Transient Time-resolved Photoluminescence Research Articles

Type-I CdSe@CdS@ZnS Heterostructured Nanocrystals with Long Fluorescence Lifetime.

Conventional single-component quantum dots (QDs) suffer from low recombination rates of photogenerated electrons and holes, which hinders their ability to meet the requirements for LED and laser applications. Therefore, it is urgent to design multicomponent heterojunction nanocrystals with these properties. Herein, we used CdSe quantum dot nanocrystals as a typical model, which were synthesized by means of a colloidal chemistry method at high temperatures. Then, CdS with a wide band gap was used to encapsulate the CdSe QDs, forming a CdSe@CdS core@shell heterojunction. Finally, the CdSe@CdS core@shell was modified through the growth of the ZnS shell to obtain CdSe@CdS@ZnS heterojunction nanocrystal hybrids. The morphologies, phases, structures and performance characteristics of CdSe@CdS@ZnS were evaluated using various analytical techniques, including transmission electron microscopy, X-ray diffraction, UV-vis absorption spectroscopy, fluorescence spectroscopy and time-resolved transient photoluminescence spectroscopy. The results show that the energy band structure is transformed from type II to type I after the ZnS growth. The photoluminescence lifetime increases from 41.4 ns to 88.8 ns and the photoluminescence quantum efficiency reaches 17.05% compared with that of pristine CdSe QDs. This paper provides a fundamental study and a new route for studying light-emitting devices and biological imaging based on multicomponent QDs.

Comparing defect-assisted interfacial charge transfer in CdS/PSA and CdS/TiO2[sbnd]NTAs nanoheterojunctions for enhanced photodegradation and biosensing

Firstly, oxygen vacancy defects are intentionally introduced in the preparation process of TiO2 nanotube arrays (TiO2-NTAs) and porous silicon arrays (PSA) films through the anodic oxidation method after annealing treatment, as expected. Then, CdS/TiO2-NTAs and CdS/PSA heterostructures are successfully fabricated using the electrodeposition method, incorporating abundant surface vacancy defects. The crafted CdS/TiO2-NTAs nanoheterojunctions presented elevated photoelectrochemical (PEC) performances and sensing activities compared with that of CdS/PSA, validating by photodegradation and PEC biosensing tests, which mainly ascribed to the synergistic effect between the promoted carriers separation efficiency and boosted surface active sites induced by vacancies defects. Furthermore, a comparative and comprehensive interfacial charge transfer mechanism and qualitative dynamic processes for CdS/TiO2-NTAs and CdS/PSA type-II nanoheterojunctions are proposed, revealing an enlarged staggered band offset between CdS and TiO2-NTAs. The evaluation of this band offset was conducted using nanosecond time-resolved transient photoluminescence and steady photoluminescence spectra, respectively. Following this scheme, the photodegradation rate for CdS/TiO2-NTAs nanocomplex toward degradation of MO was 92.8% and 1.06 times than that of CdS/PSA; while the PEC biosensing performance for CdS/TiO2-NTAs showed a detection range 0 – 600 μM and limit of detection of 2.3 μM, with a sensitivity of 456 mA cm−2M−1.

Comprehension of the Synergistic Effect between m&t-BiVO4/TiO2-NTAs Nano-Heterostructures and Oxygen Vacancy for Elevated Charge Transfer and Enhanced Photoelectrochemical Performances.

