This thesis studies State-of-Charge (SOC) method for widely used lead-acid batteries and the most prospective lithium-ion batteries. First, the relationship between the battery capacity and the open-circuit-voltage under different charging/discharging currents is investigated based on the equivalent circuit. Experimental results indicate that the open-circuit-voltage of the lead-acid battery varies regularly with the charging/discharging current and the duration of time for the battery disconnected from the load. Accordingly, a dynamic open-circuit-voltage method in considerations the open-circuit-time and the previous operating current is capable of precisely estimating the battery capacity in a shorter time. As for the lithium-ion batteries, their charging/discharging characteristics reveal that the Coulomb/Ampere-Hour Counting method is capable of yielding accurate estimations. Finally, through the experiments that emulate practical operations, the SOC estimations of batteries are verified to demonstrate the effectiveness and accuracy of the proposed methods.

Organic solar cells have the potential to be portable power sources that are light-weight, flexible, and inexpensive. However, the highest power conversion efficiency for organic solar cells to date is ~8%, and most high-efficiency solar cells have an area of less than 1 cm². This thesis advances the field of organic solar cells by studying the physics and engineering of the devices to understand the reverse saturation current, which is related to efficiency, and the effects of area scaling. The most commonly accepted models to describe the physics of organic photovoltaic devices are reviewed and applied to planar heterojunction solar cells based on pentacene / C60 as a model system. The equivalent circuit model developed for inorganic solar cells is shown to work well to describe the behavior of organic devices and parameterize their current-voltage characteristics with five parameters. Changes in the parameters with different material combinations or device structures are analyzed to better understand the operation of the presented organic solar cells. A one-dimensional diffusion model for the behavior of excitons and treatment of the organic layers as planes is demonstrated to adequately model the external quantum efficiency and photocurrent in pentacene / C60 solar cells. The origin of the open-circuit voltage is studied using cells with different electrodes and different donor materials. While changing the electrodes does not affect open-circuit voltage, it is greatly modified by changes in the donor. Tests with additional semiconductors show the change in open-circuit voltage is not consistent from donor to donor as the acceptor is varied, suggesting a more complex relation than just the difference in energy levels. Study of the temperature dependence of the equivalent circuit parameters shows that the reverse saturation current, which has a significant role in determining the open-circuit voltage, has a thermally activated behavior. From this behavior, the reverse saturation current is related back to charge transfer at the donor / acceptor heterojunction to suggest that both the effective energy barrier presented by the energy levels and the electronic coupling are important in determining the reverse saturation current and open-circuit voltage. This marks a shift from just considering a built-in voltage or the energy levels to also considering the electronic coupling of the donor and acceptor materials. Temperature-dependent performance characteristics are also used to show key differences between organic and inorganic devices. Finally, the effect of area scaling is explored with pentacene / C60 solar cells having areas of 0.11, 7, and 36.4 cm². Analysis with the equivalent circuit model shows that performance decreases as area increases because of an increasing series resistance presented by the transparent electrode. A metal grid, to provide low resistance pathways for current, fabricated on top of the transparent electrode is proposed to reduce the effective resistance. The grid is unique in that it is placed between the electrode and the semiconductor layer and must be passivated to prevent shorts through the thin semiconductor to the back metal electrode. Analysis of the grid predicts greatly reduced series resistance, and experimental results show reduced resistance and improved performance for the 7 cm² and 36.4 cm² devices when including the grid.

