Structural Defects in Donor-Acceptor Blends: Influence on the Performance of Organic Solar Cells

Defects play an important role in the performance of organic solar cells. The investigation of trap states and their origin can provide ways to further improve their performance. Here, we investigate defects in a system composed of the small-molecule oligothiophene derivative DCV5T-Me blended with C-60, which shows power conversion efficiencies above 8% when used in a solar cell. Froma reconstruction of the density of trap states by impedance spectroscopy, we obtain a Gaussian distribution of trap states with E-t = 470 meV below the electron transport level, N-t = 8 x 10(14) cm(-3), and s(t) = 41 meV. From V-oc vs illumination intensity and open-circuit corrected charge carrier extraction measurements, we find that these defects lead to trap-assisted recombination. Moreover, drift-diffusion simulations show that the trap states decrease the fill factor by 10%. By conducting degradationmeasurements and varying the blend ratio, we find that the observed trap states are structural defects in the C-60 phase due to the distortion of the natural morphology induced by the mixing.