Fabrication and Characterization of Hybrid Metal-Oxide/Polymer Light-Emitting Diodes

Hybrid metal-oxide/polymer light-emitting diodes (HyLEDs) are a novel class of electronic devices based on a combination of electroluminescent organic and charge-injecting metal-oxide components. These devices employ air-stable electrodes, such as ITO and Au, and are therefore well suited for fabrication of encapsulation-free light-emitting devices. The current work is intended to provide an insight into operating mechanisms and limitations of the HyLEDs, and, on the basis of this knowledge, aims at modifying the device architecture in order to improve the performance. The choice of optically transparent metal-oxide charge-injection layers appears to be critical in this respect in order to optimize the electron-hole balance within the polymer layer. Starting from the original device architecture, ITO/TiO2/F8BT/MoO3/Au, which uses ITO as a cathode and Au as an anode, we follow different approaches, such as the use of dipolar self-assembled monolayers and nanoscale structuring of the electron-injecting interface, pursuing the goal of enhancing electron injection into the emissive layer. However, substitution of the electron-injecting layer of TiO2 with ZrO2 is demonstrated to be the most efficient of the approaches employed herein. Further, optimization of the device utilizing the latter metal oxide is demonstrated in terms of deposition and post-deposition treatment of the electron-injecting and electroluminescent layers. Substrate temperature during spray pyrolysis deposition of the electron-injecting layer is found to have a strong influence on the HyLED performance, as well as the precursor solution spraying rate and the layer thickness. On the other hand, post-deposition annealing of the polymer layer is shown to improve the device efficiency and brightness significantly, possible explanations lying in enhancement in polymer luminescence efficiency and formation of a more intimate contact between the electron-injecting and the active polymer layers. Combining electron-transporting (TiO2 and ZnO) and hole-blocking (Al2O3 and ZrO2) materials into a single electron-injecting layer is demonstrated to be an effective strategy of enhancing efficiency in the HyLEDs. The search for a hole-injecting electrode alternative to the conventionally used MoO3/Au leads to the device employing the PEDOT:PSS/VPP-PEDOT system, which though resulting in a poorer device efficiency, provides route for fabrication of vacuum deposition-free organic light-emitting devices. Finally, the HyLED architecture is demonstrated to offer better stability than the conventional architecture using LiF/Al as a cathode. It is hoped that the current work provides a better understanding of the requirements for fabrication of encapsulation-free organic light-emitting devices

ELECTRICAL AND STRUCTURAL CHARACTERIZATION OF FEW-LAYER GRAPHENE SHEETS ON QUARTZ

Despite the impressive performance and incredible promise for a variety of applications, the wide-scale commercialization of graphene is still behind its full potential. One of the main challenges is related to preserving graphene’s unique properties upon transfer onto practically desirable substrates. In this work, few-layer graphene sheets deposited via liquid-phase transfer from copper onto a quartz substrate have been studied using a suite of experimental techniques, including scanning electron microscopy (SEM), Raman spectroscopy, admittance spectroscopy, and four-point probe electrical measurements. SEM measurements suggest that the transfer of graphene from copper foil to quartz using the aqueous solution of ammonium persulfate was accompanied by unintentional etching of the entire surface of the quartz substrate and, as a result, the formation of microscopic facet structures covering the etched surface of the substrate. As revealed by Raman spectroscopy and the electrical measurements, the transfer process involving the etching of the copper foil in a 0.1Msolution of (NH4)2S2O8 resulted in its p-type doping. This was accompanied by the appearance of an electronic gap of 0.022 eV, as evidenced by the Arrhenius analysis. The observed increase in the conductance of the samples with temperature can be explained by thermally activated carrier transport, dominating the scattering processes

Self-Doping of the Transport Layers Decreases the Bimolecular Recombination by Reducing Static Disorder

