Facilitate Carrier Transport Research Articles

Electrophoretic‐Driven In Situ Polymerization Depositing High‐Quality Perovskite Films for Photodetectors

AbstractPerovskite quantum dots (PQDs) have wide applications in optoelectronic devices due to their excellent stability and processability. However, PQD films are still seeking to overcome the problems of disorder and porosity. This seriously hinders the carrier transport and the application of PQDs in photodetectors (PDs). In this work, the electrophoretic deposition (EPD) method is employed to fabricate PQDs with conjugated linoleic acid (CLA) as the ligands, and the highly oriented and dense films are obtained. In addition, CLA molecules can polymerize during electrophoresis, which shortens the quantum dot spacing. It facilitates carrier transport while improving the stability of the film. PDs prepared by electrophoresis have a high detectivity (D*) up to 5.1 × 1012 Jones, which is 3.9 times higher than PDs prepared by spin coating (1.3 × 1012 Jones) and reaches the highest record under the same conditions. In addition, the device exhibits a remarkable stability, maintaining 95% of the initial performance when left in the air for 3 months. This work provides a new perspective on the assembly of PQDs in the application of optoelectronic devices.

Implementing an intermittent spin-coating strategy to enable bottom-up crystallization in layered halide perovskites

Two-dimensional halide perovskites (2D PVSKs) have drawn tremendous attentions owing to their outstanding ambient stability. However, the random orientation of layered crystals severely impedes the out-of-plane carrier transport and limits the solar cell performance. An in-depth understanding coupled with an effective control of the crystallization in 2D PVSKs is the crux for highly efficient and durable devices. In this contribution, we accidentally discovered that the crystallization of 2D PVSKs can be effectively regulated by so-called ′intermittent spin-coating (ISC)′ process. Combined analyses of in(ex)-situ grazing-incidence wide-angle X-ray scattering with time-of-flight secondary ion mass spectrometry distinguish the interface initialized bottom-up crystallization upon ISC treatment from the bi-directional one in the conventional spin-coating process, which results in significantly enhanced crystal orientation and thus facilitated carrier transport as confirmed by both electrical measurements and ultrafast spectroscopies. As a result, the p-i-n architecture planar solar cells based on ISC fabricated paradigm PEA2MA3Pb4I13 deliver a respectable efficiency of 11.2% without any treatment, which is three-fold improvement over their spin-coated counterparts and can be further boosted up to 14.0% by NH4Cl addition, demonstrating the compatibility of ISC method with other film optimization strategies.

Capacitive β-Ga2O3 solar-blind photodetector with graphene electrode

Conventional solar-blind photodetectors based on the conduction of photoexcited carriers are energy inefficient owing to the power dissipation caused by a resistive sensing mechanism and the narrow bandgap energy of the photon-absorbing layer. Herein, we demonstrate the energy-efficient capacitive sensing of deep-UV wavelengths by integrating an intrinsically solar-blind ultrawide bandgap (UWBG) β-Ga2O3 semiconductor with UV-transparent and conductive graphene electrode. A UWBG β-Ga2O3 eliminates the requirement of a solar-blind deep-UV bandpass filter. The high optical transmittance of the graphene enables UV-C light to be absorbed in the underlying β-Ga2O3, thereby facilitating carrier transport between the graphene electrode and β-Ga2O3. A capacitance change under UV-C excitation is observed, along with excellent reproductivity and spectral selectivity at various frequencies and bias conditions; the sensing performance improves with an increase in frequency. The average power dissipation of the fabricated photodetector in the stand-by (dark) and active (UV-C illumination) modes is 37.7 and 53.3 μW, respectively. Overall, this work introduces a new strategy for developing next-generation compact and energy-efficient solar-blind photodetectors.

Interfacial gradient energy band alignment modulation via ion exchange reaction toward efficient and stable methylammonium-free Dion-Jacobson quasi-2D perovskite solar cells

Dion-Jacobson (DJ) quasi-2D perovskite solar cells (PSCs) have attracted significant attention owing to its greater potentials in realizing efficient and stable quasi 2D PSCs as compared to Ruddlesden–Popper counterpart. To further enhance power conversion efficiency (PCE) and stability, the fabrication of methylammonium-free formamidinium (FA)-based DJ quasi-2D PSCs is highly desirable. Herein, we report a strategy for constructing gradient energy band alignment by achieving gradient Br doping (GBD) via in situ ion exchange reaction between I− and Br− in FA-based DJ quasi-2D PSCs. First, the gradient energy band alignment can facilitate carrier transport, extraction and transfer. Second, the improved crystallinity and reduced defect density due to recrystallization process are realized after FABr treatment. Finally, the incorporation of Br also contributes to increased device stability. The device with GBD achieves a much higher PCE of 16.75% than control device (13.78%), which is mainly as a consequence of a significantly boosted VOC from 0.970 V to 1.107 V due to suppressed bulk and interfacial nonradiative recombination. The unencapsulated device with GBD maintains 93% of its initial PCE after aging under the relative humidity range of 15–20% for 1600 h, and 91% after aging at 60 °C for 400 h.

