Publications by authors named "Dong Hwan Wang"

39 Publications

A Facile and Effective Ozone Exposure Method for Wettability and Energy-Level Tuning of Hole-Transporting Layers in Lead-Free Tin Perovskite Solar Cells.

ACS Appl Mater Interfaces 2021 Sep 31;13(36):42935-42943. Epub 2021 Aug 31.

SKKU Advanced Institute of Nanotechnology (SAINT) and Department of Nano Engineering, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea.

Lead-free perovskite solar cells (PSCs) have attracted interest among scientists searching for eco-friendly energy harvesting devices. Herein, the effects of ozone exposure on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) in lead-free tin halide PSCs as a facile and low-cost process for improving device performance are analyzed. Two types of tin-based PSCs and one typical lead-based PSC were fabricated. The ozone exposure on PEDOT:PSS increases the short-circuit current density () and the fill factor (FF) of PSCs in all cases with perovskite grain enlargement and hole-mobility enhancement of the devices, respectively. For open-circuit voltage (), the outcome depends on the band gap and the energy levels of the perovskite films. While ozone exposure treatment is favorable for PEAFASnI-based tin PSCs, decreases with ozone exposure in the case of Ge:EDAFASnI-based tin PSCs because of a misalignment of the energy levels. Regardless, the efficiency of PEAFASnI-based tin PSCs increases from 8.7 to 10.1% when measured inside a glovebox upon ozone exposure of PEDOT:PSS. The efficiency of Ge:EDAFASnI-based tin PSCs increases from 6.8 to 8.1%, and the devices retain an efficiency of 5.0% even after 50 days in air.
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http://dx.doi.org/10.1021/acsami.1c13546DOI Listing
September 2021

Versatile Pendant Polymer for Selective Charge Carrier Transport via Controlling the Supramolecular Self-Assembly.

ChemSusChem 2021 Aug 30. Epub 2021 Aug 30.

Chung-Ang University, School of Integrative Engineering, 221 Heukseok-dong, Dongjak-gu, 156-756, Seoul, KOREA, REPUBLIC OF.

Polyvinyl carbazole (P0)-based pendant polymers were synthesized by modifying carbazole motifs with pyrene derivatives (P1 and P4) to manipulate the bandgap and frontier orbital energy levels. To establish the electronic properties of pendant polymers according to structural differences, the polymers were utilized as additional hole transport layers in planar-type perovskite solar cells and organic photovoltaic cells. When P4 with thiophene-pyrene pendant was used as hole transport layer, all device parameters, except Voc, were significantly improved in comparison with P0 and P1 (conjugated with t-butyl pyrene derivatives). Since P4 had more electrically conductive thiophene units than benzene units with fewer alkyl groups, the supramolecular assembly of P4 was found to be more favourable in electronic devices. Furthermore, devices with P4 demonstrated lower dark current than others, which could potentially be useful for charge carrier transport and sensitive photo detecting devices.
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http://dx.doi.org/10.1002/cssc.202101785DOI Listing
August 2021

High-Valent Iodoplumbate-Rich Perovskite Precursor Solution Solar Illumination for Reproducible Power Conversion Efficiency.

J Phys Chem Lett 2021 Feb 9;12(6):1676-1682. Epub 2021 Feb 9.

Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea.

The power conversion efficiency (PCE) of solution-processed organic-inorganic hybrid perovskite solar cells has been drastically improved. Despite this considerable progress, systematic research on precursor solution chemistry and its effects on photovoltaic parameters has been limited thus far. Herein, we report on the tracking of changes in chemical species in a precursor solution under solar illumination and investigate the correlation between the equilibrium change and the corresponding perovskite film formation. The illuminated perovskite precursors display a higher density of high-valent iodoplumbate, where the resulting perovskite film exhibits reduced defect density with uniform film formation. Conclusively, the perovskite solar cells prepared by the photoaged precursor solution demonstrate not only improved average PCE but also enhanced reproducibility with a narrow PCE distribution. This discovery shows robust control of perovskite precursor solutions from a simple treatment and suggests that the resulting uniform film may be applicable to various halide perovskite-based devices.
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http://dx.doi.org/10.1021/acs.jpclett.0c03849DOI Listing
February 2021

Fully Inorganic CsSnI-Based Solar Cells with >6% Efficiency and Enhanced Stability Enabled by Mixed Electron Transport Layer.

ACS Appl Mater Interfaces 2021 Jan 31;13(1):1345-1352. Epub 2020 Dec 31.

Department of Materials Science and Engineering, Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania 16802, United States.

Fully inorganic black orthorhombic (B-γ) CsSnI has become a promising candidate for perovskite solar cell (PSC) thanks to its low toxicity and decently high theoretical power conversion efficiency (PCE). However, so far, the reported PCE of the B-γ CsSnI PSC is still not comparable with its lead-based or organotin-based counterparts. Herein, a mixed electron transport layer (ETL) composed of ZnO nanoparticles (NPs) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is incorporated into inverted B-γ CsSnI PSCs. The mixed ETL exhibits the merits of both ZnO and PCBM. The highest PCE of 6.08% was recorded for the PSC with mixed ZnO-PCBM ETL, which is 34.2% higher than that of the device with plain PCBM ETL (PCE of 4.53%) and 28.8% superior to that of plain ZnO ETL-based device (PCE of 4.72%). Meanwhile, the mixed ZnO-PCBM ETL-based PSC retained 71% of its initial PCE under inert conditions at room temperature after 60 days of storage and maintained 67% PCE after 20 days of storage under ambient air at 30% relative humidity and room temperature.
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http://dx.doi.org/10.1021/acsami.0c16634DOI Listing
January 2021

Light-Emitting Transistors with High Color Purity Using Perovskite Quantum Dot Emitters.

