| 1. |
- Kuang, Chaoyang, 1988-
(author)
-
Interface-Assisted Perovskite Modulations for High-Performance Light-Emitting Diodes
- 2021
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Doctoral thesis (other academic/artistic)abstract
- Metal halide perovskites have emerged as a class of promising materials for a wide range of optoelectronic devices. Compared with traditional inorganic and organic semiconductors, perovskite materials can be easily processed via solution-based techniques at low temperatures and exhibit high photo-luminescence efficiency, outstanding colour purity, and superior charge transport properties, showing great promise for cost-effective and high-performance light-emitting diodes (LEDs).Since the first demonstration of room-temperature operating perovskite-based LEDs (PeLEDs) in 2014, various useful strategies on optimizing perovskite emissive materials and device structures have been developed, leading to notably enhanced device performance of PeLEDs during the last several years. Nevertheless, despite rapid progress in improving the external quantum efficiencies (EQEs) of PeLEDs, which are now approaching those of commercialized technologies, the operational stability of state-of-the-art PeLEDs remains poor, presenting a critical challenge for their practical applications and commercialization. Besides, a majority of the optimization strategies demonstrated for PeLEDs derivate from those developed for either perovskite photovoltaics or prevailing light-emitting technologies, e.g., organic- and quantum-dot-based LEDs. Although these strategies are helpful, more comprehensive investigations and in-depth understanding of factors affecting the property of perovskite emissive layers and the device performance of ensuing PeLEDs are highly desirable to foster further advancements of this promising technology.In this thesis, we focus our study on near-infrared PeLEDs based on triiodide perovskite emissive layers processed from precursor solutions. We systematically investigate the critical effects of precursors, substrates, and additives on the film quality of perovskite emissive layers. With the indepth understanding of the perovskite crystallization process, we developed a range of effective interface-assisted strategies on modulating the perovskite emissive layers, which enable us to achieve PeLEDs with high EQEs and excellent long-term operational stability beyond the state-of-the-art.In the first study, we unveiled the synergistic effect of precursor stoichiometry and interfacial reactions for PeLEDs. We reveal that ZnO efficiently deprotonates the organic cations, which promotes the formation of highly emissive perovskites from precursor solution with excess organic components, leading to the achievement of PeLEDs with a high EQE of 19.6 %. In the second study, we presented that such ZnO deprotonation process of excess organic cations can also assist the cation exchange process between cesium-formamidinium (FA-Cs) cation exchange, enabling low-temperature fabrication of pure-phase Cs-FA mixed cation perovskite films with widely tunable emissions peaking between 715 nm and 800 nm as well as high-performance devices with peak EQEs over 15%.In spite of enhanced device efficiency realized by the perovskite crystallization modulation, this ZnO deprotonation process places a detrimental effect on the stability of the PeLEDs, which can be accelerated by Joule heating and high electric fields during the device operation. In the third study, we, therefore, demonstrated the role of ZnO in catalyzing an efficient amidation reaction between incorporated dicarboxylic acid additives and excess FAI, preventing the above-mentioned harmful interfacial reaction. With this strategy, the operational half lifetime of the resulting PeLEDs was improved to 682 hours at 20 mA/cm2 while maintaining a high device efficiency of 18.6%.In the last work, we emphasized that the rational design of molecular reactions between two additives (diamine and triacrylate) and perovskite components with the assistance of ZnO substrates can subsequently eliminate the negative effect introduced by additive, reduce the defect density and enhance the crystal orientation in the perovskite emissive layers. The rational understanding of interfacial interactions between perovskite, additives, and ZnO, enabled us to achieve PeLEDs with a device efficiency of 23.8% as well as an outstanding operational stability T70 (reduction to 70% of initial efficiency) lifetime of 290 hours at 20 mA/cm2.The study in this thesis developed effective interface-assisted modulation strategies for high-quality perovskites towards highly efficient and stable PeLEDs for commercialization. A thorough understanding of perovskite chemistry-property-performance modulation assisted by interfaces is indispensable for the future development of PeLEDs and our study took an important step.