Crystal Orientation Modulation and Defect Passivation for Efficient and Stable Methylammonium-Free Dion-Jacobson Quasi-2D Perovskite Solar Cells.

Dion-Jacobson (DJ) quasi-2D perovskite solar cells (PSCs) have received increasing attention due to their greater potentials in realizing efficient and stable quasi-2D PSCs relative to their Ruddlesden-Popper counterpart. The substitution of methylammonium (MA+) with formamidinium is expected to be able to further increase the stability and power conversion efficiency (PCE) of DJ quasi-2D PSCs. Herein, we report a multifunctional additive strategy for preparing high-quality MA-free DJ quasi-2D perovskite films, where 1,1'-carbonyldi(1,2,4-triazole) (CDTA) molecules are incorporated into the perovskite precursor solution. CDTA modification can control phase distribution, enlarge grain size, modulate crystallinity and crystal orientation, and passivate defects. After CDTA modification, more favorable gradient phase distribution and accordingly gradient band alignment are formed, which is conducive to carrier transport and extraction. The improved crystal orientation can facilitate carrier transport and collection. The enlarged grain size and effective defect passivation contribute to reduced defect density. As a result, the CDTA-modified device delivers a PCE of 16.07%, which is one of the highest PCEs ever reported for MA-free DJ quasi-2D PSCs. The unencapsulated device with CDTA maintains 92% of its initial PCE after aging under one sun illumination for 360 h and 86% after aging at 60 °C for 360 h.

Engineering doping level for enhanced thermoelectric performance of carbon nanotubes/polyaniline composites

Thermoelectric (TE) properties of pure polyaniline (PANI) have been extensively investigated and significant progress has been achieved with proper modulation of doping level, there is however few reports to illustrate the crucial role of doping level in modulating the microstructure and optimizing the TE performance of carbon nanotubes (CNTs)/PANI composites. Herein, a series of CNTs/PANI composites with varied CNTs loadings are doped with various amount of camphorsulfonic acid to control doping level of PANI. As the doping level decreases, PANI chains are transformed from extended coil with delocaliztion polarons to compact with polarons localized. All the CNTs/PANI composites exhibit increased Seebeck coefficient, while show less deteriorated conductivity compared with pure PANI film. With high CNTs loading more than 69 wt%, greatly enhanced power factor of 321 ± 24 μW m−1 K−2 could be achieved at low doping level, ascribing to not only strong π-π interaction and doping effect of CNTs which promote PANI chains ordered stacking, but also intrinsic conductive CNTs network which provides extra pathways to facilitate carrier transport. This work demonstrates the effective strategy of precise tuning doping level to improve TE properties of CNTs/PANI composites.

A facile solution processed ZnO@ZnS core–shell nanorods arrays for high-efficiency perovskite solar cells with boosted stability

Zinc Oxide (ZnO) has been extensively applied as electron transport material (ETM) in perovskite solar cells (PSCs) since the emergence of PSCs. However, some chemisorbed oxygen species on the surface of ZnO can cause the degradation of CH3NH3+ (MA+) based perovskite. To avoid the destructive effect of ZnO, a facile solution strategy was proposed to produce a ZnS shell around the ZnO nanorods arrays (ZnO-NRs), i. e. ZnO@ZnS core–shell nanorods (ZnO-NRs@ZnS). The ZnO-NRs@ZnS cascade structure can not only facilitate carrier transport, but also enhance the stability of ZnO based PSCs. A power conversion efficiency (PCE) of 20.6% was finally yielded, which is the-state-of-the-art efficiency for PSCs with one-dimensional (1D) ZnO electron transport materials (ETMs). Moreover, over 90% of the initial efficiency was retained for the unencapsulated device with ZnO-NRs@ZnS ETMs at 85 °C for 500 h, demonstrating excellent stability. This work provides a simple and efficient avenue to simultaneously enhance the photovoltaic (PV) performance and stability of 1D ZnO nanostructure-based PSCs.

