Mixed Heterojunction Research Articles

High-Responsivity Vis–NIR photodetectors based on Bi2Te3 thin films and Ag2S QDs heterojunction

In this paper, high quality Bi2Te3 films were prepared by double temperature zone vapor deposition. On this basis, Ag2S/Bi2Te3 heterojunction photodetectors were prepared by spin-coating technique. Due to the synergistic absorption effect of the two materials, and the carrier extraction and separation enhanced by the built-in electric field, the device exhibits a wide spectral response and high performance. Under 405 nm, the responsivity is 18.34 mA/W, and the response time is 33/21 ms. This work provides research experience for mixed dimensional heterojunction photodetectors based on two-dimensional materials.

A high performance piezoelectric hetero-junction based on the configuration reform on interfacial potential barrier

In this paper, the barrier region of a mixed hetero-junction consisting of a p-type Si and an n-type ZnO was fully opened by abandoning the depletion layer approximation in advance. The barrier configuration was reconstructed by the artificial potential barrier based on a new fully coupled model in which the interaction between physical fields and charge carriers was taken into account. Then, we put forward an interesting design idea of elevating the mechanical regulation performance of a PN junction by manufacturing it with a third-generation piezoelectric semiconductor and a narrow bandgap semiconductor. It is through the deformation of the third-generation piezoelectric semiconductor on one side under mechanical loadings to excite polarization charges at the PN interface such that the movement of charge carriers in the narrow bandgap semiconductor on the other side is able to be tuned by the interface polarization charges. Besides, it is found a specific current-stress characteristic for such a mixed hetero-junction, which will present three stages, i.e., rising stage, platform stage and falling stage. The rising stage is caused by the improving recombination rate, which can be used as a high sensitivity stress sensor (output current improved by more than 10 times in a small stress). Oppositely, the falling stage is caused by the weakened recombination rate due to carrier type-inversion, which can be used to determine overload (output current reduced dramatically when the stress increased to a specific value). Finally, the platform stage is caused by the competition of the above two cases. At this stage, the device operates very stable and can resist the external perturbation. Obviously, the study possesses referential significance to the design and mechanical tuning on performance of piezotronic devices.

Interactions between nonfullerene acceptors lead to unstable ternary organic photovoltaic cells

For organic photovoltaic (OPV) devices to achieve consistent performance and long operational lifetimes, organic semiconductors must be processed with precise control over their purity, composition, and structure. This is particularly important for high volume solar cell manufacturing where control of materials quality has a direct impact on yield and cost. Ternary-blend OPVs containing two acceptor-donor-acceptor (A-D-A)-type nonfullerene acceptors (NFAs) and a donor have proven to be an effective strategy to improve solar spectral coverage and reduce energy losses beyond that of binary-blend OPVs. Here, we show that the purity of such a ternary is compromised during blending to form a homogeneously mixed bulk heterojunction thin film. We find that the impurities originate from end-capping C=C/C=C exchange reactions of A-D-A-type NFAs, and that their presence influences both device reproducibility and long-term reliability. The end-capping exchange results in generation of up to four impurity constituents with strong dipolar character that interfere with the photoinduced charge transfer process, leading to reduced charge generation efficiency, morphological instabilities, and an increased vulnerability to photodegradation. As a consequence, the OPV efficiency falls to less than 65% of its initial value within 265 h when exposed to up to 10 suns intensity illumination. We propose potential molecular design strategies critical to enhancing the reproducibility as well as reliability of ternary OPVs by avoiding end-capping reactions.

Detrimental Effects of “Universal” Singlet Photocrosslinkers in Organic Photovoltaics

The use of singlet photocrosslinkers based on diazirine or azide has been investigated for organic photovoltaics (OPVs) to stabilize the morphology of mixed bulk heterojunction (BHJ) films. The photocrosslinking reactions in BHJs have major detrimental effects on the performance of OPVs. This is in striking contrast to previous reports on organic field-effect transistors, where these singlet crosslinkers can solidify the films without affecting the charge transport properties. Based on the OPV results with the layer-by-layer-deposited films and the transferred films, we found that the crosslinking reactions had greater effects on the non-fullerene acceptor than on the donor polymer. The photoluminescence measurements suggested that the crosslinkers could react with the π-conjugated backbone of the non-fullerene acceptor to form trace amounts of byproducts that cause severe exciton quenching. These results indicate that the use of the “universal” singlet photocrosslinkers in OPVs requires careful selection of the organic semiconducting materials and crosslinkers.

