b$.$ Explain what are 'direct' and 'indirect' band gap semiconductors and discuss

their band structure in terms of efficiency of adsorption of solar radiation in

solar cells. In particular, which type is more efficient at adsorption of the solar

visible light spectrum and why?

$30\%$

c$.$ On the basis of the continuous equation describe what physics phenomena

contribute to the overall generated current in solar cells. Why does the

efficiency of solar cells vary for different semiconductor materials? Discuss at

least $5$ reasons.