Excited State Relaxation in Vacuum Deposited and Solution Processed Films of Merocyanine/Fulerene Blends

Exciton dynamics in merocyanine/fulerene blend films made by vacuum deposition and solution processing techniques were investigated by means of steady-state and time resolved fluorescence and absorption spectroscopy. Intermolecular charge transfer states are formed during several ps in neat merocianine films, which determine their fluorescence properties. Fullerene additives cause formation of new heterogeneous charge transfer states. Even a small fullerene concentration significantly influences the exciton dynamics by quenching inherent merocianine fluorescent states and causing appearance of new fluorescence bands caused by the charge transfer states between merocyanine and fullerene molecules. All fluorescence bands are quenched in films with high fulerence concentration due to the charge carrier generation, and the quenching effect is stronger in vacuum deposited films. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3529

Electrode Materials, Thermal Annealing Sequences, and Lateral/Vertical Phase Separation of Polymer Solar Cells from Multiscale Molecular Simulations

The nanomorphologies of the bulk heterojunction (BHJ) layer of polymer solar cells are extremely sensitive to the electrode materials and thermal annealing conditions. In this work, the correlations of electrode materials, thermal annealing sequences, and resultant BHJ nanomorphological details of P3HT:PCBM BHJ polymer solar cell are studied by a series of large-scale, coarse-grained (CG) molecular simulations of system comprised of PEDOT:PSS/P3HT:PCBM/Al layers. Simulations are performed for various configurations of electrode materials as well as processing temperature. The complex CG molecular data are characterized using a novel extension of our graph-based framework to quantify morphology and establish a link between morphology and processing conditions. Our analysis indicates that vertical phase segregation of P3HT:PCBM blend strongly depends on the electrode material and thermal annealing schedule. A thin P3HT-rich film is formed on the top, regardless of bottom electrode material, when the BHJ layer is exposed to the free surface during thermal annealing. In addition, preferential segregation of P3HT chains and PCBM molecules toward PEDOT:PSS and Al electrodes, respectively, is observed. Detailed morphology analysis indicated that, surprisingly, vertical phase segregation does not affect the connectivity of donor/acceptor domains with respective electrodes. However, the formation of P3HT/PCBM depletion zones next to the P3HT/PCBM-rich zones can be a potential bottleneck for electron/hole transport due to increase in transport pathway length. Analysis in terms of fraction of intra- and interchain charge transports revealed that processing schedule affects the average vertical orientation of polymer chains, which may be crucial for enhanced charge transport, nongeminate recombination, and charge collection. The present study establishes a more detailed link between processing and morphology by combining multiscale molecular simulation framework with an extensive morphology feature analysis, providing a quantitative means for process optimization

Ultrafast charge carrier mobility dynamics in poly(spirobifluorene-co-benzothiadiazole): Influence of temperature on initial transport

In this work we examine the influence of temperature on the transport dynamics of photogenerated charge carriers in pi-conjugated poly(spirobifluorene-co-benzothiadiazol) films. Investigations were performed by picosecond time-resolved electric field induced second-harmonic technique. The mobility is independent of temperature during the initial 100 ps. During this time, the carriers drift about 6 nm under an applied field of 1.2 x 10(6) V/cm, and the mobility decreases from 3 x 10(-2) cm(2)/V s by one order of magnitude. We attribute this change in mobility to the intrachain charge transport during carrier relaxation down in the density of states. At a time delay of 1 ns the transport is temperature dependent with an activation energy of about 22 meV

Hierarchical charge carrier motion in conjugated polymers

Motion of photogenerated charge carriers in p-conjugated polymer films with different disorder and chain orientation and also in a blend of conducting and insulating polymers has been investigated by means of time-resolved electric field-induced second harmonic generation technique. Experimental results and Monte Carlo simulation enabled us to distinguish three time domains of charge transport. The charge carriers become separated by about 10 nm in a strong electric field within 1 ps. Subsequently, carriers drift another 10-15 nm on a picosecond time scale with a high mobility. The third and the slowest carrier motion phase is well described by the stochastic drift and determines the macroscopic equilibrium mobility. We attribute the two ultrafast drift phases to carrier motion inside a conjugated segment and along a single polymer chain, respectively, whilst the slow motion phase involves interchain jumps. (C) 2010 Elsevier B.V. All rights reserved

Time-independent, high electron mobility in thin PC61BM films: Relevance to organic photovoltaics

Ultrafast optical probing of electric field by means of electroabsorption combined with conventional photocurrent measurements was employed to investigate the drift and mobility dynamics of photo-generated charge carriers in the pristine PC61BM film and in the blend with a merocyanine dye. Electrons passed a 40 nm thick PC61BM film within a few picoseconds with time-independent and weakly dispersive mobility. The electron mobility is 1 cm(2)/(V s) at 1 MV/cm and an estimate of the zero-field mobility yields 5 . 10(-2) cm(2)/(V s). The initial electron mobility in the blend is of the order of 10(-2) cm(2)/(V s) and decreases rapidly. We conclude that electron motion in PC61BM based organic bulk hetero-junction solar cells is limited by barriers between PC61BM domains rather than by intrinsic PC61BM properties. (C) 2014 Elsevier B.V. All rights reserved

Electric field assisted charge carrier photogeneration in poly(spirobifluorene-co-benzothiadiazole)

The dynamics of charge carrier generation in poly (spirobifluorene-co-benzothiadiazole) was investigated by electric field-induced fluorescence quenching and differential absorption measurements. Three different time domains of carrier generation have been identified: an ultrafast phase, a subnanosecond phase, and an entire lifetime phase. The charge generation efficiencies during the first and second phases were found to be almost independent of temperature, being about 25% and 10%, respectively, at an applied electric field of 1.3 x 10(6) V/cm, while the generation efficiency during the third phase increases from 2% at 80 K to 10% at room temperature. The results of transient spectroscopy measurements and quantum chemical calculations suggest an intramolecular charge transfer for about 1 ps from the alkoxy-substituted fluorene side group to the benzothiadiazole subunit of the main chain. The formation and evolution of the resulting charge transfer states determine the way of charge carrier generation. (C) 2010 American Institute of Physics. [doi:10.1063/1.3493589

Carrier Tunneling from Charge Transfer States in Organic Photovoltaic Cells

Charge transfer (CT) states play a key role in the functioning of organic solar cells; however, understanding the mechanism by which CT states dissociate efficiently into free charges remain a conceptual challenge. Here, the electric field dependent dynamics of charge generation in planar cyanine/fullerene photovoltaic cells is probed over a wide temperature range using time-resolved Stark effect experiments, transient absorption, and photocurrent measurements. Results indicate that dissociation of thermalized CT states is the rate-limiting step for all temperatures. The dissociation rate strongly depends on the field, but is temperature independent. The results also suggest that the yield of generated charges is temperature independent. Model electrostatic calculations illustrate that specific orientations of the cyanine crystal relative to C60 create a repulsive potential for an electron near the interface that is largely due to the quadrupole moment of the unit cell. In combination with the electron-hole coulomb attraction and the electric field-induced barrier lowering, a high-energy potential barrier forms with a narrow width of a few nanometers. It is proposed that charge separation occurs via a field-dependent electron tunneling mechanism through that barrier, which is temperature independent. The results support a thus far overlooked pathway for CT state dissociation via carrier tunneling