Amorphous electron-accepting materials for organic optoelectronics

The importance of organic materials for use in electronic devices such as OLEDs, OFETs and photovoltaic cells has increased significantly over the past decade. Organic materials have been attractive candidates for such electronic devices because of their compatibility with high-throughput, low-cost processing techniques and their capability to be precisely functionalized to afford desired performance attributes. This has already lead to commercial applications of OLED in car-audio, cell phones and digital cameras. To further improve the performance of these materials, many research groups are focusing on controlling the morphology of the organic films by carefully fine tuning the degree of crystallinity. Synthetic organic chemistry plays a pivotal role in this, as the toolbox of organic chemistry allows the formation of tailor-made materials that form a uniform film in their solid state.Organic photovoltaics is a rapidly growing field since the exponential growth of energy needs and the rapid depletion of the fossil fuels have led to a compelling demand for alternative sources of energy. The traditional inorganic solar cells are based on silicon. Although energy efficiencies around 25 % have been reached in silicon-based solar cells, for many climatological conditions they are not cost effective since the production of such photovoltaic device requires demanding conditions like high processing temperature, clean room facilities, etc., which prevents the commercially attractive bulk production. Therefore, a significant research efforts are focused on easily processable organic materials for use in photovoltaic devices. Usually the organic photovoltaic devices consist of an electron donating polymer (p-type material) and a fullerene-based electron-accepting material (n-type material). For such devices energy efficiencies up to 5.2 % have been reported. A significant issue in these devices is the crystallization of fullerenes, which easily leads to excessive phase separation p and n-type materials.Amorphous molecular materials may exhibit isotropic properties due to the absence of grain boundaries. Naphthalene diimides (NDI) are known to have a high conductivity and electron-accepting capability from a variety of electron donors. This thesis aims at making use of the properties of novel amorphous materials with NDIs to obtain uniform films without phase separation and crystallization for use in organic solar cells.Chapter 1 gives an overview of the organic (opto-)electronic materials and of the working principles of several devices that are based on such materials. Solar cells, in particular organic heterojunction cells, are described in detail. The importance of the nanoscale morphology in such heterojunction devices is discussed, together with expected advantages of amorphous materials to obtain films with the desired morphology. Finally the outline of the thesis is given.A new approach towards the design and synthesis of amorphous n-type materials with NDIs is presented in Chapter 2. The tetrahedral shape of the molecule yields the amorphous material properties, which are decoupled from its optoelectronic properties. In the first tetrahedral molecule the non-directionality available via tetrahedral cores, as present in tetra(phenyl) methane, is used. This tetrahedral material with 4 NDIs has been characterized for its steady-state and transient optical behavior and for its ground-state electrochemical properties. It has been shown to display a conductivity of 0.03 cm2 V^s"1 in neat film, and exhibited a near-complete quenching of the p-type. polymer fluorescence. The blended films of this tetrahedral molecule with polymeric p-type materials have a very uniform morphology and demonstrated high transient charge carrier mobility.The photophysical properties of the tetrahedral molecule with naphthalene diimide (NDI) moieties and of two model compounds are described in Chapter 3. One of the model compound is a symmetrically dialkyl substituted NDI and the other model compound is an NDI with an alkyl chain and a phenyl ring substitution. The steady-state absorption and fluorescence spectra of dialkyl-substituted NDI are in agreement with literature. While the absorption spectra of the phenyl-substituted molecules are similar to all other NDIs, their fluorescence showed a broad band between 500-650 nm. This band is sensitive to the polarity of the solvent, and is attributed to a charge-transfer (CT) state. The absorption spectra and lifetime (10 ± 1 ps) of the electronically excited singlet state of a dialkyl-substituted NDI was determined by femtosecond transient absorption spectroscopy, and the latter was confirmed by picosecond fluorescence spectroscopy. Nanosecond flash photolysis showed the subsequent formation of the triplet state. The presence of a phenyl substituent on the imide nitrogen of NDI resulted in faster deactivation of the singlet state (lifetime 0.5- 1 ps). This is attributed to the formation of a short-lived CT state, which decays to the local triplet state. The faster deactivation was confirmed by fluorescence-lifetime measurements in solution and in a low-temperature methyl-tetrahydrofuran (MTHF) glass.Another new class of amorphous materials with NDIs is described in Chapter 4. Cyclic siloxanes are known to exhibit a size-dependent structure with amorphous properties. Novel cyclic siloxanes with pendent naphthalene diimides were synthesized via a hydrosilylation reaction, to form amorphous electron-accepting materials. These materials were studied for their basic photophysical properties using steady state and time-resolved techniques. The fluorescence spectra revealed the formation of excimers, which was shown to be solvent dependent. Fluorescence quenching studies of blends of these siloxanes with p-type polymers (P3HT, MDMO-PPV) showed>99.9 % fluorescence quenching of the latter polymers. Mixtures of these siloxanes and p-type polymers gave homogeneous amorphous films from chloroform solution, and films with micro-crystallinity were obtained from o-dichlorobenzene solutions. The time-resolved microwave conductance in films formed from o-dichlorobenzene was higher than in films formed from chloroform, which is attributed to nanoscopic phase separation that enhances the interfacial charge separation. Due to this reason, they also showed a better conductivity than the tetrahedral molecule.For a good charge transport in the active organic heterojunction films, it is necessary to have a bicontinuous film with nanoscale phase separation. For this reason it is essential that the NDIs are interacting with each other. In order to achieve this, four novel naphthalenediimide (NDI) side-chain polymers were synthesized by grafting NDI onto poly(R-alt-maleic anhydride) backbone polymers with various R groups and molecular weights [R= styrene, 1-octene and 1-octadecene]. The synthesis and other characterizations of these materials are described in Chapter 5. These polymers were obtained with a degree of substitution up to 60 %, and showed a high solubility in solvents like chloroform. Their absorption and fluorescence spectra were studied both in solution and in thin films, with specific attention to the fluorescence quenching of P3HT in thin films. The results show that in all four polymers the NDI chromophores form n-stacked dimers in solution exhibiting excimer fluorescence. The morphology of the blends of the grafted polymers with P3HT was studied at various weight ratios, and revealed phase separation into domains of um dimensions. These blends were also studied using time-resolved microwave-conductivity for their photo-induced charge carrier generation efficiency, which showed appreciable generation of charge carriers, although significantly lower than observed in blends of P3HT with PCBM or oligomeric n-type siloxanes described in the previous chapter.Overall it could be summari2ed that the formation of amorphous films with structural elements based on a tetrahedral organization or flexible siloxane rings provide a novel way to construct materials that can be used as p-type or n-type materials in optoelectronic devices. Use of these elements with appropriate aromatic systems containing more extended xc-systems seems a viable route to further improve the potential of organic optoelectronic materials.