Room temperature processed protective layer for printed silver electrodes.

Low-temperature processed printed silver electrodes pave the way for electrical connections in flexible substrates with reduced energy consumption. Despite their excellent performance and simple process, printed silver electrodes' poor stability limits their applications. This study demonstrates a transparent protective layer without thermal annealing for printed silver electrodes, which maintains its electrical properties for a long period of time. A fluoropolymer, specifically a cyclic transparent optical polymer (CYTOP), was used as a protective layer for silver. The CYTOP is room temperature processable and chemically stable against carboxyl acid. The introduction of the CYTOP film on the printed silver electrodes mitigates the chemical reaction between silver and carboxyl acid, thereby elongating its lifetime. Under heated acetic acid, the printed silver electrodes with a CYTOP protective layer maintained their initial resistance for up to 300 hours, while the electrodes without a protective layer were damaged within a few hours. A microscopic image shows that the protective layer enables printed electrodes to maintain their shape without damage. Hence, the protective layer guarantees the accurate and reliable performance of electronic devices with printed electrodes under actual operating conditions. This research will contribute to designing chemically reliable flexible devices in the near future.

Coherent heteroepitaxial growth of I-III-VI2 Ag(In,Ga)S2 colloidal nanocrystals with near-unity quantum yield for use in luminescent solar concentrators.

Colloidal Ag(In,Ga)S2 nanocrystals (AIGS NCs) with the band gap tunability by their size and composition within visible range have garnered surging interest. High absorption cross-section and narrow emission linewidth of AIGS NCs make them ideally suited to address the challenges of Cd-free NCs in wide-ranging photonic applications. However, AIGS NCs have shown relatively underwhelming photoluminescence quantum yield (PL QY) to date, primarily because coherent heteroepitaxy has not been realized. Here, we report the heteroepitaxy for AIGS-AgGaS2 (AIGS-AGS) core-shell NCs bearing near-unity PL QYs in almost full visible range (460 to 620 nm) and enhanced photochemical stability. Key to the successful growth of AIGS-AGS NCs is the use of the Ag-S-Ga(OA)2 complex, which complements the reactivities among cations for both homogeneous AIGS cores in various compositions and uniform AGS shell growth. The heteroepitaxy between AIGS and AGS results in the Type I heterojunction that effectively confines charge carriers within the emissive core without optically active interfacial defects. AIGS-AGS NCs show higher extinction coefficient and narrower spectral linewidth compared to state-of-the-art heavy metal-free NCs, prompting their immediate use in practicable applications including displays and luminescent solar concentrators (LSCs).

Perovskite solar cells integrated with blue cut-off filters for mitigating light-induced degradation.

The stability of methylammonium (MA)-based perovskite solar cells (PSCs) remains one of the most urgent issues that need to be addressed. Inherent weak binding forces between MAs and halides cause the perovskite structure to become unstable under exposure to various external environmental factors such as moisture, oxygen, ultraviolet radiation, and heat. In particular, the degradation of perovskite films under light exposure accelerates the deterioration of the device, mainly due to the migration of halide ions. In this study, we investigated the effect of light energy on the degradation of inverted PSCs by introducing red ( = 610-800 nm), green (500-590 nm), and blue (300-500 nm) light-pass filters. After 30 h, the inverted PSCs of blue-light-induced devices retained a power conversion efficiency (PCE) of 70%, while those of the green and red light-induced devices retained PCEs of 85% and 90%, respectively. Direct evidence of light-induced degradation was obtained by investigating morphological changes in the perovskite films and the amount of ion accumulation on the Ag electrode. This evidence highlights the varying effect of light with different energies on device degradation. Furthermore, to minimize light-induced device degradation, we designed two types of blue cut-off filters that can selectively block light ranging from = 400 to 500 nm, comprising a multilayered inorganic metasurface. An optical simulation was used to optimize the performance of the designed filters. By investigating the changes in the photovoltaic parameters and the amount of ion accumulation on the Ag electrode, we confirmed that integrating blue cut-off filters into PSCs greatly improved the operational lifetime of the devices.

External validation and comparison of the Pediatric Emergency Care Applied Research Network and Canadian Assessment of Tomography for Childhood Head Injury 2 clinical decision rules in children with minor blunt head trauma.