π-Conjugated polymers have attracted considerable attention since they are potential candidates for various opto-electronic devices such as solar cells, light emitting iodes, photodiodes, and transistors. Electronic de vices based on conjugated polymers can be easily processed at low temperature using inexpensive technologies. This leads to cost reduction, a key-deriving factor for choosing conjugated polymers for various types of applications. In particular, polymer based solar cells are of special interest due to the fact that they can play a major role in generating clean and cheap energy in the future. The investigations described in thesis are aimed mainly at understanding charge transport and the role of energy le vels in solar cells based on polymer/acceptor bulk heterojunction (BHJ) active films. Best polymer based solar cells, with efficiency 4 to 5%, rely on polymer/fullerene BHJ active films. These solar cells are in an immature state to be used for energy conversion purposes. In order to enhance their performance, it is quite important to understand the efficiency-limiting factors. Solid films of conjugated polymers compose conjugation segments that are randomly distributed in space and energy. Such distributio n gives rise to the localization of charge carriers and hence broadening of electron density of states. Consequently, electronic wave functions have quite poor overlap resulting into absence of continuous band transport. Charge transport in polymers and organic materials, in general, takes place by hopping among the localized states. This makes a bottleneck to the performance of polymer-based solar cells. In this context, the knowledge of charge transport in the solar cell materials is quite important to develop materials and device architectures that boost the efficiency of such solar cells. Most of the transport studies are based on polyfluorene copolymers and fullerene electron acceptor molecules. Fullerenes are blended with polymers to enhance the dissociation of excited state into free carriers and transport free electrons to the respective electrode. The interaction within the polymer-fullerene complex, therefore, plays a major role in the generation and transport of both electrons and holes. In this thesis, we present and discuss the effect of various polymer/fullerene compositions on hole percolation paths. We mainly focus on hole transport since its mobility is quite small as compared to electron mobility in the fullerenes, leading to creation of spa ce charges within the bulk of the solar cell composite. Changing a polymer band gap may necessitate an appropriate acceptor type in order to fulfill the need for sufficient driving force for dissociation of photogenerated electron-hole pairs. We have observed that different acceptor types give rise to completely different hole mobility in BHJ films. The change of hole transport as a function of acceptor type and concentration is mainly attributed to morphological changes. The effect of the acceptors in connection to hole transport is also discussed. The later is supported by studies of bipolar transport in pure electron acceptor layers. Moreover, the link between charge carrier mobility and photovoltaic parameters has also been studied and presented in this thesis. The efficiency of polymer/fullerene-based solar cells is also significantly limited by its open-circuit voltage (Voc), a parameter that does not obey the metal-insulator-metal principle due to its complicated characteristics. In this thesis, we address the effect of varying polymer oxidation potential on Voc of the polymer/fullerene BHJ based solar cells. Systematic investigations have been performed on solar cells that comprise several polythiophene polymers blended with a fullerene derivative electron acceptor molecule. The Voc of such solar cells was found to have a strong correlation with the oxidation potential of the polymers. The upper limit to Voc of the aforementioned solar cells is thermodynamically limited by the net internal electric filed generated by the difference in energy levels of the two materials in the blend. The cost of polymer-based solar cells can be reduced to a great extent through realization of all-plastic and flexible solar cells. This demands the replacement of the metallic components (electrodes) by highly conducting polymer films. While hole conductor polymers are available, low work function polymer electron conductors are rare. In this thesis, prototype solar cells that utilizes a highly conducting polymer, which has a work function of ~ 4.3 eV, as a cathode are demonstrated. Development of this material may eventually lead to fabrication of large area, flexible and cheap solar cells. The transparent nature of the polymer cathode may also facilitate fabrication of multi-layer and tandem solar cells. In the last chapter of this thesis, we demonstrate generation of red and near infrared polarized light by employing thermally converted thin films of polyfluorene copolymers in light emitting diodes. This study, in particular, aims at fabricating polarized infrared light emitting devices. / On the day of the defence day the status of article III was In press and article VI was Manuscript.