Through the utilization of a facile procedure combined with anodization and hydrothermal synthesis, highly ordered alignment TiO2 nanotube arrays (TiO2-NTAs) were decorated with BiVO4 with distinctive crystallization phases of monoclinic scheelite (m-BiVO4) and tetragonal zircon (t-BiVO4), favorably constructing different molar ratios and concentrations of oxygen vacancies (Vo) for m&t-BiVO4/TiO2-NTAs heterostructured nanohybrids. Simultaneously, the m&t-BiVO4/TiO2-NTAs nanocomposites significantly promoted photoelectrochemical (PEC) activity, tested under UV-visible light irradiation, through photocurrent density testing and electrochemical impedance spectra, which were derived from the positive synergistic effect between nanohetero-interfaces and Vo defects induced energetic charge transfer (CT). In addition, a proposed self-consistent interfacial CT mechanism and a convincing quantitative dynamic process (i.e., rate constant of CT) for m&t-BiVO4/TiO2-NTAs nanoheterojunctions are supported by time-resolved photoluminescence and nanosecond time-resolved transient photoluminescence spectra, respectively. Based on the scheme, the m&t-BiVO4/TiO2-NTAs-10 nanohybrids exhibited a photodegradation rate of 97% toward degradation of methyl orange irradiated by UV-visible light, 1.14- and 1.04-fold that of m&t-BiVO4/TiO2-NTAs-5 and m&t-BiVO4/TiO2-NTAs-20, respectively. Furthermore, the m&t-BiVO4/TiO2-NTAs-10 nanohybrids showed excellent PEC biosensing performance with a detection limit of 2.6 μM and a sensitivity of 960 mA cm-2 M-1 for the detection of glutathione. Additionally, the gas-sensing performance of m&t-BiVO4/TiO2-NTAs-10 is distinctly superior to that of m&t-BiVO4/TiO2-NTAs-5 and m&t-BiVO4/TiO2-NTAs-20 in terms of sensitivity and response speed.

Self-assembly synthesis of S-scheme g-C3N4/Bi8(CrO4)O11 for photocatalytic degradation of norfloxacin and bisphenol A

To realize the high-efficiency photodegradation of antibiotics, a novel S-scheme heterojunction photocatalyst g-C3N4/Bi8(CrO4)O11 was proposed and successfully prepared in this work. The 10% g-C3N4/Bi8(CrO4)O11 heterojunction exhibits the highest degradation rate of norfloxacin (NOR) and bisphenol A (BPA). The degradation rate of NOR on 10% g-C3N4/Bi8(CrO4)O11 is about 1.38 and 2.33 times higher than that of pure Bi8(CrO4)O11 and g-C3N4, respectively. Further, the degradation rate of BPA over 10% g-C3N4/Bi8(CrO4)O11 heterojunction is bout 1.35 and 9.11 times higher than that of pure Bi8(CrO4)O11 and g-C3N4, respectively. The formation of S-scheme heterojunction facilitates the separation of photogenerated electron-hole pairs and reduces the recombination of charge carriers, which was confirmed by photocurrent, electrochemical impedance spectroscopy, steady-state and time-resolved transient photoluminescence spectrum, etc. The in-situ X-ray photoelectron spectroscopy, radical trapping experiments and electron paramagnetic resonance results demonstrate that the charge transfer is in accord with S-scheme mechanism.

Copper phosphide decorated g-C3N4 catalysts for highly efficient photocatalytic H2 evolution

Designing functional heterojunctions to enhance photocatalytic hydrogen evolution is still a key challenge in the field of efficient solar energy utilization. Copper phosphides become an ideal material to serve as the cocatalysts during photocatalytic hydrogen evolution by virtue of the lower prices. In this study, we synthesized graphitic carbon nitride (g-C3N4) based catalysts loaded with copper phosphide (Cu3P, Cu97P3), which exhibit superior performance in photocatalytic H2 evolution. Ultraviolet (UV)-visible spectroscopy illustrated that the absorption of light strengthened after the loading of copper phosphide, and the time-resolved transient photoluminescence (PL) spectra showed that the separation and transfer of the photoexcited carriers greatly improved. Moreover, both copper phosphide/g-C3N4 photocatalysts exhibited a relatively high H2 evolution rate: Cu3P/g-C3N4 (maximum 343 μmol h−1 g−1), Cu97P3/g-C3N4 (162.9 μmol h−1 g−1) while copper phosphide themself exhibit no photocatalytic activity. Thus, the copper phosphides (Cu3P, Cu97P3) work as a cocatalyst during photocatalytic H2 evolution. The cycling experiments illustrated that both copper phosphide/g-C3N4 photocatalysts perform excellent stability in the photocatalytic H2 evolution. It is worth noting that while the NaH2PO2 was heated in the tube furnace for phosphorization to obtain Cu3P, the excessive PH3 could pass through the solution of CuSO4 to obtain Cu97P3 at the same time, which significantly improved the utilization of PH3 and reduced the risk of toxicity. This work could provide new strategies to design photocatalysts decorated with copper phosphide for highly efficient visible-light-driven hydrogen evolution.

