TiO2 films as electron collecting interlayers are important in determining the efficiency of organic photovoltaics (OPVs). Various methods of film deposition have been explored, and they revealed the tradeoff between pinhole free coverage (large shunt resistance) and small film thickness (small series resistance). It is hypothesized that atomic layer deposition (ALD) with its self-limiting nature and sub-nanometer level control would be able to circumvent this problem and provide TiO2 films of pinhole free coverage and small thickness. TiO2 films made by chemical vapor deposition (CVD) and ALD were investigated and compared. Conductive atomic force microscopy (CAFM) was used to characterize film morphology and conductivity. X-ray photoelectron spectroscopy (XPS) was utilized to analyze film composition and chemical state. Cyclic voltammetry (CV) was able to reveal the hole blocking capability of films. Finally, organic photovoltaic devices were made with different TiO2 films to reveal the relationship between device property and film characteristic. It is found that both CVD and ALD created TiO2 films with Ti4+ species containing oxygen from hydroxyl groups. They both showed conformal coverage of the electrode via CAFM and CV measurements, and clearly ALD method achieved this with a thinner film and smaller series resistance. This work provided the evidence of effective and surprising capabilities of electron harvesting and hole blocking of ultrathin ALD TiO2 films for OPVs.

Wide-bandgap metal halide perovskite solar cells have become an alluring next-generation solar panel technology because of their simple manufacturing and rising efficiencies by up to 25.7%. When the single junction devices face the ultimate S-Q limit, the incorporation of wide-bandgap perovskite materials with low-bandgap absorbers to form multi-junction cells offers a promising route to surpass the theoretical efficiency. Monolithic perovskite-organic tandem cells are appealing among other compositions owing to the combination of the sub-cells advantages: low-cost, flexibility, and solution processing. In this work, we focused on optimizing the hole transporting materials (HTMs) separately for the two components in tandem devices. In the 1.76 eV perovskite subcell, three commonly seen HTMs are selected (2PACz, NiOx and PTAA) to investigate the influence on device performance. An MgF2 interlayer at perovskite/C60 is deposited as passivation to enhance the voltage and overall performance. It is found that 2PACz is most suitable for triple cation FA0.7MA0.15Cs0.15Pb(I0.6Br0.4)3, giving good crystallinity, energy match and absorption with a champion PCE of 16.12%. Then, we performed a similar optimization for ternary PM6: BTP-eC9: PC70BM with MoOx, MoOx/2PACz, and PEDOT: PSS as HTMs, where MoOx/2PACz present the best statistics. Finally, two terminal tandem devices were fabricated based on the two optimized subcells, and a promising efficiency of 23.6% and a Voc of 2.09V were reached free of hysteresis. More passivation methods or perovskite bandgap engineering are expected to further improve the performance and break the record.

This dissertation has focused on the study of the ITO/organic heterojunction and the chemistries therein, it proposes appropriate strategies that enhance the interfacial physical and electronic properties for charge injection with application to organic thin-layer devices. We focused on four major aspects of this work: i) To characterize the ITO surface and chemistries that may be pertinent to interaction with adjacent organic layers in a device configuration. This developed a working model of surface and provided a foundation for modification strategies. Characterization of the electronic properties of the surface indicate less than 5% of the geometrical surface is responsible for the bulk of current flow while the rest is electrically inactive. ii) To determine the extent to which these chemistries are variable and propose circumstances where compositional changes can occur. It is shown that the surface chemistry of ITO is heterogeneous and possible very dynamic with respect to the surrounding environment. iii) To propose a strategy for modification of the interface. Modification of ITO surfaces by small molecules containing carboxylic acid functionalities is investigated. Enhancements in the electron transfer rate coefficient were realized after modification of the ITO electrode. The enhancements are found to stem from a light etching mechanism. Additionally, an elecro-catalytic effect was observed with some of the modifiers. iv) Apply these modifications to organic light emitting diodes (OLEDs) and organic photovoltaic devices (OPVs). Enhancements seen in solution electrochemical experiments are indicative of the enhancements seen for solid state devices. Modifications resulted in substantially lower leakage currents (3 orders of magnitude in some cases) as well as nearly doubling the efficiency.An additional chapter describes the creation and characterization of electrochemically grown polymer nano-structures based on blazed angle diffraction gratings. The discussion details the micro-contact printing process and the electro-catalytic growth of the conductive polymers PANI and PEDOT to form diffraction grating structures in their own right. The resulting diffraction efficiency of these structures is shown to be sensitive to environmental conditions outlining possible uses as chemical sensors. This is demonstrated by utilizing these structures as working pH and potentiometric sensors based on the changing diffraction efficiency.

