Tongchuan Gao

Uniform and Ordered Copper Nanomeshes by Microsphere Lithography for Transparent Electrodes

We report a comprehensive simulation and experimental study on the optical and electronic properties of uniform and ordered copper nanomeshes (Cu NMs) to determine their performance for transparent conductors. Our study includes simulations to determine the role of propagating modes in transmission and experiments that demonstrate a scalable, facile microsphere-based method to fabricate NMs on rigid quartz and flexible polyethylene terephthalate substrates. The fabrication method allows for precise control over NM morphology with near-perfect uniformity and long-range order over large areas on rigid substrates. Our Cu NMs demonstrate 80% diffuse transmission at 17 Ω/square on quartz, which is comparable to indium tin oxide. We also performed durability experiments that demonstrate these Cu NMs are robust from bending, heating, and abrasion.

Hierarchical Graphene/Metal Grid Structures for Stable, Flexible Transparent Conductors

We report an experimental study on the fabrication and characterization of hierarchical graphene/metal grid structures for transparent conductors. The hierarchical structure allows for uniform and local current conductivity due to the graphene and exhibits low sheet resistance because the microscale silver grid serves as a conductive backbone. Our samples demonstrate 94% diffusive transmission with a sheet resistance of 0.6 Ω/sq and a direct current to optical conductivity ratio σdc/σop of 8900. The sheet resistance of the hierarchical structure may be improved by over 3 orders of magnitude and with little decrease in transmission compared with graphene. Furthermore, the graphene protects the silver grid from thermal oxidation and better maintains the sheet resistance of the structure at elevated temperature. The graphene also strengthens the adhesion of the metal grid with the substrate such that the structure is more resilient under repeated bending.

Designing metal hemispheres on silicon ultrathin film solar cells for plasmonic light trapping

We systematically investigate the design of two-dimensional silver (Ag) hemisphere arrays on crystalline silicon (c-Si) ultrathin film solar cells for plasmonic light trapping. The absorption in ultrathin films is governed by the excitation of Fabry-Perot TEMm modes. We demonstrate that metal hemispheres can enhance absorption in the films by (1) coupling light to c-Si film waveguide modes and (2) exciting localized surface plasmon resonances (LSPRs). We show that hemisphere arrays allow light to couple to fundamental TEm and TMm waveguide modes in c-Si film as well as higher-order versions of these modes. The near-field light concentration of LSPRs also may increase absorption in the c-Si film, though these resonances are associated with significant parasitic absorption in the metal. We illustrate how Ag plasmonic hemispheres may be utilized for light trapping with 22% enhancement in short-circuit current density compared with that of a bare 100 nm thick c-Si ultrathin film solar cell.

Copper nanowire arrays for transparent electrodes

Metallic nanowires have demonstrated high optical transmission and electrical conductivity with potential for application as transparent electrodes that may be used in flexible devices. In this paper, we systematically investigated the electrical and optical properties of 1D and 2D copper nanowire (Cu NW) arrays as a function of diameter and pitch and compared their performance to that of Cu thin films and our recent results on silver (Ag) NW arrays. Cu NWs exhibit enhanced transmission over thin films due to propagating resonance modes between NWs. For the same geometry, the transmission of Cu NW arrays is about the same as that of Ag NW arrays since the dispersion relation of propagating modes in metal nanowire arrays are independent of the metal permittivity. The sheet resistance is also comparable since the conductivity of Cu is about the same as that of Ag. Just as in Ag NWs, larger Cu NW diameters and pitches are favored for achieving higher solar transmission at a particular sheet resistance. Cu NW...

The role of propagating modes in silver nanowire arrays for transparent electrodes.

Silver nanowires have been shown to demonstrate enhanced transmission and promising potential for next-generation transparent electrodes. In this paper, we systematically investigated the electrical and optical properties of 1D and 2D silver nanowire arrays as a function of diameter and pitch and compared their performance to that of silver thin films. Silver nanowires were found to exhibit enhanced transmission over thin films due to propagating resonance modes between nanowires. We evaluated the angular dependence and dispersion relation of these propagating modes and demonstrate that larger nanowire diameters and pitches are favored for achieving higher solar transmission at a particular sheet resistance. Silver nanowires may achieve achieve solar transmission > 90% with sheet resistances of a few Ω/sq and figure of merit σdc/σop > 1000.

Hierarchical metal nanomesh/microgrid structures for high performance transparent electrodes

We report a comprehensive study on the optical and electronic properties of hierarchical metal nanomesh (NM)/microgrid (MG) structures to determine their performance as transparent conductors (TCs). The NM helps deliver or collect carriers locally while the lower resistance MG transports carriers over larger distances. The structures exhibit high uniformity of optical and electronic properties. Hierarchical Ag NM/Ag MG structures demonstrate 83% diffusive transmission at a sheet resistance of 0.7 Ω per square when fabricated directly on quartz and 81% at 0.7 Ω per square when fabricated directly on flexible plastic. The direct current to optical conductivity ratios σdc/σop of these structures are 2900 and 2300, respectively. This corresponds to over an order of magnitude reduction in sheet resistance with a negligible to slight reduction in transmission compared to NMs. The haze factor of these structures may be tuned by modifying the NM hole diameter. Furthermore, the hierarchical structures exhibit good durability under bending and heating.

