Showing papers by "Min Jae Ko published in 2017"

Inorganic Rubidium Cation as an Enhancer for Photovoltaic Performance and Moisture Stability of HC(NH2)2PbI3 Perovskite Solar Cells

Abstract: Perovskite solar cells (PSCs) based on organic monovalent cation (methylammonium or formamidinium) have shown excellent optoelectronic properties with high efficiencies above 22%, threatening the status of silicon solar cells. However, critical issues of long-term stability have to be solved for commercialization. The severe weakness of the state-of-the-art PSCs against moisture originates mainly from the hygroscopic organic cations. Here, rubidium (Rb) is suggested as a promising candidate for an inorganic–organic mixed cation system to enhance moisture-tolerance and photovoltaic performances of formamidinium lead iodide (FAPbI3). Partial incorporation of Rb in FAPbI3 tunes the tolerance factor and stabilizes the photoactive perovskite structure. Phase conversion from hexagonal yellow FAPbI3 to trigonal black FAPbI3 becomes favored when Rb is introduced. The authors find that the absorbance and fluorescence lifetime of 5% Rb-incorporated FAPbI3 (Rb0.05FA0.95PbI3) are enhanced than bare FAPbI3. Rb0.05FA0.95PbI3-based PSCs exhibit a best power conversion efficiency of 17.16%, which is much higher than that of the FAPbI3 device (13.56%). Moreover, it is demonstrated that the Rb0.05FA0.95PbI3 film shows superior stability against high humidity (85%) and the full device made with the mixed perovskite exhibits remarkable long-term stability under ambient condition without encapsulation, retaining the high performance for 1000 h.

Effect of Molecular Orientation of Donor Polymers on Charge Generation and Photovoltaic Properties in Bulk Heterojunction All‐Polymer Solar Cells

Abstract: All-polymer solar cells (all-PSCs) utilizing p-type polymers as electron-donors and n -typepolymers as electron-acceptors have attracted a great deal of attention, and their efficiencies have been improved considerably. Here, five polymer donors with different molecular orientations are synthesized by random copolymerization of 5-fluoro-2,1,3-benzothiadiazole with different relative amounts of 2,2′-bithiophene (2T) and dithieno[3,2-b;2′,3′-d]thiophene (DTT). Solar cells are prepared by blending the polymer donors with a naphthalene diimide-based polymer acceptor (PNDI) or a [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) acceptor and their morphologies and crystallinity as well as optoelectronic, charge-transport and photovoltaic properties are studied. Interestingly, charge generation in the solar cells is found to show higher dependence on the crystal orientation of the donor polymer for the PNDI-based all-PSCs than for the conventional PC71BM-based PSCs. As the population of face-on-oriented crystallites of the donor increased in PNDI-based PSC, the short-circuit current density (JSC) and external quantum efficiency of the devices are found to significantly improve. Consequently, device efficiency was enhanced of all-PSC from 3.11% to 6.01%. The study reveals that producing the same crystal orientation between the polymer donor and acceptor (face-on/face-on) is important in all-PSCs because they provide efficient charge transfer at the donor/acceptor interface.

Solution-Processed Ultrathin TiO2 Compact Layer Hybridized with Mesoporous TiO2 for High-Performance Perovskite Solar Cells

Abstract: The electron transport layer (ETL) is a key component of perovskite solar cells (PSCs) and must provide efficient electron extraction and collection while minimizing the charge recombination at interfaces in order to ensure high performance. Conventional bilayered TiO2 ETLs fabricated by depositing compact TiO2 (c-TiO2) and mesoporous TiO2 (mp-TiO2) in sequence exhibit resistive losses due to the contact resistance at the c-TiO2/mp-TiO2 interface and the series resistance arising from the intrinsically low conductivity of TiO2. Herein, to minimize such resistive losses, we developed a novel ETL consisting of an ultrathin c-TiO2 layer hybridized with mp-TiO2, which is fabricated by performing one-step spin-coating of a mp-TiO2 solution containing a small amount of titanium diisopropoxide bis(acetylacetonate) (TAA). By using electron microscopies and elemental mapping analysis, we establish that the optimal concentration of TAA produces an ultrathin blocking layer with a thickness of ∼3 nm and ensures that ...

Single‐Mode Distributed Feedback Laser Operation in Solution‐Processed Halide Perovskite Alloy System

Abstract: A single-mode laser operation from a solution-processed halide perovskite alloy system, CH(NH2)2Pb(I1−xBrx)3 (0 ≤ x ≤ 1), is reported. Despite its simplicity, the solution process results in halide perovskite alloy films of smooth and uniform surface morphology that exhibits characteristics of bandgap engineering, where t he bandgap and thus emission wavelength can be tuned (from near-infrared to green) by controlling the anion composition ratio x. A set of thin films of the halide perovskite alloy are coated on top of the fused quartz substrates with engraved surface gratings and are optically excited to induce distributed feedback lasing action. Laser devices operate in single modes over a wide range of wavelengths, depending on the composition ratio, up to room temperature for x = 0 and 1 and up to 200 K for alloys. This study demonstrates control of the polarization of the single-mode laser output by carefully choosing the grating period. The achievement is an important milestone toward the advancement in the applicability of halide perovskite materials to optoelectronic devices.

Development of Highly Crystalline Donor–Acceptor-Type Random Polymers for High Performance Large-Area Organic Solar Cells

Abstract: We developed donor–acceptor (D–A)-type random polymers based on 3,3′-difluoro-2,2′-bithiophene with various relative amounts of 5,6-difluoro-4,7-bis(5-bromo-(2-decyltetradecyl)thiophen-2-yl)-2,1,3-benzothiadiazole (2FBT) and 5,6-difluoro-4,7-bis(5-bromo-(2-octyldodecyl)thiophen-2-yl)-2-(3,4-dichlorobenzyloxybutyl)-2H-benzo[d][1,2,3]triazole (DCB-2FBTZ). Introducing small relative amounts of DCB-2FBTZ into the polymer was found to effectively enhance its solar cell performance, resulting in a power conversion efficiency of 9.02%, greater than the 7.29% that resulted from the PFBT-FTh copolymer. Moreover, when the active area of the BHJ film was increased to 1 cm2, the solar cell reproducibly showed a high performance, here with an efficiency of 8.01% even when the thickness of the active layer was 313 nm. Our studies revealed that including the DCB-2FBTZ group in the polymer simultaneously improved the solution processability and crystallinity of the polymer. These improvements resulted in the formation of...

Electron transport layer for flexible perovskite solar cell and flexible perovskite solar cell including the same

Abstract: Disclosed is an electron transport layer for a flexible perovskite solar cell. The electron transport layer includes transition metal-doped titanium dioxide particles. The titanium dioxide particles are densely packed in the electron transport layer. The electron transport layer is transparent. The use of the electron transport layer enables the fabrication of a flexible perovskite solar cell with high power conversion efficiency. Also disclosed is a flexible perovskite solar cell employing the electron transport layer.

Organic solar cell and method for fabricating the same

Abstract: An organic solar cell is provided. The organic solar cell includes a photoactive layer in which a low molecular weight conjugated compound as a first organic semiconductor material is mixed with an appropriate amount of a second organic semiconductor material. The first organic semiconductor material includes both electron donors and electron acceptors. The presence of the electron donors and the electron acceptors in the first organic semiconductor material improves the morphology of the photoactive layer, leading to high efficiency of the organic solar cell.