Doo Kyung Moon

An organic–inorganic hybrid interlayer for improved electron extraction in inverted polymer solar cells

We fabricated inverted polymer solar cells (PSCs) using an organic–inorganic hybrid interlayer for electron extraction. The surface energy and surface defects of an organic–inorganic ZnO–PFN hybrid film, which was prepared by dissolving the conjugated polymer electrolyte poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl-fluorene)] (PFN) in a ZnO solution, were reduced, compared to ZnO film. By introducing the ZnO–PFN electron extraction layer, the interfacial contact between the active and electron extraction layers was improved and the series resistance of the PSC device was decreased. As a result, electron extraction from the active layer to the electrode was enhanced. The highest power conversion efficiency (PCE) of the inverted PSCs was 9.2%. Moreover, the ZnO–PFN-based inverted PSCs showed improved long-term stability compared to ZnO-based devices. The ZnO–PFN interlayer aimed to overcome the drawbacks of the conventional hydrophilic surface of ZnO, based on the properties of the conjugated polymer (PFN) without the need for additional processes. It was therefore simple to fabricate the inverted PSCs, making the devices commercially viable.

An effect on the side chain position of D–π–A-type conjugated polymers with sp2-hybridized orbitals for organic photovoltaics

A D–π–A-type poly[alkylidenefluorene-alt-di-2-thienyl-2,1,3-benzothiadiazole] (P1) was synthesized via Suzuki coupling reaction. Based on the positions of the spacer (π) and acceptor (A) in the polymers, a dodecyl chain (P2) and an octyloxy chain (P3), respectively, were introduced. Both chains were introduced to the spacer and acceptor of P4. The obtained polymers (P2–P4) were soluble in organic solvents such as chlorobenzene, THF and o-dichlorobenzene at room temperature. Also, the introduction of a dodecyl chain to the spacer reduced the energy of the highest occupied molecular orbital (HOMO) level (−5.5 to −5.56 eV) but increased the tilt angle (45.4–53.0°), which had prevented the main chain from π–π stacking. The orientation of the obtained polymers in thin films was confirmed by XRD measurement. P3 with only an octyloxy chain showed a face-on-rich structure with a π–π stacking distance of 3.7 A compared to other polymers. The polymer solar cells were fabricated through a solution process, and showed a power conversion efficiency (PCE) of 3.6%, with a short-circuit current density (Jsc) of 8.9 mA cm−2, an open-circuit voltage (Voc) of 0.88 V, and a fill factor (FF) of 45.7%. With the introduction of poly[9,9-bis(6′-(diethanolamino)hexyl)fluorene] (PFN-OH), a PCE of 3.9% was confirmed.

Alkylidenefluorene–isoindigo copolymers with an optimized molecular conformation for spacer manipulation, π–π stacking and their application in efficient photovoltaic devices

D–(π)n-A-type copolymers with different thienyl spacers (n = 0–2) between alkylidenefluorene and isoindigo (P1, P2, P3) were synthesized via a Suzuki coupling reaction. As the number of spacers was increased in a polymer (P1), the UV-Vis absorption maximum (λmax) red-shifted, and the band gap decreased from 1.83 to 1.64 eV. The highest occupied molecular orbital (HOMO) levels of the polymers were increased by increasing the number of spacers. In addition, the facile intermixing due to better accessibility with the PCBM led to an increase in the hole mobility and JSC. In contrast to P1 and P2, when the P3 thin films were blended with PC70BM in the X-ray diffraction (XRD) measurement, an increased face-on structure of the crystal was observed. The power conversion efficiency (PCE) of P3 was 3.0% and it reached 4.2% for the inverted device fabricated at 1 : 2 ratio with PC70BM.

Enhanced performance in polymer light emitting diodes using an indium–zinc–tin oxide transparent anode by the controlling of oxygen partial pressure at room temperature

The amount of indium in indium zinc tin oxide (IZTO) was reduced by over 20% by manufacturing an IZTO target containing ZnO for application as the transparent conducting oxide (TCO) anode of polymer light emitting diodes (PLEDs). The IZTO target was manufactured with the composition In2O3 (70 at%)–ZnO (15 at%)–SnO2 (15 at%), and IZTO films were formed at room temperature using a pulsed DC magnetron sputtering system at oxygen partial pressures of 0–4%. The optical and electrical properties of the IZTO films and the device performance of PLEDs with IZTO film were characterized. Amorphous IZTO films prepared at an oxygen partial pressure of 3% showed the best properties. The resistivity, mobility, transmittance, figure of merit and work function of the IZTO film were 5.6 × 10−4 Ω cm, 44.59 cm2 V s−1, 81% (visible region), 3.0 × 10−3 ohm−1, and 5.56 eV, respectively. The PLEDs with the IZTO film deposited under the optimum conditions showed the maximum brightness and the maximum luminance efficiency of 23 485 cd cm−2 and 2.29 cd A−1, respectively, which showed a 21% enhancement in device performance compared to PLEDs with commercial ITO film. In addition, the stability of the fabricated device was improved.

Synthesis of Random Copolymers of Pyrrole and Aniline by Chemical Oxidative Polymerization

Random copolymers have been synthesized in high yields (94%) by chemical oxidative polymerization of aniline/pyrrole comonomers using H2O2 in the presence of Fe catalyst. The oxidative polymerization proceeds via successive coupling that gives the copolymer structure similar to polyaniline and polypyrrole. The copolymers are black powders and soluble in dimethylsulfoxide (DMSO), dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP). The weight–average molecular weight (Mw) of the copolymers was in a range, from 6200 to 15,000 g/mol.

