Semiconductor nanochemistry and hybrid polymer photovoltaics

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.

Author dc:creator

• Kumar, Sandeep

Contributors dc:contributor

• Nann, Thomas

• Photoelement
• semiconductor nanocrystals
• photovoltaics

Repository record source_url

https://freidok.uni-freiburg.de/data/1695

OAI identifier oai:identifier

oai:freidok.uni-freiburg.de:1695