ACS Appl Mater Interfaces 2020 Aug 27;12(31):35175-35180. Epub 2020 Jul 27.

School of Integrative Engineering, Chung-Ang University, Seoul 06974, Republic of Korea.

The class of organic-inorganic lead halides with perovskite crystal structures has recently emerged as promising materials for a variety of practical optoelectronic applications. In particular, hybrid halide perovskite quantum dots possess excellent intrinsic optoelectronic properties such as high color purity (full width at half-maximum of 24.59 nm) and photoluminescence quantum yields (92.7%). In this work, we demonstrate the use of perovskite quantum dot materials as an emissive layer of hybrid light-emitting transistors. To investigate the working mechanism of perovskite quantum dots in light-emitting transistors, we investigated the electrical and optical characteristics under both p-channel and n-channel operation. Using these materials, we have achieved perovskite quantum dot light-emitting transistors with high electron mobilities of up to 12.06 cm·V s, high brightness of up to 1.41 × 10 cd m, and enhanced external quantum efficiencies of up to 1.79% operating at a source-drain potential of 40 V.
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http://dx.doi.org/10.1021/acsami.0c05537DOI Listing
August 2020

Selective Soxhlets extraction to enhance solubility of newly-synthesized poly(indoloindole-selenophene vinylene selenophene) donor for photovoltaic applications.

Nano Converg 2020 Mar 10;7(1). Epub 2020 Mar 10.

School of Integrative Engineering, Chung-Ang University, 84 Heukseok-Ro, Dongjak-gu, Seoul, 156-756, Republic of Korea.

An electron-rich fused indoloindole-based poly(indoloindole-selenophene vinylene selenophene) was synthesized and characterized. Soxhlet can be obtained by continuously purifying the product with a specific solvent and obtaining a pure polymer with a high concentration. Molecular weight is affected by the vapor pressure of marginal solvent, and the polymer was fractionated using tetrahydrofuran, chloroform, and chlorobenzene. Solubility is closely related to the morphology of bulk heterojunction and device parameters. In the solution process of fabricating the organic solar cell, securement of solubility has a great effect on the performance of the device, because morphology and orientation of a photo-active layer which significantly affect charge transport in the device. Since tetrahydrofuran (THF) Soxhlet solvents have high vapor pressure and appropriate solubility parameters, THF induced the best solubility of P-IDI-SVS materials for organic solvents. And through additive optimization, the performance of the device based on P-IDI-SVS from THF-Soxhlet extraction was enhanced. This is expected to be a meaningful study because the effect on solubility of Soxhlet solvent suggests factors to be considered in the solution process in organic solar cell research. In addition, surface modified bulk heterojunction was observed using atomic force microscopy, photoluminescence, time-correlated single photon counting and Raman spectroscopy analysis.
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http://dx.doi.org/10.1186/s40580-020-0219-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7062981PMC
March 2020

Acidity Suppression of Hole Transport Layer via Solution Reaction of Neutral PEDOT:PSS for Stable Perovskite Photovoltaics.

Polymers (Basel) 2020 Jan 6;12(1). Epub 2020 Jan 6.

School of Integrative Engineering, Chung-Ang University, Seoul 06974, Korea.

Poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT:PSS) is typically used for hole transport layers (HTLs), as it exhibits attractive mechanical, electrical properties, and easy processability. However, the intrinsically acidic property can degrade the crystallinity of perovskites, limiting the stability and efficiency of perovskite solar cells (PSCs). In this study, inverted CHNHPbI photovoltaic cells were fabricated with acidity suppressed HTL. We adjusted PEDOT:PSS via a solution reaction of acidic and neutral PEDOT:PSS. And we compared the various pH-controlled HTLs for PSCs devices. The smoothness of the pH-controlled PEDOT:PSS layer was similar to that of acidic PEDOT:PSS-based devices. These layers induced favorable crystallinity of perovskite compared with acidic PEDOT:PSS layers. Furthermore, the enhanced stability of pH optimized PEDOT:PSS-based devices, including the prevention of degradation by a strong acid, allowed the device to retain its power conversion efficiency (PCE) value by maintaining 80% of PCE for approximately 150 h. As a result, the pH-controlled HTL layer fabricated through the solution reaction maintained the surface morphology of the perovskite layer and contributed to the stable operation of PSCs.
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http://dx.doi.org/10.3390/polym12010129DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7022435PMC
January 2020

Tuning the energy level of TAPC: crystal structure and photophysical and electrochemical properties of 4,4'-(cyclohexane-1,1-diyl)bis[N,N-bis(4-methoxyphenyl)aniline].

Acta Crystallogr C Struct Chem 2019 Jul 14;75(Pt 7):919-926. Epub 2019 Jun 14.

Department of Chemistry, Seoul Women's University, 621 Hwarang-ro, Nowon-gu, Seoul 01797, Republic of Korea.

