{"id":{"repo_id":"freiburg-diss","oai_identifier":"oai:freidok.uni-freiburg.de:1695"},"canonical_url":"https://search.dev.ndltd.org/etd/freiburg-diss/oai:freidok.uni-freiburg.de:1695","repository":{"repo_id":"freiburg-diss","name":"University of Freiburg","base_url":"https://freidok.uni-freiburg.de/oai/oai2.php"},"display":{"title":"Semiconductor nanochemistry and hybrid polymer photovoltaics","abstract":"The dissertation focuses on the synthesis, mechanism, and structural stability of semiconducting nanoparticles as well as on the development of new hybrid polymeric photovoltaic devices based on nanoparticle/polymer composites. A series of experiments was performed where highly monodisperse nanoparticles of
CdSe, CdTe, and InP were synthesized via high temperature organometallic routes. The prepared nanoparticles were characterized by powder XRD, TEM, CV, AFM, HRTEM, EDX, XPS, absorption, and photoluminescence spectroscopy. The first solar cells were produced based on CdTe nanoparticle - polymer composites.
CdSe is an important material used for a large number of applications like solar cells, LEDs, biological sensors, catalysts etc. However, the reported synthesis methods for CdSe nanoparticles were based on dimethyl cadmium as Cd source, which is explosive and pyrophoric in nature. In the present work, new high temperature organometallic synthetic methods were developed for the production of good quality CdSe nanoparticles using non pyrophoric cadmium naphthenate and cadmium stearate. A series of surfactants was tried for synthesizing these
nanoparticles starting from TOPO to a series of amines. Synthesis of CdSe of various morphologies viz. dots, rods, and various intermediate morphologies were achieved. A detailed mechanistic investigation was carried out for the factors governing the shape control of these nanoparticles. A new theory based on stericity of surfactants for the growth of these nanoparticles was proposed. The results suggest that the surfactant has dominant influence on deciding the morphology of resulting nanoparticles.
CdTe nanoparticles can be a key material for polymer photovoltaic devices due to its absorption in the NIR region. However the synthesis of CdTe nanoparticles is not as advanced as CdSe, and the first step for realization of such a cell should be to develop new synthetic strategy for a high quality of CdTe nanoparticles. CdTe nanoparticles of high quality were synthesized using Cd naphthenate as well as cadmium stearate. A series of surfactants was tried for synthesis of such nanoparticles. Shape controls of these nanoparticles were achieved using this method and various morphologies viz. dots, rods, wires, and tetrapods were stabilized. Most importantly the nanoparticles prepared using this method were having predominantly wurtzite lattice which is a metastable phase for bulk CdTe. Further experiments were carried out to check the stability of this metastable wurtzite CdTe nanoparticles using XRD. It was observed that the nanoparticles gradually changes to bulk CdTe with a gradual increase in cubic content. Finally, a pure cubic CdTe was achieved at high temperature.
The realization of first solar cells needed some fundamental property measurements like band edge positions to confirm that they satisfy the thermodynamics of charge transfer followed by evidence of charge transfer using photoluminescence quenching measurements. The band edge positions of these nanoparticles and MEH-PPV were measured using cyclic voltammetry. It was
found that the thermodynamics of charge transfer was satisfied for hybrids of CdTe nanoparticles/MEH-PPV for an active solar cell. Further evidence for charge transfer was confirmed by PL quenching measurements. After measuring the fundamental properties of CdTe nanoparticles and MEH-PPV, solar cells were fabricated using ITO as anode and Al/LiF as cathode. The as prepared solar cell had an efficiency of 0.05\\% under AM 1.5 conditions.
Furthermore, solar cells were fabricated using CdTe nanoparticles and copolymer of polyfluorene and polycarbazole and an efficiency of 0.03 \\% was achieved. The spectral response and absorption measurements suggest that the CdTe is actively involved in charge carrier generation and transport to electrodes, extending the photon harvesting to near infrared region up to around 760 nm. However, it was observed that the surface of the composite films were rough which might be responsible for low efficiency of the resulting device and needs to be improved for a future development of these type of devices. The work presented describes new synthetic methods for preparation of shape and structural controlled CdTe nanoparticles, as well as fabrication of new photovoltaic devices based on them. Further work focused on device engineering of the presented system will surely provide an improvement in power conversion efficiency.
