Publications 

First author papers (†equal contribution) 

12. S. B. Kang , G.Huang, G.Singhal, D. Xie, D. Hsieh , Y. Lee, A.A. Kulkarni, J. Smith, Q.Chen, K.Thornton, S. Sinha, and P. V. Braun “Highly Ordered Eutectic Mesostructures via Template-Directed Solidification within Thermally Engineered Templates”, Advanced Materials, 2308720 (2024)  

Template-directed self-assembly of solidifying eutectics results in emergence of unique microstructures due to diffusion constraints and thermal gradients imposed by the template. Here, we demonstrate the importance of selecting the template material based on its conductivity to control heat transfer between the template and the solidifying eutectic and thus the thermal gradients near the solidification front. Simulations elucidate the relationship between the thermal properties of the eutectic and template and the resultant microstructure. The overarching finding is that templates with low thermal conductivities are generally advantageous for forming highly organized microstructures. When we use electrochemically porosified silicon pillars (thermal conductivity < 0.3 Wm-1K-1) as the template into which an AgCl-KCl eutectic is solidified, 99% of the unit cells in the solidified structure exhibit the same pattern. In contrast, when higher thermal conductivity crystalline silicon pillars (~100 Wm-1K-1) are utilized, the expected pattern is only present in 50% of the unit cells. The thermally engineered template resulted in mesostructures with tunable optical properties and reflectances nearly identical to the simulated reflectances of perfect structures, indicating highly ordered patterns are formed over large areas. Our work highlights the importance of controlling heat flows in template-directed self-assembly of eutectics.


11. S.B Kang†, A Sanger†, M.H Jeong, J.M Baik, K.J Choi  "Heterogeneous Stacking of Reduced Graphene Oxide on ZnO Nanowires for NO2 Gas Sensors with Dramatically Improved Response and High Sensitivity ” Sensors and Actuators B: Chemical, 133196  (2023) 

Graphene or two-dimensional materials have been intensively studied as a new generation of gas sensing materials due to their large specific surface area and high mobility. However, fabrication processes for oxide and 2D materials lead to non-uniform structures of flakes of graphene or its derivatives and oxide nanowire, are randomly suspended to devices, resulting in poor and unrepeatable sensing performances. Here, we report on the heterogeneous stacked interface of reduced graphene oxide (rGO) on the surface of ZnO nanowires and their demonstration as a NO2 gas sensor. Compared to the conventional surface decoration using noble metals such as Au, Ag, and Pd, the present sensor shows excellent sensing performances including 22 times faster response behavior. Moreover, this interface-based rGO-ZnO gas sensor showed outperforming sensitivity and recovery time to reported 2D and 2D/oxide based gas sensor. The active sites of rGO are more favorable for chemisorption of oxygen molecules due to functional groups on rGO surfaces. Moreover, the gas-sensing mechanism is firstly elucidated by the finite-difference time-domain (FDTD) simulation, confirming that mono-to-few layers of rGO on ZnO act a role of bridge, facilitating the migration of electrons from ZnO to NO2, leading to higher increment of depletion region and corresponding sensor response. Our approaches may offer the new opportunities and strategies for highly sensitive and fast recoverable 2D materials/oxide hybrid sensors. 


10. S.B Kang, R Sharma, M Jo, S.I Kim, J Hwang, S.H Won, J.C Shin, K.J Choi

  "Catalysis-Free Growth of III-V Core-Shell Nanowires on p-Si for Efficient Heterojunction Solar Cells with Optimized Window Layer ” Energies 15 (5), 1772  (2022) 

