Low-temperature Solution Process Research Articles

Fine-Tuning Conformation of PEDOT Chains Enables Simultaneously Enhanced Conductivity, Work Function, Transmittance, and Waterproofness in PEDOT:PSS Interlayer for Highly Efficient and Stable Organic Photovoltaics.

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is widely utilized as the hole transport layer (HTL) inorganic photovoltaics (OPVs) because of its low-temperature solution processing peculiarity, high optical transmittance, and excellent mechanical flexibility. However, the core-shell structure of PSS coated PEDOT results in relatively low conductivity, work function, transmittance and waterproofness of PEDOT:PSS interlayer, limiting the photovoltaic performance and stability of OPVs. Here, the conformation of PEDOT chains are regulated from helical benzoyl to linear quinone structure via incorporation of 2D Cd0.85PS3Li0.15H0.15dopant into the conventional PEDOT:PSS interlayer, promoting an interpenetrating network structure in PEDOT:PSS interlayer and forming an efficient hole transport channel from active layer to ITO electrode. Such features significantly improve the electrical conductivity, work function, and transmittance of PEDOT:PSS interlayer. In consequence, the maximum power conversion efficiency (PCE) of D18:L8-BO, PBDB-T:ITIC, as well as PTzBI-dF:L8-BO based OPVs ameliorated from 18.37%, 8.94%, and 15.80% to 19.26%, 10.00%, and 16.83%, respectively. The application of Cd0.85PS3Li0.15H0.15 doping PEDOT:PSS strategy demonstrates great potential for the development of strongly conductive, large-work-function, highly transparent, and excellent-waterproof PEDOT:PSS interlayer toward highly efficient and stable OPVs.

Solution Processed Bilayer Metal Halide White Light Emitting Diodes.

Metal halide perovskites and perovskite-related organic metal halide hybrids (OMHHs) have recently emerged as a new class of luminescent materials for light emitting diodes (LEDs), owing to their unique and remarkable properties, including near-unity photoluminescence quantum efficiencies, highly tunable emission colors, and low temperature solution processing. While substantial progress has been made in developing monochromatic LEDs with electroluminescence across blue, green, red, and near-infrared regions, achieving highly efficient and stable white electroluminescence from a single LED remains a challenging and under-explored area. Here, a facile approach to generating white electroluminescence is reported by combining narrow sky-blue emission from metal halide perovskites and broadband orange/red emission from zero-dimensional (0D) OMHHs. For the proof of concept, utilizing TPPcarz+ passivated two-dimensional (2D) CsPbBr3 nanoplatelets (NPLs) as sky blue emitter and 0D TPPcarzSbBr4 as orange/red emitter (TPPcarz+ = triphenyl (9-phenyl-9H-carbazol-3-yl) phosphonium), white LEDs (WLEDs) with a solution processed bilayer structure have been fabricated to exhibit a peak external quantum efficiency (EQE) of 4.8% and luminance of 1507 cd m-2 at the Commission Internationale de L'Eclairage (CIE) coordinate of (0.32, 0.35). This work opens a new pathway for creating highly efficient and stable WLEDs using metal halide perovskites and related materials.

Fabrication and Characterization of Flexible CuI-Based Photodetectors on Mica Substrates by a Low-Temperature Solution Process

Both CuI and CuI:Zn semiconductor thin films, along with MSM-structured UV photodetectors, were prepared on flexible mica substrates at low temperature (150 °C) by a wet chemical method. The two CuI-based films exhibited a polycrystalline phase with an optical bandgap energy close to 3.0 eV. Hall effect measurements indicated that the CuI thin film sample had p-type conductivity, while the CuI:Zn thin film sample exhibited n-type conductivity, with the latter showing a higher carrier mobility of 14.78 cm2/Vs compared to 7.67 cm2/Vs for the former. The I-V curves of both types of photodetectors showed asymmetric rectification characteristics with rectification ratios at ±3 V of 5.23 and 14.3 for the CuI and CuI:Zn devices, respectively. Flexible CuI:Zn devices exhibited significantly better sensitivity, responsivity, and specific detectivity than CuI devices both before and after static bending tests. It was found that, while the optoelectronic performance of flexible CuI-based photodetectors degraded under tensile stress during static bending tests, they still exhibited good reproducibility and repeatability in their photoresponses.

