Sang Hyuk Im

Efficient and thermally stable inverted perovskite solar cells by introduction of non-fullerene electron transporting materials

Highly efficient and thermally stable inverted CH3NH3PbI3 (MAPbI3) and HC(NH2)2PbI3−xBrx (FAPbI3−xBrx) perovskite planar solar cells are demonstrated by using a N,N′-bis(phenylmethyl)naphthalene-1,4,5,8-tetracarboxylic diimide (NDI-PM)-based electron transporting material (ETM) instead of a conventional fullerene-based phenyl-C61-butyric acid methyl ester (PCBM) ETM. The MAPbI3 and FAPbI3−xBrx devices with the NDI-PM-based ETM exhibit 18.4% and 19.6% power conversion efficiency under an illumination of 1 Sun (100 mW cm−2), respectively, which are comparable to the efficiency of PCBM ETM-based ones (18.9% and 20.0%). The improved thermal stability of NDI-based perovskite solar cells is attributed to much stronger hydrogen bonds in the NDI-PM molecular crystals than the PCBM crystals.

Highly flexible, high-performance perovskite solar cells with adhesion promoted AuCl3-doped graphene electrodes

Super flexible TCO-free FAPbI3−xBrx planar type inverted perovskite solar cells with a 17.9% power conversion efficiency under 1 sun conditions were demonstrated by introducing an APTES (3-aminopropyl triethoxysilane) adhesion promoter between a PET flexible substrate and a AuCl3-doped single-layer graphene transparent electrode (TCE). Due to the formation of covalent bonds by the APTES inter-layer, the AuCl3-GR/APTES/PET substrate had excellent flexibility, whereas the AuCl3-GR/PET substrate and the ITO/PET substrate had significant degradation of the sheet resistance after a bending test due to the break-off or delamination of AuCl3-GR from the PET substrate and the cracking of ITO. Accordingly, the perovskite solar cells constructed on the AuCl3-GR/APTES/PET TCE substrate exhibited excellent bending stability and they maintained their PCE at over 90% of the initial value after 100 bending cycles at R ≥ 4 mm.

Memory effect behavior with respect to the crystal grain size in the organic-inorganic hybrid perovskite nonvolatile resistive random access memory

The crystal grain size of CH3NH3PbI3 (MAPbI3) organic-inorganic hybrid perovskite (OHP) film was controllable in the range from ~60u2009nm to ~600u2009nm by non-solvents inter-diffusion controlled crystallization process in dripping crystallization method for the formation of perovskite film. The MAPbI3 OHP non-volatile resistive random access memory with ~60u2009nm crystal grain size exhibited >0.1u2009TB/in2 storage capacity, >600 cycles endurance, >104u2009s data retention time, ~0.7u2009V set, and ~−0.61u2009V re-set bias voltage.

Formation of uniform PbS quantum dots by a spin-assisted successive precipitation and anion exchange reaction process using PbX2 (X = Br, I) and Na2S precursors

We devised a straightforward spin-assisted successive precipitation and anion exchange reaction (spin-SPAER) process in order to deposit relatively uniform PbS quantum dots (QDs) on mesoporous TiO2 (mp-TiO2). For the spin-SPAER process, we used PbX2 (X = I, Br, and Cl) precursors instead of a Pb(NO3)2 precursor and consequently deposited individual PbS QDs on mp-TiO2 due to the suppressed overgrowth of PbS QDs, whereas the conventional spin-assisted successive ionic layer adsorption and reaction (spin-SILAR) process formed aggregated PbS QDs on the mp-TiO2 surface due to continuous adsorption and reaction. In addition, the PbS QDs prepared by spin-SPAER showed better air stability than the PbS QDs prepared by spin-SILAR possibly due to the passivation by halogen elements such as I and Br. Accordingly, we could improve the overall power conversion efficiency of PbS QD-SSCs prepared by the spin-SPAER process using PbI2 and PbBr2 precursors to ∼26.7% and ∼44.2%, respectively, compared to the PbS QD-SSCs prepared by spin-SILAR using the Pb(NO3)2 precursor.

Inverted CH3NH3PbI3 perovskite hybrid solar cells with improved flexibility by introducing a polymeric electron conductor

Inverted-type CH3NH3PbI3 flexible perovskite solar cells (FPeSCs) with improved flexibility were prepared by incorporating a polymeric electron conductor, poly([N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalene bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)) (PNDI-2T), into small-molecule electron conductor phenyl-C61-butyric acid methyl ester (PCBM). PCBMu2006:u2006PNDI-2T electron conductor films with different compositions (100u2006:u20060, 75u2006:u200625, 50u2006:u200650, 25u2006:u200675 and 0u2006:u2006100) were employed in PeSCs. The 75u2006:u200625 PCBMu2006:u2006PNDI-2T film showed slightly improved power conversion efficiency (average η of 30 samples = 16.59 ± 1.13%, best = 18.4%) compared with PCBM film (average η of 30 samples = 16.49 ± 1.15%, best = 17.8%), as well as similar electrical conductivity and electron mobility, and lower absorption loss. Similarly, inverted planar FPeSCs of 75u2006:u200625 PCBMu2006:u2006PNDI-2T films showed improved performance (average η of 20 samples = 13.91 ± 0.84%, best = 15.4%) compared with PCBM (average η of 20 samples = 13.04 ± 0.97%, best = 15.0%). Furthermore, the 75u2006:u200625 PCBMu2006:u2006PNDI-2T inverted planar FPeSC films showed significantly improved flexibility compared with PCBM-based devices, due to the entangled polymeric PNDI-2T matrix relieving stress on the PCBM domains.

