Bilayer Heterojunction Research Articles

Carbon Nanodots as a Potential Transport Layer for Boosting Performance of All-Inorganic Perovskite Nanocrystals-Based Photodetector

A low-cost and simple drop-casting method was used to fabricate a carbon nanodot (C-dot)/all-inorganic perovskite (CsPbBr3) nanosheet bilayer heterojunction photodetector on a SiO2/Si substrate. The C-dot/perovskite bilayer heterojunction photodetector shows a high performance with a responsivity (R) of 1.09 A/W, almost five times higher than that of a CsPbBr3-based photodetector (0.21 A/W). In addition, the hybrid photodetector exhibits a fast response speed of 1.318/1.342 µs and a highly stable photocurrent of 6.97 µA at 10 V bias voltage. These figures of merits are comparable with, or much better than, most reported perovskite heterojunction photodetectors. UV–Vis absorption and photoluminescent spectra measurements reveal that the C-dot/perovskite bilayer heterojunction has a band gap similar to the pure perovskite layer, confirming that the absorption and emission in the bilayer heterojunction is dominated by the top layer of the perovskite. Moreover, the emission intensity of the C-dot/perovskite bilayer heterojunction is less than that of the pure perovskite layer, indicating that a significant number of charges were extracted by the C-dot layer. The studied band alignment of the C-dots and perovskites in the dark and under emission reveals that the photodetector has a highly efficient charge separation mechanism at the C-dot/perovskite interface, where the recombination rate between photogenerated electrons and holes is significantly reduced. This highly efficient charge separation mechanism is the main reason behind the enhanced performance of the C-dot/perovskite bilayer heterojunction photodetector.

Planar Organic Bilayer Heterojunctions Fabricated on Water with Ultrafast Donor‐to‐Acceptor Charge Transfer

Herein, planar heterojunctions comprising a nonfullerene small molecular acceptor (NFA) and a polymer donor are demonstrated by transferring polymer films on a water surface on top of NFA layers. So far, most solution‐processed layer‐by‐layer architectures have been reported as sequentially deposited bulk heterojunctions or pseudo‐bilayers because mixed regions at the donor/acceptor interface are inevitable in these methods. By virtue of the unique properties of conjugated polymers such as hydrophobicity and spontaneous film formation on a water surface, the fabrication of NFA/polymer bilayer nanostructures is clearly demonstrated by dramatically simplified methods. These bilayers are successfully rendered into bilayer organic solar cells achieving a power conversion efficiency of up to 7.47%. This reflects that these bilayers have appropriate morphological and optoelectrical properties to be operated as photoactive layers in photovoltaic devices. Further, ultrafast charge transfer from the polymer donor to the NFA and fast carrier mobility are investigated by transient‐absorption spectroscopy and photoinduced charge‐extraction measurements. Fast carrier dynamics are observed, which are essential for the efficient harvest of excitons in photovoltaic devices. It is believed that the formation of planar heterojunctions on water can offer technical diversity for the fabrication methods of the photovoltaic devices.

Preparation and optimization of heterojunction donor(DLC) – acceptor(SI) as a solar cell by DFT and PLD

Hybrid bilayer heterojunction (DLC) thin-film P-type as an active donor layer and (Si) thin-film n-type as an acceptor with (Electron Transport Layer ), were fabricated using Qswitching Nd-YAG Pulsed Laser Deposition (PLD) method under vacuum condition 10-3 torr on two electrodes for (AL) Aluminum material is used to construct the hydride bilayer heterojunction photovoltaic solar cell (PVSC). The electrical properties of hybrid heterojunction Al / DLC / SI / AL thin film were studied. The result showed the voltage of open circuit (Vm=0.145V ), a short circuit (Jm=9.3mA/cm2 ), and the fill factor (FF) of (0.4709), with Xenon lamp with an intensity (235mw/cm2 ), the conversion efficiency (η=3.54%) at thickness(75nm). While the theoretical calculations and optimization of the HETEROJUNCTION confirmed the possibility of using it as a successful solar cell, the calculation of the small energy gap and thus the high electrical conductivity, and the determination of the type of charge carriers(p), as well as some electronic properties such as electrostatic potential surfaces and 3-D electron density, hardness, softness and electrophilicity, were clarified.

