1,721,014 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Wide-bandgap Donor Polymers Based on Thiophene-Vinyl-Thiophene Ester (TVT-Ester) or Thiophene-Alkyloxime (TO) Units in Organic Solar Cells
Full text linkThe organic solar cells (OSCs), a branch of the third-generation solar cell as a viable future energy resource has received a lot of attention in lab and industry today. The OSCs have advantages of low cost, light weight, flexible, and roll-to-roll printing for large area manufacture. The bulk heterojunction structure (BHJ) is wildly accepted in the active layer of OSCs, which is a blend film of donor polymer and acceptor material for efficient charge transfer. Recently, the combination of non-fullerene acceptors (NFAs) with wide-bandgap donor polymers as active layer has come to the forefront of OSCs research, which has achieved high power conversion efficiency (PCE) over 18%.
In this work, we will introduce donor polymers in D-A copolymer structure, based on benzodithiophene (BDT) or polythiophene (PT) as the donor backbone unit combining with electron-withdrawing sidechains as the acceptor unit. Two novel series of polymers, thiophene-vinyl-thiophene ester (TVT-ester) polymer and thiophene-alkyloxime (TO) polymer based on different electron-withdrawing sidechains, were designed, synthesized, and characterized to function as wide-bandgap donor polymers in organic solar cells.
Among the designed TVT-ester polymers, the PEBDT exhibits planar backbone structure, low synthetic complexity, and high quenching efficiency with the non-fullerene acceptors (3PS)2-SiPc and Y6, which is a promising candidate for OSCs. The solar cell device based on PEBDT:Y6 blend film had the highest PCE of 1.23%, with Jsc of 7.32 mA/cm2, Voc of 0.54V, and FF of 31%. The carrier mobilities of blend film were calculated to be 1.37E-6 cm2/Vs and 0.84E-6 cm2/Vs. The low PCE and mobilities were mainly due to poor film morphology with incontinuous network of the blend films. The amorphous property and low carrier mobility of PEBDT could relate to its disordered sidechain packing, so increasing the planarity and rigidity of the backbone maybe beneficial to achieve a more ordered molecule packing and enhanced charge carrier mobility. Further optimized will focus on novel polymer structures by replacing TVT unit with fused ring on the backbone.
For the TO based polymers, our group has reported two TO polymers based on BDT and PT, respectively. To further improve the crystallinity of those polymers, two novel polymers based on BDT-terthiophene backbone with alkyloxime side chains substituted on spacing thiophenes were designed by taking the advantages of previous polymers’ structures. The novel polymers P3TOBDT and P4TOBDT both demonstrated wide optical bandgaps and low-lying HOMO energy levels, which can obtain the complementary light absorption spectra and large Voc when blending with acceptor Y6. P3TOBDT neat film achieved a face-on orientation tendency with higher crystallinity than P4TOBDT after thermal annealing, which also exhibited higher SCLC hole mobility. The highest PCE up to 10.17% was achieved based on P4TOBDT:Y6 bland film, which was mostly contributed to high Jsc of 25.95 mA/cm2, indicating the TO unit is a promising acceptor unit on donor polymers. The P3TOBDT:Y6 system exhibited lower PCE and carrier mobilities than P4TOBDT:Y6 indicating the poor phase separation in blend film, which may be due to high miscibility of materials in processing solvent chlorobenzene. However, both systems showed extremely low electron mobilities, resulting from poor interconnecting network of Y6. Thus, there are still large spaces for optimizing the OSC devices, and the improvement of film morphology will be focused in the future direction
Development of Benzodithiophene (BDT)-Based Wide Bandgap Donor-Acceptor (D-A) Polymers Containing Novel Thiophene and Benzene Acceptor Building Blocks for Organic Solar Cells
No full textOrganic solar cells (OSCs) are an emerging renewable energy technology with advantages of low cost, flexibility, lightweight, and easy processability. High performing OSCs often consist of a wide bandgap polymer donor and a small molecule non-fullerene acceptor (NFA) for complementary absorption. These materials are usually mixed in solution to form a bulk heterojunction (BHJ) structure to achieve efficient charge transfer. Currently, leading materials can reach a power conversion efficiency (PCE) greater than 18 %. However, several challenges remain before OSCs can be commercially viable including further enhancement of the PCE, long-term stability, large area device fabrication, low-cost materials development, and environmentally friendly synthesis. It is critical to target solution-processable polymer donors, with well-matched opto-electronic properties and morphological compatibility with NFAs through effective side chain engineering for further progress in this field.