Organic photovoltaics (OPVs) are mainly based on organic semiconducting small molecules, macromolecules, and polymers, which form an active layer in photovoltaics. They act as an active material in absorbing light and causing charge mobility to generate electricity from sunlight. This thesis describes the molecular systems derived from fused arenes such as anthracene, pyrene, carbazole and thiophene for use as either a donor or an acceptor component of the active layer of OPVs. Two novel molecular systems (9- anthracenecarboxy-1-methylpyrene, (1) and Py-bi-TH-ANT, (2) were prepared using Steglich esterification and Grignard metathesis followed by Kumada coupling. The molecular structure of each was confirmed by 1H-NMR and IR analysis respectively. The photophysical properties of the products were also evaluated in solution. The potential applicability of these two novel systems for OPVs will be studied in the future.

The tandem solar cell concept is a promising approach to improve the efficiency of photovoltaic devices. However, characterization of tandem solar cell devices is challenging since correct efficiency determination demands special experimental infrastructure as well as suitable characterization procedures. Even though the appropriate IEC and ASTM measurement standards define all that very precisely, they cannot be applied without special care to organic photovoltaics (OPV) because they were originally developed for inorganic devices. As a consequence, nowadays almost all tandem organic solar cell publications are not using correct characterization procedures, often resulting in questionable efficiency values. The aim of this work is developing a measurement procedure for tandem organic solar cells assuring their correct characterization. Therefore, at first the existing standards and measurement procedures for tandem solar cells are reviewed and challenges when applying these standards to organic solar cells are identified. As main challenges the relatively low fill factors and distinct nonlinearities of organic solar cells are identified. As preliminary experiments, single junction organic solar cells are investigated to analyze the influence of measurement parameters like bias irradiance, bias voltage, and chopper frequency on the external quantum efficiency (EQE) of organic solar cells. This results in parameter sets assuring minimized artifacts for the subsequent EQE determination of the subcells of tandem organic solar cells. The main part of this thesis presents the detailed characterization of a tandem OPV example device. First, EQE is measured and validated by two independent institutes. The EQE results are used to calculate the illumination conditions to reach AM1.5g conditions for both subcells with a multi-source sun simulator. The resulting efficiency value under standard reporting conditions (SRC) is found to be 5% lower than the efficiency measured with a single-source sun simulator. A full spectrometric characterization shows that differing fill factors of the subcells are the reason for this behavior. To overcome the main reason for the complicated measurement procedure of tandem solar cells, the inaccessibility of the individual subcells, three different approaches for the jV-characteristics determination of the subcells are presented. The so-called Bias Voltage Approach is based on EQE-measurements under varying bias voltage and needs no additional electrical contacts. Therefore, it can be applied to existing devices. The Voltage Contact Approach as well as the Current Contact Approach require in changed stack designs. Therefore, they cannot be applied to existing devices but give more accurate results. Finally, a procedure for characterizing tandem organic solar cells is formulated. This procedures aims at giving practical advice how to characterize tandem organic solar cells to achieve results conforming to the measurement standards and being as accurate and reproducible as possible. Hence, this thesis attempts to establish standards for a correct measurement of tandem organic solar cells of which other emerging solar cell technologies can profit as well.

