Inorganic and Organic Solution-Processed Thin Film Devices

Thin films and thin film devices have a ubiquitous presence in numerous conventional and emerging technologies. This is because of the recent advances in nanotechnology, the development of functional and smart materials,conducting polymers, molecular semiconductors, carbon nanotubes, and graphene, and the employment of unique properties of thin films and ultrathin films, such as high surface area, controlled nanostructure for effective charge transfer, and special physical and chemical properties, to develop new thin film devices. This paper is therefore intended to provide a concise critical review and research directions on most thin film devices, including thin film transistors, data storage memory, solar cells, organic light-emitting diodes, thermoelectric devices, smart materials, sensors, and actuators. The thin film devices may consist of organic, inorganic, and composite thin layers, and share similar functionality, properties, and fabrication routes. Therefore, due to the multidisciplinary nature of thin film devices, knowledge and advances already made in one area may be applicable to other similar areas. Owing to the importance of developing low-cost, scalable, and vacuum-free fabrication routes, this paper focuses on thin film devices that may be processed and deposited from solution.

Construction of a cement-rebar nanoarchitecture for a solution-processed and flexible film of a Bi_(2)Te_(3)/CNT hybrid toward low thermal conductivity and high thermoelectric performance

Solution processability and flexibility still remain major challenges for many thermoelectric(TE)materials,including bismuth telluride(Bi_(2)Te_(3)),a typical and commercially available TE material.Here,we report a new solutionprocessed method to prepare a flexible film of a Bi_(2)Te_(3)/single-walled carbon nanotube(SWCNT)hybrid,where the dissolved Bi_(2)Te_(3) ion precursors are mixed with dispersed SWCNTs in solution and recrystallized on the SWCNT surfaces to form a“cement-rebar”-like architecture.The hybrid film shows an n-type characteristic,with a stable Seebeck coefficient of^(−1)00.00±1.69μVK^(−1) in air.Furthermore,an extremely low in-plane thermal conductivity of∼0.33Wm^(−1) K^(−1) is achieved at 300 K,and the figure of merit(ZT)reaches 0.47±0.02.In addition,the TE performance is independent of mechanical bending.The unique“cement-rebar”-like architecture is believed to be responsible for the excellent TE performances and the high flexibility.The results provide a new avenue for the fabrication of solution-processable and flexible TE hybrid films and will speed up the applications of flexible electronics and energy conversion.

Recent Progress in All-Solution-Processed Organic Solar Cells

All-solution-processed organic solar cells(OSCs)(from the bottom electrode to the top electrode)are highly attractive thanks to their low cost,lightweight and high-throughput production.However,achieving highly efficient all-solution-processed OSCs remains a significant challenge.One of the key issues is the lack of high-quality solution-processed electrode systems that can replace indium tin oxide(ITO)and vacuum-deposited metal electrodes.In this paper,we comprehensively review recent advances in all-solution-processed osCs,and classified the devices as the top electrode materials,including silver nanowires(AgNWs),conducting polymers and composite conducting materials.The correlation between electrode materials,properties of electrodes,and device performance in all-solution-processed OSCs is elucidated.In addition,the critical roles of the active layer and interface layer are also discussed.Finally,the prospects and challenges of all-solution-processed OSCs are presented.

Non-conjugated Polynorbornene Hosts with High Triplet Energy Levels for Solution-processed Narrowband Blue OLEDs

Three polymer hosts(namely PNB-tBuCz,PNB-Ac,PNB-TAc)containing non-conjugated polynorbornene(PNB)backbone and hole-transporting arylamine segments(carbazole,acridan and dendritic teracridan)in side chains are developed for solution-processed narrowband blue organic light-emitting diodes(OLEDs).It is found that the non-conjugated polynorbornenes can keep high triplet energy(ET)levels in range of 3.12-3.20 eV by interrupting the conjugation of repeating units,making them capable as host materials for blue emitters.Meanwhile,by increasing the electron-donating capability of side chain arylamine from carbazole to acridan and dendritic teracridan,the highest occupied molecular orbital(HOMO)levels for the polymer hosts are elevated from-5.50 eV to-5.11 eV,beneficial for reducing the hole injection barrier from anode to emissive layer.As a result,solution-processed OLEDs employing polynorbornenes with dendritic teracridan side chain(PNB-TAc)as host and boron,selenium,nitrogen-containing multiple resonance thermally activated delayed fluorescence emitter as dopant reveal efficient narrowband blue electroluminescence with emission peak at 474 nm,full-width at half maximum of 30 nm,together with maximum external quantum efficiency of 20.2%,representing the state-of-the-art device efficiency for solution-processed OLEDs with narrowband blue emission.

