Challenges in Going Beyond 17% Efficiencies in CdTe/CdS Solar Cells

CdTe has a theoretical efficiency of 29%, due to its optimal direct bandgap (1.56eV) for solar irradiation and excellent absorption properties. However; record efficiencies only go up to 17%.  In this talk, the challenges to achieve higher efficiencies will be discussed as well as some of the approaches taken by researchers to increase it.

Material defects in solar cells produce material boundaries which are potential recombination sites for the carriers.   Defect concentration in CdTe cells is created during the growth in the interface between the CdTe(p material) and CdS(n material) due to the lattice mismatch between the two (~11%) .  Currently, researchers at the University of Texas at El Paso have been able to create nano-structures that control the crystallinity and crystal growth of CdTe in close spaced sublimation.  The use of this approach could lead to lower defects and higher efficiencies.   Other challenges discussed in a general form will be: the tradeoff of the CdS layer thickness and doping with absorption and depletion region with respectively; the selection of the conductive oxide (balance between resistance and transparency); and finally but not less important the contacting procedure to achieve an ohmic contact to the semiconductor.