Controllable Distribution of Oxygen Vacancies in Grain Boundaries of p‐Si/TiO2 Heterojunction Photocathodes for Solar Water Splitting

AbstractSilicon is a promising photocathode material in photoelectrochemical water splitting for hydrogen production, but it is primarily limited by photocorrosion in aqueous electrolytes. As an extensively used protective material, crystalline TiO2 could protect Si photoelectrode against corrosion. However, a large number of grain boundaries (GBs) in polycrystalline TiO2 would induce excessive recombination centers, impeding the carrier transport. This paper describes the introduction of oxygen vacancies (Ovac) with controllable spatial distribution for GBs to promote carrier transport. Two kinds of Ovac distribution, Ovac along GBs and Ovac inside grains, are compared, where the latter one is demonstrated to facilitate carrier transport owing to the formation of tunneling paths across GBs. Consequently, a simple p‐Si/TiO2/Pt heterojunction photocathode with controllable Ovac distribution in TiO2 shows a +400 mV onset potential shift and yields an applied bias photon‐to‐current efficiency of 5.9 %, which is the best efficiency reported among silicon photocathodes except for silicon homojunction.

Controllable Distribution of Oxygen Vacancies in Grain Boundaries of p-Si/TiO2 Heterojunction Photocathodes for Solar Water Splitting.

Silicon is a promising photocathode material in photoelectrochemical water splitting for hydrogen production, but it is primarily limited by photocorrosion in aqueous electrolytes. As an extensively used protective material, crystalline TiO2 could protect Si photoelectrode against corrosion. However, a large number of grain boundaries (GBs) in polycrystalline TiO2 would induce excessive recombination centers, impeding the carrier transport. This paper describes the introduction of oxygen vacancies (Ovac ) with controllable spatial distribution for GBs to promote carrier transport. Two kinds of Ovac distribution, Ovac along GBs and Ovac inside grains, are compared, where the latter one is demonstrated to facilitate carrier transport owing to the formation of tunneling paths across GBs. Consequently, a simple p-Si/TiO2 /Pt heterojunction photocathode with controllable Ovac distribution in TiO2 shows a +400 mV onset potential shift and yields an applied bias photon-to-current efficiency of 5.9 %, which is the best efficiency reported among silicon photocathodes except for silicon homojunction.

Uncovering the unusual effect of halogenation on crystal packing in an azzaacee-based electron transporting material

Due to strong interaction with perovskite, azzaacee-based derivative TDTP is promising electron-transporting material (ETM) in the inverted perovskite solar cell. However, the low electron mobility of TDTP seriously limits the photoelectric conversion efficiency (PCE) of the device. Herein, three TDTP-type material molecules, FT, ClT and BrT, are designed based on the paternal TDTP to boost the electron mobility. The results demonstrate that the introduction of halogen (electron-withdrawing group) improves the electron mobility of the materials due to the enhanced intermolecular π-π stacking. Unexpectedly, ClT with chlorine substitution violates the electronegative order and shows the highest electron mobility, which results from the head-to-head stacking manner only in ClT crystal. This special stacking produces stronger electron coupling between molecules, thus facilitating carrier transport. This work provides theoretical guidance for the design of ETMs with excellent electron mobility by deeply understanding the relationship between the charge transporting properties of ETMs and their molecular/crystal structures.

Interfacial ZnS passivation for improvement of transparent ZnO/CuI diode characteristics

To fabricate a n-ZnO/p-CuI heterojunction diode with a high rectification ratio at a large junction area, γ-CuI film was spin-coated on ZnO. Since the n-ZnO/p-CuI diode showed a rectification ratio as small as 2.38 × 102, the ZnS interfacial layer was prepared by successive ionic layer adsorption and reaction. By placing a ZnS layer between the n-ZnO/p-CuI, the rectification ratio from the diode increased to 1.71 × 107 at a junction area of 1 cm2. Modification of the valence band offset and the conduction band offset of the n-ZnO/p-CuI junction in the presence of the interfacial ZnS layer may facilitate carrier transport between ZnO and CuI. In addition, it was confirmed that two types of defects (oxygen vacancy and unpaired electron trapped on an oxygen vacancy) were reduced by insertion of an interfacial ZnS layer. A significant improvement in the rectification ratio resulted from the large decrease in the reverse current in the ZnO/CuI diode by a reduced amount of oxygen vacancies. Therefore, the improvement of band alignment with the decrease in the interfacial defects by the introduction of wide bandgap ZnS passivation layer enhanced the electrical characteristics of ZnO/CuI diode.