P3HT single layer, P3HT/PCBM bilayer and P3HT/PCBM inverted bilayer devices were produced by spin coating organic layers onto ITO patterned glass in air, and clamping it with an Au coated silicon wafer, as top electrode, at the end (Figure13). Normal and inverted bilayer devices were also fabricated with and without PEDOT:PSS. All devices were divided into two groups by changing concentration of P3HT solution. The first group of devices contained 1.0 wt. % P3HT solution (P3HT in dichlorobenzene); the second group 0.56wt %. Power conversion efficiency, short circuit current, open circuit voltage, fill factor and maximum extracted power were measured on all produced devices. In contrast, all devices with 1.0wt % P3HT concentration showed better result than the devices with 0.56wt %. The highest result was obtained for P3HT single layer devices in both cases with short circuit current 56uA/cm2, open circuit voltage 0.94mV, maximum power 11.4uW/cm2 and power conversion efficiency of 0.11%. Inverted bilayer devices performed better than the non-inverted one. The devices with PEDOT:PSS got slightly better performance than the non-PEDOT:PSS used one. Charge carrier mobility measurement was done for all fabricated devices with charge extraction by linearly increasing voltage (CELIV) and dark injected space charge limited current (DI-SCLC) methods. All devices showed same magnitude of charge carrier mobility 10-5 cm2/V.s, the highest value still belongs to P3HT single layer device. The charge carrier mobility in all devices observed by DI-SCLC technique is one order of magnitude higher than by CELIV technique. This may be due to DI-SCLC method`s restriction on ohmic contacts between material and electrode. / بۇ تەتقىقاتتا ئورگانىك ماتېرىيالدىن پايدىلنىپ ئۈچ خىل قۇياش ئىنىرگىيەلىك باتارىيە ئادەتتىكى ئۆي مۇھىتىدا ياساپ چىقىلدى. ئەڭ چوڭ توك كۈچى، ئەڭ                                                    يۇقىرى بېسىم، ئەڭ يۇقىرى قۇۋەت ۋە زەرەت يۆتكۈلۈش تېزلىكى ئۆلچەپ چىقىلدى ئۇيغۇر

Battery Management System (BMS) is an essential part in battery powered applications where large battery packs are in use. BMS ensures protection, controlling, supervision and accurate state estimation of battery pack to provide efficient energy management. However the particular application determines the accuracy and requirements of BMS where it has to implement; in electric vehicles (EVs) accuracy cannot be compromised. The software part of BMS estimates the states of the battery pack and takes the best possible decision. In EVs one of the key tasks of BMS’s software part is to provide the actual state of charge (SOC), which represents a crucial parameter to be determined, especially in lithium iron phosphate (LiFePO4) batteries, due to the presence of the high hysteresis behavior in the open circuit voltage than other kind of lithium batteries. This hysteresis phenomena appears with two different voltage curves during the charging and discharging process. The value of the voltage that the battery is going to assume during the off-loading operation depends on several factors, such as temperature, loop direction and ageing. In this research work, hybrid method is implemented in which advantages of several methods are achieved by implementing one technique combined with another. In this work SOC is calculated from coulomb counting method and in order to correct the error of SOC, an hysteresis model is developed and used due to presence of hysteresis effect in LiFePO4 batteries. An hysteresis model of the open circuit voltage (OCV) for a LiFePO4 cell is developed and implemented in MATLAB/Simulink© in order to reproduce the voltage response of the battery when no current from the cell is required (no load condition). Then the difference of estimated voltage and measured voltage is taken in order to correct the error of SOC calculated from coulomb counting or current integration method. To develop the hysteresis model which can reproduce the same voltage behavior, lot of experiments have been carried out practically in order to see the hysteresis voltage response and to see that how voltage curve change with the variation of temperature, ageing and loop direction. At the end model is validated with different driving profiles at different ambient temperatures.

