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2018 Volume 34 Issue 11
2018, 34(11): 1189-1190
doi: 10.3866/PKU.WHXB201804101
Abstract:
2018, 34(11): 1191-1192
doi: 10.3866/PKU.WHXB201804091
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2018, 34(11): 1193-1994
doi: 10.3866/PKU.WHXB201804093
Abstract:
2018, 34(11): 1195-1196
doi: 10.3866/PKU.WHXB201804094
Abstract:
2018, 34(11): 1197-1201
doi: 10.3866/PKU.WHXB201804097
Abstract:
Perovskite CH3NH3PbI3 is an ionic crystal with suitable band gap and conductivity for optoelectronic applications. The sensitivity of the CH3NH3PbI3 crystal and its derivatives to chemical composition, film-forming process, and even moisture lead to difficulties in evaluating its electronic structure and redox behavior using electrochemical techniques. Nevertheless, full understanding of the electrochemical behavior of the perovskite crystal is certainly beneficial for tuning its redox properties and chemical stability, especially for device fabrication. We show that the band structure of CH3NH3PbI3 can be successfully evaluated based on electrochemical square wave voltammetry. The energy level of the bottom of the conduction band of the perovskite crystal was determined directly from the onset reduction potential with reference to the onset oxidation potential of ferrocene, and estimated to be −3.56 eV; the top of the valence band, at –5.07 eV, was determined indirectly after taking into consideration the bandgap, because the oxidation current of the iodide ions shields that corresponding to the valence band of the CH3NH3PbI3 crystal. The overlap of the oxidation currents from the iodide ions and the valence band of the crystal suggests that there are excess iodide ions in CH3NH3PbI3 not involved in the development of the valence band. In addition, the alternating current (AC) impedance spectra of CH3NH3PbI3 indicate that the iodide ions are not completely immobilized. These imply that the defects in the crystal are related to the iodide ions to a large extent. The electrochemistry of CH3NH3PbI3 in an organic electrolyte reveals its coupling degradation during the redox processes in square wave and cyclic voltammetry. The degradation reactions result from the reduction of lead ions and oxidation of iodide ions in the perovskite crystal. In electrochemical reduction, along with the reduction that occurs in the conduction band, the lead ions in the crystal are reduced to metallic lead, which introduces a phase change in the crystal, as revealed in cyclic voltammetry; the metallic lead can be re-oxidized electrochemically. In the case of electrochemical oxidation, the iodide ions, as well as the valence band of CH3NH3PbI3, lose electrons. The electrochemically generated iodine diffuses into the organic electrolyte gradually, which results in the loss of iodide ions in the crystal. Such loss of iodide ions continues during the cyclic redox reaction. Apparently, the electrochemical investigations on perovskite CH3NH3PbI3 show that the crystal is extremely reactive during the redox process; attention should be paid on controlling the excess iodide ions and the irreversible phase change resulting from the oxidation of lead ions.
Perovskite CH3NH3PbI3 is an ionic crystal with suitable band gap and conductivity for optoelectronic applications. The sensitivity of the CH3NH3PbI3 crystal and its derivatives to chemical composition, film-forming process, and even moisture lead to difficulties in evaluating its electronic structure and redox behavior using electrochemical techniques. Nevertheless, full understanding of the electrochemical behavior of the perovskite crystal is certainly beneficial for tuning its redox properties and chemical stability, especially for device fabrication. We show that the band structure of CH3NH3PbI3 can be successfully evaluated based on electrochemical square wave voltammetry. The energy level of the bottom of the conduction band of the perovskite crystal was determined directly from the onset reduction potential with reference to the onset oxidation potential of ferrocene, and estimated to be −3.56 eV; the top of the valence band, at –5.07 eV, was determined indirectly after taking into consideration the bandgap, because the oxidation current of the iodide ions shields that corresponding to the valence band of the CH3NH3PbI3 crystal. The overlap of the oxidation currents from the iodide ions and the valence band of the crystal suggests that there are excess iodide ions in CH3NH3PbI3 not involved in the development of the valence band. In addition, the alternating current (AC) impedance spectra of CH3NH3PbI3 indicate that the iodide ions are not completely immobilized. These imply that the defects in the crystal are related to the iodide ions to a large extent. The electrochemistry of CH3NH3PbI3 in an organic electrolyte reveals its coupling degradation during the redox processes in square wave and cyclic voltammetry. The degradation reactions result from the reduction of lead ions and oxidation of iodide ions in the perovskite crystal. In electrochemical reduction, along with the reduction that occurs in the conduction band, the lead ions in the crystal are reduced to metallic lead, which introduces a phase change in the crystal, as revealed in cyclic voltammetry; the metallic lead can be re-oxidized electrochemically. In the case of electrochemical oxidation, the iodide ions, as well as the valence band of CH3NH3PbI3, lose electrons. The electrochemically generated iodine diffuses into the organic electrolyte gradually, which results in the loss of iodide ions in the crystal. Such loss of iodide ions continues during the cyclic redox reaction. Apparently, the electrochemical investigations on perovskite CH3NH3PbI3 show that the crystal is extremely reactive during the redox process; attention should be paid on controlling the excess iodide ions and the irreversible phase change resulting from the oxidation of lead ions.
2018, 34(11): 1202-1210
doi: 10.3866/PKU.WHXB201803271
Abstract:
Solution-processed bulk-heterojunction organic solar cells (BHJ-OSCs), with their advantages of light weight, low cost, and easy fabrication, are a photovoltaic technology with practical potentials. In BHJ-OSCs, the exciton dissociation and charge transport are highly sensitive to the molecular packing pattern and phase separation morphology in the active layer. On the other hand, when using photovoltaic small molecules (SMs), the purity can be controlled due to their well-defined chemical structure, and therefore there is better reproducibility in device performance. Especially, the non-fullerene acceptors are easier to tune in their light absorption and energy level. Hence, there has been considerable interest in small non-fullerene SM organic solar cells (NF-SM-OSCs). Although these cells have the dual advantages of non-fullerene acceptor materials and SMs, the fabrication of high-efficiency cells still possess great challenges. For example, efficient photovoltaic SMs typically possess an acceptor-donor-acceptor (A-D-A) structure that causes intrinsic anisotropy, making it more complicated to modulate and control the morphology of the nanoscale active layer. In this article, we will summarize recent advances in high-performance NF-SM-OSCs, and present an introduction of the specific requirements for SM donors in the small NF-SM-OSCs. We first summarize our works on SM donors with the A-D-A structure. The trialkylthienyl-substituted benzodithiophene (TriBDT-T) unit is employed as the D-core unit, and the A end groups include rhodanine (RN), cyano-rhodanine (RCN), and 1, 3-indanone (IDO). The band gap (Eg) of these compounds is about 2.0 eV, with the low-lying highest occupied molecular orbital (HOMO) level of -5.51 eV. First, NF-SM-OSCs with DRTB-T and a non-fullerene acceptor (IDIC) were constructed. The morphology of the active layer was fine-tuned by solvent vapor annealing (SVA), leading to the formation of the desired interconnected nanoscale structure. Our results demonstrate that the molecular design of a wide band gap (WBG) donor to create a well-matched donor-acceptor pair with a low band gap (LBG) non-fullerene SM acceptor, as well as subtle morphological control, provides great potential to realize high-performance NFSM-OSCs. We also studied the molecular orientation optimization from the aspect of molecular design. We designed and synthesized a group of SM compounds having identical π-conjugated backbones and end groups with different alkyl chain lengths. Since these compounds have identical photoelectric properties, they allow us to focus on the significant influence of the end alkyl chains on the molecular orientation and intermolecular aggregation behavior in solid-state films. Characterization of the DRTB-T-CX films using 2D grazing incidence wide-angle X-ray scattering (GIWAXS) revealed an obvious transition of orientation from edge-on to face-on relative to the substrate when the end alkyl chain is lengthened. This demonstrates that the length of the end alkyl chain can be used to modify the molecular orientation. A DRTB-T-C4/IT-4F-based device achieved a maximum power conversion efficiency (PCE) of up to 11.24%, which is the best performance reported for state-of-the-art NF-SM-OSCs. On this basis, the challenges and prospects of NF-SM-OSCs are discussed.
