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Namyoung Ahn

Other affiliations: Los Alamos National Laboratory
Bio: Namyoung Ahn is an academic researcher from Seoul National University. The author has contributed to research in topics: Perovskite (structure) & Perovskite solar cell. The author has an hindex of 17, co-authored 37 publications receiving 4277 citations. Previous affiliations of Namyoung Ahn include Los Alamos National Laboratory.

Papers
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Journal ArticleDOI
TL;DR: High efficiency perovskite solar cells were fabricated reproducibly via Lewis base adduct of lead(II) iodide through interaction between Lewis base DMSO and/or iodide and Lewis acid PbI2 through spin-coating of a DMF solution.
Abstract: High efficiency perovskite solar cells were fabricated reproducibly via Lewis base adduct of lead(II) iodide. PbI2 was dissolved in N,N-dimethyformamide with equimolar N,N-dimethyl sulfoxide (DMSO) and CH3NH3I. Stretching vibration of S═O appeared at 1045 cm–1 for bare DMSO, which was shifted to 1020 and 1015 cm–1 upon reacting DMSO with PbI2 and PbI2 + CH3NH3I, respectively, indicative of forming the adduct of PbI2·DMSO and CH3NH3I·PbI2·DMSO due to interaction between Lewis base DMSO and/or iodide (I–) and Lewis acid PbI2. Spin-coating of a DMF solution containing PbI2, CH3NH3I, and DMSO (1:1:1 mol %) formed a transparent adduct film, which was converted to a dark brown film upon heating at low temperature of 65 °C for 1 min due to removal of the volatile DMSO from the adduct. The adduct-induced CH3NH3PbI3 exhibited high charge extraction characteristics with hole mobility as high as 3.9 × 10–3 cm2/(V s) and slow recombination rate. Average power conversion efficiency (PCE) of 18.3% was achieved from 41 ...

1,960 citations

Journal ArticleDOI
TL;DR: In this article, the grain boundaries in thin-film perovskite solar cells are passedivated by using excess CH3NH3I in the precursor solution, achieving an average power conversion efficiency of 20.1% over 50 cells (best cell at 20.4%).
Abstract: Perovskite solar cells have attracted significant research efforts due to their remarkable performance, with certified power conversion efficiency now reaching 22%. Solution-processed perovskite thin films are polycrystalline, and grain boundaries are thought to be responsible for causing recombination and trapping of charge carriers. Here we report an effective and reproducible way of treating grain boundaries in CH3NH3PbI3 films deposited by means of a Lewis acid–base adduct approach. We show by high-resolution transmission electron microscopy lattice images that adding 6 mol% excess CH3NH3I to the precursor solution resulted in a CH3NH3I layer forming at the grain boundaries. This layer is responsible for suppressing non-radiative recombination and improving hole and electron extraction at the grain boundaries by forming highly ionic-conducting pathways. We report an average power conversion efficiency of 20.1% over 50 cells (best cell at 20.4%) together with significantly reduced current–voltage hysteresis achieved by this grain boundary healing process. The grain boundaries in thin-film perovskite solar cells are responsible for non-radiative carrier recombination, which is deleterious for the optoelectronic performance. Son et al. show how to passivate the grain boundaries by using excess CH3NH3I in the precursor solution, achieving efficiencies of 20.4%.

855 citations

Journal ArticleDOI
TL;DR: A mechanism for irreversible degradation of perovskite materials in which trapped charges, regardless of the polarity, play a decisive role is uncovered.
Abstract: Perovskite solar cells have shown unprecedent performance increase up to 22% efficiency. However, their photovoltaic performance has shown fast deterioration under light illumination in the presence of humid air even with encapulation. The stability of perovskite materials has been unsolved and its mechanism has been elusive. Here we uncover a mechanism for irreversible degradation of perovskite materials in which trapped charges, regardless of the polarity, play a decisive role. An experimental setup using different polarity ions revealed that the moisture-induced irreversible dissociation of perovskite materials is triggered by charges trapped along grain boundaries. We also identified the synergetic effect of oxygen on the process of moisture-induced degradation. The deprotonation of organic cations by trapped charge-induced local electric field would be attributed to the initiation of irreversible decomposition.