Insights on interfacial charge transfer across MoS2/TiO2-NTAs nanoheterostructures for enhanced photodegradation and biosensing&gas-sensing performance

Novel MoS2 nanobelts (NBs) decorated highly ordered alignment TiO2 nanotubes arrays (TiO2-NTAs) with promoted photoelectrocatalytic (PEC) photodegradation and sensing performance under UV-Visible light irradiation were successfully synthesized via a facile combination of anodization and electrodeposition procedure. Simultaneously, the MoS2/TiO2-NTAs nanohybrids presented a significantly enhanced for PEC activity tests, which were mainly ascribed to the synergistic effect between MoS2 NBs and TiO2-NTAs for the efficient separation of photogenerated carriers induced by the enlarged staggered band offset associated with sulfur vacancies defects. Furthermore, self-consistent interfacial charge transfer mechanism and convincing quantitative dynamic process (i.e. rate constant of charge transfer) for MoS2/TiO2-NTAs type-II nanoheterojunctions mediated with the p-type doping effect are proposed, which were evaluated by nanosecond time-resolved transient photoluminescence (NTRT-PL) and time-resolved photoluminescence (TRPL) spectra, respectively. Following this scheme, the photodegradation rate (η) toward degradation of methyl orange for optimum MoS2/TiO2-NTAs-2 nanocomplex was 96% irradiated by UV-Visible light, which are 1.2 and 1.1 times than that of MoS2/TiO2-NTAs-1 and MoS2/TiO2-NTAs-5, respectively. Furthermore, the PEC biosensing analysis of MoS2/TiO2-NTAs-2 was performed for detecting the glutathione, leading to an excellent detect limit of 1.6 μM and the sensitivity of 930 mA cm-2 M-1. Additionally, the sensitivity of gas-sensing for MoS2/TiO2-NTAs-2 is 3.1, which are 1.3 and 1.1 folds than that of MoS2/TiO2-NTAs with MoS2 deposited times 1 min and 5 min, respectively. It is distinctly manifested that the gas sensing performance of MoS2/TiO2-NTAs-2 toward ammonia gas is superior than that of MoS2/TiO2-NTAs-1 and MoS2/TiO2-NTAs-5 in terms of higher sensitivity degree and faster response speed.

Dependence of the intrinsic phase structure of Bi2O3 catalysts on photocatalytic CO2 reduction

The combination of time-resolved transient photoluminescence with in-situ Fourier transform infrared spectroscopy has been conducted to investigate the intrinsic phase structure-dependent activity of Bi2O3 catalyst for CO2 reduction.

Oxygen Vacancy-Mediated Interfacial Charge Transfer of Au/ZnO Schottky Heterojunctions for Enhanced UV Photodegradation