One of the major disadvantages of using nanoparticle based solar cells is the toxicity issue of Cd compounds. Therefore, attempts were carried out to develop non toxic InP nanoparticles for photovoltaic applications. Synthesis of InP nanoparticles were carried out and the band edge positions were measured by CV. It was observed that the band edge positions of InP nanoparticles are favorably aligned for the thermodynamics of charge transfer with MEH-PPV, P3OT, PVP, and PVK. Thereby indicating that the active solar cells can be produced with
composites of InP nanoparticles and any of these polymers. However, in the present course of experiments it was not possible to assemble solar cells using these composites, but the fundamental measurements were carried out and using these measurements the non toxic hybrid polymeric solar cells are expected to be realized soon.","abstract_html":"The dissertation focuses on the synthesis, mechanism, and structural stability of semiconducting nanoparticles as well as on the development of new hybrid polymeric photovoltaic devices based on nanoparticle/polymer composites. A series of experiments was performed where highly monodisperse nanoparticles of <br>CdSe, CdTe, and InP were synthesized via high temperature organometallic routes. The prepared nanoparticles were characterized by powder XRD, TEM, CV, AFM, HRTEM, EDX, XPS, absorption, and photoluminescence spectroscopy. The first solar cells were produced based on CdTe nanoparticle - polymer composites. <br>CdSe is an important material used for a large number of applications like solar cells, LEDs, biological sensors, catalysts etc. However, the reported synthesis methods for CdSe nanoparticles were based on dimethyl cadmium as Cd source, which is explosive and pyrophoric in nature. In the present work, new high temperature organometallic synthetic methods were developed for the production of good quality CdSe nanoparticles using non pyrophoric cadmium naphthenate and cadmium stearate. A series of surfactants was tried for synthesizing these <br>nanoparticles starting from TOPO to a series of amines. Synthesis of CdSe of various morphologies viz. dots, rods, and various intermediate morphologies were achieved. A detailed mechanistic investigation was carried out for the factors governing the shape control of these nanoparticles. A new theory based on stericity of surfactants for the growth of these nanoparticles was proposed. The results suggest that the surfactant has dominant influence on deciding the morphology of resulting nanoparticles. <br> <br>CdTe nanoparticles can be a key material for polymer photovoltaic devices due to its absorption in the NIR region. However the synthesis of CdTe nanoparticles is not as advanced as CdSe, and the first step for realization of such a cell should be to develop new synthetic strategy for a high quality of CdTe nanoparticles. CdTe nanoparticles of high quality were synthesized using Cd naphthenate as well as cadmium stearate. A series of surfactants was tried for synthesis of such nanoparticles. Shape controls of these nanoparticles were achieved using this method and various morphologies viz. dots, rods, wires, and tetrapods were stabilized. Most importantly the nanoparticles prepared using this method were having predominantly wurtzite lattice which is a metastable phase for bulk CdTe. Further experiments were carried out to check the stability of this metastable wurtzite CdTe nanoparticles using XRD. It was observed that the nanoparticles gradually changes to bulk CdTe with a gradual increase in cubic content. Finally, a pure cubic CdTe was achieved at high temperature. <br> <br>The realization of first solar cells needed some fundamental property measurements like band edge positions to confirm that they satisfy the thermodynamics of charge transfer followed by evidence of charge transfer using photoluminescence quenching measurements. The band edge positions of these nanoparticles and MEH-PPV were measured using cyclic voltammetry. It was <br>found that the thermodynamics of charge transfer was satisfied for hybrids of CdTe nanoparticles/MEH-PPV for an active solar cell. Further evidence for charge transfer was confirmed by PL quenching measurements. After measuring the fundamental properties of CdTe nanoparticles and MEH-PPV, solar cells were fabricated using ITO as anode and Al/LiF as cathode. The as prepared solar cell had an efficiency of 0.05\\% under AM 1.5 conditions. <br>Furthermore, solar cells were fabricated using CdTe nanoparticles and copolymer of polyfluorene and polycarbazole and an efficiency of 0.03 \\% was achieved. The spectral response and absorption measurements suggest that the CdTe is actively involved in charge carrier generation and transport to electrodes, extending the photon harvesting to near infrared region up to around 760 nm. However, it was observed that the surface of the composite films were rough which might be responsible for low efficiency of the resulting device and needs to be improved for a future development of these type of devices. The work presented describes new synthetic methods for preparation of shape and structural controlled CdTe nanoparticles, as well as fabrication of new photovoltaic devices based on them. Further work focused on device engineering of the presented system will surely provide an improvement in power conversion efficiency. <br> <br>One of the major disadvantages of using nanoparticle based solar cells is the toxicity issue of Cd