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In this study, the performance of a multi-split VRF system using the bypass cycle and injection cycle is evaluated using the numerical simulation as a possible sub-cooling method to prevent flash gas generation in liquid pipelines. The simulation for the multi-split VRF system is developed by considering the applications of the bypass cycle and injection cycle, and is validated with experimental data. The bypass cycle and injection cycle in the multi-split VRF system yield improvements in their cooling capacities of the order of 3.22% and 13.43%, respectively, and energy efficient ratio (EER) of the order of 1.98% and 1.72%, respectively. The input power of the injection cycle is reduced by up to 4.45% when the performance of the multi-split VRF systems with bypass cycle and injection cycle is compared under the same cooling capacity conditions.

The performance of the variable refrigerant flow (VRF) system becomes poor at a very low ambient temperature because of the decrease in the suction mass flow rate. The vapor injection is applied to the heat pump system to enhance the performance of the systems in a cold season. In this study, the influence of the vapor injection under different intermediate pressures and outdoor ambient temperatures was investigated mathematically, and the results were validated with experimental data. In addition, the performance variation was observed with different indoor ambient temperatures in nine indoor units. The intermediate pressure affected the heating capacity, input power, coefficient of performance (COP), and isentropic efficiency of the compressor. The improvement of the COP with vapor injection was dominant at a low ambient temperature. The heating capacity at different indoor ambient temperatures was observed with different refrigerant mass flow rates in each indoor unit.

This study describes a systematic study on side chain effect in the poly(thieno[3,4-b]thiophene)benzo[1,2-b:4,5-b′]dithiophene (PTB) polymer series based regioregular polymers with D1-A-D2-A configuration, denoted as PTT-BDT(R1)-TT-BDT(R2). The synthesized three regioregular polymers employ the same conjugated backbone but with different side chains (R1, R2), P1; hexyldecyl thiophene (HD, R1) and ethylhexyl thiophene (EH, R2), P2; hexyldecyl thiophene (HD, R1) and ethylhexylthio thiophene (S-EH, R2), P3; hexyldecylthio thiophene (S-HD, R1) and ethylhexylthio thiophene (S-EH, R2). Introduction of different side chains influenced optical, electrochemical, molecular packing properties, and device performance. The P3 polymer with alkylthio thiophene groups as R1 and R2 showed much broader light absorption and lower HOMO level than other polymers with relatively less alkylthio thiophene groups. Furthermore, the P3 polymer exhibited favorable polymer orientation and short π-π stacking distance for efficient charge transport in the BHJ PSCs. The P3 based bulk-heterojunction (BHJ) polymer solar cell (PSC) showed an improved PCE of 7.20% while the P1 and P2 based BHJ PSC devices showed PCEs of 5.27% and 6.45%, respectively. This result, within our knowledge, is the highest record among BHJ PSCs based PTB polymer series composed of benzo[1,2-b:4,5-b’]dithiophene (BDT) segment and non-fluorinated alkyl ester (COOR) side chain substituted thieno[3,4-b]thiophene (TT) segment.

A series of highly microporous carbon materials was produced by carbonization of a triazine-based covalent organic polymer (TCOP) followed by carbonization and CO2 physical activation. The N-containing porous COP was prepared from easily available economic monomer precursors via a simple Friedel-Crafts reaction, which produced a predominantly microporous structure with a high surface area. Carbonization at 600–900 °C produced predominantly microporous carbons with a narrow pore size distribution in the range of 0.5–1.5 nm. Upon further activation using CO2, more micropores were formed, accompanied by an increase in the surface area (to 2003 m2 g−1) and the nitrogen level in the carbon structure was maintained at ca. 2 wt%. The electrochemical properties of the samples were measured by employing a three-electrode system with 6 M KOH electrolyte. Among the prepared carbon samples, the electrode fabricated using the carbon activated at 900 °C (AC-900) had a specific capacitance of 278 F g−1 at a current density of 1 A g−1, which is significantly higher than that of a commercial activated carbon (130 F g−1) and ranks among the highest reported so far. This improved performance was attributed to the highly microporous structure of the nitrogen-doped carbon with a narrow pore size distribution.