Constructing an efficient deep-blue TADF emitter by host-guest interactions towards solution-processed OLEDs with narrowband emission

High-efficiency thermally activated delayed fluorescence(TADF) emitters and corresponding well-designed solution-processed organic light emitting diodes(OLEDs) are presently attractive due to their potential for exploiting large-area flexible displays. In this context, we innovatively integrate 2,12-di-tert-butyl-5,9-dioxa-13b-boronaphtho [3,2,1] anthracene as the acceptor with 3,6-bis(3,6-di-tert-butylcarbazol-N-yl) carbazole as the donor to construct a rigid deep-blue emitter, TB-3t BuCz, which exhibits a narrow emission with full-width-at-half-maximum(FWHM) of 46 nm. TB-3t BuCz itself dispalys no TADF characteristics both in solution or in pure film states. However, the significant TADF behavior can be observed when TB-3t BuCz is doped with 2,6-DCzPPy host due to the formation of exciplex-like species in 2,6-DCzPPy/TB-3t BuCz system. It is also found that the discernible spin-flip of triplet excitons is feasible when the S1/T1states of the formed exciplex stay slightly lower than S1 and T1states of TB-3t BuCz for the other host/TB-3t BuCz systems. Eventually, thanks to the synergetic effect of rigid structure and favorable photophysical properties of TB-3t BuCz, the solution-processed OLEDs based on 2,6-DCzPPy/TB-3t BuCz as emitting layer has achieved the significantly improved external quantum efficiency(EQE) of 14.6% with suppressed efficiency roll-off.The CIE1931 coordinate of(0.158, 0.052) is typically in deep-blue region. Note that, this EQE value is among the highest echelon of solution-processed OLEDs with deep-blue emission by utilizing boron-containing TADF emitters.

Phosphonium-Based Ionic Thermally Activated Delayed Fluorescence Emitters for High-Performance Partially Solution-Processed Organic Light-Emitting Diodes

Ionic thermally activated delayed fluorescence(TADF)emitters are rarely investigated due to their poor photoluminescence and electroluminescence performance.Herein,highly efficient ionic TADF emitters with charged donor–acceptor(D–A^(+))and D–A^(+)–D architectures are designed,innovatively based on the phosphonium cation electron acceptor.The symmetric D–A^(+)–D compound in doped film exhibits a high photoluminescence quantum yield of 0.91 and a short emission lifetime of 1.43 microseconds.Partially solution-processed organic lightemitting diodes based on these ionic TADF emitters achieve a maximum external quantum efficiency(EQE)of 18.3%and a peak luminance of 14,532 candelas per square meter(cd/m^(2))and show a small efficiency roll-off of 7.1%(EQE=17%)at a practical high luminance of 1000 cd/m^(2).These results demonstrate the high potential of phosphonium cations as promising electron acceptors to construct TADF emitters for high-performance electroluminescence devices.The current study opens up an appealing way for future exploitation of high-efficiency ionic TADF materials.