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| 2. |
- Qian, Deping
(author)
-
Studies of Voltage Losses in Organic Solar Cells
- 2017
-
Doctoral thesis (other academic/artistic)abstract
- Organic photovoltaic (OPV) devices based on semiconducting polymers and small molecules are potential alternatives to inorganic solar cells, owing to their advantages of being inexpensive, lightweight, flexible and suitable for roll-to-roll production. The state of art organic solar cells (OSCs) performed power conversion efficiencies (PCEs) over 13%.The quantum efficiency losses in OSCs have been significantly reduced within the charge generation and extraction processes, resulting in high EQEPV (70-90%) and high FF (70-80%). Whereas, large voltage losses (Δ? = ??/? − ???) were observed in conventional fullerene based solar cells, and it has been the main limiting factor for further OPV advancement. Therefore, strategies to reduce the voltage losses are required.In this thesis, newly designed non-fullerene (NF) acceptors are used to construct novel material systems for high efficiency solar cells. In particular, we studied the hole transfer in these fullerene free systems. We also reported a NF system that exhibit ultrafast and efficient charge separation despite a negligible driving force, as ECT is nearly identical to ??. Moreover, the small driving force is found to have minimal detrimental effects on charge transfer dynamics of the OSCs. We demonstrate a NF based OSC with efficiency of 9.5% and internal quantum efficiency nearly 90% despite a low voltage loss of 0.61 V. This creates a path towards highly efficient OSCs with a low voltage loss.CT states in OSCs are also investigated, since VOC is governed by the CT energy (ECT), which is found as ???? = ??? − 0.6 in a large set of fullerene based solar cells. In order to reduce these recombination losses from CT states, we explored polymer-diPDI systems which exhibited weakened D-A coupling strength, due to the steric hindrance effect. The radiative recombination losses at D/A interface in these NF devices are all reduced to less than 0.18 eV. In particular, in some cases, the additional emission from pure material is favorable for suppressing the non-radiative CT states decay. Consequently, the recombination losses in these NF systems are reduced to 0.5 eV, while the charge generation is still efficient as confirmed by PL quenching and EQEPV.Novel material systems based on non-fullerene acceptors are investigated. The systems performed energy offsets (ΔHOMO or ΔLUMO) less than 0.15eV, resulting in the same energy of CT states and bulk excitons. In this regard, the charge transfer energy loss is minimized. We also found that the EL spectra as well as the EQEEL of the blend solar cells are similar with that of lower gap components in blends. Thus the non-radiative voltage losses are reduced to < 0.3V and small voltage loss of 0.5-0.7V are obtained. Meanwhile, the charge generation in systems are still efficient and high EQEPV of 50-70% can be achieved. It confirms that there is no intrinsic limit for the VOC and efficiency of OPVs as compared with other photovoltaic technologies.
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| 3. |
- Wang, Heyong, 1989-
(author)
-
High-Quality Perovskite Films for Efficient and Stable Light-Emitting Diodes
- 2020
-
Doctoral thesis (other academic/artistic)abstract
- Metal halide perovskites have attracted significant attention for light-emitting applications, because of their excellent properties, such as high photoluminescence quantum yields (PLQYs), good charge mobility, narrow emission bandwidth, readily tunable emission spectra ranging from ultraviolet to near-infrared, and solution processability. Since the first room-temperature perovskite-based light-emitting diodes (PeLEDs) reported in 2014, tremendous efforts have been made to promote the efficiencies of PeLEDs, including theoretical simulation, materials design, and device engineering. To reach the ultimate goal of commercialization, PeLEDs with both high-efficiency and long-term operational stability are desired. Achieving high-quality perovskite emissive films is key towards this goal. Centering around the high-quality perovskite films, in this thesis, we demonstrate effective synthesis strategies for the deposition of high-quality perovskite films (including both three-dimensional and mixed-dimensional perovskites) and investigate the effects of ion migration in the perovskite films on the performance of PeLEDs.Due to the fast crystallization nature of perovskites and the low formation energy of defects, controlling the crystallization processes of these films has proved to be an effective approach for achieving high-quality perovskite films. For three-dimensional (3D) perovskite films, we have controlled the formation of these films through the assistance of molecules with the amino group. Herein, we have chosen an electron-transport molecule with two amino groups, 4,4’-diaminodiphenyl sulfone (DDS), to control the crystallization process of perovskite films (Paper 1). The resulting perovskite films consists of in-situ formed high quality perovskite nanocrystals embedded in the electron-transport molecular matrix, resulting in improved PLQYs and structural stability. PeLEDs based on these perovskite films have exhibited both high efficiency and long operational stability.In addition, we have investigated the formation of mixed-dimensional perovskite films. Efficient