Electron-transport layers (ETLs) have a crucial role in the solar cells’ performance. Generally, ETLs are characterized in terms of the interface properties and conductivity rather than their effect on the photoactive layer. Herein, two ETLs, 2,9-bis(3-((3-(dimethylamino)propyl)amino)propyl)anthra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline-1,3,8,10(2H,9H)-tetraone (PDINN) and 2,9-bis[3-(dimethyloxidoamino)propyl]anthra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline-1,3,8,10(2H,9H)-tetrone, are compared in the conventional PM6:Y6 organic solar cell (OSC) structure and the influence of the ETL on the photoactive layer is shown. It is shown that a significant portion of the unpaired electrons of PDINN is mobile by combining electron paramagnetic resonance and Hall effect measurements. It is established that the high doping in PDINN ETL changes the dark electron concentration of the photoactive layer. The impacts of this change in the photoactive layer can be observed in the reduced static energetic disorder, and subsequently in the (nonradiative) recombination of free carriers. The results can be used to suppress nonradiative recombination in OSC, which can significantly boost their efficiency

DETERMINATION OF THE CHARGE CARRIER DENSITY IN ORGANIC SOLAR CELLS: A TUTORIAL

The increase in the performance of organic solar cells observed over the past few years has reinvigorated the search for a deeper understanding of the loss and extraction processes in this class of device. A detailed knowledge of the density of free charge carriers under different operating conditions and illumination intensities is a prerequisite to quantify the recombination and extraction dynamics. Differential charging techniques are a promising approach to experimentally obtain the charge carrier density under the aforementioned conditions. In particular, the combination of transient photovoltage and photocurrent as well as impedance and capacitance spectroscopy have been successfully used in past studies to determine the charge carrier density of organic solar cells. In this Tutorial, these experimental techniques will be discussed in detail, highlighting fundamental principles, practical considerations, necessary corrections, advantages, drawbacks, and ultimately their limitations. Relevant references introducing more advanced concepts will be provided as well. Therefore, the present Tutorial might act as an introduction and guideline aimed at new prospective users of these techniques as well as a point of reference for more experienced researcher

On the critical competition between singlet exciton decay and free charge generation in non-fullerene based organic solar cells with low energetic offsets†

Reducing voltage losses while maintaining high photocurrents is the holy grail of current research on non-fullerene acceptor (NFA) based organic solar cell. Recent focus lies in understanding the various fundamental mechanisms in organic blends with minimal energy offsets – particularly the relationship between ionization energy offset (ΔIE) and free charge generation. Here, we quantitatively probe this relationship in multiple NFA-based blends by mixing Y-series NFAs with PM6 of different molecular weights, covering a broad power conversion efficiency (PCE) range: from 15% down to 1%. Spectroelectrochemistry reveals that a ΔIE of more than 0.3 eV is necessary for efficient photocurrent generation. Bias-dependent time-delayed collection experiments reveal a very pronounced field-dependence of free charge generation for small ΔIE blends, which is mirrored by a strong and simultaneous field-dependence of the quantified photoluminescence from the NFA local singlet exciton (LE). We find that the decay of singlet excitons is the primary competition to free charge generation in low-offset NFA-based organic solar cells, with neither noticeable losses from charge-transfer (CT) decay nor evidence for LE–CT hybridization. In agreement with this conclusion, transient absorption spectroscopy consistently reveals that a smaller ΔIE slows the NFA exciton dissociation into free charges, albeit restorable by an electric field. Our experimental data align with Marcus theory calculations, supported by density functional theory simulations, for zero-field free charge generation and exciton decay efficiencies. We conclude that efficient photocurrent generation generally requires that the CT state is located below the LE, but that this restriction is lifted in systems with a small reorganization energy for charge transfer.A quantitative study, supported by Marcus theory and DFT, showing why the fate of singlet excitons is the pivot to free charge generation in low-energy offset organic solar cells.Fonds Wetenschappelijk Onderzoek 10.13039/501100003130European Research Council 10.13039/501100000781Deutsche Forschungsgemeinschaft 10.13039/501100001659China Scholarship Council 10.13039/50110000454

On the critical competition between singlet exciton decay and free charge generation in non-fullerene based organic solar cells with low energetic offsets

