I'm reading a paper on photovoltaic hydrogen production and the author claims that for reasonable solar efficiencies the band gap must be less than 2.0eV. This I understand. My question is how will it effect the photon absorption if the band gap is much smaller. Say the band gap energy is 0.5eV and a photon of energy 6eV strikes the semi--conductor. Will the photon be absorbed or is its energy too high? For efficient absorption of solar light, there is clearly an upper limit for band edge energy but is there a lower limit? Basically I want to know if I can make the following statement - "the lower the band edge energy, the greater the absorption."