Solution-processed bulk-heterojunction organic solar cells (BHJ-OSCs), with their advantages of light weight, low cost, and easy fabrication, are a photovoltaic technology with practical potentials. In BHJ-OSCs, the exciton dissociation and charge transport are highly sensitive to the molecular packing pattern and phase separation morphology in the active layer. On the other hand, when using photovoltaic small molecules (SMs), the purity can be controlled due to their well-defined chemical structure, and therefore there is better reproducibility in device performance. Especially, the non-fullerene acceptors are easier to tune in their light absorption and energy level. Hence, there has been considerable interest in small non-fullerene SM organic solar cells (NF-SM-OSCs). Although these cells have the dual advantages of non-fullerene acceptor materials and SMs, the fabrication of high-efficiency cells still possess great challenges. For example, efficient photovoltaic SMs typically possess an acceptor-donor-acceptor (A-D-A) structure that causes intrinsic anisotropy, making it more complicated to modulate and control the morphology of the nanoscale active layer. In this article, we will summarize recent advances in high-performance NF-SM-OSCs, and present an introduction of the specific requirements for SM donors in the small NF-SM-OSCs. We first summarize our works on SM donors with the A-D-A structure. The trialkylthienyl-substituted benzodithiophene (TriBDT-T) unit is employed as the D-core unit, and the A end groups include rhodanine (RN), cyano-rhodanine (RCN), and 1, 3-indanone (IDO). The band gap (Eg) of these compounds is about 2.0 eV, with the low-lying highest occupied molecular orbital (HOMO) level of -5.51 eV. First, NF-SM-OSCs with DRTB-T and a non-fullerene acceptor (IDIC) were constructed. The morphology of the active layer was fine-tuned by solvent vapor annealing (SVA), leading to the formation of the desired interconnected nanoscale structure. Our results demonstrate that the molecular design of a wide band gap (WBG) donor to create a well-matched donor-acceptor pair with a low band gap (LBG) non-fullerene SM acceptor, as well as subtle morphological control, provides great potential to realize high-performance NFSM-OSCs. We also studied the molecular orientation optimization from the aspect of molecular design. We designed and synthesized a group of SM compounds having identical π-conjugated backbones and end groups with different alkyl chain lengths. Since these compounds have identical photoelectric properties, they allow us to focus on the significant influence of the end alkyl chains on the molecular orientation and intermolecular aggregation behavior in solid-state films. Characterization of the DRTB-T-CX films using 2D grazing incidence wide-angle X-ray scattering (GIWAXS) revealed an obvious transition of orientation from edge-on to face-on relative to the substrate when the end alkyl chain is lengthened. This demonstrates that the length of the end alkyl chain can be used to modify the molecular orientation. A DRTB-T-C4/IT-4F-based device achieved a maximum power conversion efficiency (PCE) of up to 11.24%, which is the best performance reported for state-of-the-art NF-SM-OSCs. On this basis, the challenges and prospects of NF-SM-OSCs are discussed.
2018, 34(11): 1211-1220
doi: 10.3866/PKU.WHXB201804095
Abstract:
Intrinsically conductive polymers are a class of exciting materials since they combine the advantages of both metals and plastics. But their application is limited due to the issues related to their electronic properties, stability and processibility. For example, although polyacetylene can have electrical conductivity comparable to metals, it degrades fast in air. Most of the conductive polymers in the conductive state, such as polypyrrole and polythiophene, cannot be dispersed in any solvent and cannot be turned to a melt. It is thus difficult to process them into thin films with good quality, while thin films with good quality are important for many applications. In terms of the materials processing, polyaniline (PANi) and poly(3, 4-ethylenedioxythiophene) (PEDOT) have gained great attention. PANi doped with some large cations can be dispersed in some toxic organic solvents, and poly(3, 4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) can be dispersed in water and some polar organic solvents. But the PANi and PEDOT:PSS films prepared from their solutions are usually low. Recently, great progress was made in improving the properties of intrinsically conductive polymers. The conductivity of PEDOT:PSS can be enhanced from 10-1 S·cm-1 to > 4000 S·cm-1 through the so-called "secondary doping". The high conductivity together with the solution processibility enables the application of conductive polymers in many areas, such as electrodes and thermoelectric conversion. In addition, due to their electrochemical activity, conductive polymers or their composites with inorganic materials can have high capacity of charge storage. Conductive polymers can also be added into the electrodes of batteries, because they can facilitate the charge transport and alleviate the large volume change problem of silicon electrode of batteries. It has been demonstrated that conductive polymers can have important application in many areas, such as transparent electrode, stretchable electrode, neural interfaces, thermoelectric conversion and energy storage system. This article provides a brief review on the enhancement of the electrical conductivity of intrinsically conductive polymers and their application as electrodes and in thermoelectric conversion, supercapacitors and batteries.
