Sub-THz thermally activated-electrical conductivity of CdS thin films

TL;DR

This study measures the sub-THz electrical conductivity of thin CdS films using an open cavity resonator, revealing how annealing affects conductivity and carrier concentration, with implications for solar cell window materials.

Contribution

It introduces a novel electromagnetic measurement technique for thin photovoltaic films and analyzes the impact of annealing on their electrical properties at sub-THz frequencies.

Findings

Annealed CdS films have about 100 times lower conductivity than unannealed.

Carrier concentration decreases significantly after annealing.

Electrical conductivity varies with temperature and grain structure.

Abstract

The electrical conductivity of a CdS thin film, controlled by grain structures is essential to enhance its photoconductivity to be able to be fit as a window material in CdS/CdTe heterojunction solar cell. In order to characterize a thin film, electromagnetically, we employed an open cavity resonator with a sub-millimeter VNA (Vector Network Analyzer). Our technique is capable of measuring complex dielectric permittivity, $\tilde{\epsilon}$, of a photovoltaic film as thin as 0.1 $\mu$m. We measured the real part of the complex dielectric permittivity, $\epsilon_{re}$, and electrical conductivity, $\sigma_{re}$ (derived from the imaginary part, $\epsilon_{im}$), of unannealed and annealed CdS films with thicknesses $\sim$ 0.15 $\mu$m on $\sim$ 3 mm thick-borosillicate glass substrates, at room temperature. We obtain the (thermally activated) electrical conductivity between 100 and 312 GHz, which is less in annealed samples than in unannealed one by $\sim$ 2 orders of magnitude. Contrary to our expectations, the carrier concentrations extracted from these data by fitting a Drude model, are $\sim$ 10$^{16}$ cm$^{-3}$ (unannealed) and $\sim$ 10$^{14}$ cm$^{-3}$ (annealed). We investigate the connection between grain size and carrier concentration.