The growth of high-quality compound semiconductor materials on silicon substrates has long been studied to overcome the high price of compound semiconductor substrates. In this study, we successfully fabricated nanowire solar cells by utilizing high-quality hetero p-n junctions formed by growing n-type III-V nanowires on p-silicon substrates. The n-InAs0.75P0.25 nanowire array was grown by the Volmer–Weber mechanism, a three-dimensional island growth mode arising from a lattice mismatch between III-V and silicon. For the surface passivation of n-InAs0.75P0.25 core nanowires, a wide bandgap InP shell was formed. The nanowire solar cell was fabricated by benzocyclobutene (BCB) filling, exposure of nanowire tips by reactive-ion etching, electron-beam deposition of ITO window layer, and finally metal grid electrode process. In particular, the ITO window layer plays a key role in reducing light reflection as well as electrically connecting nanowires that are electrically separated from each other. The deposition angle was adjusted for conformal coating of ITO on the nanowire surface, and as a result, the lowest light reflectance and excellent electrical connectivity between the nanowires were confirmed at an oblique deposition angle of 40°. The solar cell based on the heterojunction between the n-InAs0.75P0.25/InP core-shell nanowire and p-Si exhibited a very high photoelectric conversion efficiency of 9.19% with a current density of 27.10 mA/cm2, an open-circuit voltage of 484 mV, and a fill factor of 70.1%. 


9. S. B. Kang†, S. Baurzhan†, W.J Park, M.H Jeong, J. Kim and K.J. Choi  "Colorful transparent silicon photovoltaics with unprecedented flexibility” Advanced Functional Materials, 2110435 (2021) 

Integrated photovoltaics regarded as next-generation photovoltaic technologies that can generate electricity in urban areas with limited available land while also serving as aesthetic architectural elements. The criteria for integrating photovoltaics into buildings and electronic devices are flexibility, color tunability, efficiency, scalability, and stability. It is very challenging for BIPVs to demonstrate all-around performance benefits because photovoltaic performances exist in a trade-off relation, such as that between transparency and efficiency. Here, we demonstrated great all-around TSC; highly-flexible, high-transparency, and the highest output solar cells. The TSC devices exhibit PCE values of 7.38% and 5.52% at the average visible transparencies (AVT) of 45% and 60%, respectively. By introducing a periodic hole array structure, the flexibility of TSCs was dramatically improved. The minimum bending radius decreased from 100 mm to 6 mm; it further decreased to 3 mm after PDMS embedding. The results of the FDTD simulation show that the periodic hole array structure uniformly distributes the stress across the entire area as a self-stress relief structure. The PDMS-embedded TSCs demonstrate unprecedently high flexibility and long-term stability without significant degradation even after cyclic bending deformations up to 1000 cycles and 1500 h of the standard damp heat test.


8.  S. B. Kang, W. J. Park, M. H. Jeong, S. Kang, C. Yang and K.J. Choi "Ambipolar passivated back surface field layer for Silicon Photovoltaics” Advanced Functional Materials , 2004943 (2020) (IF = 16.836) 

The suppression of surface recombination is of primary importance for realizing efficient silicon (Si) photovoltaics, which is usually achieved by introducing passivation or back-surface field (BSF) layers. In this study, we demonstrated for the first time that self-assembled, ferroelectric and organic thin films can be used as passivating BSF layers for both n- and p-type Si solar cells. The n-Si / PEDOT:PSS heterojunction solar cell with the P(VDF-TrFE) exhibited an efficiency of 18.37 %, which is a record-high efficiency for Si/organic hybrid solar cells. In addition, homojunction p-Si solar cells with the P(VDF-TrFE) yielded superior performance compared to the standard aluminium-BSF cells. Finite-difference time-domain simulations revealed that the electric field due to the poled ferroelectric layer penetrated deep into the backside of Si, causing band bending and, consequently, reducing surface recombination. Moreover, the solar cell with the ferroelectric interlayer maintained > 95% of its initial performance even after 1000 h of the standard damp heat test. This work now endows the Si-based photovoltaics with the superior passivation and high-performance which are previously exclusive to inorganics. 

7.  J.H. Kim†, S. B. Kang†, H.H. Yu†, J. Kim, J. Ryu, J.W. Lee, K.J. Choi, C.M. Kim, C. Yi, "Augmentation of absorption channels induced by wave-chaos effects in free-standing nanowire arrays" Optics Express 28, 23569-23583 (2020) (IF = 3.551) 

Plenty of issues on quantal features in chaotic systems have been raised since chaos was accepted as one of the intrinsic properties of nature. Through intensive studies, it was revealed that resonance spectra in chaotic systems exhibit complicated structures, which is deeply concerned with sophisticated resonance dynamics. Motivated by these phenomena, we investigate light absorption characteristics of chaotic nanowires in an array. According to our results, a chaotic cross-section of a nanowire induces a remarkable augmentation of absorption channels, that is, an increasing number of absorption modes leads to substantial light absorption enhancement, as the deformation of cross-section increases. We experimentally demonstrate the light absorption enhancement with free-standing Si-nanowire polydimethylsiloxane (PDMS) composites. Our results are applicable not only to transparent solar cells but also to complementary metal-oxide-semiconductor (CMOS) image sensors to maximize absorption efficiency. 

