A ternary composite supercapacitor electrode consisting of phosphomolybdic acid (HMA), a renewable biopolymer, lignin, and polypyrrole was synthesized by a simple one-step simultaneous electrochemical deposition and characterized by electrochemical methods. It was found that the addition of HMA increased the specific capacitance of the polypyrrole-lignin composite from 477 to 682 F g(-1) ( at a discharge current of 1 A g(-1)) and also significantly improved the charge storage capacity from 6(to 128 mA h g(-1).

Organic electronics based on organic semiconductors offer tremendous advantages compared to traditional inorganic counterparts such as low temperature processing, light weight, low manufacturing cost, high throughput and mechanical flexibility. Many key electronic processes in organic electronic devices, e.g. charge injection/extraction, charge recombination and exciton dissociation, occur at interfaces, significantly controlling performance and function. Understanding/modeling the interface energetics at organic-electrode/organic-organic heterojunctions is one of the crucial issues for organic electronic technologies to provide a route for improving device efficiency, which is the aim of the research presented in this thesis.

Integer charge transfer (ICT) states pre-existed in the dark and created as a consequence of Fermi level equilibrium at donor-acceptor interface have a profound effect on open circuit voltage in organic bulk heterojunction photovoltaics. ICT state formation causes vacuum level misalignment that yields a roughly constant effective donor ionization potential to acceptor electron affinity energy difference at the donor-acceptor interface, even though there is a large variation in electron affinity for the fullerene series. The large variation in open circuit voltage for the corresponding device series instead is found to be a consequence of trap-assisted recombination via integer charge transfer states. Based on the results, novel design rules for optimizing open circuit voltage and performance of organic bulk heterojunction solar cells are proposed.

Doping and insertion of interlayer are two established methods for enhancing charge injection/extraction properties at organic-electrode interface. By studying the energy level alignment behavior at low to intermediate doping levels for molecule-doped conjugated polymer/electrode interfaces, we deduce that two combined processes govern the interface energetics: (i) equilibration of the Fermi level due to oxidation (or reduction) of polymer sites at the interface as per the ICT model and (ii) a double dipole step induced by image charge from the dopant-polymer charge transfer complex that causes a shift of the work function. Such behavior is expected to hold in general for low to intermediate level doped organic semiconductor systems. The unified model is further extended to be suitable for conjugated electrolyte/electrode  interfaces, revealing the design rules for achieving the smallest charge injection/extraction barrier for both thin tunneling and thick charge transporting conjugated electrolyte interlayers.

To probe into the energy level spatial extension at interfaces, we employ the original approach of building and characterizing multilayers composed of a well-defined number of polymer monolayers with the Langmuir-Shäfer method to control polymer film uniformity and thicknesses, avoiding the problems associated with spin-coating ultrathin films. The disordered/amorphous films feature smaller, and in fact negligible, energy level bending compared to the more well-ordered films, in contradiction with existing models. It is found that that energy level bending depends on the ICT state distribution rather than the density of states of the neutral polymer chains in relation to the Fermi energy, thus taking into account the Coulomb energy associated with charging the polymer chain and transferring a charge across the interface. Based on this work, a general model for energy level bending in absence of significant doping of conjugated polymer films is proposed.

Organic semiconductors are sensitive to ambient atmosphere that can influence the energetics. The degradation effects of common PCBM film induced by oxygen and water are found to be completely different. Upon exposure to oxygen, the work function is down-shifted by ~ 0.15 eV compared to the ICT curve of the pristine PCBM film, originating from the weak interaction between the fullerene part of PCBM and oxygen, and this can be reversed by thermal treatment in vacuum. The down-shift in energetics will cause a loss in open circuit voltage at electrode interface, but aids free charge generation at donor-acceptor interface. Upon exposure to water, there is irreversible extensive broadening and bleaching of the valence electronic structure features as well as a substantial decrease of work function and ionization potential, severely degrading the transport properties.

Overall, the research results in this thesis thus give a deeper understanding of interface phenomena in organic electronics, especially regard to organic solar cells, aimed to further improve the device operation efficiency and lifetime.

