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.

<p>Organic solar cells have caught considerable attention in the past few years due to their potential for providing environmentally safe, flexible, lightweight, inexpensive, and roll-to-roll feasible production solar cells. However, the efficiency achieved in current organic solar cells is quite low, yet quick and successive improvements render it as a promising alternative. A hopeful approach to improve the efficiency is by exploiting the tandem concept which consists of stacking two or more organic solar cells in series.</p><p>One important constituent in tandem solar cells is the middle electrode layer which is transparent and functions as a cathode for the first cell and an anode for the second cell. Most studies done so far have employed noble metals such as gold or silver as the middle electrode layer; however, they suffered from several shortcomings especially with respect to reproducibility.</p><p>This thesis focuses on studying a new trend which employs an oxide material based on nano-particles as a transparent cathode (such as Zinc-oxide-nano-particles) along with a transparent anode so as to replace the middle electrode.</p><p>Thus, this work presents a study on solution processable zinc oxide (ZnO) nanostructures, their proper handling techniques, and their potential as a middle electrode material in Tandem solar cells in many different configurations involving both polymer and small molecule materials. Moreover, the ZnO-np potential as a candidate for acceptor material is also investigated.</p>

<p>This project is an extension of a pre-existing simulation program (‘Simulation_2dioden’). This simulation program was first developed in Konarka Technologies. The main purpose of the project ‘Simulation_2dioden’ is to calibrate the values of different parameters like, Shunt resistance, Series resistance, Ideality factor, Diode current, epsilon, tau, contact probability, AbsCT, intensity, etc; This is one of the curve fitting procedure’s. This calibration is done by using different equations. Diode equation is one of the main equation’s used in calculating different currents and voltages, from the values generated by diode equation all the other parameters are calculated.</p><p>The reason for designing this simulation_2dioden is to calculate the values of different parameters of a device and the researcher would know which parameter effects more in the device efficiency, accordingly they change the composition of the materials used in the device to acquire a better efficiency. The platform used to design this project is ‘Microsoft Excel’, and the tool used to design the program is ‘Visual basics’. The program could be otherwise called as a ‘Virtual Solar cell’. The whole Virtual Solar cell is programmed in a single excel sheet.</p><p>An Automated working solution is suggested which could save a lot of time for the researchers, which is the main aim of this project. To calibrate the parameter values, one has to load the J-V characteristics and simulate the program by just clicking one button. And the parameters extracted by using this automated simulation are Parallel resistance, Series resistance, Diode ideality, Saturation current, Contact properties, and Charge carrier mobility.</p><p>Finally, a basic working solution has been initiated by automating the simulation program for calibrating the parameter values.</p>

