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.

Figure 1

Schematic of the OFET structure and KPFM tip above the channel.

Figure 2

(a) Inset: $V_L$ profiles measured across an undoped transistor for $V_{\mathrm{DS}}=0\mathrm{V}$ and $V_{\mathrm{GS}}$ ranging from 0 to $-16.5\mathrm{V}$. (a) $V_L$ as a function of $V_{\mathrm{GS}}-V_t$ measured on undoped ($A$) and doped ($B$) samples. (b) DOS vs energy relative to $\mu$ for $V_{\mathrm{GS}}=V_t$ for undoped (solid triangles) and doped (solid circles) samples. The solid curves are a fitting of a Gaussian function [Eq. (2); curve $A$] and an exponential function [Eq. (4); curve $B$] to given ranges in the undoped sample DOS curve and a fitting of Eq. (3) (curve $C$) to the doped sample DOS curve.

Figure 3

(a) Inset: $V_L$ profile measured across the channel of an undoped transistor. (a) DOS distributions measured at different lateral locations across the channel marked as $a$–$f$ in the inset. (b) DOS distributions measured at different lateral locations across the channel of doped transistor. Each DOS curve is an average of three curves that were measured at the same location.