more accurately, the parasitic absorption within contact and interfacial layers has also to be taken into account, which can be obtained by using optical simulations of your organic solar cells, e.g. with the transfer matrix method.
A comprehensive study of different attempts to calculate the IQE for organic solar cells can be found here: http://pubs.acs.org/doi/abs/10.1021/ph400044k
IQE is defined as the number of absorbed photons in the solar cell to the number of collected carriers. Therefore it is very important to consider parasitic absorption via optical modelling.
An interesting discussion on the definition of IQE can be found in this paper
A. W. B. D.N. Micha, A. Walker, G. Siefer, F. Dimroth, “Influence of a Very Efficient Back Reflectors on the Quantum Efficiency of Solar Cells,” in EU-PVSEC, 2016, no. 100, pp. 20–24
recently, we made organic solar cells with 2 parameters. Device with higher absoprtion (UV-vis) and higher EQE gave lower density current ( and lower efficiency). How to explain this situation. Thank you very much.