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1.
ACS Appl Mater Interfaces ; 11(24): 21766-21774, 2019 Jun 19.
Artigo em Inglês | MEDLINE | ID: mdl-31185565

RESUMO

Organic solar cells are thought to suffer from poor thermal stability of the active layer nanostructure, a common belief that is based on the extensive work that has been carried out on fullerene-based systems. We show that a widely studied non-fullerene acceptor, the indacenodithienothiophene-based acceptor ITIC, crystallizes in a profoundly different way as compared to fullerenes. Although fullerenes are frozen below the glass-transition temperature Tg of the photovoltaic blend, ITIC can undergo a glass-crystal transition considerably below its high Tg of ∼180 °C. Nanoscopic crystallites of a low-temperature polymorph are able to form through a diffusion-limited crystallization process. The resulting fine-grained nanostructure does not evolve further with time and hence is characterized by a high degree of thermal stability. Instead, above Tg, the low temperature polymorph melts, and micrometer-sized crystals of a high-temperature polymorph develop, enabled by more rapid diffusion and hence long-range mass transport. This leads to the same detrimental decrease in photovoltaic performance that is known to occur also in the case of fullerene-based blends. Besides explaining the superior thermal stability of non-fullerene blends at relatively high temperatures, our work introduces a new rationale for the design of bulk heterojunctions that is not based on the selection of high- Tg materials per se but diffusion-limited crystallization. The planar structure of ITIC and potentially other non-fullerene acceptors readily facilitates the desired glass-crystal transition, which constitutes a significant advantage over fullerenes, and may pave the way for truly stable organic solar cells.

2.
Polymers (Basel) ; 11(1)2019 Jan 17.
Artigo em Inglês | MEDLINE | ID: mdl-30960146

RESUMO

The thermal and electrical properties of a polymer nanocomposite are highly dependent on the dispersion of the CNT filler in the polymer matrix. Non-covalent functionalisation with a PVP polymer is an excellent driving force towards an effective dispersion of MWNTs in the polymer matrix. It is shown that the PVP molecular weight plays a key role in the non-covalent functionalisation of MWNT and its effect on the thermal and electrical properties of the polymer nanocomposite is reported herein. The dispersion and crystallisation behaviour of the composite are also evaluated by a combination of scanning electron microscopy (SEM) and differential scanning calorimetry (DSC).

3.
Polymers (Basel) ; 11(3)2019 Mar 22.
Artigo em Inglês | MEDLINE | ID: mdl-30960528

RESUMO

More environmentally friendly polymer solar cells were constructed using a conjugated polymer, poly (2,5-thiophene-alt-4,9-bis(2-hexyldecyl)-4,9-dihydrodithieno[3,2-c:3',2'h][1,5] naphthyridine-5,10-dione, PTNT, as a donor material in combination with PC71BM as an acceptor in a bulk heterojunction device structure. A non-halogenated processing solvent (o-xylene) and solvent additives that are less harmful to the environment such as 1-methoxynaphthalene (MN) and 1-phenylnaphthalene (PN) were used throughout the study as processing solvents. The most widely used halogenated solvent additives (1,8-diiodooctane (DIO) and 1-chloronaphthalene (CN)) were also used for comparison and to understand the effect of the type of solvent additives on the photovoltaic performances. Atomic force microscopy (AFM) was employed to investigate the surface morphology of the films prepared in the presence of the various additives. The best-performing polymer solar cells provided a high open-circuit voltage of 0.9 V, an efficient fill factor of around 70%, and a highest power conversion efficiency (PCE) of over 6% with the use of the eco-friendlier o-xylene/MN solvent systems. Interestingly, the solvent blend which is less harmful and with low environmental impact gave a 20% rise in PCE as compared to an earlier reported device efficiency that was processed from the chlorinated solvent o-dichlorobenzene (o-DCB).