Fabrication Strategies for 2D Halide Perovskite Towards Next-Generation Optoelectronic Applications

AbstractHalide perovskites have emerged as promising materials in high-performance optoelectronics due to their exceptional optoelectrical properties, such as long carrier lifetime and tunable bandgap. Despite the promising capabilities of three-dimensional (3D) halide perovskites in applications like solar cells and light-emitting diodes, their operational stability remains a critical challenge. This review focuses on quasi-two-dimensional (2D) halide perovskites, which offer enhanced stability through their reduced dimensionality. We discuss the unique properties of these materials, including the ability to modify optical and electronic characteristics by altering the organic cations and the layer number in the perovskite structure. Additionally, we review various fabrication techniques, highlighting the shift from traditional low-temperature solution processes to more advanced solid, liquid, and vapor-phase methods, which address the limitations of conventional fabrication and enhance material quality. This comprehensive review aims to provide insights into the development of stable and efficient 2D halide perovskite-based optoelectronic devices, paving the way for their integration into next-generation optoelectronic applications.

Multidimensional Deep Ultraviolet (DUV) Synapses Based on Organic/Perovskite Semiconductor Heterojunction Transistors for Antispoofing Facial Recognition Systems.

Reliably discerning real human faces from fake ones, known as antispoofing, is crucial for facial recognition systems. While neuromorphic systems offer integrated sensing-memory-processing functions, they still struggle with efficient antispoofing techniques. Here we introduce a neuromorphic facial recognition system incorporating multidimensional deep ultraviolet (DUV) optoelectronic synapses to address these challenges. To overcome the complexity and high cost of producing DUV synapses using traditional wide-bandgap semiconductors, we developed a low-temperature (≤70 °C) solution process for fabricating DUV synapses based on PEA2PbBr4/C8-BTBT heterojunction field-effect transistors. This method enables the large-scale (4-in.), uniform, and transparent production of DUV synapses. These devices respond to both DUV and visible light, showing multidimensional features. Leveraging the unique ability of the multidimensional DUV synapse (MDUVS) to discriminate real human skin from artificial materials, we have achieved robust neuromorphic facial recognition with antispoofing capability, successfully identifying genuine human faces with an accuracy exceeding 92%.

Experimental Characterisation of BGBC OTFT for Indoor CO<sub>2</sub> Gas Sensing

Global warming is a concern nowadays due to excessive release of harmful gasses to the environment, leading to greenhouse effect phenomena worldwide. Based on the data provided by global pollution agencies, the release of greenhouse gasses to the atmosphere is the main cause of pollution and the increase in atmospheric temperature due to warming. Greenhouse gasses (GHGs) contents released to the environment is worrying, with carbon dioxide (CO2) is reported at the highest concentration compared to other gasses. There are many studies conducted to develop and evaluate the performance of harmful gas sensors incorporating inorganic and organic semiconductive materials. Organic semiconductors (OSCs) are environmentally friendly materials, relatively cheaper technology, and comprised of a wide range of materials with good carrier mobility. Therefore, in this work, Organic Thin Film Transistor (OTFT) is developed for gas sensor application. As global warming is becoming more serious, this solution is instead a sustainable solution to the environment, as organic molecules which are held together via Van der Waals bond are easily processed via low-temperature deposition and solution processing as compared to more complicated processes involved in conventional inorganic counterpart. In addition, the developed sensor is generally robust due to the ability to withstand high humidity conditions and can be fabricated on flexible substrates. In this work, suitable materials are identified in basic OTFT construction, which are the electrodes, dielectric and substrate. The scope is mainly focusing on the development of bottom gate OTFT construction, incorporating p-type active material which are Trisisopropylsilylethynyl Pentacene (TIPS Pentacene), Aluminium (Al) as drain and source electrodes, PEDOT: PSS as gate electrode and Polyvinyl alcohol (PVA) as gate dielectric. The materials in bottom gate bottom contact (BGBC) configuration, fabricated via screen printing technique is experimentally tested towards CO2 detection. CO2 is initially detected at 1618 ppm with contact resistance of 15 kΩ, and at 10 ml/minute flow rate, the developed configuration is demonstrated able to achieve sensitivity of 2.069 Ω/ppm. In conclusion, the studied BGBC OTFT has demonstrated suitability and applicability in CO2 gas sensing for sustainable environmental condition monitoring, that could lead to safer environment for the living things on earth. With the proposed dimensions, in the future it is possible to proceed with this work to be fabricated by using more advanced techniques such as photolithography and many others.

