Ken Xingze Wang

Absorption Enhancement in Ultrathin Crystalline Silicon Solar Cells with Antireflection and Light-Trapping Nanocone Gratings

Enhancing the light absorption in ultrathin-film silicon solar cells is important for improving efficiency and reducing cost. We introduce a double-sided grating design, where the front and back surfaces of the cell are separately optimized for antireflection and light trapping, respectively. The optimized structure yields a photocurrent of 34.6 mA/cm(2) at an equivalent thickness of 2 μm, close to the Yablonovitch limit. This approach is applicable to various thicknesses and is robust against metallic loss in the back reflector.

Large-area free-standing ultrathin single-crystal silicon as processable materials.

Silicon has been driving the great success of semiconductor industry, and emerging forms of silicon have generated new opportunities in electronics, biotechnology, and energy applications. Here we demonstrate large-area free-standing ultrathin single-crystalline Si at the wafer scale as new Si materials with processability. We fabricated them by KOH etching of the Si wafer and show their uniform thickness from 10 to sub-2 μm. These ultrathin Si exhibits excellent mechanical flexibility and bendability more than those with 20-30 μm thickness in previous study. Unexpectedly, these ultrathin Si materials can be cut with scissors like a piece of paper, and they are robust during various regular fabrication processings including tweezer handling, spin coating, patterning, doping, wet and dry etching, annealing, and metal deposition. We demonstrate the fabrication of planar and double-sided nanocone solar cells and highlight that the processability on both sides of surface together with the interesting property of these free-standing ultrathin Si materials opens up exciting opportunities to generate novel functional devices different from the existing approaches.

Two-dimensional chalcogenide nanoplates as tunable metamaterials via chemical intercalation.

New plasmonic materials with tunable properties are in great need for nanophotonics and metamaterials applications. Here we present two-dimensional layered, metal chalcogenides as tunable metamaterials that feature both dielectric photonic and plasmonic modes across a wide spectral range from the infrared to ultraviolet. The anisotropic layered structure allows intercalation of organic molecules and metal atoms at the van der Waals gap of the host chalcogenide, presenting a chemical route to create heterostructures with molecular and atomic precision for photonic and plasmonic applications. This marks a departure from a lithographic method to create metamaterials. Monochromated electron energy-loss spectroscopy in a scanning transmission electron microscope was used to first establish the presence of the dielectric photonic and plasmonic modes in M2E3 (M = Bi, Sb; E = Se, Te) nanoplates and to observe marked changes in these modes after chemical intercalation. We show that these modal properties can also be tuned effectively by more conventional methods such as thickness control and alloy composition of the nanoplates.

Fundamental bounds on decay rates in asymmetric single-mode optical resonators.

We derive tight upper and lower bounds of the ratio between decay rates to two ports from a single resonance exhibiting Fano interference, based on a general temporal coupled-mode theory formalism. The photon transport between these two ports involves both direct and resonance-assisted contributions, and the bounds depend only on the direct process. The bounds imply that, in a lossless system, full reflection is always achievable at Fano resonance, even for structures lacking mirror symmetries, while full transmission can only be seen in a symmetric configuration where the two decay rates are equal. The analytic predictions are verified against full-field electromagnetic simulations.

Condition for perfect antireflection by optical resonance at material interface

Reflection occurs at an air–material interface. The development of antireflection schemes, which aims to cancel such reflection, is important for a wide variety of applications including solar cells and photodetectors. Recently, it has been demonstrated that a periodic array of resonant subwavelength objects placed at an air–material interface can significantly reduce reflection that otherwise would have occurred at such an interface. Here, we introduce the theoretical condition for complete reflection cancellation in this resonant antireflection scheme. Using both general theoretical arguments and analytical temporal coupled-mode theory formalisms, we show that in order to achieve perfect resonant antireflection, the periodicity of the array needs to be smaller than the free-space wavelength of the incident light for normal incidence, and also the resonances in the subwavelength objects need to radiate into air and the dielectric material in a balanced fashion. Our theory is validated using first-principles full-field electromagnetic simulations of structures operating in the infrared wavelength ranges. For solar cell or photodetector applications, resonant antireflection has the potential for providing a low-cost technique for antireflection that does not require nanofabrication into the absorber materials, which may introduce detrimental effects such as additional surface recombination. Our work here provides theoretical guidance for the practical design of such resonant antireflection schemes.

Optical Impedance transformer for transparent conducting electrodes

We elucidate the physics of optical impedance transformation and use this concept to design nanophotonic structures that provide broadband and omnidirectional improvement of transmission in transparent electrodes without compromising their electrical performances.

Optical impedance transformer for transparent conducting electrodes

We present a practical and robust concept to bypass the typical trade-off between optical transparency and electrical conductivity of transparent conducting electrodes. A transparent conducting electrode serves to transmit photons and conduct electrons, and the frequencies of the corresponding optical and dc electric fields differ by at least 12 orders of magnitude. Therefore, we could engineer the optical electric field to influence the optical property, which is not intrinsic, of the transparent electrode without sacrificing its electrical performance. For a given light power input, the optical impedance transformer reduces the loss in a transparent electrode by raising the refractive index of its surrounding medium. The concept of optical impedance transformer can be realized by nanocone arrays, and we use it to design nanophotonic structures that provide broadband and omnidirectional reduction of optical loss in an ultrathin graphene electrode. In addition, the concept applies to thicker or nanostructured transparent electrodes. The results are verified against first-principles full-field electromagnetic simulations.

Condition for perfect resonant antireflection in solar cells

Antireflection coatings are necessary components in solar cells. It has recently been experimentally demonstrated that a periodic array of resonant subwavelength objects placed at an air-dielectric interface can significantly reduce reflection. We introduce the theoretical condition for complete reflection cancellation in this resonant antireflection scheme. First, the periodicity of the array needs to be smaller than the free-space wavelength of the incident light for normal incidence; second, the resonances in the subwavelength objects need to radiate into air and the dielectric in a balanced fashion. We use both intuitive arguments and the temporal coupled-mode theory, validated by full-field electromagnetic simulations.

Radiative cooling for solar cells

Standard solar cells heat up under sunlight, and the resulting increased temperature of the solar cell has adverse consequences on both its efficiency and its reliability. We introduce a general approach to radiatively lower the operating temperature of a solar cell through sky access, while maintaining its sunlight absorption. We present first an ideal scheme for the radiative cooling of solar cells. For an example case of a bare crystalline silicon solar cell, we show that the ideal scheme can passively lower the operating temperature by 18.3 K. We then show a microphotonic design based on realistic material properties, that approaches the performance of the ideal scheme. We also show that the radiative cooling effect is substantial, even in the presence of significant non-radiative heat change, and parasitic solar absorption in the cooling layer, provided that we design the cooling layer to be sufficiently thin.

Absorption Enhancement in Ultrathin Solar Cells with Antireflection and Light-Trapping Nanocone Gratings

We combine optimized front gratings primarily for antireflection at shorter wavelengths and back gratings primarily for light-trapping at longer wavelengths in ultrathin crystalline silicon solar cells to achieve near Yablonovitch limit absorption.