4.
Adv Mater ; 31(22): e1807275, 2019 May.
Artigo em Inglês | MEDLINE | ID: mdl-30790384

RESUMO

All-polymer solar cells (all-PSCs) based on n- and p-type polymers have emerged as promising alternatives to fullerene-based solar cells due to their unique advantages such as good chemical and electronic adjustability, and better thermal and photochemical stabilities. Rapid advances have been made in the development of n-type polymers consisting of various electron acceptor units for all-PSCs. So far, more than 200 n-type polymer acceptors have been reported. In the last seven years, the power conversion efficiency (PCE) of all-PSCs rapidly increased and has now surpassed 10%, meaning they are approaching the performance of state-of-the-art solar cells using fullerene derivatives as acceptors. This review discusses the design criteria, synthesis, and structure-property relationships of n-type polymers that have been used in all-PSCs. Additionally, it highlights the recent progress toward photovoltaic performance enhancement of binary, ternary, and tandem all-PSCs. Finally, the challenges and prospects for further development of all-PSCs are briefly considered.

5.
Phys Chem Chem Phys ; 21(10): 5705-5715, 2019 Mar 06.
Artigo em Inglês | MEDLINE | ID: mdl-30801074

RESUMO

A modified synthesis method for aqueous nanoparticle printing inks, based upon vacuum-assisted solvent removal, is reported. Poly(3-hexylthiophene):phenyl C61 butyric acid methyl ester nanoparticle inks were prepared via this modified miniemulsion method, leading to both an improvement in photoactive layer morphology and a substantial reduction in the ink fabrication time. A combination of UV-visible spectroscopy, photoluminescence spectroscopy and scanning transmission X-ray microscopy measurements revealed a nanoparticle morphology comprising highly intermixed donor-acceptor domains. Consistent with these measurements, dynamic mechanical thermal analysis of the nanoparticles showed a glass transition temperature (Tg) of 104 °C, rather than a pure polymer phase or pure fullerene phase Tg. Together the spectroscopy, microscopy and thermomechanical data indicate that rapid solvent removal generates a more blended nanoparticle morphology. As such, this study highlights a new experimental lever for optimising nanostructure in the photoactive layer of nanoparticulate organic photovoltaic devices by enabling highly intermixed donor-acceptor architectures to be built from customised nanoparticulate inks.

6.
Chemphyschem ; 19(24): 3393-3400, 2018 12 19.
Artigo em Inglês | MEDLINE | ID: mdl-30381883

RESUMO

Donor-acceptor (D-A) copolymers typically show two absorption peaks in the visible region, flanking a valley region of limited absorptivity. One strategy for more panchromatic light harvesting is to incorporate side-groups orthogonal to the polymer backbone, which enable 2D π conjugation and can give rise to additional absorption peaks. Here we design and synthesize two D-A polymers which both carry a fluorinated quinoxaline acceptor unit, but while P1 includes a benzodithiophene donor moiety with thiophene side-groups (2D-BDT), the P2 polymer lacks 2D conjugation in its simpler pentathiophene donor segment. The P1 polymer consequently shows an atypical absorption profile with more panchromatic absorption with no apparent valley in the spectrum. In order to understand the structure-electronic relations, the optical and electrochemical properties were predicted using a previously developed computational approach. The predicted optical properties show very good agreement with the experimental results. Solar cells made from P1 show a short-circuit current more than twice as large as P2, attributed to its enhanced spectral coverage. However, poor fill factors limit the preliminary power conversion efficiencies to 3.3 % for P1 and 1.0 % for P2 as blended with PCBM[70] in a 1 : 1.5 (w/w) ratio.

7.
Adv Mater ; : e1706584, 2018 Jul 10.
Artigo em Inglês | MEDLINE | ID: mdl-29987856

RESUMO

Due to the so-called energy-gap law and aggregation quenching, the efficiency of organic light-emitting diodes (OLEDs) emitting above 800 nm is significantly lower than that of visible ones. Successful exploitation of triplet emission in phosphorescent materials containing heavy metals has been reported, with OLEDs achieving remarkable external quantum efficiencies (EQEs) up to 3.8% (peak wavelength > 800 nm). For OLEDs incorporating fluorescent materials free from heavy or toxic metals, however, we are not aware of any report of EQEs over 1% (again for emission peaking at wavelengths > 800 nm), even for devices leveraging thermally activated delayed fluorescence (TADF). Here, the development of polymer light-emitting diodes (PLEDs) peaking at 840 nm and exhibiting unprecedented EQEs (in excess of 1.15%) and turn-on voltages as low as 1.7 V is reported. These incorporate a novel triazolobenzothiadiazole-based emitter and a novel indacenodithiophene-based transport polymer matrix, affording excellent spectral and transport properties. To the best of knowledge, such values are the best ever reported for electroluminescence at 840 nm with a purely organic and solution-processed active layer, not leveraging triplet-assisted emission.

