Direct Determination of the Hole Density of States in Undoped and Doped Amorphous Organic Films with High Lateral Resolution

We investigate the density of states (DOS) for hole transport in undoped and doped amorphous organic films using high lateral resolution Kelvin probe force microscopy. Measurements are done on field effect transistors made of $N,N^{\mathrm{I}}$-diphenyl-$N$, $N^{\mathrm{I}}$-bis(1-naphthyl)-1,10-biphenyl–4,$4^{\mathrm{II}}$-diamine undoped or $p$ doped with tetrafluoro-tetracyanoquinodimethane. We determine the DOS structure of the undoped material, including an anomalous peak related to interfaces between regions of different surface potential, the DOS doping-induced broadening, and doping-induced sharp peaks on the main DOS distribution.