Intrinsically conductive polymers are a class of exciting materials since they combine the advantages of both metals and plastics. But their application is limited due to the issues related to their electronic properties, stability and processibility. For example, although polyacetylene can have electrical conductivity comparable to metals, it degrades fast in air. Most of the conductive polymers in the conductive state, such as polypyrrole and polythiophene, cannot be dispersed in any solvent and cannot be turned to a melt. It is thus difficult to process them into thin films with good quality, while thin films with good quality are important for many applications. In terms of the materials processing, polyaniline (PANi) and poly(3, 4-ethylenedioxythiophene) (PEDOT) have gained great attention. PANi doped with some large cations can be dispersed in some toxic organic solvents, and poly(3, 4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) can be dispersed in water and some polar organic solvents. But the PANi and PEDOT:PSS films prepared from their solutions are usually low. Recently, great progress was made in improving the properties of intrinsically conductive polymers. The conductivity of PEDOT:PSS can be enhanced from 10-1 S·cm-1 to > 4000 S·cm-1 through the so-called "secondary doping". The high conductivity together with the solution processibility enables the application of conductive polymers in many areas, such as electrodes and thermoelectric conversion. In addition, due to their electrochemical activity, conductive polymers or their composites with inorganic materials can have high capacity of charge storage. Conductive polymers can also be added into the electrodes of batteries, because they can facilitate the charge transport and alleviate the large volume change problem of silicon electrode of batteries. It has been demonstrated that conductive polymers can have important application in many areas, such as transparent electrode, stretchable electrode, neural interfaces, thermoelectric conversion and energy storage system. This article provides a brief review on the enhancement of the electrical conductivity of intrinsically conductive polymers and their application as electrodes and in thermoelectric conversion, supercapacitors and batteries.
2018, 34(11): 1221-1238
doi: 10.3866/PKU.WHXB201803231
Abstract:
Organic-inorganic perovskite solar cells (PSCs) have become one of the most promising solar cells, as the power conversion efficiency (PCE) has increased from less than 5% in 2009 to certified values of over 22%. In the typical PSC device architecture, hole transport materials that can effectively extract and transmit holes from the active layer to the counter electrode (HTMs) are indispensable. The well-known small molecule 2, 2', 7, 7'-tetrakis-(N, N-di-4-methoxy-phenyl amino)-9, 9'-spirobifluorene (spiro-OMeTAD) is the best choice for optimal perovskite device performance. Nevertheless, there is a consensus that spiro-OMeTAD by itself is not stable enough for long-term use in devices due to the sophisticated oxidation process associated with undesired ion migration/interactions. It has been found that spiro-OMeTAD can significantly contribute to the overall cost of materials required for the PSC manufacturing, thus its market price makes its use in large-scale production costly. Besides, another main drawback of spiro-OMeTAD is its poor reproducibility. To engineer HTMs that are considerably cheaper and more reproducible than spiro-OMeTAD, shorter reaction schemes with simple purification procedures are required. Furthermore, HTMs must possess a number of other qualities, including excellent charge transporting properties, good energy matching with the perovskite, transparency to solar radiation, a large Stokes shift, good solubility in organic solvents, morphologically stable film formation, and others. To date, hundreds of new organic semiconductor molecules have been synthesized for use as HTMs in perovskite solar cells. Successful examples include azomethine derivatives, branched methoxydiphenylamine-substituted fluorine derivatives, enamine derivatives, and many others. Some of these have been incorporated as HTMs in complete, functional PSCs capable of matching the performance of the best-performing PSCs prepared using spiro-OMeTAD while showing even better stability. In light of these results, we describe the advances made in the synthesis of HTMs that have been tested in perovskite solar cells, and give an overview of the molecular engineering of HTMs. Meanwhile, we highlight the effects of molecular structure on PCE and device stability of PSCs. This review is organized as follows. In the first part, we give a general introduction to the development of PSCs. In the second part, we focus on the introduction of the perovskite structure, device architecture, and relevant work principles in detail. In the third part, we discuss all kinds of molecular HTMs applied in PSCs. Special emphasis is placed on the relationship between HTM molecular structure and device function. Last but not least, we point out some existing challenges, suggest possible routes for further HTM design, and provide some conclusions.
Organic-inorganic perovskite solar cells (PSCs) have become one of the most promising solar cells, as the power conversion efficiency (PCE) has increased from less than 5% in 2009 to certified values of over 22%. In the typical PSC device architecture, hole transport materials that can effectively extract and transmit holes from the active layer to the counter electrode (HTMs) are indispensable. The well-known small molecule 2, 2', 7, 7'-tetrakis-(N, N-di-4-methoxy-phenyl amino)-9, 9'-spirobifluorene (spiro-OMeTAD) is the best choice for optimal perovskite device performance. Nevertheless, there is a consensus that spiro-OMeTAD by itself is not stable enough for long-term use in devices due to the sophisticated oxidation process associated with undesired ion migration/interactions. It has been found that spiro-OMeTAD can significantly contribute to the overall cost of materials required for the PSC manufacturing, thus its market price makes its use in large-scale production costly. Besides, another main drawback of spiro-OMeTAD is its poor reproducibility. To engineer HTMs that are considerably cheaper and more reproducible than spiro-OMeTAD, shorter reaction schemes with simple purification procedures are required. Furthermore, HTMs must possess a number of other qualities, including excellent charge transporting properties, good energy matching with the perovskite, transparency to solar radiation, a large Stokes shift, good solubility in organic solvents, morphologically stable film formation, and others. To date, hundreds of new organic semiconductor molecules have been synthesized for use as HTMs in perovskite solar cells. Successful examples include azomethine derivatives, branched methoxydiphenylamine-substituted fluorine derivatives, enamine derivatives, and many others. Some of these have been incorporated as HTMs in complete, functional PSCs capable of matching the performance of the best-performing PSCs prepared using spiro-OMeTAD while showing even better stability. In light of these results, we describe the advances made in the synthesis of HTMs that have been tested in perovskite solar cells, and give an overview of the molecular engineering of HTMs. Meanwhile, we highlight the effects of molecular structure on PCE and device stability of PSCs. This review is organized as follows. In the first part, we give a general introduction to the development of PSCs. In the second part, we focus on the introduction of the perovskite structure, device architecture, and relevant work principles in detail. In the third part, we discuss all kinds of molecular HTMs applied in PSCs. Special emphasis is placed on the relationship between HTM molecular structure and device function. Last but not least, we point out some existing challenges, suggest possible routes for further HTM design, and provide some conclusions.
