Elucidating the optical properties of novel heterolayered materials based on MoTe2-InN for photovoltaic applications

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Cu nanoparticles enable plasmonic-improved silicon photovoltaic devices

This work examines the effect of copper nanoparticles (Cu NPs) on the photocurrent efficiency of silicon photovoltaic (Si PV) devices. An optimized synthesis of stable Cu NPs is reported together with a procedure for their immobilization on the Si PV surface. A comprehensive analysis of the photocurrent and power dependence of the Cu NPs surface coverage and size is presented. A decrease in photoconversion was observed for wavelengths shorter than similar to 500 nm, due to the Cu interband absorption. In the low surface coverage limit, where the level of aggregation was found to be low, the surface plasmon resonance absorption dominates leading to a modest effect on the photocurrent response. As the number of aggregates increased with the surface coverage, the photocurrent efficiency also increased, and a maximum enhancement power conversion of 16% was found for a 54 +/- 6 NPs per mu m(2) PV cell. This enhancement was attributed to SPR light scattering and trapping into the Si PV device. Higher surface coverage yielded numerous aggregates which acted as a bulk coating and caused a decrease in both photocurrent and power measurements.

Photoconduction action spectra of regio-regular poly(3-hexylthiopene):TiO2 diodes

The development of polymer-based photovoltaic devices brings the promise of low-cost and lightweight solar energy conversion systems. This technology requires new materials and device architectures with enhanced efficiency and lifetime, which depends on the understanding of charge-transport mechanisms. Organic films combined with electronegative nanoparticles may form systems with efficient dissociation of the photogenerated excitons, thus increasing the number of carriers to be collected by the electrodes. In this paper we investigate the steady-state photoconductive action spectra of devices formed by a bilayer of regio-regular poly(3-hexylthiophene) (RRP3HT) and TiO2 sandwiched between ITO and aluminum electrodes (ITO/TiO2:RRP3HT/Al). Photocurrents were measured for distinct bias voltages with illumination from either side of the device. Heterojunction structures were prepared by spin coating a RRP3HT film on an already deposited TiO2 layer on ITO. Symbatic and antibatic curves were obtained and a model for photocurrent action spectra was able to fit the symbatic responses. The quantum yield increased with the electric field, indicating that exciton dissociation is a field-assisted process as in an Onsager mechanism. Furthermore, the quantum yield was significantly higher when illumination was carried out through the ITO electrode onto which the TiO2 layer was deposited, as the highly electronegative TiO2 nanoparticles were efficient in exciton dissociation.