Cadmium sulfide (CdS) and cadmium telluride (CdTe) are two leading semiconductor materials used in the fabrication of thin film solar cells of relatively high power conversion efficiency and low manufacturing cost. In this work, CdS/CdTe solar cells with a varying set of processing parameters and device designs were fabricated and characterized for comparative evaluation. Studies were undertaken to elucidate the effects of (i) each step in fabrication and (ii) parameters like thickness, sheet resistance, light absorptivity solution concentration, inert gas pressure etc. Best results were obtained when the thickness of CdS planar film for the window layer was in the range of 150 nm to 200 nm. Also, CdS nanowires were fabricated for use as the window layer in CdS-CdTe solar cells. Their materials characteristics were studied with scanning electron microscopy (SEM) and X-ray Diffraction (XRD). Spectral absorption measurements on the planar CdS films and nanowire CdS layers were performed and results compared. It was established that the nanowire CdS design was superior because its absorption of sunlight was far less than that of planar CdS film, which would lead to enhanced performance in the CdS-CdTe solar cell through higher short circuit current density and higher open circuit voltage. Diode behavior of CdS-CdTe devices on planar CdS and nanowire CdS was analyzed and compared. KEYWORDS: Thin Film Solar Cell, Nanowire, UV Absorption, Open-circuit Voltage, Close Space Sublimation

In a context of an expected increasing use of Lithium-ion batteries in the transportation sector, Volvo AB is developing its own solutions for large electric vehicles. It is then beneficial to reduce the costs, the energy demand and the raw materials demand by improving the battery systems’ performances. For that purpose, understanding the physical phenomena which come into play in lithium ion cells is necessary. This project’s motivation has been to deepen the existing knowledge on one or a group of these phenomena which include those at the origin of the Open Circuit Voltage (OCV) hysteresis. It is characterized by the difference in charging and discharging voltage when the cell is at a resting state. These voltage differences might result in heat losses in the cells. In this thesis, the behaviour of the Open Circuit Voltage (OCV) under different operating conditions is studied, and a Preisach empirical hysteresis model is developed. The core part of the work consisted in experimental measurements of the Open-Circuit-Voltage of 10 Ah prismatic LiFePO4 (Lithium Iron Phosphate) cells. These measurements were completed using the Galvanostatic Intermittent Titration Technique (GITT) that consists of alternative current pulse and relaxation phases. The tests were performed using relaxation times ranging from 1 hour to 48 hours with the cells being under various cycles (series of charge and discharge). The impacts of the temperature, various current rates from 0.1C to 1C (1 A to 10 A) on the OCV and the voltage relaxation were evaluated. The amplitude of the OCV hysteresis that does not vanish after full relaxation, which is defined by the difference between the OCV charge and the OCV after discharge, was found to vary between 5 mV and 20-25 mV depending on the State-of-Charge of the cells. Two peaks are identified around 20-30% and 65-70% State-of-Charge. The measured OCV hysteresis with 24 hours relaxation is about half of the measured OCV hysteresis with 2-5 hours relaxation. The experiments also measured an apparent smaller OCV hysteresis when the magnitude of the current increases; this trend is to be verified after full relaxation. The temperature has an impact on the OCV which is averaging around ±0.2 mV/K. The analysis of the voltage relaxation behaviour described that at low temperatures and low C-Rates, the cells get closer to equilibrium voltage at a slower pace. In addition, a higher test time, characterized by longer relaxation times after each step and/or a larger number of steps within the same State of Charge (SOC) range, tends to increase the time required for the cell to reach an equilibrium. After completing the OCV measurements, a Preisach hysteresis model is developed based on the experimental results. The model predicts the OCV variations of an Lithium ferrophosphate (LFP) cell at ambient temperature when going through various charge and discharge cycles. Its estimated Root Mean Square Error (RMSE) is 3 mV, but the accuracy of the model could be partially confounded with measurement uncertainty. The main outcomes are a more accurate description of the voltage relaxation behaviour and a new estimation of the amplitude of the OCV hysteresis in LFP cells. / I en värld där det finns en förväntad ökning av användandet av litiumjonbatterier inom transportsektorn, utvecklar AB Volvo sina egna lösningar för stora elfordon som lastbilar. För att reducera kostnaderna, energibehovet och efterfrågan på råvaror, är det nödvändigt att förstå fysiska fenomen inom litiumjoncellerna eftersom det kan hjälpa till att förbättra systemens prestanda. Examensarbetets motivation är att fördjupa kunskapen om fenomenen vid uppkomsten av öppen kretsspänningshysteres inom litiumjärnfosfatceller. Denna hysteres definieras av skillnaden mellan öppen