Electron transport in polyfluorene-based sandwich-type devices: Quantitative analysis of the effects of disorder and electron traps

Results of a combined experimental and modeling study of electron transport in a blue-emitting polyfluorene-based copolymer in sandwich-type devices are presented. We show how, for wide temperature and layer thickness ranges, an accurate and internally consistent drift-diffusion model description of the voltage-dependent current density can be obtained. We employ an adapted form of the "extended Gaussian disorder model," within which the density of states (DOS) is described as a superposition of a Gaussian DOS and an exponential DOS ("trap states"), characterized by only a small set of physically meaningful parameters. A comparison is made with the hole mobility reported for related polymers

Hole transport in polyfluorene-based sandwich-type devices : quantitative analysis of the role of energetic disorder

The current density versus voltage [J (V)] curves of hole-only sandwich-type devices containing a blue-emitting polyfluorene-based copolymer were measured for a wide range of temperatures and for several thicknesses of the active organic layer. We show that the J (V) curves cannot be accurately described using a commonly used model within which the mobility depends only on the electric field, but that a consistent and quantitatively precise description of all curves can be obtained using the recently introduced extended Gaussian disorder model (EGDM). Within the EGDM, the mobility depends on the electric field and on the carrier concentration. Two physically interpretable parameters, viz. the width of the density of states, ?, and the density of transport sites, Nt, determine the shape of the curves. For the semiconductor studied, we find ?=0.13Ý0.01 eV and Nt = (6Ý1) × 1026 m-3. Consistent with the EGDM, the logarithm of the mobility in the low carrier concentration and low-field limit is found to show a 1/ T2 temperature dependence. It is shown that analyses which neglect the carrier-concentration dependence of the mobility yield an apparent 1/T temperature dependence, as reported for many different materials, and that the incorrectness of such an approach would readily follow from a study of the layer thickness dependence of the mobility

Functional LH1 antenna complexes influence electron transfer in bacterial photosynthetic reaction centers

The effect of the light harvesting 1 (LH1) antenna complex on the driving force for light-driven electron transfer in the Rhodobacter sphaeroides reaction center has been examined. Equilibrium redox titrations show that the presence of the LH1 antenna complex influences the free energy change for the primary electron transfer reaction through an effect on the reduction potential of the primary donor. A lowering of the redox potential of the primary donor due to the presence of the core antenna is consistently observed in a series of reaction center mutants in which the reduction potential of the primary donor was varied over a 130 mV range. Estimates of the magnitude of the change in driving force for charge separation from time-resolved delayed fluorescence measurements in the mutant reaction centers suggest that the mutations exert their effect on the driving force largely through an influence on the redox properties of the primary donor. The results demonstrate that the energetics of light-driven electron transfer in reaction centers are sensitive to the environment of the complex, and provide indirect evidence that the kinetics of electron transfer are modulated by the presence of the LH1 antenna complexes that surround the reaction center in the natural membrane