Cellulose based Lithium ion polymer electrolytes for Lithium batteries

The separator membrane in batteries and fuel cells is of crucial importance for the function of these devices. In lithium ion batteries the separator membrane as well as the polymer matrix of the electrodes consists of polymer electrolytes which are lithium ion conductors. To overcome the disadvantage of currently used polymer electrolytes which are highly swollen with liquids and thus mechanically and electrochemically unstable, the goal of this work is a new generation of solid polymer electrolytes with a rigid backbone and a soft side chain structure. Moreover the novel material should be based on cheap substrates and its synthesis should not be complicated aiming at low overall costs. The new materials are based on hydroxypropylcellulose and oligoethyleneoxide derivatives as starting materials. The grafting of the oligoethyleneoxide side chains onto the cellulose was carried out following two synthetic methods. One is based on a bromide derivative and another based on p-toluolsulfonyl as a leaving group. The side chain reagents were prepared form tri(ethylene glycol) monoethyl ether. In order to improve the mechanical properties the materials were crosslinked. Two different conceptions have been engaged based on either urethane chemistry or photosensitive dimethyl-maleinimide derivatives. PEO - graft - cellulose derivatives with a high degree of substitution between 2,9 and 3,0 were blended with lithium trifluoromethane-sulfonate, lithium bis(trifluorosulfone)imide and lithium tetrafluoroborate. The molar ratios were in the range from 0,02 to 0,2 [Li]/[O]. The products have been characterized with nuclear magnetic resonance (NMR), gel permeation chromatography (GPC) and laserlight scattering (LS) with respect to their degree of substitution and molecular weight. The effect of salt concentration on ionic conductivity, thermal behaviour and morphology has been investiga-ted with impedance spectroscopy, differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). The crosslinking reactions were controlled with dynamic mechanical analysis (DMS). The degree of substitution of our products is varying between 2,8 and 3,0 as determined by NMR. PEO - graft - cellulose derivatives are highly viscous liquids at room temperature with glass transition temperatures around 215 K. The glass transition temperature for the Lithium salt complexes of PEO - graft - cellulose deri-vatives increase with increasing salt content. The maximum conductivity at room temperature is about 10-4 and at 100°C around 10-3 Scm-1. The presence of lithium salt decreases the thermal stability of the complexes in comparison to pure PEO - graft - cellulose derivatives. Complexes heated over 140 – 150°C completely lose their ionic conductivity. The temperature dependence of the conductivity presented as Arrhenius-type plots for all samples is similar in shape and follows a VTF behaviour. This proofs that the ionic transport is closely related to the segmental motions of the polymer chains. Novel cellulose derivatives with grafted oligoethylen-oxide side chains with well-defined chemical structure and high side chain grafting density have been synthesized. Cellulose was chosen as stiff, rod like macromolecule for the backbone while oligoethylen-oxides are chosen as flexible side chains. A maximum grafting density of 3.0 have been obtained. The best conductivity reaches 10-3 Scm-1 at 100°C for a Li-triflate salt complex with a [Li]/[O] ratio of 0.8. The cross-linked complexes containing the lithium salts form elastomeric films with convenient mechanical stability. Our method of cellulose modification is based on relatively cheap and commercially available substrates and as such appears to be a promising alternative for industrial applications.

Thin functional conducting polymer films

In the present study, thin functional conducting polyaniline (PANI) films, either doped or undoped, patterned or unpatterned, were prepared by different approaches. The properties of the obtained PANI films were investigated in detail by a combination of electrochemistry with several other techniques, such as SPR, QCM, SPFS, diffraction, etc. The sensing applications (especially biosensing applications) of the prepared PANI films were explored. Firstly, the pure PANI films were prepared by the electropolymerisation method and their doping/dedoping properties in acidic conditions were investigated in detail by a combination of electrochemistry with SPR and QCM. Dielectric constants of PANI at different oxidation states were obtained quantitatively. The results obtained here laid a good foundation for the following investigations of PANI films in neutral pH conditions. Next, PANI multilayer films doped by a variety of materials were prepared by the layer-by-layer method in order to explore their biosensing applications, because of the loss of redox activity of pure PANI in neutral pH conditions. The dopants used include not only the traditionally used linear polyelectrolytes, but also, for the first tim, some other novel materials, like modified gold nanoparticles or modified carbon nanotubes. Our results showed that all the used dopants could form stable multilayer films with PANI. All the obtained PANI multilayer films showed good redox activity in a neutral pH environment, which makes them feasible for bioassays. We found that all the prepared PANI multilayer films can electrocatalyze the oxidation of NADH in neutral conditions at a low potential, although their catalytic efficiencies are different. Among them, PANI/carbon nanotube system showed the highest catalytic efficiency toward the oxidation of NADH, which makes it a good candidate as a NADH sensor. Besides, because some of the prepared PANI multilayer systems were end-terminated with –COOH groups (like PANI/Au nanoparticles system), which can be utilized to easily link biomolecules for biosensing applications. Here, we demonstrated, for the first time, to use the prepared PANI multilayer films for the DNA hybridisation detection. The detection event was monitored either by direct electrochemical method, or by enzyme-amplified electrochemical method, or by surface plasmon enhanced fluorescence spectroscopic method. All the methods can effectively differentiate non-complementary DNA from the complementary ones, even at the single-base mismatch level. It should also be noted that, our success in fabricating PANI multilayer films with modified Au nanoparticles or carbon nanotubes also offered another novel method for incorporating such novel materials into (conducting) polymers. Because of the unique electrochemical and optical properties of each component of the obtained PANI multilayer films, they should also find potential applications in many other fields such as microelectronics, or for electrochromic and photovoltaic devices. Finally, patterned PANI films were fabricated by the combination of several patterning techniques, such as the combination of electrocopolymerization with micromolding in capillaries (EP-MIMIC), the combination of microcontact printing with the layer-by-layer technique (µCP-LBL), and the polystyrene (PS) template induced electropolymerisation method. Using the obtained stripe-shaped PANI/PSS film, a redox-switchable polymer grating based on the surface-plasmon-enhanced mode was constructed and its application in the field of biosensing was explored. It was found that the diffraction efficiency (DE) of the grating was very sensitive to the applied potential (i.e. redox state of the film) as well as the pH environment of the dielectric medium. Moreover, the DE could also be effectively tuned by an electrocatalytic event, such as the electrocatalytic oxidation of NADH by the grating film. By using PS colloidal crystal assemblies as templates, well-ordered 3D interconnected macroporous PANI arrays (PANI inverse opals) were fabricated via electropolymerisation method. The quality of the obtained inverse opals was much higher than those reported by chemical synthesis method. By electrochemical method, the structures of the prepared inverse opals can be easily controlled. To explore the possible biosensing applications of PANI inverse opals, efforts were also done toward the fabrication of PANI composite inverse opals. By selecting proper dopants, high quality inverse opals of PANI composites were fabricated for the first time. And the obtained opaline films remained redox-active in neutral pH conditions, pointing to their possible applications for electrobioassays.

