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Investigations on semiconductor material synthesis and on morphology manipulation towards high efficiency and stable solar technologies.

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Tan, Long (2017). Investigations on semiconductor material synthesis and on morphology manipulation towards high efficiency and stable solar technologies. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 204 p.

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Résumé

Solar cells, which harvest renewable solar energy via the photovoltaic (PV) effect, have been considered as one of the cleanest and most promising technologies to address the press energy crisis and environmental issues. In order to make solar cells the feasible and best replacement of fossil fuels, the concept of the third-generation PV devices based on inorganic nanomaterials, dye molecules, conjugated polymers and perovskite materials was proposed to largely decrease the cost of the solar cell devices, at the same time maintaining comparable or even achieving higher power conversion efficiency (PCE) compared to that of the existing ones. Additionally, luminescent solar concentrators (LSCs), which convert a wide range of photons into concentrated light in a specific range, are another potential approach to achieve these goals by reducing the use of expensive PV materials. In principle, the development of both PV and LSC devices depends on the material chemistry and device engineering, which are the two main directions to optimise the performance of these solar technologies. In this thesis, the work performed focuses on the synthesis of near infrared (NIR) absorbing quantum dots (QDs), their characterizations, and solar-related applications, as well as morphology optimisation of the photoactive film of polymer solar cells (PSCs), leading to enhanced PCE and stabilities. In the first part, we successfully synthesized high-quality NIR PbS QDs in very small sizes using PbCl2 and elemental sulfur (S) as lead and sulfur precursor, respectively, by introducing tributylphosphine (TBP) into the reaction. This route is much “greener” and facile as compared to the glove-box-involved method using toxic bis(trimethylsilyl)sulfide as sulfur precursor. Afterwards, the synthesis mechanism of very small QDs and their optical properties, morphology, dispersity, surface properties and LSC application were systematically investigated. In detail, this work can be divided into three sections (I-III): two sections are related to the PbS QDs synthesis, and the last section is about LSC application based on these synthesized QDs. In section Ⅰ, the S-oleylamine (OLA) solution was mixed with different contents of TBP prior to its injection into the pre-heated PbCl2-OLA solution. For the TBP-free reaction, the shortest absorption wavelength of synthesized PbS QDs was limited to ~1056 nm. With increasing the TBP content to 40 μL, the first-excitonic absorption peak of obtained PbS QDs was gradually blue-shifted to ~705 nm, due to the decreased QD size. Further increasing the contents of TBP, no QDs could be collected. The key factor leading to the blue-shifted first-excitonic absorption peak and final disappearance of QDs with increasing TBP content is the formation of strong bond between S and TBP. The S-TBP could have participated into the QD growth process, which prevented the QDs further growing to larger size with TBP as a type of rigid ligands on the surface. However, the stable S-TBP itself was not able to start the nucleation reaction with the lead precursor, then failed to form the QD once the S-OLA concentration was below a critical point. In addition to decreasing the size of QDs, as extra ligands located at the S sites on the QD surface (rather than on the Pb site as OLA ligands), TBP could change the QD surface composition, morphology and dispersity, and benefit its optical properties. In brief, TBP-assisted synthesis of PbS QDs shows narrow QD size distributions without any QD aggregation and demonstrates high photoluminescent quantum yield (PL QY) in the range of 60–90%, depending on the QD size. All of these results underline that this route is convenient for synthesis of high-quality PbS QDs in very small sizes. Different from that of section Ⅰ, a two-step injection synthesis is introduced in section Ⅱ. For instance, large amounts of TBP were first injected into the preheated lead precursor, followed by the sulfur precursor solution. Based on this synthesis, the first-excitonic peak of the PbS QDs can be largely extended to ~780 nm (corresponding to diameter ~2.5 nm) as compared to that of the TBP-free reaction. The resulted PbS QDs show excellent dispersity without any aggregation and high PL QY around 80%. Moreover, the TBP chemical was revealed to assist the transformation of PbCl₂-OLA into more reactive Pb(OH)Cl, that can directly participate into the nucleation process, yielding ultrasmall PbS QDs. It indicates that Pb(OH)Cl can be potentially applied for synthesis of other lead-based small size QDs as a new precursor. In section Ⅲ, LSC devices using PbS and PbS/CdS QDs as the phosphor were fabricated, respectively, and their performance of concentrating the wide range of photons into a specific range of light was also tested. The high performance is mainly attributed to their high PL QY, wide separation of absorption and PL spectrum and good photo-stability. Clearly, ultrasmall PbS QDs are promising for LSC application. More specifically, the LSC device showed optical efficiency around 1.2% at a geometric factor of 50 (10 cm in length) by using the 2.5 nm QDs with a 0.1 nm CdS shell, which is record-high compared to other QD-based LSCs. The devices using pure PbS QDs and PbS/CdS QDs with thick shell were also fabricated and tested, and all of them showed promising results. Over all, these results confirmed the high quality of our synthesized QDs via TBP involved synthetic route, which is promising for large-area LSCs