Synthesis and application of pi-conjugated polymers for organic solar cells

Organic photovoltaic cells (OPCs) hold the promise of being a cheap and environmentally benign large scale renewable energy resource. The photoactive layer in OPCs is based on a blend, or bulk heterojunction, of an electron donor (p-type) and an electron acceptor (n-type) material. The operational principle involves the absorption of light to create an exciton that diffuses to the p-n interface and is split into charges that are subsequently transported to the electrodes and collected. All of these processes have to occur with high quantum efficiencies and minimal energy losses to make a high efficiency solar cell. In this thesis new materials have been developed by means of synthetic chemistry to improve the efficiency of existing materials in OPCs by absorbing a larger part of the solar spectrum and minimizing energy losses in the conversion process. The morphologies of the new donor-acceptor bulk heterojunction layers have been optimized for high efficiency by influencing the degree of mixing of the two materials. In designing new small band gap polymers for bulk heterojunctions that can absorb and convert a large part of the solar spectrum various aspects need to be considered. The frontier energy levels of the new polymers must be designed to minimize energy losses by increasing the open-circuit voltage with respect to the optical band gap, while maintaining a high coverage of the absorption with the solar spectrum. Another balance is the amount of interface between the two materials that should be large to create free charge carriers, while maintaining percolating domains of pure material to transfer these charges to the electrodes. To prevent recombination during the transport, free charge carrier mobility in the two materials should be high. In a first approach, alternating copolymers based on cyclopentadithiophene (CPDT) and five different electron deficient aromatic units with reduced optical band gaps have been synthesized via palladium catalyzed coupling. All polymers showed a significant photovoltaic response when mixed with a fullerene (PCBM) as acceptor. The best cells have been obtained for a copolymer of CPDT and benzooxadiazole with a band gap of 1.5 eV. This cell gives a power conversion efficiency (PCE) of about 2.5%. Next, a series of polythiophenes (PTn) based on CPDT units alternating with short oligothiophenes of different length n along the chain has been synthesized to control the morphology of PTn:PCBM blends via the chemical structure of the polymer, rather than via processing conditions. The degree of phase separation in PTn:PCBM blends can be controlled via n, because with increasing n the number of solubilizing side chains per thiophene is reduced. The best cells and most intimately mixed morphology were obtained for PT2 that exhibits a PCE of about 1.5% when mixed with PCBM. Although the final efficiency is moderate, the study represents an example of a rational approach towards morphology control via chemical structure. To maximize the open-circuit voltage of polymer:PCBM cells, a new polymer has been synthesized based on benzothiadiazole and a substituted thienothiophene, to create a material with a relatively deep lying level of the highest occupied molecular orbital. To obtain working and reproducible devices, the poly(ethylenedioxythiophene):poly(styrenesulfonate) hole conducting layer was treated with UV-ozone. This treatment increased the work function of the electrode, facilitating an Ohmic contact with the polymer in the active layer. The open-circuit voltage of 1.15 V corresponds to the highest value obtained for any polymer:PCBM cell to date and matches with the previously predicted ultimate limit for PCBM-based OPCs. In a new approach a semiconducting polymer with alternating diketopyrrolopyrrole (DPP) and terthiophene units has been developed with a small band gap of 1.3 eV. This polymer exhibits high, nearly balanced hole and electron mobilities of 0.04 cm2 V-1 s-1 and 0.01 cm2 V-1 s-1, respectively in field-effect transistors (FETs). When the polymer was combined with [60]PCBM or [70]PCBM, photovoltaic cells were made that provide a photoresponse up to 900 nm and a PCE of 3.8 and 4.7% in sunlight, respectively. The efficiency of the photovoltaic cells was found to be strongly dependent on the molecular weight of the polymer and the use of processing agents during film formation. In a further development, a new easily accessible, alternating DPP and dithienylphenylene co-polymer has been developed, again with high electron and hole mobilities, exceeding 0.01 cm2 V-1 s-1. PCEs of 4.6 and 5.5% were obtained with [60]PCBM and [70]PCBM. The performance of these cells strongly depends on the use of a processing agent, 1,8-diiodooctane (DIO), during film formation. The active layers have been studied with atomic force microscopy and transmission electron microscopy and show vast variations in size of the PCBM clusters upon application of DIO, together with appearance of fiber-like structures. Photoinduced absorption measurements support the generation of more charges in the optimized morphology. The use of thienothiophene (TT) as a co-monomer was further explored by alternation with DPP units in an attempt to combine the successful strategies for improved performance described above. Two polymers, with substituted and unsubstituted thienothiophenes were prepared. No significant difference was observed in charge carrier mobility, but the photovoltaic behavior was better for the polymer with more and shorter side chains (2.3% vs. 1.2%). A further co-polymer with extra thiophene units between TT and DPP in the chain was also synthesized. This polymer has a lower charge carrier mobility in an FET, but outperformed the other polymers in an OPC, with the best cell exhibiting a PCE of 3.4%. One reason for the overall lower PCE in the TT-DPP copolymers is the moderate molecular weight of the materials obtained. In further exploring the electron deficient DPP structural motif for photovoltaic polymers, furan rings were considered as a building block in the main chain. Four small band gap copolymers based on DPP alternating with electron rich trimers of benzene, furan, and thiophene have been synthesized via Suzuki polymerization. The polymers show optical band gaps between 1.4 and 1.6 eV, optimized for solar energy conversion, and exhibit ambipolar charge transport in field-effect transistors with hole and electron mobilities higher than 10-2 cm2 V-1 s-1. In solar cells the polymers are used as electron donor and provide power conversion efficiencies up to of 3.7% in simulated solar light when mixed with [70]PCBM as acceptor. Again the reduced molecular weight of the new materials is possibly limiting the PCE compared to higher efficiencies reached earlier. With the development of DPP type polymers a new, highly successful, class of promising materials for OPCs has been created. The final efficiencies obtained, compare favorably to the state of the art in the field. Several of the materials made have close to optimal energy levels for very efficient OPCs. Hence further improvements can be expected when improved control over molecular weight, morphology, and charge carrier mobility can be obtained