We designed and synthesized new hole-blocking materials (HBMs), mPyrPPB and pPPyrPB, consisting of pyrimidine and phenylene segments for high-performance blue phosphorescent OLEDs. The thermal, electrochemical, and optical properties of mPyrPPB and pPPyrPB were systemically investigated. It was found that the Tg values of mPyrPPB and pPPyrPB were 118 and 137 °C, respectively. Especially, the triplet energy and highest occupied molecular orbital (HOMO) energy level of mPyrPPB were 2.77 eV and −6.86 eV, respectively, indicating that it had sufficiently high triplet energy and deep HOMO energy level for the hole-blocking layer (HBL) in blue phosphorescent OLED devices. It was found that all the meta conjugation of mPyrPPB molecular structure effectively prevented π-electron delocalization and thus increased the triplet energy and electron transport property. In addition, mPyrPPB exhibited higher electron-transporting property than pPPyrPB because mPyrPPB possessed effective intermolecular hydrogen bonds. When mPyrPPB was utilized as a HBM for a blue phosphorescent OLED device, external quantum efficiency (EQE), current efficiency (CE), and power efficiency (PE) values effectively increased to 16.4%, 36.7 cd/A, and 13.4 lm/W, respectively. Compared to the reference device without HBM, EQE, CE, and PE increased by 38%, 35%, and 54% respectively, mainly due to the confinement of triplet excitons and holes and improved electron-transporting ability.

Herein, 3D in-situ hollow carbon fiber/carbon nanosheet/Fe3C@Fe3O4 (h-CNF/CNS/Fe3C@Fe3O4) was constructed using ferrocene and oleic acid by a solventless one-step process that did not require additional filtering, neutralization, drying, or calcination steps. The iron-oleate chelating structure that self-assembled during annealing facilitated the fabrication of 3D in-situ h-CNF/CNS/Fe3C@Fe3O4 by undergoing graphitization at 600 °C. This carbon shell of the 3D in-situ h-CNF/CNS/Fe3C@Fe3O4 was derived from the two cyclopentadienyl rings in ferrocene. This hybrid material made from economically cheap oleic acid exhibited superior supercapacitive behavior: high specific capacitance of 327 F g−1 at 5 mV s−1 and 210 F g−1 at 10 mV s−1, good rate capability of 60 F g−1 at 10 A g−1 (compared to 70 F g−1 at 1 A g−1 for pure Fe3O4), and superior retention of 108% after 6000 cycles at 100 mV s−1. These superior supercapacitive properties were ascribed to the 3D graphitic h-CNF/CNS for enhancing electrical conductivity and the carbonaceous shell over Fe3C@Fe3O4 core for buffering the bulk expansion of iron-related particles.

Aligned carbon nanofibers (ACNFs) were fabricated by electrospinning using a high speed of rotary collector (2000 rpm). To achieve a synergy effect of the electron double layer and redox faradaic reaction, NiCo 2 O 4 was deposited on the surface of the ACNFs using an electrodeposition method. The improved electrochemical performance of the electrodes was investigated by cyclic voltammetry, galvanostatic charge-discharge testing, and electrochemical impedance analysis. Among the prepared electrodes, which were deposited for different times (5, 10, 20, and 30 s), the sample deposited by NiCo 2 O 4 for 10 s had the highest specific capacitance (90.1 F g −1 at 5 mV s −1 ), good rate capability (64.7%), and cycle stability (85.2% after 1000 cycles). In addition, to examine the effects of the alignment and redox faradaic reaction of NiCo 2 O 4, the ACNFs covered with NiCo 2 O 4 for 10 s (NC-ACNFs) were compared with the ACNFs deposited by Co 3 O 4 (C-ACNFs) and randomly oriented carbon nanofibers covered with NiCo 2 O 4 (NC-RCNFs) . The NC-ACNFs had a 15.8% and 11.3% higher specific capacitance than that of the C-ACNFs and NC-RCNFs, respectively. These results suggested that the electrochemical properties of carbon nanofibers could be improved through the structural effects of aligned nanofibers and a redox faradaic reaction of NiCo 2 O 4 .

