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Semiconducting Polymer Nanofibers and Quantum Dot based Nanocomposites for Optoelectronic Applications
KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Functional Materials, FNM. (Functional Materials Division)
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Nanostructured materials have attracted a broad interest in various technologies such as optoelectronics. In this thesis, nanostructured semiconductor nanocrystals, including inorganic and organic materials, were fabricated by solution based methods. The reaction conditions were optimized to control the size and morphology of the obtained nanocrystals. The optical and photoelectric properties of nanocrystals were evaluated for potential optoelectronic applications.

Colloidal CdSe quantum dots (QDs) were synthesized via thermolysis method and layers of CdS was further grown on the core CdSe QDs to form a core-shell heterostructure quantum dots (HQDs). The optical properties of HQDs were evaluated and showed the characteristics of quasi-type-II alignment of energy levels, which has potential for excitonic solar cell (XSC) application.

Nanofibers of the semiconducting polymer poly-(3-hexylthiophene) (P3HT) were synthesized via a modified whisker method. In order to control the size (both the length and the diameter) of nanofibers, we systematically studied the ratio between mixture solvents and the solute concentration. In addition, the degradation processes of P3HT nanofibers on different substrates under various environments were investigated. We found that the degradation of P3HT nanofibers can be effectively suppressed by using the substrate of higher conductivity. A nanocomposite consisting of HQDs and P3HT nanofibers was fabricated and its photoelectric properties were evaluated by I-V measurements. A ‘turn-on’ voltage was found and revealed the localization of excited holes within the HQDs, which confirmed the quasi-type-II alignment between core and shell energy levels.

In addition, we aligned the P3HT nanofibers by applying the external electric field. Alternating current (AC) and direct current (DC) induced alignments of P3HT nanofibers were investigated respectively to study the effects of different electric fields on the alignment behavior. It was determined that the AC electric field allowed a better alignment of nanofibers. Moreover, two different lengths of P3HT nanofibers were aligned and their absorption spectra were measured. Under polarized light beams, we observed a better aligned pattern in the case of longer nanofibers, shown as a higher dichroic ratio calculated from optical absorption spectra. These aligned nanofibers may find applications in optoelectronic devices.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. , 57 p.
National Category
Nano Technology
Research subject
Physics
Identifiers
URN: urn:nbn:se:kth:diva-187255ISBN: 978-91-7595-946-7OAI: oai:DiVA.org:kth-187255DiVA: diva2:929478
Public defence
2016-06-15, Sal C, Electrum, Isafjordsgatan 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20160519

Available from: 2016-05-19 Created: 2016-05-18 Last updated: 2016-05-20Bibliographically approved
List of papers
1. Synthesis of tetrahedral quasi-type-II CdSe-CdS core-shell quantum dots
Open this publication in new window or tab >>Synthesis of tetrahedral quasi-type-II CdSe-CdS core-shell quantum dots
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2011 (English)In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 22, no 42, 425202- p.Article in journal (Refereed) Published
Abstract [en]

Synthesis of colloidal nanocrystals of II-VI semiconductor materials has been refined in recent decades and their size dependent optoelectronic properties have been well established. Here we report a facile synthesis of CdSe-CdS core-shell heterostructures using a two-step hot injection process. Red-shifts in absorption and photoluminescence spectra show that the obtained quantum dots have quasi-type-II alignment of energy levels. The obtained nanocrystals have a heterostructure with a large and highly faceted tetrahedral CdS shell grown epitaxially over a spherical CdSe core. The obtained morphology as well as high resolution electron microscopy confirms that the tetrahedral shell have a zinc blende crystal structure. A phenomenological mechanism for the growth and morphology of the nanocrystals is discussed.

National Category
Nano Technology
Identifiers
urn:nbn:se:kth:diva-95416 (URN)10.1088/0957-4484/22/42/425202 (DOI)000295467800003 ()2-s2.0-80053281966 (ScopusID)
Note
QC 20120524Available from: 2012-05-24 Created: 2012-05-24 Last updated: 2016-05-19Bibliographically approved
2. Size-tuneable synthesis of photoconducting poly-(3-hexylthiophene) nanofibres and nanocomposites
Open this publication in new window or tab >>Size-tuneable synthesis of photoconducting poly-(3-hexylthiophene) nanofibres and nanocomposites
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2012 (English)In: Physica Status Solidi. C, Current topics in solid state physics, ISSN 1610-1634, E-ISSN 1610-1642, Vol. 9, no 7, 1546-1550 p.Article in journal (Refereed) Published
Abstract [en]