Functionally graded ferroelectric polyetherimide composites for high temperature sensing

High temperature ferroelectrics for thermally stable devices that can detect pressure and temperature are of great industrial interest. Here we describe composites of lead titanate (PT) particle-polyetherimide (PEI) polymers with stable dielectric and piezoelectric properties over a broad range of temperature and frequency. The reported materials have a low dielectric loss (tanδ ∼ 0.001 at 1 kHz) and a high piezoelectric voltage coefficient of 100 mV m N-1 at record temperatures of 175 °C. We demonstrate that a small ceramic loading leads to a significant change in thermally stable piezoelectric behavior, while the processability as well as mechanical properties remain comparable to those of the neat polymer. Careful design of the microstructure is performed by dielectrophoretic assembly of ferroelectric PT micro-particles to induce micro-wire configurations, which is shown to be a key element in attaining high functionality at low ceramic loading. Thermal imidization of the composites is performed in two steps, first partial imidization at 60 °C to form free standing films containing polyamic acid, followed by full imidization at 200 °C and 300 °C. The presence of highly polar polyamic acid results in higher dielectric permittivity and electrical conductivity that facilitate efficient poling. Upon complete imidization of the films at 300 °C the dielectric and piezoelectric properties are tested at elevated temperatures. A fully imidized composite contains completely closed imide groups, resulting in a thermally stable material with a very low dielectric loss that maintains more than 85% of its room temperature piezoelectric sensitivity up to 175 °C. The room temperature piezoelectric voltage coefficient shows more than 400% improvement over that of PT ceramics

Large expert-curated database for benchmarking document similarity detection in biomedical literature search

Document recommendation systems for locating relevant literature have mostly relied on methods developed a decade ago. This is largely due to the lack of a large offline gold-standard benchmark of relevant documents that cover a variety of research fields such that newly developed literature search techniques can be compared, improved and translated into practice. To overcome this bottleneck, we have established the RElevant LIterature SearcH consortium consisting of more than 1500 scientists from 84 countries, who have collectively annotated the relevance of over 180 000 PubMed-listed articles with regard to their respective seed (input) article/s. The majority of annotations were contributed by highly experienced, original authors of the seed articles. The collected data cover 76% of all unique PubMed Medical Subject Headings descriptors. No systematic biases were observed across different experience levels, research fields or time spent on annotations. More importantly, annotations of the same document pairs contributed by different scientists were highly concordant. We further show that the three representative baseline methods used to generate recommended articles for evaluation (Okapi Best Matching 25, Term Frequency-Inverse Document Frequency and PubMed Related Articles) had similar overall performances. Additionally, we found that these methods each tend to produce distinct collections of recommended articles, suggesting that a hybrid method may be required to completely capture all relevant articles. The established database server located at https://relishdb.ict.griffith.edu.au is freely available for the downloading of annotation data and the blind testing of new methods. We expect that this benchmark will be useful for stimulating the development of new powerful techniques for title and title/abstract-based search engines for relevant articles in biomedical research.Peer reviewe

Re-structuring of a Dutch mono-industrial region; example of Twente: Hoofdstuk uit: The social and economic problems of monotowns

Re-structuring of a Dutch mono-industrial region; example of Twente Table of contents of the chapter Introduction Geography and location of Twente Industrialization of Twente and development of the Textile Industry Decline of the Textile Industry Restructuring Twente: arguments for a regional innovation strategy Moving towards a more diversified economy Stronger co-operation between governments, universities, and industries The role of universities and the example of ‘Kennispark Twente’ Further regional and international co-operation Twente toda

Diketopyrrolopyrroles as acceptor materials in organic photovoltaics

In the search of new electron acceptor, n-type materials for organic solar cells that combine a strong absorption over a broad range with good electrical characteristics, the use of diketopyrrolopyrrole (DPP) derivatives with low reduction potentials is explored. A series of small molecule DPP derivatives is presented and the compounds are tested as electron acceptors in combination with poly(3-hexylthiophene) (P3HT) as the donor material. Working photovoltaic devices are obtained that show a photoresponse in the wavelength region where the DPP molecules absorb. The best device shows a power conversion efficiency of 0.31% in simulated solar light, with a photon-to-electron conversion efficiency of ~10% up to 700¿nm. The efficiency seems to be limited by the coarse morphology of the blend

