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We report an organic luminescent small molecule, Bis(1-phenylisoquinoline) (acetylacetonate) iridium(III) or Ir(piq)2(acac), that can function as a stable and efficient hole selective contact (HSC) for crystalline silicon (c-Si) solar cells. The devices with the Ir(piq)2(acac) HSC exhibit superior charge transport properties and high stability for up to 30 days in the air without packaging. The photovoltaic characteristics with the solution-processed Ir(piq)2(acac) HSC exhibit little dependence on the blade coating speed and film thickness, demonstrating tolerance to coating and thickness variations. Moreover, the series resistance of the solar cells and the surface work function of the Ir(piq)2(acac) HSCs exhibit analogous correlations to the annealing temperature, suggesting that the fill factor (FF) enhancement originates from an upward energy band bending and a reduced barrier height which facilitates hole transport and collection. The conventional c-Si solar cell incorporating an Ir(piq)2(acac) HSC achieves a 17.8% power conversion efficiency (PCE) with a 78.9% FF, both exceeding the reference counterpart with a 16.9% PCE and 76.8% FF. This work opens up possibilities for exploring a variety of organic luminescent small molecules as efficient hole selective contacts in high-efficiency and low-cost silicon photovoltaics. Graphic Abstract

We report response of photoluminescence (PL) from GaN nanowires without protection in solutions. The distinct response is not only toward pH but toward ionic concentration under same pH. The nanowires appear to be highly stable under aqueous solution with high ionic concentration and low pH value down to 1. We show that the PL has a reversible interaction with various types of acidic and salt solutions. The quantum states of nanowires are exposed to the external environment and have a direct physical interaction which depends on the anions of the acids. As the ionic concentration increases, the PL intensity goes up or down depending on the chemical species. The response results from a competition of change in surface band bending and charge transfer to redox level in solution. That of GaN films is reported for comparison as the effect of surface band bending can be neglected so that there are only slight variations in PL intensity for GaN films. Additionally, such physical interaction does not impact on the PL peaks in acids and salts, whereas there is a red shift on PL when the nanowires are in basic solution, say NH4OH, due to chemical etching occurred on the nanowires. Supplementary Information The online version contains supplementary material available at 10.1186/s11671-021-03473-7.

In this work, insulating hygroscopic polymer thin films have been applied as the vertical-channel sensing layer of extremely sensitive ammonia sensors to detect 5–1000 ppb ammonia. Hygroscopic polymers such as poly(ethylene glycol) (PEG), polyvinyl alcohol (PVA), and poly(acrylic acid) (PAA) serve as good absorbents of ammonia gas. The high-density nano-porous structure covered by a hygroscopic thin film acted like a nano-porous sponge (Nano Sponge) to absorb water, which forms an ionic conduction path in the 300 nm vertical channels. The influences of the polymer functional groups, pH value, relative humidity (RH), and other interfering gas analytes on the sensing performances of the Nano Sponge sensors have also been investigated. After adding glycerol, the Glycerol/PEG Nano Sponge sensors exhibited a stable response. Breath detection in a healthy group and a chronic kidney group of patients was examined using the Glycerol/PEG Nano Sponge sensor in a sensing system with a NaOH tube to control the relative humidity and to absorb the interfering species such as CO and H2S.

Organic semiconductor (OSC) gas sensors have grown into a widely discussed technology because of the presence of nanoscale fabrication processes. Thanks to the rapid development of nanotechnology, ...

The degradation of the polymer solar cells in the outdoor condition was studied. It was found that the devices with normal structure (ITO/PEDOT:PSS/PBDTTT-EFT:PC71BM/ZrOx/Al) show better stability than those with inverted device structure (ITO/ZnO/PBDTTT-EFT:PC71BM/MoO3/Ag). PBDTTT-EFT is poly[4, 8-bis(5-(2-ethylhexyl) thiophen-2-yl) benzo [1,2-b;4,5-b'] dithiophene-2, 6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno [3,4-b]thiophene-) −2-carboxylate-2-6-diyl)] and PC71BM is [6,6]-phenyl-C71-butyric acid methyl ester. The major cause of degradation was attributed to the ultraviolet (UV) part of the sunlight. When the UV filter with cut-off wavelength of 400 nm was used to protect the device, the decay rate was improved by 50 times and the protected device retains 95% of their initial power conversion efficiency after 13 days outside. In the meanwhile, the decay of device fabricated by toluene was found 8 times faster than device by chlorobenzene. Thin 20 nm of Ag capping layer can further improve the stability. Degradation under water immersion indicates that the major effect of Ag is to protect against the residual moisture inside the encapsulation.