OBJECTIVE: Among the pediatric population with minor head trauma, it is difficult to determine an indication for the usage of brain computerized tomography (CT). Our study aims to compare the efficiency of the most commonly used clinical decision rules: the Pediatric Emergency Care Applied Research Network (PECARN) and Canadian Assessment of Tomography for Childhood Head Injury 2 (CATCH2). METHODS: This retrospective study investigated whether the PECARN and CATCH2 rules were applicable to Korean children with minor head trauma for reducing the use of brain CT imaging, while detecting intracranial pathology. RESULTS: Overall, 251 patients (0-5 years old) admitted to emergency rooms within 24 hours of injury were included between August 2015 to August 2018. The performance results are as follows: the PECARN and CATCH2 rules had a sensitivity of 80.00% (51.91%-95.67%) and 100% (78.20%-100.00%) with a specificity of 28.39% (22.73%-34.60%) and 15.25% (10.92%-20.49%), respectively; the negative predictive values were 98.58% and 100%, respectively. Overall, the CATCH2 rule was more successful than the PECARN rule in detecting intracranial pathology; however, there was no significant difference between them. Furthermore, the PECARN and CATCH2 rules lowered the rate of head CT imaging in our study group. CONCLUSION: Both the rules significantly lowered the rate of indicated brain CT. However, since the CATCH2 rule had higher sensitivity and negative predictive value than the PECARN rule, it is more appropriate to be used in emergency rooms for detecting intracranial pathology in children with minor head trauma.

Study on graphene oxide as a hole extraction layer for stable organic solar cells.

The development of an efficient and stable hole extraction layer (HEL) is crucial for commercializing organic solar cells (OSCs). Although a few candidates have been widely utilized as HELs for OSCs, the most appropriate material has been lacking. A few articles have recently reported graphene oxide (GO) as a well-working HEL that offers comparable performance to conventional HELs. However, a systematic study providing comprehensive insight into the GO-based OSC behavior is lacking. This article discusses broad topics, including the material properties, device efficiency, shelf lifetime, and impedance properties. We found that GO offers excellent properties, which are identical to those of conventional HELs, while the shelf lifetime shows a significant 6-fold increase. Furthermore, we discuss the significantly reduced space-charge limited region of an aged GO-based OSC compared with a PEDOT:PSS-based device, which is revealed to be a reason for the different shelf lifetime. We believe that the results will accelerate the development of GO as an HEL for OSCs and other optoelectronic devices.

Effects of cholesterol levels on outcomes of out-of-hospital cardiac arrest: a cross-sectional study.

OBJECTIVE: High cholesterol level is a risk factor for coronary artery disease, and coronary artery disease is a major risk factor for out-of-hospital cardiac arrest (OHCA). However, the effect of cholesterol level on outcomes of OHCA has been poorly studied. This study aimed to determine the effect of cholesterol level on outcomes of OHCA. METHODS: This cross-sectional study used the CAPTURES (Cardiac Arrest Pursuit Trial with Unique Registration and Epidemiologic Surveillance) project database in Korea. Multivariable conditional logistic regression analysis was performed to estimate the effect of cholesterol level on outcomes in OHCA. RESULTS: In all, 584 cases of OHCA were analyzed; those with cholesterol levels <120 mg/dL were classified as having low total cholesterol (TC) (n=197), those with levels ranging from 120-199 mg/dL as middle TC (n=322), and those with ≥200 mg/dL as high TC (n=65). Compared to low TC, more patients with middle TC and high TC survived to discharge (9.1% vs. 22.0% and 26.2%, respectively, P=0.001). The good cerebral performance category also increased in that order (4.1 % vs. 14.6% and 23.1%, respectively, P≤0.001). Comparing middle TC and high TC with low TC, adjusted odds ratios (95% confidence intervals) were 1.97 (1.06 to 3.64) and 2.53 (1.08 to 5.92) for survival to discharge, respectively, and 2.53 (1.07 to 5.98) and 4.73 (1.63 to 13.71) for good neurological recovery, respectively. CONCLUSION: Higher cholesterol is associated with better outcomes in OHCA; cholesterol level is a good predictor of outcomes of OHCA.

Influence of Electrical Traps on the Current Density Degradation of Inverted Perovskite Solar Cells.