This thesis work targets low-cost polymer donor materials through relatively easy synthesis routes and low-cost starting materials. Side chains will be selected to target good solubility, low EHOMO levels (to match with high performance NFAs like Y6), and good morphology to achieve high performing polymer donor materials. In this work, several novel donor polymers are developed, which have a donor-acceptor (D-A) structure. D-A polymer building blocks have become increasingly important as donor materials in OSCs since the energy levels and bandgaps are easily tunable via intramolecular charge transfer. Four novel series of polymers: ethynyl thiophene-benzodithiophene (BDT), triazole thiophene-BDT, tetrafluorobenzene-BDT, and acetal thiophene-BDT, with a variety of electron withdrawing group (EWG) side chains were designed, synthesized, and characterized as wide bandgap polymer donors for OSC applications.
An ethynyl series of polymers were designed with the goal of extending the conjugation into the side chain. This can effectively polarize the polymers and increase the exciton lifetime, resulting in improved performance. The ethynyl series included the following side chains: a trimethyl silyl group (PSETBDT), an unsubstituted benzene ring (PBETBDT), and an alkyloxime-substituted benzene ring (POBETBDT). The solar cell devices based on PSETBDT:Y6 and PSETBDT:IT-4F had a PCE of 1.44 and 0.77 %, respectively. The poor performance was attributed to low, unbalanced mobility, and a high polydispersity index (PDI), indicating that cross-linking occurred. Solubility issues and low molecular weight were experienced with polymer PBETBDT; therefore, this polymer was only preliminarily tested for opto-electrochemical measurements. POBETBDT had a PCE of 3.65 % with Y6, while only achieving an open circuit voltage (Voc) of 0.71 and a fill factor (FF) of 0.33. Low mobility and the bulky side chain are potential issues with this material. Future work will look to assess the surface roughness and domain size of these polymers using atomic force microscopy (AFM) to further assess morphological issues.
Similarly, a triazole series of polymers were designed with the goal of extending the conjugation into the side chain. The triazole series involved converting an aldehyde to a triazole ring, which was then substituted at the middle nitrogen with an alkyl chain (PTTBDT), and a carbamate chain (PCTBDT). Polymer PTTBDT achieved a PCE of 5.00 % with Y6, while PCTBDT only achieved 3.29 % due to lower short circuit current density (Jsc), Voc, and FF. Both polymers suffered from low mobility, while PCTBDT had exceptionally low hole mobility (10-7 cm2V-1s-1). DFT calculations indicated these polymers suffer from backbone and side chain twisting, which can negatively affect the charge transfer. Additionally, PCTBDT has a thermally removable side chain that could result in extensive hole trapping, which would limit charge extraction. Future work will look to assess the surface roughness via AFM and further optimize PTTBDT:Y6 devices by altering the donor:acceptor (D:A) ratio to improve the charge mobility.
A tetrafluorobenzene-BDT (PFBBDT) polymer was designed to improve upon the EHOMO and co-planarity characteristics of an unsubstituted benzene-BDT polymer by adding fluorine atoms as an electron withdrawing group (EWG). This can help energy level matching with NFAs such as Y6 and allow for good charge transfer. PFBBDT achieved a PCE of 5.14 %; however, the performance was limited by low molecular weight and strong lamellar stacking interactions. The latter is thought to cause potential aggregation in the active layer and contribute to the low and unbalanced mobility observed. The lamellar stacking indicates this material has potential for future transistor applications.