Investigation of graphene/InN nanowire based mixed dimensional barristors with widely tunable Schottky barrier for highly sensitive multimodal gas sensing applications

In this work, an electrically tunable mixed dimensional heterojunction of graphene and planar InN nanowires (NW) has been investigated as a potential candidate for highly sensitive gas sensing applications. The high surface carrier concentration of InN, which prevents it from forming good rectifying contact with graphene, has been effectively suppressed through controlled partial surface oxidation in a plasma reactor. The resulting surface modified InN NWs were found to form excellent Schottky junction with graphene, with a barrier height that is widely tunable by the gate voltage, turning it into a mixed dimensional barristor device. When utilized as a multimodal molecular sensor, utilizing both capacitive and conductive modes, it exhibited very high sensitivity, with detection limits reaching down to sub-ppb range for NO2 and low ppb range for NH3 and CO, tested in both N2 and dry air environments. Additionally, surface functionalization, using hemin/benzoic acid, of this non-conventional barristor device yielded excellent selectivity for specific analytes while maintaining the high sensitivity. The responses of the sensor were found to be minimally affected with long term exposure to atmospheric humidity, which underscores the potential of this barristor device as a highly sensitive gas sensor for practical applications.

Suppressed recombination loss in organic photovoltaics adopting a planar–mixed heterojunction architecture

Suppressed recombination loss in organic photovoltaics adopting a planar–mixed heterojunction architecture

Charge transfer dynamics and interlayer exciton formation in MoS2/VOPc mixed dimensional heterojunction.

Mixed-dimensional van der Waals heterojunctions involve interfacing materials with different dimensionalities, such as a 2D transition metal dichalcogenide and a 0D organic semiconductor. These heterojunctions have shown unique interfacial properties not found in either individual component. Here, we use femtosecond transient absorption to reveal photoinduced charge transfer and interlayer exciton formation in a mixed-dimensional type-II heterojunction between monolayer MoS2 and vanadyl phthalocyanine (VOPc). Selective excitation of the MoS2 exciton leads to hole transfer from the MoS2 valence band to VOPc highest occupied molecular orbit in ∼710fs. On the contrary, selective photoexcitation of the VOPc layer leads to instantaneous electron transfer from its excited state to the conduction band of MoS2 in less than 100fs. This light-initiated ultrafast separation of electrons and holes across the heterojunction interface leads to the formation of an interlayer exciton. These interlayer excitons formed across the interface lead to longer-lived charge-separated states of up to 2.5ns, longer than in each individual layer of this heterojunction. Thus, the longer charge-separated state along with ultrafast charge transfer times provide promising results for photovoltaic and optoelectronic device applications.

Unveiling a Three Phase Mixed Heterojunction via Phase‐Selective Anchoring of Polymer for Efficient Photocatalysis

AbstractThe ligand‐to‐metal charge transfer (LMCT) facilitated activation of TiO2 has noteworthy potential for solar energy harvesting. However, the fast back electron transfer from TiO2 to an oxidized sensitizer is a key limiting factor causing low photocatalyst efficiency. Herein, a new catalyst design to both increase LMCT efficiency and minimize the back electron transfer is presented. A phase‐selective modification of mixed‐phase TiO2 (anatase: rutile interface) with poly‐salophen organic polymer is developed. The salophen and salen family organic monomers are selectively bound and polymerized on the anatase phase but not the rutile phase, which results in the formation of a three‐phase system. Such a three‐phase system converts an unfavorable polymer TiO2 core‐shell structure to an intimately mixed blend morphology, consisting of interfaced crystalline rutile TiO2 and an amorphous polymer‐covered anatase‐phase TiO2. The developed mixed‐blend morphology poly‐S@P25 can produce H2 of 37 410 µmol h–1 g–1 of polymer, which is ≈3.4 times higher than core‐shell poly‐S@anatase TiO2. This approach overcomes the drawback of the traditional core‐shell structured system for efficient electron harvesting from the LMCT process.

