CdTe-Based Solar Cells with Variations in Mg Concentration in the MgZnO Emitter

The optimal fraction of Mg incorporation in sputter-deposited MgXZn1-XO (MZO) emitters for thin-film CdTe-based solar cells is evaluated by varying it over a range of x from 0 to 0.35. This range allows a variation in the conduction band offset from -0.1 eV (cliff like) to +0.32 eV (spike like). A maximum efficiency of 18.5% for cells with the bilayer CdSeTe/CdTe absorber occurs at x = 0.15, which corresponds to a spike-like band offset near 0.2 eV, as confirmed by X-ray photoelectron spectroscopy. In addition, good cell performance is seen over a fairly broad range of x extending from 0.1 to 0.25. The MZO optical bandgap increases with the Mg fraction, consistent with an increasing conduction band offset. Temperature-dependent current-voltage measurements and time-resolved photoluminescence show improvement in the emitter/absorber interface with the incorporation of Mg. Capacitance-voltage measurements show that the depletion region extends further into the absorber with more Mg, and X-ray diffraction confirms a change from a hexagonal-dominant crystal structure toward zinc blende at x = 0.35.

Automated summary

By adjusting the magnesium content in the MgXZn1-XO (MZO) emitter, the optimal efficiency for CdTe-based solar cells can be achieved, with good cell performance seen in a range of x values from 0.1 to 0.25.