Ce travail traite de la croissance épitaxiale et de la caractérisation d’hétérostructures àbase de multi-puits quantiques (MPQ) pour des applications dans le photovoltaïque. Leséchantillons ont été obtenus par la technique d’épitaxie en phase vapeur aux organométalliques(EPVOM) sur des substrats de saphir (0001). La caractérisation structurale etoptique est réalisée par diffraction de rayons X, microscopie électronique en transmission,spectroscopie de photoluminescence et de transmission. Pour étudier la présence de l’effetphotovoltaïque et pour estimer la performance électrique des échantillons, les MPQ ont étéintégrés dans la géométrie de cellules solaires en utilisant de la photolithographie, desattaques réactives ioniques assistées par plasma inductif et des métallisations pour contacterles parties dopées n et p.Nous avons étudié l’influence de différents designs des régions actives InGaN/GaN surles propriétés optiques et électriques des échantillons, c’est-à-dire le nombre de puitsquantiques InGaN, les épaisseurs des puits et des barrières et la composition en indium dansles puits. Deux mécanismes principaux doivent être pris en compte pour une optimisationefficace de composants photovoltaïques: l’absorption des photons et la collections desporteurs. Nous avons montré qu’une augmentation du nombre de MPQ, de leur épaisseur etde la composition d’In améliorait l’absorption, mais causait aussi des pertes dans l’efficacitéde collection du fait de l’augmentation de l’épaisseur de la couche active (champ électriqueplus faible), de la difficulté des porteurs pour s’échapper de puits plus profonds et derelaxation des contraintes (création de défauts structuraux). La décroissance de l’épaisseur desbarrières peut résoudre les deux premiers points, mais le problème de la relaxation de lacontrainte reste entier. Pour notre meilleur design, nous obtenons une efficacité de conversionde 2 % pour des couches 15×In0.18Ga0.82N/GaN qui ont une réponse spectrale qui s’étendjusqu’à 465 nm. / In this work we report on epitaxial growth and characterization of InGaN/GaN multiquantumwells (MQWs) heterostructures for application in photovoltaic devices. The sampleswere grown by metal-organic vapor phase epitaxy (MOVPE) on (0001) sapphire substrate.The structural and optical characterization is performed by X-ray diffraction, transmissionelectron microscopy, photoluminescence spectroscopy and transmission measurements. Toinvestigate the presence of photovoltaic effect and estimate the electrical performance of thesamples, they were processed into solar cells by means of the photolithography, inductivelycoupled plasma reactive-ion etching and metallization to manage n and p contacts.We studied the influence of different InGaN/GaN active region designs on thestructural, optical and electrical properties of the samples, i.e. number of InGaN quantumwells, QW and quantum barrier thicknesses and indium composition in the wells. Two mainmechanisms have to be taken into account for an efficient optimization of photovoltaicdevices: photon absorption and carrier collection. We showed that an increase of the MQWsnumber, their thickness and the In-content allows absorption improvement, but causes lossesin the carrier collection efficiency due to: the increase of the active region thickness (lowerelectric field), the difficulty of the carrier to escape from deeper QWs and the strain relaxation(structural defect creation). The decrease of the barrier thickness can solve the first two issues,but the problem with strain relaxation remains. In the best design, we report the value of2.00% of conversion efficiency for 15×In0.18Ga0.82N/GaN samples with spectral responseextending to 465 nm.

In this dissertation, the multiple facets of benzodithiophene (BDT) units are explored, with a focus on understanding how the isomerism of the BDT structure affects the macroscopic properties of the oligomeric and polymeric materials created. First, the story focuses on an overview of the BDT synthons and their applications in organic electronics. A straightforward synthesis of BDT and its derivatization to seven π-conjugated building blocks and seven polymers is presented. Then, symmetric (donor)2-acceptor (D2-A) dye architecture for application in dye-sensitized solar cells are investigated. Two isomeric systems are studied, and the branched sensitizers show a greater incident photon-to-current efficiency than the linear dyes. The nature of the accepting core is also varied between dibenzophenazine to dithienophenazine. The sensitizer with the weakest accepting core displays the best photovoltaic performance, due to an increase in the open-circuit voltage of ~100 mV caused by the favorable shift of the metal oxide conduction band. Lastly, a study of the donating building blocks in these (D2-A) sensitizers demonstrates that increasing the number of donor units from two to six thiophene moiety doubles the solar cell performance, due to the improvement of the light harvesting ability.

Obtaining abundant, clean, sustainable energy has become an increasingly large need globally. To date, solar cells have had a limited impact in meeting energy demands. This is primarily due to their relatively high cost and low power conversion efficiencies. Sensitized solar cells, or Grätzel cells, have the potential for being made with low cost materials, and achieving power conversion efficiency high enough to economically compete with fossil fuels. Understanding the dynamics of charge carriers as they separate at the interface of the light absorbing donor and their semiconducting acceptor becomes an important first step in the realization of an inexpensive and efficient sensitized solar cell. Presented is the theory of treating electrons at donor-acceptor interfaces, and why time-resolved surface second harmonic generation (TR-SHG) is used to probe the dynamics of charge carriers at these interfaces. A series of experiments are described where various preparations of thin films of sensitizers on single crystal titanium dioxide, a common acceptor in Grätzel cells, are prepared and studied. TR-SHG studies of thin films of colloidal PbSe and CdSe QDs showed remarkably different electron cooling and transfer dynamics. The electron cooling in PbSe is thermally activated in PbSe QDs. By cooling samples, electron transfer from higher excited “hot” states was observed. Contrary, for CdSe QDs electron transfer rates were dependent on the energy of the excited state. When higher states were excited, charge transfer rates decreased, indicating that only low energy, electrically “cold”, states participate in charge transfer. When carbon based grapheme QDs are used, the electron dynamics mimic PbSe QDs. In this system, increasing the pump energy leads to slower recombination rates, indicating that electrons have to drift further back to the interface. / text