Electrically or chemically tunable photodetector with ultra high responsivity using graphene/InN nanowire based mixed dimensional barristors

In this work, an electrically/chemically tunable highly sensitive photodetector based on mixed dimensional heterojunction of graphene and planar InN nanowires (NW) is presented. Controlled partial oxidation of InN has been employed to effectively reduce the high surface carrier concentration of InN, which normally prevents it from forming good rectifying contact with graphene. The resulting surface modified InN NWs have been found to form excellent Schottky junction with graphene, with an increase in effective Schottky barrier height (SBH) by over 1.1 eV and a ratio of forward and reverse bias currents exceeding 4 orders of magnitude. Moreover, very strong barristor (gate tunable heterojunction) action has been observed, with I on/I off ≈ 4 orders of magnitude, and SBH increase by >0.3 eV. The barristor has been demonstrated to be highly sensitive to light, especially in the ultra-voilet, visible and near IR spectra. Responsivity was found to be widely tunable by gate voltage, with the highest value exceeding 1000 A W−1. Rise and fall times being in the range of hundreds of ms are indicative of photoconductive gain, which can be attributed to the ultra high responsivity. A method of semi-permanent molecular doping has been demonstrated to realize a two-terminal version of the photodetector, where the desired responsivity can still be achieved without requiring a back gate terminal, enabling the device to be realized on insulating substrates. The effect of encapsulation has been studied as a function of time, which has showed the long term stability of the dopant-induced enhancement and ultra high responsivity of the barristor photodetector.

A Mixed Heterojunction Layer for High Performance and Stability P-I-N-Based Perovskite Solar Cells

Perovskite solar cells (PSCs) have attracted a lot of attention due to high efficiency and low-cost fabrication processing. For planar structure PSCs, the electron transport layer (ETL) plays an important role to enhance the electron extraction capacity, device stability and balance the energy level between perovskite materials and metal electrode, which can effectively optimize the performance of PSCs. However, traditional ETL materials are not comfortable for commercialization of PSCs due to their inherent characteristics like strong aggregate, relatively low electrical conductivity, and poor ohmic contact with the metal electrodes. Here, a mixed heterojunction layer (MHL) combined an amphiphilic polymer material of poly[(9, 9-bis(3′-(N, N-dimethylamino)propyl)-2, 7-fluo-rene)-alt-2, 7-(9, 9-dioctylfluorene)] with conventional ETL materials [6, 6] phenyl C61-butyric acid methyl ester (PC61BM) was developed to optimize the interface characteristics between perovskite and metal electrode. Atomic force microscopy, the space charge limited current, photoluminescence measurements, and impedance spectroscopy have been measured to reveal the interface contact, electronic, and optical properties of the MHL modified PSCs device. As a result, the power conversion efficiency (PCE) of PSCs with MHL has dramatically increased from 15.17% to 18.39%. More importantly, the stability of the MHL based PSCs was improved significantly, with reduced PCE degradation (from 29% to 18%) after 160 h under ambient conditions. This research provides a simple but effective method to improve the efficiency and stability of PSCs.

Self-powered photodetectors based on 0D/2D mixed dimensional heterojunction with black phosphorus quantum dots as hole accepters

Abstact Two-dimensional (2D) materials exhibit outstanding performance and good application prospects in the field of photodetectors due to their excellent optical and electronic properties. However, weak light absorption capacity the rapid recombination of photogenerated electron-hole pairs is still a serious obstacle that limits their light detection performance. Black phosphorus (BP) can act as a hole acceptor due to its excellent hole mobility, which can induce the rapid transfer of photogenerated holes, and the heterojunction at the interface promotes the injection of photogenerated electrons onto the surface of MoS2, thereby effectively promoting the separation of photogenerated electron-hole pairs. In addition, recent reports indicate that quantum dots (QDs) have excellent local photon capture capabilities due to their quantum confinement and surface effects, which are considered to be excellent light absorbers that enhance the performance of photodetectors in 2D materials. Here, we prepared black phosphorus QDs-molybdenum disulfide (BP QDs-MoS2) heterojunction and constructed a photo-electrochemical-type (PEC-type) photodetector based on the heterojunction. Experimental results show that the BP QDs-MoS2 heterojunction-based PEC-type photodetector exhibits significantly enhanced photo-response performance. Meanwhile, the PEC-type photodetector also exhibits self-powered photo-response behavior, good photoresponse performance and stability in a liquid environmentand. This work strongly proves that BP QDs are used as hole acceptors to promote the separation of photogenerated electron-hole pairs is an effective means.