Poly-(3-hexylthiophene) (P3HT) has been applied in many fields such as organic solar cells, printed electronic circuits, due to superior semiconducting properties compared to other semiconducting polymers. The presence of p-p interaction causes regio-regular P3HT to form ordered lamellar stacks during crystallisation. Here we report a simple room temperature, solution based method to synthesise P3HT nanofibres with controllable sizes. Our method is based on differing solubility of P3HT in various solvents. In a mixed solvent environment, we could control the precipitation of P3HT to obtain nanofibres with various diameters by varying the ratios of the solvents. We found that the lengths of the nanofibres could be controlled with concentration of the solution. Other methods to obtain nanofibres of P3HT invariably involves heating and controlled cooling which makes reproducibility and morphology control difficult. Furthermore, we synthesised a nanocomposite consisting of P3HT nanofibres and quasi-type-II quantum dots and evaluated the photoelectric properties of the nanofibres as well as the nanocomposites using interdigitated gold microelectrodes.

Keyword
P3HT, nanofibres, quantum dots, nanocomposites
National Category
Nano Technology
Identifiers
urn:nbn:se:kth:diva-95407 (URN)10.1002/pssc.201100540 (DOI)000306479300006 ()2-s2.0-84863997565 (ScopusID)
Conference
16th International Semiconducting and Insulating Materials Conference (SIMC-XVI) Location: Royal Inst Technol (KTH), Stockholm, Sweden Date: JUN 19-23, 2011
Note

QC 20121221

Available from: 2012-05-24 Created: 2012-05-24 Last updated: 2016-05-19Bibliographically approved
3. Relaxation is the key to longer life: suppressed degradation of P3HT films on conductive substrates
Open this publication in new window or tab >>Relaxation is the key to longer life: suppressed degradation of P3HT films on conductive substrates
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2014 (English)In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 2, no 33, 13270-13276 p.Article in journal (Refereed) Published
Abstract [en]

Here we show the dependence of the degree of degradation of poly-3-hexylthiophene (P3HT) films on the conductivity of the supporting substrate. P3HT is widely used for organic solar cells and electronic devices because it allows simple, low cost fabrication and has potential for the fabrication of flexible devices. However, P3HT is known to have a relatively low photostability, and investigating the photodegradation mechanism is an active research field. We find that P3HT films on conductive substrates show significantly retarded degradation and retain their chemical and morphological features when compared to similar films on glass substrates. This 'substrate effect' in retarding the degradation of P3HT films is evident even upon prolonged exposure to air for up to five months.

National Category
Other Materials Engineering Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-150905 (URN)10.1039/c4ta02345e (DOI)000340514500011 ()2-s2.0-84905169082 (ScopusID)
Funder
Swedish Foundation for Strategic Research , EM11-0002
Note

QC 20140915

Available from: 2014-09-15 Created: 2014-09-11 Last updated: 2016-05-19Bibliographically approved
4. Direct Determination of Spatial Localization of Carriers in CdSe-CdS Quantum Dots
Open this publication in new window or tab >>Direct Determination of Spatial Localization of Carriers in CdSe-CdS Quantum Dots
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2015 (English)In: Journal of Nanomaterials, ISSN 1687-4110, E-ISSN 1687-4129, 321354Article in journal (Refereed) Published
Abstract [en]

Colloidal quantum dots (QDs) have gained significant attention due to their tunable band gap, simple solution processability, ease of scale-up, and low cost. By carefully choosing the materials, core-shell heterostructure QDs (HQDs) can be further synthesized with a controlled spatial spread of wave functions of the excited electrons and holes for various applications. Many investigations have been done to understand the exciton dynamics by optical characterizations. However, these spectroscopic data demonstrate that the spatial separation of the excitons cannot distinguish the distribution of excited electrons and holes. In this work, we report a simple and directmethod to determine the localized holes and delocalized electrons in HQDs. The quasi-type-II CdSe-CdS core-shell QDs were synthesized via a thermolysis method. Poly(3-hexylthiophene) (P3HT) nanofiber and ZnO nanorods were selected as hole and electron conductor materials, respectively, and were combined with HQDs to form two different nanocomposites. Photoelectrical properties were evaluated under different environments via a quick and facile characterization method, confirming that the electrons in the HQDs were freely accessible at the surface of the nanocrystal, while the holes were confined within the CdSe core.

