistic material radiative efficiencies must be included in a credible assessment of any materials‟ prospects as a solar cell technology. (Physics required to approach SQ) The escape of photons through the front surface is an impor-tant loss mechanism in these cells. The escape cone depends onthe ambient index,n, but diffraction effects in the ARC are ex-pected to increase the escape cone somewhat. Photon transportthrough the ARC is not treated. Instead, we use an ambient in-dex ofn=1.35 in an attempt to mimic transmission through thestack of GaAs/ARC (160 nm)/Air by making the front escapecone slightly larger than that of air/GaAs interface. ("Design Solar Cell Close to QS" page 739) The escape of photons through the front surface is an impor-tant loss mechanism in these cells. The escape cone depends onthe ambient index,n, but diffraction effects in the ARC are ex-pected to increase the escape cone somewhat. Photon transportthrough the ARC is not treated. Instead, we use an ambient in-dex ofn=1.35 in an attempt to mimic transmission through thestack of GaAs/ARC (160 nm)/Air by making the front escapecone slightly larger than that of air/GaAs interface. ("Design Solar Cell Close to QS" page 741)