Solution-processed multi-resonance organic light-emitting diodes with high efficiency and narrowband emission

With excellent color purity(full-width half maximum(FWHM)<40 nm)and high quantum yield,multiresonance(MR)molecules can harvest both singlet and triplet excitons for highly efficient narrowband organic light-emitting diodes(OLEDs)owing to their thermally activated delayed fluorescence(TADF)nature.However,the highly rigid molecular skeleton with the oppositely positioned bo ron and nitrogen in generating MR effects results in the intrinsic difficulties in the solution-processing of MR-OLEDs.Here,we demonstrate a facile strategy to increase the solubility,enhance the efficiencies and modulate emission color of MR-TADF molecules by extending aromatic rings and introducing tert-butyls into the MR backbone.Two MR-TADF emitters with smaller singlet-triplet splitting energies(ΔE~(ST))and larger oscillator strengths were prepared conveniently,and the solution-processed MR-OLEDs were fabricated for the first time,exhibiting efficient bluish-green electroluminescence with narrow FWHM of 32 nm and external quantum efficiency of 16.3%,which are even comparable to the state-of-the-art performances of the vacuum-evaporated devices.These results prove the feasibility of designing efficient solutionprocessible MR molecules,offering important clues in developing high-performance solution-processed MR-OLEDs with high efficiency and color purity.

Solution-processed quantum dot SnO_(2) as an interfacial electron transporter for stable fully-air-fabricated metal-free perovskite solar cells

Perovskite solar cells could strongly compete with the silicon solar cells in the market soon as illustrated in recent studies.In this work,promising and stable metal-free perovskite solar cells(PSCs)has been successfully fabricated using an inorganic SnO_(2)/Quantum dot SnO_(2)(QD-SnO_(2))double layer as an efficient electron transport layer via a low-temperature solution process.The fully-air fabricated PSCs in the form of FTO/SnO_(2)/QD-SnO_(2)/CH3NH3PbI3/Carbon were tested at different annealed QD-SnO_(2) between 300 and 500℃.The as-prepared QD-SnO_(2) and the fabricated devices are characterized by various techniques,including XRD,XPS,HR-TEM,FE-SEM,UVeviseNIR spectroscopy,PL,and solar simulator.The prepared QD-SnO_(2) at 300℃ has shown well-ordered nanoparticles of 5.6 nm in diameter with superior carrier density(1.5×10^(15) cm^(-3))and highest carrier mobility(64.1 cm^(2)·V^(-1)·s^(-1)),accelerating the carriers separation process within the cell.The best devices demonstrated a maximum power conversion effi-ciency(PCE)of 11.7%,VOC 0.81 V,JSC 19.5 mA·cm^(-2),and FF 74%.The presence of an interfacial layer of QDSnO_(2) over the blocking SnO_(2) upsurges the band gaps alignment and accelerates the carriers extraction rate affecting the performance of the fabricated perovskite devices.Moreover,the optimized fabricated devices revealed a shelf stability-life of four months in humid air(40%-50%)with>83%of its initial PCE.This simple synthetic approach can develop the opportunities to transfer the cell from the lab to the market,which will be compatible with large-scale production.

Resonance hosts for high efficiency solution-processed blue and white electrophosphorescent devices

The exploration of high-performance solution-processible host materials for blue and white electrophosphorescent devices is a key and fundamental challenge in the ongoing development of organic semiconductors.Herein,two solution-processible resonance host materials with self-adaptive characteristics are delicately designed and constructed.Because of the dynamic tautomerization upon resonance variation,these smart hosts show self-adaptive and selectively enhanced charge carrier flux at high triplet energy levels.Conferred by the resonance molecules,solution-processed blue and white devices exhibit excellent maximum current efficiencies(CEs)of 29.8 and 57.3 cd A−1,and external quantum efficiencies(EQEs)up to 14.5%and 23.5%,respectively.Our works highlight the significant progress of the solution-processed phosphorescent organic light-emitting diodes(PhOLEDs)using resonance host molecules,potentially furnishing a leap forward in constructing advanced organic semiconductors for next-generation optoelectronic devices.

Solution-processed CuOx as an efficient hole-extraction layer for inverted planar heterojunction perovskite solar cells

