| 1. |
- Bolognesi, Margherita, et al.
(författare)
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2D π-conjugated benzo[1,2-b:4,5-b']dithiophene- and quinoxaline-based copolymers for photovoltaic applications
- 2013
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Ingår i: RSC Advances. - 2046-2069. ; 3:46, s. 24543-24552
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Tidskriftsartikel (refereegranskat)abstract
- Two medium gap semiconducting polymers, P(1)-Q-BDT-4TR and P(2)-FQ-BDT-4TR, based on alternate units of alkyl-dithiophene substituted benzodithiophene (BDT) and quinoxaline units (without or with fluorine substitution), are synthesized and fully characterized. The polymers exhibit optical and electrical properties favorable for being employed as donors in BHJ OPV devices, such as: absorption spectra extending up to around 720 nm for a high solar spectrum coverage, deep lying HOMO energy levels for a high device open circuit voltage and LUMO energy levels higher than those of PC61BM and PC71BM for an efficient exciton dissociation. In particular, the presence of alkyl-dithiophene side chains allows us to obtain a high 2D π-conjugation which promotes red shifted absorption profiles, low HOMO energy levels (
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| 2. |
- Gedefaw, Desta Antenehe, 1971, et al.
(författare)
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Conjugated polymers based on benzodithiophene and fluorinated quinoxaline for bulk heterojunction solar cells: thiophene versus thieno[3,2-b]thiophene as π-conjugated spacers
- 2014
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Ingår i: Polymer Chemistry. - 1759-9954 .- 1759-9962. ; 5:6, s. 2083-2093
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Tidskriftsartikel (refereegranskat)abstract
- Two conjugated donor–acceptor copolymers based on a benzodithiophene donor unit and a fluorinated quinoxaline acceptor unit, spaced with either thiophene or thieno[3,2-b]thiophene π-bridges, were designed and synthesized. The effect of different π-bridges and of the processing conditions on the optical, electrical, morphological and photovoltaic properties of the polymer:fullerene blend films were investigated. The polymer containing the thieno[3,2-b]thiophene π-bridge (PBDTFQ-TT) showed a red-shifted absorption and enhanced charge carrier mobility, as compared to its analogue with the thiophene π-bridge (PBDTFQ-T), due to its narrower optical gap (by ~ 0.1 eV) and stronger inter-chain interactions, favored by the structural planarity and increased linearity of the polymer backbone, as also supported by DFT calculations. The blend of PBDTFQ-TT and PC61BM ([6,6]-phenyl-C61-butyric acid methyl ester), compared to the PBDTFQ-T:PC61BM one processed in the same conditions (by blade-coating technique), showed greatly enhanced photovoltaic performance, with more than doubled power conversion efficiency (PCE up to 5.60% for the best device) due to the increased short-circuit current density and fill factor. However, similar PCEs were also achieved for PBDTFQ-T:PC61BM-based devices by optimizing the processing conditions through the addition of 1,8-diiodooctane (DIO) as the solvent additive. Through morphological and electrical analysis of the films, produced with and without additive, it was observed that the addition of DIO greatly enhances the self-organization, and consequently the charge mobility, of the thiophene π-bridge-based polymer, while it was detrimental for the nanoscale morphology and photovoltaic performances of the thieno[3,2-b]thiophene π-bridge-based polymer in the corresponding blend.
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| 3. |
- Gedefaw, Desta Antenehe, 1971, et al.
(författare)
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Induced photodegradation of quinoxaline based copolymers for photovoltaic applications
- 2016
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Ingår i: Solar Energy Materials and Solar Cells. - : Elsevier BV. - 0927-0248. ; 144, s. 150-158
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Tidskriftsartikel (refereegranskat)abstract
- We report here the synthesis and characterization of a series of p-type copolymers, which combine a fluorinated quinoxaline (FQ) acceptor unit either with a differently substituted benzodithiophene (BDT) or an unsubstituted thieno[3,2-b]thiophene (TT). The effect of the structural modifications on the photochemical stability of the resulting films is investigated and then correlated with the photovoltaic performance and lifetime measurements of corresponding photovolatic devices. To this end, we firstly studied the intrinsic stability of each polymer film by monitoring the UV-vis absorption decay, under simulated sunlight, as a function of ageing time. Bulk heterojunction solar cells, based on these polymers as donor materials, were fabricated and tested. Beside the initial values, we monitored the photovoltaic performance during prolonged light soaking in order to evaluate and compare the photostability of more complex systems such as working solar cells.
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| 4. |
- Gedefaw, Desta Antenehe, 1971, et al.