National Category
Nano Technology
Identifiers
urn:nbn:se:kth:diva-174001 (URN)10.1155/2015/321354 (DOI)000360743400001 ()2-s2.0-84941255127 (ScopusID)
Funder
Swedish Foundation for Strategic Research , EM11-0002Swedish Research Council, VR-SRL 2013-6780
Note

QC 20150930

Available from: 2015-09-30 Created: 2015-09-24 Last updated: 2016-05-19Bibliographically approved
5. Electric field induced optical anisotropy of P3HT nanofibers in a liquid solution
Open this publication in new window or tab >>Electric field induced optical anisotropy of P3HT nanofibers in a liquid solution
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2015 (English)In: Optical Materials Express, ISSN 2159-3930, E-ISSN 2159-3930, Vol. 5, no 11, 2642-2647 p.Article in journal (Refereed) Published
Abstract [en]

The nanofiber morphology of regioregular Poly-3- hexylthiophene (P3HT) is a 1D crystalline structure organized by π - π stacking of the backbone chains. In this study, we report the impact of electric field on the orientation and optical properties of P3HT nanofibers dispersed in liquid solution. We demonstrate that alternating electric field aligns nanofibers, whereas static electric field forces them to migrate towards the cathode. The alignment of nanofibers introduces anisotropic optical properties, which can be dynamically manipulated until the solvent has evaporated. Time resolved spectroscopic measurements revealed that the electro-optical response time decreases significantly with the magnitude of applied electric field. Thus, for electric field 1.3 V ·μm-1 the response time was measured as low as 20 ms, while for 0.65 V ·μm-1 it was 110-150 ms. Observed phenomenon is the first mention of P3HT supramolecules associated with electrooptical effect. Proposed method provides real time control over the orientation of nanofibers, which is a starting point for a novel practical implementation. With further development P3HT nanofibers can be used individually as an anisotropic solution or as an active component in a guest-host system.

Place, publisher, year, edition, pages
Optical Society of America, 2015
Keyword
Anisotropy, Electric fields, Nanofibers, Real time control, Alternating electric field, Anisotropic optical properties, Crystalline structure, Electric field induced, Electrooptical response, Poly-3-hexylthiophene, Spectroscopic measurements, Static electric fields, Optical properties
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-181212 (URN)10.1364/OME.5.002642 (DOI)000364467700027 ()2-s2.0-84947753945 (ScopusID)
Note

QC 20160210

Available from: 2016-02-10 Created: 2016-01-29 Last updated: 2016-05-19Bibliographically approved
6. Electrical Field Induced Alignment of P3HT Nanofibers
Open this publication in new window or tab >>Electrical Field Induced Alignment of P3HT Nanofibers
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Abstract: Poly 3-hexylthiophene (P3HT) is one of the most studied conjugated polymers for organic solar cell applications due to its light weight, flexible processing methods and low cost fabrication. However, the hole mobility in P3HT is still relatively low compared to that of the inorganic semiconductors, which is one of the main challenges to achieve better performance of organic solar cells. The P3HT nanofibers with aligned by inducing an external electric field have been studied to improve the hole mobility in P3HT nanofibers. Here we present an AC electric field (1.3 V/µm, 50 Hz) induced alignment of P3HT nanofibers with two different lengths. The optical absorption spectra of aligned nanofibers were measured under different polarizations of incident light. The longer nanofibers showed higher dichroic raitos than that of shorter nanofibers, revealing a better alignment pattern. The photoconductivity of non-aligned and aligned P3HT nanofibers were measured and compared, where the aligned P3HT nanofibers showed a ~270% higher dark current than that of non-aligned sample. Moreover, the current measured under the illumination showed ~110% enhancement in the aligned P3HT nanofibers while only ~70% enhancement was obseved in non-aligned nanofibers, revealing that the alignment process have the potential to improve the mobility for optoelectronic applications. 

Keyword
P3HT nanofibers, electric field induced alignment, J-V measurement
National Category
Nano Technology
Identifiers
urn:nbn:se:kth:diva-187315 (URN)
Note

QC 20160520

Available from: 2016-05-19 Created: 2016-05-19 Last updated: 2016-05-20Bibliographically approved

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