First principles study on electronic and optical properties of Cu<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"><mml:msub><mml:mrow /><mml:mn>2</mml:mn></mml:msub></mml:math>CoGeS<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si33.svg"><mml:msub><mml:mrow /><mml:mn>4</mml:mn></mml:msub></mml:math> for photovoltaic conversion and photocatalytic applications