Determination of the intrinsic and the injection dependent charge carrier density in organic solar cells using the suns-VOC method
A method is presented to calculate the intrinsic and injection dependent average charge carrier density from the transport resistance of an organic solar cell. The latter is determined using a combination of a Suns-VOC and a current-voltage measurement under illumination which allows to split the total series resistance of the solar cell into an (ohmic) contribution of the circuitry and an injection dependent part which is caused by the transport of the charge carriers through the photoactive layer. In the derivation of the formula for the average charge carrier density, spatially homogeneous generation rate and gradients of the quasi Fermi levels as well as balanced mobilities had to be assumed. However, numerical simulations revealed that even for strongly inhomogeneous generation and unbalanced mobilities the results are reasonably accurate proving the practical applicability of the presented method. Using an inverted ITO-free P3HT:PCBM solar cell, we determined a value for the intrinsic charge carrier density of n i = 2.88 × 1011 cm−3 at T = 300 K and for the average density of mobile charge carriers at ""1 sun"" under open circuit conditions we obtained n av = 3.2 × 1016 cm−3.