A solution-processed CuO_x film has been successfully integrated as the hole-transporting layer(HTL) for inverted planar heterojunction perovskite solar cells(PVSCs). The CuO_x layer is fabricated by simply spin-coating a copper acetylacetonate(Cu(acac)_2) chloroform solution onto ITO glass with high transparency in the visible range. The compact and pinhole-free perovskite film with large grain domains is grown on the CuO_x film. The inverted PVSCs with the structure of ITO/CuO_x/MAPbI_3/PC_(61)BM/ZnO/Al are fabricated and show a best PCE of 17.43% under standard AM 1.5G simulated solar irradiation with a VOCof 1.03 V, aJ_(SC) of 22.42 mA cm^(-2), and a fill factor of 0.76, which is significantly higher and more stable than that fabricated from the often used hole-transporting material PEDOT:PSS(11.98%) under the same experimental conditions. The enhanced performance is attributed to the efficient hole extraction through the CuO_x layer as well as the high-quality CH_3NH_3PbI_3 films grown on the CuO_x. Our results indicate that low-cost and solution-processed CuO_x film is a promising HTL for high performance PVSCs with better stability.

A solution-processed nanoscale COF-like material towards optoelectronic applications

Two-dimensional(2 D)covalent organic frameworks(COFs)with periodic functionalπ-electron systems are an emerging class of optoelectronic materials.However,almost all conjugated COFs so far are insoluble and hard to process,which hampers severely their optoelectronic applications.Here,a solution-processable,nanoscale and sp2 carbon-conjugated COF-like material,PDPP-C20 was successfully designed and synthesized.The solution-processed PDPP-C20 films exhibit high crystallinity and excellent charge transport properties along out-of-plane directions,combined with the highest occupied molecular orbital(HOMO)/lowest unoccupied molecular orbital(LUMO)levels of-5.36/-3.75 e V,making PDPP-C20 suitable for electronic device applications.An efficiency as high as 21.92%has been demonstrated when it was used as a functional interfacial layer in perovskite solar cells,coupled with dramatically improved stability in comparison with the control device due to the superior hydrophobicity of PDPP-C20 layer as well as its passivation effect on perovskite surface.Furthermore,the soluble PDPP-C20 could also be used as donor in bulk-heterojunction organic solar cells and an initial efficiency of 2.46%has been achieved.These results indicate that this new class of soluble and nanoscale COF-like materials should offer a new arena of functional materials for optoelectronic devices.

Flexible and stretchable photodetectors and gas sensors for wearable healthcare based on solution-processable metal chalcogenides

Wearable smart sensors are considered to be the new generation of personal portable devices for health monitoring.By attaching to the skin surface,these sensors are closely related to body signals(such as heart rate,blood oxygen saturation,breath markers,etc.)and ambient signals(such as ultraviolet radiation,inflammable and explosive,toxic and harmful gases),thus providing new opportunities for human activity monitoring and personal telemedicine care.Here we focus on photodetectors and gas sensors built from metal chalcogenide,which have made great progress in recent years.Firstly,we present an overview of healthcare applications based on photodetectors and gas sensors,and discuss the requirement associated with these applications in detail.We then discuss advantages and properties of solution-processable metal chalcogenides,followed by some recent achievements in health monitoring with photodetectors and gas sensors based on metal chalcogenides.Last we present further research directions and challenges to develop an integrated wearable platform for monitoring human activity and personal healthcare.

Rational molecular design of efficient yellow-red dendrimer TADF for solution-processed OLEDs: a combined effect of substitution position and strength of the donors

The development of high-performance solution-processed red organic light-emitting diodes(OLEDs) remains a challenge,particularly in terms of maintaining efficiency at high luminance. Here, we designed and synthesized four novel orange-red thermally activated delayed fluorescence(TADF) dendrimers that are solution-processable: 2GCz BP, 2DPACz BP, 2FBP2GCz and 2FBP2DPACz. We systematically investigated the effect of substitution position and strength of donors on the optoelectronic properties. The reverse intersystem crossing rate constant(kRISC) of the emitters having donors substituted at positions 11and 12 of the dibenzo[a,c]phenazine(BP) is more than 10-times faster than that of compounds substituted having donors substituted at positions 3 and 6. Compound 2DPACz BP, containing stronger donors than 2GCz BP, exhibits a red-shifted emission and smaller singlet-triplet energy splitting, ΔE_(ST), of 0.01 e V. The solution-processed OLED with 10 wt% 2DPACz BP doped in m CP emitted at 640 nm and showed a maximum external quantum efficiency(EQE_(max)) of 7.8%, which was effectively maintained out to a luminance of 1,000 cd m-2. Such a device's performance at relevant display luminance is among the highest for solution-processed red TADF OLEDs. The efficiency of the devices was improved significantly by using 4Cz IPN as an assistant dopant in a hyperfluorescence(HF) configuration, where the 2DPACz BP HF device shows an EQEmaxof 20.0% at λEL of 605 nm and remains high at 11.8% at a luminance of 1,000 cd m-2, which makes this device one of the highest efficiency orange-to-red HF SP-OLEDs to date.