Photophysical pathways in efficient bilayer organic solar cells: The importance of interlayer energy transfer

The development of organic photovoltaic (OPV) cells has long been guided by the idea that excitons – bound electron-hole pairs created by light absorption – diffuse only 5–10 nm. True for many materials, this constraint led to an inherently complex device architecture – the bulk heterojunction – that has obscured our understanding of device physics, and handicapped rational material design. Here, we investigate the photophysics of a series of planar bilayer heterojunction devices incorporating fused-ring electron acceptors with power conversion efficiencies up to 11%. Using ultrafast optical spectroscopy, we demonstrate the importance of long-range layer-to-layer energy transfer in planar structures, isolating this effect by including an insulating layer between the donor and acceptor layers to eliminate charge transfer effects. We show that the slab geometry facilitates substantially longer-range energy transfer than between isolated molecules or small domains. Along with high molecular packing densities, high absorption coefficients, and long exciton diffusion lengths, we show that these effects amount to exciton harvesting length scales that match the light absorption lengths and thereby enable efficient bilayer devices. Our quantitative analysis of bilayer structures also accounts for large domain sizes in bulk-heterojunction devices including fused-ring electron acceptors, and it quantifies the importance of strong resonant spectral overlap is for material selection and design for highly efficient OPVs. • Bilayer organic solar cells can be as efficient as their bulk heterojunction counterparts. • The photophysics of bilayer devices is fundamentally different to bulk heterojunctions. • Long range interlayer energy transfer plays a key role in boosting bilayer device efficiency. • Quantifying light absorption, exciton diffusion and energy transfer enables improved device design.

DSSC Heterojunction photoelectrode based ZnO-nanorod/TiO2; Dependency of photoelectrode morphology investigated by calculating I-V curves

The simple diode model equation can be used to derive solar cell parameters based on current density-voltage curve. This calculation has an objective to study the working of ZnO nanorod/TiO2 as photoelectrode in dye-sensitized solar cell (DSC). As a function to collect and transport the electron inside the cell, ZnO-Nr/TiO2 bilayer heterojunction works as photoanode. The equivalent circuit in real solar cell is usually based on two diode model by considering presence a large of recombination process inside the cell (non-ideal diode). We simplified the equivalent circuit with only one diode and one resistor (Rsh) in parallel with a series resistor (Rs) related to photovoltage occurs. The current loss due to back reaction and recombination process which is appeared at interfacing area between photoanode/dye and electrolyte corresponds as parallel resistance (Rp). An ideality factor (m) due to non-linear b-recombination process which is occurred inside the cell is also derived using this model. The comparison between commonly photoanode (single type of TiO2 mesoporous), multilayer ZnO nanorod/TiO2, and planar heterojunction ZnO/TiO2 is also observed in order to emphasize the interface quality of photonode to the such solar cell parameters

Large-area (64 × 64 array) inkjet-printed high-performance metal oxide bilayer heterojunction thin film transistors and n-metal-oxide-semiconductor (NMOS) inverters

It is a big challenge to construct large-scale, high-resolution and high-performance inkjet-printed metal oxide thin film transistor (TFT) arrays with independent gates for the new printed displays. Here, a self-confined inkjet printing technology has been developed to construct large-area (64 × 64 array), high-resolution and high-performance metal oxide bilayer (In2O3/IGZO) heterojunction TFTs with independent bottom gates on transparent glass substrates. Inkjet printing In2O3 dot arrays with the diameters from 55 to 70 μm and the thickness of ∼10 nm were firstly deposited on UV/ozone treated AlOx dielectric layers, and then IGZO dots were selectively printed on the top of In2O3 dots by self-confined technology to form In2O3/IGZO heterojunction channels. When the inkjet-printed IO layers treated by UV/ozone for more than 30 min or oxygen plasma for 5 min prior to print IGZO thin films, the mobility of the resulting printed In2O3/IGZO heterojunction TFTs are correspondingly enhanced to be 18.80 and 28.44 cm2 V−1 s−1 with excellent on/off ratios (>108) and negligible hysteresis. Furthermore, the printed N-Metal-Oxide-Semiconductor (NMOS) inverter consisted of an In2O3/IGZO TFT and an IGZO TFT has been demonstrated, which show excellent performance with the voltage gain up to 112. The strategy demonstrated here can be considered as general approaches to realize a new generation of high-performance printed logic gates, circuits and display driving circuits.