Phenol degradation behavior via photocatalytic of ZnO/Ag2CO3/Ag2O nanoparticles

The present study successfully conducted the synthesis of ZnO/Ag 2 CO 3 /Ag 2 O nanocomposite using co-precipitate technique and phase transformation route. The resulting nanocomposite photocatalyst were characterized by TEM and UV–vis spectra, while their photocatalytic activities were subsequently tested in the mineralization of phenol solution. Meanwhile, the heterojunction of Ag 2 CO 3 /Ag 2 O over ZnO lattice that influenced the surface-phase structure of the nanocomposites managed to generate higher absorption in visible light region in UV NIR spectra. In addition, the surface phase structure was produced via Ag 2 O crystal growth over Ag 2 CO 3 which was heterojunctioned on ZnO lattice, thus leading to an effective charge carrier transfer that indirectly suppressed the recombination of photogenerated electrons and holes. The results of the current research on the photocatalytic behaviour of the nanocomposites demonstrated that the phenol peak area for ZnO/Ag 2 CO 3 /Ag 2 O under UV light decreased 25.4 % compared to visible light radiation of 14.1 %. The presence of photo-oxidation products was detected in the liquid phase products in phenol-oxidation even though phenol was not completely removed. Overall, it was remarkable to discover that the formation of Ag 2 CO 3 /Ag 2 O mixed phase heterojunction over the surface of ZnO significantly enhanced the photocatalytic activity under visible light irradiation.

Composition-controllable p-CuO/n-ZnO hollow nanofibers for high-performance H2S detection

In virtue of their fantastic sensing performance, heterojunction-based gas sensors has gained ascending attention and gradually become a new generation of high-performance gas sensors. However, due to the poor control on the element composition and distribution of heterostructure, it remains a great challenge to optimize and repeat the gas-sensing performance. In this paper, hollow p-CuO/n-ZnO nanofibers with controllable compositions were successfully synthesized by combining the electrospinning and atomic layer deposition (ALD) method, and the effect of compositions on the gas-sensing performance were systematically studied. The ratios of Zn to Cu (RZn/Cu) were well controlled by adjusting the amounts of raw materials and the deposition cycles of ALD in orthogonal experiments, which offers a good opportunity to optimize the gas sensing performances. Interestingly, with the increase of RZn/Cu, the gas responses to 100 ppm H2S at 250 °C first increased to 60.5 (RZn/Cu = 15.6) and then decreased gradually. The optimum response of these materials was improved about 6-fold versus the pure ZnO and 45-fold versus pure CuO. Meanwhile, the selectivity and stability of these H2S sensors were also got much optimized. The enhanced sensing performance is believed to be mainly attributed to the optimal ratio of p-CuO/n-ZnO and the mixed heterojunction with radial concentration gradient.

Synthesis and enhanced photocatalytic performance of Ag/AgCl/TiO2 nanocomposites prepared by ion exchange method

Abstracts Ag/AgCl/TiO2 nanocomposite was synthesized through the combination of sol-gel process and hydrothermal (HT) method followed by ion exchange method in the paper. Transmission electron microscope, UV–vis absorption property and X-ray diffraction were utilized to characterize the morphology, optical absorption property and phase structure of the as-prepared photocatalysts. The morphological and performance studies of Ag/AgCl/TiO2 revealed that a well-interconnected mixed phase heterojunction photocatalyst was achieved through as-prepared TiO2 and the subsequent ion exchange method. By the degradation of Rhodamine B under xenon lamp, Ag/AgCl/TiO2 exhibited a significant enhancement in photocatalytic activity compared with TiO2 and Ag/TiO2. The band gap of Ag/AgCl/TiO2 can reach to 2.53eV compared with 3.22eV of the prepared TiO2. Furthermore, the synthesis process and photoreaction mechanisms of TiO2, Ag/TiO2 and Ag/AgCl/TiO2 photocatalysts were proposed. It demonstrated that the enhancement of photocatalytic was mainly due to the reducing of electron-hole recombination in Ag/AgCl/TiO2.

