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Organic-inorganic perovskites have arrived at the forefront of solar technology due to their impressive carrier lifetimes and superior optoelectronic properties. By having the cm-sized perovskite single crystal and employing device patterning techniques, and the transfer length method (TLM), we are able to get the insight into the metal contact and carrier transport behaviors, which is necessary for maximizing device performance and efficiency. In addition to the metal work function, we found that the image force and interface charge pinning effects also affect the metal contact, and the studied single crystal CH3NH3PbBr3 features Schottky barriers of 0.17 eV, 0.38 eV, and 0.47 eV for Au, Pt, and Ti electrodes, respectively. Furthermore, the surface charges lead to the thermally activated transport from 207 K to 300 K near the perovskite surface. In contrast, from 120 K to 207 K, the material exhibited three-dimensional (3D) variable range hopping (VRH) carrier transport behavior. Understanding these fundamental contact and transport properties of perovskite will enable future electronic and optoelectronic applications.

Numerical studies of laminar forced convective heat transfer and fluid flow in a 2D louvered microchannel with Al2O3/water nanofluids are performed by the lattice Boltzmann method (LBM). Eight louvers are arranged in tandem within the single-pass microchannel. The Reynolds number based on channel hydraulic diameter and bulk mean velocity ranges from 100 to 400, where the Al2O3 fraction varies from 0 to 4%. A double distribution function approach is adopted for modeling fluid flow and heat transfer. Code validations are performed by comparing the streamwise Nusselt number (Nu) profiles and Fanning friction factors of the present LBM and those of the analytical solutions. Good agreements are obtained. Simulated results show that the louver microstructure can disturb the core flow and guide coolant toward the heated walls, thus enhancing the heat transfer significantly. Furthermore, the addition of nanoparticles in microchannels can also augment the heat transfer, but it creates an unnoticeable pressure loss. With both the louver microstructure and nanofluid, a maximum overall Nu enhancement of 7.06 is found relative to that of the fully developed smooth channel.

The present study reports the effects of flow pulsation on laminar forced convective heat transfer in an innovative louvered microchannel heat sink (MCHS) with lattice Boltzmann method (LBM). Two to eight louver-like microstructures are arranged in tandem within a microchannel, corresponding to a pitch ratio (PR) of 0.25 to 1.75. The Reynolds number (Re) based on channel hydraulic diameter and periodically averaged bulk mean velocity ranges from 100 to 300. The pulsating inlet velocity is in triangular waveform with Strouhal number (St) varying from 0 to 2.8. In terms of numerical models, a double distribution function LBM approach is employed for modeling both fluid flow and heat transfer. Simulation results show that the profiles of Nusselt number (Nu) and Fanning friction factor (f) with St are all multi-peak for the present louvered MCHS. This finding is different from the previous single-peak trend. The correlations of overall Nu and f ratios ( N u ¯ / N u 0 and f ¯ / f 0 ) versus Re, PR, and St for the present MCHS are established for the first time. Furthermore, the present design attains a maximum N u ¯ / N u 0 of 6.16 at St = 1.8, PR = 0.58, and Re = 300, which is the highest for f ¯ / f 0 50 among previously reported ribbed and baffled MCHSs.

By employing graphene quantum dots (GQDs), we have achieved a high efficiency of 16.55% in n-type Si heterojunction solar cells. The efficiency enhancement is based on the photon downconversion phenomenon of GQDs to make more photons absorbed in the depletion region for effective carrier separation, leading to the enhanced photovoltaic effect. The short circuit current and the fill factor are increased from 35.31 to 37.47 mA/cm(2) and 70.29% to 72.51%, respectively. The work demonstrated here holds the promise for incorporating graphene-based materials in commercially available solar devices for developing ultrahigh efficiency photovoltaic cells in the future.