8.
ACS Appl Mater Interfaces ; 10(15): 12937-12946, 2018 Apr 18.
Artigo em Inglês | MEDLINE | ID: mdl-29589432

RESUMO

A conjugated donor-acceptor polymer, poly[4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro- s-indaceno[1,2- b:5,6- b']dithiophene-2,7-diyl- alt-5-(2-ethylhexyl)-4 H-thieno[3,4- c]pyrrole-4,6(5 H)-dione-1,3-diyl] (PIDT-TPD), is blended with the fullerene derivative [6,6]phenyl-C61-butyric acid methyl ester (PC61BM) for the fabrication of thin and solution-processed organic photodetectors (OPDs). Systematic screening of the concentration ratio of the blend and the molecular weight of the polymer is performed to optimize the active layer morphology and the OPD performance. The device comprising a medium molecular weight polymer (27.0 kg/mol) in a PIDT-TPD:PC61BM 1:1 ratio exhibits an external quantum efficiency of 52% at 610 nm, a dark current density of 1 nA/cm2, a detectivity of 1.44 × 1013 Jones, and a maximum 3 dB cutoff frequency of 100 kHz at -5 V bias. These results are remarkable among the state-of-the-art red photodetectors based on conjugated polymers. As such, this work presents a functional organic active material for high-speed OPDs with a linear photoresponse at different light intensities.

9.
Chem Mater ; 29(13): 5654-5662, 2017 Jul 11.
Artigo em Inglês | MEDLINE | ID: mdl-28713199

RESUMO

Thermal annealing strongly impacts the nano- and microstructure of conjugated polymers. Despite the fundamental importance for the resulting optoelectronic behavior of this class of materials, the underlying crystallization processes have not received the same attention that is encountered in other disciplines of materials science. The question arises whether classical treatment of nucleation and growth phenomena is truly applicable to conjugated polymers? Here, the isothermal crystallization behavior of the conjugated polymer poly(3-(2'-ethyl)hexylthiophene) (P3EHT) is monitored with differential scanning calorimetry (DSC). Avrami analysis reveals growth- and nucleation-limited temperature regimes that are separated by the maximum rate of crystallization. The molecular weight of the polymer is found to strongly influence the absolute rate of crystallization at the same degree of undercooling relative to the melting temperature. A combination of optical microscopy and grazing-incidence wide-angle X-ray scattering (GIWAXS) confirms that the resulting nano- and microstructure strongly correlate with the selected isothermal annealing temperature. Hence, this work establishes that classical nucleation and growth theory can be applied to describe the solidification behavior of the semicrystalline conjugated polymer P3EHT.

10.
Ultramicroscopy ; 176: 23-30, 2017 05.
Artigo em Inglês | MEDLINE | ID: mdl-28341555

RESUMO

We show by in situ microscopy that the effects of electron beam irradiation during transmission electron microscopy can be used to lock microstructural features and enhance the structural thermal stability of a nanostructured polymer:fullerene blend. Polymer:fullerene bulk-heterojunction thin films show great promise for use as active layers in organic solar cells but their low thermal stability is a hindrance. Lack of thermal stability complicates manufacturing and influences the lifetime of devices. To investigate how electron irradiation affects the thermal stability of polymer:fullerene films, a model bulk-heterojunction film based on a thiophene-quinoxaline copolymer and a fullerene derivative was heat-treated in-situ in a transmission electron microscope. In areas of the film that exposed to the electron beam the nanostructure of the film remained stable, while the nanostructure in areas not exposed to the electron beam underwent large phase separation and nucleation of fullerene crystals. UV-vis spectroscopy shows that the polymer:fullerene films are stable for electron doses up to 2000kGy.

