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In detailed balance model, the efficiency of single-junction solar cells can be potentially as high as 33.5% under AM 1.5G illumination. However the best state-of-the-art devices are still far lower than those figures, even the electronic quality is nearly perfect. Therefore the efficiency gap should stem from the light management inside solar cells. Recently, external radiation efficiency (�� ext) derived from detailed balance model is emphasized to evaluate light management and photon recycling, which aggregates the loss of backward emission into substrate and non-radiative recombination. This factor can be highly relevant to the cell’s performance, especially open-circuit voltage (Voc), and maximizing Voc is generally considered as the last mile to approach ultra-high efficiency limit. In this work, we try to quantify the Voc enhancement in GaAs solar cells by enhancing light extraction. The simulation tools are RCWA simulation and photon recycling model NREL developed recently. The top structures we simulate here are TiO2 cones arranged in three PC/QPC lattices. After our calculation, the QPC 12-folds symmetry can make the biggest Voc enhancement 11.21meV compared with bare one, and the structure also possess extraordinary omni-directional anti-reflection ability for maintaining high Jsc. Our results also show that using this way to enhance Voc is especially suitable for cells with ordinary material quality. Therefore, the requests of ideal top structures for solar cells’ use are not only near-perfect anti-reflection, but the ability to maximize light extraction if no feature of angular filter exists.

We demonstrate the GaAs solar cells which utilize the high-transmittance textured polydimethylsiloxane (PDMS) film can outstanding increase the short circuit current density and power conversion efficiency of solar cells. The transmittance of PDMS film is exceeded 90%, which can pass through almost all the light of GaAs Solar cells can be absorbed. We used a special imprint technology to let the PDMS film possess a highly textured surface. Then we measured the characteristics of textured PDMS film and found out that it has a very excellent Haze performance. The effect of flexible textured PDMS film on the suppression of surface reflection in GaAs solar cells is also investigated. The presented technology provides an inexpensive surface anti-reflection process, which can potentially replace typically complex anti-reflection coating (ARC) layer. The GaAs solar cells with textured PDMS layer can effectively enhance the short-circuit current density from 22.91 to 26.54 mA/cm 2 and the power conversion efficiency from 18.28 to 21.43 %, corresponding to a 17 % enhancement compared to the one without textured PDMS. The open-circuit voltage (V oc ) and the fill-factor (FF) of GaAs solar cells exhibit negligible change, because the textured PDMS film was pasted up on the surface of GaAs solar cells and did not interfere with the diode operation. At the same time, we observed through the EQE measurement that the textured PDMS film not only proved wonderful light scattering effect but also generated more electron-hole pairs in all absorption spectrum range. Finally, through this simple PDMS process, we believe this technology shall be a great candidate for next generation of highly efficient and low-cost photovoltaic devices.

Triple-junction solar cells offer extremely high power conversion efficiency with minimal semiconductor material usage, and hence are promising for large-scale electricity generation. To fully exploit the broad absorption range, antireflective schemes based on biomimetic nanostructures become very appealing due to sub-wavelength scale features that can collectively function as a graded refractive index (GRIN) medium to photons. The structures are generally fabricated with a single-type dielectric material which guarantees both optical design robustness and mechanical durability under concentrated illumination. However, surface recombination and current matching issues arising from patterning still challenge the realization of biomimetic nanostructures on a few micrometer thick epitaxial layers for MJSCs. In this presentation, bio-inspired antireflective structures based on silicon nitride (SiN x ) and titanium dioxide (TiO 2 ) materials are demonstrated on monolithically grown Ga 0.5 In 0.5 P/In 0.01 Ga 0.99 As/Ge triple-junction solar cells. The nano-fabrication employs scalable polystyrene nanosphere lithography, followed by inductively-coupled-plasma reactive-ion-etching (ICP-RIE). We show that the fabricated devices exhibit omni-directional enhancement of photocurrent and power conversion efficiency, offering a viable solution to concentrated illumination with large angles of incidence. Moreover, a comprehensive design scheme is also presented to tailor the reflectance spectrum of sub-wavelength structures for maximum photocurrent output of tandem cells.

Source optimization (SO) becomes increasingly important to resolution enhancement in sub-32 nm lithography nodes because the dense pattern configurations significantly limit the capability of mask correction. A key step in SO is the image formation by Abbe's method, which is a linear operation of integrating all source points' images incoherently to form aerial images. However, the aerial images are usually converted to resist images through the nonlinear sigmoid function. Such operation loses the merit of linearity in optimization and leads to slow convergence and time-consuming calculation. In this paper we propose a threshold-based linear resist model to replace the sigmoid model in SO. The effectiveness of our proposed model can be clearly seen from mathematical analysis. We also compare results based on linear and sigmoid models. Highly similar optimal sources are obtained, but the linear model has a significant advantage over the sigmoid in terms of convergence rate and simulation time. Furthermore, the process variations characterized by exposure-defocus (E-D) windows are still in similar trends for optimal sources based on two different resist models.

