Vacancy ordered Cs2SnX6 (X = Cl, Br, I) double perovskites as an absorber and antiferromagnetic NiO with GO as a hole transport layer for highly efficient heterojunction solar cell application

•A new class of vacancy ordered double perovskites Cs2SnX6 (X = Cl, Br, I) systems for photovoltaic applications.•Cs2SnI6 VODP with bandgap ∼ 1.45 eV and large absorption coefficient for single junction solar cell.•27 % photovoltaic efficiency for Cs2SnI6 absorber with GO and NiO as hole transport layers in inverted device structure.•GO as protecting layer for NiO to avoid its degradation such as formation of iodides of nickel.•The possibility of tandem junction solar cells using higher bandgap Cs2SnBr6 (Eg ∼ 2.74 eV) with lower bandgap Cs2SnI6 VODPs.