11.
ACS Appl Mater Interfaces ; 9(12): 10929-10936, 2017 Mar 29.
Artigo em Inglês | MEDLINE | ID: mdl-28262016

RESUMO

Poly(4-vinylpyridine) (P4VP) was used as a cathode interface layer in inverted organic solar cells (OSCs) fabricated using poly[2,3-bis(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1) and PC71BM (phenyl C71 butyric acid methyl ester) as the donor and acceptor materials, respectively. We successfully demonstrate that the work function of underlying indium tin oxide (ITO) electrode can be significantly reduced by ∼0.7 eV, after modification of the surface with a thin film of P4VP. Photoconversion efficiency of 4.7% was achieved from OSCs incorporating P4VP interface layer between the ITO and bulk heterojunction (BHJ). Thin P4VP layer, when used to modify ZnO electron transport layer in inverted OSCs, reduced the ZnO work function from 3.7 to 3.4 eV, which resulted in a noteworthy increase in open-circuit voltage from 840 to 890 mV. On simultaneous modification of ZnO with P4VP and optimization of the BHJ morphology by using solvent additive chloronapthalene, photoconversion efficiency of OSCs was significantly increased from 4.6% to 6.3%. The enhanced device parameters are also attributed to an energetically favorable material stratification, as a result of an enrichment of PC71BM toward the P4VP interface.

12.
Ultramicroscopy ; 173: 16-23, 2017 02.
Artigo em Inglês | MEDLINE | ID: mdl-27902941

RESUMO

We show by in situ microscopy that the effects of electron beam irradiation during transmission electron microscopy can be used to lock microstructural features and enhance the structural thermal stability of a nanostructured polymer:fullerene blend. Polymer:fullerene bulk-heterojunction thin films show great promise for use as active layers in organic solar cells but their low thermal stability is a hindrance. Lack of thermal stability complicates manufacturing and influences the lifetime of devices. To investigate how electron irradiation affects the thermal stability of polymer:fullerene films, a model bulk-heterojunction film based on a thiophene-quinoxaline copolymer and a fullerene derivative was heat-treated in-situ in a transmission electron microscope. In areas of the film that exposed to the electron beam the nanostructure of the film remained stable, while the nanostructure in areas not exposed to the electron beam underwent large phase separation and nucleation of fullerene crystals. UV-vis spectroscopy shows that the polymer:fullerene films are stable for electron doses up to 2000kGy.

13.
J Am Chem Soc ; 138(34): 10935-44, 2016 08 31.
Artigo em Inglês | MEDLINE | ID: mdl-27479751

RESUMO

Growing interests have been devoted to the design of polymer acceptors as potential replacement for fullerene derivatives for high-performance all polymer solar cells (all-PSCs). One key factor that is limiting the efficiency of all-PSCs is the low fill factor (FF) (normally <0.65), which is strongly correlated with the mobility and film morphology of polymer:polymer blends. In this work, we find a facile method to modulate the crystallinity of the well-known naphthalene diimide (NDI) based polymer N2200, by replacing a certain amount of bithiophene (2T) units in the N2200 backbone by single thiophene (T) units and synthesizing a series of random polymers PNDI-Tx, where x is the percentage of the single T. The acceptor PNDI-T10 is properly miscible with the low band gap donor polymer PTB7-Th, and the nanostructured blend promotes efficient exciton dissociation and charge transport. Solvent annealing (SA) enables higher hole and electron mobilities, and further suppresses the bimolecular recombination. As expected, the PTB7-Th:PNDI-T10 solar cells attain a high PCE of 7.6%, which is a 2-fold increase compared to that of PTB7-Th:N2200 solar cells. The FF of 0.71 reaches the highest value among all-PSCs to date. Our work demonstrates a rational design for fine-tuned crystallinity of polymer acceptors, and reveals the high potential of all-PSCs through structure and morphology engineering of semicrystalline polymer:polymer blends.

14.
Beilstein J Org Chem ; 12: 1629-37, 2016.
Artigo em Inglês | MEDLINE | ID: mdl-27559416

RESUMO

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.

