The main difference between the theoretical limit of solar energy conversion, like that of a Mueser engine at maximum concentration (see Sect. 4.1.4, where we found ^Mues = 0.86) and the Shockley-Queisser efficiency, representing the radiative limit of a single-gap absorber illuminated by sunlight without concentration (^Sq = 0.29, [1]) results from
• the excess energy of photons > eg which is converted into heat,
• the amount of photons hrn < eg not absorbed,
• the low photon solid angle Qin = ^Sun = 5.3 x 10“6 of non-concentrated sunlight1 compared to the solid angle for emission ^out (e. g., for flat absorbers with highly reflecting rear contacts ^out = 2ж)
Accordingly, amongst the many technological attempts to increase the optoelectronic properties of absorber semiconductors, such as reduction of bulk and surface recombination rates, two issues have been regarded as the most promising options for increasing the performance of photovoltaic solar-light conversion:
• Use of photons with energy above the optical band gap („« > eg).
• Stronger trapping of light in the absorber.
