Halogenated graphene derivatives as an absorber layer for solar cell applications : A DFT study with vdw correction

Abstract

Due to the flexible nature and high transparency of Graphene (G) in the ultraviolet to infrared spectrum, it is ideal for thin-film solar cell applications. To use graphene for such a purpose, there is a need to tune its band gap at Fermi level, and the most convenient method is chemical functionalization. So, in this study, we will investigate electronic and mechanical properties, along with the structure and stability analysis of halogenated (X) bilayer and trilayer graphene B(T)LG using density functional theory with van der Waals correction (vdw-GGA). These halogenated B(T)LGs are designed by adsorbing the top and bottom surfaces with fluorine (F) and chlorine (Cl) at the maximum possible separation. BLGs are designed as X/graphene/graphene/X with bernal (AB) and non-bernal (AA) stacking. TLGs are designed as X/graphene/graphene/graphene/X as AAA, ABA, and ABC stacking with 50% X termination at the top and 50% on the bottom side. Chlorination results in weak physisorption while fluorination results in chemisorption. Binding energies Eb found quite higher for fluorinated bilayer graphene derivatives. Halogenation of TLG disturbs Dirac cones at Fermi level in case of fluorine adsorption while remaining unaffected under chlorination. Due to repositioning of some states above EF, provide an additional path for intraband transition, made systems more conductive except for fluorinated bilayer graphenes F-BLGs. A small band gap of 0.2 meV and 0.04 eV appeared at Fermi level of fluorinated non-bernal and bernal configuration, respectively. An E-field applied normal to the plane of F-BLGs can further tune the band gap. Due to underestimation of Band gap using GGA, hybrid functionals HSE06 were used for better estimation.