The energy level of a hole-transporting material (HTM) in organic electronics, such as organic light-emitting diodes (OLEDs) and perovskite solar cells (PSCs), is important for device efficiency. In this regard, we prepared 4,4'-(cyclohexane-1,1-diyl)bis[N,N-bis(4-methoxyphenyl)aniline] (TAPC-OMe), CHNO, to tune the energy level of 4,4'-(cyclohexane-1,1-diyl)bis[N,N-bis(4-methylphenyl)aniline] (TAPC), which is a well-known HTM commonly used in OLED applications. A systematic characterization of TAPC-OMe, including H and C NMR, elemental analysis, UV-Vis absorption, fluorescence emission, density functional theory (DFT) calculations and single-crystal X-ray diffraction, was performed. TAPC-OMe crystallized in the triclinic space group P-1, with two molecules in the asymmetric unit. The dihedral angles between the central amine triangular planes and those of the phenyl groups varied from 26.56 (9) to 60.34 (8)° due to the steric hindrance of the central cyclohexyl ring. This arrangement might be induced by weak hydrogen bonds and C-H...π(Ph) interactions in the extended structure. The emission maxima of TAPC-OMe showed a significant bathochomic shift compared to that of TAPC. A strong dependency of the oxidation potentials on the nature of the electron-donating ability of substituents was confirmed by comparing oxidation potentials with known Hammett parameters (σ).
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http://dx.doi.org/10.1107/S2053229619007101DOI Listing
July 2019

Facile NiO Sol-Gel Synthesis Depending on Chain Length of Various Solvents without Catalyst for Efficient Hole Charge Transfer in Perovskite Solar Cells.

Polymers (Basel) 2018 Nov 6;10(11). Epub 2018 Nov 6.

School of Integrative Engineering, Chung-Ang University, 84 Heukseok-Ro, Dongjak-gu, Seoul 06974, Korea.

Nickel oxide (NiO)⁻based perovskite solar cells (PSCs) have recently gained considerable interest, and exhibit above 20% photovoltaic efficiency. However, the reported syntheses of NiO sol-gel used toxic chemicals for the catalysts during synthesis, which resulted in a high-temperature annealing requirement to remove the organic catalysts (ligands). Herein, we report a facile "NiO sol-gel depending on the chain length of various solvents" method that eschews toxic catalysts, to confirm the effect of different types of organic solvents on NiO synthesis. The optimized conditions of the method resulted in better morphology and an increase in the crystallinity of the perovskite layer. Furthermore, the use of the optimized organic solvent improved the absorbance of the photoactive layer in the PSC device. To compare the electrical properties, a PSC was prepared with a p-i-n structure, and the optimized divalent alcohol-based NiO as the hole transport layer. This improved the charge transport compared with that for the typical 1,2-ethanediol (ethylene glycol) used in earlier studies. Finally, the optimized solvent-based NiO enhanced device performance by increasing the short-circuit current density (), open-circuit voltage (), and fill factor (), compared with those of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)⁻based devices.
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http://dx.doi.org/10.3390/polym10111227DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6290588PMC
November 2018

The Investigation of the Seebeck Effect of the Poly(3,4-Ethylenedioxythiophene)-Tosylate with the Various Concentrations of an Oxidant.

Polymers (Basel) 2018 Dec 24;11(1). Epub 2018 Dec 24.

School of Integrative Engineering, Chung-Ang University, 84 Heukseok-Ro, Dongjak-gu, Seoul 06974, Korea.

Poly(3,4-ethylenedioxythiophene)-tosylate (PEDOT-Tos) can be synthesized through an in situ polymerization and doping process with iron(III) p-toluenesulfonate hexahydrate as an oxidant. Both the Seebeck coefficient and the electrical conductivity were modified by varying the concentration of the oxidant. We investigated the effects of varying the concentration of the oxidant on the particle sizes and doping (oxidation) levels of PEDOT-Tos for thermoelectric applications. We demonstrated that an increase in the oxidant enabled an expansion of the particle sizes and the doping levels of the PEDOT-Tos. The modification of the doping levels by the concentration of the oxidant can provide another approach for having an optimal power factor for thermoelectric applications. De-doping of PEDOTs by reduction agents has been generally investigated for changing its oxidation levels. In this study, we investigated the effect of the concentration of the oxidant of PEDOT-Tos on the oxidation levels, the electrical conductivities and the Seebeck coefficients. As loading the oxidant of PEDOT-Tos, the Seebeck coefficient was compromised, while the electrical conductivity increased.
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http://dx.doi.org/10.3390/polym11010021DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6401917PMC
December 2018

Long-Term Stable Transferred Organic Photoactive Layer-Based Photodiode with Controlled Wetting through Interface Stabilization.

ACS Appl Mater Interfaces 2018 Nov 24;10(44):38603-38609. Epub 2018 Oct 24.

School of Integrative Engineering , Chung-Ang University , 221 Heukseok-dong , Dongjak-gu, Seoul 156-756 , Republic of Korea.