Photon energy transfer by quantum dots in organic–inorganic hybrid solar cells through FRET

Organic–inorganic hybrid solar cells were fabricated with InP QDs (5 wt%) in a BHJ active layer (PTB7 + PC71BM). InP QDs were distributed on the top of the hybrid active layer (BHJ + QDs), determined through XPS depth profiling and AFM analyses. InP QDs showed strong emission characteristics at λmax = 650 nm in the PL spectra. They played an important role in increasing Jsc by Forster resonance energy transfer (FRET) to PTB7. The carrier recombination of the hybrid active layer (BHJ + QDs) was reduced by morphology control through introducing a PFN interlayer. The carrier mobility of the device with the hybrid active layer (BHJ + QDs) increased 1.5–1.58 times over that of the device with the BHJ active layer. By means of the synergy effect of InP QDs and the PFN interlayer, the PCE of the fabricated hybrid solar cells was enhanced from 4.9% (Jsc = 13.2 mA cm−2, FF = 60.0%) to 8.4% (Jsc = 14.5 mA cm−2, FF = 72.5%).

Molecular design through computational simulation on the benzo[2,1-b;3,4-b′]dithiophene-based highly ordered donor material for efficient polymer solar cells

Donor–acceptor (D–A) copolymers have been proved to be excellent candidates for efficient polymer solar cells (PSCs). An easy and powerful strategy of D–A copolymer design to enhance the performance of PSCs would advance their industrialization. Here we demonstrate an effective molecular design method using the simple molecular mechanics function of MM2 (Molecular Mechanic program 2) & MMFF94 (Merck Molecular Force Field 94) calculation and noncovalent conformational locking effects in the D–A polymer backbone. It is shown to play an important role in D–A copolymers with a highly ordered structure through intra- and/or intermolecular interactions. We report a newly designed D–A copolymer donor, poly(benzodithiophene-dibenzophenazine), P(BDP-DTPz), using our strategy which exhibits good solubility, high molecular ordering and excellent charge carrier mobility balance. A maximum power conversion efficiency (PCE) of 6.2% is achieved with a P(BDP-DTPz):PC71BM blend under 1.5 G solar irradiation. This work will provide a new perspective for molecular design of D–A copolymers in PSCs.

Enhanced performance in bulk heterojunction solar cells with alkylidene fluorene donor by introducing modified PFN-OH/Al bilayer cathode

We report bulk heterojunction solar cells with benzothiadiazole-containing polyalkylidene fluorene (PAFBToBT) donors made by introducing 2,4,7,9-tetramethyldec-5-yne-4,7-diol (Surfynol 104)-doped poly[9,9-bis(6′-(diethanolamino)hexyl)-fluorene] (PFN-OH) as an interfacial layer to enhance power conversion efficiency (PCE) and stability. Surfynol 104 has amphiphilic characteristics, and it supports the formation of a homogeneous interfacial layer on the photoactive layer. In addition, the application of the Surfynol 104-doped PFN-OH interfacial layer prevented the decrease in absorption of photons in the photoactive layer and improved the photocurrent density (JSC) by increasing the electron mobility. Therefore, JSC and the fill factor increased to 10.5 mA cm−2 and 49%, respectively, and the calculated PCE was 4.8%. In addition, a modified bilayer cathode system was introduced to the PCDTBT and the P3HT-based PSCs, and the PCE improved to 5.3% and 3.9%, respectively. Moreover, the in-air stability was enhanced.

Synthesis of Conjugated Materials Based on Benzodithiophene - Benzothiadazole and Their Application of Organic Solar Cells

Electron acceptor-donor-acceptor type oligomers based on benzothiadiazole (BT) unit and a (4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl) bis(trimethylstannane) (BDT) unit have been designed and prepared as electron-donating materials, which are 2,6-[5-(7-methyl-benzo[1,2,5]thiadiazol-4-yl)-thiophen-2-yl]-4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene) (BDT-TBT) and 2,6-[5′-(3′-hexyl-[2,2′]bithiophenyl-5-yl)-7-methyl-benzo[1,2,5]thiadiazole]-4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene) (BDT-THTBT), for Organic Solar Cells (OSCs) with PC71BM as electron-accepting materials. The HOMO energy level is elevated by the number of thiophene ring as a p-bridge, which lowers the band gap. Inverted-type organic solar cells (OSCs) with a configuration of ITO/ZnO/BDT-TBT (or BDT-THTBT):PC71BM/MoO3/Al are fabricated. OSC based on BDT-THTBT exhibits the highest power conversion efficiency (PCE) of 1.04% with the best Jsc of 4.20 mA/cm2.

Effect on Electrode Work Function by Changing Molecular Geometry of Conjugated Polymer Electrolytes and Application for Hole-Transporting Layer of Organic Optoelectronic Devices

In this study, we synthesized three conjugated polymer electrolytes (CPEs) with different conjugation lengths to control their dipole moments by varying spacers. P-type CPEs (PFT-D, PFtT-D, and PFbT-D) were generated by the facile oxidation of n-type CPEs (PFT, PFtT, and PFbT) and introduced as the hole-transporting layers (HTLs) of organic solar cells (OSCs) and polymer light-emitting diodes (PLEDs). To identify the effect on electrode work function tunability by changing the molecular conformation and arrangement, we simulated density functional theory calculations of these molecules and performed ultraviolet photoelectron spectroscopy analysis for films of indium tin oxide/CPEs. Additionally, we fabricated OSCs and PLEDs using the CPEs as the HTLs. The stability and performance were enhanced in the optimized devices with PFtT-D CPE HTLs compared to those of PEDOT:PSS HTL-based devices.