Kyung-Geun Lim

Boosting the Power Conversion Efficiency of Perovskite Solar Cells Using Self‐Organized Polymeric Hole Extraction Layers with High Work Function

A self-organized hole extraction layer (SOHEL) with high work function (WF) is designed for energy level alignment with the ionization potential level of CH3 NH3 PbI3 . The SOHEL increases the built-in potential, photocurrent, and power conversion efficiency (PCE) of CH3 NH3 PbI3 perovskite solar cells. Thus, interface engineering of the positive electrode of solution-processed planar heterojunction solar cells using a high-WF SOHEL is a very effective way to achieve high device efficiency (PCE = 11.7% on glass).

Planar heterojunction organometal halide perovskite solar cells: roles of interfacial layers

Organometal halide perovskites are promising photo-absorption materials in solar cells due to their high extinction coefficient, broad light absorption range and excellent semiconducting properties. The highest power conversion efficiency (PCE) of perovskite solar cells (PrSCs) is now 20.1%. However, a high-temperature processed mesoscopic metal oxide (e.g., TiO2) must be removed to realize flexible PrSCs on plastic substrates using low temperature processes. Although the planar heterojunction (PHJ) structure can be considered as the most appropriate structure for flexible PrSCs, they have shown lower PCEs than those with a mesoscopic metal oxide layer. Therefore, development of interfacial layers is essential for achieving highly efficient PHJ PrSCs, and necessary in fabrication of flexible PrSCs. This review article gives an overview of progress in PHJ PrSCs and the roles of interfacial layers in the device, and suggests a practical strategy to fabricate highly efficient and flexible PHJ PrSCs. We conclude with our technical suggestion and outlook for further research direction.

Universal energy level tailoring of self-organized hole extraction layers in organic solar cells and organic–inorganic hybrid perovskite solar cells

Tailoring the interface energetics between a polymeric hole extraction layer (HEL) and a photoactive layer (PAL) in organic photovoltaics (OPVs) and organic–inorganic hybrid perovskite solar cells (PrSCs) is very important to maximize open circuit voltage (Voc), power conversion efficiency (PCE), and device lifetime. In principle, when Fermi-level pinning and a vacuum level shift take place between the HEL and PAL, they give rise to an energy level offset between the HEL and the valence band maximum (VBM) (or the highly occupied molecular orbital (HOMO) in the case of organic photoactive materials) of the PAL and then Voc loss. However, here we show that the Voc loss at the interface can be overcome by universal energy level tailoring of a self-organized HEL (SOHEL) between the HEL and PAL irrespective of photoactive materials. A SOHEL composed of a conducting polymer and a perfluorinated ionomer (PFI) is effectively used to study the interface energetics in OPVs and PrSCs. We systematically tailored the interface energy level of the SOHEL to remove the energy offset at the interface and understand clearly the universal energy level alignment with the diverse photoactive materials of OPVs and PrSCs. The Fermi-level of the HEL is pinned to the midgap state of photoactive materials, which is about 0.6–0.7 eV above the VBM or HOMO. However, the interface energy state of the PFI-enriched surface layer of the SOHEL can be formed deeper below the Fermi-level by self-organized molecules so that it can match the top of the valence band of the photoactive materials. As a result, the energy offset at the interface between photoactive materials and the SOHEL can be significantly decreased to achieve high Voc and PCE. Furthermore, our SOHEL significantly prolonged the stability of OPVs (half lifetime: 2.84 year) compared with pristine PEDOT:PSS (half lifetime: 0.2 year) under continuous irradiation of air mass-1.5 global simulated sunlight at 100 mW cm−2 due to the diffusion-blocking ability of the self-organized PFI at the surface of SOHELs for impurities from indium tin oxide.