The stamping transfer process, which provides a precise patterning of the target material without the limitation of an underlying layer, has attracted significant attention for large-scale roll-to-roll fabrication. Despite the need to minimize the peeling energy, expressed as the sum of adhesion energies, for a simple transfer process, many studies have not considered this effect. In this study, we introduced a wetting coefficient related with adhesions between polymers for the transfer design of organic photosensitive materials. We observed a difference in adhesion between polymer blends depending on the surface energy of the mold. We designed high-surface-energy polyurethane acrylate to enable a residue-free transfer process. The transfer process significantly contributed to the device stability through changes in dark currents, photocurrents, responsivity, and detectivity over time, compared to spin coating. In particular, the detectivity was maintained over 95% after 360 h, and no burn-in loss of internal resistance was observed in the device with a transferred active layer. X-ray photoelectron spectroscopy showed that a large interfacial change between poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and poly(4,8-bis[(2-ethylhexyl)oxy]benzo[1,2- b:4,5- b']dithiophene-2,6-diyl- alt-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4- b]thiophene-4,6-diyl):[6,6] phenyl C butyric acid methyl ester obtained through spin coating occurred owing to solution penetration, whereas the transfer process provided a constant interface owing to morphology stabilization. Therefore, the transfer process with optimized adhesion properties can improve the device operation durability without burn-in loss, enabling a cost-effective fabrication of organic optoelectronic devices.
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http://dx.doi.org/10.1021/acsami.8b13375DOI Listing
November 2018

Covalent organic nanosheets for effective charge transport layers in planar-type perovskite solar cells.

Nanoscale 2018 Mar;10(10):4708-4717

School of Integrative Engineering, Chung-Ang University, 84 Heukseok-Ro, Dongjak-gu, Seoul 06974, Republic of Korea.

Herein, solvent-treated bandgap-tunable covalent organic nanosheets (CONs) were prepared via the Stille cross-coupling reaction. These materials are considered useful as interlayers in photovoltaic devices upon the alignment of energy levels between other components. Among various types of solar cells, according to the organic-interlayer study, inverted planar perovskite solar cells (PSCs) are mostly demanded to effectively transport and collect charge carriers due to their high performance. At first, the C-V analysis proved the energy levels of the frontier orbitals for CON-10 and CON-16 nanosheets; this verified the suitability of these nanosheets as hole transport layers (HTLs) with the PEDOT:PSS upon casting both films from DMSO. It became evident, however, that the hole transport property of the PEDOT:PSS on the CON-16 layer was unfavorable with the increasing UPS-proven hole injection barrier. In addition, both CONs induced a rough surface morphology; however, CON-10 showed a relatively smooth surface as compared to CON-16 based on the Scanning electron microscopy (SEM) and Atomic force microscopy (AFM) profiles; furthermore, their surface properties influenced both the PEDOT:PSS layers and the perovskite layers. Especially, the XRD profiles presented an enhanced crystallinity of the perovskite layers with CON-10. All these aspects indicate that CON-10 is a more effective HTL material, and several versions of perovskite solar cells (PSCs) have been fabricated with/without CON-10 and CON-16 together with the PEDOT:PSS to determine the more-HTL-suitable CON. As a result, the power conversion efficiencies (PCEs) of the optimized devices with CON-10 exhibited a value of 10.2%, which represented a 1% increase over those of the reference devices without the CONs and was 4% higher than that of the CON-16 devices. Moreover, the devices with CON-10 were further optimized with TiO using Al electrodes, leading to a PCE increase of these devices that became slightly higher than the PCEs of the device with CON-10 and without TiO. This tendency was supported by photoluminescence (PL) spectroscopy, photocurrent density (J), and space-charge-limited current (SCLC) mobility results.
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http://dx.doi.org/10.1039/c7nr08797gDOI Listing
March 2018

Rapid Formation of a Disordered Layer on Monoclinic BiVO : Co-Catalyst-Free Photoelectrochemical Solar Water Splitting.

ChemSusChem 2018 03 5;11(5):933-940. Epub 2018 Feb 5.

Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul, 03722, Republic of Korea.

A surface disordered layer is a plausible approach to improve the photoelectrochemical performance of TiO . However, the formation of a crystalline disordered layer in BiVO and its effectiveness towards photoelectrochemical water splitting has remained a big challenge. Here, we report a rapid solution process (within 5 s) that is able to form a disordered layer of a few nanometers thick on the surface of BiVO nanoparticles using a specific solution with a controllable reducing power. The disordered layer on BiVO alleviates charge recombination at the electrode-electrolyte interface and reduces the onset potential greatly, which in turn results in a photocurrent density of approximately 2.3 mA cm at 1.23 V versus the reversible hydrogen electrode (RHE). This value is 2.1 times higher than that of bare BiVO . The enhanced photoactivity is attributed to the increased charge separation and transfer efficiencies, which resolve the intrinsic drawbacks of bare BiVO such as the short hole diffusion length of around 100 nm and poor surface oxygen evolution reactivity.
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http://dx.doi.org/10.1002/cssc.201702173DOI Listing
March 2018

Dry-Stamping-Transferred PCBM Charge Transport Layer via an Interface-Controlled Polyurethane Acrylate Mold Film for Efficient Planar-Type Perovskite Solar Cells.

ACS Appl Mater Interfaces 2017 May 28;9(18):15623-15630. Epub 2017 Apr 28.

School of Integrative Engineering, Chung-Ang University , 221 Heukseok-dong, Dongjak-gu, Seoul 156-756, Republic of Korea.