Organic solar cells promise electricity generation at very low cost, and higher installation flexibility as compared to inorganic solar cells. The lower cost is achieved by cheaper semiconductors and easier manufacturing processes. The flexibility is naturally given by these ultra-thin, amorphous layers. Also the power conversion efficiency can be high enough for many applications. The organic molecules have to withstand the constant excitation by photons, transport of energy in form of excitons and charge. A small but significant amount of these photons has energy over the absorption gap, the excess of energy must be released without breaking the molecular bonds. In consequence, the solar cells can also heat up to temperatures at above 80°C. The objective of this work is to answer the question if the small molecules organic solar cells can be stable enough to operate under a very long time. The stability of organic doped layers in an organic solar cell is also addressed. This work starts with a general introduction followed by the description of the experimental procedures. The aging experiments of the solar cell were done with a self developed equipment. The fabrication of this equipment (a set of measurement boxes) was necessary to maintain the conditions, under which a solar cell can be aged, as constant as possible. The measurement boxes were used to control the electrical load of the cell, its temperature, the illumination intensity, and its electric connection to the IxV measurement equipment. A software package was also developed to control the equipment and to facilitate the work and visualization of the high volume of collected data. The model solar cells chosen for the aging experiments were donor-acceptor heterojunctions devices formed with the well-known materials C60 and ZnPc. Two basic different structures were analyzed, because they offered reasonable performance and potentially long lifetime: the flat heterojunction (FHJ) and the mixed heterojunction in a Metal-Insulator-p-Semiconductor (m-i-p) configuration. Variations of the FHJ and of the m-i-p structures are also used to verify the limits of the stability of electrically p- and n- doped organic semiconducting layers. The least stable solar cells are the FHJ devices. These devices show a fast initial decrease of all their characteristic conversion parameters but the Voc. After a few hundred hours, the saturation current (current under a reverse bias of 1 V) was almost stable. The saturation current is related to the number of absorbing centers, the decrease indicates that the degradation of the absorbing centers has stopped. With wavelength resolved external quantum efficiency measurements and chemical analysis, it was found that the degradation is related to the oxidation of C60. It was also shown that the use of organic dopants do not significantly affect the lifetime. The results show that the m-i-p solar cells are more stable than the FHJ devices. They are also stable under high temperatures up to 105°C. Outdoor testing also showed that the solar cells remained chemically, electrically and mechanically stable during a 900 h test.

En vue de limiter la dégradation par l’humidité et l’oxygène des dispositifs photovoltaïquesorganiques flexibles, les cellules solaires sont encapsulées entre des films barrières aux gaz.Malgré l’importance des procédés d’encapsulation et leur potentiel impact sur les performancesinitiales et lors du vieillissement des dispositifs, ils sont rarement étudiés dans la littérature. Enoutre, plusieurs études de vieillissement sur le terrain ont montré que la détérioration mécaniquelimitait la durée de vie des échantillons bien avant que leur stabilité photochimique ne soit miseen cause. L’adhésion entre les différentes couches composant les cellules est donc un facteurcritique afin d’obtenir des dispositifs flexibles fiables après leur mise en oeuvre et lors de leurutilisation.Dans ce travail, deux procédés d’encapsulation ont été étudiés : la lamination à rouleauxd’un adhésif sensible à la pression et la lamination sous vide d’un thermoplastique. Afin dequantifier l’adhésion de chacune des interfaces comprises dans les échantillons, la technique decaractérisation mécanique par pelage à 180° a été adaptée et ensuite appliquée aux dispositifsflexibles. De plus, des techniques de caractérisation des dispositifs par imagerie non-destructiveont été développées : la cartographie du courant induit par faisceau laser (plus courammentconnu sous l’acronyme anglais « LBIC ») et l’imagerie de luminescence sous excitation optiqueet électrique. Grace à ces techniques, l’hypothèse d’une dégradation mécanique des dispositifsdurant le procédé d’encapsulation par lamination à rouleaux a été émise. Des solutions permettantl’amélioration des interfaces identifiées comme mécaniquement faibles ont été recherchées etensuite évaluées par rapport aux performances photovoltaïques des dispositifs de référence.Les techniques d’imagerie développées précédemment ont également été appliquées durant levieillissement en condition accélérées des cellules encapsulées. Un mécanisme a été proposé,qui permet d’expliquer la localisation spatiale de la dégradation mais également le type dedégradation, optique ou électrique, survenu à chaque étape du vieillissement. / In order to limit the flexible organic photovoltaic devices degradation induced by moisture andoxygen, the solar cells are encapsulated between two gas-barrier films. Despite the importanceof the encapsulation processes and their potential influence on the initial performances as well asduring aging of the devices, they are scarcely described in the literature. Furthermore, severalfield aging studies showed that mechanical degradation could limit the devices lifetime beforetheir photo-chemical stability became an issue. Thus, adhesion between the different layerscomposing the devices is a critical factor in order to develop flexible OPV devices reliable aftertheir manufacturing and during their use.In this work, two encapsulation protocols were studied: the roll-to-roll lamination of apressure sensitive adhesive and the vacuum lamination of a hotmelt thermoplastic. In order toquantify the adhesion strength of every interface comprised in the samples, the 180° peelingtest mechanical characterization was adapted for and then applied to the flexible devices. Inaddition, non-destructive imaging characterization techniques were developed: the laser-beaminduced-current mapping and the luminescence emission imaging under optical and electricalexcitation. Thank to these techniques, the hypothesis of a mechanical degradation occurringduring the roll-to-roll lamination process was made. Answers allowing for the improvement ofthe interfaces identified as weak were searched for and evaluated with respect to the photovoltaicperformances of the reference devices. The imaging techniques previously developed were alsoapplied along the accelerated aging of encapsulated cells. A mechanism was proposed, whichallows one to explain the localization of the degradation but also the failure type, either opticalor electrical, occurring during each aging step.