Previously, synthesis of a polymer based on BDT and a formaldehyde-substituted thiophene was attempted by our group but was unsuccessful. A polymer (PATBDT) was designed with an acetal side chain substituted thiophene to facilitate an acid-catalyzed post polymerization modification to obtain a soluble formaldehyde-substituted thiophene-BDT polymer (PXTBDT). DFT calculations indicated that both these materials had potential for good organic photovoltaic performance (OPV). When PATBDT was paired with Y6, a PCE of 8.20 % was achieved. Low electron mobility resulted in unbalanced charge transfer and low FF for this material. PXTBDT had a quite low EHOMO of -5.67 eV but still worked well with Y6, achieving a better PCE of 9.97 %, mainly due to higher Voc and FF. The low FF of these materials leaves room for process optimization to improve performance. Furthermore, future work will explore other D units, such as bithiophene and thienothiophene, to pair with acetal-protected aldehyde A units to investigate if this simple chemistry can produce any other high performing materials
Thiophene-S,S-dioxidized indophenine for use in organic field effect transistors
No full textTo address the need for better n-type organic semiconductors, thiophene-S,S-dioxidized
indophenine (IDTO) was developed. IDTO is a planar quinoidal molecule with deep energy levels
which facilitate stable and efficient electron transport. IDTO is a promising material in the design and
fabrication of high performance n-type organic field-effect transistors because it offers simple
synthesis, as well as excellent processability and good performance. The work presented in this thesis
follows the development of IDTO and its various uses in organic field-effect transistors. Firstly, the
synthetic method for IDTO was systematically improved. Two important factors were improved,
safety was improved by replacing the solvent, benzene, with the much less hazardous toluene and the
yield of the synthesis was increased from less than 10% to ~40%. Next, IDTO and two derivatives (5-
bromo-IDTO and 6-bromo-IDTO) were synthesized and evaluated as new n-type small molecule
organic semiconductors. The three compounds all exhibited unipolar n-type characteristics in the
range of 10-2 to 10-1 cm2V-1s-1. The highest electron mobility of 0.11 cm2V-1s-1 was measured for 6-
bromo-IDTO. IDTO was then utilized as an electron accepting building block for the synthesis of two
donor-acceptor polymers: PIDTOBT and PIDTOBTz. Both polymers showed unipolar n-type
performance with electron mobilities on the order of 10-2 to 10-1 cm2V-1s-1. PIDTOBT had the highest
electron mobility of 0.18 cm2V-1s-1 after annealing at 200 °C. From the small molecule and polymer
results, it is clear that IDTO is a promising material for unipolar n-type organic semiconductors. Since
IDTO has a strong electron accepting feature, it was thought that IDTO would have another use as an
electron trap. By blending IDTO with an ambipolar polymer that has a LUMO energy level above
that of IDTO; electron transport could be suppressed under the correct conditions. The three small
molecule IDTO compounds were blended with PINDFBT (TT) and PINDFBT (HH) to investigate the
electron trapping ability of IDTO. Complete suppression of electron transport was not observed but
the overall trend agreed with the theory. 6-bromo-IDTO having the lowest LUMO energy level
showed the strongest electron trapping effect while IDTO had the weakest effect due to its higher
LUMO energy level. This thesis provides a comprehensive study of the development and application
of the novel material, IDTO
Study on Solution-Processable Polypyrrole-Based Conducting Polymers
Full text linkDue to the adjustable conductivity, cost-effective synthesis, and easy device fabrication, conducting polymers have wide-ranging applications in numerous areas, including sensors, solar cells, supercapacitors, and electrodes. Among conducting polymers, polypyrrole (PPy) has attracted lots of attention because of its high conductivity, biocompatibility, and mass producibility. However, PPy is insoluble in most solvents, which lowers its processability and limits its application. In the past few decades, many works by researchers around the world have been done to improve the solubility of PPy. Unfortunately, the two major strategies, including doping with large contour ions and introducing an alkyl sidechain to the PPy backbone, were found to significantly decrease the conductivity of PPy by several orders of magnitude. For the doping method, the restricted dopant options limit the conductivity of PPy, whereas the alkyl sidechains twist the polymer structure thus influencing the conductivity. As a result, new methods need to be explored to improve the solubility of PPy without sacrificing its electrical properties.