6. S. B. Kang, J. Kim, M. H. Jeong, A. Sanger, C. Kim and K.J. Choi "Stretchable and colorless free-standing microwire arrays for transparent solar cells with flexibility” Light: science and applications, 8, 121 (2019) (Nature Publishing Group, IF = 13.714) 

Transparent solar cells (TSC) are emerging devices that combine the advantages of visible transparency and light-to-electricity conversion. Currently, existing TSCs are based predominantly on organics, dyes, and perovskites; however, rigidity and the color-tinted transparent nature of those devices strongly limits the utility of the resulting TSCs for real-world applications. Here, we demonstrate a flexible, color-neutral, and high-efficiency TSC based on a free-standing-form of n-silicon microwires (SiMW). Flat-tip-of SiMWs with controllable spacing are fabricated via deep reactive ion etching and embedded in the freestanding transparent polymer matrix. The light transmittance can be tuned from ~10% to 55% by adjusting the spacing between the microwires. For the TSCs, a hetero-junction is formed with p-type polymer in the top portion of the n-type flat-tip-of-SiMW, ohmic contact with indium-doped-ZnO film occurs at the bottom and the side surface has an Al2O3 passivation layer. Furthermore, the slanted-tip-of-SiMW is developed by the novel solvent-assisted wet etching to manipulate light absorption. Finite-difference time-domain simulation revealed that the reflected light from slanted-tip-of-SiMW helps light-matter interactions in adjacent microwire. The TSC based on the slanted-tip-of-SiMPF demonstrates 8 % efficiency at the visible transparency of 10 % with flexibility. This efficiency is the highest among Si based TSCs and comparable with state-of-the-art neutral-color TSCs based on organic-inorganic hybrid perovskite and organics. Moreover, unlike others, the stretchable and transparent platform in this study is promising for future TSCs. 

5. A.Sanger† , S. B. Kang†  M. H. Jeong,  C. U. Kim, J.M. Baik,  K. J. Choi "All-transparent NO2 gas sensor based on free-standing Al doped ZnO nanofibers ” ACS applied electronic materials, 17, 1261-1268  (2019) (IF = pending) 

Transparent optoelectronics can enable new class of applications such as transparent displays, smart windows, and invisible sensors. Here, we demonstrate all-transparent NO2 gas sensors based on aluminum-doped zinc oxide (AZO) free-standing hollow nanofibers. Free-standing AZO nanofibers are fabricated by sputtering AZO on template polyvinylpyrrolidone (PVP) nanofibers, which are electrospun on a glass frame with indium zinc oxide (IZO) transparent electrodes, followed by a heat treatment to remove the PVP template nanofibers. Not only the gas sensing active material, but also other components such as the substrate and electrodes are all transparent in the visible region. The optical transparency of the nanofibers is controlled by changing the AZO nanofibers density without compromising the sensitivity. The gas sensing measurements of transparent sensor depicts n-type response behavior with full recovery even at low NO2 concentrations (0.5 ppm). The high sensitivity of the transparent sensors is attributed to the higher surface area of the hollow nanofibers and the high impact frequency of trapped NO2 gas inside the hollow compared to solid counterpart nanofibers. The unique combination of transparency and high sensitivity can potentially have applications in advanced sensor systems that can be attached to windows integrated with IoT (Internet of Things). 