kretsspänning (Open-Circuit-Voltage eller OCV) under laddning och OCV under urladdning. Det orsakar över- och underspänning som ökar värmeförlusterna i litiumjoncellerna. Detta projekt studerar beteendet av både spänningsrelaxation och OCV för ett valt intervall av parametrar. Sedan utvecklas en Preisach empirisk modell. Huvuddelen av arbetet bestod i den experimentella mätningen av OCV av 10 Ah prismatiska LiFePO4 celler (Litiumjärnfosfatceller). Dessa experiment genomfördes medelst en mätprocedur som kallas Galvanostatic Intermittent Titration Technique eller Galvanostatisk intermittent titreringsteknik (GITT). Testerna innehåller växelvis strömpuls- och relaxationsfaser. Spänningsrelaxationsfaserna varade mellan 1 och 48 timmar under olika laddnings- och urladdningscykler. Inverkan av båda temperaturen och strömstyrkan (mellan 0.1C och 1C) på OCV utvärderades. Amplituden för OCV hysteresen som kvarstår efter full relaxation beräknades ligga mellan 5 mV och 20-25 mV beroende på cellersladdningstillstånd. Två hysterestoppar identifierades: en runt 70% och en andra mellan 20% och 30% laddningstillstånd. Hysteresen som mäts med 24 timmar av relaxation är runt hälften av hysteresen som mäts med två till fem timmar av relaxation. Med större strömstyrka är den uppmätta hysteresen lite lägre. Ytterligare tester bör göras för att verifiera att hysteresen fortfarande är lägre efter full relaxation. Temperaturen har en begränsad effekt på den totala hysteresen, men entropikoefficientensvärdet är i genomsnitt runt ± 0.2 mV/K. Analysen av relaxations beteende beskriver att en högre temperatur och strömstyrka ökar hastigheten med vilken jämviktspotentialen nås efter strömpulsen. Dessutom orsakaren ökning av den totala testlängden en långsammare relaxering. En längre testtid karaktäriseras av en längre relaxationstid efter varje strömpuls och/eller flera steg för laddningstillstånd. Efter OCV-mätningarna, byggdes en Preisach hysteresmodel med hjälp av de experimentella resultaten. OCV-variationer under olika laddnings och urladdnings cykler modellerades vid rumstemperatur med ett uppskattat minsta kvadratfel på cirka 3 mV. Modellen testades inte med ett begränsat antal cykler så den exakta noggrannheten behöver ytterligare verifieras för att få ner mätosäkerheten. Det huvudsakliga bidraget från detta examensarbete är uppskattning av amplituden för den hysteresen och beskrivningen av spänningsrelaxering efter olika strömpulser, såväl i längd som i amplitud.

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Foram desenvolvidos sistemas de medidas visando a caracterização de células solares de sílico monocristalino. Para isso, foram determinadas as características I x V no escuro para diferentes níveis de iluminação. Curvas de resposta espectral e capacitância em função da tensão inversa aplicada foram também obtidas. Foi feita uma avaliação do comportamento dessas células em função da temperatura e realizadas medidas de profundidade de junção utilizando-se três métodos distintos. Os principais parâmetros, que determinam o desempenho dessas células, foram obtidos boa concordância com a teoria e com os resultados apresentados na literatura. / Systems of measurements were developed for the characterization of single crystal silicon solar cells. For that, the curves I x V were measured in the dark and for different intensity of illumination. Curves of spectral response and of capacitance as a function of the reciprocal of the voltage were also measured. The behavior of the cells as a function of temperature was analysed and also measurements of junction depth were made by three different methods. Values for the parameters that characterize the cells were obtained, showing a good agreement with theoretical values and also with already reported values.

This thesis deals with the device physics of organic solar cells. Organic photovoltaics (OPV) is a field of applied research which has been growing rapidly in the last decade leading to a current record value of power-conversion efficiency of 10 percent. One major reason for this boom is a potentially low-cost production of solar modules on flexible (polymer) substrate. Furthermore, new application are expected by flexible or semitransparent organic solar cells. That is why several OPV startup companies were launched in the last decade. Organic solar cells consist of hydrocarbon compounds, deposited as ultrathin layers (some tens of nm) on a substrate. Absorption of light leads to molecular excited states (excitons) which are strongly bound due to the weak interactions and low dielectric constant in a molecular solid. The excitons have to be split into positive and negative charges, which are subsequently collected at different electrodes. An effective dissociation of excitons is provided