Tuning Charge-Transfer States by Interface Electric Fields

Abstract

Intermolecular charge-transfer (CT) states are extended excitons with a charge separation on the nanometer scale. Through absorption and emission processes, they couple to the ground state. This property is employed both in light-emitting and light-absorbing devices. Their conception often relies on donor-acceptor (D-A) interfaces, so-called type-II heterojunctions, which usually generate significant electric fields. Several recent studies claim that these fields alter the energetic configuration of the CT states at the interface, an idea holding prospects like multicolor emission from a single emissive interface or shifting the absorption characteristics of a photodetector. Here, we test this hypothesis and contribute to the discussion by presenting a new model system. Through the fabrication of planar organic p-(i-)n junctions, we generate an ensemble of oriented CT states that allows the systematic assessment of electric field impacts. By increasing the thickness of the intrinsic layer at the D-A interface from 0 to 20 nm and by applying external voltages up to 6 V, we realize two different scenarios that controllably tune the intrinsic and extrinsic electric interface fields. By this, we obtain significant shifts of the CT-state peak emission of about 0.5 eV (170 nm from red to green color) from the same D-A material combination. This effect can be explained in a classical electrostatic picture, as the interface electric field alters the potential energy of the electric CT-state dipole. This study illustrates that CT-state energies can be tuned significantly if their electric dipoles are aligned to the interface electric field.

Keywords: charge-transfer states; color tuning; exciplex emission; interface electric fields; organic p–n junction.

Figure 1

(A) Sketch of several parameters contributing to the CT-state energy E_CT, cf. eq 1. Molecular structures of (B) the donor material m-MTDATA, (C) the p-dopant F₆-TCNNQ, and (D) the acceptor molecule TPBi. Photographs of (E) the general device layout, see the Experimental Section for fabrication details, and (F) the shift of the emission color of the m-MTDATA/TPBi CT state with increasing interface electric fields.

Figure 2

Sketch of the energy level diagrams, materials, CT excitons, and interface electric field magnitudes F_el = F_int + F_ex for different experimental conditions: (A) abrupt p–n junction at short-circuit (SC) condition, (B) p-i-n junction with intrinsic layers reducing F_int at the interface, (C) abrupt p–n junction under forward bias causing positive F_ex and a positive total electric field F_el. Energy values are taken from refs (−35). The dashed lines indicate the (quasi) Fermi levels.

Figure 3

Estimation of electric interface fields: (A) Classical understanding of F_int at a semiconductor p-(i-)n junction following ref (22). (B) Dependence of F_int on the intrinsic layer thickness using the classical p-i-n junction approach and a drift-diffusion (DD) model. (C) F_ex at the D–A interface (x = 70 nm in D) as a function of the external bias for the p–n junction (i = 0 nm). (D) Total electric field distribution in the p–n stack (i = 0 nm) under varying external bias, according to the DD model. (E) Shift of E_CT relative to the donor singlet (D*), free charge (FC) level, and ground state (GS) with changing interface electric field. (F) Rationalization of the E_CT shift by eqn 2, where the pure Coulomb potential (red) is superimposed by F_el = F_int + F_ex, which shifts E_CT by several 100 meV for typical CT exciton radii in our devices.

Figure 4

Experimental data for tuning the intrinsic layer thickness and thus F_int: EL spectra over (A) wavelength and (B) energy at V_ex = 5 V normalized to the maximum intensity of each spectrum. (C) Emission peak shift obtained by Gaussian fits of the emission spectra at V_ex = 5 V. (D) sEQE_PV characteristics and (E) the relative changes to the abrupt p–n junction under increasing intrinsic layer thickness at V_ex = 0 V.

Figure 5

Tuning the external bias, i.e., F_ex, in the abrupt p–n junction: EL over (A) wavelength and (B) energy at increasing V_ex normalized to the maximum intensity of each spectrum. (C) The corresponding shift of the emission peak as obtained by Gaussian fits. (D) sEQE_PV characteristics and (E) the relative change to short-circuit conditions.