BiOCl/group-IV Xene bilayer heterojunctions: stability and electronic and photocatalytic properties.

Vertical van der Waals heterojunctions (HJs) composed of a photocatalytic star material BiOCl monolayer and group-IV Xene monolayer (silicene, germanene etc.) were studied by using first-principles calculations. Formation energy analysis and molecular dynamics simulation show that the BiOCl/Xene bilayer HJs can exist stably up to room temperature. Owing to evident charge redistribution and accumulation occurring between the bilayers, electron-hole puddles form and charge carrier transfer and separation occur in the HJs, which are beneficial to the improvement of photocatalytic performance. The HJ energy bands maintain the Dirac cones with almost linear dispersion curves, suggesting low effective mass and high mobility of carriers, and can be effectively tuned by strain. Our results show that the BiOCl/Xene bilayer HJs with high separation efficiency and high mobility of carriers and strain-adjustable bandgaps provide varieties in the functionalities of 2D van der Waals HJs and show great potentials in photocatalytic applications.

High-performance near-infrared polymeric phototransistors realized by combining cross-linked polymeric semiconductors and bulk heterojunction bilayer structures

Flexible polymeric phototransistors highly responsive to near-infrared (NIR) light are crucial for the room temperature biological imaging due to some unique properties of organic semiconductors, such as light weight, low cost, and good biocompatibility. However, the low mobility and/or the high dark off-current (DOC) of low-bandgap polymeric phototransistors limit their NIR light detectability. This work proposes the development of a solution-processable bilayer heterostructure (BHS) for polymeric phototransistors, which uses cross-linked polymeric semiconductors (CLPS) as the bottom conducting channel and a bulk heterojunction (BHJ) as the photoactive layer. The BHS architecture ensures fast carrier transport and efficient exciton separation. Besides, the electron-trapping effect generated between the CLPS and BHJ results in a significantly lower DOC compared with other controlled devices. Thus, the BHS phototransistor based on low-bandgap polymer can detect NIR light with wavelengths of 808 nm, 1064 nm, and even 1550 nm, going from NIR-I (750–1000 nm) to NIR-II (1000–1700 nm). Meanwhile, its responsivity (R) is two or three orders of magnitude higher than the controlled device. Furthermore, an ultra-flexible NIR phototransistor array (~5 μm thick) using this strategy could detect image barcodes with different gray levels, demonstrating its potential application in flexible NIR imaging.

Hybrid Bilayer Heterojunction Al/ZnPc/ZnO /ITO Thin Films Solar Cell Prepared by Pulsed Laser Deposition

Hybrid bilayer heterojunction Zinc Phthalocyanine (ZnPc) thin-film P-type is considered as a donor active layer as well as the Zinc Oxide (ZnO) thin film n-type is considered as an acceptor with (Electron Transport Layer). In this study, using the technique of Q-switching Nd-YAG Pulsed Laser Deposition (PLD) under vacuum condition 10-3 torr on two ITO (Indium Tin Oxide) and (AL) electrodes and aluminum, is used to construct the hydride bilayer photovoltaic solar cell heterojunction (PVSC). The electrical properties of hybrid heterojunction Al/ZnPc/ZnO/ITO thin film are studied. The results show that the voltage of open circuit (V_oc=0.567V), a short circuit (I_sc=36 ?A), and the fill factor (FF) of 0.443. In addition, the conversion efficiency of (n=3.4%) is recorded with Xenon lamp with an intensity 235mw/cm2 .

Characterization of Bilayer Organic Solar Cells with Carbon Nanotubes Using Impedance Analysis.

Four organic solar cell (OSC) devices with the bilayer heterojunction architecture were investigated, where carbon nanotubes (CNTs) were doped within the acceptor layer. The power conversion efficiency (PCE) of the CNT-incorporated device with a concentration of 0.004 wt% is approximately 20% point higher than that of the reference one. As the concentration of CNTs became higher, the PCE of the devices deteriorated; this could be caused by the percolative connection of CNTs within the layer. The voltage dependence on the effective lifetime of the charge carriers, determined by Cole-Cole curves of the impedance analysis, was different for the reference and CNT-incorporating devices-the lifetime of the CNT-incorporated ones was shorter, possibly owing to the high local electric field near the CNTs. Controlling the concentration of CNTs below the critical concentration of percolation is a key factor in achieving high photovoltaic performance.