In the pursuit of novel functionalities by utilizing the lattice degree of freedom in complex oxide heterostructure, the control mechanism through direct strain manipulation across the interfaces is still under development, especially with various stimuli, such as electric field, magnetic field, light, etc. In this study, the superlattices consisting of colossal-magnetoresistive manganites La0.7Sr0.3MnO3 (LSMO) and photostrictive SrRuO3 (SRO) have been designed to investigate the light-dependent controllability of lattice order in the corresponding functionalities and rich interface physics. Two substrates, SrTiO3 (STO) and LaAlO3 (LAO), have been employed to provide the different strain environments to the superlattice system, in which the LSMO sublayers exhibit different orbital occupations. Subsequently, by introducing light, we can modulate the strain state and orbital preference of LSMO sublayers through light-induced expansion of SRO sublayers, leading to surprisingly opposite changes in photoresistivity. The observed photoresistivity decreases in the superlattice grown on STO substrate while increases in the superlattice grown on LAO substrate under light illumination. This work has presented a model system that demonstrates the manipulation of orbital–lattice coupling and the resultant functionalities in artificial oxide superlattices via light stimulus.

Graphene dots (GDs) are used for enhancing the performance of the poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS)-based counter electrodes in Pt-free dye-sensitized solar cells (DSSCs). As compared to PEDOT:PSS CEs, GD–PEDOT:PSS films possess a rough surface morphology, high conductivity and electrocatalytic activity, and low charge-transfer resistance toward I − /I 3 − redox reaction, pushing cell efficiency to 7.36%, which is 43% higher than that of the cell with PEDOT:PSS CEs (5.14%). Without much impact on efficiency, the DSSCs with GD–PEDOT:PSS CEs work well under low-light conditions (light intensity −2 and angle of incidence >60°), such as indoor and low-level outdoor lighting and of the sun while the other traditional cells would fail to work. The concurrent advantage in low cost in Pt-free materials, simple fabrication processes, comparable efficiency with Pt CEs, and high performance under low-light conditions makes the DSSC with GD–PEDOT:PSS CEs suitable to harvest light for a diverse range of indoor and low-level outdoor lighting locations.

Semipolar {101¯1} InGaN quantum wells are grown on (001) Si substrates with an Al-free buffer and wafer-scale uniformity. The novel structure is achieved by a bottom-up nano-heteroepitaxy employing self-organized ZnO nanorods as the strain-relieving layer. This ZnO nanostructure unlocks the problems encountered by the conventional AlN-based buffer, which grows slowly and contaminates the growth chamber.

Harvesting solar energy at extremely harsh environments is of practical interest for building a self-powered harsh electronic system. However, working at high temperature and radiative environments adversely affects the performance of conventional solar cells. To improve the performance, GaN-based multiple quantum wells (MQWs) are introduced into the solar cells. The implementation of MQWs enables improved efficiency (+0.52%/K) and fill factor (+0.35%/K) with elevated temperature and shows excellent reliability under high-temperature operation. In addition, the GaN-based solar cell exhibits superior radiation robustness (lifetime >30 years under solar storm proton irradiation) due to their strong atomic bonding and direct-bandgap characteristics. This solar cell employing MQW nanostructures provides valuable routes for future developments in self-powered harsh electronics.

Flexible electronics will form the basis of many next-generation technologies, such as wearable devices, biomedical sensors, the Internet of things, and more. However, most flexible devices can bear strains of less than 300% as a result of stretching. In this work, we demonstrate a simple and low-cost paper-based photodetector array featuring superior deformability using printable ZnO nanowires, carbon electrodes, and origami-based techniques. With a folded Miura structure, the paper photodetector array can be oriented in four different directions via tessellated parallelograms to provide the device with excellent omnidirectional light harvesting capabilities. Additionally, we demonstrate that the device can be repeatedly stretched (up to 1000% strain), bent (bending angle ±30°), and twisted (up to 360°) without degrading performance as a result of the paper folding technique, which enables the ZnO nanowire layers to remain rigid even as the device is deformed. The origami-based strategy described herein suggests avenues for the development of next-generation deformable optoelectronic applications.

Nitrogen-doped graphene (NGR) was utilized in dye-sensitized solar cells for energy harvesting. NGR on a Pt-sputtered fluorine-doped tin oxide substrate (NGR/Pt/FTO) as counter electrodes (CEs) achieves the high efficiency of 9.38% via the nitrogen doping into graphene. This is due to (i) the hole-cascading transport at the interface of electrolyte/CEs via controlling the valence band maximum of NGR located between the redox potential of the I–/I– redox couple and the Fermi level of Pt by nitrogen doping, (ii) the extended electron transfer surface effect provided by large-surface-area NGR, (iii) the high charge transfer efficiency due to superior catalytic characteristics of NGR via nitrogen doping, and (iv) the superior light-reflection effect of NGR/Pt/FTO CEs, facilitating the electron transfer from CEs to I3– ions of the electrolyte and light absorption of dye. The result demonstrated that the NGR/Pt hybrid structure is promising in the catalysis field.