2018, 34(11): 1239-1249
doi: 10.3866/PKU.WHXB201803272
Abstract:
Organic solar cells (OSCs) have received widespread attention for their advantages of cheap, light, flexible characteristics and roll-to-roll printing technology. However, the efficiencies of OSCs are still lower than 50% of the theoretical Shockley-Queisser detailed-balance efficiency limit. Consequently, to further improve device performance, it is significant to develop molecular design strategies to lower the energy loss and enhance the utilization of absorbed photons. From the molecular design aspects, down-shifting energy levels is an effective way to lowering the energy loss in order to obtain a high open circuit voltage, and optimizing the morphology is an efficient approach to lowering the fill factor and current density loss. Introduction of fluorine atom in molecules is an effective molecular design strategy to realize both above-mentioned requirements. In this review, starting from the characteristics of fluorine atoms, we summarized the fluorination effects on adjusting molecular levels. Whether the fluorine attached to the donor units, acceptor units or π-bridge units, it could efficiently downshift the energy levels. However, fluorinating the molecular backbone affects the energy levels more significantly than fluorinating the side chains of the two-dimensional structures. The introduction of fluorine is also an effective approach to optimize molecular packing and morphology. Generally, whether the fluorine attached to the donor units, acceptor units or π-bridge units, it can effectively increase molecular coherence length, decrease π–π stacking distance, and enhance domain purity. However, there is a saturation of the fluorine on the backbone, further introduction of the fluorine can accelerate molecular aggregation and induce disorder. In addition, the position of fluorination is important. In this review, we also briefly discuss the fluorination strategy for representative and high-efficiency photovoltaic material designs, including small molecule, polymer, and non-fullerene OSCs, mainly focusing on improving efficiency by reducing the efficiency losses. Fluorination is advantageous only for OSCs with high HOMO energy levels or poor molecular packing; otherwise, it can compromise device performance. OSCs based on narrow band-gap non-fullerene acceptors with low energy loss show promise for highly efficient device performance. Fluorination provides an effective means to fine-tune energy levels and form ideal microstructures to further reduce the efficiency loss and achieve a breakthrough in device performance.
Organic solar cells (OSCs) have received widespread attention for their advantages of cheap, light, flexible characteristics and roll-to-roll printing technology. However, the efficiencies of OSCs are still lower than 50% of the theoretical Shockley-Queisser detailed-balance efficiency limit. Consequently, to further improve device performance, it is significant to develop molecular design strategies to lower the energy loss and enhance the utilization of absorbed photons. From the molecular design aspects, down-shifting energy levels is an effective way to lowering the energy loss in order to obtain a high open circuit voltage, and optimizing the morphology is an efficient approach to lowering the fill factor and current density loss. Introduction of fluorine atom in molecules is an effective molecular design strategy to realize both above-mentioned requirements. In this review, starting from the characteristics of fluorine atoms, we summarized the fluorination effects on adjusting molecular levels. Whether the fluorine attached to the donor units, acceptor units or π-bridge units, it could efficiently downshift the energy levels. However, fluorinating the molecular backbone affects the energy levels more significantly than fluorinating the side chains of the two-dimensional structures. The introduction of fluorine is also an effective approach to optimize molecular packing and morphology. Generally, whether the fluorine attached to the donor units, acceptor units or π-bridge units, it can effectively increase molecular coherence length, decrease π–π stacking distance, and enhance domain purity. However, there is a saturation of the fluorine on the backbone, further introduction of the fluorine can accelerate molecular aggregation and induce disorder. In addition, the position of fluorination is important. In this review, we also briefly discuss the fluorination strategy for representative and high-efficiency photovoltaic material designs, including small molecule, polymer, and non-fullerene OSCs, mainly focusing on improving efficiency by reducing the efficiency losses. Fluorination is advantageous only for OSCs with high HOMO energy levels or poor molecular packing; otherwise, it can compromise device performance. OSCs based on narrow band-gap non-fullerene acceptors with low energy loss show promise for highly efficient device performance. Fluorination provides an effective means to fine-tune energy levels and form ideal microstructures to further reduce the efficiency loss and achieve a breakthrough in device performance.
2018, 34(11): 1250-1263
doi: 10.3866/PKU.WHXB201804041
Abstract:
As new types of carbon nanomaterials, carbon quantum dots (CQDs) have received widespread attention for their potential applications in optoelectronic device owing to their unique properties such as long hot-electron lifetime, high electron mobility, tunable bandgap, strong stable florescence, solution-processability, stability, non-toxicity, and low cost. Correspondingly, there has been several interesting developments in researches focusing on CQDs. In this review, we will present an update the on the latest research on the synthesis, morphology, structural characteristics, and optoelectronic properties of CQDs. The latter are determined by quantum confinement effect and surface defects. Using bottom-up synthesis methods, CQDs with higher crystallinity and less surface defects could be obtained by accurately designing the precursors and reaction conditions. The structures could be characterized by high-resolution transmission electron microscopy. Secondly, the latest progress on photoelectric devices, including light-emitting diodes (LEDs), solar cells (SCs), and photodetectors (PDs), are summarized in detail. CQDs-based LEDs are divided into photoluminescence (PL) and electroluminescence (EL) LEDs owing to their different excitation modes. Recently, PL LEDs leveled with developed QDs-based LEDs in both luminous efficiency and color rendering index (CRI). With the discovery of their bandgap emission, CQDs overcame carrier injection, which is determined by surface defects and molecule states, and presented excellent potential in EL applications. Moreover, their broad absorption in the ultraviolet-to-visible light range and high electron mobility make CQDs preferable for improving energy conversion efficiency of SCs and responsivity of PDs. Finally, we delineate current challenges on studying CQDs. Its indefinite fluorescence mechanism and structural characterizations limit the development of CQDs. Furthermore, large-scale synthesis methods for CQDs with high quantum yields and crystallinity are not yet established, which hinders their utility in optoelectronic devices. Moreover, CQDs with narrow emission bandwidth (full width at half maximum, FWHM ≤ 35 nm) still do not exist, which restrains their applications in display and laser. Hence, researches on CQDs-based optoelectronic applications are still in the first stages of development. We hope that this review will indicate future directions and encourage critical thinking to elicit new discoveries on CQDs from both fundamental and applied researches. Consequently, the potential of environment-friendly CQDs can be realized in optoelectronics and more areas.
As new types of carbon nanomaterials, carbon quantum dots (CQDs) have received widespread attention for their potential applications in optoelectronic device owing to their unique properties such as long hot-electron lifetime, high electron mobility, tunable bandgap, strong stable florescence, solution-processability, stability, non-toxicity, and low cost. Correspondingly, there has been several interesting developments in researches focusing on CQDs. In this review, we will present an update the on the latest research on the synthesis, morphology, structural characteristics, and optoelectronic properties of CQDs. The latter are determined by quantum confinement effect and surface defects. Using bottom-up synthesis methods, CQDs with higher crystallinity and less surface defects could be obtained by accurately designing the precursors and reaction conditions. The structures could be characterized by high-resolution transmission electron microscopy. Secondly, the latest progress on photoelectric devices, including light-emitting diodes (LEDs), solar cells (SCs), and photodetectors (PDs), are summarized in detail. CQDs-based LEDs are divided into photoluminescence (PL) and electroluminescence (EL) LEDs owing to their different excitation modes. Recently, PL LEDs leveled with developed QDs-based LEDs in both luminous efficiency and color rendering index (CRI). With the discovery of their bandgap emission, CQDs overcame carrier injection, which is determined by surface defects and molecule states, and presented excellent potential in EL applications. Moreover, their broad absorption in the ultraviolet-to-visible light range and high electron mobility make CQDs preferable for improving energy conversion efficiency of SCs and responsivity of PDs. Finally, we delineate current challenges on studying CQDs. Its indefinite fluorescence mechanism and structural characterizations limit the development of CQDs. Furthermore, large-scale synthesis methods for CQDs with high quantum yields and crystallinity are not yet established, which hinders their utility in optoelectronic devices. Moreover, CQDs with narrow emission bandwidth (full width at half maximum, FWHM ≤ 35 nm) still do not exist, which restrains their applications in display and laser. Hence, researches on CQDs-based optoelectronic applications are still in the first stages of development. We hope that this review will indicate future directions and encourage critical thinking to elicit new discoveries on CQDs from both fundamental and applied researches. Consequently, the potential of environment-friendly CQDs can be realized in optoelectronics and more areas.
