Investigation of electronic quality of chemical bath deposited cadmium sulphide layers used in thin film photovoltaic solar cells

Abstract

The investigation of electronic quality of chemical bath deposited cadmium sulphide (CdS) layers was the main objective of this work. For completeness, the CdS layers were characterised using X-ray diffraction, atomic force microscopy, optical absorption, photoelectrochemical cell, DC electrical conductivity measurements, current–voltage and capacitance–voltage measurements using Gold/CdS Schottky contacts. It has been found that the CdS layers grown are hexagonal with (002) preferential orientation. The n-type CdS materials show 1–2 μm clusters consisting of 0.3–0.4 μm size crystallites. The optical band gap is 2.42 eV, which shows a red-shift to 2.25 eV upon heat treatment. Gold Schottky contacts produce large Schottky barriers of 1.02 eV with ideality factors of 1.50, indicating excellent electronic qualities. Schottky–Mott plots indicate a moderate doping concentration of 1.2×10 17 cm −3, suitable for electronic device fabrication. However, the DC electrical conductivity measurements carried out at room temperature indicate a very low electrical conductivity in the range (4–11)×10 −5 (Ω cm) −1. This indicates a very low mobility value of (2–5)×10 −3 cm 2 V −1 s −1, which are five orders of magnitude below that of single crystal CdS. The way forward for further improvement of the electrical conductivity is discussed.