447 citations

Journal ArticleDOI
TL;DR: Frequency (f)-dependent capacitance revealed that the normal planar structure with the TiO2/MAPbI3/spiro-MeOTAD configuration showed most significant I-V hysteresis along with highest capacitance (10(-2) F/cm(2)) among the studied cell configurations.
Abstract: Mismatch of current (I)-voltage (V) curves with respect to the scan direction, so-called I–V hysteresis, raises critical issue in MAPbI3 (MA = CH3NH3) perovskite solar cell. Although ferroelectric and ion migration have been proposed as a basis for the hysteresis, origin of hysteresis has not been apparently unraveled. We report here on the origin of I–V hysteresis of perovskite solar cell that was systematically evaluated by the interface-dependent electrode polarizations. Frequency (f)-dependent capacitance (C) revealed that the normal planar structure with the TiO2/MAPbI3/spiro-MeOTAD configuration showed most significant I–V hysteresis along with highest capacitance (10–2 F/cm2) among the studied cell configurations. Substantial reduction in capacitance to 10–3 F/cm2 was observed upon replacing TiO2 with PCBM, indicative of the TiO2 layer being mainly responsible for the hysteresis. The capacitance was intensively reduced to 10–5 F/cm2 and C–f feature shifted to higher frequency for the hysteresis-fre...

423 citations

Journal ArticleDOI
TL;DR: In this paper, a super flexible perovskite solar cells using graphene as a transparent electrode is presented, achieving a performance of 16.8% with no hysteresis comparable to that of the counterpart fabricated on a flexible indium-tin-oxide electrode showing a maximum efficiency of 17.3%.
Abstract: With rapid and brilliant progress in performance over recent years, perovskite solar cells have drawn increasing attention for portable power source applications. Their advantageous features such as high efficiency, low cost, light weight and flexibility should be maximized if a robust and reliable flexible transparent electrode is offered. Here we demonstrate highly efficient and reliable super flexible perovskite solar cells using graphene as a transparent electrode. The device performance reaches 16.8% with no hysteresis comparable to that of the counterpart fabricated on a flexible indium-tin-oxide electrode showing a maximum efficiency of 17.3%. The flexible devices also demonstrate superb stability against bending deformation, maintaining >90% of its original efficiency after 1000 bending cycles and 85% even after 5000 bending cycles with a bending radius of 2 mm. This overwhelming bending stability highlights that perovskite photovoltaics with graphene electrodes can pave the way for rollable and foldable photovoltaic applications.

358 citations


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Journal ArticleDOI
17 Feb 2017-Science
TL;DR: A contact-passivation strategy using chlorine-capped TiO2 colloidal nanocrystal film that mitigates interfacial recombination and improves interface binding in low-temperature planar solar cells is reported.
Abstract: Planar perovskite solar cells (PSCs) made entirely via solution processing at low temperatures (<150°C) offer promise for simple manufacturing, compatibility with flexible substrates, and perovskite-based tandem devices. However, these PSCs require an electron-selective layer that performs well with similar processing. We report a contact-passivation strategy using chlorine-capped TiO2 colloidal nanocrystal film that mitigates interfacial recombination and improves interface binding in low-temperature planar solar cells. We fabricated solar cells with certified efficiencies of 20.1 and 19.5% for active areas of 0.049 and 1.1 square centimeters, respectively, achieved via low-temperature solution processing. Solar cells with efficiency greater than 20% retained 90% (97% after dark recovery) of their initial performance after 500 hours of continuous room-temperature operation at their maximum power point under 1-sun illumination (where 1 sun is defined as the standard illumination at AM1.5, or 1 kilowatt/square meter).