2018, 34(11): 1264-1271
doi: 10.3866/PKU.WHXB201804096
Abstract:
In the past decade, perovskite solar cells (Pero-SCs) have attracted a great deal of attention owing to their soaring power conversion efficiency (PCE), up to 22.7% in 2017. In p-i-n type Pero-SCs, one of the most commonly used hole transport layer (HTL) materials is poly(3, 4-ethylene-dioxythiophene):polystyrenesulfonate (PEDOT: PSS), which possesses a high coverage and an extremely smooth surface. However, the inferior electrical conductivity (or large series resistance) and lower work function (WF) of PEDOT:PSS relative to many other HTL materials limits the open-circuit voltages of Pero-SCs. Furthermore, the hygroscopic property and the acidic nature of PEDOT:PSS can readily cause the degradation of perovskite, and thereby affect the long-term stability of Pero-SCs. The abovementioned disadvantages can hinder the application of PEDOT:PSS in high-performance and stable Pero-SCs; therefore, many efforts have been made to modify PEDOT:PSS to prevent these disadvantages, for instance, adding various organic solvents, surfactants, salts, or acids to PEDOT:PSS as dopants. In this paper, we report a simple codoping method to modify PEDOT:PSS, i.e., employing L-3, 4-dihydroxyphenylalanine (DOPA) and dimethyl sulfoxide (DMSO) as codopants in PEDOT:PSS, and applying it as a HTL in p-i-n type Pero-SCs. Herein, DOPA and DMSO were mixed separately with PEDOT:PSS to obtain HTLs for comparison. The DMSO-doped PEDOT:PSS improved the conductivity of the PEDOT:PSS film, while the DOPA-doped PEDOT:PSS tuned the WF of the PEDOT:PSS film. Hence, codoping of DMSO and DOPA not only allows for a good match of the energy levels between PEDOT:PSS and the perovskite but also leads to an improvement in the conductivity of PEDOT:PSS. The champion PCE of the Pero-SCs increased from 13.35% to 17.54% after DOPA and DMSO were codoped in PEDOT:PSS. Owing to their aligned energy levels and enhanced charge transportation, the detailed photovoltaic parameters were greatly improved. Scanning electron microscope and X-ray diffraction were used to characterize the morphological change and crystallinity of the perovskite films. Morphological characterization also revealed that the density of grain boundaries in the perovskite films decreased, which should alleviate the charge recombination occurring in the photoactive layer. Both steady-state photoluminescence (PL) and time-resolved PL characterizations were carried out, and they indicated that nonradiative recombination increased for the perovskite films prepared on the doped PEDOT:PSS films. This result explains the improved short-circuit current density. Electrochemical impedance spectroscopy was employed to determine the resistances of the solar cells. The results are consistent with device performance and that reflected in the PL spectra.
In the past decade, perovskite solar cells (Pero-SCs) have attracted a great deal of attention owing to their soaring power conversion efficiency (PCE), up to 22.7% in 2017. In p-i-n type Pero-SCs, one of the most commonly used hole transport layer (HTL) materials is poly(3, 4-ethylene-dioxythiophene):polystyrenesulfonate (PEDOT: PSS), which possesses a high coverage and an extremely smooth surface. However, the inferior electrical conductivity (or large series resistance) and lower work function (WF) of PEDOT:PSS relative to many other HTL materials limits the open-circuit voltages of Pero-SCs. Furthermore, the hygroscopic property and the acidic nature of PEDOT:PSS can readily cause the degradation of perovskite, and thereby affect the long-term stability of Pero-SCs. The abovementioned disadvantages can hinder the application of PEDOT:PSS in high-performance and stable Pero-SCs; therefore, many efforts have been made to modify PEDOT:PSS to prevent these disadvantages, for instance, adding various organic solvents, surfactants, salts, or acids to PEDOT:PSS as dopants. In this paper, we report a simple codoping method to modify PEDOT:PSS, i.e., employing L-3, 4-dihydroxyphenylalanine (DOPA) and dimethyl sulfoxide (DMSO) as codopants in PEDOT:PSS, and applying it as a HTL in p-i-n type Pero-SCs. Herein, DOPA and DMSO were mixed separately with PEDOT:PSS to obtain HTLs for comparison. The DMSO-doped PEDOT:PSS improved the conductivity of the PEDOT:PSS film, while the DOPA-doped PEDOT:PSS tuned the WF of the PEDOT:PSS film. Hence, codoping of DMSO and DOPA not only allows for a good match of the energy levels between PEDOT:PSS and the perovskite but also leads to an improvement in the conductivity of PEDOT:PSS. The champion PCE of the Pero-SCs increased from 13.35% to 17.54% after DOPA and DMSO were codoped in PEDOT:PSS. Owing to their aligned energy levels and enhanced charge transportation, the detailed photovoltaic parameters were greatly improved. Scanning electron microscope and X-ray diffraction were used to characterize the morphological change and crystallinity of the perovskite films. Morphological characterization also revealed that the density of grain boundaries in the perovskite films decreased, which should alleviate the charge recombination occurring in the photoactive layer. Both steady-state photoluminescence (PL) and time-resolved PL characterizations were carried out, and they indicated that nonradiative recombination increased for the perovskite films prepared on the doped PEDOT:PSS films. This result explains the improved short-circuit current density. Electrochemical impedance spectroscopy was employed to determine the resistances of the solar cells. The results are consistent with device performance and that reflected in the PL spectra.