4.  S. B. Kang† , K.C Kwon† , K.S Choi, R. Lee, A. Sanger, S.Y Kim, H.W. Jang, K.J. Choi, “Transfer of ultrathin molybdenum disulfide and transparent nanomesh electrode onto silicon for efficient heterojunction solar cells” Nano Energy, 50, 649-658 (2018) (IF = 16.602) 

Two-dimensional transition-metal dichalcogenides (TMDCs) are very promising for photovoltaic (PV) applications due to their excellent light absorption properties and appropriate bandgap energy, Although multifunctional applications of TMDCs in photovoltaic devices have been achieved, the photovoltaic conversion efficiency under 1 sun is still very low with small active area because of their inexpedient high sheet resistance and limitation of synthesis techniques. In this study, we demonstrate uniform synthesis of 4-in. wafer-scale MoS2 thin films by thermal decomposition of solution precursors. The solar cells are fabricated by transferring n-MoS2 thin films on p-Si substrates to form p-n heterojunctions and then transferring Au nanomeshes prepared in a novel surface treatment as transparent top electrodes onto MoS2. The circular n-MoS2/p-Si heterojunction solar cell exhibited a power conversion efficiency of 5.96% at a diameter of 0.3 in. and proved to be easily scalable to 1-in. diameter with 5.18% efficiency. To the best of our knowledge, the solar cells of this study are the most efficient and the largest in all types of solar cells based on TMDC reported so far. Finally, based on finite difference time domain simulation, we proposed a strategy for implementing n-MoS2/p-Si heterojunction solar cell with efficiency higher than 15% by introducing optimal doping control of n-MoS2 and efficient anti-reflection layers.

3. A.Sanger† , S. B. Kang†  M. H. Jeong, M. J. Im, I. Y. Choi, C. U. Kim, H.M. Lee, K. J. Choi “Morphology-controlled aluminum-doped zinc oxide nanofibers for highly sensitive NO2 sensor with full recovery at room temperature” Advanced Science, 1800816 (2018) (IF = 15.840) 

Room-temperature (RT) gas sensitivity of morphology-controlled freestanding hollow aluminum-doped zinc oxide (AZO) nanofibers for NO2 gas sensors is presented. The free-standing hollow nanofibers are fabricated using a polyvinylpyrrolidone fiber template electrospun on a copper electrode frame followed by radio-frequency sputtering of an AZO thin overlayer and heat treatment at 400 °C to burn off the polymer template. The thickness of the AZO layer is controlled by the deposition time. The gas sensor based on the hollow nanofibers demonstrates fully recoverable n-type RT sensing of low concentrations of NO2 (0.5 ppm). A gas sensor fabricated with Al2O3-filled AZO nanofibers exhibits no gas sensitivity below 75 °C. The gas sensitivity of a sensor is determined by the density of molecules above the minimum energy for adsorption, collision frequency of gas molecules with the surface, and available adsorption sites. Based on finite-difference time domain simulations, the RT sensitivity of hollow nanofiber sensors is ascribed to the ten times higher collision frequency of NO2 molecules confined inside the fiber compared to the outer surface, as well as twice the surface area of hollow nanofibers compared to the filled ones. This approach might lead to the realization of RT sensitive gas sensors with 1D nanostructures.

2. S. B. Kang, M. H. Jeong, I. Y. Choi, S.-D. Sohn, S. H. Kim, H.-J. Shin, W. I. Park, J. C. Shin, M. H. Song, K. J. Choi, “Self-Assembled, Highly Crystalline Porous Ferroelectric Poly (Vinylidene Fluoride-co-Trifluoroethylene) interlayer for Si/Organic Hybrid Solar Cells,” Nano Energy 41, 243-250 (2017) (IF = 16.602) 

Ferroelectric polymers can effectively improve the photovoltaic performance of solar cells, inducing an electric field to promote the dissociation of electron-hole pairs, with the thus generated charges collected from open pores. Since such performance enhancement requires materials with a unique porous crystalline structure, we herein present a novel route to highly crystalline and porous poly(vinylidene fluoride-co-trifluoroethylene) (P (VDF-TrFE)) thin films utilizing a modified breath figure method based on spin coating. The key feature of the above method is the addition of small amounts of water to the acetone/P(VDF-TrFE) solution to produce porous ferroelectric thin films which have significantly higher crystallinity values than nanostructures or films prepared by other methods. Furthermore, n-Si / poly(3,4-ethylene dioxy thiophene):poly(styrene sulfonate) hybrid solar cells with porous P(VDF-TrFE) interlayers are demonstrated to exhibit spontaneous polarization sufficient for increasing their open circuit voltages and fill factors. Finite-difference time-domain simulation reveals that the electric field due to the above spontaneous polarization increases the built-in electric field of the Schottky junction between n-Si and poly(3,4-ethylene dioxy thiophene):poly(styrenesulfonate) and reduces the reverse leakage current of the Schottky diode. Thus, the organic ferroelectric thin films with controlled porosity proposed in this study are well suited for a broad range of optoelectronic applications.