The semiconductor-electrode interface impacts the function and the performance of (opto) electronic devices. For printed organic electronics the electrode surface is not atomically clean leading to weakly interacting interfaces. As a result, solution-processed organic ultrathin films on electrodes typically form islands due to dewetting. It has therefore been utterly difficult to achieve homogenous ultrathin conjugated polymer films. This has made the investigation of the correct energetics of the conjugated polymer-electrode interface impossible. Also, this has hampered the development of devices including ultrathin conjugated polymer layers. Here, LangmuirShafer-manufactured homogenous mono-and multilayers of semiconducting polymers on metal electrodes are reported and the energy level bending using photoelectron spectroscopy is tracked. The amorphous films display an abrupt energy level bending that does not extend beyond the first monolayer. These findings provide new insights of the energetics of the polymer-electrode interface and opens up for new high-performing devices based on ultrathin semiconducting polymers.

The semiconductor-electrode interface impacts the function and the performance of (opto-)electronic devices. For printed organic electronics the electrode surface is not atomically clean leading to weakly interacting interfaces. As a result, solution-processed organic ultra-thin films on electrodes typically form islands due to de-wetting. It has therefore been utterly difficult to achieve homogenous ultrathin conjugated polymer films. This has made the investigation of the correct energetics of the conjugated polymer-electrode interface impossible. Also, this has hampered the development of devices including ultra-thin conjugated polymer layers. Here, we report Langmuir-Shäfer-manufactured homogenous mono- and multilayers of semiconducting polymers on metal electrodes and track the energy level bending using photoelectron spectroscopy. The amorphous films display an abrupt energy level bending that does not extend beyond the first monolayer. Our findings provide new insights of the energetics of the polymer-electrode interface and opens up for new high-performing devices based on ultra-thin semiconducting polymers.

Effects of in situ oxygen/water exposure on the energetics of [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) films are presented. For oxygen exposure, the work function is downshifted by ≈0.15 eV compared to the ideal integer charge transfer (ICT) curve for pristine PCBM, which is incompatible with significant introduction of electron trap states or p-doping. Water induces the highest occupied molecular orbital (HOMO) structure to undergo strong, irreversible modifications accompanied by a chemical interaction with PCBM.

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The interface energetics and intermixing effects of the donor/acceptor bulk heterojunction (BHJ) blends of poly[2,3‐bis‐(3‐octyloxyphenyl) quinoxaline‐5, 8‐dilyl‐alt‐thiophene‐2, 5‐diyl]: [6,6]‐phenyl C71butyric acid methyl ester (TQ1:PC71BM) have been investigated using ultraviolet photoemission spectroscopy (UPS) in combination with the integer charge transfer model. The TQ1:PC71BM represents the useful model system for BHJ organic photovoltaics featuring effective charge generation and transport. It finds out that the positive integer charge state of TQ1 are equal in energy to the negative integer charge state of PC71BM, leading to a negligible potential step at TQ1:PC71BM interface and thus the vacuum level alignment. It is observed that the TQ1 accumulates on the top of TQ1:PC71BM BHJ and UPS spectra as function of various blend ratios suggest that the TQ1 mixes finely with PC71BM with the little work function modification in a wide range. In addition, no significant influence of the long‐range Coulomb interactions or the intermolecular hybridization on the occupied electronic structures is present for the well‐intermixed TQ1:PC71BM BHJs. These findings provide deep insights into the properties of BHJ blends and are beneficial for the performance optimization in organic photovoltaics.

Regular energetics at a conjugated electrolyte/electrode modifier are found and controlled by equilibration of the Fermi level and an additional interface double dipole step induced by ionic functionality. Based on the results, design rules for conjugated electrolyte/electrode modifiers to achieve the smallest charge injection/exaction barrier and break through the current thickness limitation are proposed.

We systematically show the effect of UV-light soaking on surface electronic structures and chemical states of solution processed ZnO nanoparticle (ZnONP) films in UHV, dry air and UV-ozone. UV exposure in UHV induces a slight decrease in work function and surface-desorption of chemisorbed oxygen, whereas UV exposure in the presence of oxygen causes an increase in work function due to oxygen atom vacancy filling in the ZnO matrix. We demonstrate that UV-light soaking in combination with vacuum or oxygen can tune the work function of the ZnONP films over a range exceeding 1 eV. Based on photovoltaic performance and diode measurements, we conclude that the oxygen atom vacancy filling occurs mainly at the surface of the ZnONP films and that the films consequently retain their n-type behavior despite a significant increase in the measured work function.