This paper demonstrates the high-temperature operation of fully transparent solar-blind deep ultraviolet (DUV) metal–semiconductor–metal (MSM) photodetectors (PDs) employing $\beta \hbox{-Ga}_{2}\hbox{O}_{3}$ thin films with transmittance up to 80% from 400 to 900 nm without image blurring. Even at a bias up to 200 V, the $\beta \hbox{-Ga}_{2}\hbox{O}_{3}$ MSM PDs show dark current as low as ∼1 nA. The dark current of $\beta \hbox{-Ga}_{2}\hbox{O}_{3}$ MSM PDs under significantly different oxygen concentration in the ambiences are similar, indicating that the high inertness to surface effect. Moreover, the responsivity and the working temperature of $\beta \hbox{-Ga}_{2}\hbox{O}_{3}$ MSM PDs at 10 V bias are 0.32 mA/W and as high as 700 K, respectively. Full recovery after 700-K operation demonstrates reliability and robustness of $\beta \hbox{-Ga}_{2}\hbox{O}_{3}$ PDs. The superior see-through features, electrical tolerance, inertness to surface effect, thermal stability, and solar-blind DUV photoresponse of $\beta \hbox{-Ga}_{2}\hbox{O}_{3}$ MSM PDs support the use in next-generation DUV PDs applications under harsh environments.

Organic semiconductors demonstrate several advantages over conventional inorganic materials for novel electronic and optoelectronic applications, including molecularly tunable properties, flexibility, low-cost, and facile device integration. However, before organic semiconductors can be used for the next-generation devices, such as ultrafast photodetectors (PDs), it is necessary to develop new materials that feature both high mobility and ambient stability. Toward this goal, a highly stable PD based on the organic single crystal [PtBr2 (5,5'-bis(CF3 CH2 OCH2 )-2,2'-bpy)] (or "Pt complex (1o)") is demonstrated as the active semiconductor channel-a material that features a lamellar molecular structure and high-quality, intraligand charge transfer. Benefitting from its unique crystal structure, the Pt-complex (1o) device exhibits a field-effect mobility of ≈0.45 cm2 V-1 s-1 without loss of significant performance under ambient conditions even after 40 days without encapsulation, as well as immersion in distilled water for a period of 24 h. Furthermore, the device features a maximum photoresponsivity of 1 × 103 A W-1 , a detectivity of 1.1 × 1012 cm Hz1/2 W-1 , and a record fast response/recovery time of 80/90 µs, which has never been previously achieved in other organic PDs. These findings strongly support and promote the use of the single-crystal Pt complex (1o) in next-generation organic optoelectronic devices.

To explore the surface effect on resistive random-access memory (ReRAM), the impact of surface roughness on the characteristics of ZnO ReRAM was studied. The thickness-independent resistance and the higher switching probability of ZnO ReRAM with rough surfaces indicate the importance of surface oxygen chemisorption on the switching process. Furthermore, the improvements in switching probability, switching voltage, and resistance distribution observed for ReRAM with rough surfaces can be attributed to the stable oxygen adatoms under various ambience conditions. The findings validate the surface-controlled stability and the uniformity of ReRAM and can serve as the guideline for developing practical device applications.

Trihalide perovskites have arrived at the forefront of next-generation solar technology due to their impressive light absorption coefficients, long carrier lifetimes, high carrier mobility, and intense photoluminescence. The metal/perovskite junction interface and carrier transport behaviors along the perovskite surface were studied on single crystalline CH3NH3PbBr3 by employing transfer length method with lithography patterned Au, Pt, and Ti electrodes. The charge pinning effect and image force were observed at perovskite junction interface for different contact metals. Furthermore, the surface charges at perovskite surface can result in the thermally activated carrier transport with activation energy of 0.2 eV when temperature is above 207 K; meanwhile variable range hopping dominates the transport below 207 K. This fundamental study of metal contact and surface transport of halide perovskite will guide the future electronic applications, and are especially important for low-dimensional perovskites which have high specific surface area.