1.  S. B. Kang, S. H. Won, M. J. Im, C. U. Kim, W. I. Park, J. M. Baik, K. J. Choi, “Enhanced piezoresponse of highly aligned electrospun poly (vinylidene fluoride) nanofibers,” Nanotechnology 28, 395402 (2017) (IF = 3.551)

Well-ordered nanostructure arrays with controlled densities can potentially improve material properties; however, their fabrication typically involves the use of complicated processing techniques. In this work, we demonstrate a uniaxial alignment procedure for fabricating poly (vinylidene fluoride) (PVDF) electrospun nanofibers (NFs) by introducing collectors with additional steps. The mechanism of the observed NF alignment, which occurs due to the concentration of lateral electric field lines around collector steps, has been elucidated via finite difference time-domain simulations. The membranes composed of well-aligned PVDF NFs are characterized by a higher content of the PVDF β-phase, as compared to those manufactured from randomly orientated fibers. The piezoelectric energy harvester, which was fabricated by transferring well-aligned PVDF NFs onto flexible substrates with Ag electrodes attached to both sides, exhibited a 2-fold increase in the output voltage and a 3-fold increase in the output current as compared to the corresponding values obtained for the device manufactured from randomly oriented NFs. The enhanced piezoresponse observed for the aligned PVDF NFs is due to their higher β-phase content, denser structure, smaller effective radius of curvature during bending, greater applied strain, and higher fraction of contributing NFs.

Contributing Author 

11.  S. Baurzhan, J. E. Kang, N.A. Raju, S. B. Kang, K.J. Choi, “ Three Dimensionally Stretchable Photovoltaic Module with Improved Mechanical Robustness through Elastomeric Interconnections ”, Solar RRL (2024) 

Stretchable electronics have attracted a lot of attention due to their potential applications in various innovative fields such as e-skins, health care monitoring, soft robotics, wearable energy harvesting, etc. In this work, stretchable solar modules are demonstrated by sandwiching electrically connected rigid solar cells between top and bottom polydimethylsiloxane (PDMS) layers, which are then attached onto an elastomer with an island and trench structure. The electrical connection of solar cells is achieved by dispenser-printing of electrically conductive adhesive (ECA). The fabricated stretchable solar modules have a maximum bi-axial elongation of 75%, and the electrical connection of the solar cells using ECA resulted in a negligible efficiency loss of 1.6%. In particular, embedding of solar cells and interconnects between PDMS layers enhances mechanical stability by placing these key components at the mechanical neutral plane and the long-term stability against humidity due to the encapsulation role of the PDMS layer. The module demonstrates only a 5% of efficiency drop after 1000 hours in the dump-heat test (85°C/85%). The results in this study are not limited to solar cells but can be applied to various types of rigid devices, paving the way for the commercialization of stretchable electronics.


10.  M.H Jeong, S. B. Kang and K.J. Choi “ Ambient-temperature-independent power generation in wearable thermoelectric generator with CNTs/MoS2 solar absorber” ACS Applied Electronic Materials (2024) 

The low-temperature difference (ΔT) of the body-heat driven wearable thermoelectric generators (WTEG) is one of the major issues that set back the application of the device. Recently, we proposed a WS-TEG that achieves high ΔT by introducing a solar absorber on the hot side of the device, which has inspired many subsequent studies. Here, we propose a systematic approach to further increase the ΔT by considering heat conduction and convection in WS-TEG. For the fabrication of WS-TEGs, TE legs are dispenser-printed with BiTe-based ink, and the CNT/MoS2 solar absorber was spin-coated on a polyimide substrate. The ΔT was ramped up by incorporating a PDMS lid to prevent convective heat loss on the hot side and Cu foams as heat sinks to accelerate heat dissipation on the cold side. Upon exposure to sunlight, the wearable TEG exhibits the ΔT of 33.9 ℃. In addition, unlike conventional WTEG, the ΔT of the WS-TEG is maintained almost independent of ambient temperature due to the stable and sustained absorption of sunlight. We believe the suggested methodology is a pragmatic and viable solution to the ΔT issue, one of the biggest hurdles for wearable TEGs 