Soluble Self‐Doped Conducting Polymer Compositions with Tunable Work Function as Hole Injection/Extraction Layers in Organic Optoelectronics

Charge injection/extraction layers greatly affect the efficiency and lifetime of organic optoelectronic devices. Therefore, reliable and soluble charge injection/extraction buffer layers, which are widely applicable to various organic optoelectronic devices, should be developed. Solution processable conducting polymers doped with poly(4-styrenesulfonate) are especially good candidates for use as hole injection buffer layers (HILs) and hole extraction buffer layers (HELs) in organic light-emitting diodes (OLEDs) and organic photovoltaic cells (OPVs). However, these polymers have practical limitations for large area electronics because the particles are dispersed in water and thus form poor-quality films with defects caused by aggregation of the particles; they also present a high hole injection/extraction barrier to overlying organic layers, and are hygroscopic. Therefore, to improve the device lifetime and hole injection/extraction capability in organic optoelectronics, a soluble, efficient, and stable buffer layer should be developed. We introduce soluble, self-doped conducting polyaniline graft copolymer compositions based on poly(4-styrenesulfonate)-g-polyaniline (PSS-g-PANI) (Figure 1) that have tunable work functions WF for HILs/ HELs in OLEDs and OPVs. We systematically controlled the self-organized surface-enriched layer of the HIL/HELs after incorporating a perfluorinated ionomer (PFI) into the PSS-gPANI solutions. Then, we investigated the influence of the surface layer on the values of WF of spin cast films and the correlation of WF values with hole injection/extraction capabilities and device lifetimes in OLEDs and OPVs. Water-soluble PSS-g-PANI with a 8:1 weight ratio (Figure 1) was synthesized (see the Supporting Information) by oxidative polymerization of aniline with an oxidant and an aqueous solution of random copolymer composed of stryenesulfonate and p-aminostyrene derivatives. Doping of the polymeric acid dopant PSS is structurally stable because this graft type conducting polymer bonds covalently to the PSS. We used ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) at the 4B1 beam line of Pohang Acceleration Laboratory (PAL) to characterize the surfaces of spin-cast films of the soluble conducting polymer and of the blends with the PFI. The value of WF was affected by the compositions of the solutions (Table 1). In PSS-g-PANI, WF= 5.28 eV, which is higher than that of the Figure 1. Self-organized film of PSS-g-PANI compositions after spincoating, and chemical structures of PSS-g-PANI and PFI.

Electromagnetic wave absorption properties of amorphous alloy-ferrite-epoxy composites in quasi-microwave band

Reports on the electromagnetic wave absorption properties of amorphous alloy-ferrite-epoxy composites with various amorphous alloy fractions in the frequency range from 1 to 5 GHz. We varied the fraction of amorphous alloy in amorphous alloy-ferrite-epoxy composites from 0 to 50 vol.% at a fixed 50 vol.% fraction of epoxy resin polymer. We measured the complex permeability /spl mu/ and permittivity /spl epsiv/ of composites by the reflection/transmission technique, and determined the theoretical matching frequency and thickness with maximum reflection loss by plotting the measured /spl mu/ and /spl epsiv/ on an impedance matching solution map. The increasing amorphous alloy fraction in amorphous alloy-ferrite-epoxy composites resulted in a larger matching frequency and thinner thickness than those of ferrite-epoxy composite. On the basis of these results, we propose a new, thinner wave absorber, made by optimizing the amorphous fraction and thickness in amorphous alloy-ferrite-epoxy composites.

Amorphous phase formation of Zr-based alloy coating by HVOF spraying process

This investigation was conducted to clarify the effects of process parameters on the formation of the new amorphous coating using a Zr-based alloy, which is known as bulk metallic glass forming alloy, by a HVOF (High Velocity Oxygen Fuel) spraying process. Powders used for spraying was prepared by vacuum gas atomization and then crushed by a centrifugal mill. HVOF spraying experiments were carried out using a Tafa JP-5000 spraying gun. DTA (Differential Thermal Analysis) measurements have shown that the amorphous content of the coatings was measured up to about 62% depending on the spraying process parameters. The amorphous fraction of the coatings is decreased with increasing the spray distance and the fuel flow rate. Microstructural observations and X-ray diffraction analysis of the spray coated layers reveal that the amorphization behavior during the spraying is attributed to the degree of the solidification of droplets and the oxide (ZrO2) formation in spray coated layers. Therefore, flame temperature and spray distance that can control the carrier gas temperature and undercooling effects of the droplets are the most crucial factors for the evolution of the amorphous phase using this bulk metallic glass forming alloy.