by a heterojunction of two molecules with different frontier orbital energies, such that the electron is transfered to the (electron) acceptor and the positive charge (hole) remains on the donor molecule. This junction can be realized by two distinct layers forming a planar heterojunction or by an intermixed film of donor and acceptor, resulting in a bulk heterojunction. Electrodes are attached to the absorber to collect the charges by providing an ohmic contact in the optimum case. This work focuses on the electrical processes in organic solar cells developing and employing a one-dimensional drift-diffusion model. The electrical model developed here is combined with an optical model and covers the diffusion of excitons, their separation, and the subsequent transport of charges. In contrast to inorganics, charge-carrier mobilities are low in the investigated materials and charge transport is strongly affected by energy barriers at the electrodes. The current-voltage characteristics (J-V curve) of a solar cell reflect the electrical processes in the device. Therefore, the J-V curve is selected as means of comparison between systematic series of simulation and experimental data. This mainly qualitative approach allows for an identification of dominating processes and provides microscopic explanations. One crucial issue, as already mentioned, is the contact between absorber layer and electrode. Energy barriers lead to a reduction of the power-conversion efficiency due to a decrease in the open-circuit voltage or the fill factor by S-shaped J-V curve (S-kink), which are often observed for organic solar cells. It is shown by a systematic study that the introduction of deliberate barriers for charge-carrier extraction and injection can cause such S-kinks. It is explained by simulated electrical-field profiles why also injection barriers lead to a reduction of the probability for charge-carrier extraction. A pile-up of charge carriers at an extraction barrier is confirmed by measurements of transient photocurrents. In flat heterojunction solar cells an additional reason for S-kinks is found in an imbalance of electron and hole mobilities. Due to the variety of reasons for S-kinks, methods and criteria for a distinction are proposed. These include J-V measurements at different temperatures and of samples with varied layer thicknesses. Most of the studies of this this work are based on experimental data of solar cells comprisiing the donor dye zinc phthalocyanine and the acceptor fullerene C60. It is observed that the open-circuit voltage of these devices depends on the mixing ratio of ZnPc:C60. A comparison of experimental and simulation data indicates that the reason is a changed donor-acceptor energy gap caused by a shift of the ionization potential of ZnPc. A spatial gradient in the mixing ratio of a bulk heterojunction is also investigated as a donor(acceptor)-rich mixture at the hole(electron)-collecting contact is supposed to assist charge extraction. This effect is not observed, but a reduction of charge-carrier losses at the “wrong” electrode which is seen at an increase in the open-circuit voltage. The most important intrinsic loss mechanism of a solar cell is bulk recombination which is treated at the example of ZnPc:C60 devices in the last part of this work. An examination of the dependence of the open-circuit voltage on illumination intensity shows that the dominating recombination mechanism shifts from trap-assisted to direct recombination for higher intensities. A variation of the absorption profile within the blend layer shows that the probability of charge-carrier extraction depends on the locus of charge-carrier generation. This results in a fill factor dependent on the absorption profile. The reason is an imbalance in charge-carrier mobilities which can be influenced by the mixing ratio. The work is completed by a simulation study of the influence of charge-carrier mobilities and different recombination processes on the J-V curve and an identification of a photoshunt dominating the experimental linear photocurrent-voltage characteristics in reverse bias. / Diese Dissertation beschäftigt sich mit der Physik organischer Solarzellen. Die organische Photovoltaik ist ein Forschungsgebiet, dem in den letzten zehn Jahren enorme Aufmerksamkeit zu Teil wurde. Der Grund liegt darin, dass diese neuartigen Solarzellen, deren aktueller Rekordwirkungsgrad bei 10 Prozent liegt, ein Potential für eine kostengünstige Produktion auf flexiblem (Polymer)substrat aufweisen und aufgrund ihrer Vielfältigkeit neue Anwendungsbereiche für die Photovoltaik erschließen. Organische Solarzellen bestehen aus ultradünnen (einige 10 nm) Schichten aus Kohlenwasserstoffverbindungen. Damit der photovoltaische Effekt genutzt werden kann, müssen