9. A.K. Ambedkar, Y. K. Gautam, D. Gautam, A. Kumar, A. Sanger, S. B. Kang, M. Singh, S. Vikal, B. P. Singh, “ Experimental and theoretical investigation of palladium-doped zinc oxide nanorods for NO2 gas sensor, Journal of materials science: materials in electronics (2023) 

The high concentration of nitrogen dioxide (NO2) gets it one of the most popular and harmful air pollutants. This work examines the NO2 gas sensing properties of palladium-doped zinc oxide (Pd-ZnO) nanorods. The Pd-ZnO nanorods were synthesized by chemical hydrothermal method with different Pd doping concentrations (0–1 wt%). The Pd-ZnO nanorods were characterized by XRD, FESEM, and XPS for their structural and morphological properties, respectively. The ZnO nanostructures show hexagonal structures, and XRD and XPS results confirmed the doping of Pd on ZnO nanostructures. The Pd (1 wt%)-ZnO nanorods-based sensor shows high response of 22.1 with response/recovery time of 67/118 s toward 100 ppm NO2, while it exhibits a response of 7 with response/recovery times of 80/145 s for 1 ppm NO2, at 200 °C. The sensor is observed very selective for NO2 compared to other gases like carbon monoxide (CO), ammonia (NH3), and hydrogen (H2). The sensor has strong stability for a longer time (35 days) in a dry and humid (RH 60%) environment. The mechanism of gas sensors is further explained by the Crowell-Sze model in Finite-Difference Time-Domain (FDTD) simulation using COMSOL Multiphysics using drift Diffusion-Poisson equations to simulate the electric potential distribution in the nanorods during the gas sensing. 

8.  A. Kumar, A. Sanger, S. B. Kang, and R. Chandra, “Interface engineering-driven room temperature ultra-low gas sensing performance of heterostructure transition metal dichalcogenide thin films: Experimental and Theoretical Investigation”, ACS Sensor (2023) 

In this report, we investigate the room-temperature gas sensing performance of heterostructure transition metal dichalcogenide (MoSe2/MoS2, WS2/MoS2, and WSe2/MoS2) thin films grown over a silicon substrate using a pulse laser deposition technique. The sensing response of the aforementioned sensors to a low concentration range of NO2, NH3, H2, CO, and H2S gases in air has been assessed at room temperature. The obtained results reveal that the heterojunctions of metal dichalcogenide show a drastic change in gas sensing performance compared to the monolayer thin films at room temperature. Nevertheless, the WSe2/MoS2-based sensor was found to have an excellent selectivity toward NO2 gas with a particularly high sensitivity of 10 ppb. The sensing behavior is explained on the basis of a change in electrical resistance as well as carrier localization prospects. Favorably, by developing a heterojunction of diselenide and disulfide nanomaterials, one may find a simple way of improving the sensing capabilities of gas sensors at room temperature. 

7.  V. V. Sharma, K. N. Kim, G. H. Han, E. J. Gwak, J. H. Woo, S. B. Kang, K. J. Choi, J. Y Kim, J.M. Baik "3D Multiscale Gradient Pores Impregnated with Ag Nanowires for Simultaneous Pressure and Bending Detection with Enhanced Linear Sensitivity” Advanced Materials Technologies, 1901041 (2020)

Developing a successful strategy to fabricate multifunctional sensors with high linear sensitivity is crucial for enhancing tactile interactions of the human body with the surrounding environment. Here, a stretchable multifunctional sensor consisting of stretchable porous film with multiscale aligned pores and Ag nanomesh electrode is reported. Due to the gradient in pore size and porosity inside the film, the sensor can detect bending and pressure simultaneously and independently as well as bending direction with quite high linear sensitivity up to 2 kPa. The 12 sensors are attached onto the tip and two joints each of four human fingers of one hand, and the capacitance changes measured by grabbing two different balls provide exact information associated with various hand gestures.

