We present a microscopic study of photoinduced charge generation in polyfluorene-based photovoltaic structures. The sub-100 nm lateral resolution of scanning Kelvin probe microscopy allows characterizing the three-dimensional structure of thin films of blends of poly-(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT) and poly-(9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylenediamine) (PFB). From the strong correlation between surface photovoltage and blend morphology, we propose a simple model for the lateral and vertical film structure identifying in particular those regions with the most efficient conduction pathway for the photocurrent.

We present an analytical and experimental investigation into the origin of the open-circuit voltage in the solid-state dye-sensitized solar cell. Through Kelvin probe microscopy, we demonstrate that a macroscopically uniform electric field exists throughout the nanocomposite between the electrodes. Considering a balance between drift and diffusion currents, and between charge generation and recombination, we develop an analytical expression for the open-circuit voltage which accurately follows experimental data. We find the opencircuit voltage increases with light intensity as 1.7 kT / q, where T is absolute temperature, however it decreases with increasing temperature and device thickness. The intensity dependence arises from the charge generation rate increasing more strongly with intensity than the recombination rate constant, resulting in increased chemical potential within the device. We find that the temperature dependence arises from a reduction in the charge lifetime and not from increased charge diffusion and mobility. The thickness dependence is found to derive from two factors; first, charge recombination sites are distributed throughout the film, enabling more charges to recombine in thicker films before influencing the potential at the electrodes, and second, the average optical power density within the film reduces with increasing film thickness.

A scanning Kelvin probe microscopy (SKPM) study of the surface potential of vacuum sublimed pentacene transistors under bias stress and its correlation with the film morphology is presented. While for thicker films there are some trapping centers inhomogeneously distributed over the film, as previously reported by other authors, by decreasing the film thickness the effect of thin intergrain regions (IGRs) becomes clear and a very good correlation between the topography and the potential data is observed. It is shown that in the thick pentacene grains the potential is homogeneous and independent of the gate bias applied with negligible charge trapping, while in the thin IGRs the potential varies with the applied gate bias, indicating that only an incomplete accumulation layer can be formed. Clear evidence for preferential charge trapping in the thin IGRs is obtained.

No abstract

No abstract