| 1. |
- Geng, Xinjian, 1992-
(författare)
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Interfacial Analysis and Charge Transfer in Solar Cells
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Harnessing sunlight through solar cells is vital for sustainable energy production. The conventional architecture of a solar cell consists of multilayers of materials, each serving a particular function. The absorbing layer converts the solar energy into valuable electricity by the photovoltaic effect. Positioned on either side of the absorbing layer are charge transport layers, which collect the generated electricity. Charge transfer and recombination occur at interfaces, affecting the solar cells' open-circuit voltage (Voc). This doctoral thesis focuses on the analysis of the perovskite/n-type semiconductor interface and plasmonic/p-type semiconductor interface, provides an understanding of the charge generation and transfer in solar cells, and helps to establish a selection of the ideal charge transport layer for efficient charge extraction.Initially, photogenerated charge carriers were investigated by time- and energy-resolved photoluminescence (TER-PL) from the thin formamidinium lead-bromide (FAPbBr3) perovskite absorber film. Both exciton and free charge dynamics under various excitation power intensities were taken into consideration. Following studies on the charge transfer at interfaces were performed by incorporating n-type semiconductors (TiO2 and SnO2) as the charge transport layer. The results revealed that the TiO2 is more efficient as a fluorescence quencher due to the higher density of states in its conduction band. While conductivity measurements suggested that SnO2 was better for avoiding the accumulation of charges. Thus, solely charge extraction efficiency is not sufficient for determining the solar cell performance.To alleviate the energy losses in solar cells, extraction of hot electrons would be a viable way. Then, hot carriers generated through plasmon excitation in metal nanoparticles, which is different from semiconductors, were collected in the solid-state direct plasmonic solar cell (DPSC). Also, plasmonics show high absorption across the visible–infrared spectrum. Early progress mainly focused on hot electron transfer. The second part of the thesis investigated the effect of Li-TFSI dopant concentration in the hole transport layer (spiro-OMeTAD) on solar cell performance. 33% oxidised species (spiro+TFSI-) gave the optimal efficiency. By using transient absorption spectroscopy, the injection of hot electrons and holes was verified in a complete solar cell device as well.
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| 2. |
- Hattori, Yocefu, 1990-
(författare)
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Charge Separation on Localized Surface Plasmon and Hot Carrier Transfer to Semiconductors
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The relatively recent discovery that plasmonic nanoparticles generate energetic electron-hole pairs known as hot carriers has been the source of interest from many scientific groups. The capability to extract these short-lived hot carriers from metal nanoparticles (NPs) might potentially lead to applications in solar cells, photodetection, and photocatalysis. However, a better understanding of the hot carrier dynamics, starting from the formation process, is required. This thesis seeks to elucidate some aspects of charge formation, extraction, and hot carriers' recombination in plasmonic composite systems.First, two systems based on Ag and Au NPs were designed and studied to elucidate charge carriers' dynamics. The studies revealed that electrons and holes were effectively extracted and injected into suitable acceptors. Additionally, the electron injection and back transfer on TiO2 was significantly affected by the interface's status. The result motivated the following study that consisted of Au plasmonic NPs supported on different metal oxides, namely TiO2, ZnO, SnO2, and Al-ZnO (AZO). The electron dynamics on these systems were widely different. They could not be attributed solely to differences in the Schottky barrier height values, which suggested that interface status, electron bulk mobility, and oxide conduction band density of states are relevant factors to explain electron dynamics. The insertion of an insulator layer between the Au NPs and the metal oxides improved charge separation, which could be further explored to improve device efficiencies.In situ measurements on Au NPs/TiO2 samples were performed to investigate the effect of an increase of temperature in the range expected for device applications. This increase resulted in a higher number of electrons injected, which was attributed to the enhancement of plasmon decay by phonons.The last chapter investigates the change in the electron-phonon relaxation upon electron and hole injection, separately. Ab initio methods allowed theoretical investigation of this process and were used to predict the hole injection efficiency.