Synthesis and characterization of Poly(vinylphosphonic acid) for proton exchange membranes in fuel cells

Vinylphosphonic acid (VPA) was polymerized at 80 ºC by free radical polymerization to give polymers (PVPA) of different molecular weight depending on the initiator concentration. The highest molecular weight, Mw, achieved was 6.2 x 104 g/mol as determined by static light scattering. High resolution nuclear magnetic resonance (NMR) spectroscopy was used to gain microstructure information about the polymer chain. Information based on tetrad probabilities was utilized to deduce an almost atactic configuration. In addition, 13C-NMR gave evidence for the presence of head-head and tail-tail links. Refined analysis of the 1H NMR spectra allowed for the quantitative determination of the fraction of these links (23.5 percent of all links). Experimental evidence suggested that the polymerization proceeded via cyclopolymerization of the vinylphosphonic acid anhydride as an intermediate. Titration curves indicated that high molecular weight poly(vinylphosphonic acid) PVPA behaved as a monoprotic acid. Proton conductors with phosphonic acid moieties as protogenic groups are promising due to their high charge carrier concentration, thermal stability, and oxidation resistivity. Blends and copolymers of PVPA have already been reported, but PVPA has not been characterized sufficiently with respect to its polymer properties. Therefore, we also studied the proton conductivity behaviour of a well-characterized PVPA. PVPA is a conductor; however, the conductivity depends strongly on the water content of the material. The phosphonic acid functionality in the resulting polymer, PVPA, undergoes condensation leading to the formation of phosphonic anhydride groups at elevated temperature. Anhydride formation was found to be temperature dependent by solid state NMR. Anhydride formation affects the proton conductivity to a large extent because not only the number of charge carriers but also the mobility of the charge carriers seems to change.