Terpolymers consisting of three monomers with one electron donor unit and two electron acceptor units are promising p-type polymers for polymer solar cells (PSCs) because the incorporation of a third monomer into a copolymer backbone provides synergetic effect on physical properties such as absorption ability, charge transport, and photovoltaic performance. Currently, novel p-type terpolymers need to be developed for high-efficiency PSCs, which can be processed with eco-friendly non-halogenated solvents. In this study, a new series of terpolymers composed of 4,8-di(2,3-didecylthiophen-5-yl)-benzo[1,2-b:4,5-b′]dithiophene (BDT), 4,7-di(thien-2-yl)-5,6-difluoro-2,1,3-benzothiadiazole (DTffBT), and benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD) segments was synthesized and characterized for high-performance PSCs processed with non-halogenated solvents. PBDTBD terpolymers (i.e., PBDTBD-25, PBDTBD-50, and PBDTBD-75) were synthesized by adjusting different ratios of DTffBT to BDD segment (25%, 50%, and 75% of DTffBT). PBDTBD terpolymers exhibited excellent solubility in non-halogenated solvents. The optical, electrochemical, and morphological properties of PBDTBD terpolymers were successfully controlled by modulating the molar ratio of DTffBT and BDD. Moreover, a PBDTBD-50:IT-4F blended film showed homogeneous film with a favorable face-on orientation. The PBDTBD-50:IT-4F blended film showed excellent hole and electron mobility, which resulted in a superior carrier balance. PBDTBD-50-based PSCs, processed with o-xylene, achieved the highest PCE of 10.03%, which is four times higher than those of copolymer-based PSCs. The novel terpolymers composed of one electron donor unit and two electron acceptor units are expected to make a considerable contribution to the development of high-performance PSCs.

Many researchers have reported dibenzo[b,d]furan segment as a building block of organic light-emitting diode (OLED) materials because it has high thermal stability and triplet energy. However, most of the research has focused on symmetrically substituting the same functional groups at 2-position due to easy functionalization or substituting the same functional groups at different positions of dibenzo[b,d]furan. Herein, we design and synthesize three new hole blocking materials based on asymmetrically difunctionalized dibenzo[b,d]furan, diphenyl (2-(pyridin-3-yl)dibenzo[b,d]furan-6-yl)phosphine oxide (DBFPO-Py), diphenyl(2-(pyrimidin-5-yl)dibenzo[b,d]furan-6-yl)phosphine oxide (DBFPO-Pyr), and diphenyl(2-(4-(triphenylsilyl)phenyl)dibenzo[b,d]furan-6-yl)phosphine oxide (DBFPO-Si) for high-performance phosphorescent OLEDs. Phosphine oxide, tetraphenylsilane, pyridine, and pyrimidine segments are successfully introduced into the asymmetric position of a dibenzo[b,d]furan. It is found that DBFPO-Py, DBFPO-Pyr, and DBFPO-Si possess high thermal stability; high triplet energies of 2.96, 2.98, and 2.80 eV; and deep highest occupied molecular orbital (HOMO) energy levels of −7.13, −7.23 and −7.07 eV; respectively. Blue phosphorescent OLEDs with DBFPO-Py, DBFPO-Pyr, and DBFPO-Si show low turn-on voltages, high current and power efficiencies, and superior external quantum efficiencies. Blue phosphorescent OLEDs with DBFPO-Py and DBFPO-Pyr showed improved performance in terms of current and power efficiencies, etc, compared with the device with 1,3-bis(3,5-dipyrid-3-yl-phenyl)benzene (BmPyPB), which is commonly used as a hole blocking layer. More importantly, the blue phosphorescent OLEDs with DBFPO-Pyr showed the best performance with maximum external quantum efficiency of 23.6%, current efficiency of 29.8 cd A−1, power efficiency of 26.0 lm W−1, and low efficiency roll-off of 6.38%. Novel hole blocking materials based on asymmetrically difunctionalized dibenzo[b,d]furan are expected to make a significant contribution to the development of blue phosphorescent OLEDs.