Physical properties of rf-sputtered ZnS and ZnSe thin films used for double-heterojunction ZnS/ZnSe/CdTe photovoltaic structures

Polycrystalline ZnX (X=S, Se) and CdTe thin films were prepared by rf magnetron sputtering and by thermal vacuum evaporation (CdTe films), respectively. The structural properties were studied by X-ray diffraction (XRD), which revealed that ZnX films are polycrystalline with a marked (111) texture. After irradiation with protons crystallite sizes decreased while mechanical strains increased. Thicknesses of ZnX films and surface roughness have been measured by X-ray reflectometry (XRR) with thickness values between 58 nm and up to 163 nm and with surface roughness between 1.7 nm and 2.4 nm. Morphological investigations were made by scanning electron microscopy (SEM), drops - free surfaces with compact and uniform aspect being deposited. Absorption and transmission measurements were carried out for all samples deposited on optical glass substrates. Experimentally determined bandgap energies were between 2.31-2.75 eV for ZnSe layers, respectively, 3.10-3.65 eV for ZnS films. Optical transmissions in VIS-NIR regions are higher than 60% for both ZnSe and ZnS films. Glass/ITO/ZnS/ZnSe/CdTe/Cu:Au structures in superstrate configuration were produced by depositing CdTe absorber layers by thermal vacuum evaporation (TVE). Action spectra of the external quantum energy (EQE) and the current-voltage (I-V) characteristics in AM 1.5 conditions (the density power of the incident light is equal with 100 mW/cm(2)) corresponding for double-heterojunction ZnS/ZnSe/CdTe photovoltaic structures were investigated before and after irradiations with high energy protons (3 MeV).