Formation of intermediate bands in silicon by transition metal doping - A DFT/DFTB computational study

Abstract

The intermediate band solar cell offers the possibility of increasing the efficiency of modern-day solar cells. By having a narrow intermediate band placed in the conventional band gap, sequential absorption of photons is attainable. With suitable extraction of charge carriers, the photocurrent can increase whilst the voltage is preserved, leading to higher efficiencies. By using electronic structure calculations within density functional theory (DFT) and density-functional tight-binding (DFTB), we study the possible formation of intermediate bands when silicon is doped with a transition metal impurity. We particularly consider the 4d transition metal silver, but also investigate doping by transition metals of the 3d series V, Cr, Mn, Fe and Co. We employ density functional theory in the Kohn-Sham approach, using mainly the Perdew-Wang exchange-correlation functional (PW91) within the generalized gradient approximation. Both DFT and DFTB calculations are performed by using modeling suites in the Amsterdam density functional (ADF) package. It is found, by method of DFTB, that substitutional doping of bulk Si with silver at 0.4% impurity concentration, exhibits an intermediate band at the Fermi level, 0.28 eV above the valence band edge. However, it is also found that our DFTB calculations have poor accuracy, and that this result should be cross-examined by a more accurate method. DFT calculations reveal only contours of formation, yet no intermediate bands are found in silicon with silver impurity concentrations down to 6.25%. Doping with 3d transition metals at 6.25% impurity concentration have not revealed any fully formed intermediate bands.