Multifunctional lead-free double perovskites <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si15.svg" display="inline" id="d1e2459"> <mml:mrow> <mml:mi mathvariant="normal">Cs</mml:mi> <mml:mn>2</mml:mn> <mml:mi mathvariant="normal">CuScX</mml:mi> <mml:mn>6</mml:mn> </mml:mrow> </mml:math> (X = Br, I) for photovoltaic and thermoelectric applications: A DFT and SCAPS-1D study