Strain Mapping of CdTe Grains in Photovoltaic Devices

Strain within grains and at grain boundaries (GBs) in polycrystalline thin-film absorber layers limits the overall performance because of higher defect concentrations and band fluctuations. Yet, the nanoscale strain distribution in operational devices is not easily accessible using standard methods. X-ray nanodiffraction offers the unique possibility to evaluate the strain or lattice spacing at nanoscale resolution. Moreover, the combination of nanodiffraction with additional techniques in the framework of multimodal scanning X-ray microscopy enables the direct correlation of the strain with material and device parameters such as the elemental distribution or local performance. This method is applied for the investigation of the strain distribution in CdTe grains in fully operational photovoltaic solar cells. It is found that the lattice spacing in the (111) direction remains fairly constant in the grain cores but systematically decreases at the GBs. The lower strain at GBs is accompanied by an increase of the total tilt. These observations are both compatible with the inhomogeneous incorporation of smaller atoms into the lattice, and local stress induced by neighboring grains.