2018, 34(11): 1272-1278
doi: 10.3866/PKU.WHXB201803221
Abstract:
Polymer solar cells (PSCs) with bulk heterojunction (BHJ) structures have seen rapid development in recent years. In comparison with their inorganic counterparts, PSCs have some advantages such as low cost, light weight, solution processability, and good mechanical flexibility. However, improvement of the power conversion efficiency (PCE) of PSCs is required for commercial applications. In order to achieve high-performance PSCs, active layers, including donor polymers and acceptors, are very important. Several design principles for conjugated donor polymers in PSCs have emerged, including optimization of the conjugated backbone, side-chains, and substituents. In the past few decades, various classes of electron-donating polymers have been reported for PSCs. Among them, quinoxaline (Qx) is a unique building block for the construction of different optoelectronic polymers because of its planar, rigid, and conjugated structure. Qx derivatives have proven interesting and have been widely employed in many fields. Qx-based conjugated polymers (or small molecules) can be easily modified to match with ball-like fullerene derivatives such as PCBM ([6, 6]-phenyl-C61 or C71-butyric acid methyl ester) or weak crystalline non-fullerene acceptors such as 2, 2'-[[6, 6, 12, 12-tetrakis(4-hexylphenyl)-6, 12, -dihydrodithieno[2, 3-d:2', 3'-d']-s-indaceno[1, 2-b:5, 6-b']dithiophene-2, 8-diyl]bis[methylidyne(3-oxo-1H-indene-2, 1(3H)-diylidene)]]bispropanedinitrile (ITIC). Herein, we synthesized a Qx-based polymer with asymmetric side-chains (TPQ-1). The molecular weight, optical properties, molecular energy levels, and mobilities of TPQ-1 were investigated. Furthermore, the blend morphologies and photovoltaic properties of TPQ-1 using a strong crystalline non-fullerene (NF) acceptor (o-IDTBR) were systematically explored. The photovoltaic performance of TPQ-1 and its symmetric side-chain counterpart, HFQx-T, was compared. The introduction of asymmetric side-chains led to a favorable phase separation when blended with o-IDTBR. As expected, the TPQ-1:o-IDTBR-based devices exhibited a high PCE of 8.6% after thermal annealing (TA). In contrast, the HFQx-T:o-IDTBR-based devices showed a moderate PCE of 5.7%, moreover, the PCE was decreased to 4.6% after TA treatment. More importantly, a low bandgap material, PTB7-Th, was specifically selected as a third component to mix with the TPQ-1:o-IDTBR blend to form highly-efficient ternary PSCs. At an optimal weight ratio (15%) of PTB7-Th addition, a PCE of 9.6% was achieved. In the systems that were investigated, TPQ-1 demonstrated significantly better photovoltaic properties than the HFQx-T-based devices. These results indicate that Qx-based polymers with asymmetric side chains have a bright future in photovoltaic devices.
Polymer solar cells (PSCs) with bulk heterojunction (BHJ) structures have seen rapid development in recent years. In comparison with their inorganic counterparts, PSCs have some advantages such as low cost, light weight, solution processability, and good mechanical flexibility. However, improvement of the power conversion efficiency (PCE) of PSCs is required for commercial applications. In order to achieve high-performance PSCs, active layers, including donor polymers and acceptors, are very important. Several design principles for conjugated donor polymers in PSCs have emerged, including optimization of the conjugated backbone, side-chains, and substituents. In the past few decades, various classes of electron-donating polymers have been reported for PSCs. Among them, quinoxaline (Qx) is a unique building block for the construction of different optoelectronic polymers because of its planar, rigid, and conjugated structure. Qx derivatives have proven interesting and have been widely employed in many fields. Qx-based conjugated polymers (or small molecules) can be easily modified to match with ball-like fullerene derivatives such as PCBM ([6, 6]-phenyl-C61 or C71-butyric acid methyl ester) or weak crystalline non-fullerene acceptors such as 2, 2'-[[6, 6, 12, 12-tetrakis(4-hexylphenyl)-6, 12, -dihydrodithieno[2, 3-d:2', 3'-d']-s-indaceno[1, 2-b:5, 6-b']dithiophene-2, 8-diyl]bis[methylidyne(3-oxo-1H-indene-2, 1(3H)-diylidene)]]bispropanedinitrile (ITIC). Herein, we synthesized a Qx-based polymer with asymmetric side-chains (TPQ-1). The molecular weight, optical properties, molecular energy levels, and mobilities of TPQ-1 were investigated. Furthermore, the blend morphologies and photovoltaic properties of TPQ-1 using a strong crystalline non-fullerene (NF) acceptor (o-IDTBR) were systematically explored. The photovoltaic performance of TPQ-1 and its symmetric side-chain counterpart, HFQx-T, was compared. The introduction of asymmetric side-chains led to a favorable phase separation when blended with o-IDTBR. As expected, the TPQ-1:o-IDTBR-based devices exhibited a high PCE of 8.6% after thermal annealing (TA). In contrast, the HFQx-T:o-IDTBR-based devices showed a moderate PCE of 5.7%, moreover, the PCE was decreased to 4.6% after TA treatment. More importantly, a low bandgap material, PTB7-Th, was specifically selected as a third component to mix with the TPQ-1:o-IDTBR blend to form highly-efficient ternary PSCs. At an optimal weight ratio (15%) of PTB7-Th addition, a PCE of 9.6% was achieved. In the systems that were investigated, TPQ-1 demonstrated significantly better photovoltaic properties than the HFQx-T-based devices. These results indicate that Qx-based polymers with asymmetric side chains have a bright future in photovoltaic devices.
2018, 34(11): 1293-1298
doi: 10.3866/PKU.WHXB201804111
Abstract:
DNA can adopt a diverse range of structural conformations, including duplexes, triplexes, and quadruplexes. Among these structures, G-quadruplexes have attracted much more attention of researchers. For G-rich DNA sequences, they can fold into multiple G-quadruplex conformations, such as parallel, antiparallel, or hybrid, and the exact conformation is influenced by G-rich DNA sequence, strand concentration, and binding cations. Among the factors influencing the G-quadruplex conformation and stability, cations played a really important role. Numerous studies have reported cation-dependent stability and topological changes of G-quadruplexes; however, most of studies have focused on the effect of individual cation (such as charge, radii, or hydration, etc.), and only few have assessed the effect of competition between cations at different concentrations. Actually, most biological and aqueous systems contained multiple cations and each of the cations had very different concentrations. Thus, investigation of the competitions between different cations (at different concentrations) for binding with G-quadruplexes and their effects on polymorphism of G-quadruplex is critical, which would improve our understanding of the roles of G-quadruplexes and assist us in further exploring their potential applications in biochemical, biomedical, and environmental systems. Under this situation, we focused on K+- and Pb2+-stabilized G-quadruplex, two major cations that are usually used to stabilize G-quadruplex. It has been shown that for a given G-quadruplex forming DNA sequence, Pb2+-stabilized G-quadruplex was more stable than K+-stabilized G-quadruplex, and Pb2+ could substitute K+ in K+-stabilized G-quadruplex. However, the concentrations of K+ that allow such a substitution are not completely studied. Previous studies have used G-quadruplex-based fluorescent, colorimetric, and electrochemical sensors for detecting Pb2+, and these methods show excellent selectivity for Pb2+ over K+. Although G-quadruplex-based Pb2+ sensors were developed, their applications in real samples containing K+ were greatly limited. Thus, how K+ and Pb2+ compete for binding to G-quadruplex and how K+ concentrations affect the stability of Pb2+-stabilized G-quadruplex remain elusive. In this study, eight G-rich DNA sequences were selected to investigate the effect of K+ concentration on Pb2+-stabilized G-quadruplex. Previous studies have established that the presence of K+ cannot alter the stability of Pb2+-stabilized G-quadruplex. In contrast to this, our results indicated that K+ could induce a conformational switch in Pb2+-stabilized T2TT (G-rich DNA sequence, forming G-quadruplex in the presence of Pb2+), and further replace Pb2+ in Pb2+-stabilized T2TT and transform it into 2K+-stabilized T2TT, which is strictly K+ concentration-dependent. Importantly, such a conformational switch could be observed for other seven selected G-rich sequences as well. Therefore, our findings provide a new insight into the exchange and competition of cations in G-quadruplex.