application and has high potential for other NIR-related applications. In the second part, we focused on improving the air-stability of the PSCs using the blend of Poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C₆₁-butyric acid methyl ester (PCBM) as the research system. To do so, the inorganic NIR QDs were selected as a stabilizer, which extended the photoresponse of the device to the NIR range in the same time. Considering the energy level alignment of P3HT and PCBM, PbS QDs sample with an average diameter of ~3.3 nm was preferred, which can facilitate the charge transfer processes in the device. Moreover, further surface treatments of QDs were performed before applying them for PV application. First, a thin CdS shell (0.1 nm) on the surface of PbS QDs was formed by cation-exchange reaction. It can significantly improve the air-stability of QDs as well as their thermo- and photo-stability, without blocking charge carrier transport. Second, large amounts of insulating, long-chain oleic acid ligands (OA-) on the QD surface were replaced by short inorganic Cl- via metal halide treatment. The atomic Cl- ligands can form a dense “ligand shell” on the surface and reach the mid-gap trap states, which can’t be achieved by the long-chain ligands, due to the “steric effect”. Therefore, the QD stability can be further enhanced and the density of trap states can be effectively reduced, which makes these ligand-exchanged QDs suitable for PV application. It is worth mentioning that the surface ligand manipulation also affected the morphology of the P3HT:PCBM:QDs film, which played a significant role in device stability. As evidenced by the images obtained from atomic force microscopy (AFM) measurements, the QDs with Cl- ligands formed continuous QD-networks in the P3HT:PCBM film, whereas QDs without Cl- ligands were homogeneously distributed in the film. Importantly, PSCs device based on the film with the unusual QD-networks showed excellent long-term stability under relative high humidity air (50-60%), while accomplishing over 3% of PCE simultaneously. After 30 days storage without any encapsulation, around 91% of the pristine PCE can be retained. It was a remarkable improvement as compared to that of the devices based on pure P3HT:PCBM film (~53%). We verified that such improvement was attributed to the prevention of PCBM aggregation and oxidation of the thiophene ring in P3HT. Furthermore, the presence of QD-networks efficiently improved the thermal stability of the device as well by suppressing the thermal stress/oxidation under relative high humidity air. Around 60% of pristine PCE was retained after 12 h thermal treatment at 85 °C, which was more than twice higher than that of the device without QD networks. To the best of our knowledge, this work represents the first unambiguous demonstration of the formation of QD networks in the photoactive layer and of their important contribution to stability of PSCs. This strategy is highly promising for other fullerene based PSCs, and opens a new avenue towards achieving PSCs with high PCE and excellent stability. The third part was focused on morphology manipulation of the P3HT:PCBM blend, in order to improve the PCE of the PSCs. The key point is to control the mobility of the film components in the fabrication process. To do so, butylamine was introduced as an additive into the P3HT:PCBM solution with dichlorobenzene as the solvent. It was confirmed that PCBM has a higher solubility than P3HT in butylamine. As a result of different solubilities, the addition of butylamine improved the accumulation of P3HT and PCBM at the top and bottom of the blend film, respectively, leading to a P3HT-enriched top surface and buried abundance of PCBM as evidenced by the results from AFM and X-ray photoelectron spectroscopy characterizations. Then, solar cell devices with the configuration of ITO/ZnO/P3HT:PCBM/MoO3/Ag were fabricated to evaluate the effect of resulted morphology change by butylamine on the performance of device. The butylamine affected device showed largely enhanced fill factor (FF: ~70%) and PCE (~4.03%) compared to the standard device, which can be attributed to the improved charge transport in the P3HT:PCBM film and enhanced electrode selectivity. The formation of such morphology offers a possibility to further optimize the thickness of the active layer in order to absorb more light, without prohibiting charge transport. In other words, it is possible to further enhance the PCE by increasing the short-circuit current (Jsc) without significantly sacrificing the FF, if the same preferable bicontinuous interpenetrating morphology can also be achieved in a thicker film. Therefore, we further fabricated devices based on thicker P3HT:PCBM film by doubling the concentration of precursor solution. The results from the current-voltage measurement indicate that FF of ~63% was obtained for the butylamine involved device, which is even higher than that of the standard device based on relatively thin film. In the same time, the Jsc was also largely enhanced due to the increased thickness of the active layer, leading to a high PCE of 4.61%, among the best efficiencies of P3HT:PCBM-based devices. Over all, these results well confirmed that butylamine as an additive can effectively improve the performance of P3HT:PCBM device. Besides, it has the advantages of free of malodorousness and halogen ions compared to commonly applied alkane dithiols and halogen additives, which makes it promising for the use in large scale fabrication of PSCs in the future.

Type de document: Thèse Thèse
Directeur de mémoire/thèse: Ma, Dongling
Co-directeurs de mémoire/thèse: Chaker, Mohamed
Mots-clés libres: photovoltaic devices; solar cells technologies; solar energy; photons; polymers; perovskite materials
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 03 juill. 2018 13:46
Dernière modification: 03 juill. 2018 13:46
URI: http://espace.inrs.ca/id/eprint/6933

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