Atomic-Scale Studies of Overlapping Grain Boundaries between Parallel and Quasi-Parallel Grains in Low-Symmetry Monolayer ReS2

Summary Grain boundaries (GBs) are significant microstructures that dominate properties of polycrystalline two-dimensional (2D) materials. Low-symmetry rhenium disulfide monolayers provide an ideal platform to investigate diverse configurations of GBs and their orientation-dependent characteristics. Here, we utilize the preferential deposition of platinum-based nanoparticles on grain edges to rapidly locate nanoscale-wide overlapping GBs during microscopic-scale observation. Atomic-resolution investigations by aberration-corrected annular dark-field scanning transmission electron microscopy reveal diverse overlapping GBs constructed by parallel grains with transversal displacement. They prefer to align along several primary lattice directions with different interlayer atomic registries. Calculations show that the electronic band structures of overlapping GBs are strongly direction dependent, suggesting their potential in tunable electronic and photonic devices. Incommensurate overlapping GBs formed by quasi-parallel grains with ultrasmall twist angles are also observed. These results unveil the rich polymorphism of GBs and new opportunities to tailor properties of anisotropic 2D materials via GB engineering.

Electronic and optical properties of graphene/molybdenite bilayer composite

Since its discovery, graphene has been the object of study for several and manifold applications, ranging from photocatalysis to mechanics and electronics of materials. However, the use of pure graphene in certain opto- and microelectronic applications is sometimes limited because of its zero band gap. Among the different methods to widen the band gap, in the present work the attention is focused on the so-called van der Waals composites (or heterostructures, or heterojunctions), namely the stacking of two monolayers of different materials that are held together by weak van der Waals interactions. An interesting composite is the graphene/molybdenite (MoS2) heterostructure, where the second material is a bidimensional semiconductor characterized by strong in-plane covalent bonds and weak out-of-plane interactions, which means that it is possible to exfoliate MoS2 into monolayers of atomic thickness. Also, from the crystallographic point of view, both graphene and the monolayer of molybdenite (MoS2-1H) have a 2D hexagonal lattice and their stacking could be structurally favourable. In this work, the electronic band structure and density of states, complex dielectric function and optical properties of the stacked van der Waals bilayer heterojunction graphene/MoS2-1H were calculated and compared to both the single monolayers of graphene and molybdenite, to understand how the interaction between the two materials may alter the above cited properties. The analysis of the band structure in this van der Waals composite clearly showed a small direct band gap related to the transition π→π∗ (ca. 2.5 meV) in correspondence of the high symmetry point K. The heterojunction showed also, as expected, some important variations in the complex dielectric function and related properties in the visible-light spectral region. The results obtained in the present work could be of use for future development and applications of this kind and similar 2D composite materials with tailored electronic and optical properties.

Optimizing the working mechanism of the CsPbBr3-based inorganic perovskite solar cells for enhanced efficiency

Recently, inorganic perovskite solar cells (PSCs) based on CsPbBr3 have triggered incredible interest due to the demonstrated excellent stability against thermal and high humidity environmental conditions. However, the power conversion efficiency (PCE) of the CsPbBr3-based PSCs is still lower than that of the organic-inorganic hybrid one, because of the large band gap and serious charge recombination at the interface or inside the device. Here, the working mechanism of the devices with normal n-i-p planar structure is modeled and investigated using SCAPS 1D simulation software. The simulation results state that the proper band structure of PSCs is crucial to carrier separation and transport. The high interface recombination, originated from the large band offsets of the electron transport material (ETM)/absorber and absorber/hole transport material (HTM) respectively, can be effectively diminished with the continuous gradient junction design of the absorber, and a PCE of 11.58% is obtained with a high open-circuit voltage (VOC) of 1.68 V. Moreover, by building a heterojunction bilayer absorption scenario of CsPbIBr2/CsPbBr3 and employing ZnOS and Cu2ZnSnS4 films as the ETM and HTM respectively, the PCE of PSCs is further increased to 15.89%, caused mainly by the enhancement in short-current density (JSC). Moreover, reducing the interface defect density is also very important to improve the performance of PSCs. These results will provide theoretical guidance for improving the performance of the CsPbBr3-based PSCs.