Efficient and stable planar all-inorganic perovskite solar cells based on high-quality CsPbBr3 films with controllable morphology

All-inorganic cesium lead bromide(CsPbBr3)perovskite is attracting growing interest as functional materials in photovoltaics and other optoelectronic devices due to its superb stability.However,the fabrication of high-quality CsPbBr3 films still remains a big challenge by solution-process because of the low solubility of the cesium precursor in common solvents.Herein,we report a facile solution-processed approach to prepare high-quality CsPbBr3 perovskite films via a two-step spin-coating method,in which the Cs Br methanol/H2 O mixed solvent solution is spin-coated onto the lead bromide films,followed by an isopropanol-assisted post-treatment to regulate the crystallization process and to control the film morphology.In this fashion,dense and uniform CsPbBr3 films are obtained consisting of large crystalline domains with sizes up to microns and low defect density.The effectiveness of the resulting CsPbBr3 films is further examined in perovskite solar cells(PSCs)with a simplified planar architecture of fluorine–doped tin oxide/compact Ti O2/CsPbBr3/carbon,which deliver a maximum power conversion efficiency of 8.11%together with excellent thermal and humidity stability.The present work offers a simple and effective strategy in fabrication of high-quality CsPbBr3 films for efficient and stable PSCs as well as other optoelectronic devices.

Solution‑Processed Transparent Conducting Electrodes for Flexible Organic Solar Cells with 16.61% Efficiency

Nonfullerene organic solar cells(OSCs)have achieved breakthrough with pushing the efficiency exceeding 17%.While this shed light on OSC commercialization,high-performance flexible OSCs should be pursued through solution manufacturing.Herein,we report a solution-processed flexible OSC based on a transparent conducting PEDOT:PSS anode doped with trifluoromethanesulfonic acid(CF3SO3H).Through a low-concentration and low-temperature CF3SO3H doping,the conducting polymer anodes exhibited a main sheet resistance of 35Ωsq−1(minimum value:32Ωsq−1),a raised work function(≈5.0 eV),a superior wettability,and a high electrical stability.The high work function minimized the energy level mismatch among the anodes,hole-transporting layers and electron-donors of the active layers,thereby leading to an enhanced carrier extraction.The solution-processed flexible OSCs yielded a record-high efficiency of 16.41%(maximum value:16.61%).Besides,the flexible OSCs afforded the 1000 cyclic bending tests at the radius of 1.5 mm and the long-time thermal treatments at 85°C,demonstrating a high flexibility and a good thermal stability.

PEROVSKITE SEMICONDUCTOR OPTOELECTRONIC DEVICES Rational molecular passivation for high-performance perovskite light-emitting diodes

Solution-processed metal halide perovskites (MHPs) have received significant interest for cost-effective, high-performance optoelectronic devices. In addition to the great successes in photovoltaics, their excellent luminescence and charge transport properties also make them promising for light emitting diodes (LEDs). To achieve high-efficiency perovskite LEDs (PeLEDs), extensive efforts have been carried out to enhance radiative recombination rates by confining the electrons and holes. In addition to enhancing radiative recombination rates, it is equally important to decrease the non-radiative recombination for improving the device performance. Passivation of the defects could be an efficient way for reducing the non-radiative recombination.