Photovoltaics from discotic liquid crystalline HBCs & poly(2,7-carbazole)s

In this study, the use of the discotic liquid crystalline HBCs and conjugated polymers based on 2,7-carbazole were investigated in detail as donor materials in organic bulk-heterojunction solar cells. It has been shown that they perform efficiently in photovoltaic devices in combination with suitable acceptors. The efficiency was found to depend strongly dependent on the morphology of the film. By investigation of a series of donor materials with similar molecular structures based on both discotic molecules and conjugated polymers, a structure-performance relation was established, which is not only instructive for these materials but also serves as a guideline for improved molecular design. For the series of HBCs used in this study, it is found that the device efficiency decreases with increasing length of the alkyl substituents in the HBC. Thus, the derivative with the smallest alkyl mantle, being more crystalline compared to the HBCs with longer alkyl chains, gave the highest EQE of 12%. A large interfacial separation was found in the blend of HBC-C6,2 and PDI, since the crystallization of the acceptor occurred in a solid matrix of HBC. This led to small dispersed organized domains and benefited the charge transport. In contrast, blends of HBC-C10,6/PDI or HBC-C14,10/PDI revealed a rather homogeneous film limiting the percolation pathways due to a mixed phase. For the first time, poly(2,7-carbazole) was incorporated as a donor material in solar cells using PDI as an electron acceptor. The good fit in orbital energy levels and absorption spectra led to high efficiency. This result indicates that conjugated polymers with high band-gap can also be applied as materials to build efficient solar cells if appropriate electron acceptors are chosen. In order to enhance the light absorption ability, new ladder-type polymers based on pentaphenylene and hexaphenylene with one and three nitrogen bridges per repeat unit have been synthesized and characterized. The polymer 2 with three nitrogen bridges showed more red-shifted absorbance and emission and better packing in the solid-state than the analogous polymer 3 with only one nitrogen bridge per monomer unit. An overall efficiency as high as 1.3% under solar light was obtained for the device based on 1 and PDI, compared with 0.7% for the PCz based device. Therefore, the device performance correlates to a large extent with the solar light absorption ability and the lateral distance between conjugated polymer chains. Since the lateral distance is determined by the length and number of attached alkyl side chains, it is possible to assume that these substituents insulate the charge carrier pathways and decrease the device performance. As an additional consequence, the active semiconductor is diluted in the insulating matrix leading to a lower light absorption. This work suggests ways to improve device performance by molecular design, viz. maintaining the HOMO level while bathochromically shifting the absorption by adopting a more rigid ladder-type structure. Also, a high ratio of nitrogen bridges with small alkyl substituents was a desirable feature both in terms of adjusting the absorption and maintaining a low lateral inter-chain separation, which was necessary for obtaining high current and efficiency values.

Advanced Numerical Simulation of Silicon-Based Solar Cells

Photovoltaic (PV) conversion is the direct production of electrical energy from sun without involving the emission of polluting substances. In order to be competitive with other energy sources, cost of the PV technology must be reduced ensuring adequate conversion efficiencies. These goals have motivated the interest of researchers in investigating advanced designs of crystalline silicon solar (c-Si) cells. Since lowering the cost of PV devices involves the reduction of the volume of semiconductor, an effective light trapping strategy aimed at increasing the photon absorption is required. Modeling of solar cells by electro-optical numerical simulation is helpful to predict the performance of future generations devices exhibiting advanced light-trapping schemes and to provide new and more specific guidelines to industry. The approaches to optical simulation commonly adopted for c-Si solar cells may lead to inaccurate results in case of thin film and nano-stuctured solar cells. On the other hand, rigorous solvers of Maxwell equations are really cpu- and memory-intensive. Recently, in optical simulation of solar cells, the RCWA method has gained relevance, providing a good trade-off between accuracy and computational resources requirement. This thesis is a contribution to the numerical simulation of advanced silicon solar cells by means of a state-of-the-art numerical 2-D/3-D device simulator, that has been successfully applied to the simulation of selective emitter and the rear point contact solar cells, for which the multi-dimensionality of the transport model is required in order to properly account for all physical competing mechanisms. In the second part of the thesis, the optical problems is discussed. Two novel and computationally efficient RCWA implementations for 2-D simulation domains as well as a third RCWA for 3-D structures based on an eigenvalues calculation approach have been presented. The proposed simulators have been validated in terms of accuracy, numerical convergence, computation time and correctness of results.

