Energy Barrier For Electron Injection Research Articles

Modulating J-V hysteresis of planar perovskite solar cells and mini-modules via work function engineering

Commercialization of perovskite solar cells (PSCs) requires the development of high-efficiency devices with none current density-voltage (J-V) hysteresis. Here, electron transport layers (ETLs) with gradual change in work function (WF) are successfully fabricated and employed as an ideal model to investigate the energy barriers, charge transfer and recombination kinetics at ETL/perovskite interface. The energy barrier for electron injection existing at ETL/perovskite is directly assessed by surface photovoltage microscopy, and the results demonstrate the tunable barriers have significant impact on the J-V hysteresis and performance of PSCs. By work function engineering of ETL, PSCs exhibit PCEs over 21% with negligible hysteresis. These results provide a critical understanding of the origin reason for hysteresis effect in planar PSCs, and clear reveal that the J-V hysteresis can be effectively suppressed by carefully tuning the interface features in PSCs. By extending this strategy to a modified formamidinium-cesium-rubidium (FA-Cs-Rb) perovskite system, the PCEs are further boosted to 24.18%. Moreover, 5 cm × 5 cm perovskite mini-modules are also fabricated with an impressive efficiency of 20.07%, demonstrating compatibility and effectiveness of our strategy on upscaled devices.

Conjugated oxazole‐based interfacial materials for efficient and stable inverted polymer solar cell with an efficiency of 16.52%

Organic solar cells (OSCs) using non-fullerene acceptors have delivered the highest efficiencies in the overall reported literatures. In order to improve the efficiency and stability of OSCs, great efforts are being made in designing and synthesizing new materials. Inverted OSCs were fabricated using three commercially available and inexpensive materials, 5-methylbenzoxazole (E1), 2-(4-biphenyl)-5-phenyloxazole (E2), and 4-bis(5-phenyl-2-oxazolyl)benzene (E3), as an interlayer between zinc oxide and the active layer. The new interlayer materials enhance the carrier injection/extraction properties and thus, the polymer solar cells (PSCs) exhibited an improved JSC and power conversion efficiency (PCE). PSCs showed an improved JSC of 27.18 from 24.88 mA/cm2 with the introduction of E3. The new interlayer, E3 in particular, forms well-aligned cascading energy levels between the PM6:Y6 active layer and zinc oxide layer. These cascading energy levels reduce the energy barrier for electron injection and collection at the interface. Thus, the PCE of devices reached a remarkable 16.52% using interlayer at the ZnO/active layer interface. The devices also exhibited improved device stability under continuous illumination, annealing, and high humidity.

Photovoltaic and spectroscopic properties of bacteriochlorin-based photosensitizer: molecular approach

The biophotovoltaic features of the porphyrin- and bacteriochlorin-based solar cells in different media were investigated through quantum chemistry calculations. The results show that a decrease in the chemical potential and electrophilicity facilitates the charge transfer process. Also, the bacteriochlorins have a lower energy barrier of electron injection than that of porphyrin, because of a longer charge transfer distance and less electron–hole overlap. Moreover, nonpolar solvents accelerate the free charge production, due to the favorable changes in the exciton radius and exciton binding. Improved spectroscopic properties are observed for the dyes in the solvent medium, which may be related to an increment in the probability of the electronic transition and reduced band gaps. Based on the simulated absorption spectra, bacteriochlorins are the preferred light-harvesting materials than porphyrin in the visible region. Moreover, the development of the push–pull model in the bacteriochlorin structure increases their energy conversion ability, due to advanced quantum chemistry reactivity properties. According to the final efficiency, bacteriochlorins are the preferred candidates in comparison with porphyrin to be applied in the dye-sensitized solar cells, in agreement with the experimental results.

