Organic Semiconductor Heterojunctions Research Articles

Unraveling the Effect of Stacking Configurations on Charge Transfer in WS2 and Organic Semiconductor Heterojunctions.

Photoinduced interfacial charge transfer plays a critical role in energy conversion involving van der Waals (vdW) heterostructures constructed of inorganic nanostructures and organic materials. However, the effect of molecular stacking configurations on charge transfer dynamics is less understood. In this study, we demonstrated the tunability of interfacial charge separation in a type-II heterojunction between monolayer (ML) WS2 and an organic semiconducting molecule [2-(3″',4'-dimethyl-[2,2':5',2':5″,2″'-quaterthiophen]-5-yl)ethan-1-ammonium halide (4Tm)] by rational design of relative stacking configurations. The assembly between ML-WS2 and the 4Tm molecule forms a face-to-face stacking when 4Tm molecules are in a self-aggregation state. In contrast, a face-to-edge stacking is observed when 4Tm molecule is incorporated into a 2D organic-inorganic hybrid perovskite lattice. The face-to-face stacking was proved to be more favorable for hole transfer from WS2 to 4Tm and led to interlayer excitons (IEs) emission. Transient absorption measurements show that the hole transfer occurs on a time scale of 150 fs. On the other hand, the face-to-edge stacking resulted in much slower hole transfer without formation of IEs. This inefficient hole transfer occurs on a similar time scale as A exciton recombination in WS2, leading to the formation of negative trions. These investigations offer important fundamental insights into the charge transfer processes at organic-inorganic interfaces.

Weak Light‐Stimulated Synaptic Transistors Based on MoS2/Organic Semiconductor Heterojunction for Neuromorphic Computing

Photonic synapses are expected to play an important role in implementing brain‐like computing owing to the wide bandwidth and low mutual interference of optical signals. Herein, photonic synaptic transistors based on inorganic semiconductor molybdenum disulfide (MoS2) and organic semiconductor heterojunction with adjustable short‐term/long‐term plasticity are proposed. Benefitting from the outstanding photosensitive characteristics originating from the heterojunction, the devices have the capability of weak light detection and can exhibit distinct synaptic responses even under an ultraweak light intensity of 40 nW cm−2, which is considerably lower than that of most previously reported photonic synaptic transistors. Furthermore, a low energy consumption of 0.4 fJ per synaptic event can be obtained at a low operating voltage of 0.001 V, lower than that required for a single synaptic event observed in human brains. In addition, the devices can implement high‐pass filtering function and illustrate the potential in image sharpening processing. More significantly, logic functions as well as associative learning behavior are dramatically simulated through all‐optical stimulation. This work demonstrates a feasible approach to developing multifunctional photonic synaptic transistors based on inorganic/organic semiconductor heterojunction for neuromorphic computing.

Air-stable ambipolar charge transport behaviors of organic-inorganic hybrid bilayer and application to Au nanoparticle-based floating gate memory

Ambipolar-based devices are considered effective alternatives in various applications, including reconfigurable logic, light-emitting transistors, and neuromorphic devices. On the other hand, the previously reported intrinsic single-layer ambipolar materials have limitations, such as high instability in ambient air and asymmetric transport of electrons and holes. In the present study, high air-stable and symmetrical ambipolar behavior were implemented using an organic-inorganic bilayer. An ambipolar-based memory application was implemented using these characteristics. This fabricated memory device used electrons and holes together with dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene organic p-type and zinc tin oxide inorganic n-type semiconductor heterojunction layers, resulting in wide memory window and high retention. In addition, these behaviors were analyzed at the band diagram level through ultraviolet photoelectron spectroscopy and ultraviolet-visible light spectroscopy, and chemical bonding was analyzed using x-ray photoelectron spectroscopy. These reports can provide useful strategies for applying various ambipolar applications in the future.

Interfacial charge transfer states enable efficient solid-state upconversion

While high-efficiency upconversion is widely demonstrated in solution, solid-state upconversion has lagged significantly behind. Recently, Hiramoto et al. reported a novel solid-state upconversion system based on organic semiconductor heterojunctions, leading to more than 10-fold improvement in efficiency.

