The Influence of Isomer Purity on Trap States and Performance of Organic Thin‐Film Transistors

Organic field‐effect transistor (OFET) performance is dictated by composition and geometry, as well as the quality of the organic semiconductor (OSC) film, which strongly depends on purity and microstructure. When present, impurities and defects give rise to trap states in the band gap of the OSC, lowering device performance. Here, 2,8‐difluoro‐5,11‐bis(triethylsilylethynyl)‐anthradithiophene is used as a model system to study the mechanism responsible for performance degradation in OFETs due to isomer coexistence. The density of trapping states is evaluated through temperature‐dependent current–voltage measurements, and it is discovered that OFETs containing a mixture of syn and anti‐isomers exhibit a discrete trapping state detected as a peak located at ≈0.4 eV above the valence‐band edge, which is absent in the samples fabricated on single‐isomer films. Ultraviolet photoelectron spectroscopy measurements and density functional theory calculations do not point to a significant difference in electronic band structure between individual isomers. Instead, it is proposed that the dipole moment of the syn‐isomer present in the host crystal of the anti‐isomer locally polarizes the neighboring molecules, inducing energetic disorder. The isomers can be separated by applying gentle mechanical vibrations during film crystallization, as confirmed by the suppression of the peak and improvement in device performance. Density of states calculations on organic field‐effect transistors of the mix‐isomer semiconductor 2,8‐difluoro‐5,11‐bis(triethylsilylethynyl) anthradithiophene reveal a discrete trapping state that is not present in the syn or anti‐isomer devices. The presence of both isomers in the same electrically active layer introduces this energetic state and can be suppressed by phase separation through vibration‐assisted crystallization.