Organic photovoltaics are under intense development and significant focus has been placed on tuning the donor ionization potential and acceptor electron affinity to optimize open circuit voltage. Here, it is shown that for a series of regioregular-poly(3-hexylthiophene): fullerene bulk heterojunction (BHJ) organic photovoltaic devices with pinned electrodes, integer charge transfer states present in the dark and created as a consequence of Fermi level equilibrium at BHJ have a profound effect on open circuit voltage. The integer charge transfer state formation causes vacuum level misalignment that yields a roughly constant effective donor ionization potential to acceptor electron affinity energy difference at the donor-acceptor interface, even though there is a large variation in electron affinity for the fullerene series. The large variation in open circuit voltage for the corresponding device series instead is found to be a consequence of trap-assisted recombination via integer charge transfer states. Based on the results, novel design rules for optimizing open circuit voltage and performance of organic bulk heterojunction solar cells are proposed.

Optical spacers based on metal oxide layers have been intensively studied in poly(3-hexylthiophene) (P3HT) based polymer solar cells for optimizing light distribution inside the device, but to date, the potential of such a metal oxide spacer to improve the electronic performance of the polymer solar cells simultaneously has not yet be investigated. Here, a detailed study of performance improvement in high efficient polymer solar cells by insertion of solution-processed ZnO optical spacer using ethanolamine surface modification is reported. Insertion of the modified ZnO optical spacer strongly improves the performance of polymer solar cells even in the absence of an increase in light absorption. The electric improvements of the device are related to improved electron extraction, reduced contact barrier, and reduced recombination at the cathode. Importantly, it is shown for the first time that the morphology of optical spacer layer is a crucial parameter to obtain highly efficient solar cells in normal device structures. By optimizing optical spacer effects, contact resistance, and morphology of ZnO optical spacers, poly[[4,8-bis[(2-ethylhexyl) oxy] benzo[1,2-b: 4,5-b] dithiophene-2,6diyl] [3-fluoro-2-[(2-ethylhexyl) carbonyl] thieno[3,4-b] thiophenediyl]] (PTB7):[6,6]-phenyl-C71-butyric acid (PC70 BM) bulk heterojunction solar cells with conversion efficiency of 7.6% are obtained in normal device structures with all-solution-processed interlayers.

The application of solution-processable graphene oxide (GO) as a hole injection layer in organic light-emitting diodes (OLEDs) is demonstrated. High luminance of over 53 000 cd m(-2) is obtained at only 10 V. The results will unlock a route to apply GO in flexible OLEDs and other electrode applications.

The function of approximate to 3-nm thick lithium fluoride (LiF) buffer layers in combination with high work function metal contacts such as coinage metals and ferromagnetic metals for use in organic electronics and spintronics is investigated. The energy level alignment at the organic/LiF/metal interface is systematically studied using photoelectron spectroscopy and the integer charge transfer model. The thick-LiF buffer layer is found to pin the Fermi level to approximate to 3.8 eV, regardless of the work function of the initial metal due to energy level bending in the LiF layer caused by depletion of defect states. At 3-nm thickness, the LiF buffer layer provides full coverage, and the organic semiconductor adlayers are found to physisorb with the consequence that the energy level alignment at the organic/LiF interface follows the integer charge transfer models predictions.

We demonstrate the use of low cost paper as an efficient light-trapping element for thin film photovoltaics. We verify its use in fully-solution processed organic photovoltaic devices with the highest power conversion efficiency and the lowest internal electrical losses reported so far, the architecture of which - unlike most of the studied geometries to date - is suitable for upscaling, i.e. commercialization. The use of the paper-reflector enhances the external quantum efficiency (EQE) of the organic photovoltaic device by a factor of approximate to 1.5-2.5 over the solar spectrum, which rivals the light harvesting efficiency of a highly-reflective but also considerably more expensive silver mirror back-reflector. Moreover, by detailed theoretical and experimental analysis, we show that further improvements in the photovoltaic performance of organic solar cells employing PEDOT:PSS as both electrodes rely on the future development of high-conductivity and high-transmittance PEDOT:PSS. This is due optical losses in the PEDOT:PSS electrodes.