Donor-acceptor organic semiconductor heterojunction nanoparticles for efficient photocatalytic H2 evolution

Donor-acceptor organic semiconductor heterojunction nanoparticles are currently among the most efficient visible-light-active H2 evolution photocatalysts due to their long-lived reactive charges. In the current issue of Nature Energy, McCulloch et al. developed two donor-acceptor organic semiconductors with heterojunction nanoparticles for highly efficient photocatalytic H2 evolution. Donor-acceptor organic semiconductor heterojunction nanoparticles are currently among the most efficient visible-light-active H2 evolution photocatalysts due to their long-lived reactive charges. In the current issue of Nature Energy, McCulloch et al. developed two donor-acceptor organic semiconductors with heterojunction nanoparticles for highly efficient photocatalytic H2 evolution.

Flexible Perovskite and Organic Semiconductor Heterojunction Devices for Tunable Band-Selective Photodetection

Filter-free band-selective photodetectors with tunable band edges possess extensive applications in smart sensors, artificial intelligence, the internet of everything, and so forth. However, such photodetectors operated with the charge collection narrowing effect require a thick active layer over millimeters, leading to rather a large device size, high material cost, and limited mechanical flexibility. In this work, we report a flexible thin-film perovskite and organic semiconductor (OSC) heterojunction device, which can achieve adjustable band-selective photodetection by utilizing the easy tunability of the perovskite band gap. First, heterojunction photodiodes with only two active layers of perovskites and OSCs are fabricated, in which the perovskite contributes to light absorption and rectification characteristics simultaneously. The fundamental device operation mechanism is investigated by combining the band alignments of the heterojunctions and their potential distributions observed with Kelvin probe force microscopy. It uncovers that the presence of built-in electric fields in the heterojunctions caused by moderate band alignments and appropriate difference in hole and electron concentrations between the perovskites and the OSCs is significant for the properties of the photodiodes. The resultant photodiodes exhibit a high rectification ratio of up to 103, decent photodetection performance, and mechanical flexibility. Furthermore, photodetectors based on perovskite/OSC/perovskite stack heterojunctions can realize the functions of filterless band-selective photodetection in a single device, and the band edges in the photodetection can be modulated by tuning the perovskite band gap. Therefore, this work provides flexible thin-film photodiodes with the potential of miniaturization and low-cost fabrication and demonstrates tunable band-selective photodetection for advanced applications.

Generation of long-lived charges in organic semiconductor heterojunction nanoparticles for efficient photocatalytic hydrogen evolution

Organic semiconductor photocatalysts for the production of solar fuels are attractive as they can be synthetically tuned to absorb visible light while simultaneously retaining suitable energy levels to drive a range of processes. However, a greater understanding of the photophysics that determines the function of organic semiconductor heterojunction nanoparticles is needed to optimize performance. Here, we show that such materials can intrinsically generate remarkably long-lived reactive charges, enabling them to efficiently drive sacrificial hydrogen evolution. Our optimized hetereojunction photocatalysts comprise the conjugated polymer PM6 matched with Y6 or PCBM electron acceptors, and achieve external quantum efficiencies of 1.0% to 5.0% at 400 to 900 nm and 8.7% to 2.6% at 400 to 700 nm, respectively. Employing transient and operando spectroscopies, we find that the heterojunction structure in these nanoparticles greatly enhances the generation of long-lived charges (millisecond to second timescale) even in the absence of electron/hole scavengers or Pt. Such long-lived reactive charges open potential applications in water-splitting Z-schemes and in driving kinetically slow and technologically desirable oxidations.