PeLEDs based on mixed-dimensional perovskite films were fabricated with tin dioxide (SnO2) as an electron transport layer (Paper 3). We also note that the deposition methods have a significant impact on the morphology and optical properties of prepared mixed-dimensional perovskite films (Paper 4). In addition, we provide an effective method to extend the deposition of mixed-dimensional perovskite films, replacing organic ammonium halides with amines in the perovskite precursor solutions to form organic spacer cations through the in-situ protonation process of amines (Paper 2).In spite of these efforts, the performance of PeLEDs is still far from the commercialization standard, partially limited by ion migration. In Paper 5, we discuss impacts of mobile ions in the perovskite films on the performance of PeLEDs. We find that a dynamic redistribution of mobile ions can change current density of a device, leading to EQE/hysteresis during forward and reverse voltage scan and enhanced EQE under constant driving voltages. In addition, we have found that excess mobile ions in the perovskite layer can aggravate the hysteresis and shorten the operational stability of PeLEDs.In this thesis, we also discuss the remaining key challenges in the PeLED field, including the achievement of high-performance blue, white, and lead-free PeLEDs, as well as possible strategies to address these challenges. We hope that our research findings provide insights into the basic science behind the perovskite materials, and broadly benefit other optoelectronic communities, such as perovskite solar cells, flexible electronics, and so on.
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| 4. |
- Yu, Hongling, 1987-
(author)
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Color Tuning for Perovskite Light-Emitting Diodes
- 2020
-
Doctoral thesis (other academic/artistic)abstract
- Metal halide perovskites (MHPs) are recognized as promising semiconductor materials for a variety of optical and electrical device applications due to their cost-effective and outstanding optoelectronic properties. As one of the most significant applications, perovskite light-emitting diodes (PeLEDs) hold promise for future lighting and display technologies, attributed to their high photoluminescence quantum yield (PLQY), high color purity, and tunable emission color. The emission colors of PeLEDs can be tuned by mixing the halide anions, adjusting the size of perovskite nanocrystals, or changing the dimensionality of perovskites. However, in practice, all these different approaches have their own advantages and challenges. This thesis centres around the color tunability of perovskites, aiming to develop PeLEDs with different colors using different approaches.We first demonstrate red and near-infrared PeLEDs using a straightforward approach – in situ solution-processed perovskite quantum dots (PQDs). PQDs prepared from colloidal approaches are widely reported and used in LEDs. In contrast, PQDs prepared from the in situ approaches are hardly reported, although they have advantages for device applications. By employing aromatic ammonium iodide (1-naphthylmethyl ammonium iodide, NMAI) as an agent into perovskite precursor solutions, together with annealing temperature modulation, we obtain in situ grown PQDs delivering high external quantum efficiencies (EQEs) of up to 11.0% with tunable electroluminescence (EL) spectra (667 - 790 nm). Our in situ generated PQDs based on pure-halogen perovskites can be easily obtained through a simple deposition process and free of phase segregation, making them a more promising approach for tuning the emission colors of perovskite LEDs.We then move to blue PeLEDs using cesium-based mixed-Br/Cl perovskites. Although mixed halides are a straightforward strategy to tune the emission color, PeLEDs based on this approach suffer from poor color stability, which is attributed to surface defects at grain boundaries. Under the condition of optical excitations, light density over a certain value (a threshold), oxygen, and surface defects at perovskite grain boundaries are found to be key factors inducing photoluminescence (PL) spectral instability of CsPb(Br1−xClx)3 perovskites. Upon electrical bias, defects at grain boundaries provide undesirable halide migration channels, responsible for EL spectral instability issues. Through effective defect passivation, the PL spectral resistance to oxygen is enhanced; moreover, high-performance and color-stable blue PeLEDs are achieved, delivering a maximum luminance of 5351 cd m–2 and a peak EQE of 4.55% with a peak emission wavelength at 489 nm. These findings provide new insights into the color instability issue of mixed halide blue perovskites, against which we also demonstrate an effective strategy.We finally realize single-emissive-layer (EML) white PeLEDs by employing a mixed halide perovskite film as the EML. In spite of high-performance monochromatic blue, green, and red colors, the development of white PeLEDs, especially for single-EML ones, remains a very big challenge. By effective modulation of the halide salt precursors, we achieve single-EML white PeLEDs with Commission Internationale de L’Eclairage (CIE) coordinates of (0.33, 0.33), close to those (0.3128, 0.3290) of the CIE standard illuminant D65. This work not only provides a successful demonstration of a single-EML white PeLED, but also provides useful guidelines for the future development of highperformance single-EML white PeLEDs.