On the physical and photo-electrical properties of organic photovoltaic cells based on 1,10-Phenanthroline and 5,10,15,20-Tetra(4-pyridyl)-21H,23H-porphine non-fullerene thin films

The active layer of the bulk heterojunction organic photovoltaic cells is typically based on a polymeric blend containing a donor polymer and a fullerene-based polymer electron acceptor. However, in the last decade non-fullerene polymer-based solar cells have seen a rapid progress, due to their extraordinary chemical, thermal and photo stability, and because of their ability to decrease the voltage loss. The conventional Organic Photovoltaic Cells (OPVs), structures based on either P3HT:PCBM bulk heterojunction (BHJ) polymeric blend or P3HT/PCBM bilayer heterojunction, have limited efficiency and stability, in principal due to the fullerene derivatives used as acceptor component. PCBM for example, as many other fullerene derivatives, exhibits some deficiencies, such as: limited chemical and energetic tunability, small range of absorption spectra, morphological instability and relatively high production costs. That is why in the last years, novel non-fullerene materials started to be used in order to overcome the above limits associated with the fullerene acceptors. In this paper two different non-fullerene acceptor materials have been tested as potential candidates for the efficient OPVs based on P3HT donor polymer as main absorber: a new conjugated polymer containing chelating ligands like 1,10-Phenanthroline (Phen), and a small-molecule acceptor like 5,10,15,20-Tetra(4-pyridyl)-21H,23H-porphine (TPyP). For both cell configurations a decrease of the voltage loss was obtained, and it was also observed that the thickness of the P3HT donor layer has an important contribution to the output power (Pmax) and fill factor (FF) of the OPV structures (FF and Pmax increased almost two times by decreasing the thickness of the P3HT donor layer).

CuPc/C60 heterojunction for high responsivity zero bias organic red light photodetector

Organic photodetector (OPD), having copper phthalocyanine (CuPc) and fullerene (C60) bilayer heterojunction as an active layer, has been fabricated by employing thermal evaporation method for the red light detection applications. The spectral response has been observed to cover from 300 to 800 nm for the OPD. Experimental results of the OPD reflect good photodiode behavior. Noteworthy, our fabricated OPD has exhibited responsivity of 0.052 A/W and external quantum efficiency (EQE) of 9.28% at wavelength 700 nm and 0 V. Maximum photocurrent density to dark current density ratio has been found to be 4.5 × 104 under the illumination of red light. Detectivity at wavelength 700 nm and 0 V has been found to be on the order of 1011 Jones for the same OPD. Response time and recovery time are found to be 16 and 36 ms, respectively. Our fabricated OPD exhibits excellent stability. Thus, the CuPc/C60 based OPD has been found to be quite promising for the low cost molecular photodetector applications due to its zero bias high responsivity and detectivity.

Synthesis and Characterization of a 2,3-Dialkoxynaphthalene-Based Conjugated Copolymer via Direct Arylation Polymerization (DAP) for Organic Electronics.

Poly[(5,5’-(2,3-bis(2-ethylhexyloxy)naphthalene-1,4-diyl)bis(thiophene-2,2′-diyl))-alt-(2,1,3-benzothiadiazole-4,7-diyl)] (PEHONDTBT) was synthesized for the first time and through direct arylation polymerization (DAP) for use as p-donor material in organic solar cells. Optimized reaction protocol leads to a donor-acceptor conjugated polymer in good yield, with less structural defects than its analog obtained from Suzuki polycondensation, and with similar or even higher molecular weight than other previously reported polymers based on the 2,3-dialkoxynaphthalene monomer. The batch-to-batch repeatability of the optimized DAP conditions for the synthesis of PEHONDTBT was proved, showing the robustness of the synthetic strategy. The structure of PEHONDTBT was corroborated by NMR, exhibiting good solubility in common organic solvents, good film-forming ability, and thermal stability. PEHONDTBT film presented an absorption band centered at 498 nm, a band gap of 2.15 eV, and HOMO and LUMO energy levels of −5.31 eV and −3.17 eV, respectively. Theoretical calculations were performed to understand the regioselectivity in the synthesis of PEHONDTBT and to rationalize its optoelectronic properties. Bilayer heterojunction organic photovoltaic devices with PEHONDTBT as the donor layer were fabricated to test their photovoltaic performance, affording low power-conversion efficiency in the preliminary studies.

