Characterization of In2O3, ITO and CdS thin films for photovoltaic application
Frau Shuangzhou Wang (SONDERTERMIN AM MITTWOCH)

Abstract:
A wide variety of characterization methods were used to determine the key properties of undoped In2O3 and ITO as well as CdS thin films grown by molecular beam epitaxy for CdTe/CdS solar cell systems. The In2O3 and ITO films have been grown on quartz glass substrate and (111)‐oriented YSZ within a substrate temperature range of 200 to 690. Examinations on these films revealed optical, electrical and structural properties. UV‐VIS‐NIR spectroscopy shows highest transmittance of over 80% in the visible range for both In2O3 and ITO films. Tin doping gives rise to networks of nanowires, leading to soft, rough and thick ITO films on quartz glass and (111)‐YSZ substrates. X‐ray diffraction reveals a random orientation of grains in films grown on quartz glass, whereas a growth orientation of (222) in films on (111)‐YSZ, even in the presence of nanowires. The obtained transparent conductive films contain activated charge carrier concentrations of up to 8.2 x 10¹⁸ cm^‐3 in In2O3 and up to 6.2 x 10¹⁹ cm^‐3 in ITO. In case of CdS films grown on quartz glass substrate, X‐ray diffraction reveals coexistence of cubic and hexagonal phase. AFM images depict smooth surfaces with RMS roughness in the range of 2,8 to 6,0 nm and grain sizes varying from 20 to 100 nm. Optical transmission spectra show average transmittance of more than 60% in the visible region. Photoluminescence measurement suggests sulfide vacancies (V_S ²⁺) and complexes (Cd_i‐V_Cd) as possible defect levels which might be responsible for low mobility and conductivity of the CdS films.