Evaluating the Optoelectronic Quality of Hybrid Perovskites by Conductive Atomic Force Microscopy with Noise Spectroscopy

Lee, Byungho,Lee, Sangheon,Cho, Duckhyung,Kim, Jinhyun,Hwang, Taehyun,Kim, Kyung Hwan,Hong, Seunghun,Moon, Taeho,Park, Byungwoo American Chemical Society 2016 ACS APPLIED MATERIALS & INTERFACES Vol.8 No.45

<P>Organic-inorganic hybrid perovskite solar cells have emerged as promising candidates for next-generation solar cells. To attain high photovoltaic efficiency, reducing the defects in perovskites is crucial along with a uniform coating of the films. Also, evaluating the quality of synthesized perovskites via facile and adequate methods is important as well. Herein, CH3NH3PbI3 perovskites were synthesized by applying second solvent dripping to nonstoichiometric precursors containing excess CH3NH3I. The resulting perovskite films exhibited a larger average grain size with a better crystallinity compared to that from stoichiometric precursors. As a result, the performance of planar perovskite solar cells was significantly improved, achieving an efficiency of 14.3%. Furthermore, perovskite films were effectively analyzed using a conductive AFM and noise spectroscopy, which have been uncommon in the field of perovskite solar cells. Comparing the topography and photocurrent maps, the variation of photocurrents in nanoscale was systematically investigated, and a linear relationship between the grain size and photocurrent was revealed. Also, noise analyses with a conductive probe enabled examination of the defect density of perovskites at specific grain interiors by excluding the grain-boundary effect, and reduced defects were clearly observed for the perovskites using CH3NH3I-rich precursors.</P>

Nanoscale Direct Mapping of Noise Source Activities on Graphene Domains

Lee, Hyungwoo,Cho, Duckhyung,Shekhar, Shashank,Kim, Jeongsu,Park, Jaesung,Hong, Byung Hee,Hong, Seunghun American Chemical Society 2016 ACS NANO Vol.10 No.11

<P>An electrical noise is one of the key parameters determining the performance of modern electronic devices. However, it has been extremely difficult, if not impossible, to image localized noise sources or their activities in such devices. We report a 'noise spectral imaging' strategy to map the activities of localized noise sources in graphene domains. Using this method, we could quantitatively estimate sheet resistances and noise source densities inside graphene domains, on domain boundaries and on the edge of graphene. The results show high activities of noise sources and large sheet resistance values at the domain boundary and edge of graphene. Additionally, we showed that the top layer in double layer graphene had lower noises than single-layer graphene. This work provides valuable insights about the electrical noises of graphene. Furthermore, the capability to directly map noise sources in electronic channels can be a major breakthrough in electrical noise research in general.</P>

Nanoscale Mapping of Molecular Vibrational Modes via Vibrational Noise Spectroscopy

Cho, Duckhyung,Shekhar, Shashank,Lee, Hyungwoo,Hong, Seunghun American Chemical Society 2018 NANO LETTERS Vol.18 No.2

<P>We have developed a 'vibrational noise spectroscopy (VNS)' method to identify and map vibrational modes of molecular wires on a solid substrate. In the method, electrical-noises generated in molecules on a conducting substrate were measured using a conducting atomic force microscopy (AFM) with a nanoresolution. We found that the bias voltage applied to the conducting AFM probe can stimulate specific vibrational modes of measured molecules, resulting in enhanced electrical noises. Thus, by analyzing noise-voltage spectra, we could identify various vibrational modes of the molecular wires on the substrates. Further, we could image the distribution of vibrational modes on molecule patterns on the substrates. In addition, we found that VNS imaging data could be further analyzed to quantitatively estimate the density of a specific vibrational mode in the layers of different molecular species. The VNS method allows one to measure molecular vibrational modes under ambient conditions with a nanoresolution, and thus it can be a powerful tool for nanoscale electronics and materials researches in general.</P>

Nanoscale mapping of noise-source-controlled hopping and tunneling conduction in domains of reduced graphene oxide

Shekhar, Shashank,Lee, Hyungwoo,Cho, Duckhyung,Yang, Myungjae,Lee, Minju,Hong, Seunghun Elsevier 2019 Carbon Vol.148 No.-