Novel photoelectrochemical immunosensor for MCF-7 cell detection based on n-p organic semiconductor heterojunction

In this work, we developed a novel photoelectrochemical (PEC) sensor based on n-p organic semiconductor heterojunction for sensitive detecting MCF-7 cancer cells. BTA-C4Ph and PM6 were designed as photoactive materials to form n-p heterojunction, which greatly enhanced the photoelectric conversion efficiency. Antibody-modified magnetic nanoparticles were utilized to capture and separate MCF-7 cells from samples. Detection of MCF-7 is ascribed to the loading of MCF-7 onto BTA-C4Ph-PM6 modified electrode that resulted in the decrease of photocurrent intensity. The PEC immunosensor displayed a linear concentration ranging from 50 cell/mL to 1 × 104 cell/mL with a limit of detection (LOD) of 41 cell/mL (S/N = 3) for MCF-7. Additionally, the senor also exhibited good stability, excellent selectivity and prominent reproducibility. Furthermore, the sensor was successfully applied to detect MCF-7 in whole blood. This work illustrates that n-p heterojunction of organic semiconductor may find wide applications for the preparation of different photoelectrochemical sensors.

High current output direct-current triboelectric nanogenerator based on organic semiconductor heterojunction

The high demand on direct current (DC) power source generated through triboelectric effect drives the research on semiconductor-based triboelectric nanogenerators (TENGs), which have lower internal resistance. With high flexibility, organic semiconductor-based DC TENGs are more intriguing. In this work, we use organic semiconductor Poly (3,4-ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT: PSS) as a friction layer to make organic semiconductor-based TENGs. By experimentally analyzing the work functions of PEDOT: PSS, Al alloy, Si, ITO, Cu, and graphene, the DC devices with PEDOT: PSS/Al alloy triboelectric layers exhibit impressive performance. Our PEDOT: PSS/Al alloy DC TENGs achieves open-circuit voltage about 1 V, and short-circuit current up to 309 μA. The internal resistance of the DC TENG is tested to be about 2×104 Ω, and the power density reaches 11.67 mW/m2. Effects of the load pressure, sliding speed, and size of the devices are systematically studied in this work. The charging probability of our DC TENGs as power sources and human body motion detecting capability as force sensors are all experimentally tested, indicating further potential applications of our PEDOT: PSS/Al alloy DC TENGs as flexible energy sources and motion detector.

Migration of Charge-Transfer States at Organic Semiconductor Heterojunctions.

Charge-transfer (CT) states formed at organic donor-acceptor (D-A) semiconductor heterojunctions play a critical role in optoelectronic devices. While mobile, their migration has not been extensively characterized. In addition, the factors impacting the CT state diffusion length (LD) have not been elucidated. Here, CT state LD is measured by using photoluminescence quenching for several D-A mixtures, with migration occurring along the bulk heterojunction. All D-A pairings considered yield a similar LD ∼ 5 nm in equal mixtures despite variations in the CT state energy and the constituent molecular structures. The CT state LD varies strongly with mixture composition and is well-correlated to the slowest charge carrier mobility, suggesting a direct method to tune CT state transport. These findings may be applied to elucidate the role of CT state migration in organic photovoltaic and light-emitting devices as well as to broadly explain the transport of interfacial excited states along inorganic and hybrid organic-inorganic heterojunctions.

Enhanced photocatalytic hydrogen evolution from organic semiconductor heterojunction nanoparticles.

Photocatalysts formed from a single organic semiconductor typically suffer from inefficient intrinsic charge generation, which leads to low photocatalytic activities. We demonstrate that incorporating a heterojunction between a donor polymer (PTB7-Th) and non-fullerene acceptor (EH-IDTBR) in organic nanoparticles (NPs) can result in hydrogen evolution photocatalysts with greatly enhanced photocatalytic activity. Control of the nanomorphology of these NPs was achieved by varying the stabilizing surfactant employed during NP fabrication, converting it from a core-shell structure to an intermixed donor/acceptor blend and increasing H2 evolution by an order of magnitude. The resulting photocatalysts display an unprecedentedly high H2 evolution rate of over 60,000 µmol h-1 g-1 under 350 to 800 nm illumination, and external quantum efficiencies over 6% in the region of maximum solar photon flux.

