Influence of active layer thickness on contribution of hole and electron trapping to threshold voltage instability in organic field effect transistors

Abstract

Effect of active layer thickness on threshold voltage stability was studied. Threshold voltage instability was found to be minimum for an optimum thickness of 15nm when operated under normal room light conditions. Influence of active layer thickness on individual contribution of hole and electron trapping effect to hysteresis was estimated. Hole and electron trap concentrations were found to exhibit different pattern of variation with thickness compared to each other. Film with 15nm thickness exhibited minimum hole trapping and maximum electron trapping effect. This could be attributed to reduced grain boundary density and availability of large photogenerated carrier concentration at the dielectric/semiconductor interface. Along with electron trap, hole trap concentrations were also shown to be significantly influenced by photogenerated carrier concentration at the interface. For further increase in thickness, increase in hole trapping and reduction in electron trapping were observed again. This could be attributed to (a) increase in grain boundary density due to volume effect and (b) lesser number of photogenerated carriers at the interface due to limitations in exciton quenching length. Hole and electron trapping effect were suggested to be mutually affecting each other, depending on active layer thickness. Effect of active layer thickness was further confirmed with frequency and voltage dependent capacitance measurements.