Structural stability of naphthyl end-capped oligothiophenes in organic field-effect transistors measured by grazing-incidence X-ray diffraction in operando

We report on microstructural durability of 5,5′-bis(naphth-2-yl)-2,2′-bithiophene (NaT2) in organic field-effect transistors (OFETs) in operando monitored by grazing-incidence X-ray diffraction (GIXRD). NaT2 maintains its monoclinic bulk motif in operating OFETs with a=20.31±0.06 Å, b=6.00±0.01 Å, c=8.17±0.04 Å and β=96.64±0.74°. Crystallites appear as a mosaic of single crystals reaching through the whole 50 nm thick active layer. The lattice parameters variation (<1%) falls within the statistical error of structure refinement when the OFET gate voltage is varied from 0 V to 40 V; or when the OFET is continuously cycled within this voltage interval over more than 10 h period. Within the first few cycles, both the hole mobility and threshold voltage are changing but then reach stable levels with an average mobility of ð3:25±0:04Þ 104 cm2=Vs and an average threshold voltage of 13:6±0:2 V, both varying less than 4% for the remainder of the 10 h period. This demonstrates crystalline stability of NaT2 in operating OFETs.