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Morphology control and photovcoltaic performance enhancement for non-fullerene ased organic solar cells.


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Yu, Ting (2023). Morphology control and photovcoltaic performance enhancement for non-fullerene ased organic solar cells. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 193 p.

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As people become more aware of environmental protection, the replacement of non-renewable fossil fuels is imminent to alleviate the unsustainable consequences. Solar energy is a promising candidate as an alternative to meet the increasing energy demand, which can be converted to electricity through photovoltaic devices. In 1954, Bell laboratories successfully fabricated crystalline silicon (c-Si) solar cells for the first time, and achieved an efficiency of 6%, thus opening new horizons for photovoltaic technology to harness solar energy. After the development of first- and second-generation solar cells, c-Si solar cells have reached an efficiency of 26% and realized commercial and industrial production. However, the complicated manufacturing processes of Si and inorganic film solar cells, the high cost of raw materials, and the inherent limitations of inorganic materials limit the further use of solar energy. In order to address the above issues, third-generation solar cells have been widely developed, with organic solar cells (OSCs) being one of the most promising photovoltaic devices due to their significant advantages such as low cost, flexibility, lightweight, and simple manufacturing process. The best power conversion efficiency (PCE) of single-junction OSCs has now reached over 19% in the laboratory, making them more attractive than ever for possible commercial applications.

For decades, significant efforts have been made to enhance the photovoltaic performance of OSCs through the innovation of the device configuration and the development of materials in photoactive layers, interfacial layers, and electrodes. Researchers developed three main configurations for the enhancement of photovoltaic performance, including single-layer OSCs, bilayer OSCs and bulk heterojunction (BHJ) OSCs. Among them, BHJ-OSCs are considered to be the most efficient device structure because of the largely increased contact area between the donor and acceptor materials, which favors exciton dissociation and charge transport. Moreover, BHJ-OSC configurations can be further optimized by employing a ternary blend strategy, which endows the photovoltaic efficiency of over 19%. It’s worth noting that the ideal morphology of the BHJ blend film with interpenetrating nanostructures, suitable phase separation and proper domain size is of vital importance for the achievement of state-of-the-art OSCs. The introduction of the guest components in ternary OSCs is a simple and effective approach to yield the desirable morphology. However, the ternary blend film with three components usually brings more complex morphology. Therefore, the morphology of ternary blend film requires to be characterized in depth, which helps to optimize the device performance. In addition, while breakthroughs in photoactive layer materials have led to rapid improvements in device efficiency, the contribution of interface materials to device performance cannot be ignored. Appropriate interfacial materials can also optimize the surface morphology of the photoactive layer, enhance optical absorption, promote Ohmic contact, and tune work functions of electrodes. Accordingly, some researchers have dedicated to developing interfacial materials, which further enhance photovoltaic performance. Based on the above discussion, the following three parts in this thesis provide meaningful explorations.

In the first part, in order to achieve the high-efficient OSCs, we integrated highly air-stable perylene diimide (PDI) based non-fullerene material (PDI-DPP-PDI) into OSCs as the guest acceptor. These three components exhibited cascaded energy band structure and complimentary absorption, thus facilitating the increase of photovoltage and photocurrent. Besides, the morphology of the ternary blend film was optimized by adjusting the content of the PDI-DPP-PDI. The three-dimensional nanoscale morphology was characterized in depth by photo-induced force microscopy (PiFM) coupled with infrared laser spectroscopy and energy-filtered transmission electron microscopy (EF-TEM), which could qualitatively and quantitatively “view” the surface and cross-sectional morphology. The results provided strong evidence that the PDI-DPP-PDI guest acceptor could suppress the aggregation of the fullerene molecules and generate the homogenous morphology with a higher level of the molecularly mixed phase, thus preventing the charge recombination and stabilizing the morphology of the photoactive layer. This work represented very promising approaches to deeply explore the morphology of the ternary blend film, which contributes to achieving a stable device and simultaneously improved photovoltaic performance.

In the second part, we fabricated non-fullerene based ternary OSCs by incorporating the PDI-EH non-fullerene guest acceptor. The nearly orthogonal structure of the PDI-EH molecule was designed to suppress its aggregation. In addition to obtaining broadened spectral absorption and a well-matched energy band, the PDI-EH guest acceptor serves to adjust the miscibility of the blend film, facilitating the acquisition of ‘ideal’ morphology. Flory-Huggins interaction parameters and differential scanning calorimetry (DSC) measurements revealed that the PDI-EH guest acceptor was compatible with host donor and acceptor materials so that it could fine-tune the miscibility to form an optimally intermixed phase. Selective PiFM imaging of donor and acceptor materials suggested the formation of well-mixed films after the incorporation of PDI-EH. In addition, grazing incidence wide-angle X-ray scattering (GIWAXS) measurements demonstrated that the PDI-EH could promote favorable molecular ordering, leading to effective charge transport. Lastly, the relationship between the optimized morphology and local charge carrier dynamics at the nanoscale was explored by a novel transient photo-response atomic force microscopy (TP-AFM) technique. The results demonstrated that the ternary blend film containing 10% PDI-EH showed reduced charge transport time, increased charge recombination lifetime and extended charge diffusion length, thus improving the performance of OSCs under 1-sun, and indoor 2700 K and 6500 K LED illuminations with efficiencies enhancement of 12%, 14% and 16%, respectively. This work offers insights into morphology modulation and the resulting charge carrier dynamics, thereby facilitating the development of OSCs.

In the third part, to further enhance the photovoltaic performance of non-fullerene-based OSCs for their practical applications, integrating plasmonic metallic nanoparticles as interfacial modifiers and sunlight concentrators offers an attractive pathway to optimize the surface morphology of the photoactive layer to reduce surface defects, improve sunlight capture to promote more photon harvesting, and increase interfacial contact to facilitate charge extraction. However, at present, this approach has not attracted enough attention in the field of non-fullerene OSCs. In this work, the facile polyelectrolyte polystyrene sulfonate (PSS)-coated plasmonic gold nanorods (named GNRs@PSS) were synthesized by Dr. Zhonglin Du. I firstly incorporated GNRs@PSS into the inverted non-fullerene OSCs as rear interfacial modifiers, which led to nearly 20% PCE enhancement due to the dramatically increased photocurrent. It was found that GNRs could improve sunlight absorption and exciton generation via the near-field plasmonic and back scattering effects. The electric field distribution of individual GNRs was measured by PiFM measurements under tunable laser irradiation to reveal the surface plasmon resonance (SPR) effect. Meanwhile, advanced EF-TEM measurements exhibited that the PSS organic shell was uniformly coated on the surface of GNRs with a thickness of ~2.2 nm. This ultrathin surface organic layer not only ensured the strong SPR effect but also prevented direct contact between the GNRs and the photoactive layer. As a result, the integration of plasmonic GNRs@PSS contributed to sunlight absorption, surface morphology optimization, and interfacial contact, leading to an enhancement of photovoltaic performance.

Type de document: Thèse Thèse
Directeur de mémoire/thèse: Ma, Dongling
Mots-clés libres: non-fullerene organic solar cells ; ternary blend strategy ; morphology optimization ; interfacial modification; photovoltaic performance enhancement ; advanced characterization techniques
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 20 déc. 2023 20:24
Dernière modification: 20 déc. 2023 20:24
URI: https://espace.inrs.ca/id/eprint/13812

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