(författare)
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Recent Development of Quinoxaline Based Polymers/Small Molecules for Organic Photovoltaics
- 2017
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Ingår i: Advanced Energy Materials. - : Wiley. - 1614-6840 .- 1614-6832. ; 7:21
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Forskningsöversikt (refereegranskat)abstract
- Among the various molecular designs developed for the synthesis of conjugated polymers and small molecules for optoelectronic applications, the donor: acceptor (D-A) approach is the most widely explored method over the past decades. Through the covalent linkage of electron-rich and electron-deficient units, a plethora of medium-low band gap materials has been developed and tested in organic photovoltaic devices. In particular, the quinoxaline aromatic structure and its derivatives are among the most studied electron deficient aromatic units used in D-A structures. Quinoxaline based materials are endowed with characteristics that are useful for large scale production in real world applications, such as easy synthetic procedures and excellent stability in air. Moreover, the use of quinoxaline based polymers/small molecules in bulk heterojunction (BHJ) devices led to power conversion efficiencies over 9%. Considering the potential of quinoxaline based materials, this review gathers together quinoxaline based polymers and small molecules reported in the literature during the last 5 years, summarizing and discussing the structure-properties relationships for this class of organic semiconductors, aiming to serve as a background and to promote efforts for the further development of new quinoxaline derivatives with improved and advanced properties for future applications.
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| 5. |
- Gedefaw, Desta Antenehe, 1971, et al.
(författare)
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Synthesis and characterization of benzodithiophene and benzotriazole-based polymers for photovoltaic applications
- 2016
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Ingår i: Beilstein Journal of Organic Chemistry. - : Beilstein Institut. - 1860-5397. ; 12, s. 1629-1637
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Tidskriftsartikel (refereegranskat)abstract
- Two high bandgap benzodithiophene-benzotriazole-based polymers were synthesized via palladium-catalysed Stille coupling reaction. In order to compare the effect of the side chains on the opto-electronic and photovoltaic properties of the resulting polymers, the benzodithiophene monomers were substituted with either octylthienyl (PTzBDT-1) or dihexylthienyl (PTzBDT-2) as side groups, while the benzotriazole unit was maintained unaltered. The optical characterization, both in solution and thin-film, indicated that PTzBDT-1 has a red-shifted optical absorption compared to PTzBDT-2, likely due to a more planar conformation of the polymer backbone promoted by the lower content of alkyl side chains. The different aggregation in the solid state also affects the energetic properties of the polymers, resulting in a lower highest occupied molecular orbital (HOMO) for PTzBDT-1 with respect to PTzBDT-2. However, an unexpected behaviour is observed when the two polymers are used as a donor material, in combination with PC61BM as acceptor, in bulk heterojunction solar cells. Even though PTzBDT-1 showed favourable optical and electrochemical properties, the devices based on this polymer present a power conversion efficiency of 3.3%, considerably lower than the efficiency of 4.7% obtained for the analogous solar cells based on PTzBDT-2. The lower performance is presumably attributed to the limited solubility of the PTzBDT-1 in organic solvents resulting in enhanced aggregation and poor intermixing with the acceptor material in the active layer.
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| 6. |
- Prosa, M., et al.
(författare)
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Enhanced Ultraviolet Stability of Air-Processed Polymer Solar Cells by Al Doping of the ZnO Interlayer
- 2016
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Ingår i: ACS Applied Materials & Interfaces. - : American Chemical Society (ACS). - 1944-8252 .- 1944-8244. ; 8:3, s. 1635-1643
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Tidskriftsartikel (refereegranskat)abstract
- Photostability of organic photovoltaic devices represents a key requirement for the commercialization of this technology. In this field, ZnO is one of the most attractive materials employed as an electron transport layer, and the investigation of its photostability is of particular interest. Indeed, oxygen is known to chemisorb on ZnO and can be released upon UV illumination. Therefore, a deep analysis of the UV/oxygen effects on working devices is relevant for the industrial production where the coating processes take place in air and oxygen/ZnO contact cannot be avoided. Here we investigate the light-soaking stability of inverted organic solar cells in which four different solution-processed ZnO-based nanoparticles were used as electron transport layers: (i) pristine ZnO, (ii) 0.03 at %, (iii) 0.37 at %, and (iv) 0.8 at % aluminum-doped AZO nanoparticles. The degradation of solar cells under prolonged illumination (40 h under 1 sun), in which the ZnO/AZO layers were processed in air or inert atmosphere, is studied. We demonstrate that the presence of oxygen during the ZnO/AZO processing is crucial for the photostability of the resulting solar cell. While devices based on undoped ZnO were particularly affected by degradation, we found that using AZO nanoparticles the losses in performance, due to the presence of oxygen, were partially or totally prevented depending on the Al doping level.
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| 7. |
- Seri, Mirko, et al.
(författare)
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A New Quinoxaline and Isoindigo Based Polymer as Donor Material for Solar Cells: Role of Ecofriendly Processing Solvents on the Device Efficiency and Stability
- 2017
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Ingår i: Journal of Polymer Science, Part A: Polymer Chemistry. - : Wiley. - 1099-0518 .- 0887-624X. ; 55:2, s. 234-242
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Tidskriftsartikel (refereegranskat)abstract
- A new semiconducting polymer based on two different electron deficient (quinoxaline and isoindigo) and electron rich (benzodithiophene) moieties is synthesized, characterized and used as donor material for photovoltaic devices. Blade-coated bulk heterojunction solar cells are fabricated in air by using chlorinated (o-dichlorobenzene) and nonchlorinated (o-xylene) solvents for the deposition of the active layer. The use of o-xylene allows a similar to 10% improvement of the device efficiency in comparison to the analogous system processed from o-dichlorobenzene. In addition, the evolution of the photovoltaic parameters of the resulting devices during thermal stress is monitored and compared, demonstrating a nearly identical resistance against temperature. The reported results not only highlight the promising properties of the new polymer in terms of environmental stability and compatibility with nonhalogenated solvents, but also show an easy and ecofriendly way to further improve the device performance without altering the corresponding thermal stability.