<P><B>Abstract</B></P> <P>We report a nanoscale mapping of noise-source-controlled transport characteristics in the domains of reduced graphene oxide by utilizing noise-source imaging strategies. In this method, current and noise images were measured simultaneously using a scanning noise microscopy and analyzed to map sheet−resistances (<I>R</I> <SUB>□</SUB>) and noise−source densities (<I>n</I> <SUB>eff</SUB>). The maps showed the formation of conducting and insulating domains, where the insulating domains exhibited up to three-four orders of higher <I>R</I> <SUB>□</SUB> and <I>n</I> <SUB>eff</SUB> than those of conducting domains. Interestingly, the sheet−conductance (<I>Σ</I> <SUB>□</SUB>) and <I>n</I> <SUB>eff</SUB> followed rather opposite power−law behaviors like <I>Σ</I> <SUB>□</SUB>∝ <I>n</I> <SUB>eff</SUB> <SUP>−0.5</SUP> and <I>Σ</I> <SUB>□</SUB> ∝ <I>n</I> <SUB>eff</SUB> <SUP>0.5</SUP> in <I>conducting</I> and <I>insulating</I> domains, respectively, which could be attributed to the difference in mesoscopic charge transport mechanisms controlled by <I>n</I> <SUB>eff</SUB> in domains. Notably, high biases resulted in the increased conductance (Δ<I>Σ</I> <SUB>□</SUB>) and decreased noise−source density (Δ<I>n</I> <SUB>eff</SUB>) following a relationship like Δ<I>Σ</I> <SUB>□</SUB> ∝−Δ<I>n</I> <SUB>eff</SUB> <SUP>0.5</SUP> for both conducting and insulting domains, which could be explained by the passivation of noise−sources at high biases. Furthermore, Δ<I>Σ</I> <SUB>□</SUB> <I>versus</I> Δ<I>n</I> <SUB>eff</SUB> plot on the annealing also followed a power−law dependence (Δ<I>Σ</I> <SUB>□</SUB> ∝−Δ<I>n</I> <SUB>eff</SUB> <SUP>0.5</SUP>) in conducting domains, which could be attributed to carrier generation on the annealing. Our results about mesoscopic charge transports could be significant advancements in fundamental researches and applications.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Extraction of Vertical, Lateral Locations and Energies of Hot-Electrons-Induced Traps through the Random Telegraph Noise

Kang, Daewoong,Kim, Jinho,Lee, Duckhyung,Park, Byung-Gook,Lee, Jong Duk,Shin, Hyungcheol IOP Publishing 2009 Japanese journal of applied physics Vol.48 No.4

Investigation of chlorine-mediated microstructural evolution of CH₃NH₃PbI₃(Cl) grains for high optoelectronic responses

Hwang, Taehyun,Cho, Duckhyung,Kim, Jinhyun,Kim, Jaewon,Lee, Sangheon,Lee, Byungho,Kim, Kyung Hwan,Hong, Seunghun,Kim, Chunjoong,Park, Byungwoo Elsevier 2016 Nano energy Vol.25 No.-

