Marlus Koehler

Temperature dependent tunnelling current at metal/polymer interfaces—potential barrier height determination

It has been demonstrated that the expression for the temperature dependent tunneling current through a triangular potential barrier at metal/polymer interfaces fits experimental data including the deviation from a straight line observed in Fowler–Nordheim plots. The fitting parameters can be used to calculate the potential barrier height at the metal/polymer interface, which is of crucial importance for the control of charge injection in organic light emitting diodes. The tunneling current density expression reproduces the observed dependence of the current with temperature.

Density functional theory study of the dipole across the P3HT : PCBM complex: the role of polarization and charge transfer

Using density functional theory (DFT) calculations we study the electronic structure and the dipole moment of the complex formed by an oligomer of poly(3-hexylthiopene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). We scanned the total energy of the P3HT : PCBM dimer as a function of the relative angular orientation between the two molecules in the search for stable angular configurations. After finding four stable geometries, we applied DFT calculations to characterize the electronic properties of those configurations. Using extended charge decomposition analysis, we calculated the contributions of the occupied and unoccupied orbitals from the individual species to form the electronic states of the complex. This method allowed us to determine the charge effectively transferred from the polymer to the PCBM upon the formation of the complex. Our results indicate that this amount of transferred charge is small but extremely sensitive to the detailed arrangement of the molecules in the dimer. However, the direction of the dipole moment of the complex is strongly influenced by the orientation of the permanent dipole of the PCBM relative to the chain of the oligomer. In the light of the dipole-assisted exciton dissociation models, we identified only one configuration that has a dipole moment with the right orientation to help the dissociation of a photo-excited electron–hole pair in the polymer. This configuration is just the molecular arrangement with the lowest total energy. Moreover, we study the contributions of charge transfer and electronic polarization to the composition of this dipole as a function of the intermolecular separation. We finally discuss how the results found using our simple dimer model can be related to electronic properties obtained using more-realistic models of the P3HT/PCBM interface.

Hole mobility effect in the efficiency of bilayer heterojunction polymer/C60 photovoltaic cells

We report here bilayer heterojunction solar cells fabricated by using poly[9,9′-hexyl-fluorene-alt-bithiophene] (LaPPS43) polymer as active layer. The power conversion efficiency (η) displays a sevenfold increase upon annealing at 200 °C, reaching the value of 2.8%. This result is comparable to the highest η reported so far for bulk heterojunction solar cells using the same polymer. Simulation, absorbance spectra, and current versus voltage results indicate that the π–π stacking in solid state is enhanced after annealing with a reduction in traps and thus reflecting in higher hole mobility.

Modeling bilayer polymer/fullerene photovoltaic devices

The electrical transport and charge generation of polymer/fullerene photovoltaic devices were investigated. The polymer/fullerene photodiodes were formed by a heterojunction of fullerene and a semiconducting polymer poly (3,4-ethylenedioxythiophene). The current-voltage characteristic of the devices were measured with variable thickness of the C60 layer, under monochromatic light of different wavelengths. The results show that the values of the electrical photoconductivity are related to the optical absorption coefficient of the fullerene, and thus implying a large contribution of the C60 films to the diode photocurrent.

Conditions for ohmic electron injection at the Mg/Alq3 interface

We show that the contacts formed by magnesium on tris (8-hydroxyquinoline) aluminum (Alq3) are intrinsically ohmic when they are fabricated and operated in ultrahigh vacuum. Under the same conditions, the injected current shows a steep increase approximately proportional to the seventh power of the applied voltage that we assign to trap filling. Only a subsequent contact with oxygen leads to an injection-limited behavior, where the observed steep current increase is caused by potential barriers at the contacts. In addition, we observe that electron injection in oxidized structures can be very well described by Fowler–Nordheim tunneling in the case when electrons are injected from the magnesium (Mg) contact evaporated onto Alq3.

