This work is focused on developing novel techniques for characterizing organic bulk heterojunction (BHJ) materials for organic photovoltaic (OPV) applications. Polymer:fullerene BHJs are a promising class of photovoltaic materials, but an improved understanding of the charge transport processes and materials science of BHJs is required for them to effectively compete with other photovoltaic systems. Key parameters of BHJ systems that need to be evaluated include both electron and hole mobilities, the carrier concentrations, the recombination mechanism and the recombination coefficient. For these studies, poly(3-hexylthiophene) (P3HT):(6,6)-phenyl C₆₁-butyric acid methyl ester (PCBM) have been characterized due to its wide use among researchers. Traditional characterization techniques have focused on transient measurements in a vertical device configuration, but we demonstrate the use of lateral BHJ devices as materials diagnostic platforms. Lateral devices allow for direct access to the active layer for spatially resolved and environmental effect measurements. The devices are also measured under steady state operation, similar to a working OPV cell. Under these conditions, lateral BHJ devices exhibit space charge limited transport behavior. A detailed charge transport model is presented to describe the potential, electric field, and carrier concentration profiles of lateral BHJ devices, as well as the current versus voltage characteristics of different regions of the device. We are able to calculate the slower carrier mobility from photocurrent measurements of lateral devices and the carrier mobility ratio from the device potential profile, even in ambipolar BHJ systems. In situ potentiometry is used to construct detailed potential profiles of the device channel and calculate both carrier mobilities. The carrier concentration and recombination coefficient are calculated from lateral conductivity measurements, and we show that bimolecular recombination is the dominant mechanism in bulk P3HT:PCBM. A simplified in situ potentiometry and photocurrent measurement technique is presented to measure the time evolution of organic BHJ performance. Due to the open geometry of the lateral BHJ device, we are also able to monitor the change in key charge transport parameters, including the recombination mechanism, in response to environmental degradation, analyte exposure, and ambient temperature. We show increased geminate recombination in P3HT:PC₇₁BM after prolonged light exposure. Lateral BHJ device measurements offer a useful complement to measurements on vertical photovoltaic structures and provide a more complete and detailed picture of OPV materials. / text

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In this thesis, we propose the analytical calculations of single layer organic light emitting diode (OLED) characteristics using a device model which includes charge injection, trap charge limited, recombination, and the effect background dopping. Meanwhile, we report the results of a theoretical and simulated study of carrier transport between Schottky (ohmic) and organic contact in organic emitting diodes . In this thesis, my model shows that in a doped organic material with ohmic contacts the current is ohmic at low voltage. If the ohmic contact at the cathode is replaced by an Schottky contact the current varies exponentially with the applied voltage V. The current changes to space charge limited current (SCLC) at high voltage. The voltage at which the change takes place depends on the dopant concentrations. In the SCLC regime the current varies according to well-known V2 law if there are no traps and the mobility is independent of the electric field. In this model, we attempt to capture the salient features of organic LEDs with the minimal numberof physical assumptions. The aim is to describle intrinsic properties of the organic materical with the simplest model and add complexity as needed.

Conductivity and electron spin resonance measurements have been performed on solution doped polyaniline (PANi). It is proposed that both camphor-sulphonic acid (CSA) and 2-acrylamido-2-methyl-l-propanesulphonic acid (AMPSA) doped PANi can be described by the same model. It is suggested that the polyaniline materials are composed of differently ordered layers, a highly ordered region forming the core of the crystallites. The core of the crystallites are believed to be encapsulated within a semi- ordered region, with the crystallites themselves being dispersed in an amorphous polymer matrix. The conductivity measurements and ESR results described in this work support the proposal that within the highly ordered region of doped polyaniline crystallites, a polaronic lattice exists. The polaronic lattice facilitates "free" carriers which are responsible for "metallic" conduction within the crystallites. Encapsulating the polaronic lattice is a semi-ordered region in which (partially) mobile polarons (and possibly bipolarons) are present. The highly conductive crystallites are randomly dispersed in a less conductive polymer matrix. Charge transport within this heterogeneous system is well described by a heterogeneous metal - fluctuation induced tunnelling (FIT) model. The differences in the temperature dependent conductivities of the PANi-CSA and PANi-AMPSA materials are attributed to the systems having layers of different relative sizes (in the above model). AMPSA doped polyaniline films had a maximum room temperature conductivity of ~100 Scm(^-1). This material also showed potential for use as an electrode layer in polymer LEDs, to replace ITO coated glass. The conductivity of PANi-AMPSA was measured to be 50 ± 10 Scm(^-1) at thickness' of ~30nm. Layers of this thickness provide >90% optical transmission between 450 and 675 nm (most of the visible spectrum). Faraday rotation measurements have shown that the recently reported large Faraday rotation of polyaniline can not be reproduced. The limited results of the Faraday rotation experiments described in this work provide support for the theory that charge carriers in polyaniline have an effective mass of at least 100 times that of a free electron. It has also been shown that the claims of a polyaniline derivative (namely the Marcoussis polymer) being an entirely organic ferromagnet are unsubstantiated, despite intense investigation.

Electro-transmittance of molecular junctions was characterized electrically and studied optically at 410nm and 532nm. Between 1kHz and 100kHz there was no qualitative difference between the control samples and the molecular junction samples, however there were difficulties with reproducibility of the quantitative behaviour, so no hard conclusions could be drawn. A microfluidic capacitor device was designed and fabricated to study the electrical double layer, using standard microfabrication techniques. A complimentary flux corrected transport simulation was written using the same experimental geometry and the results of this study found qualitative agreement between the simulation and experiment. The experiment produced results about the concentration dependence of the double layer formation time which allows an estimate of the required frequency of an AC electrical signal for which the electrical double layer doesnt have time to form, and its effects can be ignored.