Numerical simulation of interdigitated back contact hetero-junction solar cells

The goal of this thesis is the application of an opto-electronic numerical simulation to heterojunction silicon solar cells featuring an all back contact architecture (Interdigitated Back Contact Hetero-Junction IBC-HJ). The studied structure exhibits both metal contacts, emitter and base, at the back surface of the cell with the objective to reduce the optical losses due to the shadowing by front contact of conventional photovoltaic devices. Overall, IBC-HJ are promising low-cost alternatives to monocrystalline wafer-based solar cells featuring front and back contact schemes, in fact, for IBC-HJ the high concentration doping diffusions are replaced by low-temperature deposition processes of thin amorphous silicon layers. Furthermore, another advantage of IBC solar cells with reference to conventional architectures is the possibility to enable a low-cost assembling of photovoltaic modules, being all contacts on the same side. A preliminary extensive literature survey has been helpful to highlight the specific critical aspects of IBC-HJ solar cells as well as the state-of-the-art of their modeling, processing and performance of practical devices. In order to perform the analysis of IBC-HJ devices, a two-dimensional (2-D) numerical simulation flow has been set up. A commercial device simulator based on finite-difference method to solve numerically the whole set of equations governing the electrical transport in semiconductor materials (Sentuarus Device by Synopsys) has been adopted. The first activity carried out during this work has been the definition of a 2-D geometry corresponding to the simulation domain and the specification of the electrical and optical properties of materials. In order to calculate the main figures of merit of the investigated solar cells, the spatially resolved photon absorption rate map has been calculated by means of an optical simulator. Optical simulations have been performed by using two different methods depending upon the geometrical features of the front interface of the solar cell: the transfer matrix method (TMM) and the raytracing (RT). The first method allows to model light prop-agation by plane waves within one-dimensional spatial domains under the assumption of devices exhibiting stacks of parallel layers with planar interfaces. In addition, TMM is suitable for the simulation of thin multi-layer anti reflection coating layers for the reduction of the amount of reflected light at the front interface. Raytracing is required for three-dimensional optical simulations of upright pyramidal textured surfaces which are widely adopted to significantly reduce the reflection at the front surface. The optical generation profiles are interpolated onto the electrical grid adopted by the device simulator which solves the carriers transport equations coupled with Poisson and continuity equations in a self-consistent way. The main figures of merit are calculated by means of a postprocessing of the output data from device simulation. After the validation of the simulation methodology by means of comparison of the simulation result with literature data, the ultimate efficiency of the IBC-HJ architecture has been calculated. By accounting for all optical losses, IBC-HJ solar cells result in a theoretical maximum efficiency above 23.5% (without texturing at front interface) higher than that of both standard homojunction crystalline silicon (Homogeneous Emitter HE) and front contact heterojuction (Heterojunction with Intrinsic Thin layer HIT) solar cells. However it is clear that the criticalities of this structure are mainly due to the defects density and to the poor carriers transport mobility in the amorphous silicon layers. Lastly, the influence of the most critical geometrical and physical parameters on the main figures of merit have been investigated by applying the numerical simulation tool set-up during the first part of the present thesis. Simulations have highlighted that carrier mobility and defects level in amorphous silicon may lead to a potentially significant reduction of the conversion efficiency.

Anhydrous proton conducting polymer electrolytes based on polymeric ionic liquids

Imidazolium types of ionic liquids were immobilized by tethering it to acrylate backbone. These imidazolium salt containing acrylate monomers were polymerize at 70oC by free radical polymerization to give polymers poly(AcIm-n) with n being the side chain lenght. The chemical structure of the polymer electrolytes obtained by the described synthetic routes was investigated by NMR-spectroscopy. The polymers were doped with various amounts of H3PO4 and LiN(SO2CF3)2, to obtain poly(AcIm-n) x H3PO4 and poly(AcIm-2-Li) x LiN(SO2CF3)2. The TG curves show that the polymer electrolytes are thermally stable up to about 200◦C. DSC results indicates the softening effect of the length of the spacers (n) as well as phosphoric acid. The proton conductivity of the samples increase with x and reaches to 10-2 Scm-1 at 120oC for both poly(AcIm-2)2H3PO4 and poly(AcIm-6)2H3PO4. It was observed that the lithium ion conductivity of the poly(AcIm-2-Li) x LiN(SO2CF3)2 increases with blends (x) up to certain composition and then leveled off independently from blend content. The conductivity reaches to about 10-5 S cm-1 at 30oC and 10-3 at 100oC for poly(AcIm-2-Li) x LiN(SO2CF3)2 where x is 10. The phosphate and phosphoric acid functionality in the resulting polymers, poly(AcIm-n) x H3PO4, undergoes condensation leading to the formation of cross-linked materials at elevated temperature which may improve the mechanical properties to be used as membrane materials in fuel cells. High resolution nuclear magnetic resonance (NMR) spectroscopy was used to obtain information about hydrogen bonding in solids. The low Tg enhances molecular mobility and this leads to better resolved resonances in both the backbone region and side chain region. The mobile and immobile protons can be distinguished by comparing 1H MAS and 1H-DQF NMR spectra. The interaction of the protons which may contribute to the conductivity is observed from the 2D double quantum correlation (DQC) spectra.

