X-ray Microscopy Studies on Dopant Activation and its Correlation to Current Collection Efficiencies in CdTe Solar Absorbers*

Copper and arsenic have been used as primary dopants in CdTe photovoltaic absorbers. However, both atomic species suffer from very low dopant activation.  Furthermore, Cu and As concentrations in these devices are on the order of 10¹⁴ cm⁻³ and 10¹⁶ cm⁻³, respectively, which makes it notoriously difficult to correlate nanoscale distributions to the local charge transport properties. To measure and correlate these properties, measurement techniques require high sensitivity to elemental concentration, large penetration depth, and operando compatibility.  In this work, we use nanoscale X-ray microscopy to correlate chemical distribution – structure – electrical properties through X-ray Fluorescence, X-ray Absorption Near edge spectroscopy, and X-ray Beam Induced Current at a pixel-to-pixel level (50-120nm).

Our cross-section studies unveil the segregation of Cu and As and the transition of the maximum charge-collection efficiency from the ZnTe-CdTe interface to the CdS-CdTe interface as a function of processing conditions, which is correlated to the balance of acceptors and donors, and thus the activation of the dopants. We will show linear combination fitting of XANES and the presence of active species (e.g. Cu_Cd) as well as inactive species through the absorber.