N to P-type transition with narrowing optical bandgap and increasing carrier concentration of spin coated Cu doped ZnS thin films for optoelectronic applications

To accomplish the demand of optimized n-type buffer layer in photovoltaic cells; we have studied and analysed various doping compositions of Copper (Cu) into Zinc Sulfide (ZnS) thin films. A series of Cu doped ZnS thin films are fabricated on glass substrate by using spin coating technique at optimized parameters. X-Ray diffraction (XRD) technique revealed that Cu doped ZnS thin films have single phase wurtzite (hexagonal) structure up to 4% doped samples. However, XRD spectra of 6% Cu doped thin films depicted an extra peak of CuS phase that confirmed the secondary phases are formed due to inadequate incorporation of Cu in ZnS structure. Scanning electron microscope (SEM) images showed that the fabricated thin films are compact, dense and homogeneous without voids and cracks on the surface. Energy dispersive X-ray (EDX) analysis indicated that stoichiometric ratio of Zinc to Sulfur is maintained except for 6% Cu doped concentration. Fourier transform infra-red (FTIR) absorption spectrum of thin films series confirmed the presence of ZnS vibrational bonds along with vibrational and stretching bonds of carbon, oxygen and hydrogen at appropriate positions. Cu doped thin films from 0% to 4% exhibited n-type character, while 6% Cu doped ZnS sample unveiled p-type nature may be due to CuS sec-ondary phase's formation. UV-Vis optical spectroscopy revealed that the energy band gap also decreased from 3.64 eV to 3.24 eV with varying Cu concentration from 0% to 6% doped ZnS thin films. The energy band gap of Cu doped ZnS is decreased may be due to the existence of Cu-3d state in the upper part of the valence band.