In this work, we proposed a rapid and easy check of the drinking water pollution level due to bacteria growth by semiconductor gas sensor. Highly sensitive vertical channel organic ammonia gas sensor was used to detect the gases emitted from the polluted water, and then determined effective ammonia concentration according to its response. Residues from meat of fish, shrimp, and fruits were mashed and added to the clean water. The water samples were stored at 35 °C for natural decay. Initially the bacteria concentration was below 100 colony forming unit per ml (cfu/ml), then it increased to10

In this research work, the gas sensing properties of halogenated chloroaluminum phthalocyanine (ClAlPc) thin films were studied at room temperature. We fabricated an air-stable ClAlPc gas sensor based on a vertical organic diode (VOD) with a porous top electrode by the solution process method. The surface morphology of the solution-processed ClAlPc thin film was examined by field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The proposed ClAlPc-based VOD sensor can detect ammonia (NH3) gas at the ppb level (100~1000 ppb) at room temperature. Additionally, the ClAlPc sensor was highly selective towards NH3 gas compared to other interfering gases (NO2, ACE, NO, H2S, and CO). In addition, the device lifetime was tested by storing the device at ambient conditions. The effect of relative humidity (RH) on the ClAlPc NH3 gas sensor was also explored. The aim of this study is to extend these findings on halogenated phthalocyanine-based materials to practical electronic nose applications in the future.

Dopant-free carrier selective contacts have garnered significant interest for the development of high-efficiency silicon solar cells due to their low-temperature processing and suppression of free-carrier absorption. In this work, we investigate thermally-evaporated molybdenum oxides (MoO x ) functioning as a hole-selective contact (HSC) for silicon heterojunction (Si HJT) solar cells. The high work function of MoO x results in upward energy band bending at the MoO x /n-type Si interface, which facilitates the hole transport and blocks the electron. We compare the optical properties of MoO x with different thickness settings of 3, 10, and 15 nm, where the optical bandgap is fitted via the tauc plot and ranging between 3.46, 3.44, and 3.42 eV, respectively. The absorption spectra of MoO x , MoO x with an intrinsic amorphous silicon (a-Si(i)) layer, and sputtered tin-doped indium oxides (ITO) are also presented. The transmission electron microscopy further reveals a 2.8nm-thick silicon oxides layer in between the MoO x and silicon interface. Preliminary results show that the Si HJT cell with a 15 nm-thick MoO x layer achieves a power conversion efficiency (PCE) of 20.3% and an open-circuit voltage (Voc) of 651.9mV, possibly limited by the pyramidal surface morphology and the a-Si(i) interface passivation layer.

High-efficiency silicon solar cells requires an effective carrier selective contact. Over the past few years, molybdenum trioxide (MoO3) has been widely used as the hole selective contact (HSC) for silicon heterojunction solar cells. The high work function and wide energy gap lead to favorable band bending for hole transport across the MoO3/Si interface. In previous studies, the MoO3 is commonly deposited by thermal evaporation. Here, we propose a simple solution-processed method to form a MoO3 HSC for conventional crystalline silicon (c-Si) solar cells. The crystalline MoO3 nanoparticle solution with a concentration of 2.3-2.7wt% was blade coated on the rear side of an n+/p silicon solar cell without an antireflective coating. By measuring the current-voltage (I-V), contact resistivity (ρ c ), and external quantum efficiency (EQE), we investigate the photoelectric properties of the solar cells incorporating the MoO3 nanoparticle layer. We show that the MoO3 HSC has a low contact resistivity of 29.5 mΩ cm between p-type Si and the silver electrode. The EQE exhibits enahcement on the near-infrared wavelength regime, indicating a field passivation effect. Overall, the n+/p c-Si solar cell incorporating the solution-processed MoO3 HSC exhibits an improved power conversion efficiency (PCE) of 11.8% due to an increased open-circuit voltage (Voc) and fill factor (FF).

