Dear Azhar

I think IPCE (incident photon conversion efficiency)is the easiest to measure, as you only need accurate measurement of the incident photons, which can be obtained using a thermal detector (blackbody absorber) or a calibrated photodiode. On the other hand,APCE (absorbed photon conversion efficiency) relies on the measurement of photons absorbed in the active layer of the cell. To obtain this amount you have to subtract the reflected photons and the transmitted ones from the incident ones (usually you need a spectrophotometer for this) Therefore you see these latter measurements can ccontribute a source of error in APCE measurements, which are therefore less reliable than IPCE ones, even though carryng some additional information. I hope these considerations are useful for you