Premature aging of perovskite solar cells (PSC) is one of the biggest challenges for its commercialization. Particularly, PSCs exhibit rapid degradation of photovoltaic parameters under ambient air exposure. To estimate the degradation mechanism of PSC under air exposure, we systematically analyzed the relationship between electrical traps of the PSC and its degradation. After 240 h of air exposure to the PSC, its power conversion efficiency degraded to 80% compared to its initial value. The loss mainly originated from reduced current density, which is affected by traps and carrier transport in the disordered semiconducting layer. Capacitance-voltage plots of the PSC showed that the ionic doping from the perovskite layer caused an increased number of trap sites at the buffer layer. Moreover, the extrapolation of temperature dependent open circuit voltage graphs indicated that the trap sites lead to poor carrier transport by increasing recombination losses in the aged device. Therefore, trap sites arose from the result of ion migration and caused an early degradation of PSC under air exposure.

CdSe tetrapod interfacial layer for improving electron extraction in planar heterojunction perovskite solar cells.

We demonstrate the improvement in the efficiency of planar heterojunction perovskite solar cells by employing cadmium selenide tetrapods (CdSe TPs) as an electron extraction layer. The insertion of the CdSe TP layer between the titanium oxide (TiO2) and perovskite film facilitates electron transfer at the TiO2/perovskite interface, as indicated by the significantly quenched steady-state photoluminescence of the perovskite film. Furthermore, we observed a conductivity enhancement of the perovskite film by introducing the CdSe TP layer. The combination of both effects induced by the TPs leads to enhancement in the carrier extraction as well as decreased recombination losses in the perovskite solar cells. As a result, an efficiency of 13.5% (1 sun condition) is achieved in the perovskite solar cells that incorporate the CdSe TP layer, which is 10% higher than that of the device without the CdSe TP layer.

Performance Limits of Luminescent Solar Concentrators Tested with Seed/Quantum-Well Quantum Dots in a Selective-Reflector-Based Optical Cavity.

Luminescent solar concentrators (LSCs) can serve as large-area sunlight collectors for photovoltaic devices. An important LSC characteristic is a concentration factor (C), which is defined as the ratio of the output and the input photon flux densities. This parameter can be also thought of as an effective enlargement factor of a solar cell active area. On the basis of thermodynamic considerations, the C-factor can reach extremely high values that exceed those accessible with traditional concentrating optics. In reality, however, the best reported values of C are around 30. Here we demonstrate that using a new type of high-emissivity quantum dots (QDs) incorporated into a specially designed cavity, we are able to achieve the C of ∼62 for spectrally integrated emission and ∼120 for the red portion of the photoluminescence spectrum. The key feature of these QDs is a seed/quantum-well/thick-shell design, which allows for obtaining a high emission quantum yield (>95%) simultaneously with a large LSC quality factor (QLSC of ∼100) defined as the ratio of absorption coefficients at the wavelengths of incident and reemitted light. By incorporating the QDs into a specially designed cavity equipped with a top selective reflector (a Bragg mirror or a thin silver film), we are able to effectively recycle reemitted light achieving light trapping coefficients of ∼85%. The observed performance of these devices is in remarkable agreement with analytical modeling, which allows us to project that the applied approach should allow one to boost the spectrally integrated concentration factors to more than 100 by further improving light trapping and/or increasing QLSC.

Phase-Transfer Ligand Exchange of Lead Chalcogenide Quantum Dots for Direct Deposition of Thick, Highly Conductive Films.