Direct observation of reach-through behavior in back-illuminated algan avalanche photodiode with separate absorption and multiplication structure

A back-illuminated heterostructure AlGaN avalanche photodiode (APD) was fabricated with a separate absorption and multiplication (SAM) structure. The SAM structure is able to modulate the electric field distribution of the photodetector and induce a single carrier to trigger an avalanche event. The fabricated p-i-n-i-n APD exhibited a superior maximum gain of 1.8 × 105 at 70 V and distinct solar-blind responsivity peak at 280 nm. However, before the avalanche occurs, the SAM APD needs to be reached through for facilitating carrier transport. By investigating the capacitance characteristics of the SAM APD, the reach-through behavior was directly observed. The capacitance of the detector demonstrated evident declining trends with the applied voltage in the range of 18 to 24 V at different frequencies. The simulated electric field and energy band also revealed that the absorption region was initially reached through at 18 V reverse bias, corresponding to the depletion of n-type interlayer into the absorption region. Meanwhile, there were large differences in capacitances between low and high frequencies, which indicated the effects of impurity and defect states. These findings explicitly reveal the reach-through mechanism of the SAM APD, which is beneficial to the better understanding of device physics and further APD design.

Asymmetrically phosphorylated carbazole host for highly efficient blue and white thermally activated delayed fluorescence diodes

Host development is crucial and challenging for thermally activated delayed fluorescence (TADF) organic light emitting diodes (OLED), which requires accurate modulation of host-dopant energy transfer and host/dopant-dopant quenching. Here, we demonstrate that phosphine oxide (PO) groups can be used to simultaneously facilitate carrier transport, positive energy transfer and quenching suppression. Two diphenylphosphine oxide (DPPO) groups at carbazole and N-phenyl of 3,6-di-tert-butyl-N-phenylcarbazole induce the spatially different steric hindrance and inductive effect of DPPOs in tBCzPPOSPO host. The asymmetric configuration with two bulky DPPOs at the same side of tBCzPPOSPO results in the lamellar structure and head-to-tail molecular packing in each layer of its single crystal. tBCzPPOSPO reveals the first singlet (S1) and triplet energy levels (T1) of 3.42 and 2.98 eV, providing the positive energy transfer to blue TADF emitter. Intermolecular interactions are mitigated by steric hindrance of tert-butyls and DPPO groups for quenching suppression. A corner of tBCzPPOSPO is uncovered, rendering electron and hole mobility by level of 10−6 cm2 V−1 s−1. Therefore, based on conventional blue and yellow TADF emitters, tBCzPPOSPO endowed its single-emissive-layer blue and white OLEDs with the state-of-the-art performance, including maximum external quantum efficiencies more than 20% and a top-rank power efficiency of 63.5 lm W−1.

The Role of Surface Termination in Halide Perovskites for Efficient Photocatalytic Synthesis.

Halide perovskites have received attention in the field of photocatalysis owing to their excellent optoelectronic properties. However, the semiconductor properties of halide perovskite surfaces and the influence on photocatalytic performance have not been systematically clarified. Now, the conversion of triose (such as 1,3-dihydroxyacetone (DHA)) is employed as a model reaction to explore the surface termination of MAPbI3 . By rational design of the surface termination for MAPbI3 , the production rate of butyl lactate is substantially improved to 7719 μg g-1 cat. h-1 under visible-light illumination. The MAI-terminated MAPbI3 surface governs the photocatalytic performance. Specially, MAI-terminated surface is susceptible to iodide oxidation, which thus promotes the exposure of PbII as active sites for this photocatalysis process. Moreover, MAI-termination induces a p-doping effect near the surface for MAPbI3 , which facilitates carrier transport and thus photosynthesis.

The Role of Surface Termination in Halide Perovskites for Efficient Photocatalytic Synthesis

AbstractHalide perovskites have received attention in the field of photocatalysis owing to their excellent optoelectronic properties. However, the semiconductor properties of halide perovskite surfaces and the influence on photocatalytic performance have not been systematically clarified. Now, the conversion of triose (such as 1,3‐dihydroxyacetone (DHA)) is employed as a model reaction to explore the surface termination of MAPbI3. By rational design of the surface termination for MAPbI3, the production rate of butyl lactate is substantially improved to 7719 μg g−1 cat. h−1 under visible‐light illumination. The MAI‐terminated MAPbI3 surface governs the photocatalytic performance. Specially, MAI‐terminated surface is susceptible to iodide oxidation, which thus promotes the exposure of PbII as active sites for this photocatalysis process. Moreover, MAI‐termination induces a p‐doping effect near the surface for MAPbI3, which facilitates carrier transport and thus photosynthesis.