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| 5. |
- Zhang, Huotian
(author)
-
Loss Mechanisms In Non-Fullerene Organic Solar Cells
- 2021
-
Doctoral thesis (other academic/artistic)abstract
- Photovoltaics are one of the most important sustainable energy sources in the 21st century. Among photovoltaics, organic solar cells (OSCs) offer many advantages such as ease of processing, lightweight, the potential for flexibility, and tunable properties. Its peculiar nature and complexity present a fascinating charm, attracting many researchers. Thanks to researchers' efforts, the power conversion efficiency (PCE) of OSCs has been boosted from 1% to 19% during the last three decades. Despite the exciting PCE, some problems remain to be solved, for example, the large voltage loss and long-term stability. The aim of this thesis is to understand the fundamental physics of the state-of-the-art OSCs, especially the loss mechanism. Ultimately, properly understanding the mechanisms will sever as the basis of OSCs further improvements and commercialization. This work focuses on the loss mechanisms of OSCs, particularly the open-circuit voltage and the fill factor. The beginning of this thesis introduces basic concepts regarding semiconductors physics and donor-acceptor OSCs. This part explains how a photon is used to generate electricity and the fundamentals of organic electronics. Subsequently, the detailed balance in a solar cell is reviewed, which is the basis of voltage loss analysis. In this part, we see how the input, recombination, and output form a balance. Then, the way to determine the voltage loss is shown, and the latest understandings in reducing the loss are reviewed. The fill factor, as a measure of the quality of a solar cell, is a complex parameter, especially in OSCs.The latter part of this thesis starts from the photophysical processes in an OSC, and then relates intrinsic parameters to the fill factor. The figure of merits has been employed to express the fill factor analytically. In the end, experimental methods and basic principles for the previous analysis are introduced, including Fourier transform infrared spectroscopy, the external quantum efficiency of photovoltaics (EQEPV), spectrograph for electroluminescence or photoluminescence, transient absorption, and time-delayed collection field. Overall, the thesis combined thermal dynamics and charge dynamics to analyze voltage losses and fill factor losses. The author hopes this work can contribute to a better understanding of the loss mechanisms OSCs.
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| 6. |
- Gabrielsson, Roger
(author)
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Electroactive Conjugated Polyelectrolytes Based on EDOT From Synthesis to Organic Electronics
- 2012
-
Doctoral thesis (other academic/artistic)abstract
- Conjugated polyelectrolytes (CP) show interesting electrical and optical properties for organic electronics as well as for life science applications. Their possibilities of supramolecular assembly with nanowire like misfolded proteins, amyloids, as well as synthetic polypeptides or DNA forming conducting nano composites is highly interesting as being a truly bottom up approach for fabrication of OLEDs, photovoltaic’s as well as logic devices.A special class of CPs is that of electroactive cojugated polymers (ECPs), which, due to their structure, will exhibits a unique combination of properties, including the following; electrically conducting, ability to store an electric charge and ability to exchange ions. The positive or negative excess charge can be introduced into the conjugated polymer by means of chemical or electrochemical oxidation/reduction (a process called doping) following the polymerization reaction. In order to preserve overall electroneutrality of the polymer during introduction of excess charge, ionexhange processes occurs between the polymer phase and the surrounding electrolyte solution. This charge/discharge process can be utilized for application such as; pseudo super capacitors (energy storage through oxidation/reduction processes), electro mechanical actuators (convert electrical energy to mechanical energy) and sensors (converts a chemical signal to electrical conductivity).In this thesis we describes the synthetic challenges with ECPs for applications vide supra. These mostly relates to solubility, ionic functionalization, conductivity and macromolecular properties such as size and shape of the ECPs. The key requirement in the synthesis of ECPs is that the conjugated nature of the monomer is conserved in the synthesis process and that insertion of excess charge (doping) can be obtained. This limits both the choice of monomer and the choice of polymerization process. Monomers of great complexity have been synthesized with this careful goal in mind. Furthermore, the development of novel monomers must also target the appropriate functionality for polymerization. As such, most ECP monomers are electron-rich molecules with pendant groups containing pyrroles, thiophenes, or 3,4-ethylenedioxythiophenes. These three well known ECP monomers are excellent additions to conjugated systems as they typically enable electrochemical polymerization and direct the polymerizations toward linear polymers with good