Femtosecond Laser Irradiation to Zirconia Prior to Calcium Phosphate Coating Enhances Osteointegration of Zirconia in Rabbits.

Calcium phosphate (CaP) coating of zirconia and zirconia-based implants is challenging, due to their chemical instability and susceptibility to thermal and mechanical impacts. A 3 mol% yttrium-stabilized tetragonal zirconia polycrystal was subjected to femtosecond laser (FsL) irradiation to form micro- and submicron surface architectures, prior to CaP coating using pulsed laser deposition (PLD) and low-temperature solution processing. Untreated zirconia, CaP-coated zirconia, and FsL-irradiated and CaP-coated zirconia were implanted in proximal tibial metaphyses of male Japanese white rabbits for four weeks. Radiographical analysis, push-out test, alizarin red staining, and histomorphometric analysis demonstrated a much improved bone-bonding ability of FsL-irradiated and CaP-coated zirconia over CaP-coated zirconia without FsL irradiation and untreated zirconia. The failure strength of the FsL-irradiated and CaP-coated zirconia in the push-out test was 6.2-13.1-times higher than that of the CaP-coated zirconia without FsL irradiation and untreated zirconia. Moreover, the adhesion strength between the bone and FsL-irradiated and CaP-coated zirconia was as high as that inducing host bone fracture in the push-out tests. The increased bone-bonding ability was attributed to the micro-/submicron surface architectures that enhanced osteoblastic differentiation and mechanical interlocking, leading to improved osteointegration. FsL irradiation followed by CaP coating could be useful for improving the osteointegration of cement-less zirconia-based joints and zirconia dental implants.

Bulk-like ferroelectricity and magnetoelectric response of low-temperature solution-processed BiFeO3–PbTiO3 films on Ni for metallic MEMS

Simple and cost-effective procedures for the direct integration of ferroelectric perovskite oxides into Ni structures are necessary to realize related multifunctional metallic microelectromechanical systems, such as dual-source energy harvesters. This is especially difficult in the case of lead-containing morphotropic phase boundary materials for high piezoelectric response because the two components are thermodynamically incompatible and the formation of NiOx or perovskite oxide reduction takes place depending on the processing conditions. We show here that low-temperature solution processing is an effective means to kinetically limit nickel oxidation, capable of providing BiFeO3–PbTiO3 films on Ni plates at only 500 °C. Bulk-like ferroelectric properties and a distinctive magnetoelectric response were attained. This perovskite system, not explored before on Ni, has a much larger switchable polarization than the widely studied Pb(Zr,Ti)O3, and it is shown here to present an excellent downscaling behavior of ferroelectric properties until the verge of the nanoscale.

Wafer-scale organic-on-III-V monolithic heterogeneous integration for active-matrix micro-LED displays.

The organic thin-film transistor is advantageous for monolithic three-dimensional integration attributed to low temperature and facile solution processing. However, the electrical properties of solution deposited organic semiconductor channels are very sensitive to the substrate surface and processing conditions. An organic-last integration technology is developed for wafer-scale heterogeneous integration of a multi-layer organic material stack from solution onto the non-even substrate surface of a III-V micro light emitting diode plane. A via process is proposed to make the via interconnection after fabrication of the organic thin-film transistor. Low-defect uniform organic semiconductor and dielectric layers can then be formed on top to achieve high-quality interfaces. The resulting organic thin-film transistors exhibit superior performance for driving micro light emitting diode displays, in terms of milliampere driving current, and large ON/OFF current ratio approaching 1010 with excellent uniformity and reliability. Active-matrix micro light emitting diode displays are demonstrated with highest brightness of 150,000 nits and highest resolution of 254 pixels-per-inch.