The role of solvent and structure in the kinetics of the excitons in porphyrin-based hybrid solar cells

In this study, we investigated the rate of exciton formation and dissociation in TiO2/Si/porphyrins hybrid solar cells in different phases. Using different porphyrin-based donors in four sets, it was found the best hybrid architectures belong the donors containing metal and alkyl group and those exposed the solvents. The proper kinetic of the excitonic processes in these hybrid solar cells are originated from lower affinity energy and exciton binding energy of their donors. Also, donors with the lowest energy barrier of electron injection as well as the highest excitonic Bohr radius showed the excitons can be dissociated in the dye/semiconductors interface, easily. The analysis of the probable correlations between the molecular descriptors and kinetic parameters of the excitons presented more credible evidences about the solvent and metal effects on the photovoltaic processes. Finally, based on the other performance characters of these solar cells, it was concluded that maximum value of the external quantum efficiency belongs to the metalloporphyrins in the solvent media.

Theoretical study on effect of thiophene substitution on the structure and phosphorescence quantum yields of red-emitting iridium(III) emitters in OLEDs

Abstract The electronic structures and photophysical properties of Ir(tpy)2(acac) complex (1) and the derivative (2, 3) with thiophene at different positions of tpy ligands have been investigated with density functional theory (DFT) and time-dependent density functional theory (TD-DFT), where tpy = 2,2-thienylpyridine, acac = acetylacetonate. 3a and 3b are studied to get insight into the influence of different positions of fluorine atoms on the photophysical properties of 3. The calculated results reveal that the introduction of thiophene are beneficial to enhance absorption intensity, decrease LUMO energy levels, and reduce the energy barrier for electron injection compared with 1. However, the decreased amount of metal orbitals involved in the transitions, small participation of metal to ligand charge transfer contribution in both the absorption (MLCT%) and emission spectra (3MLCT%), large energy difference between the S1 and Tm (ΔES1−Tm) for 2 (m = 1–2) and 3 (m = 1–3) compared with those of 1 account for their relatively low phosphorescence quantum yields (Φp) observed experimentally. Moreover, the incorporation of F atoms into complex 3 (3a, 3b) can be efficient approach of tuning the electron injection ability, balance of charge transfer process, and emitting color. Especially, the high quantum yield of 3b compared with 3a is explained based on the detailed analysis of the triplet energy (ET1), transition dipole moment (μS1) upon the S0 → S1, SOC matrix element between the Tm and Sn states, ΔES1−Tm, 3MLCT% in the phosphorescent spectra, energy difference between 3MLCT and electronic configurations of the triplet metal-centered (3MC) d–d excited states. These structure-property relationship is expected to provide useful information for synthesis highly efficiency phosphorescence emitters.

Combined theoretical and experimental approaches for development of squaraine dyes with small energy barrier for electron injection

A series of far-red sensitizing squaraine dyes has been systematically designed and synthesized in order to correlate the theoretically calculated values with their corresponding experimental parameters. Efforts have been directed towards determining the minimum thermodynamic energy barrier for the electron injection in the nanoporous TiO2 by logical molecular design. Theoretical calculations using Gaussian program package were performed for ground and excited states in both of the isolated gaseous state as well as in solution including the solvent effect using a self-consistent reaction field polarizable continuum model (PCM). Implementation of the PCM model or use of LSDA functional under TD-DFT calculations gives much better results for energetics as well as absorption maximum for all of the sensitizers used in this work. Newly designed symmetrical squaraine dye SQ-5 exhibits a minimum energy barrier of 0.16eV for electron injection and shows photon harvesting behavior in far-red region with external photoconversion efficiency of 2.02% under simulated solar irradiation.

Enhancing the performance of organic-inorganic hybrid light-emitting diodes by inserting a conjugated polyelectrolyte interlayer

Hybrid light-emitting diodes (HyLEDs) incorporating the advantages of organic and inorganic semiconductors, were promising to realise efficient electroluminescence with air-stable charge-injection interfaces. However, HyLEDs with only inorganic semiconductor such as zinc oxide (ZnO) as electron-injection layer suffered from low devices efficiencies due to large electron injection barrier. To improve the electron injection, conjugated polyelectrolyte poly[(9,9-bis(3’-(N,N-dimethylamino)propyl)-2,7-fluorene) -alt-2,7-(9,9-dioctylfluorene)] (PFN) was used together with ZnO as electron-injection layer by fabricating it on top of ZnO. Experimental data showed that the light efficiencies of HyLEDs were almost doubled after the insertion of PFN. The performances enhancement was attributed to the reduced energy barrier for electron injection from ZnO. The electron current increased. And therefore, the balance of electron and hole currents was improved.

