Intensity modulated photocurrent (IMPS) and photovoltage (IMVS) spectroscopies were used to study the mechanism of photoprocesses in P3HT:PCBM bulk heterojunction organic solar cells at various light intensities. The use of the frequency domain techniques allowed us to separate the bulk and interfacial processes and gain a valuable insight into the mechanism of losses in these devices. The results provide direct evidence that interfacial nongeminate recombination is one of the dominant loss and aging mechanisms in bulk heterojunction organic solar cells. The trapping of photoexcited holes in the P3HT phase was found to contribute to the increased recombination rate. The results suggest that promising ways of improving the efficiency of bulk heterojunction solar cells may be reducing the charge trapping both at and near the P3HT:PCBM interface, as well as improving the efficiency of charge extraction at contacts.

The mesoscopic inhomogeneity of conducting polymer films obtained by electropolymerization and spin-coating was studied using Kelvin probe force microscopy (KFM) and current-sensing atomic-force microscopy (CS-AFM). A well-pronounced correlation was established between the polymer morphology, on the one hand, and its local work function (which is related to the polymer oxidation degree) as well as polymer conductivity, on the other. The most conducting regions were associated with the tops of the polymer grains and showed Ohmic behavior. They were surrounded first by semiconducting and then by insulating polymer. The conductivity of the grain periphery could be lower by as much as 2 orders of magnitude. The grain cores also showed consistently higher values of the local work function as compared to the grain periphery. This fact suggested that the grain cores were more oxidized and/or more ordered as compared to the grain periphery, which is in good agreement with the local conductivity data. More uniform morphology corresponded to less variability in the other properties of the polymer. A model is proposed that relates the observed inhomogeneity to preferential deposition of polymer molecules with higher molecular weight at the early stages of the polymer phase formation. The polymer deposition in either electropolymerization or various solution-casting techniques involves the nucleation of a new phase from a solution containing polymer fractions of different molecular weights. The driving force of the nucleation process depends on the solubility of the polymer fractions, which decreases with an increase in the molecular weight. This gives rise to preferential deposition of more crystalline, higher molecular weight polymer at the early stages of the polymer deposition to form the cores of the polymer grains. The fractions with lower molecular weights are deposited later and form less ordered/less conducting grain periphery. On the basis of this model, we conclude that, to ensure the formation of materials with low inhomogeneity and high quality, one should use the starting polymer with as narrow molecular weight distribution as possible. Yet another possibility is to use solvents which would reduce the differences in the solubilities of polymer fractions with different molecular weight.

No abstract

The degree of crystallinity in electronically conducting polymers can affect a variety of important properties such as the work function, conductivity, and charge mobility. In our previous work (O'Neil, K. D.;Shaw, B.; Semenikhin, O. A. J. Phys. Chem. B 2007, 111, 9253), we studied the distribution of the local conductivity and doping level of conducting polymers with nanometer resolution using Kelvin probe microscopy (KFM) and current-sensing atomic force microscopy (CS-AFM). An unambiguous correlation was found between the polymer morphology, the local oxidation degree (related to the work function), and the local conductivity. One of the possible explanations leading to this behavior was a variation in the crystallinity of polymer films during their nucleation and growth. In this work, direct measurements of the local crystallinity of a conducting polymer, polybithiophene, are performed at different stages of the electropolymerization process using phase imaging atomic force microscopy (AFM). It was found that, at the early stages of the polymer nucleation and growth, the polymer films were predominantly crystalline. At the later stages, the polymer contained both crystalline and amorphous phases, with the crystalline polymer located in the grain cores and the amorphous phase found at the grain periphery. These results are in remarkable agreement with the results of the KFM and CS-AFM measurements reported in our previous work, which relates such inhomogeneity to the presence of both high and low molecular weight polymer fractions in the electropolymerization solution (polydispersity). Furthermore, our data show that the inhomogeneity is not only longitudinal (different crystallinity of grain cores and grain periphery), but also latitudinal (there is a pronounced change in crystallinity between the inner and outer layers of the polymer films).

Intensity-modulated photocurrent spectroscopy (IMPS) was applied to studying the mechanism of photoelectrochemical decomposition of a model organic compound, ethanol, on a TiO2 anatase photoelectrode in aqueous solution. The frequency spectra of the intensity-modulated photocurrents observed on the TiO2 photoelectrode at low band bending were drastically changed in the presence of alcohol. Without the alcohol, the frequency dependence of the modulated photocurrents were similar to those reported previously for TiO2 electrodes and were typical for the case of surface recombination occurring on surface states, while upon addition of alcohol the photocurrent frequency dependence was no longer observed. This fact suggests that alcohol suppresses the recombination processes at the surface of the TiO2 photoelectrode. Possible mechanisms for this phenomenon are discussed. The results demonstrate that IMPS is a powerful tool for studying the mechanism of photocatalytic decomposition of organic pollutants on semiconductor electrodes.