compounds. Therefore, attempts were carried out to develop non toxic InP nanoparticles for photovoltaic applications. Synthesis of InP nanoparticles were carried out and the band edge positions were measured by CV. It was observed that the band edge positions of InP nanoparticles are favorably aligned for the thermodynamics of charge transfer with MEH-PPV, P3OT, PVP, and PVK. Thereby indicating that the active solar cells can be produced with <br>composites of InP nanoparticles and any of these polymers. However, in the present course of experiments it was not possible to assemble solar cells using these composites, but the fundamental measurements were carried out and using these measurements the non toxic hybrid polymeric solar cells are expected to be realized soon.","abstract_has_math":false,"creators":["Kumar, Sandeep"],"institution":null,"degree_name":null,"degree_level":null,"degree_discipline":null,"degree_department":null,"school":null,"contributors":["Nann, Thomas"],"advisors":[],"committee_chairs":[],"committee_members":[],"year":null,"date_issued":"","date_published":null,"updated_at":"2026-07-24T02:22:22Z","subjects":["Photoelement","semiconductor nanocrystals","photovoltaics"],"languages":[],"rights":[],"rights_urls":[],"identifier_entries":[]},"links":{"outbound_url":"https://freidok.uni-freiburg.de/data/1695","outbound_label":"Repository record","outbound_source":"source_url"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor","label":"Contributor","values":["Nann, Thomas"]},{"key":"dc:creator","label":"Author","values":["Kumar, Sandeep"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:type","label":"Dc Type","values":["DoctoralThesis"]}]},{"id":"subjects_keywords","label":"Subjects and Keywords","entries":[{"key":"dc:subject","label":"Dc Subject","values":["Photoelement","semiconductor nanocrystals","photovoltaics"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description.abstract","label":"Abstract","values":["The dissertation focuses on the synthesis, mechanism, and structural stability of semiconducting nanoparticles as well as on the development of new hybrid polymeric photovoltaic devices based on nanoparticle/polymer composites. A series of experiments was performed where highly monodisperse nanoparticles of
CdSe, CdTe, and InP were synthesized via high temperature organometallic routes. The prepared nanoparticles were characterized by powder XRD, TEM, CV, AFM, HRTEM, EDX, XPS, absorption, and photoluminescence spectroscopy. The first solar cells were produced based on CdTe nanoparticle - polymer composites.
CdSe is an important material used for a large number of applications like solar cells, LEDs, biological sensors, catalysts etc. However, the reported synthesis methods for CdSe nanoparticles were based on dimethyl cadmium as Cd source, which is explosive and pyrophoric in nature. In the present work, new high temperature organometallic synthetic methods were developed for the production of good quality CdSe nanoparticles using non pyrophoric cadmium naphthenate and cadmium stearate. A series of surfactants was tried for synthesizing these
nanoparticles starting from TOPO to a series of amines. Synthesis of CdSe of various morphologies viz. dots, rods, and various intermediate morphologies were achieved. A detailed mechanistic investigation was carried out for the factors governing the shape control of these nanoparticles. A new theory based on stericity of surfactants for the growth of these nanoparticles was proposed. The results suggest that the surfactant has dominant influence on deciding the morphology of resulting nanoparticles.
CdTe nanoparticles can be a key material for polymer photovoltaic devices due to its absorption in the NIR region. However the synthesis of CdTe nanoparticles is not as advanced as CdSe, and the first step for realization of such a cell should be to develop new synthetic strategy for a high quality of CdTe nanoparticles. CdTe nanoparticles of high quality were synthesized using Cd naphthenate as well as cadmium stearate. A series of surfactants was tried for synthesis of such nanoparticles. Shape controls of these nanoparticles were achieved using this method and various morphologies viz. dots, rods, wires, and tetrapods were stabilized. Most importantly the nanoparticles prepared using this method were having predominantly wurtzite lattice which is a metastable phase for bulk CdTe. Further experiments were carried out to check the stability of this metastable wurtzite CdTe nanoparticles using XRD. It was observed that the nanoparticles gradually changes to bulk CdTe with a gradual increase in cubic content. Finally, a pure cubic CdTe was achieved at high temperature.
The realization of first solar cells needed some fundamental property measurements like band edge positions to confirm that they satisfy the thermodynamics of charge transfer followed by evidence of charge transfer using photoluminescence quenching measurements. The band edge positions of these nanoparticles and MEH-PPV were measured using cyclic voltammetry. It was
found that the thermodynamics of charge transfer was satisfied for hybrids of CdTe nanoparticles/MEH-PPV for an active solar cell. Further evidence for charge transfer was confirmed by PL quenching measurements. After measuring the fundamental properties of CdTe nanoparticles and MEH-PPV, solar cells were fabricated using ITO as anode and Al/LiF as cathode. The as prepared solar cell had an efficiency of 0.05\\% under AM 1.5 conditions.