Water Additive Enhanced Solution Processing of Alloy Sb2(S1−xSex)3‐Based Solar Cells

Solution process is a convenient and cost‐effective approach to the deposition of thin films for optoelectronic devices. The quality of as‐prepared films depends critically on the applied precursor materials as well as solvents. Herein, a method is developed to synthesize Sb2(S1−xSex)3 thin films and demonstrate the role played by water during the film formation. In the synthesis, antimony trichloride, selenourea, and thiourea used as Sb, Se, and S sources are dissolved in N,N‐dimethylformamide and dimethyl sulfoxide mixed solvents. It is found that a tiny amount of water (1%) in the solution enables an essential increase in grain size that in turn facilitates carrier transport and suppresses recombination. The deep‐level transient spectroscopy analysis shows that water additive helps to reduce defect density and type in the as‐obtained films. Finally, the improved film quality leads to a certified power conversion efficiency of 7.42%, a top efficiency in Sb2(S,Se)3‐based solar cells regardless of the device configurations. This study provides an effective approach to tailor the optoelectronic properties of Sb2(S,Se)3 and a practical strategy to improve the device efficiency.

Low Roll‐Off Perovskite Quantum Dot Light‐Emitting Diodes Achieved by Augmenting Hole Mobility

AbstractThe external quantum efficiencies (EQEs) of perovskite quantum dot light‐emitting diodes (QD‐LEDs) are close to the out‐coupling efficiency limitation. However, these high‐performance QD‐LEDs still suffer from a serious issue of efficiency roll‐off at high current density. More injected carriers produce photons less efficiently, strongly suggesting the variation of ratio between radiative and non‐radiative recombination. An approach is proposed to balance the carrier distribution and achieve high EQE at high current density. The average interdot distance between QDs is reduced and this facilitates carrier transport in QD films and thus electrons and holes have a balanced distribution in QD layers. Such encouraging results augment the proportion of radiative recombination, make devices with peak EQE of 12.7%, and present a great device performance at high current density with an EQE roll‐off of 11% at 500 mA cm−2 (the lowest roll‐off known so far) where the EQE is still over 11%.

Carbon Dots–Implanted Graphitic Carbon Nitride Nanosheets for Photocatalysis: Simultaneously Manipulating Carrier Transport in Inter‐ and Intralayers

Carbon dots (CDs) present unique photoinduced charge transfer and reservoir properties, showing promising application potential in photocatalysis. The in situ preparation of CDs in a graphitic carbon nitride (g‐C3N4) matrix provides not only a new approach for electronic structure modulation and heterostructure construction but also an effective way to improve their photocatalytic performance. However, incorporating CDs into ultrathin g‐C3N4 remains a challenge. Moreover, simultaneously tuning their carrier transport in inter‐ and intralayers is difficult but significant for their application as efficient photocatalysts. Herein, an unprecedented Se‐chaperoned thermal polymerization method for the synthesis of zero‐dimensional CD‐implanted g‐C3N4 nanosheets (CCNS) is reported. The CCNS simultaneously facilitate carrier transport and suppress recombination because of the seamless bonding heterostructure of CDs within the in‐plane domains of the g‐C3N4 nanosheets. Accordingly, the photocatalytic rates of water splitting for H2 evolution and CO2 reduction are enhanced 3.1 and 4.1 times, respectively. In addition, the photocatalytic RhB degradation efficiency dramatically increases 18 times. This work presents a promising solution to solving the current worldwide energy shortage and environmental pollution issues.

Facile green strategy for improving thermoelectric performance of carbon nanotube/polyaniline composites by ethanol treatment

In contrast to commonly adopted harmful reagents which are not favorable for human body and environment, a facile environmental friendly dedoping approach is proposed to modulate carrier transport and thermoelectric properties of single-walled carbon nanotube (SWCNT)/polyaniline (PANI) composites by ethanol treatment. Compared with the pristine composite films, PANI is partially dedoped after treatments, which results in typically increasing of Seebeck coefficient and deteriorative electrical conductivity. As the interconnected conductive CNT networks provide extra pathways to facilitate carrier transport, the electrical conductivity is only slightly decreased. Consequently, an enhanced power factor of 362 μW m−1 K−2 is achieved, which is superior to the pristine composite of 234 μW m−1 K−2. Furthermore, the flexible thermoelectric device based on the treated composites generates high output power of 2.86 μW and high power density of 745 μW cm−2 at ΔT = 50 K. With the advantages of low cost, facile processing, and environmental friendly, this strategy exhibits potential application for commercial thermoelectric conversion.

Nanocellulose and Nanochitin Cryogels Improve the Efficiency of Dye Solar Cells

Biobased cryogel membranes were applied as electrolyte holders in dye solar cells (DSC) while facilitating carrier transport during operation. They also improved device performance and stability. F...