voltage, high mobility, high luminous efficiency, long service life, and tunable bandgap energy by adjusting the aluminum and indium composition covering from ultraviolet to infrared. However, there are drawbacks for GaN. For example, growing GaN on a sapphire or silicon (111) substrate generates dislocation due to lattice mismatch between the GaN epitaxial layer and the substrate. In addition, the strong chemical bond at the interface makes it difficult to separate the epitaxial layer from the substrate, which limits the ability to apply GaN with excellent properties in various fields. Recently, several studies have sought to overcome these problems by adopting 2D materials between the epitaxial layer and the substrate.Xu and co-workers reported the reduced dislocation density in GaN/graphene/GaN structure using self-organized graphene as a nanomask at high-temperature in hydrogen (H 2 ) ambient with many graphene defects. [1,2] Liu and co-workers also reported that when aluminum nitride was grown on plasma-treated graphene, the dislocation density was reduced compared to when directly grown on sapphire. [3] Hong and co-workers proved that the embedded graphene oxide could be used to improve the heat dissipation in GaN light-emitting diodes. [4,5] In addition, other researchers are making efforts to separate epitaxially grown III−V films from the substrate with the help of the 2D material making the weak chemical bond between the epitaxial layer and the substrate that is transferred to the substrate such as graphene/silicon dioxide, [6] graphene/silicon carbide, [7,8] graphene/ gallium arsenide (GaAs), [9] and boron nitride/sapphire [10] structures. Very recently, Kim and co-workers reported demonstration results using various semiconductors that showed that the polarity of 2D materials and bulk play a crucial role in remote epitaxy. The article also provides the necessary general understanding of single-crystalline growth on 2D-material-coated substrates and subsequent exfoliation. [11] However, the earlyreported graphene-loss issue, particularly in GaN growth in metal-organic chemical vapor deposition (MOCVD), remains unclear and warrants investigation to take advantages of graphene in III−V compound semiconductor growth.Here, we investigated how the decomposition of substrates that support the graphene layer affect the graphene layer by Graphene has been adopted in III−V material growth since it can reduce the threading dislocations and the III−V epilayer can easily be separated from the substrate due to the weak chemical bond. However, depending on the substrate supporting the graphene, some substrates decompose in the III−V material growth environment, which results in the problem that no graphene remains. In this study, the influence of temperature-dependent substrate decomposition on graphene through an annealing process that resembles conventional growth conditions in metal-organic chemical vapor deposition (MOCVD) is investigated. It is also confirmed that trimethylgallium, hydrogen, and ammonia gases d...

Metal nanowires have been gaining increasing attention as the most promising stretchable transparent electrodes for emerging field of stretchable optoelectronic devices. Nanowelding technology is a major challenge in the fabrication of metal nanowire networks because the optoelectronic performances of metal nanowire networks are mostly limited by the high junction resistance between nanowires. We demonstrate the spontaneous and selective welding of Ag nanowires (AgNWs) by Ag solders via an electrochemical Ostwald ripening process and high electrostatic potential at the junctions of AgNWs. The AgNWs were welded by depositing Ag nanoparticles (AgNPs) on the conducting substrate and then exposing them to water at room temperature. The AgNPs were spontaneously dissolved in water to form Ag ions, which were then reduced to single-crystal Ag solders selectively at the junctions of the AgNWs. Hence, the welded AgNWs showed higher optoelectronic and stretchable performance compared to that of as-formed AgNWs. These results indicate that electrochemical Ostwald ripening-based welding can be used as a promising method for high-performance metal nanowire electrodes in various next-generation devices such as stretchable solar cells, stretchable displays, organic light-emitting diodes, and skin sensors.

We introduce high-performance metal mesh/graphene hybrid transparent conductive layers (TCLs) using prime-location and metal-doped graphene in near-ultraviolet light-emitting diodes (NUV LEDs). Despite the transparency and sheet resistance values being similar for hybrid TCLs, there were huge differences in the NUV LEDs’ electrical and optical properties depending on the location of the graphene layer. We achieved better physical stability and current spreading when the graphene layer was located beneath the metal mesh, in direct contact with the p-GaN layer. We further improved the contact properties by adding a very thin Au mesh between the thick Ag mesh and the graphene layer to produce a dual-layered metal mesh. The Au mesh effectively doped the graphene layer to create a p-type electrode. Using Raman spectra, work function variations, and the transfer length method (TLM), we verified the effect of doping the graphene layer after depositing a very thin metal layer on the graphene layers. From our results, we suggest that the nature of the contact is an important criterion for improving the electrical and optical performance of hybrid TCLs, and the method of doping graphene layers provides new opportunities for solving contact issues in other semiconductor devices.

