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Fabrication and investigation of heterojunction solar cells based on near infrared quantum dots and one dimensional nanostructures.

Gonfa, Belete Atomsa (2015). Fabrication and investigation of heterojunction solar cells based on near infrared quantum dots and one dimensional nanostructures. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 116 p.

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Solar cell technology, which harvests the solar energy and converts it to direct current electricity, is a viable alternative to provide clean and renewable energy from an abundant source to meet the energy demand of the growing world population in the future. For a solar cell technology to be applicable practically it has to compete with other sources of energy in terms of cost. Furthermore the power conversion efficiency (PCE) of a solar cell is important. Since the discovery of the photovoltaic effect in 1839 by the French scientist Alexandre- Edmond Becquerel [1], various materials and device structures have been investigated resulting in development of devices of high PCE and even commercialized for practical applications. Among figures of merit, for example, single junction devices have achieved a record PCE of 28.3 % [2] and multijunction devices (4 junctions) have achieved a PCE of 46 % [3]. The most widely commercialized single crystalline silicon solar cells have achieved a record PCE of 25.6 %. However, these solar cells are produced at very high processing cost and it is quite necessary to develop solar cells, which can be produced at lower cost and with reasonable PCE so that they can compete with other sources of energy. The solar cells investigated by a plethora of researchers so far are classified as different generations based on their cost, PCE, device characteristics, and the nature of the materials involved. While the first generation solar cells, such as single crystalline silicon solar cells, achieved high PCE, their price is high. The second generation solar cells have low cost, but their efficiency is low. The concept of the third generation solar cells is later on proposed towards making low cost and high PCE solar cells. Quantum dots (QDs) based solar cells are considered as promising third generation solar cells candidates. Among QDs synthesized by different techniques, colloidal QDs, which are synthesized in solution, are attractive for applications in solar cells because of their easy, low cost synthesis and their low temperature solution processability into solar cell devices. Moreover, they possess several attributes, which endow them high potential to achieve high PCE. QDs are strong absorbers of light due to their high extinction coefficient and thus thin layer of QDs can be used in designing solar cells. The absorption of photons by QDs can be tuned by varying their size thanks to the quantum confinement effect. Especially near infrared (NIR) QDs are very attractive as they can absorb NIR part of the solar spectrum, which is wasted by other solar cell materials, but yet comprise about 50 % of the solar spectrum, in addition to higher energy photons. QDs also offer a possibility of utilizing multiple exciton generation (MEG) and extraction of hot carriers in designing very high PCE solar cell devices and intermediate band solar cells, all of which have potential to surpass Shockley-Queisser limit for single junction solar cells. Further considering device stability, recently core-shell QDs attracted attention in solar cells due to the presence of a robust, inorganic shell on the surface. While designing solar cell devices from QDs, several factors need to be taken into consideration. In devices involving continuous film of QDs, the QDs should be closely packed for efficient mobility of charge carriers through the film and the charge carrier recombination sites should be minimized. To alleviate this problem, combining the QDs with one dimensional (1D) nanostructure to form continuous charge transport pathways is one solution in improving the performance of QD based solar cell devices. Another means of overcoming limited charge carrier diffusion in thick QD film is integrating plasmonic nanoparticles with thinner QD film to increase absorption of light, while maintaining efficient charge carrier extraction. In the first part of this work, solar cells fabricated by combining colloidal PbS and PbS/CdS coreshell QDs with rutile TiO2 nanorod arrays (2 and 4 μm long) have been investigated. The general structure of the solar cell devices investigated is fluorine doped tin oxide (FTO)/TiO2/QDs/interfacial layer/Au. Two types of QDs (PbS and PbS/CdS core-shell) and two types of interfacial layer (Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and MoO3) between the QD film and the back electrode have been investigated. It has been found out that both the processing atmosphere and the interfacial layer influence the performance of the solar cell devices. A maximum PCE of 2.14 % has been achieved under air mass (AM) 1.5 illumination with devices involving PbS/CdS core-shell QDs, 2 μm long TiO2 nanorod arrays and MoO3 interfacial layer. Moreover these devices were processed in ambient atmosphere and have shown better performance (by about 40 %) than devices involving PbS QDs processed under inert atmosphere in glove box. It has been encouraging to demonstrate the use of PbS/CdS core-shell QDs in solar cells with easy processability and better performance compared to the PbS QDs. However, there still remained room for improvement of the performance by further optimization. Accordingly, in the second part, the solar cell devices involving PbS/CdS core-shell QDs have been further optimized and a PCE as high as 4.43 % has been achieved. This PCE has been achieved by utilizing uniform sputter-deposited TiO2 seed layer on FTO glass prior to the growth of TiO2 nanorod arrays, optimizing the length of the TiO2 nanorod arrays and post deposition mild thermal annealing of the PbS/CdS core-shell QD film under inert atmosphere. The performance of the solar cell devices has been found to depend on the length of TiO2 nanorod arrays and reach maximum at optimum TiO2 nanorod array length of about 450 nm. In the third part, plasmon-enhanced bulk heterojunction (BH) solar cells involving Au nanostars incorporated into NIR PbS/CdS core-shell QD film, which was spin coated onto TiO2 nanorod arrays film on FTO glass substrate are studied. The effects of the density of the Au nanostars and their location in between the TiO2 nanorod arrays and the back electrode on the performance of the device were investigated. After optimizing the density and the location of the Au nanostars a PCE of 4.16 % has been achieved. This is about 16 % increase compared to the device without Au nanostars with a PCE of 3.59 %. The improvement in the PCE as a result of Au nanostars incorporation is mainly due to increase in the short circuit current (Jsc) (about 26 % in this case). This indicates that the presence of Au nanostars enhances the charge carriers generation by improving the absorption of photons. This was further confirmed by the enhancement of photoresponse, as evidenced by external quantum efficiency (EQE) spectra of the devices.

Type de document: Thèse
Directeur de mémoire/thèse: Ma, Dongling
Co-directeurs de mémoire/thèse: El Khakani, My Ali
Informations complémentaires: Résumé avec symboles
Mots-clés libres: points quantiques; nanostructures; nanotiges; cellules photovoltaïques
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 13 nov. 2015 20:57
Dernière modification: 18 nov. 2015 21:21
URI: http://espace.inrs.ca/id/eprint/2797

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