Efficient intermonomer thermally activated delayed fluorescence is demonstrated for the first time, opening a new route to achieving high-efficiency solution processable polymer light-emitting device materials. External quantum efficiency (EQE) of up to 10% is achieved in a simple fully solution-processed device structure, and routes for further EQE improvement identified.

We present a Monte Carlo model of electron transport in mesoporous films of TiO2 particles in which electrons execute a random walk through a chain of spherical grains with traps at the surface of the grains. This has been used to simulate transient photocurrents in dye sensitized nanocrystalline solar cells. By comparing our results with a model based on solutions of the continuity equation for the free and trapped electron densities in which the film is treated as a homogeneous medium, we find that necks between grains have a noticeable effect on reducing the photocurrent. Values of an effective electron diffusion coefficient have been deduced by comparing an analytical solution to the continuity equation for the free electrons, in which the traps and back-reaction are ignored, with the numerical results from the Monte Carlo and continuum models. To the authors’ knowledge, this is the first time that the influence of the grain connectivity on electron transport has been modeled.

We have examined the combined effects of grain morphology and electron trapping on the transient response of photoelectrons moving through the TiO2 grains in a dye-sensitized nanocrystalline solar cell using a multi-time-scale random walk Monte Carlo model. Our use of a multi-time-scale approach enables us to simulate transport for electrons moving through spherical connected grains in a three-dimensional (3D) voided network and look at the effect of the size of interparticle boundaries on carrier dynamics. We can also address similar times to those over which measurements are taken, namely, 0.1 ms. These times are long because of deep traps in the TiO2 grains. The grains have 2-fold connectivity in one dimension (linear chains) or 4-fold or 6-fold connectivity in three dimensions and traps with an exponential distribution of energies. Photoelectrons are generated by a light pulse of short duration. The spatial distribution of the photogenerated electron density from this pulse either has a uniform profile or is peaked on the electrolyte side. We show that the constrictions at the grain necks slow the electrons, making trapping more likely and hence further delaying their passage to the extracting electrode. By comparing our results for 4-fold and 6-fold coordinated particles on a cubic lattice with 2-fold coordinated particles on linear chains, we show that transport is slowed in the former case due to the additional paths available to the electrons in the 3D network. We also find that the charge and current transients cannot be fit to an analytical solution of the continuum equations with an effective diffusion coefficient even at long times. Therefore, caution must be exercised when attempting to fit experimental transient data with an effective diffusion coefficient.

Molecular dynamics simulations have been used to calculate the self-diffusion coefficient, D, of the hard sphere fluid over a wide density range and for different numbers of particles, N, between 32 and 10 976. These data are fitted to the relationship D = D(infinity) - AN(-alpha) where the parameters D(infinity), A, and alpha are all density-dependent (the temperature dependence of D can be trivially scaled out in all cases). The value alpha = 1/3 has been predicted on the basis of hydrodynamic arguments. In the studied system size range, the best value of alpha is approximately 1/3 at intermediate packing fractions of approximately 0.35, but increases in the low- and high-density extremes. At high density, the scaling follows more closely that of the thermodynamic properties, that is, with an exponent of order unity. At low packing fractions (less than approximately 0.1), the exponent increases again, appearing to approach a limiting value of unity in the zero-density limit. The origin of this strong N dependence at low density probably lies in the divergence in the mean path between collisions, as compared with the dimensions of the simulation cell. A new simple analytical fit formula based on fitting to previous simulation data is proposed for the density dependence of the shear viscosity. The Stokes-Einstein relationship and the dependence of D on the excess entropy were also explored. The product Deta(s)p with p = 0.975 was found to be approximately constant, with a value of 0.15 in the packing fraction range between 0.2 and 0.5.

We demonstrate that the fast initial decay of a prototypical fluorescent polymer based organic light emitting diode device is related to the contribution that triplet–triplet annihilation makes to the device efficiency. We show that, during typical operating conditions, approximately 20% of the device efficiency originates from the production of singlet excitons by triplet–triplet annihilation. During prolonged device operation, the triplet excitons are quenched much more easily than the emissive singlets; thus, the contribution to the efficiency from triplet–triplet annihilation is lost during the early stages of the device lifetest. The fast initial decay of the device luminance can be removed by incorporating a triplet quenching additive into the active layer to remove any effect of triplet–triplet annihilation; this yields an increase in the device lifetime of greater than 3× and an even more significant improvement in the initial luminance decay.

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Light- and voltage-induced changes in the microwave reflectivity of semiconductors can be used to study the kinetics and mechanisms of electron transfer at semiconductor|electrolyte interfaces. The theory of the method is developed and illustrated by numerical calculations of the steady-state microwave response for low-doped silicon. The results define the range of rate constants that should be experimentally accessible using microwave reflectivity methods. The time and frequency responses of light-induced microwave reflectivity changes are considered, and it is shown that they can be used to derive values of electron transfer and recombination rate constants.

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