In this work, two strategies were applied to address this problem. The first is to introduce an alkyl carbamate sidechain to the PPy backbone; the carbamate sidechain can solubilize the polymer, while allowing thermal removal at mild temperatures. Thus, after solution processing, the obtained polymer film can be thermally annealed to remove the sidechain, thereby recovering the PPy structure and conductivity. The second one is to introduce an alkoxy sidechain at the N position of the PPy backbone. This sidechain has less steric hindrance than the alkyl sidechain. Therefore, the PPy with the alkoxy sidechain was expected to have a more planar backbone compared to the alkyl chain-substituted counterpart and as such was expected to have higher conductivity.
We first designed, synthesized, and characterized poly(2-ethylhexyl 1H-pyrrole-1-carboxylate) (PEPC), with a 2-ethylhexyl carbamate chain substituted at the N position of PPy. Different synthetic routes were explored and optimized, including organometallic polymerization and oxidative polymerization. The desired polymer was successfully synthesized and was soluble in various organic solvents, including acetone, dichloromethane, and chloroform. Polymer films in a thickness of 50-60 nm could be deposited from the PEPC chloroform solution on a SiO2/Si substrate, which satisfies requirements for sensor applications. 1 However, the 2-ethylhexyl carbamate sidechain could not be completely removed by thermal annealing and acid cleavage due to the primary structure of ethylhexyl aliphatic chain of the carbamate being reluctant to undergo decomposition. This led to a low conductivity of the PEPC thin films. In the future, a carbamate sidechain with a different secondary or tertiary structure may be applied, allowing easy removal of the sidechain and restoration of the PPy structure.
Poly[1-((2-ethylhexyl)oxy)-1H-pyrrole] (PEOP) with a 2-(ethylhexyl)oxy sidechain at the N position of PPy was designed, and its synthesis was explored. Due to the difficulties in the synthesizing and purifying the monomer, 1-((2-ethylhexyl)oxy)-1H-pyrrole, the desired PEOP was not obtained. However, it was found that the 2-(ethylhexyl)oxy sidechain can undergo a thermolysis process under milder temperatures than the alkyl carbamate sidechain, as evidenced by FTIR and TGA results. Therefore, the thermal instability of 2-(ethylhexyl)oxy and other alkoxy sidechains on nitrogen may allow the development of other polymers that require the sidechains to be thermally removable at mild temperatures. In addition, new synthetic approaches can be explored in the future to obtain a more structurally defined PEOP
Organic Thin-Film Transistor Based Gas Sensors for Putrescine Detection
Full text linkThe application of organic thin film transistors (OTFTs) has been progressing in the area of sensors for decades now. For accomplishing gas sensing in ambient conditions, polymers with good air stability and high enough mobility to detect environmental variations by analytes exposure are required for the OTFT-sensors. In this work, putrescine (PUT) was selected as our primary analyte, as it has high volatility and can be emitted by decayed food products. OTFT-based sensors were made with various polymers as semiconductors to detect PUT vapor.