We intend to report an interesting phenomenon related to the different interfacial transfer processes between ellipsoidal-like ZnO (E-ZnO) and rod-like ZnO (R-ZnO) nanoheterojunctions witness by the nanosecond time-resolved transient photoluminescence (NTRT-PL) spectra. Fristly, E-ZnO and R-ZnO nanoarchitectures were fabricated via facilitating the electrochemical route; and then, they decorated it with dispersed Au nanoparticles (NPs) by the methods of ion-sputtering deposition, constituting Au/E-ZnO and Au/R-ZnO Schottky-heterojunction nanocomplex, which is characterized by SEM, XRD, Raman analysis, and UV-vis absorption spectra. Steady-state photoluminescence and NTRT-PL spectra of as-fabricated Au/E-ZnO and Au/R-ZnO nanocomposites were probed for interfacial charge transfer process under 266 nm femtosecond (fs) light irradiation. Simultaneously, a distinct diversification for the NTRT-PL spectra is observed, closely associating with oxygen vacancies (Vo), which is confirmed by X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) spectra. Furthermore, Au NPs act as an “annular bridge” and “transit depot” for interfacial charge transfer through local surface plasmon resonance (LSPR) effect and Schottky barrier, respectively, which is identified by NTRT-PL and time-resolved PL (TRPL) decay spectrum. Moreover, this mechanism is responsible for the enhanced photoelectrochemical (PEC) performances of methyl orange (MO) photodegradation under UV light irradiation.

Efficiency enhancement of perovskite solar cells based on Al2O3-passivated nano-nickel oxide film

Interface passivation plays a significant role in reducing surface defects in semiconductor materials. In this paper, we present a kind of Al2O3-passivated nickel oxide (NiO) film as hole transport layer for the perovskite solar cells (PSCs). The optimal Al2O3 insulating film between the NiO and perovskite active layers has a capability to decrease the defect states density at the surface of NiO and suppress the carriers recombination at NiO/perovskite interface. The passivation process could apparently improve the efficiencies of corresponding PSCs from 5.64 to 7.12%. Steady-state photoluminescence and transient time-resolved photoluminescence (TRPL) results demonstrate the restraint of charge recombination at the NiO/perovskite interface with passivation. The Hall effect test further shows that the electrical properties of the NiO film can be improved by adjusting the precursor concentration of the Al2O3, which has a certain significance for the application of interface engineering in PSCs. Furthermore, this method has the advantages of simple operation, low cost, and strong reproducibility, which is also good for the working stability of PSCs.

Au-Mediated Charge Transfer Process of Ternary Cu2O/Au/TiO2-NAs Nanoheterostructures for Improved Photoelectrochemical Performance.

Based on a facile three-step preparation method, Cu2O/Au/TiO2-NAs ternary heterojunction nanocomposites have been successfully synthesized by electrodepositing a Cu2O layer on the surface of Au nanoparticles (NPs) decorated highly ordered TiO2 nanotube arrays (NAs). The structure, surface morphology, chemical composition, and optical and intrinsic defects properties of the as-prepared samples are characterized by transmission and scanning electron microscopy (TEM and SEM), X-ray diffraction (XRD), UV–vis light absorbance spectra, Raman scattering, and X-ray photoelectron spectroscopy (XPS). Simultaneously, the Cu2O/Au/TiO2-NAs ternary nanohybrids exhibited progressively improved photoelectrocatalytic (PEC) performance compared with the dual Cu2O/TiO2-NAs type-II nanoheterojunctions, confirming by the photocurrent density versus testing time curve (amperometric I–t curve), open-circuit potential versus testing time curve (Voc–t curve), and electrochemical impedance spectroscopy (EIS) measurements, which were mainly ascribed to the synergistic effect of reduced interfacial charge transfer resistance and boosted energetic charge carriers generation associated with embedding Au NPs. Furthermore, the self-consistent charge transfer mechanism of Z-scheme and interband transitions mediated with Au NPs for Cu2O/Au/TiO2-NAs triple nanocomposites is proposed, which was evaluated by nanosecond time-resolved transient photoluminescence (NTRT-PL) spectra excited by 266 and 400 nm, respectively. Following this scheme, UV–vis light photocatalytic activities of Cu2O/Au/TiO2-NAs ternary nanohybrids were elaborated toward photodegradation of methyl orange (MO) in aqueous solution, and the photodegradation rate of optimum triple nanocomplex was found to be 90%.

