Yeong-Hui Seo

An 8.2% efficient solution-processed CuInSe2 solar cell based on multiphase CuInSe2 nanoparticles

Multiphase CuInSe2 (CISe) nanoparticles including the CuSe phase are synthesized by the microwave-assisted solvothermal method. Without additional processing, multiphase CISe nanoparticles facilitate the solution-processed CISe absorber layer with a dense microstructure, large grains, high crystallinity, and composition controllability, which are essential for acceptable thin-film solar cell performance. The high performance, solution-processed CISe solar cell, with a conversion efficiency of 8.2%, is obtained through phase transformation, microstructural evolution, and composition adjustment by selenization (annealing under a Se atmosphere) at 530 °C.

Air-stable, surface-oxide free Cu nanoparticles for highly conductive Cu ink and their application to printed graphene transistors

Air-stable, surface-oxide free Cu nanoparticles are, for the first time, synthesized by surrounding completely the Cu surface with oleic acid incorporated as a capping molecule. XPS analysis, in conjunction with TEM analysis, revealed that the oleic acid is chemisorbed to the Cu surface via a chemical interaction wherein a monodentate bond is included, without leaving behind free (non-interacting) oleic acid, thereby providing complete surface protection against oxidation. By eliminating the surface oxide layer that critically degrades the electrical properties, the surface-oxide free Cu nanoparticle ink facilitates the realization of a solution-processed Cu electrode layer with resistivity as low as 4 μΩ cm, comparable to the resistivity of noble metal-based, solution-processed counterparts. In addition, high resolution Cu electrode patterns with 5 μm line-width are directly printed using an electrohydrodynamic inkjet technique, and graphene transistors with the printed Cu electrodes demonstrate potential applications in printed electronics.

Metal salt-derived In–Ga–Zn–O semiconductors incorporating formamide as a novel co-solvent for producing solution-processed, electrohydrodynamic-jet printed, high performance oxide transistors

We report the previously unrecognized co-solvent, formamide (FA), which can comprehensively improve both the device performance and bias stability of metal salt-derived, solution-processed In–Ga–Zn–O (IGZO) TFTs. By incorporating FA in IGZO precursor solutions, the chemical structures are tailored adequately for reducing the content of hydroxide and encouraging the oxygen vacancy formation, which has not been fulfilled in conventional chemical/physical approaches. Owing to such distinct chemical structural evolution, the field-effect mobility is enhanced dramatically by a factor of 4.3 (from 2.4 to 10.4 cm2 V−1 s−1), and the threshold voltage shift during a positive-bias stress test is suppressed effectively by a factor of 2.3 (from 9.3 to 4.1 V) for unpassivated devices. The addition of formamide to IGZO precursor solutions also facilitates electrohydrodynamic-jet (e-jet) printability, with which the directly printed device with a channel width of ∼30 μm is demonstrated successfully. In addition, a high performance, solution-processed IGZO transistor with a mobility of 50 cm2 V−1 s−1 is suggested through coupling a FA-added IGZO oxide semiconductor with a solution-processed zirconium aluminum oxide ((Zr,Al)2Ox) gate dielectric.

Chemically improved high performance printed indium gallium zinc oxide thin-film transistors

With the aim of facilitating the high performance printed In-Ga-Zn-O (IGZO) thin-film transistors (TFTs), we present the heretofore unrecognized chemical methodology for tailoring the chemical structures of printable IGZO semiconductors through incorporation of ethylene glycol in sol–gel derived precursor solutions. With the optimal composition of ethylene glycol, the device performance of TFT employing the printed IGZO semiconducting layer annealed at 400 °C is significantly improved with the field-effect saturation mobility of 4.9 cm2 V−1s−1. In addition, by lowering the contact resistance between the source/drain electrode and printed IGZO semiconducting layer, the device performance is further improved with the field-effect saturation mobility of 7.6 cm2 V−1s−1.

3D-stacked carbon composites employing networked electrical intra-pathways for direct-printable, extremely stretchable conductors.

The newly designed materials for stretchable conductors meeting the demands for both electrical and mechanical stability upon morphological elongation have recently been of paramount interest in the applications of stretchable, wearable electronics. To date, carbon nanotube-elastomeric polymer mixtures have been mainly developed; however, the method of preparing such CNT-polymer mixtures as stretchable conductors has been limited to an ionic liquid-mediated approach. In this study, we suggest a simple wet-chemical method for producing newly designed, three-dimensionally stacked carbon composite materials that facilitate the stable morphological elongation up to a strain of 300% with normalized conductivity variation of only 0.34 under a strain of 300%. Through a comparative study with other control samples, it is demonstrated that the intraconnected electrical pathways in hierarchically structured composite materials enable the generation of highly stretchable conductors. Their direct patternability is also evaluated by printing on demand using a programmable disperser without the use of prepatterned masks.