Based on the preliminary study of gas sensing with bottom-gate, bottom-contact (BGBC) OTFTs, a gas sensing system was established with OTFT-sensors, an analyte injection platform and a signal analyzing system, which is presented in Chapter 3. Before and after gas detection, all OTFT devices were characterized by their charge carrier mobility, threshold voltage and current on/off ratio in the glovebox and in air. Additional tests on polymer films were conducted by morphology and crystallinity tests with AFM and XRD. Chapter 4 and Chapter 5 show the screening of polymers as the semiconductor in OTFT sensors. P-type polymers and n-type polymers including commercially available ones and those synthesized by our group were employed in OTFT-sensors. P3HT and N2200 as typical p-type and n-type representatives have succeeded in the detection of PUT vapor as OTFT sensors. In addition, p-type polymers with FTPDO, DPP, indigo and bithiazole cores, n-type polymers with IBDF building block also showed sensing ability towards PUT vapor as OTFT sensors. Chapter 5 especially focuses on a series of 1,4-DPP polymers that exhibited high transistor performance as well as good air stability. As main factors that determine the performance of a sensor, operational temperature, stability, response and recovery time, sensitivity and selectivity of these DPP-polymer sensors showed very promising potential in PUT vapor detection. Food spoilage detection was realized with three types of DPP-sensors to detect vapors emitted by food samples. Mechanism of DPP-sensors to detect PUT vapor was studied by responses of the sensor, IV characteristics of the device, and morphology/crystallinity of the polymer. Combined with chemical properties of DPP-polymers and putrescine molecules, it was proposed that the trapping effect of lone pairs of electrons on putrescine molecules would cause the responses of DPP-OTFT sensors. Mobility dropping, drain-source current decrease and negative shift of VTH were observed with DPP-sensors by this trapping effect with the diffused PUT in the active channel.
Future steps are expected based on this study. Better sensing ability can be achieved with the improvement on both transistors quality and polymers properties. More accurate relationships between sensors responses and PUT vapor concentrations can be built by optimizing gas sensing process. Spoilage detection of food products with OTFT sensors can be improved for more precise analysis with assistance of liquid chromatography (LC) and gas chromatography (GC). The ultimate goal of this study is to manufacture smart labels or tags with OTFT-sensors to attach on food packages, by connecting to smartphones or computers, fulfilling a real-time, in-situ detection of PUT vapor with ideal accuracy
A new series of semiconducting azine polymers and their applications in organic electronics
Full text linkThe rise of renewable energy and the urgency of sustainable development make organic solar cells (OSCs) or organic photovoltaics (OPVs) valued. Among so many donor materials for bulk heterojunction structure, P3HT is one of the most common donor polymers which is cost-effective and has been studied deeply. The room and potential for the improvement of OSCs based on polythiophene material still need further research. Therefore, Poly(4,4’-didodecyl-2,2’-bithiophene-azine) (PDDBTA), a new semiconducting azine polymer was synthesized in only three steps. As a p-channel material, PDDBTA showed hole mobilities of up to 4.1 10-2 cm2 V-1 s-1 in organic thin film transistors (OTFTs). As a donor in organic solar cells (OSCs), a 2.18% of power conversion efficiencies (PCEs) was achieved, which was the first example to use an azine-based polymer to fabricate organic photovoltaics (OPVs). These performances indicate the potential of bithiophene-azine polymers as a new type of low-cost semiconductor materials for OPVs and other organic electronics. The performances of OTFT and OPV devices have reached the same level as those fabricated with P3HT. However, the morphology of the active layer mixed by PDDBTA and PCBM was relatively unsatisfying due to the poor solubility and high crystallinity nature based-on a straight longer side chain, which had an adverse effect to charge transport and charge collection of the active layer, thereby limited the JSC and FF. In order to increase the solubility to form a better blend film morphology and lower the aggregation caused by high crystallinity, as well as obtain a better phase separation and maintain the original performance of PDDBTA, a branch sidechain polymer PDEHBTA and a shorter straight sidechain polymer with pyrrole units as backbone PHPA were synthesized. However, the low molecular weight of these two types of optimized structures became an obstacle that hindered the OPVs to gain a satisfactory JSC and FF, and only 1.58% of PCE based on PDEHBTA OSCs and 1.1% of that fabricated with PHPA were obtained, respectively
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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