Low-temperature solution processing route for potassium sodium niobate (KNN) thin films

Low-temperature solution processing route for potassium sodium niobate (KNN) thin films

Enabling low-drift flexible perovskite photodetectors by electrical modulation for wearable health monitoring and weak light imaging

Metal halide perovskites are promising for next-generation flexible photodetectors owing to their low-temperature solution processability, mechanical flexibility, and excellent photoelectric properties. However, the defects and notorious ion migration in polycrystalline metal halide perovskites often lead to high and unstable dark current, thus deteriorating their detection limit and long-term operations. Here, we propose an electrical field modulation strategy to significantly reduce the dark current of metal halide perovskites-based flexible photodetector more than 1000 times (from ~5 nA to ~5 pA). Meanwhile, ion migration in metal halide perovskites is effectively suppressed, and the metal halide perovskites-based flexible photodetector shows a long-term continuous operational stability (~8000 s) with low signal drift (~4.2 × 10−4 pA per second) and ultralow dark current drift (~1.3 × 10−5 pA per second). Benefitting from the electrical modulation strategy, a high signal-to-noise ratio wearable photoplethysmography sensor and an active-matrix photodetector array for weak light imaging are successfully demonstrated. This work offers a universal strategy to improve the performance of metal halide perovskites for wearable flexible photodetector and image sensor applications.

Flexible resistive memory device based on agar

Potential applications of natural materials in environmentally friendly electronics include for information storage. In this work, natural material–agar was used to fabricate a flexible resistive memory device. Agar is one of the most widely used biomaterials for tissue engineering, medicine and other biotechnological applications. Agar is a suitable material for flexible electronics due to its good film formation, biocompatibility, low-temperature solution processability, transparency and flexibility. The flexible agar memory device described in this article exhibits an ON/OFF ratio of 103 under a bending radius of 5 mm, good bending endurance and a stable data retention time of over 104 s. Moreover, the agar could easily use a leaf as a substrate to make a fully biodegradable device. Agar, due to its exceptional flexibility, is emerging as a promising candidate for wearable and skin-compatible electronics, particularly in memory devices.

Amino acid modified Ti3C2Tx as electron transport layers for high-performance organic solar cells

The Ti3C2Tx, as a typical representative of the MXene family, has attracted much attention due to excellent properties in terms of transmittance, conductivity, tunable work function (WF), and low temperature solution processability. However, the work function that needs to be adjusted and a large number of crystal defects in Ti3C2Tx greatly restrict its use in electronic devices. Herein, Ti3C2Tx is modified with amino acids to modulate WF of Ti3C2Tx and passivate its defect sites, thereby improving the electrical properties of Ti3C2Tx and enhancing the performance of organic solar cells (OSCs). Amino acids interact with Ti3C2Tx by charge transfer and forming a surface dipole on the surface, which reduces the WF of Ti3C2Tx. Moreover, -NH2 of amino acids passivates the negatively charged Ti vacancies in Ti3C2Tx, while -COOH of amino acids passivates the positively charged C vacancies, increasing the conductivity of Ti3C2Tx. Consequently, the OSCs with amino acid-modified Ti3C2Tx-based as new electron transport layer (ETL) exhibit more than 18% of power conversion efficiency (PCE) and good device stability. The work shows OSCs with high performance can be fabricated by using environmental-friendly amino acids, which inspires researchers to seek ideas for fabricating efficient devices from natural materials.