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| 8. |
- Tessarolo, Marta, et al.
(författare)
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Predicting thermal stability of organic solar cells through an easy and fast capacitance measurement
- 2015
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Ingår i: Solar Energy Materials and Solar Cells. - : Elsevier BV. - 0927-0248. ; 141, s. 240-247
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Tidskriftsartikel (refereegranskat)abstract
- Degradation of organic photovoltaic (OPV) devices is currently a topic under intense research as it is one of the main limitations towards the commercialization of this technology. Morphological changes at both active layer and interfaces with the outer contacts are believed to determine main key issues to be overcome. In-line techniques are essential to rule out any effect arising during sample fabrication. Unfortunately, the number of physical techniques able to provide morphological information on complete and operational devices is certainly limited. In this work, we study the thermal degradation of bulk heterojunction (BHJ) solar cells composed by different donor polymers with techniques developed to provide in-situ information on operational devices. Capacitance measurement as a function of temperature monitors the electrical integrity of the active layer and provides the threshold temperature (TMAX) at which the whole device becomes thermally unstable. We found a direct correlation between the threshold temperature TMAX, obtained by capacitanceerature measurements on complete OPV devices, and the power conversion efficiency decay measured at 85°C. Devices tend to be thermally stable when the temperature of the thermal stress is below TMAX, while above TMAX evident changes in the active layer or at the active layer/electrode interface are also detected by confocal fluorescence microscopy. The capacitance method gives precious guidelines to predict the thermal stability of BHJ solar cells using an accelerated and easy test.
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| 9. |
- Tessarolo, Marta, et al.
(författare)
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Structural tuning of quinoxaline-benzodithiophene copolymers via alkyl side chain manipulation: synthesis, characterization and photovoltaic properties
- 2014
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Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry (RSC). - 2050-7488 .- 2050-7496. ; 2:29, s. 11162-11170
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Tidskriftsartikel (refereegranskat)abstract
- We report here the synthesis and characterization of two novel semiconducting quinoxaline (FQ)-benzodithiophene (BDT) based copolymers, PFQBDT-TR1 and PFQBDT-T2R(2), in which the BDT unit is substituted with either 2-octylthienyl (-TR1) or 2,3-dihexylthienyl (-T2R(2)), respectively, as side groups. The effect of the alkyl side chain(s), linked to the thienyl side groups, on the optical, electronic and morphological properties of the resulting polymers is investigated and correlated with the photovoltaic performance. Solution-processed BHJ solar cells, using these copolymers as electron donor materials and PC61BM (or PC71BM) as an electron acceptor counterpart, are prepared by a blade-coating technique under ambient conditions. As a result, power conversion efficiencies (PCEs) of similar to 5.7% and similar to 3.4% have been achieved for PFQBDT-TR1 and PFQBDT-T2R(2) based devices, respectively, highlighting the crucial role of the alkyl portion of the pi-conjugated side segment in the optoelectronic properties of this class of copolymers.
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| 10. |
- Zhuang, Wenliu, 1979, et al.
(författare)
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Influence of Incorporating Different Electron-Rich Thiophene-Based Units on the Photovoltaic Properties of Isoindigo-Based Conjugated Polymers: An Experimental and DFT Study
- 2013
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Ingår i: Macromolecules. - : American Chemical Society (ACS). - 1520-5835 .- 0024-9297. ; 46:21, s. 8488-8499
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Tidskriftsartikel (refereegranskat)abstract
- A series of novel donor–acceptor conjugated alternating copolymers based on the isoindigo acceptor moiety have been designed, synthesized, and characterized, in order to explore the potential of isoindigo for efficient donor materials with high photovoltages in solar cells. We have systematically investigated and discussed the effect of combining different electron-rich thiophene-based units on the structural, optical, electronic, and photovoltaic properties of the resulting polymers. Morphological studies and quantum-chemical calculations are carried out to gain insights into the different properties. The power conversion efficiencies (PCEs) of the solar cells based on these polymers are increased step by step by over 3-fold through a rational structural modification. Among them, PBDTA-MIM shows a PCE of 5.4%, which is to our knowledge the best result achieved among isoindigo-based polymers for solar cells combined with PC61BM as the acceptor using the conventional device configuration. Our results further emphasize the use of isoindigo as an effective acceptor unit and highlight the importance of carefully choosing appropriate chemical structure to design efficient donor–acceptor polymers for organic solar cells. In addition, the resulting low optical gaps, the promising PCEs with PC61BM as the acceptor, and the good open-circuit voltages (up to 0.8 V) synergistically demonstrate the potential of this class of polymers as donor materials for bottom subcells in organic tandem solar cells.
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