Charge-carrier induced barrier-height reduction at organic heterojunction

In order to provide an accurate theoretical description of current density–voltage $\left(J\text{−}V\right)$ characteristics of an organic heterojunction device over a wide range of electric fields at various temperatures, it is proposed that an accumulation of charge carrier at the heterojunction will lead to a reduction in the barrier height across the heterojunction. Two well-known hole-transporting materials, $4,4^{\prime },4^{″}$-Tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine (MTDATA) and $N,N^{\prime }$-diphenyl-$N,N^{\prime }$-bis(1-naphthyl)($1,1^{\prime }$-biphenyl)-$4,4^{\prime }$ diamine (NPB), were used to fabricate unipolar heterojunction devices. It is found that the $J\text{−}V$ characteristics depend strongly on applied bias. The simulated $J\text{−}V$ characteristics of the heterojunction device, with the modified injection model, are found to be in excellent agreement with the experimental data.

Figure 1

(a) The normalized Gaussian energy distribution of the carrier density $p_{\text{int}1}\left(E_{\text{int}1}\right)$ at the injecting layer side of the interface. (b) The normalized fractional injection current densities $J\left(E_{\text{int}1}\right)$ contributed by the carriers at different energy levels. The results are compared with different values of the quasi-Fermi level $E_F$ (from $-0.2\text{eV}$ to $-0.4\text{eV}$).

Figure 2

A schematic diagram of energy-level alignment at the heterojunction with the boundary condition of (a) vacuum level alignment and (b) Fermi level alignment. The lower graphs are the corresponding alignment of the Gaussian DOS at both sides of the interface. The shaded region represents those states occupied by electrons.

Figure 3

The field and voltage dependence of the variable barrier ${\varphi }_v$ at 323 K $(◻)$, 288 K $(○)$, 266 K $(△)$, 219 K $(▽)$, and 172 K $(◇)$. Assuming thermodynamic equilibrium, ${\varphi }_v$ is calculated by Eq. (3) with $p_{\text{int}1}$ and $p_{\text{int}2}$ obtained by the steady-state simulation. The solid line is a fitting curve to an exponential decay function.

Figure 4

Measured $J\text{−}{F}_{\text{int}}$ characteristics of the ITO/MTDATA(320 nm)/NPB(390 nm)/Ag heterojunction device at 323 K $(◻)$, 288 K $(○)$, 266 K $(△)$, 219 K $(▽)$, and 172 K $(◇)$. The corresponding solid symbols are the calculation results based on the modified injection model in Eq. (1) with the variable barriers ${\varphi }_v$ in Fig. 3. The inset is the $J$ vs 1/T plot at $F_{\text{int}}=0.5\text{M}\text{V}/\text{cm}$. Fitting the calculation results (solid line) with Eq. (1) to the experiment data (opened symbols) obtains ${\varphi }_v=0.45\pm 0.1\text{eV}$, ${\sigma }_{\text{MTDATA}}=95\text{meV}$ and ${\sigma }_{\text{NPB}}=110\text{meV}$.