A Bipodal Dicyano Anchor Unit for Single‐Molecule Spintronic Devices

The conductance through single 7,7,8,8-tetracyanoquinodimethane (TCNQ) connected to gold electrodes is studied with the nonequilibrium Green's function method combined with density functional theory. The aim of the study is to derive the effect of a dicyano anchor group, =C(CN)2, on energy level alignment between the electrode Fermi level and a molecular energy level. The strong electron-withdrawing nature of the dicyano anchor group lowers the LUMO level of TCNQ, resulting in an extremely small energy barrier for electron injection. At zero bias, electron transfer from electrodes easily occurs and, as a consequence, the anion radical state of TCNQ with a magnetic moment is formed. The unpaired electron in the TCNQ anion radical causes an exchange splitting between the spin-α and spin-β transmission spectra, allowing the single TCNQ junction to act as a spin-filtering device.

Conjugated polymers with cationic iridium(iii) complexes in the side-chain for flash memory devices utilizing switchable through-space charge transfer

Polycarbazole and polyfluorene containing cationic iridium(III) complexes in the side-chain have been designed and synthesized. Both polymers have been demonstrated to show conductance switching behavior and non-volatile flash memory devices based on them were successfully realized, in which the formation and dissociation of through-space charge-transfer states from the conjugated polymer “sea” to the Ir(III) complex “island”, controlled by voltage, are responsible for the conductance switching behavior and memory effect. The devices exhibit low reading, writing, and erasing voltages and a high ON/OFF current ratio. Both ON and OFF states are stable up to 107 read cycles at a read voltage of −1.0 V. Due to the different chemical structures of the polymer main-chain, the two devices show different threshold voltages. The polycarbazole derivative exhibits higher HOMO and LUMO levels compared with the polyfluorene analogue. Thus, the threshold voltage from the OFF to ON state of the device based on the polycarbazole derivative is obviously lower than that of the polyfluorene derivative-based device because of the low energy barrier between the work function of the ITO anode and the HOMO level of the polycarbazole derivative. Similarly, the threshold voltage from the ON to OFF state is evidently higher because the energy barrier of electron injection from Al into the LUMO of the polycarbazole derivative is slightly higher than that of the polyfluorene analogue. Thus, the threshold voltages of memory devices may be rationally modulated by modifying the chemical structure of polymers.

The π-conjugated P-flowers C16(PH)8 and C16(PF)8 are potential materials for organic n-type semiconductors

Following the theme of this special issue, two new compounds, the P-flowers C(16)(PH)(8) and C(16)(PF)(8), are designed by us and subsequently characterized by quantum chemical computations. Their geometries and infrared signatures are analyzed and compared to those of the well-known sulflower C(16)S(8). Their electronic structure and aromaticity are examined using the electron localization function (ELF) and also by the total and partial densities of state (DOS). Both C(16)(PF)(6) and C(16)(PH)(8) molecules exhibit small energy barrier of electron injection (Φ(e) = 0.33 eV for the gold electrode for the former, and Φ(e) = 0.1 eV for the calcium electrode for the latter), remarkably low reorganization energy and high rate of electron hopping. Thus, both theoretically designed P-flower molecules are predicted to be excellent candidates for organic n-type semiconductors.