The instability of the organic light-emitting diodes (OLEDs) during operation can be attributed to the existence of point defects on the organic layers. In this work, the effect of mixed-host emissive layer and the thermal annealing treatment were investigated to eliminate defects and to boost the device performance. The mixed-host system includes 4,4',4''-tri (9-carbazoyl) triphenylamine (TCTA) and 2,7-bis(diphenylphosphoryl)-9, 9'-spirobi[fluorene] (SPPO13). The mixed-host emissive layer with thermal annealing treatment showed low roughness and few pinholes, and the devices fabricated from this emissive layer exhibited high efficiencies, high stabilities, and long lifetimes. The red and orange-red OLEDs exhibited efficiencies of 13.9 cd/A and 24.35 cd/A, respectively. The longest half-lifetime (L0 =500 cd/m2 ) of the red and orange-red OLEDs were 158 h and 180 h, respectively. Efforts were made to solve problems in large-area coating and to reduce the number of defects on in organic layer. Large-active-area (active area=3 cm×4 cm) red phosphorescent OLEDs (PhOLEDs) devices were realized with very high current efficiency up to 9 cd/A.

Salivary urea was reported to be a useful biomarker to reflect the blood urea nitrogen in chronic kidney disease patients. However, as a new biomarker, enormous clinical trials are required to define the intended-use and to verify the specification. In this report, we demonstrated a low-cost easy-operated real-time sensing system (optical fiber-urea-sensing, OFUS, system) to detect salivary urea. We aim to make the system easily reproduced by the community to stimulate abundant clinical tests worldwide. The OFUS system is composed of a simple three-dimensional printed tank to link with two optical fibers, one connecting with a commercial light-emitting diode to deliver the input light signal, the other connecting with a commercial cadmium sulfide photo-conductive cell to detect the sensing signal. To allow on-site detection without any sample pretreatment, only 1 μl saliva is needed to be mixed with 10 μl urease solution and 90 μl pH indicator solution in the reaction tank and the detection time is only 20 s. A stable and reproducible calibration curve can be easily built with a detection range as 24–300 mg/dL. The OFUS system successfully detected saliva with added synthetic urea and samples from chronic kidney disease patients. A good agreement between the OFUS system and the commercial kit was obtained. A good correlation between salivary urea and the blood urea nitrogen was also confirmed.

In this work, we successfully demonstrated an extremely-low-voltage (0.05 V) gas sensor based on a vertical organic diode with work-function-matched electrodes. With the designed electrodes (MoO3/Al vs. Al), the built-in field in the vertical diode is greatly reduced and hence the diode can be turned on at a very low voltage. By producing nanometer pores in the top electrode, the diode becomes a very sensitive gas sensor to detect ammonia in parts-per-billion (ppb) concentrations. Upon reducing the voltage from 5 V to 0.05 V, this sensor device shows an over 5-times enhancement in the sensitivity. The air-stable sensing film also exhibits a lifetime of 30 days in air. Finally, by connecting with an organic photovoltaic (OPV) device, the gas sensor can be directly powered and switched on by irradiating light on the OPV.

The electrical current leakage and stability are studied for solution-processed OLEDs with areas of 4.45 mm2, 3 × 3.2 cm2, and 6 × 11.5 cm2. The emission layer of the OLED has a ternary or binary mixed host with hole-transporting molecules tris(4-carbazoyl-9-ylphenyl)amine (TCTA) and 9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi), together with the electron-transporting molecule 2,7-bis(diphenylphosphoryl)-9,9′-spirobi[fluorene] (SPPO13). The phosphorescent emitters are Ir(mppy)3 for green and bis[4-(4-tert-butylphenyl)thieno[3,2-c]pyridine][N,N′-diisopropylbenamidinato]iridium(III) (PR-02) for orange. Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyl))diphenylamine)] (TFB) is used as the hole transport layer and PEDOT:PSS is used as the hole injection layer. On top of the emission layer, CsF/Al is deposited by thermal evaporation as the cathode. All organic layers are deposited by blade coating and the initial current leaking defects can be avoided by careful control of the coating conditions. The detrimental burning point caused by a local current short developed after long-time operation can be avoided by reducing the operation voltage using a ternary mixed host. The operation voltage is only 4 V at 100 cd m−2 and 5 V at 250 cd m−2 for the green emitting device. Furthermore, the crystallization defect is reduced by the ternary host. For the orange emitting device, the binary host is good enough with an operating voltage of 5 V at 100 cd m−2. For an area as large as 6 × 11.5 cm2, the OLED shows good stability and there is no burning point after an operation of over 1600 hours.

