Deep defects play an important role for the use of semiconductors in any opto-electronic device since they act as recombination centres which reduce the number of photogenerated charge carriers. Chalcopyrites are used as absorbers in thin film solar cells and have reached the highest efficiencies of all thin film technologies. Because of their importance for solar cell operation, an extended literature exists about deep defects in chalcopyrite semiconductors, mostly based on AC capacitance and transport measurements. However, it has been recently argued that the observed signals cannot be attributed to defects but to mobility freeze out. Therefore it is necessary to detect defects by optical methods which are independent of transport properties. Room-temperature photoluminescence offers an excellent tool for this detection. The luminescence spectrum is determined, like the absorption spectrum, by the density of states. However it is much easier to detect an emission of extremely low intensity than to detect weak absorption. We plan to extend our photoluminescence measurement to energies well inside the band gap and thus detect deep defects. This data will be compared to the available literature data and will help to sort out if the capacitance and transport data is due to deep defects or due to mobility freeze out.