1,912 citations

Journal ArticleDOI
TL;DR: Jeon et al. as discussed by the authors synthesize a fluorene-terminated hole-transporting material with a fine-tuned energy level and a high glass transition temperature to ensure highly efficient and thermally stable perovskite solar cells.
Abstract: Perovskite solar cells (PSCs) require both high efficiency and good long-term stability if they are to be commercialized. It is crucial to finely optimize the energy level matching between the perovskites and hole-transporting materials to achieve better performance. Here, we synthesize a fluorene-terminated hole-transporting material with a fine-tuned energy level and a high glass transition temperature to ensure highly efficient and thermally stable PSCs. We use this material to fabricate photovoltaic devices with 23.2% efficiency (under reverse scanning) with a steady-state efficiency of 22.85% for small-area (~0.094 cm2) cells and 21.7% efficiency (under reverse scanning) for large-area (~1 cm2) cells. We also achieve certified efficiencies of 22.6% (small-area cells, ~0.094 cm2) and 20.9% (large-area, ~1 cm2). The resultant device shows better thermal stability than the device with spiro-OMeTAD, maintaining almost 95% of its initial performance for more than 500 h after thermal annealing at 60 °C. Interfacial losses between device layers play a key role in determining characteristics of solar cells. Jeon et al. address this in perovskite solar cells by synthesizing a hole-transporting layer that is better matched to the surrounding layers, and show high-efficiency and high-stability devices.

1,771 citations

Journal ArticleDOI
TL;DR: The fundamentals, recent research progress, present status, and views on future prospects of perovskite-based photovoltaics, with discussions focused on strategies to improve both intrinsic and extrinsic (environmental) stabilities of high-efficiency devices are described.
Abstract: The photovoltaics of organic–inorganic lead halide perovskite materials have shown rapid improvements in solar cell performance, surpassing the top efficiency of semiconductor compounds such as CdTe and CIGS (copper indium gallium selenide) used in solar cells in just about a decade. Perovskite preparation via simple and inexpensive solution processes demonstrates the immense potential of this thin-film solar cell technology to become a low-cost alternative to the presently commercially available photovoltaic technologies. Significant developments in almost all aspects of perovskite solar cells and discoveries of some fascinating properties of such hybrid perovskites have been made recently. This Review describes the fundamentals, recent research progress, present status, and our views on future prospects of perovskite-based photovoltaics, with discussions focused on strategies to improve both intrinsic and extrinsic (environmental) stabilities of high-efficiency devices. Strategies and challenges regardi...

1,720 citations

Journal ArticleDOI
01 Jul 2016-Science
TL;DR: A simple vacuum flash–assisted solution processing method is devised to obtain shiny, smooth, crystalline perovskite films of high electronic quality over large areas, which enables the realization of highly efficient large-area PSCs for practical deployment.
Abstract: Metal halide perovskite solar cells (PSCs) currently attract enormous research interest because of their high solar-to-electric power conversion efficiency (PCE) and low fabrication costs, but their practical development is hampered by difficulties in achieving high performance with large-size devices. We devised a simple vacuum flash-assisted solution processing method to obtain shiny, smooth, crystalline perovskite films of high electronic quality over large areas. This enabled us to fabricate solar cells with an aperture area exceeding 1 square centimeter, a maximum efficiency of 20.5%, and a certified PCE of 19.6%. By contrast, the best certified PCE to date is 15.6% for PSCs of similar size. We demonstrate that the reproducibility of the method is excellent and that the cells show virtually no hysteresis. Our approach enables the realization of highly efficient large-area PSCs for practical deployment.

1,586 citations

Journal ArticleDOI
TL;DR: Zheng et al. as discussed by the authors showed that quaternary ammonium halides can effectively passivate ionic defects in several different types of hybrid perovskite with their negative-and positive-charged components.
Abstract: The ionic defects at the surfaces and grain boundaries of organic–inorganic halide perovskite films are detrimental to both the efficiency and stability of perovskite solar cells. Here, we show that quaternary ammonium halides can effectively passivate ionic defects in several different types of hybrid perovskite with their negative- and positive-charged components. The efficient defect passivation reduces the charge trap density and elongates the carrier recombination lifetime, which is supported by density-function-theory calculation. The defect passivation reduces the open-circuit-voltage deficit of the p–i–n-structured device to 0.39 V, and boosts the efficiency to a certified value of 20.59 ± 0.45%. Moreover, the defect healing also significantly enhances the stability of films in ambient conditions. Our findings provide an avenue for defect passivation to further improve both the efficiency and stability of solar cells. Losses in solar cells can be caused by material defects in the bulk or at interfaces. Here, Zheng et al. use quaternary ammonium halides to passivate various perovskite absorbers and prepare solar cells with certified efficiency above 20%, suggesting that both anionic and cation defects are affected.

1,536 citations