Although stacking of multiple subcells with complementary bandgaps via an interconnecting layer (ICL) to form multi-junction tandem devices appears to be one of the most promising strategies to increase the power conversion efficiency (PCE) of organic-inorganic hybrid perovskite solar cells (PeSCs), the realization of high efficiency and long-term stable tandem devices based on solution-processed ICL remains highly challenging. In this study, a promising solution-processed ICL consisting of cross-linked p-doped hole transport layer (HTL) and n-doped fullerene electron transport layer for efficient and stable monolithic perovskite tandem solar cells is demonstrated. For the first time, high-performance photo-cross-linkable p-doped HTL material based on poly-[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) is developed by incorporating 1,2-bis[4-(azido-methyl)phenyl]-1,2-diphenylethene (TPE-MN3) and molybdenum tris-[1-(trifluoroethanoyl)-2-(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfdCOCF3)3) as the cross-linking agent and p-type dopant, respectively. This HTL exerts multiple positive impact on the device characteristics, including good solvent resistance, appropriate energy level matching, good electrical conductivity, and high compatibility with different perovskite systems. With these desirable characteristics, the series-connected tandem solar cells afford a PCE up to 18.69%, which represents the highest efficiency reported to date for monolithic all-perovskite tandem cells. Additionally, owing to high conductivity of the interfacial layers, a promising PCE up to 16.20% is attained even when the active area is extended to 1.2 cm2. More encouragingly, the encapsulated tandem cells exhibit remarkable long-term stability, maintaining ≈ 91% of its initial PCE after 9300 h (≈387 days) of air exposure. This work represents an important step forward toward the realization of large-area solution-processed perovskite tandem solar cells with high efficiency and long-term stability.

Additional file 1: Fig. S1–S2. Experimental setup and procedure of PL measurement. Fig. S3–S4. PL spectra from GaN nanowires immersing in salts or acids and after removal of the solutions. Fig. S5–S6. PL spectra from GaN films immersing in acids or salts and after removal of the solutions. Fig. S7. PL spectra from GaN nanowires immersing in HBr, HCl, and HI. Fig. S8. The shift of PL of GaN nanowires in NH4OH.

An optical pH sensor is developed with the coating of second layer to eliminate leaching effect. A great linearity and reproducibility of pH resolution in physiological range is demonstrated.

We propose and demonstrate a received-signal-strength (RSS) based visible light positioning (VLP) system using a low-cost organic photovoltaic cell (OPVC) receiver (Rx). The OPVC is a passive device without the need of external power supply. It could detect VLC signal and harvest energy. Our developed OPVC has a high power conversion efficiency (PCE) of 9.8%. The VLP system can be operated at a low illumination of 130 lux. The regression machine learning (ML) algorithm is used to enhance the positioning accuracy.

High-efficiency solar cells require effective carrier selective contacts which are depicted by their contact resistance and carrier selectivity. Over the past few years, diffusion-free carrier selective contacts have been broadly investigated including a variety of metal oxides, poly (3,4-ethylenedioxythiophene) doped with poly(-styrenesulfonic acid) (PEDOT:PSS), amino acids, alkaline metal salts, etc.. In this work, we investigate a solution-processed, organic small-molecule materials, 2,5,8,11-Tetra-tert-butylperylene (TBPe) to serve as the carrier selective materials. TBPe is fluorescent blue emitters respectively, which have been used as a hole transport layer in organic light emitting diodes. Moreover, the highest occupied molecular (HOMO) level of the material matches to the valance band of Si which may facilitate hole transport in conventional silicon solar cells. The small-molecule material is separately dissolved in chlorobenzene (CB), chloroform (CF) and toluene, followed by blade-coating on to the rear side of a 4 cm2, conventional n+/p silicon solar cell. Without the front anti-reflective coating (ARC) the devices with TBPe achieves a fill factor of 81.6%, and power conversion efficiency (PCE) of 12.6%, respectively. We will present the contact resistance, band alignment, and device efficiency to evaluate the potential of TBPe to serve as the hole selective contact.