stability towards doping.The first topic of this thesis we demonstrate how we can obtain water soluble ECPs with good electrical conductivity by controlling the polymerization techniques and proper ionic functionalization of the monomer. We also show how these polymers can be incorporated by self-assembly with biomolecular templates, such as, DNA and amyloid fibrils, thus generating novel electrically conductive nanowires.The second topics of this thesis demonstrate how hydrogels of ECPs can be used as bioand charge storage materials, were we demonstrate electronically controlled cell release for biology applications. Both applications are based on ECPs ability to ionexhange processes during electrochemical redox reactions. As well as ions, solvent and other neutral molecules may enter the film during charge/discharge processes. This cause a swelling or shrinking of the ECP films and the expansion and contraction of the polymer network in conjugation with the sorption/desorption of solvent molecules and ions can be described in terms of mechanical work.In the first case we were able to synthesize a water soluble ECP with high amphiphilic character. The polymer was immobilized onto a flexible electrode, suitable for cell growth and subjected to a cell growth media. When the desired cell layer was formed we applied a potential to the flexible electrode. This resulted in that the mechanical work of the immobilized ECP during the applied potential overcame the week adhesive forces to the flexible electrode, which resulted in super swelling and disintegration of the ICP and the cell layer could be harvested.In the second case the possibilities of using synthetically modified ECPs as a dopant during electropolymerization of another ECP monomer to obtain a polymer integrated network with high charge density and good charge transport properties. We demonstrate how this polymer network can be used as porous electrodes suitable for supercapacitors.
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| 7. |
- Liu, Yanfeng, 1992-
(author)
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Studying Morphology Formation and Charge Separation in Organic Solar Cells
- 2021
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Doctoral thesis (other academic/artistic)abstract
- We are currently living in the era of automation and artificial intelligence, which requires more energy than ever before. Meanwhile, the reduction of carbon footprint is needed for keeping the environment sustainable. Exploring green energy is crucial. Solar power is one of the green energy sources. The apparatus that converts solar energy to electricity is a solar cell. Organic solar cells (OSCs), employing organic materials absorbing solar radiation and converting to electricity have got extensive attention in the last decades due to their unique advantages like lightweight, semi-transparency, and potential industrialization. In most cases, an OSC composes of two different organic semiconductors as electron donor and acceptor to form a photoactive layer with a bulk heterojunction (BHJ) structure, and sandwiched between the electron and hole transport layers and then two electrodes. The morphology of the BHJ plays a crucial role in the device's performance, and it is a result of a complicated interplay between donor, acceptor, and solvent during the film drying from a solution. Thus, in-situ monitoring the film drying during solvent evaporation could deepen understanding of the mechanism of the morphology formation. A versatile multiple spectroscopic setup is assembled for this purpose, which can record laser scattering, steady-state photoluminescence (PL), time-resolved photoluminescence (TRPL), and white-light absorption during film formation. By comparing the drying dynamics of three different blend systems with their corresponding pristine films, we find that the blend film formation and its final morphology are more dominated by the component with a higher molecular weight. Different PL and TRPL quenching profiles between fullerene- and non-fullerene-based systems provide hints about different donor-acceptor interactions. Moreover, with the help of TRPL, the relative change of quantum yield during film formation can be calculated. Besides, this setup is also proved suitable for studying mechanisms behind device optimization processes, like the usage of solvent additives. One of the unique features of OSCs based on non-fullerene acceptors is the highly efficient hole transfer from the acceptor to the donor, sometimes even under zero or negative energetic offsets. However, in these cases the mechanism of hole transfer has not been fully understood. By studying hole transfer at the donor:acceptor interface in different material systems and device configurations, we highlight the role of electric field on the charge separation of OSCs when energetic offsets are not enough. To achieve better device performance, engineering the photoelectric properties of interfacial layers is equally essential. A good interfacial layer can facilitate carrier extraction and reduce carrier recombination. We demonstrate that adding MXenes into the PEDOT:PSS can increase the conductivity of this composite hole transport layer, without sacrificing its optical transparency and work function.