Developing a fundamental understanding of photocurrent generation processes at organic-inorganic interfaces is critical for improving hybrid solar cell efficiency and stability. This dissertation explores processes at these interfaces by combining data from photovoltaic device performance tests with characterization experiments conducted directly on the device. The dissertation initially focuses on exploring how morphologically and chemically modifying the organic-inorganic interface, between poly(3-hexylthiophene) (P3HT) as the electron donating light absorbing polymer and titanium dioxide (TiO₂) as the electron acceptor, can result in stable and efficient hybrid solar cells. Given the heterogeneity which exists within bulk heterojunction devices, stable interfacial prototypes with well-defined interfaces between bilayers of TiO₂ and P3HT were developed, which demonstrate tunable efficiencies ranging from 0.01 to 1.6 %. Stability of these devices was improved by using Cu-based hole collecting electrodes. Efficiency values were tailored by changing TiO₂ morphology and by introducing sulfide layers like antimony trisulfide (Sb₂S₃) at the P3HT-TiO₂ interface. The simple bilayer device design developed in this dissertation provides an opportunity to study the precise role played by nanostructured TiO₂ surfaces and interfacial modifiers using a host of characterization techniques directly on a working device. Examples introduced in this dissertation include X-ray photoelectron spectroscopy (XPS) depth profiling analysis of metal-P3HT and P3HT-TiO₂ interfaces and Raman analysis of bonding between interface modifiers like Sb₂S₃ and P3HT. The incompatibility of TiO₂ with P3HT was significantly reduced by using P3HT derivatives with -COOH moieties at the extremity of a polymer chain. The role of functional groups like -COOH in interfacial charge separation phenomena was studied by comparing the photovoltaic behavior of these devices with those based on pristine P3HT. Finally, for hybrid solar cells discussed in this dissertation to become commercially viable, high temperature processing steps of the inorganic TiO₂ layer must be avoided. Accordingly, this dissertation demonstrates the novel use of electromagnetic radiation in the form of microwaves to catalyze growth of anatase TiO₂ thin films at temperatures as low as 150 °C, which is significantly lower than that used in conventional techniques. This low temperature process can be adapted to a variety of substrates and can produce patterned films. Accordingly, the ability to fabricate TiO₂ thin films by the microwave process at low temperatures is anticipated to have a significant impact in processing devices based on plastics. / text

The widespread deployment of wireless sensors in our homes, offices, factories and infrastructure has opened the door for system designers to create novel approaches for powering wireless sensor nodes. In recent years, energy harvesting has emerged as the power supply of choice for embedded system designers, enabling wireless sensors to be used in applications that previously were not feasible with conventional battery-powered designs. This report details the design and development of an energy harvesting wireless sensor from concept to production. Design constraints included the requirement to operate reliably in a wide variety of environments, the use of commercially available components, and a visually appealing form factor. The result is a very power-efficient, solar-powered wireless sensor that measures temperature, voltage, and illumination level at the solar cell and has an ultra slim form factor. / text