Hybrid PEDOT:PSS-silicon heterojunction solar cells have the advantages of low temperature, low cost and solution process. However, as a low-quality native oxide layer can easily form on the surface of silicon substrates, the hybrid solar cells still suffer from charged carrier recombination at the defective interface, hence affecting the device characteristics. In this work, we employ a thermally induced silicon hydrosilylation technique to form a self-assembled monolayer(SAM) on the hydrogen-terminated silicon substrate, in order to prevent the oxidation and passivate the surface of silicon. The quality of this monolayer passivation is characterized by the contact angle, X-ray photoelectron spectroscopy (XPS) analyses. We have found that the SAM passivation can effectively reduce the contact resistance between silicon and the aluminum electrode, leading to an improved open-circuit voltage and fill-factor. However, as the silicon surface with the SAM passivation becomes hydrophobic, it is difficult to apply aqueous PEDOT:PSS solution onto silicon. Therefore, we further apply a low-damage oxygen plasma treatment to modify the terminal functional group of the monolayer to form a hydrophilic surface. The power conversion efficiency of the modified devices is between 10–12%. Although the PCE does not surpass the reference device due to possible chemical reactions between the PEDOT:PSS and SAM, the proposed low- damage oxygen-plasma treatment provide a viable solution for modifying the functional passivation of hybrid PEDOT:PSS silicon solar cells using SAMs.

Recently, hybrid organic silicon heterojunction solar cells have attracted significant interests due to good device performance and simple solution processes. However, electron transport across the cathode interface remains one of the critical issues to be solved. In this study, we employ aluminum-doped zinc oxide (AZO) nanoparticle dispersion to form an electron transport interlayer between the n-type silicon (n-Si) and rear-side aluminum (Al) electrode via blade coating. We further investigate the correlation between the surface morphology and the device characteristics for various furnace annealing conditions: 200°C, 300°C, and 350°C. The hybrid solar cell with the AZO interlayer annealed at 300°C exhibits the highest power conversion efficiency (PCE) of 11.8% with a fill-factor (FF) of 71.3%, where the film morphology shows small and relatively smooth grains revealed by the atomic force microscopy. As a comparison, the reference counterpart without the AZO interlayer exhibits a PCE of 8.5% with a FF of 61.7%. The preliminary results demonstrate the potential of inorganic nanoparticle solution processes for forming a uniform and homogeneous interlayer. Further work on the contact resistance and carrier selectivity of the AZO thin film is still in progress and will be presented.

A fine cylindrical chemical sensor tip is developed with optical fiber in the core, surrounded by a transparent cylinder of photopolymer Norland Optical Adhesive 61 (NOA 61), and covered by a polymer hydrogel mixed with sensing molecules. The overall diameter is as small as 1.5 mm. pH response is demonstrated using two approaches of sensing materials: (i) absorbing probe Phenol Red mixed with Rhodamine 6G fluorescent dye and (ii) 8-hydroxypyrene-1,3,6-trisulfonic acid fluorescent probe. Both the optical excitation and fluorescence signal collection are through the optical fibers. A time resolution of 10 s is achieved for pH variations. Good linearity is observed in the physiological range from pH 7.0 to pH 8.6 with reversible and reproducible outcomes. For in vitro urea measurement, the sensor tip can distinguish 1, 3, and 5 mM urea solution, which is a crucial range in saliva urea concentration. The miniaturized tip with such simple cylindrical symmetry is designed to detect vital signs during minimally invasive surgeries and can be potentially accompanied with endoscopes to enter human bodies.

A metal-oxide material (indium zinc oxide [IZO]) device with near-infrared (NIR) laser annealing was demonstrated on both glass and bendable plastic substrates (polycarbonate, polyethylene, and polyethylene terephthalate). After only 60 s, the sheet resistance of IZO films annealed with a laser was comparable to that of thermal-annealed devices at temperatures in the range of 200-300 °C (1 h). XPS, ATR, and AFM were used to investigate the changes in the sheet resistance and correlate them to the composition and morphology of the thin film. Finally, the NIR-laser-annealed IZO films were demonstrated to be capable of detecting changes in humidity and serving as a highly sensitive gas sensor of hydrogen sulfide (in ppb concentration), with room-temperature operation on a bendable substrate.

A metal-oxide material (indium zinc oxide [IZO]) device with near-infrared (NIR) laser annealing was demonstrated on both glass and bendable plastic substrates (polycarbonate, polyethylene, and polyethylene terephthalate). After only 60 s, the sheet resistance of IZO films annealed with a laser was comparable to that of thermal-annealed devices at temperatures in the range of 200-300 °C (1 h). XPS, ATR, and AFM were used to investigate the changes in the sheet resistance and correlate them to the composition and morphology of the thin film. Finally, the NIR-laser-annealed IZO films were demonstrated to be capable of detecting changes in humidity and serving as a highly sensitive gas sensor of hydrogen sulfide (in ppb concentration), with room-temperature operation on a bendable substrate.