Materials issues in very thin film CdTe for photovoltaics

A study is made of extremely thin absorber layers of CdTe deposited onto planar substrates in order to characterise their properties. Metal organic chemical vapour deposition (MOCVD) was used to deposit the CdS and CdTe films under controlled conditions of substrate temperature and VI/II organometallic ratio. In situ laser reflectance was used to monitor the thickness of the films and also the roughening from loss of reflected intensity. The roughness of the final structure was measured with atomic force microscopy (AFM) and compared with that of both indium tin oxide (ITO)/glass substrates and CdS deposited on them. Absorber layer thicknesses of between 50 and 500 nm were deposited, the thicker layers being grown in two stages, with a highly As doped cap layer to act as a contact. A degradation of the current–voltage characteristics was observed from good rectification at 500 nm thick absorbers to an ohmic characteristic at 100 nm thickness. This could not be simply explained by the non-coalescence of grains, based on the AFM results, but attributed to defects on the ITO/glass substrate. Improvements in the cleaning procedure resulted in good rectification for 100 nm absorber layer. It was demonstrated that CdTe covers non-planar CdS conformally within 50 nm.