15.
ACS Appl Mater Interfaces ; 8(32): 20928-37, 2016 Aug 17.
Artigo em Inglês | MEDLINE | ID: mdl-27456294

RESUMO

Energy transfer has been identified as an important process in ternary organic solar cells. Here, we develop kinetic Monte Carlo (KMC) models to assess the impact of energy transfer in ternary and binary bulk heterojunction systems. We used fluorescence and absorption spectroscopy to determine the energy disorder and Förster radii for poly(3-hexylthiophene-2,5-diyl), [6,6]-phenyl-C61-butyric acid methyl ester, 4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]squaraine (DIBSq), and poly(2,5-thiophene-alt-4,9-bis(2-hexyldecyl)-4,9-dihydrodithieno[3,2-c:3',2'-h][1,5]naphthyridine-5,10-dione). Heterogeneous energy transfer is found to be crucial in the exciton dissociation process of both binary and ternary organic semiconductor systems. Circumstances favoring energy transfer across interfaces allow relaxation of the electronic energy level requirements, meaning that a cascade structure is not required for efficient ternary organic solar cells. We explain how energy transfer can be exploited to eliminate additional energy losses in ternary bulk heterojunction solar cells, thus increasing their open-circuit voltage without loss in short-circuit current. In particular, we show that it is important that the DIBSq is located at the electron donor-acceptor interface; otherwise charge carriers will be trapped in the DIBSq domain or excitons in the DIBSq domains will not be able to dissociate efficiently at an interface. KMC modeling shows that only small amounts of DIBSq (<5% by weight) are needed to achieve substantial performance improvements due to long-range energy transfer.

16.
ACS Appl Mater Interfaces ; 8(3): 1635-43, 2016 Jan 27.
Artigo em Inglês | MEDLINE | ID: mdl-26751271

RESUMO

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.

17.
Nanoscale ; 7(18): 8451-6, 2015 May 14.
Artigo em Inglês | MEDLINE | ID: mdl-25891364

RESUMO

The formation of fullerene crystals represents a major degradation pathway of polymer/fullerene bulk-heterojunction thin films that inexorably deteriorates their photovoltaic performance. Currently no tools exist that reveal the origin of fullerene crystal formation vertically through the film. Here, we show that electron tomography can be used to study nucleation and growth of fullerene crystals. A model bulk-heterojunction blend based on a thiophene-quinoxaline copolymer and a fullerene derivative is examined after controlled annealing above the glass transition temperature. We image a number of fullerene nanocrystals, ranging in size from 70 to 400 nanometers, and observe that their center is located close to the free-surface of spin-coated films. The results show that the nucleation of fullerene crystals predominately occurs in the upper part of the films. Moreover, electron tomography reveals that the nucleation is preceded by more pronounced phase separation of the blend components.

19.
Adv Mater ; 27(5): 897-902, 2015 Feb 04.
Artigo em Inglês | MEDLINE | ID: mdl-25504254

RESUMO

Fullerenes are shown to be efficient voltage-stabilizers for polyethylene, i.e., additives that increase the dielectric strength of the insulation material. Such compounds are highly sought-after because their use in power-cable insulation may considerably enhance the transmission efficiency of tomorrow's power grids. On a molal basis, fullerenes are the most efficient voltage stabilizers reported to date.


Assuntos
Fontes de Energia Elétrica , Fulerenos , Impedância Elétrica
20.
Phys Chem Chem Phys ; 16(45): 24853-65, 2014 Dec 07.
Artigo em Inglês | MEDLINE | ID: mdl-25318819

RESUMO

A series of nine donor-acceptor polymers, including three new and six polymers from previous work, have been investigated experimentally and theoretically. The investigation focuses on narrow band gaps and strong absorptions of the polymers, where experimentally determined first peak absorption energies range from 1.8 to 2.3 eV, and peak absorption coefficients vary between 19-67 L g(-1) cm(-1). An overall assessment of each polymer's light-harvesting capability is made, and related to the chemical structure. Oligomer calculations using density functional theory are extrapolated to obtain size-converged polymer properties, and found to reproduce the experimental absorption trends well. Accurate theoretical predictions of absorption energies to within 0.06 eV of experiments, and absorption strength to within 12%, are obtained through the introduction of an empirical correction scheme. The computational and experimental results provide insight for the design of polymers with efficient absorption, concerning the intrinsic properties of the constituent units and the use of bulky side-groups.

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