Asymmetric embedded benzene ring enhances charge transfer of carbon nitride for photocatalytic hydrogen generation

Preventing the high carrier recombination rate of graphitized C3N4 (GCN) is an urgent problem to be solved for its application as a photocatalyst for hydrogen production. Here, we first rationally embed the benzene ring in GCN to modify the local symmetry without changing its long-order structure. Theoretical calculation predicts that this design can change the electronic structure and promote the effective charge transfer in GCN. The benzene ring embedded GCN is successfully prepared by copolymerization using dicyandiamide and terephthalonitrile as precursors. Photoluminescence (PL) and time-resolved transient PL (TRPL) spectra confirm that the electron transfer efficiency of the benzene ring embed GCN is greatly improved. This nanomaterial displays 10.8 times higher photocatalytic hydrogen production rate than that of pristine GCN with the apparent quantum yield of 11.3% at 400 nm and 9% at 420 nm. This work provides a novel strategy for designing high-efficiency two-dimensional (2D) photocatalytic materials for water splitting.

Indepth Studies on Working Mechanism of Plasmon-Enhanced Inverted Perovskite Solar Cells Incorporated with Ag@SiO2 Core–Shell Nanocubes

Noble metal nanoparticles-induced localized surface plasmon resonance as a useful approach has been widely used in solar cells including perovskite solar cells (PSCs) to improve their light-harvesting. Herein, we synthesize Ag@SiO2 core–shell nanocubes and investigate their application in CH3NH3PbI3-based PSCs due to both the large local EM field induced by the nanocube with sharp corners and the effective avoidance of exciton/carrier recombination at the surfaces of Ag nanocubes via covering a ∼5 nm ultrathin SiO2 shell. Incorporating an appropriate concentration of Ag@SiO2 nanocubes into the CH3NH3PbI3 PSCs realizes a best-performing efficiency of 17.22% with an enhancement factor of 18.1%. Indepth studies on the plasmon-enhanced working mechanism of Ag@SiO2 nanocubes with UV–vis absorption spectra, steady-state and time-resolved transient photoluminescence, and electrochemical impedance spectroscopy characterizations eventually demonstrate both the increasing light harvesting and the improving charge transportation and extraction contribute to better performances of PSCs.

Electrochemical deposition synthesis of ZnO-NA/Cu2O-NPs type-II hierarchical heterojunction for enhanced photoelectrochemical degradation of methyl orange (MO)

Highly ordered ZnO nanorods array (ZnO-NA) films were fabricated on fluorine-doped tin oxide (FTO) substrates via a facilitate one-step electrochemical deposition route. Cu2O nanoparticles (Cu2O-NPs) films were then electrochemical deposited on the surface of ZnO-NA to constituted the ZnO-NA/Cu2O-NPs type-II hierarchical heterojunction. Steady-state photoluminescence (PL) spectra and nanosecond time-resolved transient photoluminescence (NTRT-PL) spectra of as-fabricated nanocomposites are studied, under light irradiation with wavelength of 266 nm. Simultaneously, a distinct 520 nm transient PL emission peak in the NTRT-PL spectra is observed owing to the presence of oxygen vacancy (OV) defect states, which is further convinced by the X-ray photoelectron spectroscopy (XPS). The PL decay spectroscopy confirms that ZnO-NA/Cu2O-NPs heterojunction could implement more facilitated avenue for transform and separation of photogenerated carriers associated with OV. Besides, the synthesized ZnO-NA/Cu2O-NPs nanocomposites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–vis spectrophotometer, Raman scattering spectra and linear sweep voltammogram (LSV) measurement, respectively. Our present research demonstrates a new approach to investigate charge separation and transport in a dual-band gap photoelectrochemical (PEC) system. Following this scheme, UV and visible-light photocatalytic activities of ZnO-NA/Cu2O-NPs heterojunction nanocomposites were evaluated through photodegradation of methyl orange (MO) in aqueous solution.

Boosted Electron Transport and Enlarged Built-In Potential by Eliminating the Interface Barrier in Organic Solar Cells.