Electronic structure, molecular orientation, charge transfer dynamics and solar cells performance in donor/acceptor copolymers and fullerene: Experimental and theoretical approaches

By combining experimental and theoretical approaches, the electronic structure, molecular orientation, charge transfer dynamics and solar cell performance in donor/acceptor copolymer poly[2,7-(9,9-bis(2-ethylhexyl)-dibenzosilole)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PSiF-DBT) films and blended with 6,6.-phenyl-C 61-butyric acid methyl ester (PSiF-DBT:PCBM) were investigated. Good agreement between experimental and theoretical PSiF-DBT UV-Vis absorption spectrum is observed and the main molecular orbitals contributing to the spectrum were determined using DFT single point calculations. Non-coplanar configuration was determined by geometric optimization calculation in isolated PSiF-DBT pentamer and corroborated by angular variation of the sulphur 1s near-edge X-ray absorption fine structure (NEXAFS) spectra. Edge-on and plane-on molecular orientations were obtained for thiophene and benzothiadiazole units, respectively. A power conversion efficiency up to 1.58%, open circuit voltage of 0.51 V...

Positional disorder enhancement of exciton dissociation at donor∕acceptor interface

We investigate the dissociation of a Coulomb bounded electron-hole geminate pair at a disordered donor∕acceptor (D-A) heterojunction by extending a previous proposal in the literature [Arkhipov et al., Appl. Phys. Lett. 82, 4605 (2003)] and using Monte Carlo simulations. We show that the presence of a layer of randomly distributed dipoles at the D-A interface creates both a potential well that confines the hole and a repulsive barrier that prevents the geminate pair recombination, even when the effective mass of the hole is around the electron rest mass. Our calculations depend strongly on the heterojunction morphology. However, contrary to what is generally believed, we find that the disorder in the position of the dipoles along the D-A interface axis enhances the pair dissociation. Inhomogeneities in the acceptor concentration at the heterojunction can then create highly efficient centers for exciton dissociation. The model explains recent experimental results for organic D-A heterojunctions and has imp...

Space-charge-limited bipolar currents in polymer/C60 diodes

We investigate carrier’s transport for devices formed by a heterojunction of the substituted polythiophene polymer and the C60 molecule, sandwiched between anode of indium-tin oxide/poly(3,4-ethylene dioxythiophene) and cathode of Al, for different thicknesses of the C60. We measure their current-voltage characteristics and find that the forward current is limited by spatial charge of holes in the polymer layer and electrons in the molecule layer. We also find that the effective mobilities of holes in the polymer and of electrons in the C60 are dependent on the different thickness of the layers. By adapting to our systems an electrical analytic model previously used in the literature to study bilayer organic diodes, we are able to explain the experimental results.

Evidence of fractal structure for charge transport in carbon-nanotube/conjugated-polymer composites

We measure the current-voltage (J-V) characteristics of organic composites as a function of carbon nanotubes concentration dispersed in a poly-3-hexilthiophene (P3HT) matrix. From a drift-diffusion-space-charge model and adapting the general effective medium and classical percolation theories, we quantify the system transport features. We find a drastic increase of the injection current due to drain channels provided by the nanotubes, probably a universal mechanism for charge injection in such type of system. We identify a percolation transition (with t=0.3, the lowest critical exponent so far reported in the literature) and a fractal structure for transport after the percolation. This nearly 1D structure is surprising since the composites do not have any peculiar orientation along some preferential direction. Supported by transmission electron microscopy we explain the fractal behavior in terms of the morphology of the conductivity paths.

Charge Injection into Thin Conjugated Polymer Films

We review several results on the charge injection into thin conjugated polymer films and discuss tunnelling and space-charge limited current models. The analysis of the currently available models reveals an apparent divergence in the literature which we try to eliminate via the consideration of both effects, tunnelling and space-charge formation, simultaneously. We analyse experimental results using a tunnelling model, which includes the contribution to the electronic potential originated by the space-charge effects due to the low mobility of charge carriers in conjugated polymers. As a consequence, the empty levels accessible for charge carriers in the polymer are shifted and the potential barrier for charge injection at the electrode/polymer interface is modified. This modification implies in a different tunnelling current and consequently in different current versus voltage dependences. We use our results to estimate the mobility of negative charge carriers in poly (p-phenylene vinylene) and to determine the trap centre concentration in the polymer layer of devices which present hysteresis in the current versus voltage curves.