6.  M. H. Jeong, A. Sanger, S. B. Kang, Y. S. Jung, I. Oh, J.W. Yoo, G.H. Kim, K. J. Choi "Increasing thermoelectric power factor of solvent-treated PEDOT : PSS thin films on PDMS by stretching” Journal of Materials chemistry A, 6, 15621-15629 (2018)

Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) is one of the most promising candidate materials for flexible thermoelectric generators. We report changes in morphological, electrical, and thermoelectric properties of strained PEDOT:PSS thin films treated with solvents, including H2SO4, MeOH, and polyethylene glycol (PEG). The stretching creates macroscopic cracks on the thin film surface and microscopically change the domain structure from a dense network to an isolated state. With increasing strain, short, fine cracks at small strains propagate into long, thick cracks. The H2SO4 treatment makes PEDOT:PSS very brittle even at  5% strain, whereas PEG improves the stretchability of PEDOT:PSS up to 50%. Stretching reduces the electrical conductivity of solvent-treated samples, which agrees well with the decreased hole concentration of strained PEDOT:PSS, as confirmed by ultraviolet photoemission spectroscopy. In contrast, the Seebeck coefficient (S) of the strained PEDOT:PSS thin films increases. The PEG sample shows the highest S, and thus exhibits an enhanced power factor (PF) with strain, while other solvent treatments reduce PF. To the best of our knowledge, this is the first report on the strain-induced enhancement of S and thermoelectric PF of PEDOT:PSS.

5.  M. G. Lee, C. W. Moon, H. Park, W. Sohn, S. B. Kang, S. Lee, K. J. Choi, H. W. Jang, “Dominance of Plasmonic Resonant Energy Transfer over Direct Electron Transfer in Substantially Enhanced Water Oxidation Activity of BiVO4 by Shape‐Controlled Au Nanoparticles,” Small 13, 1701644 (2017)

The performance of plasmonic Au nanostructure/metal oxide heterointerface shows great promise in enhancing photoactivity, due to its ability to confine light to the small volume inside the semiconductor and modify the interfacial electronic band structure. While the shape control of Au nanoparticles (NPs) is crucial for moderate bandgap semiconductors, because plasmonic resonance by interband excitations overlaps above the absorption edge of semiconductors, its critical role in water splitting is still not fully understood. Here, first, the plasmonic effects of shape-controlled Au NPs on bismuth vanadate (BiVO4) are studied, and a largely enhanced photoactivity of BiVO4 is reported by introducing the octahedral Au NPs. The octahedral Au NP/ BiVO4 achieves 2.4 mA cm−2 at the 1.23 V versus reversible hydrogen electrode, which is the threefold enhancement compared to BiVO4. It is the highest value among the previously reported plasmonic Au NPs/BiVO4. Improved photoactivity is attributed to the localized surface plasmon resonance; direct electron transfer (DET), plasmonic resonant energy transfer (PRET). The PRET can be stressed over DET when considering the moderate bandgap semiconductor. Enhanced water oxidation induced by the shape-controlled Au NPs is applicable to moderate semiconductors, and shows a systematic study to explore new efficient plasmonic solar water splitting cells.

4. J. Hwang, D. Patil, S. B. Kang, K. J. Choi, J. C. Shin, “Polyacrylic Acid / Polypyrrole / Silver Nanowires Nanocomposite Electrode for Electrochemical Supercapacitor,” Journal of Nanoscience and Nanotechnology 17, 8138-8143 (2017)

This paper reports the synthesis and enhanced electrochemical performance of polypyrrole (PPY)/polyacrylic acid (PAA)/silver nanowire (AgNW) nanocomposites. In this ternary structure, the AgNWs provide a highly conductive path to the PPY spherical granules and PAA acts as a binder. The deposition of PPY-PAA-AgNWs was carried out on a stainless steel substrate using a dip coating technique. The surface morphology of the films was examined by field emission scanning electron microscopy, which revealed the presence of AgNWs on the PPY-PAA granular background that further reduced the charge transfer resistance. The PPY-PAA-AgNWs showed a specific capacitance of 383 Fg−1 at 10 mVs−1 and an energy density of 3.75 Whkg−1 at 0.2 mA, indicating the positive synergistic effect of AgNWs and PPY.

