Scanning Kelvin probe measurements on As-doped CdTe solar cells

Abstract

Scanning Kelvin probe microscopy (SKPM) has been used to study the Fermi level shift in arsenic (As) doped cadmium telluride (CdTe) photovoltaic devices. The contact potential difference (CPD) between probe tip and sample surface revealed that increasing As concentrations in CdTe led to a decrease in CPD. This highlighted a downward shift in the CdTe Fermi level and an increase in the CdTe work function. Using a highly oriented pyrolytic graphite sample in ambient conditions as a reference, the absolute work functions of the CdTe samples were estimated to vary from 3.88 to 4.09 eV. High-resolution SKPM measurements revealed localized shifts in CPD at CdTe grain boundaries. This was directly correlated to As doping concentrations, and indicated the segregation of As to grain boundaries. A mechanism is proposed where localized band bending at grain boundaries channels minority carriers away from the grain boundary, leading to reduced carrier recombination.

Improvement to thin film CdTe solar cells with controlled back surface oxidation

“…It was inferred that the use of 02 during the CSS-CdTe deposition process enhanced p-type doping. The oxidation process after air annealing could be considered to extend into the grain boundaries with additional contribution to the acceptor concentration, where the As (dopant) tends to segregate [25]. The influence of the air anneal on carrier concentration will be assessed by capacitance -voltage (C-V) measurements in a fut1her study.…”

Direct Observation of CdCl2 Treatment Induced Grain Boundary Carrier Depletion in CdTe Solar Cells Using Scanning Probe Microwave Reflectivity Based Capacitance Measurements

“…Probing the properties of individual grain boundaries is difficult as most device characterization techniques measure macroscopic areas and hence yield results that are a convolution of the electrical properties of the grain bulk and grain boundaries. Individual grain boundary properties have been investigated via electron beam induced current (EBIC); ,,, combinations of scanning capacitance microscopy (SCM), conductive atomic force microscopy (C-AFM), and scanning Kelvin probe force microscopy; ,, near-field optical beam induced current (OBIC); , and scanning spreading resistance microscopy . These studies showed that the grain boundaries in CdTe are electrically active and provide efficient conduction pathways for electrons.…”

CdCl2 Treatment-Induced Enhanced Conductivity in CdTe Solar Cells Observed Using Conductive Atomic Force Microscopy

“…Investigating individual grain boundary properties is challenging because interaction volumes probed by conventional characterization techniques span multiple grains. Individual grain boundary properties have been investigated via electron beam-induced current (EBIC); ,,, combinations of scanning capacitance microscopy (SCM), conductive atomic force microscopy (C-AFM), and scanning Kelvin probe force microscopy; ,, near-field optical beam-induced current (OBIC); , and scanning microwave impedance microscopy. , However, the electrical conductivity of the grain boundaries as a function of CdCl 2 annealing treatment remains unclear. In order to clarify the effect of Cl incorporation, we compare close-spaced sublimation (CSS) deposited thin-film CdTe, with and without CdCl 2 annealing.…”