A smart interface modification strategy was employed to simultaneously improve short-circuit current density (Jsc) and open-circuit voltage (Voc) by incorporating a poly[(9,9-bis(3'-(N,N-dimethylamion)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl)-fluorene] (PFN) interlayer between a TiO2 film and an active layer, arising from the fact that PFN effectively eliminated the interface barrier between TiO2 and the fullerene acceptor. The work function (WF) of TiO2 was apparently reduced, which facilitated effective electron transfer from the active layer to the TiO2 electron transport layer (ETL) and suppressed charge carrier recombination between contact interfaces. Electron injection devices with and without a PFN interlayer were fabricated to prove the eliminated electron barrier, meanwhile photoluminescence (PL) and time-resolved transient photoluminescence (TRTPL) were measured to probe much easier electron transfer from [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) acceptor to TiO2 ETL, contributing to enhanced Jsc. The shift in vacuum level altered the WF of PC71BM, which enlarged the internal electrical field at the donor/acceptor interface and built-in potential (Vbi) across the device. Dark current characteristics and Mott-Schottky measurements indicated the enhancement of Vbi, benefiting to increased Voc. Consequently, the champion power conversion efficiency for a device with a PFN interlayer of 0.50 mg/mL reached to 7.14%, which is much higher than the PCE of 5.76% for the control device.

Photon-induced interfacial charge transfer mechanism of porous silicon/TiO 2 nanoparticles for photoelectrochemical performance

Abstract Highly ordered porous silicon array (PSA) films were fabricated by anodic oxidation of p-type silicon wafer. Using the methods of magnetron sputtering and thermal annealing, dispersed TiO 2 nanoparticles (NPs) were decorated onto the top of PSA films. Steady-state photoluminescence (PL) spectra and nanosecond time-resolved transient photoluminescence (NTRT-PL) spectra of as-fabricated nano-composites were studied under light irradiation with wavelengths of 266 nm and 400 nm, respectively. A distinct blue-shift in the NTRT-PL spectra is observed owing to the presence of Si 3+ and Ti 3+ cations, which is further confirmed by X-ray photoelectron spectroscopy (XPS). Such a phenomenon could be well explained by considering the competition mechanism between the photo-generated charge separation process and recombination associated with oxygen vacancy (OV). Our present research demonstrates a novel approach to investigate charge separation and transport in a dual-band gap photoelectrochemical (PEC) system. Following this scheme, UV and visible-light photocatalytic activities of TiO 2 /PSA heterojunction composites were evaluated through photodegradation of methyl orange (MO) in aqueous solution. Our current work is expected to provide a simple strategy for synthesising composite photocatalysts, with the hope that optimal sunlight harvesting would be achieved.

Exciton Quenching Behavior of Thermally Activated Delayed Fluorescence Molecules by Charge Carriers

The quenching of singlet excitons by injected charge carriers in molecules that display thermally activated delayed fluorescence (TADF) was investigated using time-resolved transient photoluminescence (PL) techniques. Injected electrons did not affect the excitons; however, injected holes caused significant quenching. Using a rate-equation analysis, the hole-induced exciton quenching rate was determined to be between 10–11 and 10–12 cm3 s–1. Interestingly, the TADF emission component was enhanced in the presence of injected holes, plausibly due to a reduction of the singlet-exciton energy level.

Optically modulated charge transfer in TiO2-Au nano-complexes

Highly ordered TiO2 nanotube array (TNA) films were fabricated by anodic oxidation of Ti foil. Au nanoparticles (NPs) are decorated onto the top of TNA films with the aid of ion-sputtering and thermal annealing. The steady-state photoluminescence (PL) spectra, as well as x-ray photoelectron spectroscopy (XPS) and Raman analysis, confirm the presence of Ti3+ valence states in the prepared TNA films. The UV–vis absorption spectra show that the photo-response of as-prepared samples is extended from UV to the visible light region. From the nanosecond time-resolved transient photoluminescence (NTRT-PL) spectra, Ti3+-related PL intensity is observed to vary distinctly with the deposition time of Au NPs. Such a phenomenon could be explained by considering the modulation of oxygen vacancy densities and charge states in TNA films by surface loading Au NPs. The enhanced visible-light photocatalytic activities of the Au-TNA composite were evaluated through the photodegradation of methyl orange (MO) in aqueous solution by UV–vis absorption spectrometry. The decoration of Au NPs plays an essential role in enhancing visible-light photocatalytic activity, because energetic photoelectrons are able to inject to the conduction band of TiO2 owing to the surface plasmon resonance (SPR) effect. It is hoped that our current work will provide a simple strategy to synthesize defect-related composite for photocatalytic applications.