Vibronic coherence contributes to photocurrent generation in organic semiconductor heterojunction diodes

Charge separation dynamics after the absorption of a photon is a fundamental process relevant both for photosynthetic reaction centers and artificial solar conversion devices. It has been proposed that quantum coherence plays a role in the formation of charge carriers in organic photovoltaics, but experimental proofs have been lacking. Here we report experimental evidence of coherence in the charge separation process in organic donor/acceptor heterojunctions, in the form of low frequency oscillatory signature in the kinetics of the transient absorption and nonlinear two-dimensional photocurrent spectroscopy. The coherence plays a decisive role in the initial ~200 femtoseconds as we observe distinct experimental signatures of coherent photocurrent generation. This coherent process breaks the energy barrier limitation for charge formation, thus competing with excitation energy transfer. The physics may inspire the design of new photovoltaic materials with high device performance, which explore the quantum effects in the next-generation optoelectronic applications.

Tandem organic light-emitting diodes with buffer-modified C60/pentacene as charge generation layer

Buffer-modified C60/pentacene as charge generation layer (CGL) is investigated to achieve effective performance of charge generation. Undoped green electroluminescent tandem organic light-emitting diodes (OLEDs) with multiple identical emissive units and using buffer-modified C60/pentacene organic semiconductor heterojunction (OHJ) as CGL are demonstrated to exhibit better current density and brightness, compared with conventional single-unit devices. The current density and brightness both can be significantly improved with increasing the thickness of Al. However, excessive thickness of Al seriously decreases the transmittance of films and damages the interface. As a result, the maximum current efficiency of 1.43 cd·A-1 at 30 mA·cm-2 can be achieved for tandem OLEDs with optimal thickness of Al. These results clearly demonstrate that Cs2CO3/Al is an effective buffer for C60/pentacene-based tandem OLEDs.

利用C60和CuPc形成的有机半导体异质结作为阳极修饰层实现高效的磷光有机发光二极管

利用C₆₀和CuPc形成的有机半导体异质结作为阳极修饰层实现高效的磷光有机发光二极管

Crystallinity of the epitaxial heterojunction of C60 on single crystal pentacene

The structure of pn heterojunctions is an important subject in the field of organic semiconductor devices. In this work, the crystallinity of an epitaxial pn heterojunction of C60 on single crystal pentacene is investigated by non-contact mode atomic force microscopy and high-resolution grazing incidence x-ray diffraction. Analysis shows that the C60 molecules assemble into grains consisting of single crystallites on the pentacene single crystal surface. The in-plane mean crystallite size exceeds 0.1µm, which is at least five time larger than the size of crystallites deposited onto polycrystalline pentacene thin films grown on SiO2. The results indicate that improvement in the crystal quality of the underlying molecular substrate leads to drastic promotion of the crystallinity at the organic semiconductor heterojunction.

Organic semiconductor heterojunctions: electrode-independent charge injectors for high-performance organic light-emitting diodes

Organic light-emitting diodes (OLEDs) are driven by injected charges from an anode and a cathode. The low and high work function metals are necessary for the effective injection of electrons and holes, respectively. Here, we introduce a fully novel design concept using organic semiconductor heterojunctions (OSHJs) as the charge injectors for achieving highly efficient OLEDs, regardless of the work functions of the electrodes. In contrast to traditional injected charges from the electrodes, the injected charges originate from the OSHJs. The device performance was shown to be significantly improved in efficiency and stability compared to conventional OLEDs. Attractively, the OLEDs based on OSHJs as charge injectors still exhibited an impressive performance when the low work function Al was replaced by air- and chemistry-stable high work function metals, such as Au, Ag, and Cu, as the cathode contact, which has been suggested to be difficult in conventional OLEDs. This concept challenges the conventional design approach for the injection of charges and allows for the realization of practical applications of OLEDs with respect to high efficiency, selectable electrodes, and a long lifetime.