Low-temperature processed natural hematite as an electron extraction layer for efficient and stable perovskite solar cells

The mixed halide perovskite solar cells (PSC) have manifested as a contender to market-dominating silicon counterparts owing to their low-temperature solution processing and high efficiency. In PSCs, the electron extraction layer (EEL) plays a vital role as it controls charge transport/extraction from the perovskite absorber layer to the EEL and also determines interfacial recombination and charge accumulation at the EEL/perovskite interface. In this work, high-purity natural hematite (α-Fe2O3) is reported for the first time as the EEL in triple-cation perovskite (CsFAMA) solar cells. To show the cost-effectiveness of this novel EEL, the entire device fabrication process was carried out at a temperature below 150 °C. The optimal α-Fe2O3 EEL shows a mobility value of 9.5 × 10−4 cm−2 V−1 s−1 and trap densities of around 2.40 × 1016 cm−3; the latter is close to state-of-the-art SnO2 EEL (1.26 × 1016 cm−3). The PSCs employing optimal EEL concentration of 10 mg/mL demonstrated a power conversion efficiency (PCE) of 13.3 %, fill factor of 68 %, and VOC of 1.03 V. X-ray diffraction studies show a high crystallinity of natural hematite whereas the photoluminescence studies show a fast carrier extraction from the perovskite to the EEL. The natural α-Fe2O3-based PSCs exhibited superior shelf-life stability of over 30 days due to less charge recombination and smoother CsFAMA thin film deposited over α-Fe2O3 EEL. The low-temperature processed natural α-Fe2O3 EEL can therefore be a promising EEL material for low-cost efficient PSCs.

RETRACTED: Interfacial passivation by Mono-ethanolamine in planar perovskite solar-cell

Aluminium doped zinc oxide (AZO) is a potential replacement for the conventionally used TiO2 as an electron transport layer (ETL) in low-temperature solution processes for hybrid perovskite solar cells (PvSCs). However, the defects and energy-band mismatch at the ETL/perovskite interface accelerate the interfacial carrier's recombination, leading to a decrease in the fill factor (FF) and current density (JSC). To address this issue, a thin interfacial modification layer of monoethanolamine (MEA) has been used which not only reduces the energy barrier between AZO/perovskite for accelerating the charge transfer but also passivates the trap states on the perovskite interface. Due to the synergistic effect of charge extraction promotion and trap density passivation , the champion PvSCs exhibit a higher value of PCE of 8.25 % with a current density (JSC) of 22.59 mA/cm2 and FF of 46.55% compared to the without MEA passivate PvSCs and the device maintains 70% of its topmost PCE after 1000 h under ambient atmosphere, without any encapsulation. This interface engineering based on MEA provides a feasible and excellent strategy to fabricate the PvSCs with improved efficiency and stability of planar device architecture.

Mechanisms behind combustion chemistry in low-temperature solution process of nanomaterials

Combustion synthesis has been widely used for low-temperature growth of metal-oxides nanomaterials. However, the mechanism behind its effectiveness remains unclear, because the current thermal analysis tools require the sample mass beyond 10 mg hardly achieved for nanomaterials. In this study, electrospinning was employed to fabricate massive In2O3 nanowires network films that enables directly study the thermal behaviours of combustion in nanomaterials via thermal analysis tools. We compared the thermal behaviours of electrospun nanomaterials and bulk materials with/without combustion. In the absence of combustion reaction in both, the nanowires shows a lower decomposition temperature than the bulk (500 versus 570 °C). In the presence of combustion reaction in both, it shows even lower decomposition temperature than the bulk (430 versus 500 °C). These indicate that combustion synthesis has two-fold mechanisms: first, it indeed provides internal thermal energy through combustion reaction for crystallizing the nanowires; second, the large surface area of the nanowires promotes the gases transport, which in turn accelerate the decomposition of organics in the nanowires. Based upon both mechanisms, a field effect transistor made of In2O3 nanowires network film was fabricated at 400 °C, whose Ion/Ioff reaches up to 105 with an Ion of 6 × 10−5 A.