The use of semiconductor nanocrystal quantum dots (QDs) in optoelectronic devices typically requires postsynthetic chemical surface treatments to enhance electronic coupling between QDs and allow for efficient charge transport in QD films. Despite their importance in solar cells and infrared (IR) light-emitting diodes and photodetectors, advances in these chemical treatments for lead chalcogenide (PbE; E = S, Se, Te) QDs have lagged behind those of, for instance, II-VI semiconductor QDs. Here, we introduce a method for fast and effective ligand exchange for PbE QDs in solution, resulting in QDs completely passivated by a wide range of small anionic ligands. Due to electrostatic stabilization, these QDs are readily dispersible in polar solvents, in which they form highly concentrated solutions that remain stable for months. QDs of all three Pb chalcogenides retain their photoluminescence, allowing for a detailed study of the effect of the surface ionic double layer on electronic passivation of QD surfaces, which we find can be explained using the hard/soft acid-base theory. Importantly, we prepare highly conductive films of PbS, PbSe, and PbTe QDs by directly casting from solution without further chemical treatment, as determined by field-effect transistor measurements. This method allows for precise control over the surface chemistry, and therefore the transport properties of deposited films. It also permits single-step deposition of films of unprecedented thickness via continuous processing techniques, as we demonstrate by preparing a dense, smooth, 5.3-µm-thick PbSe QD film via doctor-blading. As such, it offers important advantages over laborious layer-by-layer methods for solar cells and photodetectors, while opening the door to new possibilities in ionizing-radiation detectors.

Improvement in the Photocurrent of Inverted Organic Solar Cells Using MoO(x)-Doped TAPC as a P-Type Optical Spacer.

In this work, we demonstrate enhancement in the short-circuit current of inverted organic photovoltaic cells (OPVs) using a p-type optical spacer. The p-type optical spacer, which consists of molybdenum oxide (MoO(x))-doped 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC), shows improved transmittance at visible light with high electrical conductivity. The electrical field distribution of incident light at the active layer of OPVs can be controlled by tuning the thickness of the optical spacer in the OPVs. Specifically, the incorporation of the 20-nm optical spacer layer in the OPV leads to enhanced spectral response of the device in the wavelength range of 400-600 nm, which is consistent with the combined results of improved optical absorption and better charge transport characteristics. As a result, the OPV with a 20-nm p-type optical spacer shows improvement in the short-circuit current compared with a device with 10 nm of embedded MoO(x).

Simultaneous Engineering of the Substrate Temperature and Mixing Ratio to Improve the Performance of Organic Photovoltaic Cells.

In this study, we investigated the effect of the donor/acceptor mixing ratio and the substrate temperature (T(SUB)) during the co-deposition process on the performance of bulk heterojunction organic photovoltaic cells. We found that the ratio of dispersed donor islands (less than 10 nm), which hinders charge carrier transport, increased as the donor concentration (C(D)) increased in the film processed at room temperature. By contrast, the donor cluster (larger than 10 nm), providing percolation paths for the carriers, was enlarged in the film containing a high C(D) fabricated at high T(SUB) (70 degrees C). This enhanced phase separation in the mixed layer led to an improved fill factor and a decreased activation energy of the short-circuit current (J(SC)). Therefore, we demonstrated a 23% improvement in the device performance by employing an elevated T(SUB) and optimized mixing ratio in comparison with the device fabricated at room temperature.

Improved photovoltaic performance of inverted polymer solar cells through a sol-gel processed Al-doped ZnO electron extraction layer.

We demonstrate that nanocrystalline Al-doped zinc oxide (n-AZO) thin film used as an electron-extraction layer can significantly enhance the performance of inverted polymer solar cells based on the bulk heterojunction of poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) and [6,6]-phenyl C(71)-butyric acid methyl ester (PC(70)BM). A synergistic study with both simulation and experiment on n-AZO was carried out to offer a rational guidance for the efficiency improvement. As a result, An n-AZO film with an average grain size of 13 to 22 nm was prepared by a sol-gel spin-coating method, and a minimum resistivity of 2.1 × 10(-3) Ω·cm was obtained for an Al-doping concentration of 5.83 at.%. When an n-AZO film with a 5.83 at.% Al concentration was inserted between the ITO electrode and the active layer (PCDTBT:PC(70)BM), the power conversion efficiency increased from 3.7 to 5.6%.

Nanostructured Electron-Selective Interlayer for Efficient Inverted Organic Solar Cells.

We report a unique nanostructured electron-selective interlayer comprising of In-doped ZnO (ZnO:In) and vertically aligned CdSe tetrapods (TPs) for inverted polymer:fullerene bulkheterojunction (BHJ) solar cells. With dimension-controlled CdSe TPs, the direct inorganic electron transport pathway is provided, resulting in the improvement of the short circuit current and fill factor of devices. We demonstrate that the enhancement is attributed to the roles of CdSe TPs that reduce the recombination losses between the active layer and buffer layer, improve the hole-blocking as well as electron-transporting properties, and simultaneously improve charge collection characteristics. As a result, the power conversion efficiency of PTB7:PC70BM based solar cell with nanostructured CdSe TPs increases to 7.55%. We expect this approach can be extended to a general platform for improving charge extraction in organic solar cells.

