Microelectronic Structure and Doping Nonuniformity of Phosphorus-Doped CdSeTe Solar Cells.

Optimizing group-V doping and Se alloying are two main focuses for advancing CdTe photovoltaic technology. We report on nanometer-scale characterizations of microelectronic structures of phosphorus (P)-doped CdSeTe devices using a combination of two atomic force microscopy-based techniques, namely, Kelvin probe force microscopy (KPFM) and scanning spreading resistance microscopy (SSRM). KPFM on device cross-section images distribution of the potential drop across the device. SSRM taken on a delaminated front interface and further beveling into absorber bulk reveals local distributions of doping polarity and carrier concentration. The KPFM and SSRM imaging corroborate each other, suggesting that nonuniform doping revealed by SSRM is associated with nonuniform potential features observed by KPFM. These detrimental microelectronic structures were improved by enhancing P-doping. The large nonuniform potential drop and deep overall n-p transition in the device without doping were mitigated to potential fluctuation around the front interface and n-p transition depth of ∼100 nm by low-level P-doping and further mitigated to scarce and slight irregular potential and p-weighed doping at the interface by high-level P-doping. These characterizations imply sophisticated defect chemistry, atomic structure, and associated electronic structure in CdTe with Se alloying and group-V doping together and further point to the direction for improving device efficiency by mitigating and ultimately eliminating the nonuniform doping and irregular potential.