Self assembled materials for solar cell application

In der vorliegenden Arbeit wurden Materialien und Aufbauten für Hybrid Solarzellen entwickelt und erforscht. rnDer Vergleich zweier bekannter Lochleitermaterialien für Solarzellen in einfachen Blend-Systemen brachte sowohl Einsicht zur unterschiedlichen Eignung der Materialien für optoelektronische Bauelemente als auch neue Erkenntnisse in Bereichen der Langzeitstabilität und Luftempfindlichkeit beider Materialien.rnWeiterhin wurde eine Methode entwickelt, um Hybrid Solarzelle auf möglichst unkomplizierte Weise aus kostengünstigen Materialien darzustellen. Die „Eintopf“-Synthese ermöglicht die unkomplizierte Darstellung eines funktionalen Hybridmaterials für die optoelektronische Anwendung. Mithilfe eines neu entwickelten amphiphilen Blockcopolymers, das als funktionelles Templat eingesetzt wurde, konnten mit einem TiO2-Precursor in einem Sol-Gel Ansatz verschiedene selbstorganisierte Morphologien des Hybridmaterials erhalten werden. Verschiedene Morphologien wurden auf ihre Eignung in Hybrid Solarzellen untersucht. Ob und warum die Morphologie des Hybridsystems die Effizienz der Solarzelle beeinflusst, konnte verdeutlicht werden. Mit der Weiterentwicklung der „Eintopf“-Synthese, durch den Austausch des TiO2-Precursors, konnte die Solarzelleneffizienz von 0.15 auf 0.4 % gesteigert werden. Weiterhin konnte die Übertragbarkeit des Systems durch den erfolgreichen Austausch des Halbleiters TiO¬2 mit ZnO bewiesen werden.rn

Die Vielseitigkeit von Carbazol in pi-konjugierten Materialien: Makrozyklen, Doppelstrang-Strukturen und Donor-Akzeptor-Copolymere

In dieser Arbeit wurden die vielfältigen Anwendungsmöglichkeiten von Carbazol-reichen Verbindungen in unterschiedlichen Architekturen von pi-konjugierten Materialien untersucht. Die Darstellung der Makrozyklen in Kapitel 1 erfolgte über eine templatunterstützte Synthese, da die ausschließlich über ihre para-Position verknüpften Einheiten einen Zugang über ihre Geometrie nicht erlaubten. Cyclododeca-2,7-fluoren wurde als zyklische Modellverbindung für die zahlreichen bekannten linearen Oligo- und Polyfluorene dargestellt, um den Einfluss der Ringstruktur auf die optischen Eigenschaften dieser blau emittierenden Verbindungen zu untersuchen. Motiviert von den geringen Ausbeuten bei der Zyklisierung des Rings unter Yamamoto-Bedingungen wurde nach einer alternativen Kupplungsreaktion gesucht, die den Zugang zu größeren Substanzmengen erlaubt. Die Einführung von terminalen Ethinylgruppen an die zu verknüpfenden Bausteine erlaubte über die Glaser-Eglinton-Reaktion nicht nur eine Verbesserung der Reaktionsausbeute um den Faktor 10, sondern gleichzeitig auch die Erweiterung des Durchmessers auf rund 5 nm. Demonstriert wurde dies anhand der Darstellung von Cyclo[tetrakis(2,7-diethinyltriscarbazol)] und Cyclo[tetrakis(2,7-diethinyltrisfluoren)].rnDer zweite Abschnitt der Dissertation befasste sich mit ausgedehnten zweidimensionalen Strukturen, zu denen auch mehrsträngige Polymere zählen. Für die Darstellung eines Doppelstrang-Polymers mit hoher Periodizität in den beiden in regelmäßigen Abständen verknüpften Strängen wurde ein neues Konzept entwickelt, dass den Zugang zu solchen Verbindungen erlauben sollte. Dieses basierte auf einem vierfach funktionalisierten Carbazol-Makromonomer. Die Polymerisationen wurden in zwei Schritten über Glaser-Eglinton-Reaktionen durchgeführt. Um unkontrollierte Kupplungen zu vermeiden, wurden zwei der vier reaktiven Positionen am Monomer zunächst über Schutzgruppen maskiert und nach der ersten Polymerisation wieder aktiviert. Neben diesem Ansatz über oxidative Ethinylkupplungen wurde auch eine Organometall-Variante mit einem Platin-Komplex im Polymerrückgrat vorgestellt. rnIm dritten Kapitel wurde die Darstellung von Donor-Akzeptor-Polymeren mit niedriger Bandlücke für den Einsatz in organischen Solarzellen besprochen. Im Gegensatz zu den bisherigen Ansätzen in der Literatur wurden hier ganz gezielt Donor- und Akzeptor-Einheiten mit einer Länge von mindestens 1,5 nm synthetisiert und kombiniert, um zu untersuchen, welche Auswirkungen der Einsatz solcher Makromonomere auf die Eigenschaften der Materialien hat. Dabei bestand der Akzeptor-Baustein in drei Polymeren aus Bis[2,7-di-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9,18,18-tetraoctylindeno[1,2-b]-yl]-indeno[1,2-b]fluoren-12,15-dion, während die Donor-Stärke durch den Übergang von einem reinen leiterverbrückten Carbazol-Monomer über eine gemischte Carbazol-Thiophen-Einheit hin zu einem reinen Thiophen-Precursor variiert wurde. Dabei zeigte das Polymer mit Thiophen als Donor den höchsten Zellwirkungsgrad von 0,41 %.rn