Polyaniline‐Based Conducting Polymer Compositions with a High Work Function for Hole‐Injection Layers in Organic Light‐Emitting Diodes: Formation of Ohmic Contacts

It is a great challenge to develop solution-processed, polymeric hole-injection layers (HILs) that perform better than small molecular layers for realizing high-performance small-molecule organic light-emitting diodes (SM-OLEDs). We have greatly improved the injection efficiency and the current efficiency of SM-OLEDs by introducing conducting polymer compositions composed of polyaniline doped with polystyrene sulfonate and perfluorinated ionomer (PFI) as the HIL. During single spin-coating of conducting polymer compositions, the PFI layer was self-organized at the surface and greatly increased the film work function. It enhanced hole-injection efficiency and current efficiency by introducing a nearly ohmic contact and improving electron blocking. Our results demonstrate that solution-processed polyaniline HILs with tunable work functions are good candidates for reducing process costs and improving OLED performance.

High-efficiency polymer photovoltaic cells using a solution-processable insulating interfacial nanolayer: the role of the insulating nanolayer

We employed a low-cost solution-processed ultrathin insulating polymeric layer of poly(4-hydroxystyrene) (PHS), with a high glass transition temperature (Tg ∼ 185 °C), as an interfacial layer between the polymer:fullerene photoactive layer and the Al negative electrode for enhancing device power conversion efficiency (PCE) of polymer bulk-heterojunction photovoltaic cells and investigated the roles of the interfacial nanolayer by ultraviolet photoemission spectroscopy and capacitance–voltage measurement. The thin polymeric layer forms a dipole layer and causes the vacuum level of the adjacent negative electrode to shift upward, which resulted in an increase of the built-in potential. As a result, the open-circuit voltage and PCE of the device using a PHS nanolayer were remarkably improved. We finally achieved a very high PCE of 6.5% with the PHS/Al negative electrode which is even much better than that of the device using an Al electrode (5.0%). The solution-processed inexpensive PHS layer with a high Tg can be an attractive alternative to conventional vacuum-deposited low-work-function metal and insulating metal fluoride interfacial layers.

Role of Ultrathin Metal Fluoride Layer in Organic Photovoltaic Cells: Mechanism of Efficiency and Lifetime Enhancement

Although rapid progress has been made recently in bulk heterojunction organic solar cells, systematic studies on an ultrathin interfacial layer at the electron extraction contact have not been conducted in detail, which is important to improve both the device efficiency and the lifetime. We find that an ultrathin BaF2 layer at the electron extraction contact strongly influences the open-circuit voltage (Voc ) as the nanomorphology evolves with increasing BaF2 thickness. A vacuum-deposited ultrathin BaF2 layer grows by island growth, so BaF2 layers with a nominal thickness less than that of single-coverage layer (≈3 nm) partially cover the polymeric photoactive layer. As the nominal thickness of the BaF2 layer increased to that of a single-coverage layer, the Voc and power conversion efficiency (PCE) of the organic photovoltaic cells (OPVs) increased but the short-circuit current remained almost constant. The fill factor and the PCE decreased abruptly as the thickness of the BaF2 layer exceeded that of a single-coverage layer, which was ascribed to the insulating nature of BaF2 . We find the major cause of the increased Voc observed in these devices is the lowered work function of the cathode caused by the reaction and release of Ba from thin BaF2 films upon deposition of Al. The OPV device with the BaF2 layer showed a slightly improved maximum PCE (4.0 %) and a greatly (approximately nine times) increased device half-life under continuous simulated solar irradiation at 100 mW cm(-2) as compared with the OPV without an interfacial layer (PCE=2.1 %). We found that the photodegradation of the photoactive layer was not a major cause of the OPV degradation. The hugely improved lifetime with cathode interface modification suggests a significant role of the cathode interfacial layer that can help to prolong device lifetimes.

Controlling Surface Enrichment in Polymeric Hole Extraction Layers to Achieve High-Efficiency Organic Photovoltaic Cells

Hole extraction in organic photovoltaic cells (OPVs) can be modulated by a surface-enriched layer formed on top of the conducting polymer-based hole extraction layer (HEL). This tunes the surface work function of the HEL to better align with the ionization potential of the polymeric photoactive layer. Results show noticeable improvement in device power conversion efficiencies (PCEs) in OPVs. We achieved a 6.1 % PCE from the OPV by optimizing the surface-enriched layer.