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 5wt%. For 5wt%, 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.

Automated summary

In this study, the mechanisms of photocurrent in dilute donor organic solar cells (OSCs) were analyzed using kinetic Monte Carlo (kMC) simulations, with a focus on hole back transfer and long-range hopping. The research found that hole back transfer can explain the concentration dependences of photocurrents and the Jsc dependence on light intensity.