Investigation of the gate-bias induced instability for InGaZnO TFTs under dark and light illumination

Mechanism of the instability for indium–gallium–zinc oxide thin film transistors caused by gate-bias stress performed in the dark and light illumination was investigated in this paper. The parallel V t shift with no degradation of subthreshold swing ( S.S) and the fine fitting to the stretched-exponential equation indicate that charge trapping model dominates the degradation behavior under positive gate-bias stress. In addition, the significant gate-bias dependence of V t shift demonstrates that electron trapping effect easily occurs under large gate-bias since the average effective energy barrier of electron injection decreases with increasing gate bias. Moreover, the noticeable decrease of threshold voltage ( V t) shift under illuminated positive gate-bias stress and the accelerated recovery rate in the light indicate that the charge detrapping mechanism occurs under light illumination. Finally, the apparent negative V t shift under illuminated negative gate-bias stress was investigated in this paper. The average effectively energy barrier of electron and hole injection were extracted to clarify that the serious V t degradation behavior comparing with positive gate-bias stress was attributed to the lower energy barrier for hole injection.

A high performance inverted organic light emitting diode using an electron transporting material with low energy barrier for electron injection

A high performance inverted green emission organic light emitting diode with a maximum external quantum efficiency of 20% and a maximum power efficiency of 80 lm/W was realized by properly selecting an electron transporting material to have no energy barrier for electron injection between the n-doped electron transporting layer (n-ETL) and the ETL. Based on the energy levels and the current density–voltage characteristics of electron only devices, we demonstrate that the interface between an n-ETL and an ETL even in homo-junction is as important as the interface between the cathode and the n-ETL for efficient electron injection into an emitting layer.

Direct Determination of Energy Level Alignment and Charge Transport atMetal−Alq3Interfaces via Ballistic-Electron-Emission Spectroscopy

Using ballistic-electron-emission spectroscopy (BEES), we directly determined the energy barrier for electron injection at clean interfaces of Alq(3) with Al and Fe to be 2.1 and 2.2 eV, respectively. We quantitatively modeled the sub-barrier BEES spectra with an accumulated space charge layer, and found that the transport of nonballistic electrons is consistent with random hopping over the injection barrier.

Structure and Ultraviolet Electroluminescence of $n \hbox{-ZnO/SiO}_{2}\hbox{-ZnO}$ Nanocomposite/$p$ -GaN Heterostructure Light-Emitting Diodes

We fabricated and characterized ultraviolet (UV) light-emitting diodes (LEDs) composed of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> -ZnO/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -ZnO nanocomposite/ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i> -GaN heterostructures. Significant UV electroluminescence at 387 nm from the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> -ZnO layer in this heterostructure LED was observed at a forward-bias current of as low as 1.8 mA. This is ascribed to the high quality of the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> -ZnO layer and the effective function of the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -ZnO nanocomposite layer. The SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -ZnO nanocomposite layer accomplishes the role of current blocking by forming the larger energy barrier for electron injection from <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> -ZnO into <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i> -GaN and also contributes to, due to its low refractive index, higher light extraction efficiency from the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> -ZnO layer.

Improved performance in n-channel organic thin film transistors by nanoscale interface modification

We demonstrate that the electrical properties of n-channel thin film transistors can be enhanced by inserting a nanoscale interfacial layer, namely, cesium carbonate (Cs2CO3), between organic semiconductor and source/drain electrodes. Devices with the Cs2CO3/Al electrode showed a reduction of contact resistance, not only with respect to Al, but also compared to Ca. The improvement is attributed to the reduction in the energy barrier of electron injection and the prevention of unfavorable chemical interaction between the organic layer and the metal electrode. High field-effect mobility of 0.045cm2/Vs and on/off current ratios of 106 were obtained in the [6,6]-phenyl C60 butyric acid methyl ester-based organic thin film transistors using the Cs2CO3/Al electrodes at a gate bias of 40V.