Furthermore, solar cells were fabricated using CdTe nanoparticles and copolymer of polyfluorene and polycarbazole and an efficiency of 0.03 \\% was achieved. The spectral response and absorption measurements suggest that the CdTe is actively involved in charge carrier generation and transport to electrodes, extending the photon harvesting to near infrared region up to around 760 nm. However, it was observed that the surface of the composite films were rough which might be responsible for low efficiency of the resulting device and needs to be improved for a future development of these type of devices. The work presented describes new synthetic methods for preparation of shape and structural controlled CdTe nanoparticles, as well as fabrication of new photovoltaic devices based on them. Further work focused on device engineering of the presented system will surely provide an improvement in power conversion efficiency.
One of the major disadvantages of using nanoparticle based solar cells is the toxicity issue of Cd compounds. Therefore, attempts were carried out to develop non toxic InP nanoparticles for photovoltaic applications. Synthesis of InP nanoparticles were carried out and the band edge positions were measured by CV. It was observed that the band edge positions of InP nanoparticles are favorably aligned for the thermodynamics of charge transfer with MEH-PPV, P3OT, PVP, and PVK. Thereby indicating that the active solar cells can be produced with
composites of InP nanoparticles and any of these polymers. However, in the present course of experiments it was not possible to assemble solar cells using these composites, but the fundamental measurements were carried out and using these measurements the non toxic hybrid polymeric solar cells are expected to be realized soon.","Diese Arbeit zielt auf die Synthese, den Mechanismus der Entstehung, die
strukturelle Stabilität von Halbleiter Nanokristallen und auf die Entwicklung neuartiger hybrider Solarzellen, deren Funktionsweise auf Nanopartikel/Polymer-Kompositen beruht, ab. Es wurden Experimente durchgeführt, in denen hochgradig monodisperse Nanokristalle aus CdSe, CdTe und InP über einen organometallischen Syntheseweg hergestellt werden konnten.
Diese Nanopartikel wurden durch XRD, TEM, CV, AFM, HRTEM, EDX, XPS und durch Absorptions- bzw. Photolumineszenzspektroskopie charakterisiert. Es konnten dabei auch verschiedene leitfähige Polymere auf ihre Verwendungsmöglichkeit hin mittels CV charakterisiert werden. Aus diesen Kompositen wurden die ersten Solarzellen, die auf Basis von CdTe Nanokristall/Polymer-Kompositen gebaut werden konnten, realisiert. Die Solarzellen wurden sowohl bezüglich Strom-Spannung, als auch bezüglich ihrer spektralen Empfindlichkeit
charakterisiert.
CdSe ist ein bedeutendes Material, das für vielerlei Anwendungen, wie z.B. Solarzellen, LEDs, biologische Sensoren, Katalysatoren, usw., verwendet wird. Die bisher berichteten Synthesemethoden für CdSe Nanopartikel waren jedoch auf Dimethylcadmium, welches hochexplosiv und pyrophor ist, basiert. In dieser Arbeit wurden neue Hochtemperatursynthesemethoden für die Herstellung von qualitativ hochwertigen CdSe Nanopartikeln entwickelt, die nichtbrennbares Cadmiumnaphthenat und Cadmiumstearat beinhalten.
Es wurde beobachtet, dass mit dieser Methode qualitativ hochwertige Nanokristalle hergestellt werden können, wobei eine Reihe von Stabilisatoren, beginnend mit TOPO bis zu einer Reihe von Aminen, Verwendung finden kann. Dabei kann die Morphologie der synthetisierten Partikel von Kugeln über Stäbchen bis zu Dazwischenliegendem variiert werden. Dazu wurde eine detaillierte mechanistische Untersuchung derjenigen Faktoren, die für die Form der jeweiligen Partikel verantwortlich ist, durchgeführt. Es wird eine neuartige Theorie
vorgeschlagen, die auf dem sterischen Anspruch der jeweils verwendeten Stabilisatoren fußt. Die Ergebnisse stellen neue Vorstufen, nämlich
Cadmiumnaphthenat und Cadmiumstearat, für die Synthese von CdSe Nanopartikeln vor, und es konnte beobachtet werden, dass die Stabilisatoren einen dominanten Einfluß auf die Formgebung der erhaltenen Nanopartikel hat.
