Sunshot Initiative AC275 Final project

Background

CdTe photovoltaics (PV) is the only thin film technology with lower costs (\(\frac{\$}{W}\)) and carbon footprint 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% (Solar 2015a) while their commercial high volume PV modules have 17.0% (Sinha 2013).

The theoretical upper bound of efficiency, the Shockley-Queisser (SQ) limit (Shockley 1961) for a PV with a band gap of 1.49 eV (CdTe) under the standard AM1.5G flat-plate solar spectrum (Laboratory 2015) is 32.2%. This is relevant when comparing with other similar thin film PVs such as GaAs, which have a relative efficiency of \( \zeta_{SQ}=\frac{ \zeta_{real} }{ SQ_{limit} }= \frac{28.8}{33.1} \approx 0.87 \) (Vossier 2015) (Yablonovitch 2012). Under this metric, CdTe research grade PVs have a value of \(\zeta_{SQ}=0.667\). and commercial PV modules have \(\zeta_{SQ}=\frac{17.0}{32.2}\approx 0.52\). There is much room for improvement.

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 9.0-15.2% gap.