Namyoung Ahn

Highly Reproducible Perovskite Solar Cells with Average Efficiency of 18.3% and Best Efficiency of 19.7% Fabricated via Lewis Base Adduct of Lead(II) Iodide

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) cm(2)/(V s) and slow recombination rate. Average power conversion efficiency (PCE) of 18.3% was achieved from 41 cells and the best PCE of 19.7% was attained via adduct approach.

Control of I–V Hysteresis in CH3NH3PbI3 Perovskite Solar Cell

UNLABELLED 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/cm(2)) among the studied cell configurations. Substantial reduction in capacitance to 10(-3) F/cm(2) 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/cm(2) and C-f feature shifted to higher frequency for the hysteresis-free planar structures with combination of PEDOT PSS, NiO, and PCBM, which underlines the spiro-MeOTAD in part contributes to the hysteresis. This work is expected to provide a key to the solution of the problem on I-V hysteresis in perovskite solar cell.

Superflexible, high-efficiency perovskite solar cells utilizing graphene electrodes: towards future foldable power sources

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.

Trapped charge-driven degradation of perovskite solar cells

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.

Opto-electronic properties of TiO2 nanohelices with embedded HC(NH2)2PbI3 perovskite solar cells

A HC(NH2)2PbI3 solar cell of perovskite structure based on TiO2 nanohelices has been developed. Well-aligned helical TiO2 arrays of different pitch (p) and radius (r), helix-1 (p/2 = 118 nm, r = 42 nm), helix-2 (p/2 = 353 nm, r = 88 nm) and helix-3 (p/2 = 468 nm, r = 122 nm), were grown on fluorine-doped tin oxide (FTO) glass by oblique-angle electron beam evaporation. HC(NH2)2PbI3 perovskite was deposited on the TiO2 nanohelices by a two-step dipping method. Helix-1 showed higher short-circuit current density (JSC), whereas helix-3 exhibited slightly higher open-circuit voltage (VOC). HC(NH2)2PbI3 perovskite combined with helix-1 demonstrated an average power conversion efficiency of 12.03 ± 0.07% due to its higher JSC compared to helix-2 and helix-3. The higher JSC of helix-1 could be attributed to its greater light scattering efficiency and higher absorbed photon-to-current conversion efficiency. In addition, despite having the longest pathway structure, helix-1 showed rapid electron diffusion, attributed to its higher charge injection efficiency due to the larger contact area between perovskite and TiO2. We have established that fine tuning of the interface between perovskite and the electron-injecting oxide is a crucial factor in achieving a perovskite solar cell of high performance.

Thermodynamic regulation of CH3NH3PbI3 crystal growth and its effect on photovoltaic performance of perovskite solar cells

We report a theoretical analysis on the crystallization of CH3NH3PbI3 and the control of grain sizes by varying the two-step reaction temperature from −10 °C to 50 °C based on the present analysis. The thermodynamic equation for CH3NH3PbI3 crystallization is derived by considering the change in Gibbs free energy and the equilibrium concentration of the reaction between the PbI2 film and CH3NH3I solution. The photovoltaic performance of a perovskite solar cell is found to depend on the reaction temperature, which is critical in determining the crystal size of perovskite. The reaction temperature was varied between −10 °C and 50 °C, and the optimal temperature was found to be around 20 °C in our two-step process. The performance enhancement controlled by the grain size with the increase of reaction temperature could be compensated by the generation of defects for a large crystal perovskite layer device.

Biomimetic detection of aminoglycosidic antibiotics using polydiacetylene–phospholipids supramolecules

We rationally designed highly sensitive and selective polydiacetylene (PDA)-phospholipids liposomes for the facile detection of aminoglycosidic antibiotics. The detecting mechanism mimics the cellular membrane interactions between neomycin and phosphatidylinositol-4,5-bisphosphate (PIP(2)) phospholipids. The developed PDA-PIP(2) sensory system showed a detection limit of 61 ppb for neomycin and was very specific to aminoglycosidic antibodies only.

Janus-Compartmental Alginate Microbeads Having Polydiacetylene Liposomes and Magnetic Nanoparticles for Visual Lead(II) Detection

Janus-compartmental alginate microbeads having two divided phases of sensory polydiacetylene (PDA) liposomes and magnetic nanoparticles were fabricated for facile sensory applications. The sensory liposomes are composed of PDA for label-free signal generation and 1,2-dipalmitoyl-sn-glycero-3-galloyl (DPGG) lipids whose galloyl headgroup has specific interactions with lead(II). The second phase having magnetic nanoparticles is designed for convenient handling of the microbeads, such as washing, solvent exchange, stirring, and detection, by applying magnetic field. Selective and convenient colorimetric detection of lead(II) and efficient removal of lead(II) by alginate matrix at the same time are demonstrated.

Water-repellent perovskite solar cell

A water-repellent perovskite solar cell was developed based on anti-reflective lotus leaf-inspired hierarchical pyramidal arrays with enhanced self-cleaning characteristics and was prepared by the replica molding of micro pyramid structures and Ar ion surface treatment. The power conversion efficiency of CH3NH3PbI3 perovskite solar cell was improved from 13.12% to 14.01% by employing such a bifunctional polydimethylsiloxane (PDMS) film on the perovskite solar cell due to an anti-reflection effect. Moreover, the PDMS film adhered to the perovskite solar cell demonstrated excellent superhydrophobicity, which makes the perovskite solar cell water-repellent.

Precise Morphology Control and Continuous Fabrication of Perovskite Solar Cells Using Droplet-Controllable Electrospray Coating System

Herein, we developed a novel electrospray coating system for continuous fabrication of perovskite solar cells with high performance. Our system can systemically control the size of CH3NH3PbI3 precursor droplets by modulating the applied electrical potential, shown to be a crucial factor for the formation of perovskite films. As a result, we have obtained pinhole-free and large grain-sized perovskite solar cells, yielding the best PCE of 13.27% with little photocurrent hysteresis. Furthermore, the average PCE through the continuous coating process was 11.56 ± 0.52%. Our system demonstrates not only the high reproducibility but also a new way to commercialize high-quality perovskite solar cells.