Electrochemically Obtained TiO2/CuxOy Nanotube Arrays Presenting a Photocatalytic Response in Processes of Pollutants Degradation and Bacteria Inactivation in Aqueous Phase

TiO2/CuxOy nanotube (NT) arrays were synthesized using the anodization method in the presence of ethylene glycol and different parameters applied. The presence, morphology, and chemical character of the obtained structures was characterized using a variety of methods—SEM (scanning electron microscopy), XPS (X-ray photoelectron spectroscopy), XRD (X-ray crystallography), PL (photoluminescence), and EDX (energy-dispersive X-ray spectroscopy). A p-n mixed oxide heterojunction of Ti-Cu was created with a proved response to the visible light range and the stable form that were in contact with Ti. TiO2/CuxOy NTs presented the appearance of both Cu2O (mainly) and CuO components influencing the dimensions of the NTs (1.1–1.3 µm). Additionally, changes in voltage have been proven to affect the NTs’ length, which reached a value of 3.5 µm for Ti90Cu10_50V. Degradation of phenol in the aqueous phase was observed in 16% of Ti85Cu15_30V after 1 h of visible light irradiation (λ > 420 nm). Scavenger tests for phenol degradation process in presence of NT samples exposed the responsibility of superoxide radicals for degradation of organic compounds in Vis light region. Inactivation of bacteria strains Escherichia coli (E. coli), Bacillus subtilis (B. subtilis), and Clostridium sp. in presence of obtained TiO2/CuxOy NT photocatalysts, and Vis light has been studied showing a great improvement in inactivation efficiency with a response rate of 97% inactivation for E. coli and 98% for Clostridium sp. in 60 min. Evidently, TEM (transmission electron microscopy) images confirmed the bacteria cells’ damage.

The effects of mixture ratios of CuPc:C60 in nanostructured organic hybrid planar–mixed heterojunction photovoltaic cells

Efficient organic hybrid planar–mixed heterojunction photovoltaic cells based on copper phthalocyanine (CuPc) and fullerene (C60) were fabricated. This structure, in addition to the high efficiency in exciton diffusion of a mixed heterojunction, has low resistance in charge transporting of a planar heterojunction. The effects of different CuPc:C60 mixing ratios in two stages and by varying the CuPc and C60 thicknesses, respectively, were investigated. Also, we illustrate the charge transport mechanisms and specify the charge injection models for the cells. For the optimized cell, trap-charge-limited current (TCLC) model was dominant. MoO3 is one of the common materials as anodic buffer layer which is used for modifying the ITO electrode and improving the charge extraction. According to the AFM results, after depositing MoO3 layer on ITO, the surface root mean square (RMS) roughness decreases from 8.31 to 4.14 nm. Optimal thickness for both CuPc and C60 layers was obtained 20 nm. Finally, the greatest value in power conversion efficiency was achieved 3.81% using the 1:1 CuPc:C60 mixing ratio. Moreover, short-circuit current density (Jsc), fill factor (FF), and open-circuit voltage (Voc) for this optimized cell were found 10.21 (mA/cm2), 0.64 (V), and 0.58, respectively.

Concurrent growth, structural and photocatalytic properties of hybridized C, N co-doped TiO2 mixed phase over g-C3N4 nanostructured

A concurrent and facile sol-gel assisted low temperature calcination approach to homogeneous growth of TiO2 mixed phase nanoparticles over g-C3N4 for designing visible-light-driven photocatalyst is demonstrated in this study. The structural and morphological studies revealed a well-interconnected g-C3N4/TiO2 mixed phase heterojunction photocatalyst was achieved through a sol-gel process and calcination at 400°C. The well-interconnected g-C3N4/TiO2 mixed phase heterojunction photocatalyst has strong visible light absorption capability due to the presence of an in-situ nitrogen and carbon dopants. The noticeably increased in the visible-light-photocatalytic activity performance is ascertained mainly due to the improvement of electron-hole separation and charge carrier migration.

