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Article RSE 15002277

Low growth temperature MOCVD InGaP for multi-junction solar cells

Article

Energy Procedia, vol. 84 (2015) , pp. 34-402015.

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N. Armani (RSE SpA), G. Timo´ (RSE SpA) , G. Abagnale (RSE SpA), F. Trespidi (RSE SpA), M. Cornelli (RSE SpA), E. Malvisi (RSE SpA), F. Farina (RSE SpA), G. Carbi (RSE SpA), F. Rossi (IMEM CNR Parco Area delle Scienze), F. Fabbri (IMEM CNR Parco Area delle Scienze)

CONCENTRPV 2015 - Solar concentrating photovoltaic

The published manuscript summarizes the results obtained in the frame of a collaboration activity between the groups of concentrating photovoltaics in RSE-Piacenza and of electron microscopy of the CNR-IMEM institute of Parma. The structural and electro-optical properties of low growth temperature (500 °C) MOCVD InGaP layers grown on Ge substrates are presented. The study, carried out by combining high analytical and spatial resolution experimental techniques, confirms the good quality of the materials. The measurements evidence a detrimental effect of nanometric scale interface imperfections on the optical properties of the samples.

In view of the realization of high efficiency four-junction solar cells, InGaP layers, lattice matched to InGaAs, and (001) 6° off Ge substrate are grown by low pressure MOCVD at growth temperatures as low as 500 °C. The grown samples are undoped, p-type (doped by Zn) and n-type (doped by Te) materials with thickness around 1μm.

The ternary compound composition and structural properties are analysed by High Resolution X-Ray Diffraction and Transmission Electron Microscopy (TEM). Completely disordered InGaP layers are obtained with a target energy gap above 1.88 eV and a controlled Zn concentration around 1017 cm-3. The interface properties are studied by High Resolution TEM. A nanometric scale waviness is observed at the interface between InGaP and InGaAs and it is correlated to the step bunching of the substrate offcut.

In addition to this, HRTEM shows a 2-3 nm thin layer originated by atomic interdiffusion between the As- and the P- based compounds. The difference in composition of this interdiffusion layer is demonstrated by depth resolved Cathodoluminescence (CL), which reveals - approaching the InGaP/InGaAs interface, a blue shift of the InGaP related peak and the appearance of a new CL emission band ascribed to a quaternary InGaAsP compound.

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