Theoretical Study on the Optoelectronic Properties of Electron-Withdrawing Substituted Diethynylfluorenyl Gold(I) Complexes

We report on a quantum-chemical study of the electronic and optical properties of gold(I) complex AuTFT (1) and its electron-withdrawing substitutents, AuTFOT (2) and AuTFCNT (3) [where TFT = diethynylfluorenyl, TFOT = diethynylfluorenone and TFCNT = diethynyl-(9-(dicyanomethylene)fluorene)]. Our theoretical calculations indicate that for all systems the reorganization energies of electron and hole are in the same order of magnitude and similar to those of the well-known electrontransport material Alq3. The substitution of -CO and -C(CN)2 for -CH in AuTFT significantly decreases the bond length alternation and increases the electron affinity, which would effectively lower the energy barrier for electron injection from cathode and thus qualify AuTFOT (2) and AuTFCNT (3) as candidates for the electron transport layer (ETL) in light-emitting diodes (LEDs). The lowest lying excited-states of gold(I) diethynylfluorenyl derivatives have been studied by the singles configuration interaction (CIS) method and time-dependent density functional method (TDDFT). It is found that the electron-withdrawing substitutions evidently decrease the energy gap, leading to a remarkable red shift in transition energy and transformation in the direction of charge transfer. Our research is important in the development of new functional materials for the design of LEDs with enhanced performance.

Effect of magnesium oxide buffer layer on performance of inverted top-emitting organic light-emitting diodes

The effect of magnesium oxide (MgO) buffer layer between cathode and emitting materials on performance of inverted top-emitting organic light-emitting diodes (ITOLEDs) was investigated. The operation voltage at the current density of 100mA∕cm2 decreased from 14.9to9.7V for ITOLEDs with 1nm thick MgO buffer layers. The maximum luminance value increased about 78% in ITOLEDs using MgO buffer layer, which is 1000cd∕m2 at the current density of 191mA∕cm2. Synchrotron radiation photoelectron spectroscopy results revealed that the atomic concentration of Al–O bond increased after deposition of MgO on Al, indicating the oxidation of Al surface. Secondary electron emission spectra showed that the work function increased about 0.8eV by inserting the insulating MgO buffer layer. Therefore, the enhancement of device performance results from the decrease of the energy barrier for electron injection based on the tunneling model.

Efficient Electron Injection in Organic Light Emitting Diodes with Al–Mg Alloy Cathode

We report the enhancement of electron injection using an Al-Mg alloy cathode in organic light emitting diodes (OLEDs). The luminescence value at 15 V drastically increased from 800 to 5800 cd/m 2 as the Al cathode was replaced by an Al-Mg alloy with a 1:1 ratio of Al to Mg in weight percent. Synchrotron radiation photoelectron spectroscopy results showed that the work function of the Al-Mg alloy is lower by 0.2 eV than that of Al. Thus, the energy barrier for electron injection was lowered, reducing the turn-on voltage and increasing the quantum efficiency of OLEDs.

Enhancement of electron injection in inverted top-emitting organic light-emitting diodes using an insulating magnesium oxide buffer layer

We report the enhancement of the electron injection by inserting a 1-nm-thick magnesium oxide (MgO) buffer layer between Al cathode and tris (8-hydroxyquinoline) aluminum in an inverted top-emitting organic light-emitting diode (OLED). The turn-on voltage of OLEDs decreased from 10 to 6 V and the luminance increased about 61% as the MgO interfacial layer was employed. The MgO interfacial layer played a role in reducing the energy barrier of electron injection, leading to the reduction of the turn-on voltage and the enhancement of luminance.

Poly( p-phenylene ethynylene)-based light-emitting devices

We here report polymer light-emitting diodes with substituted poly( p-phenylene ethynylenes) as the emissive layer. Yellow—green electroluminescence was observed for different poly( p-phenylene ethynylene) derivatives. Surprisingly, and importantly in view of stability issues, devices with an aluminium cathode were found to have a higher external quantum efficiency (0.035%) and a lower onset voltage for electroluminescence (10.8 V) than those with a calcium cathode. These results are explained in terms of a lower energy barrier for electron injection than for hole injection, consistent with the ionization potential of poly( p-phenylene ethynylenes) which was determined to be 6.3 eV below vacuum level with ultraviolet photoelectron spectroscopy and 5.8 eV with cyclovoltammetry.