Monitoring the Charge-Carrier-Occupied Density of States in Disordered Organic Semiconductors under Nonequilibrium Conditions Using Thermally Stimulated Luminescence Spectroscopy

Abstract

The dynamics of charge carriers in disordered organic semiconductors is inherently difficult to probe by spectroscopic methods. Thermally stimulated luminescence (TSL) is an approach that detects the luminescence resulting from the recombination of spatially-well-separated geminate charge pairs, usually at low temperature. In this way, the density of states (DOS) for charges can be determined.In this study, we demonstrate that TSL can also be used for probing an occupied density of states formed by a low-temperature energetic relaxation of photogenerated charges. Another approach used to gain an insight into the charge-relaxation process is kinetic Monte Carlo (KMC) simulations.Here, we use both techniques to determine the energetic distribution of charges at low temperatures. We find that the charge dynamics is frustrated, yet this frustration can be overcome in TSL by using an infrared (IR) push pulse, and in KMC simulations by a long simulation time that allows for long-range tunneling. Applying the IR-push TSL to pristine amorphous films of 18 commonly used low-molecular-weight organic light-emitting diode mate-rials, we find that the width of the occupied DOS amounts to about 2/3 of the available DOS. The same result is obtained in KMC simulations that consider spatial correlations between site energies. Without the explicit consideration of energetic correlations, the experimental values cannot be reproduced, which testifies to the importance of spatial correlations for charges.

A spectroscopic assessment of static and dynamic disorder in a film of a polythiophene with a planarized backbone

“…40 Otherwise, the distribution of occupied states narrows up to about 2/3 of the total DOS width. 30 Since the standard deviation of the absorption spectrum of the PDOPT film is about 35 meV one would expect (i) that the PL spectrum would first bear out a red-shift upon sample cooling but tend to saturate at a temperature of (150 ± 15) K and (ii) that the width of the PL-spectrum would amount to at least 23 meV.…”

“…15–29 This spectral diffusion is well known to be temperature dependent and to address different subsets of states. 30 We wanted to examine how the inhomogeneously broadened absorption and fluorescence spectra of a neat PDOPT film evolve upon cooling from 300 K to 5 K. This should provide a handle on a correlation between inhomogeneous line-broadening, the transition energy and, concomitantly, the conjugation length distribution.…”

Charge-carrier photogeneration in single-component organic carbazole-based semiconductors via low excitation power triplet-triplet annihilation

“…Similar behavior has been observed for the action spectra of TSL measured for various PPV derivatives [20,23]; namely, the TSL signal drastically increased with increasing excitation photon energy well above the S 0 → S 1 absorption edge of these polymers. Thus, since TSL in organic semiconductors is believed to arise from the recombination of long-range geminate charge-carrier pairs [17,24,25], the TSL action spectra can be used to assess the carrier photogeneration efficiency in response to excitation photon energy. It is important to note that TSL probes geminate pairs generated predominantly through intrinsic photogeneration.…”

Impact of Photoluminescence Imaging Methodology on Transport Parameters in Semiconductors

“…We conducted PL imaging in two different ways, i.e., using delayed PL imaging and steady state PL imaging methods, and found that the values of D T or L d for the triplet excitons are different at different time scales. It is a common practice in the community to use the delayed PL imaging method, where PL images at early time scales are often used to determine D T . ,,,, This value of D T is then taken as a constant, even though it is well-known that excitons, like charges, relax in the Gaussian density of states (DOS). , Because this energetic relaxation does not require any thermal activation, transport at early times is fast until thermal equilibrium is attained . For charges and singlet excitons, this relaxation in the DOS occurs typically on a picosecond time scale .…”

“…16,17,19,20,22 This value of D T is then taken as a constant, even though it is well-known that excitons, like charges, relax in the Gaussian density of states (DOS). 37,38 Because this energetic relaxation does not require any thermal activation, transport at early times is fast until thermal equilibrium is attained. 39 For charges and singlet excitons, this relaxation in the DOS occurs typically on a picosecond time scale.…”