Transient competition between photocatalysis and carrier recombination in TiO2 nanotube film loaded with Au nanoparticles

Highly ordered TiO2 nanotube array (TNA) films are fabricated by using an anodic oxidation method. Au nanoparticles (NPs) films are decorated onto the top of TNA films with the aid of ion-sputtering and thermal annealing. An enhanced photocatalytic activity under ultraviolet C (UVC, 266 nm) light irradiation is obtained compared with that of the pristine TNA, which is shown by the steady-state photoluminescence (PL) spectra. Furthermore, a distinct blue shift in the nanosecond time-resolved transient photoluminescence (NTRT-PL) spectra is observed. Such a phenomenon could be well explained by considering the competition between the surface photocatalytic process and the recombination of the photo-generated carriers. The enhanced UV photocatalytic activities of the Au—TNA composite are evaluated through photo-degradation of methyl orange (MO) in an aqueous solution with ultraviolet—visible absorption spectrometry. Our current work may provide a simple strategy to synthesize defect-related composite photocatalytic devices.

Improved efficiency of bulk heterojunction polymer solar cells by doping low-bandgap small molecules.

We present performance improved ternary bulk heterojunction polymer solar cells by doping a small molecule, 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (DIB-SQ), into the common binary blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). The optimized power conversion efficiency (PCE) of P3HT:PC71BM-based cells was improved from 3.05% to 3.72% by doping 1.2 wt % DIB-SQ as the second electron donor, which corresponds to ∼22% PCE enhancement. The main contributions of doping DIB-SQ material on the improved performance of PSCs can be summarized as (i) harvesting more photons in the low-energy range, (ii) increased exciton dissociation, energy transfer, and charge carrier transport in the ternary blend films. The energy transfer process from P3HT to DIB-SQ is demonstrated by time-resolved transient photoluminescence spectra through monitoring the lifetime of 700 nm emission from neat P3HT, DIB-SQ and blended P3HT:DIB-SQ solutions. The lifetime of 700 nm emission is increased from 0.9 ns for neat P3HT solution, to 4.9 ns for neat DIB-SQ solution, to 6.2 ns for P3HT:DIB-SQ blend solution.

Flexible top-emitting warm-white organic light-emitting diodes with highly luminous performances and extremely stable chromaticity

The flexible top-emitting white organic light-emitting diode (FTEWOLED) with a very high efficiency but a significant color alteration is achieved with a blue/red/blue sandwiched tri-emission-layer. The voltage-dependent recombination region alternation and the emission mechanism are systematically investigated through a delta-doping method and the time-resolved transient photoluminescence lifetime measurement. By locating the main exciton recombination region at the 4,4′,4″-Tris(carbazol-9-yl)-triphenylamine (TCTA) and 9,9-spirobifluoren-2-yl-diphenyl-phosphine oxide (SPPO1) interface, replacing the carrier-trapping red dopant guest with an orange guest that utilizes energy transfer mechanism, and using a P–I–N structure together with the FIrpic blue guest dopant to balance the electron and hole carriers, an extremely color stable and a very high efficient FTEWOLED is fabricated, with the resulting high current and power efficiencies of 22.7cd/A and 14.27lm/W, and a warm white illumination with a small chromaticity variation of (−0.0087, +0.0015) over a broad luminance range of more than four orders of magnitude. In addition, the performances can be further improved to 23,340cd/m2, 24.49cd/A and 15.39lm/W with a slight concentration alteration of the orange emitter.