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| 8. |
- Ma, Zaifei, 1986-
(author)
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Studies of Morphology and Charge-Transfer in Bulk-Heterojunction Polymer Solar Cells
- 2013
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Doctoral thesis (other academic/artistic)abstract
- The work presented in this thesis focuses on the two critical issues of bulk-heterojunction polymer solar cells: morphology of active layers and energy loss during charge transfer process at electron donor/acceptor interfaces. Both issues determine the performance of polymer solar cells through governing exciton dissociation, charge carrier recombination and free charge carrier transport.The morphology of active layers (spatial percolation of the donor and acceptor) is crucial for the performance of polymer solar cells due to the limited diffusion length of excitons in organic semiconductors (5-20 nm). Meanwhile, the trade-off between charge generation and transport also needs to be considered. On the one hand, a finely mixed morphology with a large donor/acceptor interface area is preferred for charge generation because efficient exciton dissociation only occurs at the interface, but on the other hand, proper phase separation is needed to reduce charge carrier recombination and facilitate free charge carrier transport to the electrodes. In this thesis, morphologies of the active layers based on different polymeric donors and fullerene acceptors are correlated to the performance of solar cells with various microscopic and spectroscopic techniques including atomic force microscope, transmission electron microscope, grazing incidence x-ray diffraction, photoluminescence, electroluminescence and Fourier transform photocurrent spectroscopy. Furthermore, methods to manipulate the morphologies of solution processed active layers to achieve high performance solar cells are also presented. Processing solvents, chemical structures of the donor and the acceptor materials, and substrate surface properties are found critically important in determining the nanoscale phase separation and performance of polymer solar cells.Optimizing morphology of active layers alone does not guarantee high performance devices. In addition to spatial percolation, energy arrangements of donors and acceptors are also essential due to contrary requests of the photocurrent and the photovoltage: Efficient exciton dissociation or charge transfer at donor/acceptor interfaces requires large enough energetic driving force, which is also known as energy loss for charge transfer. However, the energy loss due to charge transfer will unavoidably reduce the photovoltage. In this thesis the balance between the photocurrent and the photovoltage in polymer solar cells due to charge transfer at donor/acceptor interfaces is investigated for different active material systems. The driving force tuned by synthesizing series of polymers is determined by directly measuring the optical band gap via UV-Vis spectroscopy and probing the charge transfer recombination via electroluminescence measurements. Influences of driving force on the photocurrent and the photovoltage are characterized via field dependent photoluminescence and internal quantum efficiency measurements. The results correlated well with the performance of the solar cells.
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| 9. |
- Melianas, Armantas
(author)
-
Non-Equilibrium Charge Motion in Organic Solar Cells
- 2017
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Doctoral thesis (other academic/artistic)abstract
- Organic photovoltaic (OPV) devices based on semiconducting polymers and small molecules allow for a low cost alternative to inorganic solar cells. Recent developments show power conversion efficiencies as high as 10-12%, highlighting the potential of this technology. Nevertheless, further improvements are necessary to achieve commercialization.To a large extent the performance of these devices is dictated by their ability to extract the photo-generated charge, which is related to the charge carrier mobility. Various time-resolved and steady-state techniques are available to probe the charge carrier mobility in OPVs but often lead to different mobility values for one and the same system. Despite such conflicting observations it is generally assumed that charge transport in OPV devices can be described by well-defined charge carrier mobilities, typically obtained using a single steady-state technique. This thesis shows that the relevance of such well-defined mobilities for the charge separation and extraction processes is very limited.Although different transient techniques probe different time scales after photogeneration, they are mutually consistent as they probe the same physical mechanism governing charge motion – gradual thermalization of the photo-generated carriers in the disorder broadened density of states (DOS). The photo-generated carriers gradually lose their excess energy during transport to the extracting electrodes, but not immediately. Typically not all excess energy is dissipated as the photo-generated carriers tend to be extracted from the OPV device before reaching quasi-equilibrium.Carrier motion is governed by thermalization, leading to a time-dependent carrier mobility that is significantly higher than the steady-state mobility. This picture is confirmed by several transient techniques: Time-resolved Terahertz Spectroscopy (TRTS), Time-resolved Microwave Conductance (TRMC) combined with Transient Absorption (TA), electrical extraction of photo-induced charges (photo-CELIV). The connection between