CdTe Nanopartikel können aufgrund ihrer Absorption im nahen Infrarot ein Schlüsselmaterial zur Herstellung von polymerbasierten Solarzellen darstellen. Da die Synthesen von CdTe Nanopartikeln noch nicht soweit ausgereift wie die CdSe Synthesen sind, mußte eine neue Synthesestrategie entwickelt werden, wobei sowohl Cd-Naphthenat als auch Cd-Stearat verwendet wurden, um qualitativ hochwertige Nanokristalle herzustellen. Dazu wurde auch eine Reihe verschiedener Stabilisatoren getestet. Die Kontrolle der Morphologie der Nanopartikel hinsichtlich Kugelform, sträbchenform, Drahtform und Vierfäßerform konnte etabliert werden. Die so hergestellten Nanopartikel hatten vorzugsweise Wurtzitstruktur, welche eine metastabile Phase für CdTe Volumenmaterial darstellt. Weitere Experimente, die die Stabilität dieser Wurtzitphase für CdTe mittels Röntgendiffraktometrie untersuchten, wurden ebenfalls unternommen. Es konnte gezeigt werden, dass sich die Nanokristalle eine Phasenumwandlung sowohl zu Volumenmaterial, als auch zunehmend zur kubischen Struktur unterziehen; bei hohen Temperaturen konnte eine rein kubische Phase beobachtet werden.
Bevor die ersten Solarzellen hergestellt werden konnten, mußten einige fundamentale Messungen, wie z.B. Bandkantenmessungen, durchgeführt werden, um die thermodynamische Eignung zum Ladungstransfer sicherzustellen, sowie die Erbringung des Beweises des Ladungstransfers durch Photolumineszenz-Quenchingexperimente. Die Bandkantenmessungen dieser
Nanopartikel sowie von MEH-PPV wurden mittels zyklischer Voltammetrie durchgeführt. Es wurde bewiesen, dass die Thermodynamik des Ladungstransfers von
CdTe-Nanopartikel/MEH-PPV-Kompositen für eine aktive Solarzelle ausreichend waren. Ein weiterer Beweis des Ladungstransfers wurde durch PL-Quenchingexperimente erbracht, die zeigten, dass ein Ladungstransfer vom jeweiligen Polymer zu den Nanopartikeln stattfindet.
Nach Messungen der fundamentalen Eigenschaften der CdTe Nanopartikel und MEH-PPV wurden Solarzellen, bei denen ITO als Anode und Al/LiF als Kathode benutzt wurden, hergestellt. Diese Solarzellen hatten einen Wirkungsgrad von 0.05 % unter AM 1.5 Bedingungen. Des weiteren wurden Solarzellen bestehend aus CdTe Nanopartikeln und Copolymeren wie Polyfluoren und Polycarbazol hergestellt, die einen Wirkungsgrad von 0.03 % erreichten. Die spektrale Auflösung und die absorptionsspektroskopischen Messungen lassen vermuten, dass die CdTe Nanopartikel aktiv bei der Ladungstrennung und am Ladungstransport zu den Elektroden mitwirken, wobei der Photoneneinfang auf den nahen Infrarotbereich bis hin zu 760 nm erweitert werden konnte. Es wurde festgestellt, dass die Oberflächen der Kompositfilme rauh sind, was für die geringe Effizienz des erhaltenen Gerätes verantwortlich sein könnte, wobei die Optimierung im Rahmen dieser Arbeit nicht mehr möglich war; dies könnten weitere Arbeiten belegen. Die vorgestellte Arbeit beschreibt neue Synthesemethoden für die form- und phasenkontrollierte Synthese von CdTe Nanopartikeln, wie auch die Herstellung neuer Photovoltaik Bauteile, die darauf beruhen. Weitere Arbeiten, die auf die Weiterentwicklung der Bauteile fokussiert sind, werden sicherlich eine Verbesserung des Wirkungsgrades aufzeigen können.