Crystalline structure-tunable, surface oxidation-suppressed Ni nanoparticles: printable magnetic colloidal fluids for flexible electronics

In this study, we suggest the chemical methodology that allows for the facile controllability of phase transformation between face-centered cubic and hexagonal close-packed structures for Ni nanoparticles with a 0.4–2 nm thick shallow surface oxide layer, resulting in a maximum saturation magnetization of 33.2 emu g−1. As a first proof-of-concept of the potential for the formation of flexible, printed magnetic devices on cost-effective polyethylene terephthalate (PET) and paper substrates, it is demonstrated that the resulting Ni nanoparticles, prepared in the form of magnetic fluids, are transformed into bulk-like patterned Ni architectures via air-brush printing and instant photonic annealing in a timescale of 10−3 s, exhibiting highly flexible properties under the harsh conditions of 10 000 times repeated bending tests.

Study on thermal evolution of the CuSe phase in nanoparticle-based absorber layers for solution-processed chalcopyrite photovoltaic devices.

Nanoparticle-based, solution-processed chalcopyrite photovoltaic devices have drawn tremendous attraction for the realization of low-cost, large-area solar cell applications. In particular, it has been recently demonstrated that the CuSe phase plays a critical role in allowing the formation of device-quality, nanoparticle-based chalcopyrite absorber layers. For further in-depth study, with the aim of understanding the thermal behavior of the CuSe phase that triggers the vigorous densification reaction, a requisite for high-performance chalcopyrite absorber layers, both multiphase (CuSe-phase including) and single-phase (CuSe-phase free) CISe nanoparticles are investigated from the viewpoint of compositional variation and crystalline structural evolution. In addition, with CuSe-phase including CISe particulate layers, the basic restrictions in thermal treatment necessary for activating effectively the CuSe-phase induced densification reaction are suggested, in conjunction with consideration on the thermal decomposition of organic additives that are inevitably incorporated in nanoparticle-based absorber layers.

Printable electronics-compatible silicon nanoparticles prepared by the facile decomposition of SiS2 and their application in a back-to-back Schottky diode

Crystalline silicon nanoparticles (size <5 nm) are synthesized at room temperature by the decomposition of silicon sulfide (SiS2) in a water–acid mixture followed by chemical etching. Grey free-standing silicon nanoparticles are obtained after the decomposed product is etched with a mixture of hydrofluoric acid (HF), hydrogen peroxide (H2O2) and ethanol. These silicon nanoparticles are not capped with any organic ligands, making them suitable for electronic applications. For the preparation of a functional Si nanoparticle dispersion, the silicon nanoparticle suspension is prepared by re-dispersing in benzonitrile or in ethanol by incorporating polypropylene glycol (PPG) as a binder. An Al/Si-nanoparticle/n++-Si back-to-back Schottky diode is fabricated from both the Si nanoparticle suspension and the ink, and the charge transport mechanism is studied as the working temperature increases. Such versatility of these silicon nanoparticles can be ideal for any print-type deposition with a low-cost and large-area processing method.

Thermally-derived liquid phase involving multiphase Cu(In,Ga)Se2 nanoparticles for solution-processed inorganic photovoltaic devices

In the past decade, wet chemical strategies for solution-based Cu(In,Ga)Se2 (CI(G)Se) photovoltaic devices have gained a tremendous amount of attention in solar-cell research fields. In particular, nanoparticles allowing for liquid-phase densification have been recognized as viable candidates for advancements in photovoltaic devices. In this study, multiphase CIGSe nanoparticles are synthesized by the microwave-assisted solvothermal method, in which the chemically incorporated CuSe2 and Se phases form liquid phases for inducing vigorous reactions at elevated temperatures. The morphological/crystalline structural properties of multiphase nanoparticles are analyzed, in conjunction with the temperature dependent evolution in multiphase nanoparticle-incorporating functional layers. Furthermore, we examine physical parameters including the cell performance, shunt conductance, and series resistance for multiphase CIGSe nanoparticle-derived solar cells, from which the cell performance-limiting factors are discussed.