Diketopyrrolopyrroles as acceptor materials in organic photovoltaics

In the search of new electron acceptor, n-type materials for organic solar cells that combine a strong absorption over a broad range with good electrical characteristics, the use of diketopyrrolopyrrole (DPP) derivatives with low reduction potentials is explored. A series of small molecule DPP derivatives is presented and the compounds are tested as electron acceptors in combination with poly(3-hexylthiophene) (P3HT) as the donor material. Working photovoltaic devices are obtained that show a photoresponse in the wavelength region where the DPP molecules absorb. The best device shows a power conversion efficiency of 0.31% in simulated solar light, with a photon-to-electron conversion efficiency of ~10% up to 700¿nm. The efficiency seems to be limited by the coarse morphology of the blend

Functionally graded ferroelectric polyetherimide composites for high temperature sensing

High temperature ferroelectrics for thermally stable devices that can detect pressure and temperature are of great industrial interest. Here we describe composites of lead titanate (PT) particle-polyetherimide (PEI) polymers with stable dielectric and piezoelectric properties over a broad range of temperature and frequency. The reported materials have a low dielectric loss (tan δ ∼ 0.001 at 1 kHz) and a high piezoelectric voltage coefficient of 100 mV m N−1 at record temperatures of 175 °C. We demonstrate that a small ceramic loading leads to a significant change in thermally stable piezoelectric behavior, while the processability as well as mechanical properties remain comparable to those of the neat polymer. Careful design of the microstructure is performed by dielectrophoretic assembly of ferroelectric PT micro-particles to induce micro-wire configurations, which is shown to be a key element in attaining high functionality at low ceramic loading. Thermal imidization of the composites is performed in two steps, first partial imidization at 60 °C to form free standing films containing polyamic acid, followed by full imidization at 200 °C and 300 °C. The presence of highly polar polyamic acid results in higher dielectric permittivity and electrical conductivity that facilitate efficient poling. Upon complete imidization of the films at 300 °C the dielectric and piezoelectric properties are tested at elevated temperatures. A fully imidized composite contains completely closed imide groups, resulting in a thermally stable material with a very low dielectric loss that maintains more than 85% of its room temperature piezoelectric sensitivity up to 175 °C. The room temperature piezoelectric voltage coefficient shows more than 400% improvement over that of PT ceramics.<br/

Maximizing the open-circuit voltage of polymer : fullerene solar cells

The open-circuit voltage (Voc) of bulk heterojunction solar cells based on polymers and fullerene derivatives is limited to ~ 1.15 V by the optical band gap of the fullerene of ~ 1.75 eV and the required 0.6 eV offset for efficient charge generation. In practice this limit has not yet been reached. We present a semiconducting polymer that gives Voc = 1.15 V. To reach this value the surface of the hole collecting electrode is modified by UV-ozone, which increases the work function and creates an Ohmic contact. Under simulated AM1.5 conditions optimized cells provide a power conversion efficiency of - 1%

Copolymers of diketopyrrolopyrrole and thienothiophene for photovoltaic cells

Three copolymers of thieno[3,2-b]thiophene (TT) and diketopyrrolo[3,4-c]pyrrole (DPP) with a varying number of interconnecting thiophene units (nT) were synthesized by Suzuki polymerization. The PDPPnT-TT polymers display optical band gaps in the range of 1.33 to 1.52 eV and show ambipolar charge transport when applied in a field-effect transistor (FET) with hole and electron mobilities up to 3 × 10-2 and 2 × 10-3 cm2 V-1 s-1, respectively. Organic solar cells based on the PDPPnT-TT polymers as donors and the fullerene derivative [60]PCBM as acceptor achieve power conversion efficiencies up to 3.4% when the layer is deposited with the use of a co-solvent. The co-solvent strongly affects the morphology of the active layer and improves the performance. For these polymers the LUMO–LUMO offset with [60]PCBM correlates with the photocurrent and the maximum external quantum efficiency

Controlling morphology and photovoltaic properties by chemical structure in copolymers of cyclopentadithiophene and thiophene segments

A series of polythiophenes (PTn) based on dialkyl-cyclopentadithiophene (CPDT) units alternating with short un-substituted thiophene segments of length n (n being the number of thiophenes) along the chain has been synthesized to control the morphology of PTn:PCBM blends for solar cell applications via the chemical structure of the polymer rather than via (post) processing conditions. The degree of phase separation in PTn:PCBM blends can be controlled via n, because with increase in n the number of solublizing side chains per thiophene is reduced. For the most soluble derivative, PT0, we find that PCBM crystallizes first, while for the least soluble version, PT3, polymer aggregation prevails. The most intimately mixed morphology and best solar cells were obtained for PT2, which exhibits a power conversion efficiency (PCE) of about 1.5% under AM1.5G conditions when mixed with PCBM. Although the final PCE is moderate, the study represents an example of a rational approach towards morphology control via chemical structure, rather than via processin