Small molecule donor/polymer acceptor bulk-heterojunction films with both compounds strongly absorbing have great potential for further enhancement of the performance of organic solar cells. By employing a newly synthesized small molecule donor with a commercially available polymer acceptor in a solution-processed fullerene-free system, a high power conversion efficiency of close to 4% is reported.

The effects of cathode modification by a conjugated polymer interlayer PFPA1 on the performance of reversed organic solar cells (substrate/cathode/active layer/transparent anode) based on different active material systems and different substrate electrodes are systematically investigated. A reduction of the work function irrespective of the substrate cathode used is observed upon the deposition of the PFPA1 interlayer, which is further related to an improved built-in electric field and open-circuit voltage. The amphiphilic character of the PFPA1 interlayer alters the surface energy of the substrate cathode, leading to the formation of a better active layer morphology aiding efficient exciton dissociation and photocurrent extraction in the modified solar cells. Hence, internal quantum efficiency is found to be significantly higher than that of their unmodified counterparts, while optically, the modified and unmodified solar cells are identical. Moreover, the deep highest occupied molecular orbital (HOMO) of the PFPA1 interlayer improves the selectivity for all investigated substrate cathodes, thus enhancing the fill factor.

In organic bulk-heterojunction solar cells, energy losses at the active layer/electrode interface are often observed. Modification of these interfaces with organic interlayers optimizes charge carrier injection and extraction and thus improves device performance. In this work, the effects of cathode modification by a conjugated polymer interlayer PFPA1 on the performance of reversed organic solar cells (substrate/cathode/active layer/transparent anode) based on different active material systems and different substrate electrodes are systematically investigated. A reduction of the work function irrespective of the substrate cathode used is observed upon the deposition of the PFPA1 interlayer; further related to an improved built-in electric field and open-circuit voltage. The amphiphilic character of the PFPA1 interlayer alters the surface energy of the substrate cathode, leading to the formation of a better active layer morphology aiding efficient exciton dissociation and photocurrent extraction in the modified solar cells. Hence, internal quantum efficiency is found significantly higher than that of their unmodified counterparts, while optically, the modified and unmodified solar cells are identical. Moreover, the deep HOMO of the PFPA1 interlayer improves the selectivity for all investigated substrate cathodes, thus enhancing the fill factor. We demonstrate a possibility of improving photovoltaic performance of reversed solar cells via a simple and universal interface modification and provide the basic guidelines for development and characterization of interface materials for organic solar cells in general.

The fast evolution of metal halide perovskite solar cells has opened a new chapter in the field of renewable energy. High-quality perovskite films as the active layers are essential for both high efficiency and long-term stability. Here, the perovskite films with enlarged crystal grain size and decreased defect density are fabricated by introducing the extremely low-cost and green polymer, ethyl cellulose (EC), into the perovskite layer. The addition of EC triggers hydrogen bonding interactions between EC and the perovskite, passivating the charge defect traps at the grain boundaries. The long chain of EC further acts as a scaffold for the perovskite structure, eliminating the annealing-induced lattice strain during the film fabrication process. The resulting devices with the EC additive exhibit a remarkably enhanced average power conversion efficiency from 17.11 to 19.27% and an improvement of all device parameters. The hysteresis index is found to decrease by three times from 0.081 to 0.027, which is attributed to suppressed ion migration and surface charge trapping. In addition, the defect passivation by EC significantly improves the environmental stability of the perovskite films, yielding devices that retain 80% of their initial efficiency after 30 days in ambient air at 45% relative humidity, whereas the pristine devices without EC fully degrade. This work provides a low-cost and green avenue for passivating defects that improves both the efficiency and operational stability of perovskite solar cells.

Two alcohol-soluble organic molecules are synthesized and introduced into inverted organic solar cells as the cathode interlayer. A power conversion efficiency as high as 9.22% is obtained by using the more hydrophobic molecule FTBTF-N as the cathode interlayer. Morphological studies suggest that design of the backbone can help to enhance short-circuit current density and fill factor.