Heterogeneous Capillary Interactions of Interface-Trapped Ellipsoid Particles Using the Trap-Release Method

Heterogeneous capillary interactions between ellipsoid particles at the oil-water interface were measured via optical laser tweezers. Two trapped particles were aligned in either tip-to-tip (tt) or side-to-side (ss) configurations via the double-trap method and were released from the optical traps, leading to particle-particle attractions due to the capillary forces caused by quadrupolar interface deformation. On the basis of image analysis and calculations of the Stokes drag force, the capillary interactions between two ellipsoid particles with the same aspect ratio (E) were found to vary with the particle pairs that were measured, indicating that the interactions were nondeterministic or heterogeneous. Heterogeneous capillary interactions could be attributed to undulation of the interface meniscus due to chemical and/or geometric particle heterogeneity. The power law exponent for the capillary interaction Ucap ≈ r-β was found to be β ≈ 4 and was independent of the aspect ratio and particle configuration in long-range separations. Additionally, with regard to the tt configuration, the magnitude of the capillary force proportionally increased with the E value (E > 1) when two ellipsoid particles approached each other in the tt configuration.

High-Performance Solid-State PbS Quantum Dot-Sensitized Solar Cells Prepared by Introduction of Hybrid Perovskite Interlayer

High-performance solid-state PbS quantum dot-sensitized solar cells (QD-SSCs) with stable 9.2% power conversion efficiency at 1 Sun condition are demonstrated by introduction of hybrid perovskite interlayer. The PbS QDs formed on mesoscopic TiO2 (mp-TiO2) by spin-assisted successive precipitation and anionic exchange reaction method do not exhibit PbSO4 but have PbSO3 oxidation species. By introducing perovskite interlayer in between mp-TiO2/PbS QDs and poly-3-hexylthiophene, the PbSO3 oxidation species are fully removed in the PbS QDs and thereby the efficiency of PbS QD-SSCs is enhanced over 90% compared to the pristine PbS QD-SSCs.

Enhanced Efficiency and Long-Term Stability of Perovskite Solar Cells by Synergistic Effect of Nonhygroscopic Doping in Conjugated Polymer-Based Hole-Transporting Layer

A face-on oriented and p-doped semicrystalline conjugated polymer, poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole)] (PPDT2FBT), was studied as a hole-transport layer (HTL) in methylammonium lead triiodide-based perovskite solar cells (PVSCs). PPDT2FBT exhibits a mid-band gap (1.7 eV), high vertical hole mobility (7.3 × 10-3 cm2/V·s), and well-aligned frontier energy levels with a perovskite layer for efficient charge transfer/transport, showing a maximum power conversion efficiency (PCE) of 16.8%. Upon doping the PPDT2FBT HTL with a nonhygroscopic Lewis acid, tris(pentafluorophenyl)borane (BCF, 2-6 wt %), the vertical conductivity was improved by a factor of approximately 2, and the resulting PCE was further improved up to 17.7%, which is higher than that of standard PVSCs with 2,2,7,7-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (spiro-OMeTAD) as an HTL. After BCF doping, the clearly enhanced carrier diffusion coefficient, diffusion length, and lifetime were measured using intensity-modulated photocurrent and photovoltage spectroscopy. Furthermore, compared to the standard PVSCs with spiro-OMeTAD, the temporal device stability was remarkably improved, preserving the ∼60% of the original PCE for 500 h without encapsulation under light-soaking condition (1 sun AM 1.5G) at 85 °C and 85% humidity, which is mainly due to the highly crystalline conjugated backbone of PPDT2FBT and nonhygroscopic nature of BCF. In addition, formamidinium lead iodide/bromide (FAPbI3-xBrx)-based PVSCs with the BCF-doped PPDT2FBT as an HTL was also prepared to show 18.8% PCE, suggesting a wide applicability of PPDT2FBT HTL for different types of PVSCs.

Super-flexible bis(trifluoromethanesulfonyl)-amide doped graphene transparent conductive electrodes for photo-stable perovskite solar cells

Super-flexible bis(trifluoromethanesulfonyl)-amide (TFSA)-doped graphene transparent conducting electrode (GR TCE)-based FAPbI3 − xBrx perovskite solar cells with 18.9% power conversion efficiency (PCE) for a rigid device and 18.3% for a flexible one are demonstrated because the TFSA-doped GR TCE reveals high conductivity and high transmittance. The unencapsulated TFSA-doped GR TCE-based cell maintained ∼95% of its initial PCE under a continuous light soaking of 1 Sun at 60 °C/30% relative humidity for 1000 h. In addition, the TFSA-doped GR TCE-based flexible perovskite solar cells show excellent bending stabilities, maintaining PCEs of ∼85, ∼75, and ∼35% of their original values after 5000 bending cycles, at R = 12, 8, and 4 mm, respectively.

High Yield Synthesis of Polystyrene Microspheres by Continuous Long Tubular Reactor and Their Application to Antiglare Film for High Resolution Displays

Uniform polystyrene (PS) microspheres are synthesized by dispersion polymerization in a continuous long tubular reactor (CLTR) system, whereas the batch reactor system yields ∼1.8 fold smaller PS microspheres because the CLTR system has higher conversion (∼92%) and more number of nuclei at early stage than the batch system (∼68 %) due to better heat transfer. The uniform PS microspheres can be synthesized in CLTR system with high conversion if the residence time (reaction time) in CLTR is longer than the saturation time of reaction (conversion). In addition, when we apply the PS microspheres to antiglare (AG) film for high resolution displays, the AG film exhibits good pencil hardness of 4 H under 200 g load and internal, external, and total haze of 12.50, 4.38, and 16.88, respectively.