Polimeri tiofenici funzionali e loro applicazioni nel fotovoltaico

Recentemente, sempre più attenzione è stata rivolta all' utilizzo di coloranti organici come assorbitori di luce per la preparazione di strati fotoattivi in celle solari organiche (OPV). I coloranti organici presentano un'elevata abilità nella cattura della luce solare grazie all'elevato coefficiente di estinzione molare e buone proprietà fotofisiche. Per questi motivi sono eccellenti candidati per l'incremento della conversione fotoelettrica in OPV. In questa tesi viene descritta una nuova strategia per l'incorporazione di derivati porfirinici in catena laterale a copolimeri tiofenici. Gli studi svolti hanno dimostrato che poli(3-bromoesil)tiofene può essere variamente funzionalizzato con idrossitetrafenilporfirina (TPPOH), per l'ottenimento di copolimeri utilizzabili come materiali p-donatori nella realizzazione di OPV. I copolimeri poli[3-(6-bromoesil)tiofene-co-(3-[5-(4-fenossi)-10,15,20-trifenilporfirinil]esil tiofene] P[T6Br-co-T6TPP] contenenti differenti quantità di porfirina, sono stati sintetizzati sia con metodi non regiospecifici che regiospecifici, con lo scopo di confrontarene le proprietà e di verificare se la strutture macromolecolare che presenta una regiochimica di sostituzione sempre uguale, promuove o meno il trasporto della carica elettrica, migliorando di conseguenza l'efficienza. E' stato inoltre effettuato un ulteriore confronto tra questi derivati e derivati simili P[T6H-co-T6TPP] che non contengono l'atomo di bromo in catena laterale con lo scopo di verificare se l'assenza del gruppo reattivo, migliora o meno la stabilità termica e chimica dei film polimerici, agendo favorevolmete sulle performance dei dispositivi fotovoltaici. Tutti i copolimeri sono stati caratterizzati con differenti tecniche: spettroscopia NMR, FT-IR e UV-Vis, analisi termiche DSC e TGA, e GPC. Le celle solari Bulk Heterojunction, preparate utilizzando PCBM come materiale elettron-accettore e i copolimeri come materilai elettron-donatori, sono state testate utilizzando un multimetro Keithley e il Solar Simulator.