transient and steady-state mobility measurements (space-charge limited conductivity, SCLC) is described. Unification of transient opto-electric techniques to probe charge motion in OPVs is presented.Using transient experiments the distribution of extraction times of photo-generated charges in an operating OPV device has been determined and found to be strongly dispersive, spanning several decades in time. In view of the strong dispersion in extraction times the relevance of even a well-defined time-dependent mean mobility is limited.In OPVs a continuous ‘percolating’ donor network is often considered necessary for efficient hole extraction, whereas if the network is discontinuous, hole transport is thought to deteriorate significantly, limiting device performance. Here, it is shown that even highly diluted donor sites (5.7-10 %) in a buckminsterfullerene (C60) matrix enable reasonably efficient hole transport. Using transient measurements it is demonstrated that hole transport between isolated donor sites can occur by long-range hole tunneling (over distances of ~4 nm) through several C60 molecules – even a discontinuous donor network enables hole transport
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| 10. |
- Xia, Yuxin, 1986-
(author)
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Polymer/polymer blends in organic photovoltaic and photodiode devices
- 2018
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Doctoral thesis (other academic/artistic)abstract
- Organic photovoltaics devices (OPV) have attracted attentions of scientist for their potential as inexpensive, lightweight, flexible and suitable for roll-to-roll production. In recent years, considerable attention has been focused on new acceptor materials, either polymeric or small molecules, to replace the once dominating fullerene derivatives. The emergence of numerous new non-fullerene materials has driven power conversion efficiency (PCE) up to 17%, attracting more and more interests of commercialization.Polymer acceptors with more morphology stability, more absorption and more desired energy levels has been intensively studied and show great potential for large area and low-cost production in the future. OPV at this moment is not yet competitive with inorganic solar cells in PCE but is more attractive in flexibility, low weight and semitransparency. In this thesis, some basic knowledges of OPV is introduced in the first few chapters, while the next chapters are focusing on polymer-polymer blends and investigating novel structures and techniques for large scale production of solar cells and photodetectors aiming at maximizing these advantages to compete with inorganic counterpart.Thermal annealing effects on polymer-polymer solar cells based is studied. Annealed devices show doubled power conversion efficiency compared to non-annealed devices. Based on the morphology—mobility examination, we conclude that the better charge transport is achieved by higher order and better interconnected networks of the bulk heterojunction in the annealed active layers. The annealing improves charge transport and extends the conjugation length of the polymers, which do help charge generation and meanwhile reduce recombination. The blend of an amorphous polymer and a semi-crystalline polymer can thus be modified by thermal annealing to double the power conversion efficiency.A novel concept of all-polymer organic photovoltaics device is demonstrated in this thesis where all the layers are made out of polymers. We use PEDOT:PSS as semitransparent anode and polyethyleneimine modified PEDOT:PSS as semitransparent cathode, both of which are slot-die printed on polyethylene terephthalate(PET). Active layers are deposited on cathode and anode surfaces by spin coating separately. These layers are then joined through a roll-to-roll compatible lamination process. This forms a semitransparent and flexible solar cell. By laminating a thin layer acceptor polymer to a thick polymer-polymer blend, we can further improve the performance by reducing traps comparing to laminating blend to blend.Flexible and semitransparent all-polymer photodiodes with different geometries can be fabricated through lamination. By choosing high band gap polymers and appropriate combination of two or more polymers, organic photodiode with low noise and high specific detectivity can be obtained. Comparison between bilayer and bulk heterojunction devices gives better understanding of the origin of noise and provides ways to improve the performance of photodiodes as detector.Noise level is a critical parameter for photodetectors. The difficulties of measuring the noise of photodetectors make some researchers prefer the estimated shot noise as the dominating one and ignore the thermal noise and 1/f noise. The latter two terms are sometimes several orders higher than the former, noting the importance of experimentally measuring noise.The use of semi-transparent photovoltaic devices causes an inevitable loss of photocurrent, as light transmitted has not been absorbed. This trivial effect also leads to a loss of photovoltage, an effect partially due to the lower photocurrent but also due to the geometry of the semitransparent photovoltaic device. We here demonstrate and evaluate this photovoltage loss in semi-transparent organic photovoltaic devices, compared with non-transparent solar cells of the same material. Semi-transparent solar cells in addition introduce photovoltage loss when formed by lamination. We document and analyze these effects for a number of polymer blends in the form of bulk heterojunctions.
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