High-efficiency inverted organic solar cells with polyethylene oxide-modified Zn-doped TiO2 as an interfacial electron transport layer.

High efficiency inverted organic solar cells are fabricated using the PTB7:PC71BM polymer by incorporating Zn-doped TiO2 (ZTO) and 0.05 wt% PEO:ZTO as interfacial electron transport layers. The 0.05 wt% PEO-modified ZTO device shows a significantly increased power conversion efficiency (PCE) of 8.10%, compared to that of the ZTO (7.67%) device.

Plasmonic organic solar cells employing nanobump assembly via aerosol-derived nanoparticles.

We report the effect of a nanobump assembly (NBA) constructed with molybdenum oxide (MoO3) covering Ag nanoparticles (NPs) under the active layer on the efficiency of plasmonic polymer solar cells. Here, the NPs with precisely controlled concentration and size have been generated by an atmospheric evaporation/condensation method and a differential mobility classification and then deposited on an indium tin oxide electrode via room temperature aerosol method. NBA structure is made by enclosing NPs with MoO3 layer via vacuum thermal evaporation to isolate the undulated active layer formed onto the underlying protruded NBA. Simulated scattering cross sections of the NBA structure reveal higher intensities with a strong forward scattering effect than those from the flat buffer cases. Experimental results of the device containing the NBA show 24% enhancement in short-circuit current density and 18% in power conversion efficiency compared to the device with the flat MoO3 without the NPs. The observed improvements are attributed to the enhanced light scattering and multireflection effects arising from the NBA structure combined with the undulated active layer in the visible and near-infrared regions. Moreover, we demonstrate that the NBA adopted devices show better performance with longer exciton lifetime and higher light absorption in comparison with the devices with Ag NPs incorporated flat poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Thus, the suggested approach provides a reliable and efficient light harvesting in a broad range of wavelength, which consequently enhances the performance of various organic solar cells.

Origin of the mixing ratio dependence of power conversion efficiency in bulk heterojunction organic solar cells with low donor concentration.

We studied the origin of the improvement in device performance of thermally evaporated bulk heterojunction organic photovoltaic devices (OPVs) with low donor concentration. Samples with three different donor-acceptor mixing ratios, 0:10 (C70-only), 1:9 (low-doped) and 3:7 (high-doped), were fabricated with 1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane (TAPC):C70. The power conversion efficiencies (PCEs) of these samples were 1.14%, 2.74% and 0.69%, respectively. To determine why the low-doped device showed a high PCE, we measured various properties of the devices in terms of the effective energy band gap, activation energy, charge carrier mobility and recombination loss. We found that the activation energy for charge carrier transport was increased as we increased the TAPC concentration in the blends whereas the hole and electron mobilities became more balanced as the TAPC concentration was increased. Furthermore, the recombination loss parameter alpha (from the light intensity dependence) remained alpha to approximately 0.9 in the low-doped device, but it decreased to alpha to approximately 0.77 in the high-doped device, indicating a large recombination loss as a result of space charge. Therefore, the improved PCE of low-doped OPVs can be attributed to the balance between carrier mobilities with no increase in recombination loss.

Toward mass producible ordered bulk heterojunction organic photovoltaic devices.

A strategy to fabricate nanostructured poly(3-hexylthiophene) (P3HT) films for organic photovoltaic (OPV) cells by a direct transfer method from a reusable soft replica mold is presented. The flexible polyfluoropolyether (PFPE) replica mold allows low-pressure and low- temperature process condition for the successful transfer of nanostructured P3HT films onto PEDOT/PSS-coated ITO substrates. To reduce the fabrication cost of masters in large area, we employed well-ordered anodic aluminum oxide (AAO) as a template. Also, we provide a method to fabricate reversed nanostructures by exploiting the self-replication of replica molds. The concept of the transfer method in low temperature with a flexible and reusable replica mold obtained from an AAO template will be a firm foundation for a low-cost fabrication process of ordered OPVs.