Charge transport properties of organic conjugated polymers for photovoltaic applications

Charge transport in conjugated polymers as well as in bulk-heterojunction (BHJ) solar cells made of blends between conjugated polymers, as electron-donors (D), and fullerenes, as electron-acceptors (A), has been investigated. It is shown how charge carrier mobility of a series of anthracene-containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene)s (AnE-PVs) is highly dependent on the lateral chain of the polymers, on a moderate variation of the macromolecular parameters (molecular weight and polydispersity), and on the processing conditions of the films. For the first time, the good ambipolar transport properties of this relevant class of conjugated polymers have been demonstrated, consistent with the high delocalization of both the frontier molecular orbitals. Charge transport is one of the key parameters in the operation of BHJ solar cells and depends both on charge carrier mobility in pristine materials and on the nanoscale morphology of the D/A blend, as proved by the results here reported. A straight correlation between hole mobility in pristine AnE-PVs and the fill factor of the related solar cells has been found. The great impact of charge transport for the performance of BHJ solar cells is clearly demonstrated by the results obtained on BHJ solar cells made of neat-C70, instead of the common soluble fullerene derivatives (PCBM or PC70BM). The investigation of neat-C70 solar cells was motivated by the extremely low cost of non-functionalized fullerenes, compared with that of their soluble derivatives (about one-tenth). For these cells, an improper morphology of the blend leads to a deterioration of charge carrier mobility, which, in turn, increases charge carrier recombination. Thanks to the appropriate choice of the donor component, solar cells made of neat-C70 exhibiting an efficiency of 4.22% have been realized, with an efficiency loss of just 12% with respect to the counterpart made with costly PC70BM.

Optimising thin-film solar cells by computer simulations

CIGS-Dünnschichtsolarzellen verbinden hohe Effizienz mit niedrigen Kosten und sind damit eine aussichtsreiche Photovoltaik-Technologie. Das Verständnis des Absorbermaterials CIGS ist allerdings noch lückenhaft und benötigt weitere Forschung. In dieser Dissertation werden Computersimulationen vorgestellt, die erheblich zum besseren Verständnis von CIGS beitragen. Es wurden die beiden Systeme Cu(In,Ga)Se2 und (Cu,In,Vac)Se betrachtet. Die Gesamtenergie der Systeme wurde in Clusterentwicklungen ausgedrückt, die auf der Basis von ab initio Dichtefunktionalrechnungen erstellt wurden. Damit war es möglich Monte Carlo (MC)-Simulationen durchzuführen. Kanonische MC-Simulationen von Cu(In,Ga)Se2 zeigen das temperaturabhängige Verhalten der In-Ga-Verteilung. In der Nähe der Raumtemperatur findet ein Übergang von einer geordneten zu einer ungeordneten Phase statt. Unterhalb separiert das System in CuInSe2 und CuGaSe2. Oberhalb existiert eine gemischte Phase mit inhomogen verteilten In- und Ga-Clustern. Mit steigender Temperatur verkleinern sich die Cluster und die Homogenität nimmt zu. Bei allen Temperaturen, bis hin zur Produktionstemperatur der Solarzellen (¼ 870 K), ist In-reiches CIGS homogener als Ga-reiches CIGS. Das (Cu,In,Vac)Se-System wurde mit kanonischen und großkanonischen MC-Simulationen untersucht. Hier findet sich für das CuIn5Se8-Teilsystem ein Übergang von einer geordneten zu einer ungeordneten Phase bei T0 = 279 K. Großkanonische Simulationen mit vorgegebenen Werten für die chemischen Potentiale von Cu und In wurden verwendet, um die Konzentrations- Landschaft und damit die sich ergebenden Stöchiometrien zu bestimmen. Stabilitätsbereiche wurden für stöchiometrisches CuInSe2 und für die Defektphasen CuIn5Se8 und CuIn3Se5 bei einer Temperatur von 174 K identifiziert. Die Bereiche für die Defektphasen sind bei T = 696 K verschwunden. Die Konzentrations-Landschaft reproduziert auch die leicht Cu-armen Stöchiometrien, die bei Solarzellen mit guten Effizienzen experimentell beobachtet werden. Die Simulationsergebnisse können verwendet werden, um den industriellen CIGS-Produktionspr