Plasmonic nanomesh sandwiches for ultrathin film silicon solar cells

We theoretically investigate the strategy of integrating metal nanoparticles (NPs)/nanomeshes (NMs) into the top and/or bottom of crystalline silicon (c-Si) thin film solar cells for light trapping and enhanced carrier collection. C-Si thin films exhibit absorption resonances corresponding to Fabry–Perot modes. Frontside metal NPs enhance absorption by additionally coupling light into localized surface plasmon resonances and c-Si waveguide modes, while a frontside metal NM increases absorption by coupling light into surface plasmon polaritons and c-Si waveguide modes. The frontside metal NM also functions as a flexible top electrode, which may replace conventional brittle transparent conductive oxide thin films. The backside metal NM exhibits enhanced absorption due to the coupling of light into c-Si waveguide modes and the cavity modes within the holes of metal NM. We illustrate how the optimal metal NM sandwich, consisting of NMs on both sides of a 300 nm thick c-Si with an appropriate antireflection coating (ARC), achieves a 72.9% enhancement in short-circuit current density compared with that of a 300 nm thick c-Si thin film solar cell with 100 nm thick Si3N4 ARC and 300 nm thick Ag back reflector. The current generation in the metal NM sandwich is more in the center of the thin film such that there should be less surface recombination. The uniform current generation throughout the film results in less overall recombination.

Novel Carrier Doping Mechanism for Transparent Conductor: Electron Donation from Embedded Ag Nanoparticles to the Oxide Matrix

A trade-off between the carrier concentration and carrier mobility is an inherent problem of traditional transparent conducting oxide (TCO) films. In this study, we demonstrate that the electron concentration of TCO films can be increased without deteriorating the carrier mobility by embedding Ag nanoparticles (NPs) into Al-doped ZnO (AZO) films. An increment of Ag NP content up to 0.7 vol % in the AZO causes the electron concentration rising to 4 × 1020 cm-3. A dependence of the conductivity on temperature suggests that the energy barrier for the electron donation from Ag NPs at room temperature is similar to the Schottky barrier height at the Ag-AZO interface. In spite of an increase in the electron concentration, embedded Ag NPs do not compromise the carrier mobility at room temperature. This is evidence showing that this electron donation mechanism by Ag NPs is different from impurity doping, which produces both electrons and ionized scattering centers. Instead, an increase in the Fermi energy level of the AZO matrix partially neutralizes Al impurities, and the carrier mobility of Ag NP embedded AZO film is slightly increased. The optical transmittance of mixture films with resistivity less than 1 × 10-3 Ω·cm still maintains above 85% in visible wavelengths. This opens a new paradigm to the design of alternative TCO composite materials which circumvent an inherent problem of the impurity doping.

COMPUTATIONAL SIMULATIONS OF NANOSTRUCTURED SOLAR CELLS

Simulation methods are vital to the development of next-generation solar cells such as plasmonic, organic, nanophotonic, and semiconductor nanostructure solar cells. Simulations are predictive of material properties such that they may be used to rapidly screen new materials and understand the physical mechanisms of enhanced performance. They can be used to guide experiments or to help understand results obtained in experiments. In this paper, we review simulation methods for modeling the classical optical and electronic transport properties of nanostructured solar cells. We discuss different techniques for light trapping with an emphasis on silicon nanostructures and silicon thin films integrated with nanophotonics and plasmonics.

Self-cleaning, high transmission, near unity haze OTS/silica nanostructured glass

High haze, high transparency substrates can increase the power conversion and extraction efficiency of solar cells and light emitting diodes (LEDs), respectively. In this paper, we demonstrate a new octadecyltrichlorosilane (OTS)/silica nanostructured substrate that displays high transmission (91.5 ± 0.5% at 550 nm wavelength) and near unity haze (98.1 ± 0.5% at the same wavelength) with 143° scattering angle. The OTS/silica nanostructures are fabricated through a scalable and facile maskless reactive ion etching (MRIE) process followed by OTS coating. The OTS coating enhances the transmission of the structures by merging silica nanostructures together by capillary forces and effectively grading the index of refraction. The OTS/silica nanostructures display the highest combination of both transmission and haze in the literature as defined by Pareto optimality. The OTS/silica nanostructured glass exhibits lotus leaf-like wetting with a 159.7 ± 0.6° water contact angle (WCA) and 4.9 ± 0.6° contact angle hysteresis. We demonstrate the structures have self-cleaning functionality where about 100% of transparency can be easily recovered after graphite soiled substrates are rinsed with water. This self-cleaning functionality is maintained after 200 cycles of soiling and cleaning. The OTS/silica nanostructured glass may be an important substrate in optoelectronic applications where a combination of high transmission, high haze, and self cleaning function are important.