Optical resonance formed inside a nanocavity resonator can trap light within the active region and hence enhance light absorption, effectively boosting device or material performance in applications of solar cells, photodetectors (PDs), and photocatalysts. Complementing conventional circular and spherical structures, a new type of multishelled spherical resonant strategy is presented. Due to the resonance-enhanced absorption by multiple convex shells, ZnO nanoshell PDs show improved optoelectronic performance and omnidirectional detection of light at different incidence angles and polarization. In addition, the response and recovery speeds of these devices are improved (0.8 and 0.7 ms, respectively) up to three orders of magnitude faster than in previous reports because of the existence of junction barriers between the nanoshells. The general design principles behind these hollow ZnO nanoshells pave a new way to improve the performance of sophisticated nanophotonic devices.

Researchers have recently revealed that hybrid lead halide perovskites exhibit ferroelectricity, which is often associated with other physical characteristics, such as a large nonlinear optical response. In this work, we study the non-linear optical properties of single crystal inorganic-organic hybrid perovskite CH3NH3PbBr3. By exciting the material with a 1044 nm laser, we were able to observe strong two-photon absorption-induced photoluminescence in the green spectral region. Using the transmission open-aperture Z-scan technique, we estimated the values of the two-photon absorption coefficient to be 8.5 cm GW-1, which is much higher than that of standard two-photon absorbing materials that are industrially used in nonlinear optical applications, such as LiNbO3, LiTaO3, KTiOPO4, and KH2PO4. Such a strong two-photon absorption effect in CH3NH3PbBr3 can be used to modulate the spectral and spatial profiles of laser pulses, as well as reduce noise, and can be used to strongly control the intensity of incident light. In this study, we demonstrate the superior optical limiting, pulse reshaping, and stabilization properties of CH3NH3PbBr3, opening new applications for perovskites in nonlinear optics.

Organic–inorganic hybrid perovskite materials exhibit a variety of physical properties. Pronounced coupling between phonon, organic cations, and the inorganic framework suggest that these materials exhibit strong light–matter interactions. The photoinduced strain of CH3NH3PbBr3 is investigated using high-resolution and contactless in situ Raman spectroscopy. Under illumination, the material exhibits large blue shifts in its Raman spectra that indicate significant structural deformations (i.e., photostriction). From these shifts, the photostrictive coefficient of CH3NH3PbBr3 is calculated as 2.08 × 10−8 m2 W−1 at room temperature under visible light illumination. The significant photostriction of CH3NH3PbBr3 is attributed to a combination of the photovoltaic effect and translational symmetry loss of the molecular configuration via strong translation–rotation coupling. Unlike CH3NH3PbI3, it is noted that the photostriction of CH3NH3PbBr3 is extremely stable, demonstrating no signs of optical decay for at least 30 d. These results suggest the potential of CH3NH3PbBr3 for applications in next-generation optical micro-electromechanical devices.

Self-assembled nanocomposites with a high interface-to-volume ratio offer an opportunity to overcome limitations in current technology, where intriguing transport behaviors can be tailored by the choice of proper interactions of constituents. Here we integrated metallic perovskite oxide SrRuO3-wurzite semiconductor ZnO nanocomposites to investigate the room-temperature metal-insulator transition and its effect on photoresponse. We demonstrate that the band structure at the interface can be tuned by controlling the interface-to-volume ratio of the nanocomposites. Photoinduced carrier injection driven by visible light was detected across the nanocomposites. This work shows the charge interaction of the vertically integrated multiheterostructures by incorporating a controllable interface-to-volume ratio, which is essential for optimization of the design and functionality of electronic devices.

