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# Background  CdTe photovoltaics (PV) is the only thin film technology with lower costs than conventional solar cells made of crystalline silicon in multi-kilowatt systems. In August of 2014, First Solar® was able to accomplish a CdTe solar cell efficiency of 21.5%. The theoretical upper bound of efficiency, the Shockley-Queisser (SQ) limit for a PV with a band gap of 1.49 eV (CdTe) is 32.2%.  This paper discusses the optical and electrical factors that reduce the efficiency of a single junction CdTe PV below the SQ limit and provides recommendations on closing this 9.0% gap. We use the index \( $  \zeta_{SQ} = \frac{ \zeta_{real} }{ SQ_{limit} } \) $  , which is the ratio between the real and ideal efficiency as a measure of this gap, in this case the First Solar CdTe PVs have a value of \( \zeta_{SQ}=0.667 \). This number is relevant since other high efficiency thin film PV such as GaAs have a value of \(\zeta_{SQ}= \frac{28.8}{33.1} \approx 0.87 \), which indicates it is much closer to the SQ limit.