High-performance inverters based on ambipolar organic-inorganic heterojunction thin-film transistors

This work reports on the integration of organic and inorganic semiconductors as heterojunction active layers for high-performance ambipolar transistors and complementary metal-oxide-semiconductor(CMOS)-like inverters.Pentacene is employed as a p-type organic semiconductor for its stable electrical performance,while the solution-processed scandium(Sc)substituted indium oxide(ScInO)is employed as an n-type inorganic semiconductor.It is observed that by regulating the doping concentration of Sc,the electrical performance of the n-type semiconductor could be well controlled to obtain a balance with the electrical performance of the p-type semiconductor,which is vital for achieving high-performance inverters.When the doping concentration of Sc is 10 at.%,the CMOS-like logic inverters exhibit a voltage gain larger than 80 and a wide noise margin(53%of the theoretical value).The inverters also respond well to the input signal with frequency up to 500 Hz.

Progress of Pb-Sn Mixed Perovskites for Photovoltaics: A Review

Pb-Sn mixed perovskites are becoming increasingly popular as narrowbandgap(1.2–1.3 eV)light absorbers in single-junction perovskite solar cells(PSCs)and as bottom cells for all-perovskite tandem solar cells,for highefficiency,low-cost,lightweight,roll-to-roll printable photovoltaic(PV)applications.From the first report of planar Pb:Sn mixed PSCs in 2014,the power conversion efficiencies(PCE)have increased from 10%to 21%by the end of 2020 with an exponential growth in research conducted in this field.Despite much effort,the performance and stability of Pb-Sn mixed PSCs are still limited,which constrains their long-term use in all-perovskite tandem devices.This review highlights the avenues explored in improving different aspects of Pb-Sn mixed PSCs and provides a comprehensive discussion of the interdependent factors affecting the device performance.This includes compositional engineering of the perovskite crystal,absorber layer fabrication and crystallization methods,bandgap tuning,Sn4+reduction,and surface passivation of the absorber layer,as well as the selection of interlayers and electrodes of the final PSC.

New flexible CsPbBr_(3)-based scintillator for X-ray tomography

The evolution of lead halide perovskites used for X-ray imaging scintillators has been facilitated by the development of solution-processable semiconductors characterized by large-area,flexible,fast photoresponse.The stability and durability of these new perovskites are insufficient to achieve extended computed tomography scanning times with hard X-rays.In this study,we fabricated a self-assembled CsPbBr_(3)-based scintillator film with a flexible large-area uniform thickness using a new roomtemperature solution-processable method.The sensitivity and responsivity of X-ray photon conversion were quantitatively measured and showed a good linear response relationship suitable for X-ray imaging.We also demonstrated,for the first time,that the self-assembled CsPbBr_(3)-based scintillator has good stability for hard X-ray microtomography.Therefore,such an inexpensive solution-processed semiconductor easily prepared at room temperature can be used as a hard X-ray scintillator and equipped with flexible CsPbBr3-based X-ray detectors.It has great potential in three-dimensional high-resolution phase-contrast X-ray-imaging applications in biomedicine and material science because of its heavy Pb and Br atoms.

Efficient perovskite solar cells employing a solution-processable copper phthalocyanine as a hole-transporting material

The development of alternative low-cost and high-performing hole-transporting materials(HTMs) is of great significance for the potential large-scale application of perovskite solar cells(PSCs) in the future.Here,a facilely synthesized solution-processable copper tetra-(2,4-dimethyl-3-pentoxy) phthalocyanine(CuPc-DMP) via only two simple steps,has been incorporated as a hole-transporting material(HTM) in mesoscopic perovskite solar cells(PSCs).The optimized devices based on such a HTM afford a very competitive power conversion efficiency(PCE) of up to 17.1%measured at 100 mW cm^(-2) AM 1.5G irradiation,which is on par with that of the well-known 2,2',7,7'-tetrakis(N'N'-di-p-methoxyphenylamine)-9,9'-spirobifluorene(spiro-OMeTAD)(16.7%) under equivalent conditions.This is,to the best of our knowledge,the highest value reported so far for metal organic complex-based HTMs in PSCs.The advantages of this HTM observed,such as facile synthetic procedure,superior hole transport characteristic,high photovoltaic performance together with the feasibility of tailoring the molecular structure would make solution-processable copper phthalocyanines as a class of promising HTM that can be further explored in PSCs.The present finding highlights the potential application of solution processed metal organic complexes as HTMs for cost-effective and high-performing PSCs.