We demonstrated a promising route to enhance the performance of inverted organic photovoltaic (OPV) devices by the incorporation of CuGaSe2 (CGS) quantum dots (QDs) into the ZnO buffer layer of P3HT:PCBM-based devices. The modification of QDs provides better band alignment between the organic/cathode interface, improves ZnO crystal quality, and increases photon absorption, leading to more effective carrier transport/collection. By employing this energy-harvesting scheme, short-circuit current density, open-circuit voltage, and fill factor of the OPV device after CGS QD modification are improved by 9.43%, 7.02% and 6.31%, respectively, giving rise to a 23.8% enhancement in the power conversion efficiency.

The surface effects of ZnO-based resistive random-access memory (ReRAM) were investigated using various electrodes. Pt electrodes were found to have better performance in terms of the device's switching functionality. A thermodynamic model of the oxygen chemisorption process was proposed to explain this electrode-dependent switching behavior. The temperature-dependent switching voltage demonstrates that the ReRAM devices fabricated with Pt electrodes have a lower activation energy for the chemisorption process, resulting in a better resistive switching performance. These findings provide an in-depth understanding of electrode-dependent switching behaviors and can serve as design guidelines for future ReRAM devices.

Large-area, periodic Si nanopillar arrays (NPAs) with the periodicity of 100 nm and the diameter of 60 nm were fabricated by metal-assisted chemical etching with anodic aluminum oxide as a patterning mask. The 100-nm-periodicity NPAs serve an antireflection function especially at the wavelengths of 200~400 nm, where the reflectance is decreased to be almost tenth of the value of the polished Si (from 62.9% to 7.9%). These NPAs show very low reflectance for broadband wavelengths and omnidirectional light incidence, attributed to the small periodicity and the stepped refractive index of NPA layers. The experimental results are confirmed by theoretical calculations. Raman scattering intensity was also found to be significantly increased with Si NPAs. The introduction of this industrial-scale self-assembly methodology for light harvesting greatly advances the development of Si-based optical devices.

Heteroepitaxial ZnO and SrRuO3 were grown on SrTiO3 (111) substrates and formed a self-assembled wurtzite-perovskite nanostructure. Spontaneous orientation-tuning of the SrRuO3 pillars was observed, with the growth direction changing from [111]SRO to [011]SRO as the film thickness increased, which is attributed to a misfit strain transition from the biaxial strain imposed by the SrTiO3 substrate to the vertical strain provided by the ZnO matrix. The [011]-SrRuO3 and [0001]-ZnO combination presents a favorable matching in the nanocomposite films, resulting in higher charge carrier mobility. This vertically integrated configuration and regulation on the crystallographic orientations are expected to be employed in designing multi-functional nanocomposite systems for applications in electronic devices.

We fabricate fully transparent solar-blind DUV PD devices employing β-Ga2O3 as an active layer and indium zinc oxide (IZO) as the transparent electrodes, exhibiting the average transmittance up to 80% from visible to near IR wavelength region without image blurring. The β-Ga2O3 MSM PDs for the use in harsh environments show a very low dark current (~1 nA), no sign of breakdown even at a bias higher than 200 V, excellent thermal stability, and intrinsic solar blindness. The dark current of β-Ga2O3 MSM PDs under significantly different oxygen concentration in the ambiences is similar, indicating that the high inertness to surface effect due to superior crystallinity nature of β-Ga2O3. The working temperature is up to 700 K and the responsivity is 0.32 mA/W at 10 V bias under 185-nm illumination. β-Ga2O3 PDs can be fully recovered after 700-K operation, showing excellent thermal reliability and robustness. The intriguing optoelectronic and electrical properties of β-Ga2O3 promise a new generation of stable, solar-blind DUV PDs for the extremely harsh electronic applications, such as sensing, imaging, and intrachip optical interconnects in high-temperature environments.

The oxygen chemisorption raising from surface roughness associated with the formation/rupture of conductive filaments dramatically affects the electrical properties of ZnO ReRAM, which is confirmed by the oxygen adsorption induced threshold voltage shift of the FET measurements. The lower oxygen adatom concentration changes of rougher surface under various measuring ambient condition, leading to the improvement of the electrical property stability on both SET/RESET switching voltage and HRS/LRS resistance. The thickness-independent HRS resistance and the better switching probability shown in the ZnO with rougher surface, indicating that the oxygen chemisorption on the surface dominates resistive switching process. Our findings give a guidelines for developing practically useful applications of ZnO-based ReRAM.