Interface dipole engineering at buried organic–organic semiconductor heterojunctions

A general technique for modifying energy level alignment at organic–organic heterojunctions is introduced, and is demonstrated here for phenyl-C61-butyric acid methyl ester (PCBM) and N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine (α-NPD). An ultra-thin layer (∼1nm) of TiO2 is used as an adhesion template to attach a self-assembled monolayer of dipolar phosphonate (PA) molecules to the lower interface of a two-stack ensemble. This modification induces shifts in the vacuum level and work function over ∼1.0eV depending on the molecular dipole moment of the PA, which in turn modifies the electronic level alignment across the organic heterojunction interface by up to 0.5eV.

Direct observation of ultrafast long-range charge separation at polymer-fullerene heterojunctions.

In polymeric semiconductors, charge carriers are polarons, which means that the excess charge deforms the molecular structure of the polymer chain that hosts it. This results in distinctive signatures in the vibrational modes of the polymer. Here, we probe polaron photogeneration dynamics at polymer:fullerene heterojunctions by monitoring its time-resolved resonance-Raman spectrum following ultrafast photoexcitation. We conclude that polarons emerge within 300 fs. Surprisingly, further structural evolution on ≲ 50-ps timescales is modest, indicating that the polymer conformation hosting nascent polarons is not significantly different from that near equilibrium. We interpret this as suggestive that charges are free from their mutual Coulomb potential because we would expect rich vibrational dynamics associated with charge-pair relaxation. We address current debates on the photocarrier generation mechanism at molecular heterojunctions, and our work is, to our knowledge, the first direct probe of molecular conformation dynamics during this fundamentally important process in these materials.

Quantification of energy losses in organic solar cells from temperature-dependent device characteristics

Owing to the excitonic nature of photoexcitations in organic semiconductors, the working mechanism of organic solar cells relies on the donor-acceptor (D/A) concept enabling photoinduced charge transfer at the interface between two organic materials with suitable energy-level alignment. However, the introduction of such a heterojunction is accompanied by additional energy losses compared to an inorganic homojunction cell due to the presence of a charge-transfer (CT) state at the D/A interface. By careful examination of planar heterojunctions of the molecular semiconductors diindenoperylene (DIP) and C${}_{60}$ we demonstrate that three different analysis techniques of the temperature dependence of solar-cell characteristics yield reliable values for the effective photovoltaic energy gap at the D/A interface. The retrieved energies are shown to be consistent with direct spectroscopic measurements and the D/A energy-level offset determined by photoemission spectroscopy. Furthermore, we verify the widespread assumption that the activation energy of the dark saturation current $\ensuremath{\Delta}E$ and the CT energy ${E}_{\mathrm{CT}}$ may be regarded as identical. The temperature-dependent analysis of open-circuit voltage ${V}_{\mathrm{OC}}$ and dark saturation current is then applied to a variety of molecular planar heterojunctions. The congruency of $\ensuremath{\Delta}E$ and ${E}_{\mathrm{CT}}$ is again found for all material systems with the exception of copper phthalocyanine/C${}_{60}$. The general rule of thumb for organic semiconductor heterojunctions, that ${V}_{\mathrm{OC}}$ at room temperature is roughly half a volt below the CT energy, is traced back to comparable intermolecular electronic coupling in all investigated systems.

A photodiode with high rectification ratio and low turn-on voltage based on ZnO nanoparticles and SubPc planar heterojunction

A simple photodiode based on boron subphthalocyanine chloride (SubPc) organic semiconductor and zinc oxide (ZnO) inorganic nanoparticle (NP) thin films planar heterojunction was fabricated. The current density–voltage (J–V) characteristics of ITO/ZnO NPs/SubPc/Ag device in dark and under 20mW/cm2 illumination power with a solar simulator were investigated in detail. Measurement results showed that the device exhibited good rectifying behaviors in dark and under illumination. A high rectification ratio (RR) of 607 at ±1.65V and a low turn-on voltage of 0.9V were achieved for the device in dark, such high RR was ascribed to high electron mobility of ZnO NPs and large ZnO NPs/SubPc interface in the device. Also, the photoresponsive mechanism of the photodiode was illuminated in term of the schematic band diagram and the transportation process of charge carriers in the device.