Fill factor of planar heterojunction organic solar cells with varied donor materials

The dependence of fill factor (FF) on material properties in planar heterojunction (PHJ) organic solar cells (OSCs) was investigated. The OSCs were prepared using a series of small molecular electron donating materials with different highest occupied molecular orbital (HOMO) levels ranging from 5.1 to 5.7 eV and hole mobility (μ h ) varying from 3.00 × 10 −5 to 2.43 × 10 −3 cm V −1 s −1 . The results showed that donor materials with high mobility can significantly enhance the FF, which was partly due to the low series resistance according to the dark current density–voltage ( J – V ) characteristics. An optimal Δ E of 1.1–1.2 eV, which is the energy difference between the HOMO of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor, can also increase the FF. Meanwhile, the photocurrent densities ( J ph ) of the OSCs were simulated based on the Poole–Frenkel model and charge trapping effect to intuitively reveal the effect of μ h and Δ E on FF.