Solution synthesized CdS nanoparticles for hybrid solar cell applications

Nanocrystalline semiconductor materials have attracted considerable attention due to their particle size dependent physical properties and very large specific surface area that are not present in bulk materials. Usually they are synthesized with organic ligands to disperse them from agglomeration. However, these ligands could act as series resistance in the active layer of solar cells. In this work cadmium sulfide (CdS) precipitates were prepared from cadmium and sulfide ion solutions without any ligands at different solution temperature and synthesis time. The obtained CdS precipitates were washed by rinsing with methanol or by centrifugation with water and methanol. By using Fourier transform infrared spectroscopy (FTIR) impurities were observed on the surface of rinsed CdS samples and they had been eliminated after washing by centrifugation. X-ray diffraction (XRD) patterns indicate that the obtained products were of hexagonal structure. The average size of the CdS crystallites observed by transmission electron microscopy was of about 6 nm before centrifugal washing and 3 nm after washing. The band-gap (E g ) values of CdS particles were between 2.44 and 2.56 eV, estimated by the Tauc relation using the Kubelka–Munk expression for the absorption coefficient. Photoluminiscent spectra of the centrifugal washed CdS samples indicate the presence of surface states that probably were charge recombination centers in CdS:poly(3-hexylthiophene) hybrid solar cells. The best cell prepared with CdS nanoparticles showed a photocurrent (J sc ) of 2.2 mA/cm2 and a photovoltage (V oc ) of 0.84 V, measured in air at room temperature under 100 mW/cm2 illumination in a solar simulator.