die durch Licht angeregten Molekülzustände zu freien Ladungsträgern führen, wobei positive und negative Ladung an unterschiedlichen Kontakten extrahiert werden. Für eine effektive Trennung dieser stark gebundenden lokalisierten angeregten Zustände (Exzitonen) ist eine Grenzfläche zwischen Molekülen mit unterschiedlichen Energieniveaus der Grenzorbitale erforderlich, sodass ein Elektron auf einem Akzeptor- und eine positive Ladung auf einem Donatormolekül entstehen. Diese Grenzschicht kann als planarer Heteroübergang durch zwei getrennte Schichten oder als Volumen-Heteroübergang in einer Mischschicht realisiert werden. Die Absorberschichten werden durch Elektroden kontaktiert, wobei es für effiziente Solarzellen erforderlich ist, dass diese einen ohmschen Kontakt ausbilden, da ansonsten Verluste zu erwarten sind. Diese Arbeit behandelt im Besonderen die elektrischen Prozesse einer organischen Solarzelle. Dafür wird ein eindimensionales Drift-Diffusionsmodell entwickelt, das den Transport von Exzitonen, deren Trennung an einer Grenzfläche und die Ladungsträgerdynamik beschreibt. Abgesehen von den Exzitonen gilt als weitere Besonderheit einer organischen Solarzelle, dass sie aus amorphen, intrinsischen und sehr schlecht leitfähigen Absorberschichten besteht. Elektrische Effekte sind an der Strom-Spannungskennlinie (I-U ) sichtbar, die in dieser Arbeit als Hauptvergleichspunkt zwischen experimentellen Solarzellendaten und den Simulationsergebnissen dient. Durch einen weitgehend qualitativen Vergleich können dominierende Prozesse bestimmt und mikroskopische Erklärungen gefunden werden. Ein wichtiger Punkt ist der schon erwähnte Kontakt zwischen Absorberschicht und Elektrode. Dort auftretende Energiebarrieren führen zu einem Einbruch im Solarzellenwirkungsgrad, der sich durch eine Verringerung der Leerlaufspanung und/oder S-förmigen Kennlinien (S-Knick) bemerkbar macht. Anhand einer systematischen Studie der Grenzfläche Lochleiter/Donator wird gezeigt, dass Energiebarrieren sowohl für die Ladungsträgerextraktion als auch für die -injektion zu S-Knicken führen können. Insbesondere die Tatsache, dass Injektionsbarrieren sich auch negativ auf den Photostrom auswirken, wird anhand von simulierten Ladungsträger- und elektrischen Feldprofilen erklärt. Das Aufstauen von Ladungsträgern an Extraktionsbarrieren wird durch Messungen transienter Photoströme bestätigt. Da S-Knicke in organischen Solarzellen im Allgemeinen häufig beobachtet werden, werden weitere Methoden vorgeschlagen, die die Identifikation der Ursachen ermöglichen. Dazu zählen I-U Messungen in Abhängigkeit von Temperatur und Schichtdicken. Als eine weitere Ursache von S-Knicken werden unausgeglichene Ladungsträgerbeweglichkeiten in einer Solarzelle mit flachem Übergang identifiziert und von den Barrierefällen unterschieden. Weiterer Forschungsgegenstand dieser Arbeit sind Mischschichtsolarzellen aus dem Donator-Farbstoff Zink-Phthalozyanin ZnPc und dem Akzeptor Fulleren C60. Dort wird beobachtet, dass die Leerlaufspannung vom Mischverhältnis abhängt. Ein Vergleich von Experiment und Simulation zeigt, dass sich das Ionisationspotenzial von ZnPc und dadurch die effektive Energielücke des Mischsystems ändern. Zusätzlich zu homogenen Mischschichten werden Solarzellen untersucht, die einen Gradienten im Mischungsverhältnis aufweisen. Die Vermutung liegt nahe, dass ein hoher Donatorgehalt am Löcherkontakt und ein hoher Akzeptorgehalt nahe des Elektronenkontakts die Ladungsträgerextraktion begünstigen. Dieser Effekt ist in dem hier untersuchten System allerdings vergleichsweise irrelevant gegenüber der Tatsache, dass der Gradient das Abfließen bzw. die Rekombination von Ladungsträgern am “falschen” Kontakt reduziert und somit die Leerlaufspannung erhöht. Der wichtigste intrinsische Verlustmechanismus einer Solarzelle ist die Rekombination von Ladungsträgern. Diese wird im letzten Teil der Arbeit anhand der ZnPc:C60 Solarzelle behandelt. Messungen der Leerlaufspannung in Abhängigkeit von der Beleuchtungsintensität zeigen, dass sich der dominierende Rekombinationsprozess mit zunehmender Intensität von Störstellenrekombination zu direkter Rekombination von freien Ladungsträgern verschiebt. Eine gezielte Variation des Absorptionsprofils in der Absorberschicht zeigt, dass die Ladungsträgerextraktionswahrscheinlickeit vom Ort der Ladungsträgergeneration abhängt. Dieser Effekt wird hervorgerufen durch unausgeglichene Elektronen- und Löcherbeweglichkeiten und äußert sich im Füllfaktor. Weitere Simulationsergebnisse bezüglich des Einflusses von Ladungsträgerbeweglichkeiten und verschiedener Rekombinationsmechanismen auf die I-U Kennlinie und die experimentelle Identifikation eines Photoshunts, der den Photostrom in Rückwärtsrichtung unter Beleuchtung dominiert, runden die Arbeit ab.