Einer der größten Nachteile bei der Verwendung von nanopartikelbasierenden Solarzellen ist die Giftigkeit der Cd-Komponenten. Deshalb wurden Versuche unternommen, um neue nicht- oder weniger toxische Nanopartikel für photovoltaische Anwendungen zu entwickeln. Um die Giftigkeit der CdTe Nanopartikel zu umgehen, wurden Versuche unternommen, um die Verwendungsmöglichkeit von InP Nanopartikeln auszuloten. Die Synthesen der InP
Nanopartikel wurden durchgeführt und die Bandlagen durch CV-Messungen ermittelt. Es wurde beobachtet, dass die Bandlagen der InP Nanopartikel hochgradig für die Thermodynamik des Ladungstransfers mit MEH-PPV, P3OT, PVP und PVK geeignet sind. Dadurch scheinen aktive Solarzellen mit Kompositen, bestehend aus InP Nanopartikeln und diesen Polymeren produziert werden zu können. Diese Experimente waren im Rahmen dieser Arbeit nicht mehr möglich, aber die zugrundeliegenden Messungen wurden durchgeführt, um diese nichttoxischen
Hybrid-Solarzellen baldmöglichst herstellen und untersuchen zu können."]},{"key":"dc:format.medium","label":"Dc Format Medium","values":["application/pdf"]},{"key":"dc:title","label":"Title","values":["Semiconductor nanochemistry and hybrid polymer photovoltaics","Halbleiter Nanochemie und hybrid-polimere Solarzellen"]}]}],"canonical_facts":{"dc:contributor":["Nann, Thomas"],"dc:creator":["Kumar, Sandeep"],"dc:description.abstract":["The dissertation focuses on the synthesis, mechanism, and structural stability of semiconducting nanoparticles as well as on the development of new hybrid polymeric photovoltaic devices based on nanoparticle/polymer composites. A series of experiments was performed where highly monodisperse nanoparticles of
CdSe, CdTe, and InP were synthesized via high temperature organometallic routes. The prepared nanoparticles were characterized by powder XRD, TEM, CV, AFM, HRTEM, EDX, XPS, absorption, and photoluminescence spectroscopy. The first solar cells were produced based on CdTe nanoparticle - polymer composites.
CdSe is an important material used for a large number of applications like solar cells, LEDs, biological sensors, catalysts etc. However, the reported synthesis methods for CdSe nanoparticles were based on dimethyl cadmium as Cd source, which is explosive and pyrophoric in nature. In the present work, new high temperature organometallic synthetic methods were developed for the production of good quality CdSe nanoparticles using non pyrophoric cadmium naphthenate and cadmium stearate. A series of surfactants was tried for synthesizing these
nanoparticles starting from TOPO to a series of amines. Synthesis of CdSe of various morphologies viz. dots, rods, and various intermediate morphologies were achieved. A detailed mechanistic investigation was carried out for the factors governing the shape control of these nanoparticles. A new theory based on stericity of surfactants for the growth of these nanoparticles was proposed. The results suggest that the surfactant has dominant influence on deciding the morphology of resulting nanoparticles.
CdTe nanoparticles can be a key material for polymer photovoltaic devices due to its absorption in the NIR region. However the synthesis of CdTe nanoparticles is not as advanced as CdSe, and the first step for realization of such a cell should be to develop new synthetic strategy for a high quality of CdTe nanoparticles. CdTe nanoparticles of high quality were synthesized using Cd naphthenate as well as cadmium stearate. A series of surfactants was tried for synthesis of such nanoparticles. Shape controls of these nanoparticles were achieved using this method and various morphologies viz. dots, rods, wires, and tetrapods were stabilized. Most importantly the nanoparticles prepared using this method were having predominantly wurtzite lattice which is a metastable phase for bulk CdTe. Further experiments were carried out to check the stability of this metastable wurtzite CdTe nanoparticles using XRD. It was observed that the nanoparticles gradually changes to bulk CdTe with a gradual increase in cubic content. Finally, a pure cubic CdTe was achieved at high temperature.
The realization of first solar cells needed some fundamental property measurements like band edge positions to confirm that they satisfy the thermodynamics of charge transfer followed by evidence of charge transfer using photoluminescence quenching measurements. The band edge positions of these nanoparticles and MEH-PPV were measured using cyclic voltammetry. It was
found that the thermodynamics of charge transfer was satisfied for hybrids of CdTe nanoparticles/MEH-PPV for an active solar cell. Further evidence for charge transfer was confirmed by PL quenching measurements. After measuring the fundamental properties of CdTe nanoparticles and MEH-PPV, solar cells were fabricated using ITO as anode and Al/LiF as cathode. The as prepared solar cell had an efficiency of 0.05\\% under AM 1.5 conditions.
Furthermore, solar cells were fabricated using CdTe nanoparticles and copolymer of polyfluorene and polycarbazole and an efficiency of 0.03 \\% was achieved. The spectral response and absorption measurements suggest that the CdTe is actively involved in charge carrier generation and transport to electrodes, extending the photon harvesting to near infrared region up to around 760 nm. However, it was observed that the surface of the composite films were rough which might be responsible for low efficiency of the resulting device and needs to be improved for a future development of these type of devices. The work presented describes new synthetic methods for preparation of shape and structural controlled CdTe nanoparticles, as well as fabrication of new photovoltaic devices based on them. Further work focused on device engineering of the presented system will surely provide an improvement in power conversion efficiency.
