Thermal evaporation of copper doped tin (II) sulfide absorber thin film for solar cell application

Abstract

Recent reports shows that tin sulphide (SnS) is a promising material as an absorber layer in solar cell application. However, SnS had to be doped with Copper (Cu) which could help to decrease the high resistivity. SnS with various Cu weight percentage (wt%) from 5 to 20 had been deposited on glass substrate using thermal evaporation technique. The thin films were annealed in vacuum environment at 200 oC for 2 hours using tube furnace. The structural properties was characterized using X-ray diffraction, Scanning Electron Microscope, Energy Dispersive X-ray and Atomic Force Microscopy. Optical properties was determined using ultraviolet-visible spectrometer while the resistivity was measured using Four Point Probe method at standard room pressure and temperature. The thin films were matched with polycrystalline SnS having Herzbergite crystal structure and grain size estimated to be ~ 21 to 30 nm. It had almost near stoichiometry and dopant element was found in the doped thin films. The deposited thin films had good surface coverage, good crystallinity and no pinhole or cracks on the surface. The thin films had low transmission at visible light region and absorption coeeficient were above 105 cm-1. It also revealed energy band gap around 1.52 to 2.03 eV. Doped thin film has lower band gap energy than the undoped whereas high dopant concentration caused the energy band gap to increase compared to low dopant percentage. Meanwhile, annealed thin films had lower energy band gap. The resistivity range from 0.59 to 1.80 ×103 O cm. Doped thin films has less resistivity than the undoped and annealing process tends to lower the resistivity. It was observed that the properties of SnS thin films when Cu was introduced had changed and tend to improve at lower weight percentage. This demonstrated doping could modify the optical and electrical properties of SnS thin films precisely in lowering band gap energy and resistivity, thus have potential to be used as solar cell application.