The research and development of organic electron donors is essential in the discovery of photodynamic therapy photosensitizers and catalysts, as well as in the fabrication of organic-based electronic devices. Recently, [pi]-extended tetrathiafulvalenes (exTTFs) have emerged as important organic donors due to their superb electronic properties. However, in general, exTTFs lack significant absorption in the visible and near-infrared portions of the electromagnetic spectrum, thereby limiting their utility. This doctoral dissertation depicts the author's efforts to address this inherent drawback of exTTFs by extending the electronic conjugation of tetrathiafulvalene moieties through pyrrole-based chromophores. The reported findings describe the design, synthesis, properties and potential applications of exTTFs with greatly enhanced absorption profiles. The first Chapter provides a brief historical overview on the history and development of [pi]-extended tetrathiafulvalenes. The various conjugated linkers utilized in exTTF systems are reviewed. In the latter part of the Chapter, emphasis is given to the applications in which exTTFs find use. Chapter 2, as the major focus of the dissertation, details the synthesis and characteristics of a quinoidal porphyrin-bridged exTTF, termed MTTFP. Several metalated complexes, including the Zn, Co, Cu, and Ni derivatives of MTTFP are reported. Additionally, the electrochemical, photophysical, and structural properties of MTTFPs are discussed. We also detail our efforts to synthesize and characterize both the one- and two-electron oxidized forms of MTTFPs. Finally, we discuss our efforts to reversibly switch thermodynamic electron transfer from ZnTTFP to Li@C₆₀ through coordination of axial ligands. Chapter 3 describes the formation of a 2:1 supramolecular ionic porphyrin complex between the two-electron oxidized form of ZnTTFP and a tetranionic sulfonated porphyrin. The association constants and the X-ray crystal structure of the complex are reported. A brief discussion outlining the photophysical characteristics (performed in Prof. Shunichi Fukuzumi and Prof. Dongho Kim's group) of the porphyrin donor-acceptor complexes are included. Chapter 4 details the synthesis, photophysical properties, and spectroelectrochemistry of a difluoroboradiazaindacene (BODIPY) bridged exTTF. This compound is referred to as ex-BODIPY. A singlet oxygen generation study provides initial evidence that ex-BODIPY could potentially serve as a photosensitizer. All of the experimental procedures, characterization data, and X-ray crystallographic data tables are reported in Chapter 5. / text

Semiconductor nanocrystal-based photovoltaics are an interesting new technology with the potential to achieve high efficiencies at low cost. CuInSe2 nanocrystals have been synthesized in solution using arrested precipitation and dispersed in solvent to form a “solar ink”. The inks have been deposited under ambient conditions to fabricate photovoltaic devices with efficiency up to 3%. Despite the low cost spray coating deposition technique, device efficiencies remain too low for commercialization. Higher efficiencies up to 7% have been achieved using a high temperature selenization process, but this process is too expensive. New nanocrystal film treatment processes are necessary which can improve the device efficiency at low cost. To this end, CuInSe2 nanocrystals were synthesized using a diphenyl phosphine:Se precursor which allows for precise control over the nanocrystal size. The size is controlled by changing the temperature of the reaction. The smallest size nanocrystals demonstrated extremely high device open circuit voltage. Ligand exchange procedures were used to replace the insulating oleylamine capping ligand used during synthesis with more conductive halide ions or inorganic chalcogenidometallate cluster (ChaM) ligands. These ligands led to improved charge transport in the nanocrystal films. A high-intensity pulsed light processing technique known as photonic curing was used which allows for high temperature sintering of nanocrystal films on temperature-sensitive substrates. High energy pulses cause the nanocrystals to sinter into large grains, primarily through melting and resolidification. The choice of metal back contact has a dramatic effect on the final film morphology, with Au and MoSe2 back contacts providing much better adhesion with the CuInSe2 than Mo back contacts. Nanocrystal sintering without melting can be achieved by replacing the oleylamine ligands with ChaM ligands prior to photonic curing. Low energy photonic curing pulses vaporize the oleylamine ligands without inducing sintering or grain growth. This greatly improved nanocrystal coupling and interparticle charge transport. Multiexcitons were successfully extracted from these nanocrystal films and external quantum efficiencies over 100% were observed. Transient absorption spectroscopy was used to study the multiexciton generation process in CuInSe2 nanocrystal films and colloidal suspensions. The multiexciton generation efficiency, threshold, and Auger lifetimes for CuInSe2 compare well with other nanocrystal materials. / text