One of the major disadvantages of using nanoparticle based solar cells is the toxicity issue of Cd compounds. Therefore, attempts were carried out to develop non toxic InP nanoparticles for photovoltaic applications. Synthesis of InP nanoparticles were carried out and the band edge positions were measured by CV. It was observed that the band edge positions of InP nanoparticles are favorably aligned for the thermodynamics of charge transfer with MEH-PPV, P3OT, PVP, and PVK. Thereby indicating that the active solar cells can be produced with
composites of InP nanoparticles and any of these polymers. However, in the present course of experiments it was not possible to assemble solar cells using these composites, but the fundamental measurements were carried out and using these measurements the non toxic hybrid polymeric solar cells are expected to be realized soon.","Diese Arbeit zielt auf die Synthese, den Mechanismus der Entstehung, die
strukturelle Stabilität von Halbleiter Nanokristallen und auf die Entwicklung neuartiger hybrider Solarzellen, deren Funktionsweise auf Nanopartikel/Polymer-Kompositen beruht, ab. Es wurden Experimente durchgeführt, in denen hochgradig monodisperse Nanokristalle aus CdSe, CdTe und InP über einen organometallischen Syntheseweg hergestellt werden konnten.
Diese Nanopartikel wurden durch XRD, TEM, CV, AFM, HRTEM, EDX, XPS und durch Absorptions- bzw. Photolumineszenzspektroskopie charakterisiert. Es konnten dabei auch verschiedene leitfähige Polymere auf ihre Verwendungsmöglichkeit hin mittels CV charakterisiert werden. Aus diesen Kompositen wurden die ersten Solarzellen, die auf Basis von CdTe Nanokristall/Polymer-Kompositen gebaut werden konnten, realisiert. Die Solarzellen wurden sowohl bezüglich Strom-Spannung, als auch bezüglich ihrer spektralen Empfindlichkeit
charakterisiert.
CdSe ist ein bedeutendes Material, das für vielerlei Anwendungen, wie z.B. Solarzellen, LEDs, biologische Sensoren, Katalysatoren, usw., verwendet wird. Die bisher berichteten Synthesemethoden für CdSe Nanopartikel waren jedoch auf Dimethylcadmium, welches hochexplosiv und pyrophor ist, basiert. In dieser Arbeit wurden neue Hochtemperatursynthesemethoden für die Herstellung von qualitativ hochwertigen CdSe Nanopartikeln entwickelt, die nichtbrennbares Cadmiumnaphthenat und Cadmiumstearat beinhalten.
Es wurde beobachtet, dass mit dieser Methode qualitativ hochwertige Nanokristalle hergestellt werden können, wobei eine Reihe von Stabilisatoren, beginnend mit TOPO bis zu einer Reihe von Aminen, Verwendung finden kann. Dabei kann die Morphologie der synthetisierten Partikel von Kugeln über Stäbchen bis zu Dazwischenliegendem variiert werden. Dazu wurde eine detaillierte mechanistische Untersuchung derjenigen Faktoren, die für die Form der jeweiligen Partikel verantwortlich ist, durchgeführt. Es wird eine neuartige Theorie
vorgeschlagen, die auf dem sterischen Anspruch der jeweils verwendeten Stabilisatoren fußt. Die Ergebnisse stellen neue Vorstufen, nämlich
Cadmiumnaphthenat und Cadmiumstearat, für die Synthese von CdSe Nanopartikeln vor, und es konnte beobachtet werden, dass die Stabilisatoren einen dominanten Einfluß auf die Formgebung der erhaltenen Nanopartikel hat.
CdTe Nanopartikel können aufgrund ihrer Absorption im nahen Infrarot ein Schlüsselmaterial zur Herstellung von polymerbasierten Solarzellen darstellen. Da die Synthesen von CdTe Nanopartikeln noch nicht soweit ausgereift wie die CdSe Synthesen sind, mußte eine neue Synthesestrategie entwickelt werden, wobei sowohl Cd-Naphthenat als auch Cd-Stearat verwendet wurden, um qualitativ hochwertige Nanokristalle herzustellen. Dazu wurde auch eine Reihe verschiedener Stabilisatoren getestet. Die Kontrolle der Morphologie der Nanopartikel hinsichtlich Kugelform, sträbchenform, Drahtform und Vierfäßerform konnte etabliert werden. Die so hergestellten Nanopartikel hatten vorzugsweise Wurtzitstruktur, welche eine metastabile Phase für CdTe Volumenmaterial darstellt. Weitere Experimente, die die Stabilität dieser Wurtzitphase für CdTe mittels Röntgendiffraktometrie untersuchten, wurden ebenfalls unternommen. Es konnte gezeigt werden, dass sich die Nanokristalle eine Phasenumwandlung sowohl zu Volumenmaterial, als auch zunehmend zur kubischen Struktur unterziehen; bei hohen Temperaturen konnte eine rein kubische Phase beobachtet werden.