Reducing voltage losses while maintaining high photocurrents is the holy grail of current research on non-fullerene acceptor (NFA) based organic solar cell. Recent focus lies in understanding the various fundamental mechanisms in organic blends with minimal energy offsets - particularly the relationship between ionization energy offset (ΔIE) and free charge generation. Here, we quantitatively probe this relationship in multiple NFA-based blends by mixing Y-series NFAs with PM6 of different molecular weights, covering a broad power conversion efficiency (PCE) range: from 15% down to 1%. Spectroelectrochemistry reveals that a ΔIE of more than 0.3 eV is necessary for efficient photocurrent generation. Bias-dependent time-delayed collection experiments reveal a very pronounced field-dependence of free charge generation for small ΔIE blends, which is mirrored by a strong and simultaneous field-dependence of the quantified photoluminescence from the NFA local singlet exciton (LE). We find that the decay of singlet excitons is the primary competition to free charge generation in low-offset NFA-based organic solar cells, with neither noticeable losses from charge-transfer (CT) decay nor evidence for LE–CT hybridization. In agreement with this conclusion, transient absorption spectroscopy consistently reveals that a smaller ΔIE slows the NFA exciton dissociation into free charges, albeit restorable by an electric field. Our experimental data align with Marcus theory calculations, supported by density functional theory simulations, for zero-field free charge generation and exciton decay efficiencies. We conclude that efficient photocurrent generation generally requires that the CT state is located below the LE, but that this restriction is lifted in systems with a small reorganization energy for charge transfer.A quantitative study, supported by Marcus theory and DFT, showing why the fate of singlet excitons is the pivot to free charge generation in low-energy offset organic solar cells.Fonds Wetenschappelijk OnderzoekEuropean Research CouncilDeutsche ForschungsgemeinschaftChina Scholarship Counci

Characterization of a Heterojunction Silicon Solar Cell by Means of Impedance Spectroscopy

Impedance spectroscopy provides relevant knowledge on the recombination and extraction of photogenerated charge carriers in various types of photovoltaic devices. In particular, this method is of great benefit to the study of crystalline silicon (c-Si)-based solar cells, a market-dominating commercial technology, for example, in terms of the comparison of various types of c-Si devices. This study investigates the dark and light electrophysical characteristics of a heterojunction silicon solar cell fabricated using plasma-enhanced chemical vapor deposition. The measurements are performed at various applied biases, enabling the determination of complex resistance, characteristic time, capacitive response and impurity concentration within the semiconductor junction and to correlate them with the device performance. In addition, the impedance spectra of the studied cell were investigated as a function of temperature. Studies of the frequency and temperature dependences of capacitance do not reveal a significant presence of thermally activated centers of free carrier capture, concomitant with a very small value of the activation energy extracted from an Arrhenius-type analysis. This leads to a conclusion that these centers are likely not impactful on the device operation and efficiency

Fabrication and characterization of hybrid metal-oxide/polymer light-emitting diodes

Hybrid metal-oxide/polymer light-emitting diodes (HyLEDs) are a novel class of electronic devices based on a combination of electroluminescent organic and charge-injecting metal-oxide components. These devices employ air-stable electrodes, such as ITO and Au, and are therefore well suited for fabrication of encapsulation-free light-emitting devices. The current work is intended to provide an insight into operating mechanisms and limitations of the HyLEDs, and, on the basis of this knowledge, aims at modifying the device architecture in order to improve the performance. The choice of optically transparent metal-oxide charge-injection layers appears to be critical in this respect in order to optimize the electron-hole balance within the polymer layer. Starting from the original device architecture, ITO/TiO2/F8BT/MoO3/Au, which uses ITO as a cathode and Au as an anode, we follow different approaches, such as the use of dipolar self-assembled monolayers and nanoscale structuring of the electron-injecting interface, pursuing the goal of enhancing electron injection into the emissive layer. However, substitution of the electron-injecting layer of TiO2 with ZrO2 is demonstrated to be the most efficient of the approaches employed herein. Further, optimization of the device utilizing the latter metal oxide is demonstrated in terms of deposition and post-deposition treatment of the electron-injecting and electroluminescent layers. Substrate temperature during spray pyrolysis deposition of the electron-injecting layer is found to have a strong influence on the HyLED performance, as well as the precursor solution spraying rate and the layer thickness. On the other hand, post-deposition annealing of the polymer layer is shown to improve the device efficiency and brightness significantly, possible explanations lying in enhancement in polymer luminescence efficiency and formation of a more intimate contact between the electron-injecting and the active polymer layers. Combining electron-transporting (TiO2 and ZnO) and hole-blocking (Al2O3 and ZrO2) materials into a single electron-injecting layer is demonstrated to be an effective strategy of enhancing efficiency in the HyLEDs. The search for a hole-injecting electrode alternative to the conventionally used MoO3/Au leads to the device employing the PEDOT:PSS/VPP-PEDOT system, which though resulting in a poorer device efficiency, provides route for fabrication of vacuum deposition-free organic light-emitting devices. Finally, the HyLED architecture is demonstrated to offer better stability than the conventional architecture using LiF/Al as a cathode. It is hoped that the current work provides a better understanding of the requirements for fabrication of encapsulation-free organic light-emitting devices.EThOS - Electronic Theses Online ServiceBolashak scholarship of the Ministry of Education and Science of the Republic of KazakhstanGBUnited Kingdo

