Investigating the Macroscopic and Microscopic Electronic Properties of Polycrystalline CdTe Solar Cells

Cadmium Telluride (CdTe) is an important photovoltaic (PV) material with polycrystalline (PX) CdTe solar cells having demonstrated power-conversion efficiencies (PCEs) >22%. Recently, there have been attempts to fabricate CdTe solar cells with a single-crystal (SX) CdTe absorber layer in the hopes of achieving better device efficiencies due to a reduction in defect densities in the SX CdTe films. However, apart from being much more expensive, the SX devices have lower PCEs than the PX devices.

Why are PX CdTe solar cells more efficient than their SX counterparts?

In this presentation, I will explore the macroscopic electronic band structure and microscopic electronic properties of the grains and the grain boundaries in device-grade PX CdTe films, in order to find an answer to this question. Optical Spectroscopy-based measurements have been used for macroscopic analysis while Scanning Microwave Impedance Microscopy (SMIM), an advanced Scanning Probe Microscopy-based technique, has been used to analyze the nanoscale (~30-50 nm) electronic properties of the grains and the grain boundaries in CdTe. Using SMIM, the existence of a charge-carrier depleted region along the grain boundaries in PX CdTe films was demonstrated. This charge-carrier depletion results in band bending along the grain-grain boundary interface, which aids the photo-generated charge carrier separation and hence, results in superior efficiencies of PX CdTe devices.

Apart from the specific applications to CdTe PV technology, this work demonstrates SMIM's unique capabilities and paves way for developing superior device processing steps to boost semiconductor device performances by monitoring the evolution of nanoscale electronic properties of the material during the various stages of device fabrication.

For further information please contact andrea.centrone [at] nist.gov (Andrea Centrone), 301-975-8225.