Revisiting the Charge-Transfer States at Pentacene/C60 Interfaces with the GW/Bethe-Salpeter Equation Approach.

Molecular orientations and interfacial morphologies have critical effects on the electronic states of donor/acceptor interfaces and thus on the performance of organic photovoltaic devices. In this study, we explore the energy levels and charge-transfer states at the organic donor/acceptor interfaces on the basis of the fragment-based GW and Bethe–Salpeter equation approach. The face-on and edge-on orientations of pentacene/C60 bilayer heterojunctions have employed as model systems. GW+Bethe–Salpeter equation calculations were performed for the local interface structures in the face-on and edge-on bilayer heterojunctions, which contain approximately 2000 atoms. Calculated energy levels and charge-transfer state absorption spectra are in reasonable agreements with those obtained from experimental measurements. We found that the dependence of the energy levels on interfacial morphology is predominantly determined by the electrostatic contribution of polarization energy, while the effects of induction contribution in the edge-on interface are similar to those in the face-on. Moreover, the delocalized charge-transfer states contribute to the main absorption peak in the edge-on interface, while the face-on interface features relatively localized charge-transfer states in the main absorption peak. The impact of the interfacial morphologies on the polarization and charge delocalization effects is analyzed in detail.

Zero bias high responsive visible organic photodetector based on pentacene and C60

Organic photodetector (OPD) based on simple bilayer heterojunction of pentacene and fullerene (C60) has been fabricated using thermal evaporation method. Absorption spectra of series of OPD, annealed at different temperatures, have been observed over a spectral range between 400 nm and 700 nm. At 0 V and at wavelength 650 nm, external quantum efficiency (EQE) = 5% and responsivity of 0.025 A/W have been achieved for an OPD annealed at 90 °C. For the same device, the maximum ratio of photocurrent to dark current density is found to be of the order of 2.5 × 103. Furthermore, OPD annealed at 90 °C has exhibited maximum detectivity of 2 × 1010 Jones and response time of 180 ms. Structural characterization confirms formation of small grain size of pentacene and C60 molecules thereby leading to improvement of the device performance. Our experimental signatures seem to indicate that pentacene and C60 based OPD, after suitable engineering, might be promising for red light detection applications.

(Invited) Increasing the Efficiency of Semiconducting Carbon Nanotube Photoabsorber-Based Photovoltaics

We have previously demonstrated efficient electron transfer from photoexcited small diameter (< 1 nm) semiconducting carbon nanotubes to C60 fullerene electron acceptors in planar, bilayer heterojunction diode devices, with internal quantum efficiency of 90%. However, the out-of-plane exciton diffusion length in planar nanotube thin films is poor (~ 5 nm), limiting the optical density of nanotube films that can be implemented in these devices, which in turn has limited external quantum efficiency to 50% at the nanotube bandgap and to << 50% off-resonance. Polymer solar cells also face a similar limitation (poor exciton diffusion length) and overcome this problem by blending the polymer and acceptor together into a bulk heterojunction. In this talk, our recent research aimed at addressing this shortcoming will be presented.

All‐Polymer High‐Performance Photodetector through Lamination

AbstractAll‐polymer and semitransparent organic photodetectors (OPDs) fabricated through lamination on flexible substrates are demonstrated. Through lamination both bilayer and bulk heterojunction (BHJ) configurations for two different all‐polymer blends are easily made, and characterized in terms of detectivity, linear dynamic range (LDR), and bandwidth. These devices show high detectivity up to 1011 Jones. The importance of measured noise rather than estimated shot noise in detectivity calculation is discussed and emphasized. With this high detectivity and with the geometric flexibility due to the polymer materials, these detectors can be attached to curved surfaces for optical detection. Their use at fingertips for heart rate sensing is demonstrated. These devices also have a very wide bandwidth of more than 300 kHz and an LDR of 75 dB. As such the, roll‐to‐roll compatible lamination method shows great potential for high‐performance OPDs fabrication.