Origin of Hole Transport in Small Molecule Dilute Donor Solar Cells

Dilute donor organic solar cells (OSCs) are a promising technology to circumvent the trade-off between open-circuit voltage (V_oc) and short-circuit current density (J_sc). The origin of hole transport in OSCs with donor concentrations below the percolation threshold is diversely discussed in the community. Herein, both hole back transfer and long-range hopping (tunneling) are analyzed as possible mechanisms of photocurrent in small molecule dilute donor OSCs using kinetic Monte Carlo (kMC) simulations. In contrast to previous kMC studies, the driving force for exciton dissociation is accounted for. As a study system, nitrogen-bridged terthiophene (NBTT) molecules in a [6,6]-phenyl-C70-butyric acid methyl ester (PC₇₁BM) matrix are investigated. The simulations show that hole back transfer from the small molecule donor to the fullerene matrix explains the measured concentration dependences of the photocurrents as well as the J_sc dependence on the light intensity for donor concentrations below 5 wt%. For 5 wt%, distances between NBTT molecules decrease to reasonable ranges that long-range hopping or tunneling cannot be discounted. Compared with polymer donors, larger hole localization is observed. The results emphasize that the barrier for hole back transfer is not only due to the highest occupied molecular orbital (HOMO) offset, but also by hole localization.