Photo-electrochemical characterization of CIGSn lamellar compounds: potential candidates for photoinduced applications

Metal chalcogenide semiconductors are being widely investigated for applications in solar energy conversion, such as photovoltaics and visible light photocatalysis. Herein, an initial assessment of potentialities of new lamellar chalcogenides named CIGS_n is provided, while comparing them with that of the well-known CIGS chalcopyrite. The main difference between CIGS and CIGS_n compounds concerns their electronic properties and more precisely the nature of charge carriers. Cu_0.32In_1.74Ga_0.84S₄ (CIGS₄) is an n-type semiconductor, unlike the chalcopyrite CuIn_0.7Ga_0.3S₂ (CIGS) that is a p-type semiconductor. More noticeable, Cu_1.44In_2.77Ga_0.76S₆ (CIGS₆) and in a lesser extent Cu_0.65In_1.75Ga_1.4S₅ (CIGS₅), exhibit an ambipolar character with a slight predominance of electron transport. The Fermi levels of all lamellar CIGS_n compounds are similar (-4.5 eV) and higher that of the chalcopyrite CIGS (-5.1 eV). In addition, the charge carrier densities of CIGS_n compounds (1014 - 1017 cm-3) are significantly lower than that of CIGS (1020 cm-3), which is consistent with their higher resistivity. Photoluminescence measurements and OCP decays suggest much more in-gap defect states in the lamellar compounds. These results suggest that CIGS_n compounds would not be suitable for photovoltaic applications. Nevertheless, their energy bands show an interesting positioning, with respect to redox potentials involved in water splitting and CO₂ reduction. In addition, ambipolarity could enhance the efficiency of catalytic reactions, because a type of minority charge carriers does not limit the charge transport.