Chan Il Yeo

Enhanced power generation in concentrated photovoltaics using broadband antireflective coverglasses with moth eye structures.

We present the effect of broadband antireflective coverglasses (BARCs) with moth eye structures on the power generation capability of a sub-receiver module for concentrated photovoltaics. The period and height of the moth eye structures were designed by a rigorous coupled-wave analysis method in order to cover the full solar spectral ranges without transmission band shrinkage. The BARCs with moth eye structures were prepared by the dry etching of silver (Ag) nanomasks, and the fabricated moth eye structures on coverglass showed strongly enhanced transmittance compared to the bare glass with a flat surface, at wavelengths of 300 - 1800 nm. The BARCs were mounted on InGaP/GaAs/Ge triple-junction solar cells and the power conversion efficiency of this sub-receiver module reached 42.16% for 196 suns, which is a 7.41% boosted value compared to that of a module with bare coverglass, without any detrimental changes of the open circuit voltages (Voc) and fill factor (FF).

Antireflective property of thin film a-Si solar cell structures with graded refractive index structure

We report the antireflective property of thin film amorphous silicon (a-Si) solar cell structures based on graded refractive index structure together with theoretical analysis. Optimizations of the index profile are performed using the rigorous coupled-wave analysis method. The graded refractive index structure fabricated by oblique angle deposition suppresses optical reflection over a wide range of wavelength and incident angle, compared to the conventional structure. The average reflectance of thin film a-Si solar cell structure with the graded refractive index structure is suppressed by 54% at normal incidence due to the effective refractive index matching between ITO and a-Si, indicating a reasonable agreement with calculated results.

Wafer-scale broadband antireflective silicon fabricated by metal-assisted chemical etching using spin-coating Ag ink

We report broadband antireflective disordered subwavelength structures (d-SWSs), which were fabricated on 4-inch silicon wafers by spin-coating Ag ink and metal-assisted chemical etching. The antireflection properties of the d-SWSs depend on its dimensions and heights, which were changed by the sintering temperature of the spin-coated Ag ink and etching time. The fabricated d-SWSs drastically reduced surface reflection over a wide range of wavelengths and incident angles, providing good surface uniformity. The d-SWSs with the most appropriate geometry for practical solar cell applications exhibit only 1.23% solar-weighted reflectance in the wavelength range of 300-1100 nm and average reflectance <5% up to an incident angle of 55° in the wavelength range of 300-2500 nm. This simple and low-cost nanofabrication method for antireflection could be of great importance in optical device applications because it allows mass production without any lithography processes or sophisticated equipment.

Highly tolerant a-Si distributed Bragg reflector fabricated by oblique angle deposition

We demonstrate a highly tolerant and highly reflective broadband a-Si distributed Bragg reflector fabricated by oblique angle deposition. By tuning the refractive index of an a-Si film, a high index contrast material system was achieved. The highly tolerant and broadband reflective characteristics of the a-Si distributed Bragg reflector were investigated by calculation and fabrication. A broad stop band (Δλ/λ = 33.7%, R>99%) with only a five-pair a-Si distributed Bragg reflector was achieved experimentally. The size-, feature- and substrate-independent method for highly reflective Bragg reflectors was realized by simple oblique angle evaporation.

Antireflective silicon nanostructures with hydrophobicity by metal-assisted chemical etching for solar cell applications

We present broadband antireflective silicon (Si) nanostructures with hydrophobicity using a spin-coated Ag ink and by subsequent metal-assisted chemical etching (MaCE). Improved understanding of MaCE, by conducting parametric studies on optical properties, reveals a design guideline to achieve considerably low solar-weighted reflectance (SWR) in the desired wavelength ranges. The resulting Si nanostructures show extremely low SWR (1.96%) and angle-dependent SWR (<4.0% in the range of 0° to 60°) compared to that of bulk Si (SWR, 35.91%; angle-dependent SWR, 37.11%) in the wavelength range of 300 to 1,100 nm. Relatively large contact angle (approximately 102°) provides a self-cleaning capability on the solar cell surface.