DNA can adopt a diverse range of structural conformations, including duplexes, triplexes, and quadruplexes. Among these structures, G-quadruplexes have attracted much more attention of researchers. For G-rich DNA sequences, they can fold into multiple G-quadruplex conformations, such as parallel, antiparallel, or hybrid, and the exact conformation is influenced by G-rich DNA sequence, strand concentration, and binding cations. Among the factors influencing the G-quadruplex conformation and stability, cations played a really important role. Numerous studies have reported cation-dependent stability and topological changes of G-quadruplexes; however, most of studies have focused on the effect of individual cation (such as charge, radii, or hydration, etc.), and only few have assessed the effect of competition between cations at different concentrations. Actually, most biological and aqueous systems contained multiple cations and each of the cations had very different concentrations. Thus, investigation of the competitions between different cations (at different concentrations) for binding with G-quadruplexes and their effects on polymorphism of G-quadruplex is critical, which would improve our understanding of the roles of G-quadruplexes and assist us in further exploring their potential applications in biochemical, biomedical, and environmental systems. Under this situation, we focused on K+- and Pb2+-stabilized G-quadruplex, two major cations that are usually used to stabilize G-quadruplex. It has been shown that for a given G-quadruplex forming DNA sequence, Pb2+-stabilized G-quadruplex was more stable than K+-stabilized G-quadruplex, and Pb2+ could substitute K+ in K+-stabilized G-quadruplex. However, the concentrations of K+ that allow such a substitution are not completely studied. Previous studies have used G-quadruplex-based fluorescent, colorimetric, and electrochemical sensors for detecting Pb2+, and these methods show excellent selectivity for Pb2+ over K+. Although G-quadruplex-based Pb2+ sensors were developed, their applications in real samples containing K+ were greatly limited. Thus, how K+ and Pb2+ compete for binding to G-quadruplex and how K+ concentrations affect the stability of Pb2+-stabilized G-quadruplex remain elusive. In this study, eight G-rich DNA sequences were selected to investigate the effect of K+ concentration on Pb2+-stabilized G-quadruplex. Previous studies have established that the presence of K+ cannot alter the stability of Pb2+-stabilized G-quadruplex. In contrast to this, our results indicated that K+ could induce a conformational switch in Pb2+-stabilized T2TT (G-rich DNA sequence, forming G-quadruplex in the presence of Pb2+), and further replace Pb2+ in Pb2+-stabilized T2TT and transform it into 2K+-stabilized T2TT, which is strictly K+ concentration-dependent. Importantly, such a conformational switch could be observed for other seven selected G-rich sequences as well. Therefore, our findings provide a new insight into the exchange and competition of cations in G-quadruplex.
2018, 34(11): 1279-1285
doi: 10.3866/PKU.WHXB201804098
Abstract:
Over the past two decades, bulk heterojunction polymer solar cells (PSCs) have attracted significant attention owing to their potential applications in the mass fabrication of flexible device panels by roll-to-roll printing. To improve the photovoltaic performance of PSCs, much effort has been devoted to the optimization of properties of donor-acceptor (D-A) type polymer donor materials. Until now, the development of high-performance donor polymers is mainly dependent on the design and synthesis of binary polymers with a regular D/A alternating skeleton. Compared to binary polymers, random terpolymers with three different donor or acceptor monomer units possess synergetic effects of their inherent properties, such as optical absorption ability, energy levels, crystallinity, charge mobility, and morphological compatibility with the n-OS acceptors with suitable adjustment of the molar ratio of the three monomers. However, the irregularity in the polymer backbone of the random terpolymers may have an adverse effect on molecular packing, crystallinity, and charge mobility. Therefore, design and synthesis of high-performance terpolymers for PSCs is a challenging task. In this study, a series of wide bandgap random terpolymers PSBTZ-80, PSBTZ-60, and PSBTZ-40 based on alkylthiothienyl substituted benzodithiophene as the donor unit and two weak electron-deficient acceptor units of 5, 6-difluorobenzotriazole (FBTz) and thiazolothiazole (TTz) were designed and synthesized for PSC applications. The optical, electrochemical, molecular packing, and photovoltaic properties of the polymers were effectively modulated by varying the FBTz:TTz molar ratio. Therefore, the PSC based on PSBTZ-60 as the donor material and narrow bandgap small molecule 3, 9-bis(2-methylene-(3-(1, 1-dicyanomethylene)-indanone))-5, 5, 11, 11-tetrakis(4-hexyl-phenyl)-dithieno[2, 3-d:2', 3'-d']-s-indaceno[1, 2-b:5, 6-b']di thiophene) (ITIC) as the acceptor, processed using halogen-free solvents, exhibited high power conversion efficiency (PCE) of 10.3% with high open-circuit voltage (Voc) of 0.91 V, improved short-circuit current density (Jsc) of 18.0 mA∙cm−2, and fill factor (FF) of 62.7%, which are superior to those of PSCs based on binary polymers PSBZ (a PCE of 8.1%, Voc of 0.89 V, Jsc of 14.7 mA∙cm−2, and FF of 61.5%) and PSTZ (a PCE of 8.5%, Voc of 0.96 V, Jsc of 14.9 mA∙cm−2, and FF of 59.1%). These results indicate that random terpolymerization is a simple and practical strategy for the development of high-performance polymer photovoltaic materials.