PEIE-complexed SnO2 enabled low-temperature solution fabrication of high-performance CsPbI3 all-inorganic perovskite solar cells

CsPbI3 all-inorganic perovskite has attracted much attention due to its improved thermal stability compared with organic-inorganic hybrid counterparts. The shallow conduction band of CsPbI3 leads to an energy band mismatch between it and the widely used electron transport material SnO2, causing a severe loss in the performance. As a result, most high-efficiency inorganic perovskite solar cells are based on high-temperature sintered TiO2 with better energy band alignment. To exploit the potential of SnO2 in inorganic perovskite solar cells, SnO2 nanocrystals were complexed with ethoxylated polyethyleneimine (PEIE). This strategy affords continuous adjustment of SnO2 band structure by varying PEIE content, leading to an ideal energy band alignment with CsPbI3. The introduction of PEIE also promotes the growth of CsPbI3 films, resulting in larger grains. Moreover, improved charge extraction is achieved after PEIE introduction. As a result, the power conversion efficiency of CsPbI3 all-inorganic perovskite solar cells significantly increases from 11.61 % to 13.89 %. Our work provides a simple strategy to produce high-performance CsPbI3 all-inorganic perovskite solar cells with low-temperature solution process.

Recent progress in chalcogenide perovskites: toward low-temperature solution processing

Chalcogenide perovskites, ABX<sub>3</sub> crystal structure with X=S, Se or Te, are emerging light absorber as an alternative to toxic and unstable halide perovskites. Despite their promising properties, the absence of low-temperature solution processing poses a major obstacle for chalcogenide perovskite-based optoelectronic devices. In this review, we survey various synthetic techniques for chalcogenide perovskites, focusing on BaZrS<sub>3</sub> composition. Research progress in non-solution processing methods, such as solid-state reaction and pulsed laser deposition, is briefly covered and recent reports about low-temperature solution processing for chalcogenide perovskite are highlighted. Future research perspective toward realizing chalcogenide-based optoelectronic device based on solution processing is provided. We believe this timely review will facilitate the discovery of novel solution processing for chalcogenide perovskites, the challenging materials with tantalizing prospects.

Polyethylenimine-based bifunctional interfacial layer for efficient quantum dot photovoltaics

Interface engineering, which efficiently optimizes the interfacial carrier collection and recombination, has been proven to be of importance for the emerging colloidal quantum dot solar cells (CQDSCs). Compared with the attractive modification efforts at the interface between quantum dots and the anode/electron transport layer (ETL), the interface between fluorine-doped tin-oxide (FTO) cathodes and the ETL, for which there exists a band alignment mismatch and high trap density in ZnO, has been investigated less. Herein, two kinds of ethylenimine-based polymers, branched by only ethylenimine groups (b-PEI) and by both ethylenimine/ethoxylated groups (e-PEI), respectively, are introduced as bifunctional interfacial layers (BILs) in lead sulfide (PbS) CQDSCs. PEI-based BILs were utilized to modulate the work function of an FTO cathode for optimizing the band alignment at the FTO/ZnO interface and to control the crystallinity of ZnO for reducing its traps. These BILs suppressed the interfacial carrier recombination and achieved a power conversion efficiency (PCE) of 11.28% in CQDSCs, which was much superior to the PCE of the reference device without BIL (10.29%). Also, the branched side chain of PEI-based BILs plays a crucial role in rationally modulating the Schottky barrier to gain different interface-optimization effects. Our work has laid a foundation for the commercial application of CQDSCs due to the advantage of low-temperature solution processability, low-cost, and scalable manufacturing.

Wearable Electronics Based on Stretchable Organic Semiconductors.

Wearable electronics are attracting increasing interest due to the emerging Internet of Things (IoT). Compared to their inorganic counterparts, stretchable organic semiconductors (SOSs) are promising candidates for wearable electronics due to their excellent properties, including light weight, stretchability, dissolubility, compatibility with flexible substrates, easy tuning of electrical properties, low cost, and low temperature solution processability for large-area printing. Considerable efforts have been dedicated to the fabrication of SOS-based wearable electronics and their potential applications in various areas, including chemical sensors, organic light emitting diodes (OLEDs), organic photodiodes (OPDs), and organic photovoltaics (OPVs), have been demonstrated. In this review, some recent advances of SOS-based wearable electronics based on the classification by device functionality and potential applications are presented. In addition, a conclusion and potential challenges for further development of SOS-based wearable electronics are also discussed.