Increased Light Extraction From GaN Light-Emitting Diodes by

We demonstrate GaN-based blue light-emitting diodes (GaN LEDs) with artificial compound eye structures (CESs). The GaN LEDs consist of microlens arrays (MLAs) and antireflective subwavelength structures (SWSs). The CESs, formed on the LED surface by two-step pattern transfer of reflowed photoresist micro patterns and Ag nanoparticles, play an important role in enhancing the light extraction efficiency by reducing both the Fresnel and the total internal reflection. The CES integrated GaN LEDs show an output power enhancement of 93% compared to that of conventional GaN LEDs with flat surface, without any serious degradation of electrical characteristics. Optical simulations by ray-tracing and the rigorous coupled-wave analysis method provide design guidelines for MLAs and SWSs, respectively.

{\rm SiN}_{{\rm x}}

We report on the antireflective characteristics of porous silicon (Si) nanocolumnar structures consisting of graded refractive index layers and carry out a rigorous coupled-wave analysis simulation. The refractive index of Si is gradually modified by a tilted angle electron beam evaporation method. For the fabricated Si nanostructure with a Gaussian index profile of 100 nm, reflectivity (R) of less than 7.5% is obtained with an average value of approximately 2.9% at the wavelength region of 400-800 nm. The experimental results are reasonably consistent with the simulated results for the design of antireflective Si nanostructures.

Compound Eyes

We present bifunctional nanostructured (NS) coverglasses with enhanced transmittance and self-cleaning function for improving the efficiency of a photovoltaic (PV) module with an InGaP/GaAs/Ge triple-junction solar cell. Prior to the fabrication of NS coverglasses, theoretical investigations were carried out using a rigorous coupled-wave analysis method to determine the desirable glass nanostructures that can effectively enhance the light absorption of the PV module. The transmission properties of the fabricated NS coverglasses with different glass nanostructures were systematically analyzed by considering the absorption spectrum of three subcells of the solar cell in order to find the most effective NS coverglass for enhancing the photocurrent of the PV module and thereby improving the conversion efficiency. The PV module with the most effective NS coverglass showed 4.2% enhanced short-circuit current density and 4.3% enhanced efficiency compared with the PV module with a bare coverglass measured under one sun of the air mass 1.5 global, showing the necessity of integrating a finely designed NS coverglass to effectively improve the efficiency of various PV systems with multijunction solar cells.

Antireflective properties of porous Si nanocolumnar structures with graded refractive index layers

Although recently developed bio-inspired nanostructures exhibit superior optic performance, their practical applications are limited due to cost issues. We present highly transparent glasses with grassy surface fabricated with self-masked dry etch process. Simultaneously generated nanoclusters during reactive ion etch process with simple gas mixture (i.e., CF4/O2) enables lithography-free, one-step nanostructure fabrication. The resulting grassy surfaces, composed of tapered subwavelength structures, exhibit antireflective (AR) properties in 300 to 1,800-nm wavelength ranges as well as improved hydrophilicity for antifogging. Rigorous coupled-wave analysis calculation provides design guidelines for AR surface on glass substrates.

Efficiency Enhancement of III-V Triple-Junction Solar Cell Using Nanostructured Bifunctional Coverglass With Enhanced Transmittance and Self-Cleaning Property

We present the effect of nanophotonic light trapping structures on optical absorption enhancement of crystalline silicon thin film solar cells, based on a rigorous coupled-wave analysis method. The calculation involves three different structures (i.e., hole, inverted-cone, and inverted-paraboloid), which are commonly applied on the top surface of thin film solar cells. Systematical calculation results in terms of geometrical parameters reveal sweet spots (i.e., optimum geometric structure) to obtain the highest cell efficiency for each structure, which provide a design guideline in thin film photovoltaic devices.