It is well‐known that the alkali doping of polycrystalline Cu 2 ZnSn(S,Se) 4 (CZTSSe) and Cu(In,Ga)(Se,S) 2 has a beneficial influence on the device performance and there are various hypotheses about the principles of performance improvement. This work clearly explains the effect of Na doping on the fill factor (FF) rather than on all of the solar cell parameters (open‐circuit voltage, FF, and sometimes short circuit current) for overall performance improvement. When doping is optimized, the fabricated device shows sufficient built‐in potential and selects a better carrier transport path by the high potential difference between the intragrains and the grain boundaries. On the other hand, when doping is excessive, the device shows low contact potential difference and FF and selects a worse carrier transport path even though the built‐in potential becomes stronger. The fabricated CZTSSe solar cell on a flexible metal foil optimized with a 25 nm thick NaF doping layer achieves an FF of 62.63%, thereby clearly showing the enhancing effect of Na doping.

Deep ultraviolet light-emitting diodes (DUV LEDs) (<280 nm) have been important light sources for broad applications in, e.g., sterilization, purification, and highdensity storage. However, the lack of excellent transparent electrodes in the DUV region remains a challenging issue. Here, we demonstrate an architectural engineering scheme to flexibly tune the work function of Cu@shell nanowires (NWs) as top transparent electrodes in DUV LEDs. By fast encapsulation of shell metals on Cu NWs and a shift of electron binding energy, the electronic work function could be widely tailored down to 4.37 eV and up to 5.73 eV. It is revealed that the high work function of Cu@Ni and Cu@Pt NWs could overcome the interfacial barrier to p-AlGaN and achieve direct ohmic contact with high transparency (91%) in 200−400 nm. Completely transparent DUV LED chips are fabricated and successfully lighted with sharp top emission (wall-plug efficiency reaches 3%) under a turn-on voltage of 6.4 V. This architectural strategy is of importance in providing highly transparent ohmic electrodes for optoelectronic devices in broad wavelength regions.

Cu2ZnSn­(S,Se)4 (CZTSSe) thin-film solar cells are showing great promise due to using earth-abundant and nontoxic materials and tuning the band gap through the amount of S and Se. Flexible high-efficiency CZTSSe solar cells are one of the outstanding research challenges because they currently require the use of thick glass substrates due to the high-temperature heat treatment process, and for this reason, few flexible CZTSSe solar cells have been reported. Furthermore, most researchers have used thin glass and metal substrates with little flexibility; the power conversion efficiency (PCE or η) values of the solar cells made with them have been slightly lower. To overcome these hurdles, we transferred high-efficiency CZTSSe solar cells formed on a soda-lime glass substrate to flexible substrates via an adhesive-bonding transfer method. Through this method, we were able to achieve the PCE of 5.8–7.1% on completely flexible substrates such as cloth, paper, and poly­(ethylene terephthalate) (PET). In particular, we were able to produce a CZTSSe solar cell on a PET substrate with a PCE of 7.1%, which is the highest among fully flexible CZTSSe solar cells currently known to us. In addition, we deeply analyzed the PCE degradation of the flexible CZTSSe solar cell fabricated by the transfer method through a panoramic focused ion-beam image and nanoindentation. From the results of our work, we provide an insight into the possibility of making flexible high-efficiency CZTSSe solar cells using our transfer method.

In this paper, we introduce very thin Indium tin oxide (ITO) layers (5, 10, and 15 nm) hybridized with a metal mesh to produce high-performance transparent conductive layers (TCLs) in near-ultraviolet light-emitting diodes (NUV LEDs). Using UV-vis-IR spectrometry, Hall measurement, and atomic force microscopy, we found that 10 nm was the optimal thickness for the very thin ITO layers in terms of outstanding transmittance and sheet resistance values as well as stable contact properties when hybridized with the metal mesh. The proposed layers showed a value of 4.56 Ω/□ for sheet resistance and a value of 89.1% for transmittance. Moreover, the NUV LEDs fabricated with the hybrid TCLs achieved ∼140% enhanced light output power compared to that of 150 nm thick ITO layers. Finally, to verify the practical usage of the TCLs for industrial applications, we packaged the NUV LED chips and obtained improved turn-on voltage (3.48 V) and light output power (∼116%) performance.

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