Well, this question may seem elementary to you but I simply do not have a background for it. Now the question: Sun radiation resembles a blackbody at over 5000°K. A solar cell's temperature is around 300°K. If we neglect loss in the cell, its emission and absorption coefficient are equal. Therefore, the cell's absorption is limited by the temperature of the cell and it should be very low because there is a huge gap between the radiation of a blackbody at 5000°K and 300°K. How can we understand the high efficiency of solar cells then? By the way, I have a second question: What are the main approximations in derivation of Planck's law of radiation?