The study of interlayers is important to enhance the performance of inverted perovskite solar cells (PSCs) because interlayers in PSCs align energy levels and improve charge transport. However, previous research into applying interlayers for PSCs has focused only on wet-coated methods, such as spin coating, to form the interlayer. Here, we fabricated planar-type PSCs deposited with a 6,6-phenyl-C butyric acid methyl ester (PCBM) layer onto a CHNHPbI (MAPbI) layer by stamping transfer through a relatively dry process condition. We demonstrated the effects of a stamping-transferred PCBM layer using polyurethane acrylate (PUA), the surface energy of which was modified by 2-hydroxyethyl methacrylate (HEMA) to increase the transfer reproducibility. In PSCs with a stamping-transferred PCBM layer, we observed an enhanced J and a comparable power conversion efficiency (PCE), which were caused by an enhanced coverage of the electron transport layer onto the MAPbI layer with preserved crystallinity, which occurs owing to improved electron mobility and exciton dissociation. The optimized device PCE through the dry-transferred PCBM exhibited a J, fill factor, and PCE of 21.65 mA/cm, 76.0%, and 15.46%, respectively. Moreover, morphological analysis and electrical measurements confirmed the improved durability of dry-stamping-transferred PSCs.
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http://dx.doi.org/10.1021/acsami.7b01282DOI Listing
May 2017

Counterbalancing of morphology and conductivity of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate based flexible devices.

Nanoscale 2016 Dec;8(47):19557-19563

School of Integrative Engineering, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 156-756, Republic of Korea.

The importance of conductive polymer electrodes with a balance between the morphology and electrical conductivity for flexible organic photovoltaic properties has been demonstrated. Highly transparent PEDOT:PSS anodes with controlled conductivity and surface properties were realized by insertion of dimethyl sulfoxide (DMSO) and a fluorosurfactant (Zonyl) as efficient additives and used for flexible organic photovoltaic cells (OPVs) which are based on a bulk-heterojunction of polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7):[6,6]phenyl-C-butyric acid methyl ester (PCBM). We investigated the correlation between the electrical properties of PEDOT:PSS electrodes and their influences on the surface morphology of the active materials (PTB7:PCBM). When the device was prepared from the PEDOT:PSS layer functioning as an anode of OPV through an optimized ratio of 5 vol% of DMSO and 0.1 wt% of fluorosurfactant, the devices exhibited improved fill factor (FF) due to the enhanced coverage of PEDOT:PSS films. These results correlate with reduced photoluminescence and increased charge extraction as seen through Raman spectroscopy and electrical analysis, respectively. The conductive polymer electrode with the balance between the morphology and electrical conductivity can be a useful replacement for brittle electrodes such as those made of indium tin oxide (ITO) as they are more resistant to cracking and bending conditions, which will contribute to the long-term operation of flexible devices.
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http://dx.doi.org/10.1039/c6nr05361kDOI Listing
December 2016

A Mechanistic Understanding of a Binary Additive System to Synergistically Boost Efficiency in All-Polymer Solar Cells.

Sci Rep 2015 Dec 11;5:18024. Epub 2015 Dec 11.

POSTECH Organic Electronics Laboratory, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, 790-784, Republic of Korea.

All-polymer solar cells are herein presented utilizing the PBDTTT-CT donor and the P(NDI2OD-T2) acceptor with 1,8-diiodooctane (DIO) and 1-chloronaphthalene (CN) binary solvent additives. A systematic study of the polymer/polymer bulk heterojunction photovoltaic cells processed from the binary additives revealed that the microstructures and photophysics were quite different from those of a pristine system. The combination of DIO and CN with a DIO/CN ratio of 3:1 (3 vol% DIO, 1 vol% CN and 96 vol% o-DCB) led to suitable penetrating polymer networks, efficient charge generation and balanced charge transport, which were all beneficial to improving the efficiency. This improvement is attributed to increase in power conversion efficiency from 2.81% for a device without additives to 4.39% for a device with the binary processing additives. A detailed investigation indicates that the changes in the polymer:polymer interactions resulted in the formation of a percolating nasnoscale morphology upon processing with the binary additives. Depth profile measurements with a two-dimensional grazing incidence wide-angle X-ray scattering confirm this optimum phase feature. Furthermore impedance spectroscopy also finds evidence for synergistically boosting the device performance.
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http://dx.doi.org/10.1038/srep18024DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4676009PMC
December 2015

Self-Position of Au NPs in Perovskite Solar Cells: Optical and Electrical Contribution.

ACS Appl Mater Interfaces 2016 Jan 23;8(1):449-54. Epub 2015 Dec 23.

Department of Chemical and Biomolecular Engineering, Yonsei University , 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea.

Metallic nanoparticles (NPs) exhibit a localized surface plasmon resonance (LSPR) and act as scattering centers and subwavelength antennas, so metallic NPs can be incorporated into perovskite solar cells (PSCs) to effectively improve the light absorption of light harvesting devices. Here, we have embedded Au nanoparticles (NPs) into the hole transport layer (HTL) of the PSCs to investigate the photovoltaic effect of the PSCs with Au NPs. Interestingly, it was found that Au NPs dispersed spiro-OMeTAD HTL solution could naturally end up located near the perovskite layer as the result of the spin-coating step. Solar cell performance observations indicate that the LSPR and electrical effects of Au NPs enhance the photovoltaic response of PSCs, in spite of a slight decrease in the open-circuit voltage (VOC), by causing an incredible improvement in the photocurrent density as a dominant factor.
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http://dx.doi.org/10.1021/acsami.5b09365DOI Listing
January 2016

Surface-Engineered Graphene Quantum Dots Incorporated into Polymer Layers for High Performance Organic Photovoltaics.

Sci Rep 2015 Sep 22;5:14276. Epub 2015 Sep 22.

Department of Chemistry, College of Natural Sciences, Seoul National University, Gwanak-ro 1, Gwanak-gu, Seoul, 151-747, Republic of Korea.