Fabrication and characterization of hybrid metal-oxide/polymer light-emitting diodes

Hybrid metal-oxide/polymer light-emitting diodes (HyLEDs) are a novel class of electronic devices based on a combination of electroluminescent organic and charge-injecting metal-oxide components. These devices employ air-stable electrodes, such as ITO and Au, and are therefore well suited for fabrication of encapsulation-free light-emitting devices. The current work is intended to provide an insight into operating mechanisms and limitations of the HyLEDs, and, on the basis of this knowledge, aims at modifying the device architecture in order to improve the performance. The choice of optically transparent metal-oxide charge-injection layers appears to be critical in this respect in order to optimize the electron-hole balance within the polymer layer. Starting from the original device architecture, ITO/TiO2/F8BT/MoO3/Au, which uses ITO as a cathode and Au as an anode, we follow different approaches, such as the use of dipolar self-assembled monolayers and nanoscale structuring of the electron-injecting interface, pursuing the goal of enhancing electron injection into the emissive layer. However, substitution of the electron-injecting layer of TiO2 with ZrO2 is demonstrated to be the most efficient of the approaches employed herein. Further, optimization of the device utilizing the latter metal oxide is demonstrated in terms of deposition and post-deposition treatment of the electron-injecting and electroluminescent layers. Substrate temperature during spray pyrolysis deposition of the electron-injecting layer is found to have a strong influence on the HyLED performance, as well as the precursor solution spraying rate and the layer thickness. On the other hand, post-deposition annealing of the polymer layer is shown to improve the device efficiency and brightness significantly, possible explanations lying in enhancement in polymer luminescence efficiency and formation of a more intimate contact between the electron-injecting and the active polymer layers. Combining electron-transporting (TiO2 and ZnO) and hole-blocking (Al2O3 and ZrO2) materials into a single electron-injecting layer is demonstrated to be an effective strategy of enhancing efficiency in the HyLEDs. The search for a hole-injecting electrode alternative to the conventionally used MoO3/Au leads to the device employing the PEDOT:PSS/VPP-PEDOT system, which though resulting in a poorer device efficiency, provides route for fabrication of vacuum deposition-free organic light-emitting devices. Finally, the HyLED architecture is demonstrated to offer better stability than the conventional architecture using LiF/Al as a cathode. It is hoped that the current work provides a better understanding of the requirements for fabrication of encapsulation-free organic light-emitting devices.EThOS - Electronic Theses Online ServiceBolashak scholarship of the Ministry of Education and Science of the Republic of KazakhstanGBUnited Kingdo

Fabrication and study of sol-gel ZnO films for use in Si-based heterojunction photovoltaic devices

This paper considers the use of zinc oxide thin films prepared via the sol-gel route as an n-type layer in heterojunction ZnO/Si solar cells. The ZnO films were prepared via a simple spin-coating technique using zinc acetate dihydrate as a zinc precursor, isopropanol as a solvent and monoethanolamine as a stabilizing agent. Optical, structural and morphological properties of ZnO were investigated for thin films grown from sol-gel solutions with different concentrations both on glass and silicon substrates. As such, a distribution of crystallite sizes and surface topology parameters corresponding to various zinc acetate dihydrate concentrations were obtained to elucidate optimal film deposition conditions. Correlation between thin film morphology and structural characteristics of ZnO thin films was made based on atomic-force microscopy studies. Finally, our results on fabrication, characterization and simulation of ZnO/Si heterojunctions for use as photovoltaic devices are presented. Although noticeable rectifying and photovoltaic properties were observed for Al/Si/ZnO/Ti/Au devices, there appears to exist a considerable room for device improvement with simulation studies suggesting that efficiencies of the order of 24% may be obtained for devices with optimal silicon wafer passivation, i.e. with lifetimes of the order of 1000 μs