Fabrication of solar cells from poly(3-hexylthiophene) and ZnO Nanostructures

Mestrado em Ciência dos MateriaisAs células fotovoltaicas à base de compostos orgânicos e de híbridos do tipo orgânico/inorgânico têm recebido bastante atenção devido à sua potencial aplicação como fonte de energia limpa e económica. A utilização de nanoestruturas neste tipo de dispositivos tem também recebido especial atenção já que o confinamento quântico a elas associado promove a percolação, facilitando a passagem dos portadores de carga o que aumenta a sua eficiência. Nesta tese foram preparados dispositivos fotovoltaicos “bulk heterojunction” através da mistura de poli(3-hexiltiofeno) com diferentes nanoestruturas de ZnO. As nanoestruturas de ZnO (nanopartículas, nanofios e naofibras) foram preparadas por diferentes técnicas e caracterizadas por XRD, espectroscopia no UV-Vis, SEM e TEM. As nanopartículas e os nanofios de ZnO foram preparadas por métodos químicos em solução e decomposição térmica de acetato de zinco dihidratado respectivamente. As naonofibras de ZnO foram preparadas por calcinação de nanofibras compostas por alcóol polivinílico e acetato de zinco preparadas por “electrospinning”. As nanoestruturas preparadas foram ainda funcionalizadas com o ácido pireno-1-carboxílico. As nanoestruturas preparadas, funcionalizadas ou não funcionalizadas, foram misturadas com soluções de P3HT de modo a preparar dispositivos fotovoltáticos em duas configurações distintas. Numa delas os eléctrodos consistem em ITO e o alumínio depositado por evaporação térmica, na outra, os eléctrodos consistem em ITO e tinta de prata. O primeiro tipo de configuração utilizou a seguinte sequência: vidro/ITO/PEDOT:PSS/camada fotoactiva/Al. Na segunda configuração a sequência utilizada foi: vidro/ITO/ZnO/ camada fotoactiva/ PEDOT:PSS/Ag. As camadas de PEDOT:PSS bem como as camadas fotoactivas foram depositadas por spin coating. A caracterização dos dispositivos foi feita através de medições da corrente-tensão sob condições simuladas de iluminação padrão. Os dispositivos preparados apresentaram actividade fotovoltaica mas a sua eficiência ainda precisa de ser melhorada. ABSTRACT: Organic and organic/inorganic hybrid solar cells have been receiving a significant amount of attention due to their potential to yield environmentally friendly and cheap source of energy. As a result they are being investigated widely. Making use of nanostructures in such devices has also received a great attention as they provide percolative pathways for charge carriers by quantum confinement, helping in the improvement of the efficiency of the devices. In this thesis bulk heterojunction photovoltaic devices have been produced by blending different ZnO nanostructures and surface functionalized ZnO nanostructures with poly- 3-hexylthiophene. ZnO nanostructures (nanoparticles, nanowires and nanofibers) have been produced by different techniques and characterized by XRD, UV-Visible spectroscopy and SEM. ZnO nanoparticles and ZnO nanowires were prepared by wet chemical synthesis and thermal decomposition of zinc acetate dihydrate respectively. ZnO nanofibers were prepared by calcination of polyvinyl alcohol/zinc acetate composite nanofiber, which had been produced by the electrospinning process. These nanostructures were also surface functionalized with pyrene-1-carboxylic acid and characterized. Subsequently, these nanostructures and their surface functionalized forms were used to fabricate photovoltaic devices by combining them with P3HT and its whiskers. The photovoltaic devices have been prepared in two different configurations. In some ITO and aluminium deposited by thermal evaporation were used as the electrodes, while in the others ITO and silver paste were used. The first set of devices had the order glass/ITO/PEDOT:PSS/photoactive layer/Al, while the latter had the order lass/ITO/ZnO/photoactive layer/ PEDOT:PSS/Ag paste. The PEDOT:PSS and the photoactive layers were deposited by spin coating of the suspension of PEDOT:PSS in water and the suspension of the ZnO nanostructures in the poly-3-hexylthiophene solution respectively. The photovoltaic cells were finally characterized by current-voltage characteristics measurement under simulated standard illumination conditions. The photovoltaic devices prepared have demonstrated photovoltaic properties, but their efficiencies need further improvement

Investigating photoinduced charge transfer in double- and single-emission PbS@CdS core@shell quantum dots.

International audienceWe present for the first time detailed investigation of the charge transfer behavior of PbS@CdS core@shell quantum dots (QDs) showing either a single emission peak from the core or intriguing double emission peaks from the core and shell, respectively. A highly non-concentric core@shell structure model was proposed to explain the origin of double emissions from monodisperse QDs. Their charge transfer behavior was investigated by monitoring photoluminescence (PL) intensity variation with the introduction of electron or hole scavengers. It was found that the PL quenching of the PbS core is more efficient than that of the CdS shell, suggesting more efficient charge transfer from the core to scavengers, although the opposite was expected. Further measurements of the PL lifetime followed by wave function calculations disclosed that the time scale is the critical factor explaining the more efficient charge transfer from the core than from the shell. The charge transfer behavior was also examined on a series of single-emission core@shell QDs with either different core sizes or different shell thicknesses and dominant factors were identified. Towards photovoltaic applications, these PbS@CdS QDs were attached onto multi-walled carbon nanotubes (MWCNTs) and their charge transfer behavior was compared with that in the PbS-QD/MWCNT system. Results demonstrate that although the CdS shell serves as an electron transfer barrier, the electrons excited in the PbS cores can still be transferred into the MWCNTs efficiently when the shell thickness is ∼0.7 nm. Considering their higher stability, these core@shell QDs are very promising for the development of highly efficient QD-based photovoltaic devices