Unlabelled: Graphene quantum dots (GQDs), a newly emerging 0-dimensional graphene based material, have been widely exploited in optoelectronic devices due to their tunable optical and electronic properties depending on their functional groups. Moreover, the dispersibility of GQDs in common solvents depending on hydrophobicity or hydrophilicity can be controlled by chemical functionalization, which is particularly important for homogeneous incorporation into various polymer layers. Here we report that a surface-engineered GQD-incorporated polymer photovoltaic device shows enhanced power conversion efficiency (PCE), where the oxygen-related functionalization of GQDs enabled good dispersity in a

Pedot: PSS hole extraction layer, leading to significantly improved short circuit current density (Jsc) value. To maximize the PCE of the device, hydrophobic GQDs that are hydrothermally reduced (rGQD) were additionally incorporated in a bulk-heterojunction layer, which is found to promote a synergistic effect with the GQD-incorporated hole extraction layer.
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http://dx.doi.org/10.1038/srep14276DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4585740PMC
September 2015

Enhanced fill factor of tandem organic solar cells incorporating a diketopyrrolopyrrole-based low-bandgap polymer and optimized interlayer.

ChemSusChem 2015 Jan 17;8(2):331-6. Epub 2014 Nov 17.

School of Integrative Engineering, Chung-Ang University, 84 Heukseok-Ro, Dongjak-gu, Seoul 156-756 (Republic of Korea).

We demonstrate that reproducible results can be obtained from tandem solar cells based on the wide-bandgap poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2',1',3'-benzothiadiazole] (PCDTBT) and the diketopyrrolopyrrole (DPP)-based narrow bandgap polymer (DT-PDPP2T-TT) with a decyltetradecyl (DT) and an electron-rich 2,5-di-2-thienylthieno[3,2-b]thiophene (2T-TT) group fabricated using an optimized interlayer (ZnO NPs/ph-n-PEDOT:PSS) [NPs: nanoparticles; ph-n: pH-neutral PEDOT: poly(3,4-ethylenedioxythiophene); PSS: polystyrene sulfonate]. The tandem cells are fabricated by applying a simple process without thermal annealing. The ZnO NP interlayer operates well when the ZnO NPs are dispersed in 2-methoxyethanol, as no precipitation and chemical reactions occur. In addition to the ZnO NP film, we used neutral PEDOT:PSS as a second interlayer which is not affect to the sequential deposited bulk heterojunction (BHJ) active layer of acidification. The power conversion efficiency (PCE) of a tandem device reaches 7.4 % (open-circuit voltage VOC =1.53 V, short-circuit current density JSC =7.3 mA cm(-2) , and fill factor FF=67 %). Furthermore, FF is increased to up to 71 % when another promising large bandgap (bandgap ∼1.94 eV) polymer (PBnDT-FTAZ) is used. The surface of each layer with nanoscale morphology (BHJ1/ZnO NPs film/ph-n-PEDOT:PSS/BHJ2) was examined by means of AFM analysis during sequential processing. The combination of these factors, efficient DPP-based narrow bandgap material and optimized interlayer, leads to the high FF (average approaches 70 %) and reproducibly operating tandem BHJ solar cells.
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http://dx.doi.org/10.1002/cssc.201402833DOI Listing
January 2015

Efficient solution-processed small-molecule solar cells by insertion of graphene quantum dots.

Nanoscale 2014 Dec 6;6(24):15175-80. Epub 2014 Nov 6.

School of Integrative Engineering, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 156-756, Republic of Korea.

In this work, we have demonstrated the results of several positive effects that arise from the addition of graphene quantum dots (GQDs) to solution-processed small molecule bulk-heterojunction (SM-BHJ) solar cells fabricated from a p-DTS(FBTTh(2))(2)/[6,6]-phenyl C(71) butyric acid methyl-ester (PC(71)BM). The device with an optimized ratio of GQDs exhibits increased current density and fill factor owing to 10% improved external quantum efficiency (EQE) and induction of a favorable SM-BHJ morphology. Additionally, the multiple scattering of the GQDs in the SM-BHJ leads to longer optical pathlengths according to the analysis of diffuse reflectance spectra and UV/Vis absorption spectra. The GQD inserted SM-BHJ film at the optimized concentration exhibits decreased charge transport resistance significantly by impedance measurements with effective charge extraction in the device which contributes to 15% enhancement of power conversion efficiency (PCE).
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http://dx.doi.org/10.1039/c4nr04944fDOI Listing
December 2014

Tailoring dispersion and aggregation of Au nanoparticles in the BHJ layer of polymer solar cells: plasmon effects versus electrical effects.

ChemSusChem 2014 Dec 21;7(12):3452-8. Epub 2014 Oct 21.

School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746 (Republic of Korea).

Plasmonic effects that arise from embedding metallic nanoparticles (NPs) in polymer solar cells (PSCs) have been extensively studied. Many researchers have utilized metallic NPs in PSCs by either incorporating them into the PSC interlayers (e.g., the hole extraction and electron extraction layers) or blending them into the bulk heterojunction (BHJ) active layer. In such studies, the dispersity of the metallic NPs in each layer may vary due to both the different nature of the ligands and the amount of ligands on the metallic NPs. This in turn can produce different PSC performance parameters. Here, we systematically control the amount of attached organic ligands on Au NPs to control their dispersion behavior in the BHJ active layer of PSCs. By controlling the number of capping organic ligands on the Au NPs, the dispersity of the NPs in the BHJ layer is also controlled and the positive effects (particularly the plasmonic and electrical effects) of the Au NPs in the PSCs are investigated. From the obtained results, we find that the electrical contribution of the Au NPs is a more dominant factor for enhancing cell efficiency when compared to the plasmonic effect.
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http://dx.doi.org/10.1002/cssc.201402511DOI Listing
December 2014

Enhanced performance and stability of polymer BHJ photovoltaic devices from dry transfer of PEDOT:PSS.