Nanoscale Control of Morphology in Fullerene-Based Electron-Conducting Buffers via Organic Vapor Phase Deposition

Small molecular weight organic thin film mixtures of the electron-conducting C60 in a wide energy gap matrix, 3,5,3',5'-tetra(m-pyrid-3-yl)phenyl[1,1']biphenyl (BP4mPy) forms a high efficiency electron filtering buffer in organic photovoltaics (OPV). Electrons are conducted via percolating paths of C60 whereas excitons are blocked by the BP4mPy. We find that the conductivity and exciton blocking efficiency of the blends are strongly dependent on film morphology that can be precisely controlled by the conditions used in the organic vapor phase deposition (OVPD). Specifically, we find that a background carrier gas pressure of 0.28 Torr leads to extended and highly conductive crystalline C60 domains. Furthermore, the structure is strongly influenced by carrier gas pressure. Via a combination of morphological measurements and molecular dynamics simulations, we find that this dependence is due to kinetically induced structural annealing at the growth interface. The highest electron mobility of (6.1 ± 0.5) × 10(-3) (cm(2)/V·s) is obtained at 0.28 Torr, which is approximately 2 orders of magnitude higher than for amorphous C60 films. The fill factors and power conversion efficiencies of vacuum deposited tetraphenyldibenzoperiflanthene (DBP):C70 planar mixed heterojunction OPVs using an OVPD-grown buffer layer are (8.0 ± 0.2)% compared to (6.6 ± 0.2)% using amorphous buffers grown by vacuum thermal evaporation.

Controlled donor-accepter ratio for application of organic photovoltaic cells by alternative intermittent electrospray co-deposition

Because co-deposition method has been utilized in a conventional thermal evaporation process to realize graded donor-acceptor architectures, we investigated an alternative intermittent electrospray co-deposition method for solution-processed organic photovoltaic cells. In this method, two solutions of poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C61 butyric acid methyl ester (PCBM) were alternatively deposited using high-voltage pulse. Thus, the P3HT:PCBM blend thin film could be deposited even in a vacuum-free experimental setup. The optimum pulse width was found to be greater than 6 s to avoid an unexpected charge to the adjacent glass capillary, which causes the instable electrospray. The P3HT molecular ordering estimated from Raman spectroscopy and grazing incidence X-ray diffraction patterns was comparable to that estimated from the spin-coated device. In addition, the P3HT:PCBM ratio of the deposited thin film could be controlled by changing the ratio of the pulse width for the P3HT and PCBM solutions and was evaluated from the ultraviolet–visible absorption spectrum. Finally, a two layered bulk heterojunction structure with P3HT:PCBM was successfully demonstrated, leading to a maximum photoconversion efficiency of 3.1%. This value was 1.4-fold higher than that of the uniformly mixed bulk heterojunction device because of the high carrier-collection efficiency.

Syntheses of D-A-A Type Small Molecular Donor Materials Having Various Electron Accepting Moiety for Organic Photovoltaic Application.

Small molecular donor, DTDCTB achieved a high power conversion efficiency (PCE) value of 6.6 ± 0.2% in vacuum-deposited planar mixed heterojunction (PMHJ) structure. However, the same material just recorded PCE of 0.34% in solution processed small molecule based bulk heterjunction (BHJ) organic photovoltaic cells. For the improvement of organic photovoltaic cells (OPVs), In this study, we designed and synthesized several D-A-A (donor-acceptor-acceptor) type molecular electron donating materials. Ditolylaminothienyl moiety as an electron donating group connected to 1,2,5-benzothiadiazole as a conjugated electron accepting unit, simultaneously with an electron accepting terminal group such as cyano alkyl acetate and N-alkyl rhodanine. The thermal, photophysical, and electrochemical properties of prepared small molecules were investigated by DSC, UV/Vis spectroscopy and Cyclic Voltametry, respectively. As a result, 0.89% of PCE can be obtained from OPV using a mixture of DTATBTER and PCBM as an active layer with a Voc of 0.87 V, a Jsc of 3.20 mA/cm2, and a fill factor of 31.9%.