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| 3. |
- Pavliuk, Mariia
(författare)
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Accumulative Charge Separation in Photocatalysis : From Molecules to Nanoparticles
- 2019
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Konstnärligt arbete (övrigt vetenskapligt/konstnärligt)abstract
- Photochemical energy conversion into solar fuel involves steps of light absorption, charge separation and catalysis. Nature has taught us that the effective accumulation of redox equivalents and charge separation are the key steps in sunlight conversion. The focus of this thesis is to unveil photophysical and photochemical processes that lead to accumulative charge separation. The optimization of electron transfer process will be held by minimization of losses via recombination, and extension of the lifetime of the charge separated state by usage of the electron relay.The goal is to couple light induced electron transfer process with the multi-electron catalytic process of hydrogen evolution. In this regard, light harvesters (molecules, metal nanostructures) that generate at least two electrons per absorbed photon will be studied. Additionally, semiconductors that generate long-lived charge separated states are utilized to accumulate several redox equivalents necessary for hydrogen evolution.The hybrid systems produced by the combination of the advantageous properties of molecules, semiconductors, and metal nanoparticles are under the scope of investigation. Metal nanoparticles are advantageous because of their high absorption cross-section. The molecular linkers provide control and flexibility in tuning the connection between the light absorber and the electron relay. Semiconductor nanoparticles offer the desired charge separation properties via prolonging the lifetime sufficiently to perform photocatalysis.The detailed understanding, investigation and development of the hybrid systems is at the heart of the progress of photochemical solar fuel production.
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| 4. |
- Geng, Xinjian, et al.
(författare)
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Direct Plasmonic Solar Cell Efficiency Dependence on Spiro-OMeTAD Li-TFSI Content
- 2021
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Ingår i: Nanomaterials. - : MDPI. - 2079-4991. ; 11:12
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Tidskriftsartikel (refereegranskat)abstract
- The proliferation of the internet of things (IoT) and other low-power devices demands the development of energy harvesting solutions to alleviate IoT hardware dependence on single-use batteries, making their deployment more sustainable. The propagation of energy harvesting solutions is strongly associated with technical performance, cost and aesthetics, with the latter often being the driver of adoption. The general abundance of light in the vicinity of IoT devices under their main operation window enables the use of indoor and outdoor photovoltaics as energy harvesters. From those, highly transparent solar cells allow an increased possibility to place a sustainable power source close to the sensors without significant visual appearance. Herein, we report the effect of hole transport layer Li-TFSI dopant content on semi-transparent, direct plasmonic solar cells (DPSC) with a transparency of more than 80% in the 450-800 nm region. The findings revealed that the amount of oxidized spiro-OMeTAD (spiro(+)TFSI(-)) significantly modulates the transparency, effective conductance and conditions of device performance, with an optimal performance reached at around 33% relative concentration of Li-TFSI concerning spiro-OMeTAD. The Li-TFSI content did not affect the immediate charge extraction, as revealed by an analysis of electron-phonon lifetime. Hot electrons and holes were injected into the respective layers within 150 fs, suggesting simultaneous injection, as supported by the absence of hysteresis in the I-V curves. The spiro-OMeTAD layer reduces the Au nanoparticles' reflection/backscattering, which improves the overall cell transparency. The results show that the system can be made highly transparent by precise tuning of the doping level of the spiro-OMeTAD layer with retained plasmonics, large optical cross-sections and the ultrathin nature of the devices.
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| 5. |
- Nguyen, Thinh Luong The, et al.
(författare)
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Molecular Linking Selectivity on Self-Assembled Metal-Semiconductor Nano-Hybrid Systems
- 2020
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Ingår i: Nanomaterials. - : MDPI. - 2079-4991. ; 10:7
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Tidskriftsartikel (refereegranskat)abstract
- Plasmonics nanoparticles gained prominence in the last decade in fields of photonics, solar energy conversion and catalysis. It has been shown that anchoring the plasmonics nanoparticles on semiconductors via a molecular linker reduces band bending and increases hot carriers' lifetime, which is essential for the development of efficient photovoltaic devices and photocatalytic systems. Aminobenzoic acid is a commonly used linker to connect the plasmonic metal to an oxide-based semiconductor. The coordination to the oxide was established to occur via the carboxylic functional group, however, it remains unclear what type of coordination that is established with the metal site. Herein, it is demonstrated that metal is covalently bonded to the linker via the amino group, as supported by Surface-Enhanced Resonant Raman and infrared spectroscopies. The covalent linkage increases significantly the amount of silver grafted, resulting in an improvement of the system catalytic proficiency in the 4-nitrophenol (4-NP) photoreduction.
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