Seong Min Kang

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.

Hysteresis-free low-temperature-processed planar perovskite solar cells with 19.1% efficiency

Hysteresis-free, highly efficient and stable perovskite solar cells processed at low temperatures are strongly demanded to realize flexible or perovskite-based tandem solar cells. Here, we report a hysteresis-free planar CH3NH3PbI3 perovskite solar cell with a power conversion efficiency of 19.1% using a room-temperature vacuum-processed C60 electron transport layer (ETL) without the hole blocking layer. By optimizing the thickness of the C60 layer, the highly homogeneous, uniform, and dense ETL with a thickness of 35 nm is found to not only passivate the grain boundaries and surfaces of the perovskite layer, but also enhance charge transport properties. Thus, the C60 layer deposited on perovskites eliminates the photocurrent hysteresis and improves the cell efficiency. Also, compared to the device adopting the C60 and bathocuproine (BCP) combination, the one with the C60 layer without the BCP layer shows better performance due to enhanced electron extraction properties. Furthermore, for the first time, we have demonstrated a hysteresis-free flexible perovskite solar cell using the C60 ETL on a polyethylene naphthalate (PEN) substrate with 16.0% efficiency.

Robust superomniphobic surfaces with mushroom-like micropillar arrays

We present a simple method for the fabrication of robust superomniphobic surfaces with high transmittance (>90%) and durability (<6 months). The method consists of direct micromolding of mushroom-like micropillars and C4F8 gas surface treatment. Such re-entrant structures were found to be highly resistant against wetting by various liquids and oils with a wide range of surface tensions from 22.3 (ethanol) to 72.1 mN m−1 (water). Optimal structural parameters were derived based on the measurements of static contact angle and contact angle hysteresis.

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.

Moth‐Eye TiO2 Layer for Improving Light Harvesting Efficiency in Perovskite Solar Cells

A moth-eye nanostructured mp-TiO2 film using conventional lithography, nano-imprinting and polydimethyl-siloxane (PDMS) stamping methods is demonstrated for the first time. Power conversion efficiency of the moth-eye patterned perovskite solar cell is improved by ≈11%, which mainly results from increasing light harvesting efficiency by structural optical property.

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.

Replication of flexible polymer membranes with geometry-controllable nano-apertures via a hierarchical mould-based dewetting

Membranes with nano-apertures are versatile templates that possess a wide range of electronic, optical and biomedical applications. However, such membranes have been limited to silicon-based inorganic materials to utilize standard semiconductor processes. Here we report a new type of flexible and free-standing polymeric membrane with nano-apertures by exploiting high-wettability difference and geometrical reinforcement via multiscale, multilevel architecture. In the method, polymeric membranes with various pore sizes (50-800 nm) and shapes (dots, lines) are fabricated by a hierarchical mould-based dewetting of ultraviolet-curable resins. In particular, the nano-pores are monolithically integrated on a two-level hierarchical supporting layer, allowing for the rapid (<5 min) and robust formation of multiscale and multilevel nano-apertures over large areas (2 × 2 cm(2)).

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.

Remote Manipulation of Droplets on a Flexible Magnetically Responsive Film

The manipulation of droplets is used in a wide range of applications, from lab-on-a-chip devices to bioinspired functional surfaces. Although a variety of droplet manipulation techniques have been proposed, active, fast and reversible manipulation of pure discrete droplets remains elusive due to the technical limitations of previous techniques. Here, we describe a novel technique that enables active, fast, precise and reversible control over the position and motion of a pure discrete droplet with only a permanent magnet by utilizing a magnetically responsive flexible film possessing actuating hierarchical pillars on the surface. This magnetically responsive surface shows reliable actuating capabilities with immediate field responses and maximum tilting angles of ~90°. Furthermore, the magnetic responsive film exhibits superhydrophobicity regardless of tilting angles of the actuating pillars. Using this magnetically responsive film, we demonstrate active and reversible manipulation of droplets with a remote magnetic force.

Directional Clustering of Slanted Nanopillars by Elastocapillarity.

The unidirectional clustering induced by capillary force of drying liquids between pillars is investigated and a theoretical model to set a criterion of the unidirectional clustering of the slanted nanopillars is proposed.