ChemSusChem 2014 Jul 2;7(7):1957-63. Epub 2014 Jul 2.

SKKU Advanced Institute of Nanotechnology (SAINT) and School of Chemical Engineering, Sungkyunkwan University, Suwon, 440-746 (Republic of Korea).

Polymer solar cells with enhanced initial cell performances and long-term stability were fabricated by performing a simple dry transfer of a hole extraction layer [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)] onto an indium tin oxide (ITO) substrate. Due to the very flat surface of the polyurethane acrylate/polycarbonate (PUA/PC) film, which was used as a mold and resembled the surface of the original substrate (silicon wafer), the transferred layer had a very smooth surface morphology, resulting in enhancement of the interfacial characteristics. The work function of the PEDOT:PSS layer and the morphology of bulk hetero junction (BHJ) layer were tuned by controlling the position of PSS enrichment in the PEDOT:PSS layer using the dry transfer. The power conversion efficiency of PTB7:PC71 BM BHJ device prepared by the dry transfer was 8.06%, which was significantly higher than that of the spin-cast device (7.32%). By avoiding direct contact between the ITO substrate and the PEDOT:PSS solution in the dry transfer system, etching and diffusion of indium in the ITO substrate were greatly reduced, thereby improving the stability.
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http://dx.doi.org/10.1002/cssc.201400022DOI Listing
July 2014

Balancing light absorptivity and carrier conductivity of graphene quantum dots for high-efficiency bulk heterojunction solar cells.

ACS Nano 2013 Aug 2;7(8):7207-12. Epub 2013 Aug 2.

SKKU Advanced Institute of Nano Technology and School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea.

Graphene quantum dots (GQDs) have been considered as a novel material because their electronic and optoelectronic properties can be tuned by controlling the size and the functional groups of GQDs. Here we report the synthesis of reduction-controlled GQDs and their application to bulk heterojunction (BHJ) solar cells with enhanced power conversion efficiency (PCE). Three different types of GQDs--graphene oxide quantum dots (GOQDs), 5 h reduced GQDs, and 10 h reduced GQDs--were tested in BHJ solar cells, and the results indicate that GQDs play an important role in increasing optical absorptivity and charge carrier extraction of the BHJ solar cells. The enhanced optical absorptivity by rich functional groups in GOQDs increases short-circuit current, while the improved conductivity of reduced GQDs leads to the increase of fill factors. Thus, the reduction level of GQDs needs to be intermediate to balance the absorptivity and conductivity. Indeed, the partially reduced GQDs yielded the outstandingly improved PCE of 7.60% in BHJ devices compared to a reference device without GQDs (6.70%).
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http://dx.doi.org/10.1021/nn402606vDOI Listing
August 2013

Improved light harvesting and improved efficiency by insertion of an optical spacer (ZnO) in solution-processed small-molecule solar cells.

Nano Lett 2013 Aug 3;13(8):3796-801. Epub 2013 Jul 3.

Center for Polymers and Organic Solids, University of California at Santa Barbara, Santa Barbara, California 93106-5090, United States.

We demonstrate that the power conversion efficiency can be significantly improved in solution-processed small-molecule solar cells by tuning the thickness of the active layer and inserting an optical spacer (ZnO) between the active layer and the Al electrode. The enhancement in light absorption in the cell was measured with UV-vis absorption spectroscopy and by measurements of the photoinduced carriers generation rate. The ZnO layer used to improve the light-harvesting increases the charge collection efficiency, serves as a blocking layer for holes, and reduces the recombination rate. The combined optical and electrical improvements raise the power conversion efficiency of solution-processed small-molecule solar cells to 8.9%, that is, comparable to that of polymer counterparts.
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http://dx.doi.org/10.1021/nl401758gDOI Listing
August 2013

Barium: an efficient cathode layer for bulk-heterojunction solar cells.

Sci Rep 2013 ;3:1965

Center for Polymers and Organic Solids, University of California at Santa Barbara, Santa Barbara, California 93106-5090, USA.

We report Barium (Ba) cathode layer for bulk-heterojunction solar cells which enhanced the fill factor (FF) of p-DTS(FBTTh2)2/PC71BM BHJ solar cell up to 75.1%, one of the highest value reported for an organic solar cell. The external quantum efficiency exceeds 80%. Analysis of recombination mechanisms using the current-voltage (J-V) characteristics at various light intensities in the BHJ solar cell layer reveals that Ba prevents trap assisted Shockley-Read-Hall (SRH) recombination at the interface and with different thicknesses of the Ba, the recombination shifts towards bimolecular from monomolecular. Moreover, Ba increases shunt resistance and decreases the series resistance significantly. This results in an increase in the charge collection probability leading to high FF. This work identifies a new cathode interlayer which outclasses the all the reported interlayers in increasing FF leading to high power conversion efficiency and have significant implications in improving the performance of BHJ solar cells.
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http://dx.doi.org/10.1038/srep01965DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3678135PMC
October 2013

Polymer bulk heterojunction solar cells with PEDOT:PSS bilayer structure as hole extraction layer.

