Effect of Gallium on Graded Cu(In1−xGax)S2 Thin Films for Solar Cells Prepared by Chemical Spray Pyrolysis

Abstract

The chalcopyrite semiconductor Cu(In <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1−x</inf> Ga <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> )S <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> (0 < x < 1),) is a potential candidate for absorber material in thin-film solar cells due to higher absorption coefficient (~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> ), higher radiation stability, non-WRxLFLW\ and to direct band gap adjustable. In this work, the ternary CIGS thin films have been deposited by chemical spray pyrolysis on preheated glass substrates using different concentrations of gallium in the spray solutions. The dependence structural, morphological, compositional and optical properties of the CIGS thin films have been studied using X-ray diffraction (XRD), Raman scattering measurements, scanning electron microscopy (SEM), optical absorption techniques and photoluminescence (PL) spectra respectively. The X-ray spectra reveal that the CuIn <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1−x</inf> Ga <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> S <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> thin films are of chalcopyrite crystal structure with a highly (112) preferential orientation. The main XRD peaks showed a noticeable shift to higher diffraction angles with increasing Ga content, which was attributed to Ga atoms substituting for In atoms in the chalcopyrite structure. The grain size of CIGS films decreased with increasing Ga content presumably, and pores formed on the surface. Raman spectroscopy analysis indicates that the sprayed thin films are grown with two different structures, Cu-Au (CA)-ordered phase and chalcopyrite (CH). The Optical properties was calculate from the measured spectral transmittance T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ</inf> and reflectance R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ</inf> allow us to determine the direct band gap energy value which increases by increasing the Ga content and it is in the range 1.41 – 1.50 eV, indicating that Cu(In <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1−x</inf> Ga <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> )S <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> compound has an absorbing property favorable for photovoltaic applications.