Over the past two decades, bulk heterojunction polymer solar cells (PSCs) have attracted significant attention owing to their potential applications in the mass fabrication of flexible device panels by roll-to-roll printing. To improve the photovoltaic performance of PSCs, much effort has been devoted to the optimization of properties of donor-acceptor (D-A) type polymer donor materials. Until now, the development of high-performance donor polymers is mainly dependent on the design and synthesis of binary polymers with a regular D/A alternating skeleton. Compared to binary polymers, random terpolymers with three different donor or acceptor monomer units possess synergetic effects of their inherent properties, such as optical absorption ability, energy levels, crystallinity, charge mobility, and morphological compatibility with the n-OS acceptors with suitable adjustment of the molar ratio of the three monomers. However, the irregularity in the polymer backbone of the random terpolymers may have an adverse effect on molecular packing, crystallinity, and charge mobility. Therefore, design and synthesis of high-performance terpolymers for PSCs is a challenging task. In this study, a series of wide bandgap random terpolymers PSBTZ-80, PSBTZ-60, and PSBTZ-40 based on alkylthiothienyl substituted benzodithiophene as the donor unit and two weak electron-deficient acceptor units of 5, 6-difluorobenzotriazole (FBTz) and thiazolothiazole (TTz) were designed and synthesized for PSC applications. The optical, electrochemical, molecular packing, and photovoltaic properties of the polymers were effectively modulated by varying the FBTz:TTz molar ratio. Therefore, the PSC based on PSBTZ-60 as the donor material and narrow bandgap small molecule 3, 9-bis(2-methylene-(3-(1, 1-dicyanomethylene)-indanone))-5, 5, 11, 11-tetrakis(4-hexyl-phenyl)-dithieno[2, 3-d:2', 3'-d']-s-indaceno[1, 2-b:5, 6-b']di thiophene) (ITIC) as the acceptor, processed using halogen-free solvents, exhibited high power conversion efficiency (PCE) of 10.3% with high open-circuit voltage (Voc) of 0.91 V, improved short-circuit current density (Jsc) of 18.0 mA∙cm−2, and fill factor (FF) of 62.7%, which are superior to those of PSCs based on binary polymers PSBZ (a PCE of 8.1%, Voc of 0.89 V, Jsc of 14.7 mA∙cm−2, and FF of 61.5%) and PSTZ (a PCE of 8.5%, Voc of 0.96 V, Jsc of 14.9 mA∙cm−2, and FF of 59.1%). These results indicate that random terpolymerization is a simple and practical strategy for the development of high-performance polymer photovoltaic materials.
2018, 34(11): 1286-1292
doi: 10.3866/PKU.WHXB201803222
Abstract:
The hole injection layer (HIL) plays a significant role in determining the performances of organic light-emitting diodes (OLEDs), especially when hole transport materials with deep highest occupied molecular orbital levels (HOMOs) are employed. Intensive efforts have been devoted to exploring novel hole injection materials with good solution-processing abilities in recent years. In this study, the solution-processed molybdenum trioxide (s-MoO3) is prepared via an ultra-facile method. Three different s-MoO3 layers prepared by three different methods, viz. layers annealed at 150 ℃ (s-MoO3 (150)), layers annealed at 150 ℃ and then processed in UV-ozone for 15 min (s-MoO3 (150, UVO)), and layers processed in UV-ozone for 15 min without annealing (s-MoO3 (UVO)), are obtained to investigate their influences on hole injection. The device with the s-MoO3 (150) layer has the lowest current density and the largest driving voltage, showing poor hole injection ability. In contrast, with the s-MoO3 (150, UVO) layer as HIL, the OLED produces a greatly enhanced current and sharply reduced driving voltage, comparable to the device using vacuum-evaporated MoO3. Similar results are obtained for the device with the s-MoO3 (UVO) film, suggesting that high-temperature annealing is not essential for the s-MoO3 film with UV-ozone treatment. Hole injection efficiencies of MoO3 films are quantitatively characterized by analyzing the space-charge-limited current of hole-only devices; the hole injection efficiencies of s-MoO3 (150, UVO) and s-MoO3 (UVO)-based devices are ~0.1, far exceeding that of the s-MoO3 (150)-based device (10−5). XPS analysis is performed to detect the impact of the above treatments on the surface electronic properties of the s-MoO3 films. A typical characteristic of Mo5+ species is obtained for the s-MoO3 (150) film and a high-binding-energy shoulder appears in the O 1s peak of the s-MoO3 (150) film, indicating the existence of oxygen vacancies and oxygen adsorbed at the surface of s-MoO3 (150) film. When UV-ozone treatment is applied to this s-MoO3 (150) film, it produces a decrease of Mo5+ state and elimination of oxygen-rich adsorbates, resulting in MoO3 stoichiometry similar to that of the vacuum-evaporated MoO3 film. Consequently, a maximum current efficiency of 48.3 cd∙A−1 is realized with the optimized UV-ozone treated s-MoO3 HIL. It This UV-ozone treated s-MoO3 should have widespread applications in low-cost solution-processed OLEDs as an excellent hole injection layer.
The hole injection layer (HIL) plays a significant role in determining the performances of organic light-emitting diodes (OLEDs), especially when hole transport materials with deep highest occupied molecular orbital levels (HOMOs) are employed. Intensive efforts have been devoted to exploring novel hole injection materials with good solution-processing abilities in recent years. In this study, the solution-processed molybdenum trioxide (s-MoO3) is prepared via an ultra-facile method. Three different s-MoO3 layers prepared by three different methods, viz. layers annealed at 150 ℃ (s-MoO3 (150)), layers annealed at 150 ℃ and then processed in UV-ozone for 15 min (s-MoO3 (150, UVO)), and layers processed in UV-ozone for 15 min without annealing (s-MoO3 (UVO)), are obtained to investigate their influences on hole injection. The device with the s-MoO3 (150) layer has the lowest current density and the largest driving voltage, showing poor hole injection ability. In contrast, with the s-MoO3 (150, UVO) layer as HIL, the OLED produces a greatly enhanced current and sharply reduced driving voltage, comparable to the device using vacuum-evaporated MoO3. Similar results are obtained for the device with the s-MoO3 (UVO) film, suggesting that high-temperature annealing is not essential for the s-MoO3 film with UV-ozone treatment. Hole injection efficiencies of MoO3 films are quantitatively characterized by analyzing the space-charge-limited current of hole-only devices; the hole injection efficiencies of s-MoO3 (150, UVO) and s-MoO3 (UVO)-based devices are ~0.1, far exceeding that of the s-MoO3 (150)-based device (10−5). XPS analysis is performed to detect the impact of the above treatments on the surface electronic properties of the s-MoO3 films. A typical characteristic of Mo5+ species is obtained for the s-MoO3 (150) film and a high-binding-energy shoulder appears in the O 1s peak of the s-MoO3 (150) film, indicating the existence of oxygen vacancies and oxygen adsorbed at the surface of s-MoO3 (150) film. When UV-ozone treatment is applied to this s-MoO3 (150) film, it produces a decrease of Mo5+ state and elimination of oxygen-rich adsorbates, resulting in MoO3 stoichiometry similar to that of the vacuum-evaporated MoO3 film. Consequently, a maximum current efficiency of 48.3 cd∙A−1 is realized with the optimized UV-ozone treated s-MoO3 HIL. It This UV-ozone treated s-MoO3 should have widespread applications in low-cost solution-processed OLEDs as an excellent hole injection layer.