ChemSusChem 2013 Jun 9;6(6):1070-5. Epub 2013 May 9.

School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea.

A high current density obtained in a limited, nanometer-thick region is important for high efficiency polymer solar cells (PSCs). The conversion of incident photons to charge carriers only occurs in confined active layers; therefore, charge-carrier extraction from the active layer within the device by using solar light has an important impact on the current density and the related to power conversion efficiency. In this study, we observed a surprising result, that is, extracting the charge carrier generated in the active layer of a PSC device, with a thickness-controlled PEDOT:PSS bilayer that acted as a hole extraction layer (HEL), yielded a dramatically improved power conversion efficiency in two different model systems (P3HT:PC₆₀BM and PCDTBT:PC₇₀BM). To understand this phenomenon, we conducted optical strength simulation, photocurrent-voltage measurements, incident photon to charge carrier efficiency measurements, ultraviolet photoelectron spectroscopy, and AFM studies. The results revealed that approximately 60 nm was the optimum PEDOT:PSS bilayer HEL thickness in PSCs for producing the maximum power conversion efficiency.
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http://dx.doi.org/10.1002/cssc.201200950DOI Listing
June 2013

Intensity dependence of current-voltage characteristics and recombination in high-efficiency solution-processed small-molecule solar cells.

ACS Nano 2013 May 24;7(5):4569-77. Epub 2013 Apr 24.

Center for Polymers and Organic Solids, University of California at Santa Barbara, Santa Barbara, California 93106-5090, United States.

Solution-processed small-molecule p-DTS(FBTTh2)2:PC71BM bulk heterojunction (BHJ) solar cells with power conversion efficiency of 8.01% are demonstrated. The fill factor (FF) is sensitive to the thickness of a calcium layer between the BHJ layer and the Al cathode; for 20 nm Ca thickness, the FF is 73%, the highest value reported for an organic solar cell. The maximum external quantum efficiency exceeds 80%. After correcting for the total absorption in the cell through normal incidence reflectance measurements, the internal quantum efficiency approaches 100% in the spectral range of 600-650 nm and well over 80% across the entire spectral range from 400 to 700 nm. Analysis of the current-voltage (J-V) characteristics at various light intensities provides information on the different recombination mechanisms in the BHJ solar cells with different thicknesses of the Ca layer. Our analysis reveals that the J-V curves are dominated by first-order recombination from the short-circuit condition to the maximum power point and evolve to bimolecular recombination in the range of voltage from the maximum power point to the open-circuit condition in the optimized device with a Ca thickness of 20 nm. In addition, the normalized photocurrent density curves reveal that the charge collection probability remains high; about 90% of charges are collected even at the maximum power point. The dominance of bimolecular recombination only when approaching open circuit, the lack of Shockley-Read-Hall recombination at open circuit, and the high charge collection probability (97.6% at the short circuit and constant over wide range of applied voltage) lead to the high fill factor.
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http://dx.doi.org/10.1021/nn401267sDOI Listing
May 2013

Layer-by-layer all-transfer-based organic solar cells.

Langmuir 2013 Apr 16;29(17):5377-82. Epub 2013 Apr 16.

SKKU Advanced Institute of Nanotechnology (SAINT) and School of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon 440-746, Republic of Korea.

For the first time, we describe a novel cost- and time-effective vacuum-free process to fabricate bulk-heterojunction (BHJ) organic photovoltaics (OPVs) via layer-by-layer selective stamping transfer of all layers. By controlling the surface properties of polyurethane acrylate (PUA) stamping molds with ultraviolet (UV)-ozone (UVO) exposure, poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS), BHJ layer, and metal cathode were uniformly transferred layer by layer onto each of the bottom layers. Among several interfaces between each layer, we found that the interface between the active layer and metal cathode is a critical factor in obtaining conventional device-like efficiency. To enhance the interfacial connectivity between the BHJ layer and metal cathode and increase electron extraction from the BHJ layer, a titanium oxide (TiOx) interlayer was introduced. Cell performance was optimized by controlling the concentration of TiOx solution. The poly(3-hexylthiophene-2,5-diyl)/[6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PC60BM) BHJ device fabricated by transferring PEDOT/PSS, TiOx/active layer, and Al cathode showed 2.01% power conversion efficiency. This efficiency is not comparable to those of conventional OPVs, but our approach shows the possibility of fabricating OPVs via the layer-by-layer transfer method for the first time.
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http://dx.doi.org/10.1021/la400137gDOI Listing
April 2013

Efficient solution-processed small-molecule solar cells with inverted structure.

Adv Mater 2013 May 1;25(17):2397-402. Epub 2013 Mar 1.

Center for Polymers and Organic Solids, University of California at Santa Barbara, Santa Barbara, California 93106-5090, USA.

We successfully demonstrate inverted structure small-molecule (SM) solar cells with an efficiency of 7.88% using ZnO and PEIE as an interfacial layer. Modification of ZnO with a cost-effective PEIE thin layer increases the efficiency of the inverted cell as a result of reducing the work function of the cathode and suppressing the trap-assisted recombination. In addition to the high efficiency, the inverted SM solar cells are relatively stable in air compared to conventional cells.
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http://dx.doi.org/10.1002/adma.201300295DOI Listing
May 2013
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