Bevor die ersten Solarzellen hergestellt werden konnten, mußten einige fundamentale Messungen, wie z.B. Bandkantenmessungen, durchgeführt werden, um die thermodynamische Eignung zum Ladungstransfer sicherzustellen, sowie die Erbringung des Beweises des Ladungstransfers durch Photolumineszenz-Quenchingexperimente. Die Bandkantenmessungen dieser
Nanopartikel sowie von MEH-PPV wurden mittels zyklischer Voltammetrie durchgeführt. Es wurde bewiesen, dass die Thermodynamik des Ladungstransfers von
CdTe-Nanopartikel/MEH-PPV-Kompositen für eine aktive Solarzelle ausreichend waren. Ein weiterer Beweis des Ladungstransfers wurde durch PL-Quenchingexperimente erbracht, die zeigten, dass ein Ladungstransfer vom jeweiligen Polymer zu den Nanopartikeln stattfindet.
Nach Messungen der fundamentalen Eigenschaften der CdTe Nanopartikel und MEH-PPV wurden Solarzellen, bei denen ITO als Anode und Al/LiF als Kathode benutzt wurden, hergestellt. Diese Solarzellen hatten einen Wirkungsgrad von 0.05 % unter AM 1.5 Bedingungen. Des weiteren wurden Solarzellen bestehend aus CdTe Nanopartikeln und Copolymeren wie Polyfluoren und Polycarbazol hergestellt, die einen Wirkungsgrad von 0.03 % erreichten. Die spektrale Auflösung und die absorptionsspektroskopischen Messungen lassen vermuten, dass die CdTe Nanopartikel aktiv bei der Ladungstrennung und am Ladungstransport zu den Elektroden mitwirken, wobei der Photoneneinfang auf den nahen Infrarotbereich bis hin zu 760 nm erweitert werden konnte. Es wurde festgestellt, dass die Oberflächen der Kompositfilme rauh sind, was für die geringe Effizienz des erhaltenen Gerätes verantwortlich sein könnte, wobei die Optimierung im Rahmen dieser Arbeit nicht mehr möglich war; dies könnten weitere Arbeiten belegen. Die vorgestellte Arbeit beschreibt neue Synthesemethoden für die form- und phasenkontrollierte Synthese von CdTe Nanopartikeln, wie auch die Herstellung neuer Photovoltaik Bauteile, die darauf beruhen. Weitere Arbeiten, die auf die Weiterentwicklung der Bauteile fokussiert sind, werden sicherlich eine Verbesserung des Wirkungsgrades aufzeigen können.
Einer der größten Nachteile bei der Verwendung von nanopartikelbasierenden Solarzellen ist die Giftigkeit der Cd-Komponenten. Deshalb wurden Versuche unternommen, um neue nicht- oder weniger toxische Nanopartikel für photovoltaische Anwendungen zu entwickeln. Um die Giftigkeit der CdTe Nanopartikel zu umgehen, wurden Versuche unternommen, um die Verwendungsmöglichkeit von InP Nanopartikeln auszuloten. Die Synthesen der InP
Nanopartikel wurden durchgeführt und die Bandlagen durch CV-Messungen ermittelt. Es wurde beobachtet, dass die Bandlagen der InP Nanopartikel hochgradig für die Thermodynamik des Ladungstransfers mit MEH-PPV, P3OT, PVP und PVK geeignet sind. Dadurch scheinen aktive Solarzellen mit Kompositen, bestehend aus InP Nanopartikeln und diesen Polymeren produziert werden zu können. Diese Experimente waren im Rahmen dieser Arbeit nicht mehr möglich, aber die zugrundeliegenden Messungen wurden durchgeführt, um diese nichttoxischen
Hybrid-Solarzellen baldmöglichst herstellen und untersuchen zu können."],"dc:format.medium":["application/pdf"],"dc:subject":["Photoelement","semiconductor nanocrystals","photovoltaics"],"dc:title":["Semiconductor nanochemistry and hybrid polymer photovoltaics","Halbleiter Nanochemie und hybrid-polimere Solarzellen"],"dc:type":["DoctoralThesis"]},"updated_at":"2026-07-24T02:22:22Z"}