Generating Light from Upper Excited Triplet States: A Contribution to the Indirect Singlet Yield of a Polymer OLED, Helping to Exceed the 25% Singlet Exciton Limit

Abstract

The mechanisms by which light is generated in an organic light emitting diode have slowly been elucidated over the last ten years. The role of triplet annihilation has demonstrated how the "spin statistical limit" can be surpassed, but it cannot account for all light produced in the most efficient devices. Here, a further mechanism is demonstrated by which upper excited triplet states can also contribute to indirect singlet production and delayed fluorescence. Since in a device the population of these T_N states is large, this indirect radiative decay channel can contribute a sizeable fraction of the total emission measured from a device. The role of intra- and interchain charge transfer states is critical in underpinning this mechanism.

Keywords: OLED; charge transfer states; delayed fluorescence; reverse intersystem crossing; triplet fusion, triplet–triplet annihilation.

Figure 1

a) Emission decay of PSBF:zeonex spincoated film recorded at 20 K (black circles). The initial part of the decay (up to ≈20 ns) is assigned to prompt fluorescence, the long tail appearing as a power law (from 200 ns to 1s) is assigned to delayed fluorescence arising from triplet–triplet annihilation. An intermediate decay channel is found between 20 and 200 ns. The red line is a fit with Equation (2). The curve was obtained by combining decays recorded using nanosecond gated time resolved spectroscopy (from 1 ns to 1 s, at 15 μJ per pulse excitation) and singlet photon counting techniques (from 3 ps to 10 ns, at <1 nJ per pulse excitation). b) Emission decay of PSBF spincoated film recorded at 20 K (black circles) and 296 K (red circles).

Figure 2

a) “Prompt” fluorescence (PF recorded 3 ns..–13 ns), phosphorescence (PH‐100 μs .. 5 ms), and delayed fluorescence (DF‐100 μs..5 ms) dependence on excitation energy from PSBF film recorded at 20 K. The slope of the DF power dependence being <2 indicates mixed linear and quandratic contributions to the total decay. b) Time evolution of emission from the PSBF spincoated film at 20 K. Initial vibronically resolved S₁ ¹(π, π*) emission is observed in the first nanosecond (black trace), spectral diffusion of singlet excitons red shifts this S₁ emission over ≈10 ns (red trace) and concomitant with the appearance of increased red edge emission. By 50 ns, this red unstructured emission dominates ascribed to intramolecular CT states.22 After 200 ns, and on into the microsecond regime delayed S₁ ¹(π, π*) again observed ascribed to TF. The CT state is ≈0.1 eV lower in energy than the S₁.

Figure 3

a) Decay of 0.01% PSBF in zeonex at 20 K when exciting with 3.68 eV at 0 time only (full squares) and when exciting at 3.68 eV at 0 time and at 1.96 eV at 177 ns (empty triangles, zoomed in the inset). Straight line denotes the excitation time of 177 ns with 1.96 eV pulsed laser. b) Time resolved spectra recorded from 178 ns..208 ns with 3.68 eV excitation at 0 time only, with 3.68 eV excitation at 0 time and 1.96 eV excitation at 177 ns, and with 1.96 eV excitation only (indicated appropriately). Single band pass interference filter was used to record emission signal.

Figure 4

Schematic energy level diagram for PSBF constructed from fluorescence, phosphorescence, and photoinduced absorption measurements and delayed fluorescence, see ref. 16. From this Jablowski scheme the various decay channels for initial and photoinduced excited states can be seen in PSBF. The S₁ state will be quenched by the ¹CT state by electron transfer, ¹CT can interconvert slowly by hyperfine coupling to ³CT. There is then a competition between TADF back to ¹CT and ³CT quenching to the lowest energy ³LE state of the system.