3. Y. S. Jung, D. H. Jeong, S. B. Kang, F. Kim, M. H. Jeong, K.-S. Lee, J. M. Baik, J.-S. Kim, K. J. Choi, “Wearable solar thermoelectric generator driven by unprecedentedly high temperature difference,” Nano Energy 40, 663-672 (2017)

Converting body heat into electricity using flexible thermoelectric generators can be useful for self-powered wearable electronic devices. However, the temperature difference that can be obtained by body heat is insufficient, which limits its practical applications. In this study, we present a wearable solar thermoelectric generator driven by a significantly high temperature difference created by introducing a local solar absorber and thermoelectric legs on a polyimide substrate. The solar absorber is a five-period Ti/MgF2 superlattice, in which the structure and thickness of each layer was designed for optimal absorption of sunlight. The thermoelectric legs were prepared by dispenser printing with an ink consisting of mechanically alloyed BiTe-based powders and an Sb2Te3-based sintering additive dispersed in glycerol. Thermoelectric p- and n-type legs have electrical conductivities of ~ 25,000 S m−1 with Seebeck coefficients of 166.37 and −116.38 μV K−1, respectively. When exposed to sunlight, a wearable solar thermoelectric generator comprising 10 pairs of p-n legs has an open circuit voltage of 55.15 mV and an output power of 4.44 μW. The temperature difference is as high as 20.9 °C, which is much higher than the typical temperature differences of 1.5–4.1 °C of wearable thermoelectric generators driven by body heat. The wearable solar thermoelectric generators have been demonstrated on various surfaces exposed to sunlight, such as clothes or windows.

2. M. J. Park, C. U. Kim, S. B. Kang, S. H. Won, J. S. Kwak, C.-M. Kim, K.J Choi, " 3D Hierarchical Indium Tin Oxide Nanotrees for Enhancement of Light Extraction in GaN-Based Light Emitting Diodes," Advanced Optical Materials 5, 1600684 (2017)

Recently, 3D nanostructures have attracted much interest because of their interesting electrical/optical properties such as wave guiding modes, light scattering, antireflection effects, etc. In this work, a facile yet efficient method for the fabrication of hierarchical 3D indium tin oxide (ITO) nanotrees (NTs) and their integration in GaN-based blue-light-emitting diodes (LEDs) for efficient light-extraction are reported. The ITO NTs are fabricated by the obliqueangle (≈85°) deposition method at 240 °C using electron-beam evaporation. The ITO NTs grow via a self-catalytic vapor–liquid–solid mechanism with the branches having an epitaxial relationship with the trunks. The ITO NTs successively deposited on an ITO thin film as a p-contact layer are annealed at 600 °C for 1 min under ambient air in order to form a transparent ohmic contact. The indium gallium nitrde/gallium nitride (InGaN/GaN) LED with ITO NTs presents a 29.5% enhancement in the light output power at an injection current of 20 mA, compared to the reference LED with an ITO thin film p-contact. This enhancement is ascribed to the effective light extraction of the ITO NTs due to to the gradually decreasing profile of the refractive index from 2.08 (ITO thin film), 1.15 (dense ITO NTs), 1.06 (porous ITO NTs) to 1.0 (air). These results are in good agreement with the optical simulation by the COMSOL wave optics module.

1. J. Kwon, M. J. Im, C. U. Kim, S. H. Won, S. B. Kang, S. H. Kang, I. T. Choi, H. K. Kim, I. H. Kim, J. H. Park, K. J. Choi, “Two-terminal DSSC/silicon tandem solar cells exceeding 18% efficiency,” Energy & Environmental Science 9, 3657-3665 (2016) 

The Si photovoltaics industry is one of the most promising energy industries. There is a method to add inexpensive top solar cells to crystalline silicon (c-Si) bottom solar cells to overcome the theoretical efficiency. The advantages of dye sensitizedmaterials include their adjustable band gap and easy synthesis process. In this work, we fabricated a 2-terminal DSSC/Si tandem solar cell by developing an ITO/PEDOT:FTS tunnel junction that facilitates carrier charge recombination between the DSSC and c-Si subcells. In addition, there are also consequences for water splitting due to this low-cost method.