The obtained charge carrier density is higher than observed for other layered crystals such as covalent organic frameworks, metal organic frameworks and transition metal dichalcogenides, for example MoS2 (Table ). The carrier densities closest to PTI are those of graphite and MoS2 bulk crystals (1 10 19 cm -3 and 3.7 10 19 cm -3 , respectively). The high carrier concentration might either be generated by an unknown structural defect or by adsorbed molecules like water or oxygen. Symmetric PTI devices with interdigitated indiumtin oxide electrodes are indeed sensitive to pressure, pointing towards water or oxygen acting as adsorbed dopants under environmental conditions (Figure ).