Multinary semiconducting compounds of the I-III-VI2 group (I – Cu, Ag; III – Al, Ga, In; VI – S, Se, Te) with crystal structure of chalcopyrite CuFeS2 are at the focus of current research as absorbing materials which are used in solar cells. By this reason studying the Cu-Fe-S system and determination the composition ranges of the existence of ternary compounds in this system can help to develop the methods of the improvement of the properties of the above-mentioned compounds of the I-III-VI2 group for solar cells. The goal of the present paper is to prepare thin films of haycockite Cu4Fe5S8 and to study their optical properties.
Thin films on glass substrates were deposited by flash evaporation technique from previously prepared ingots of haycockite Cu4Fe5S8. Morphology of thin films and their chemical composition were investigated using the Hitachi TM3000 Tabletop Microscope equipped by Energy Dispersive Spectrometer (EDS). The magnification was varied from ×50 to ×10000. The X-ray studies were carried out using monochromatic Cu Kα-radiation (1.5406 Å, step size 0.01° or 0.04°, counting time 10 s). The Rietveld analysis of the X-ray powder diffraction data was done using the FullProf software. X-ray photoelectron spectroscopy (XPS) measurements were performed with PHI 5000 VersaProbe II XPS system using a monochromatic Al Kα source (15 kV). The pass energy was 117.4 eV for survey scans and 29.35 eV for high-resolution scans. The pressure in the analysis chamber was maintained below 5×10-8 Pa for data acquisition. The data was processed using Multipak software, version 9.6.0. The XPS data was internally referenced to the C-C C1s peak (BE for C-C of 284.78 eV). The optical transmission was measured using Cary 500 spectrophotometer in the wave lengths interval from 400 nm to 2800 nm. Studies of the effects of illumination on the surface voltage were taken using surface photovoltage (SPV) technique in the energy range of 0.5 – 5.0 eV.
It was found that thin films have the chemical composition with the atomic content of Cu, Fe, and S of 25.21, 27.77, and 47.02 at. % with the atomic ratios of Cu/Fe and S/ (Cu + Fe) equaling to 0.91 for the atomic ration Cu/ Fe and 0.90 for the atomic ratio S/ (Cu + Fe) while these theoretical values for haycockite Cu4Fe5S8 are equal to 0.80 and 0.89 respectively. Thin films of chalcopyrite consist of separate grains with the approximately equal areas of about (1000 – 2000) μm2. It may be proposed that this structure appeared during cooling thin films because it completely covers the surface of thin films. The small inclusion of the second phase with the chemical composition close to talnakhite Cu9Fe8S16 was detected. The most common occurrence of the inclusion of the second phase along the borders of the grain shows that they are may be also responsible for the cracking of thin films. Energy band gap was estimated using both transmission and SPV methods. Usage of thin films of haycockite Cu4Fe5S8 as absorber in solar cells is discussed.
Acknowledgment. B. Korzun would like to thank PSC-CUNY for financial support of the studies under project TRADA-49-552.