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The research field of organic electronics experiences tremendous developments since the discovery of conducting polymers upon chemical doping and the developments of applications where organic materials replace the traditionally used inorganic semiconductors. Devices such as light-emitting diodes (OLEDs), solar cells, and field-effect transistors (OFETs) based on organic ð-conjugated materials as active materials represent the key applications of the domain. In OLEDs, charge carriers (holes and electrons) are injected from the electrodes into the organic semiconductor and emit light when they meet. Solar cells have an opposite working principle compared to OLEDs: light is absorbed and dissociated in charge carriers that migrate to the electrodes to give rise to an electric current. OFET plays the role of current modulator in electronic circuits by tuning the current flowing in its channel. The gain of better device performances (better conversion efficiency for OLEDs and solar cells or high ON/OFF ratios for OFETs) requires a better understanding at a molecular scale of the charge transport properties that are quantified at the experimental level by the charge carrier mobility ì. Since organic conjugated materials are typically disordered, the charge carriers are mostly localized over a single molecule and charge transfers between molecules occur via a hopping mechanism. In our Ph.D. thesis, we have characterized the charge transport properties at the molecular scale by calculating the parameters entering into the Marcus hopping rate by means of semi-empirical Hartree-Fock methods and Density Functional Theory (DFT) calculations. On that basis, we have propagated a single charge carrier in molecular assemblies by means of a Dynamic Monte-Carlo procedure that we have developed in order to estimate mobility values as the ratio of the total distance travelled by the charge divided by the product of the total time needed to travel that distance and the norm of the electric field. The systems under study were model one-dimensional array of pentacene molecules, single molecular crystals and structures simulated by Molecular Dynamics (liquid crystalline phthalocyanine derivatives) and by Molecular Mechanics (grain boundaries in pentacene layers). The principle results shows anisotropic behaviour and electric field dependence for the charge carrier mobility, the impact of energetic as well as the positional disorder on the charge migration were investigated and we emphasize the importance to describe both disorders at a molecular scale in order to get a reliable picture for the charge transport properties calculations.

This dissertation is focused on modeling charge-transport in π-conjugated organic materials, which serve as the active materials in light-weight, flexible, organic photovoltaic cells, offering the potential for cheap, ubiquitous renewable energy. In particular, we used computational chemistry to gain insight into the fundamental processes of charge transport within organic semiconductors to derive an understanding of chemical and physical phenomena that can not be explained through experiment alone in order to further the performance of organic-based electronic devices. In order to accurately model the organic materials, a combined quantum-mechanical and classical approach is needed, with the ground and excited state electronic properties of isolated organic materials determined using DFT/TD-DFT as a first step and coupled with molecular dynamics and mechanics. This allows for an understanding of the molecular order and packing within nanoscale structures as well as the impact of the intermolecular interactions. However, standard DFT methods suffer from intrinsic errors resulting from approximations to the exchange-correlation potential that can be corrected using a simple non-empirically tuning procedure. We briefly review the electronic structure methods that we use and the non-empirically procedure for DFT that allows for a substantial improvement over standard DFT methods. We then discuss the main results of this research. In Chapter 3, we detail the understanding of the limitations in DFT (currently one of our main tools) and improvements that can be achieved through non-empirically tuning a specific DFT method for the system of study. We detail the dependence of the range-separation parameter used in long-range corrected hybrid functionals on both the size and degree of conjugation for a given system. We also demonstrate the effect that self-interaction corrections employed through range-separated hybrid functionals can have in describing thermodynamic and electronic properties for large, organic π-conjugated systems. In this study, we chose a property that critically depends on the degree of delocalization (i.e., torsion potentials) to correlate the degree of delocalization with the choice of a given method in order to understand how the self-interaction errors affects this property. These results are published in C Sutton et al. “Accurate Description of Torsion Potentials in Conjugated Polymers using Density Functionals with Reduced Self-interaction Error” Journal of Chemical Physics, 140, 054310, 2014. In Chapter 4, we discuss how non-empirically tuning DFT can be used to rigorously model electron transfer in single-molecule systems (i.e., organic mixed-valence systems), where we modeled the symmetry breaking and charge (de)localization in charge-transfer complexes compared with high-level methods. The results presented in Chapter 4 are published in C Sutton et al. “Towards a Robust Quantum-Chemical Description of Organic Mixed-Valence Systems” Journal of Physical Chemistry C, 118, 3925, 2014. In Chapter 5, we applied this method to interpret photoelectron spectroscopy spectra in order to elucidate the localized nature of a charge carrier in prototypical organic semiconductors; this understanding was then extended to quantify the relaxation energy in finite molecular clusters in the presence of an excess charge from a combined multi-layer quantum-mechanical/molecular-mechanical method. In Chapter 6, we determined the effect of choosing various DFT methods on the intermolecular electronic couplings and band structure calculations in organic molecules, which are published by C Sutton et al. in "Understanding the Density Functional Dependence of DFT-Calculated Electronic Couplings in Organic Semiconductors” Journal of Physical Chemistry Letters, 4, 919, 2013. Finally, conclusions and further considerations are discussed in Chapter 7.