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- Zhang, Huotian
(författare)
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Loss Mechanisms In Non-Fullerene Organic Solar Cells
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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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| 2. |
- Melianas, Armantas
(författare)
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Non-Equilibrium Charge Motion in Organic Solar Cells
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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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| 3. |
- Zhang, Silan, 1992-
(författare)
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Organic Electrochemical Transistors : Materials and Challenges
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The use of organic mixed ionic-electronic conductors (OMIECs) has demonstrated the potential to transform the field of bioelectronics, spanning from medical diagnostics to neuromorphic computing hardware. To keep up with the fast-paced demands, it is crucial to develop customizable device fabrication, design new materials, improve operation stability, and explore the ion-electron interactions within OMIECs. This thesis explores the application of OMIECs in organic electrochemical transistors (OECTs), a crucial component of a range of organic bioelectronic devices. To meet applications requiring rapid design iterations and leveraging digitally enabled direct-write techniques, we developed a novel approach for fabricating fully 3D-printed OECTs using a direct-write additive process. This method involves utilizing 3D printable inks with conductive, semiconductive, insulating, and electrolyte properties. The resulting fully 3D-printed OECTs operate in the depletion mode and can be produced on flexible substrates, ensuring excellent mechanical durability and resilience in various environmental conditions. These 3D-printed OECTs exhibit impressive dopamine biosensing capabilities, detecting concentrations as low as 6 µM without the need for metal gate electrodes. Furthermore, they demonstrate long-term memory response lasting up to approximately 1 hour, highlighting their potential for diverse applications such as sensors and neuromorphic hardware. We have addressed the issue of sluggish response times in printed OECTs by utilizing multi-walled carbon nanotubes (MWCNTs) and the π-conjugated redox polymer called poly(benzimidazobenzo-phenanthroline) (BBL) to create high-performing n-type OECTs. By incorporating MWCNTs, we were able to improve the electron mobility of the transistors by more than 10 times, resulting in a rapid response time of just 15 ms and a high μC* value (which is the product of electron mobility and volumetric capacitance) of approximately 1 F cm–1 V−1 s−1. These breakthroughs have allowed us to develop complementary inverters that have a voltage gain of over 16, a significant worst-case noise margin at a supply voltage lower than 0.6 V and consume less than 1 µW of power. However, the operational stability of complementary inverters is hindered by the degradation of p-type OMIECs. The oxygen reduction reaction (ORR) is a common electrochemical side reaction that poses challenges to the stability of OECTs, but the underlying connection between ORR and material degradation remains poorly understood. In our investigation, we examined the influence of ORR on the stability and degradation mechanisms of thiophene-based OECTs. Our findings reveal that the polymer backbone experiences degradation as a result of the pH increase during ORR. To address this issue, we introduced a protective polymer glue layer between the semiconductor channel and the aqueous electrolyte, effectively suppressing the occurrence of ORR and significantly enhancing the stability of the OECTs. This improvement is evident in the nearly 90% retention of current during ≈2 hours of cycling in the saturation regime. Finally, we investigated the ionic-electronic transport properties in BBL-based OECTs using various electrolytes. We found that the peak drain current is achieved at a doping level of 1 electron per repeating unit, decreasing thereafter. The interaction between ions and the polymer reduces the voltage needed for this level of doping but also lowers the peak drain current. Unlike thiophene-based OECTs, larger cation sizes don't improve BBL-based OECT performance. Additionally, Lewis acids adversely affect BBL's electrical properties due to their impact on the polymer microstructure. We hope these studies will inspire our peers in the field of materials synthesis, device processing, and scalable digital techniques, paving the way for next-generation, reliable, and safe bioelectronics.
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