<P><B>Abstract</B></P> <P>An organic-inorganic hybrid perovskite is considered as a next generation solar energy harvester due to the high power conversion efficiency. The starting precursor solution for the organolead halide perovskite is of significant interests because the ionic components in the precursor can critically affect the nanostructures and thereby the optoelectronic properties. In this work, the basic and well-known precursor solution for CH<SUB>3</SUB>NH<SUB>3</SUB>PbI<SUB>3</SUB>(Cl) comprised of CH<SUB>3</SUB>NH<SUB>3</SUB>I and PbCl<SUB>2</SUB>, is specifically analyzed to unravel the phenomena in the Cl-mediated solutions. The shift in equilibrium between lead-halide complex and the solvent results into the CH<SUB>3</SUB>NH<SUB>3</SUB>PbI<SUB>3</SUB>(Cl) grain evolution with Cl incorporation, which is confirmed through x-ray fluorescence and diffraction. The effects of Cl on the optoelectronic properties are further verified by conductive atomic force microscopy, and the existing Cl leads to the 30-times-increased and inhomogeneously distributed photocurrent for CH<SUB>3</SUB>NH<SUB>3</SUB>PbI<SUB>3</SUB>(Cl) grains compared with CH<SUB>3</SUB>NH<SUB>3</SUB>PbI<SUB>3</SUB>. Moreover, photocurrent noise from the mixed-halide perovskite is reduced than that from the triiodide perovskite phase. Combining the microstructural evolution with the optoelectronic properties of mixed-halide perovskite, it is concluded that additional Cl reduces the defects of recombination centers resulting high photocurrent.</P> <P><B>Highlights</B></P> <P> <UL> <LI> CH<SUB>3</SUB>NH<SUB>3</SUB>PbI<SUB>3</SUB>(Cl) grain evolution from the equilibrium shift in the precursor. </LI> <LI> Grain-dependent and inhomogeneous conductive-AFM photocurrents in CH<SUB>3</SUB>NH<SUB>3</SUB>PbI<SUB>3</SUB>(Cl). </LI> <LI> Defects reduction in CH<SUB>3</SUB>NH<SUB>3</SUB>PbI<SUB>3</SUB>(Cl) than CH<SUB>3</SUB>NH<SUB>3</SUB>PbI<SUB>3</SUB> by scanning noise microscopy. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>Microstructural and optoelectronic roles of chlorine in CH<SUB>3</SUB>NH<SUB>3</SUB>PbI<SUB>3</SUB> perovskite are explored. The Cl-mediated CH<SUB>3</SUB>NH<SUB>3</SUB>PbI<SUB>3</SUB>(Cl) grain evolution is investigated from the shift in equlibrium between lead-halide complex and the solvent of the precursor. The evidences for the Cl incorporation in the crystalline phase are further characterized by x-ray fluorescence and diffraction. Optoelectronic properties are compared between CH<SUB>3</SUB>NH<SUB>3</SUB>PbI<SUB>3</SUB>(Cl) and CH<SUB>3</SUB>NH<SUB>3</SUB>PbI<SUB>3</SUB>, and 30-times-enhanced photocurrent in the CH<SUB>3</SUB>NH<SUB>3</SUB>PbI<SUB>3</SUB>(Cl) grains are observed with inhomogeneous distribution, compared with CH<SUB>3</SUB>NH<SUB>3</SUB>PbI<SUB>3</SUB>. The origin of high optoelectronic responses is explained by the Cl-induced reduction of defects and recombination centers, as confirmed by the noise-microscopy comparison.</P> <P>[DISPLAY OMISSION]</P>

Plasmon–Exciton Interactions in Hybrid Structures of Au Nanohemispheres and CdS Nanowires for Improved Photoconductive Devices

Maaroof, Abbas I.,Lee, Hyungwoo,Heo, Kwang,Park, June,Cho, Duckhyung,Lee, Byung Yang,Seong, Maeng-Je,Hong, Seunghun American Chemical Society 2013 JOURNAL OF PHYSICAL CHEMISTRY C - Vol.117 No.46

<P>We developed a method to prepare photoconductive channels of hybrid nanostructures based on Au nanohemispheres (Au NHs) and cadmium sulfide nanowires (CdS NWs) for photoconductive applications. The hybrid nanostructures exhibited enhanced absorbance, which was attributed to the strong electronic coupling between plasmons in Au NHs and excitons in CdS NWs. The photoconductive channels based on the hybrid nanostructures showed an increase in photocurrent level by ∼5× compared to bare CdS NWs. Because our strategy can be utilized to improve the photoconductivity of quite versatile photoconductive channels, it can be a powerful method for the development of various advanced optoelectronic and photovoltaic devices.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2013/jpccck.2013.117.issue-46/jp407446s/production/images/medium/jp-2013-07446s_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp407446s'>ACS Electronic Supporting Info</A></P>

Extraction of Interface-States Energy Distribution in Nitrided and Pure Gate Dielectrics for Metal Oxide Semiconductor Field Effect Transistor Applications

Son, Younghwan,Yang, Seungwon,Kim, Bong Chan,Kim, Jinho,Moon, Chang-Rok,Lee, Duckhyung,Lee, Jong-Duk,Park, Byung-Gook,Shin, Hyungcheol IOP Publishing 2009 Japanese journal of applied physics Vol.48 No.r5