Correlating charge transport to structure in deconstructed diketopyrrolopyrrole molecules: A case study of a monomer*

Organic co-polymer semiconductors based on a diketopyrrolopyrrole (DPP) core have attracted a lot of attention due to their high p-type as well as n-type carrier mobilities in organic field-effect transistors (FETs) and high power conversion efficiencies in solar cell structures. FETs were fabricated from both dropcasted and evaporated n-dialkyl side-chain substituted thiophene DPP (Ph-TDPP-Ph) films; the device performance is sensitive to the film thickness as well as the film deposition method. Some of the best FET performances are with evaporated films with p-type carrier mobilities on the order of 0.1 cm²/Vs and on/off ratios of 10⁶. As measured from grazing incidence X-ray diffraction (GIXRD) in bottom-gate, bottom-contact FETs, Ph-TDPP-Ph thin film reveals a triclinic crystal structure with pi-conjugated stacking direction orientated in-plane. Depending on processing, the unit cell comprises either two or one monomer and may thus signal a shift from herringbone to coplanar orientation of the molecules.