Triarylamine type materials with vastly altered physical properties are synthesized by hybridizing organic semiconducting structures with silicone and siloxane groups. By altering the silicon content of these materials, we can tune their physical composition from free flowing liquids, to amorphous glasses, to cross-linked films. Much of this modification is enabled by the unique use of a metal-free Si-H activation chemistry; the Piers-Rubinsztajn reaction. This chemistry is demonstrated to be a general and rapid way to build up hybrid semiconducting structures. Key to the utility of these materials in electronic devices, it is shown that hybridization with silicon groups has a negligible effect on the useful electrochemical properties of the base materials. Building on this, it is shown that charge carrier mobility through a prototypical liquid organic semiconductor is similar to the base materials and transport is described by existing dispersive transport theories. Finally, two side projects in the area of organic electronics are discussed. New phthalonitrile based fluorophores are characterized and their utility as deep-blue emitting dopants in organic light emitting diodes is demonstrated. These same π-extended phthalonitriles can also be used as precursors to new red-shifted BsubPcs which display exceptional electrochemical stability and tuning.

Triarylamine type materials with vastly altered physical properties are synthesized by hybridizing organic semiconducting structures with silicone and siloxane groups. By altering the silicon content of these materials, we can tune their physical composition from free flowing liquids, to amorphous glasses, to cross-linked films. Much of this modification is enabled by the unique use of a metal-free Si-H activation chemistry; the Piers-Rubinsztajn reaction. This chemistry is demonstrated to be a general and rapid way to build up hybrid semiconducting structures. Key to the utility of these materials in electronic devices, it is shown that hybridization with silicon groups has a negligible effect on the useful electrochemical properties of the base materials. Building on this, it is shown that charge carrier mobility through a prototypical liquid organic semiconductor is similar to the base materials and transport is described by existing dispersive transport theories. Finally, two side projects in the area of organic electronics are discussed. New phthalonitrile based fluorophores are characterized and their utility as deep-blue emitting dopants in organic light emitting diodes is demonstrated. These same π-extended phthalonitriles can also be used as precursors to new red-shifted BsubPcs which display exceptional electrochemical stability and tuning.

The successful realization of highly efficient bulk heterojunction OPV devices requires the development of organic donor and acceptor materials with tailored properties. Recently, non-fullerene acceptors (NFAs) have emerged as an alternative to the ubiquitously used fullerene derivatives. NFAs showed a rapid increase in efficiencies, now exceeding a PCE of 13%. In my thesis research, I used two small molecule IDT-based acceptors, namely O-IDTBR and O-IDTBCN, in combination with a wide bandgap donor polymer, GeNIDT-BT, as active material in BHJ solar cells and investigated their photophysical characteristics. The polymer combined with O-IDTBR as acceptor achieved a power conversion efficiency of only 2%, which is significantly lower than that obtained for the system of GeNIDT-BT: O-IDTBCN (5.3%). Using nano- to microsecond transient absorption spectroscopy, I investigated both systems and demonstrated that GeNIDT-BT:O-IDTBR exhibits more geminate recombination of interfacial charge-transfer states, leading to lower short circuit currents. Using time-delayed collection field experiments, I studied the field dependence of charge generation and its impact on the device fill factor. Overall, my results provide a qualitative understanding of the efficiency-limiting processes in both systems and their impact on device performance.