Light trapping techniques such as textured interfaces and highly reflective back contacts are important to thin-film solar cells. Scattering at rough interfaces inside a solar cell leads to enhanced absorption due to an increased optical path length in the active layers, which is generally characterized by a haze ratio. In this work, we demonstrate the measured haze characteristics of indium tin oxide nano-whiskers deposited on an ITO-coated glass substrate. A theoretical model based on a modified Mie theory is also employed to analyze the scattering effects of nano-whiskers. Instead of spherical model, a cylindrical condition is imposed to better fit the shapes of the whiskers. The calculated haze-ratio of an ITO whisker layer matches the measurement closely.

Improvement of efficiency for crystalline silicon (c-Si) with nanopillar arrays (NPAs) solar cell was demonstrated by deployment of CdS quantum dots (QDs). The NPAs was fabricated by colloidal lithography of self-assembled polystyrene (PS) nanospheres with a 600 nm in size and reactive-ion etching techniques, and then a colloidal CdS QDs with a concentration of 5 mg/mL was spun on the surface of c-Si with NPAs solar cell. Under a simulated one-sun condition, the device with CdS QDs shows a 33% improvement of power conversion efficiency, compared with the one without QDs. Additionally, we also found that the device with CdS QDs shows a 32% reduction in electrical resistance, compared with the one without QDs solar cell, under an ultraviolet (UV) light of 355nm illumination. This reduced electrical resistance can directly contribute to our fill-factor (FF) enhancement. For further investigation, the excitation spectrum of photoluminescence (PL), absorbance spectrum, current-voltage (I-V) characteristics, reflectance and external quantum efficiency (EQE) of the device were measured and analyzed. Based on the spectral response and optical measurement, we believe that CdS QDs not only have the capability for photon down-conversion in ultraviolet region, but also provide extra antireflection capability.

Inverse lithography which generates model-based patterns theoretically has superior patterning fidelity comparing to conventional rule-based technique. Cost functions are the determinant of performance inverse lithography that is also an optimization problem. However, the design and know-how of cost functions have rarely been discussed. In this paper, we investigate the impacts of various cost functions and their superposition for inverse lithography patterning exploiting a steepest descent algorithm. We research the most generally used objective functions, which are the resist and aerial images, and also deliver a derivation for the aerial image contrast. We then discuss the pattern fidelity and final mask characteristics for simple layouts with a single isolated contact and two nested contacts. Moreover, the convergences which are expressed by edge-placement error (EPE) and contrast versus iteration numbers rapidly attain to steady sate in most hybrid cost functions. All in all, we conclude that a cost function composed of a dominant resist-image component and a minor aerial-image or image-contrast component can carry out a good mask correction and contour targets when using inverse lithography patterning.

As lithography still pushing toward to low-k1 region, resolution enhancement techniques (RETs) including source optimization (SO) and mask optimization (MO) are expected to overcome the fundamentally physics in optics. Recently inverse lithography (IL) is widely studied for source and mask optimization (SMO) to enhance the resolution for over diffraction limit integrate circuit (IC) patterns. In this paper, we propose a gradient based SMO algorithm where the SO and MO are two sequential steps due to their different image formation mechanism. Moreover, we employ three cost functions including aerial and resist image and the image contrast which is proposed in our previous work. We show that IL patterns produced by SMO have better pattern fidelity and image contrast than MO only patterns.

In this work, we demonstrate a thorough device design, fabrication, characterization, and analysis of biomimetic antireflective structures implemented on a Ga 0.5 In 0.5 P/GaAs/Ge triple-junction solar cell. The sub-wavelength structures are fabricated on a silicon nitride passivation layer using polystyrene nanosphere lithography followed by anisotropic etching. The fabricated structures enhance optical transmission in the ultraviolet wavelength range, compared to a conventional single-layer antireflective coating (ARC). The transmission improvement contributes to the enhanced photocurrent, which is also verified by the external quantum efficiency characterization of fabricated solar cells. Under one-sun illumination, the short-circuit current of a cell with a biomimetic structures is enhanced by 24.1% and 2.2% due to much improved optical transmission and current matching, compared to cells without an ARC and with a conventional ARC, respectively. Further optimizations of the biomimetic structures including the periodicity and etching depth are conducted by performing comprehensive calculations based on a rigorous couple-wave analysis method.

In this work, we present a solution that employs combined micro- and nano-scale surface textures to increase light harvesting in the near infrared for crystalline silicon photovoltaics, and discuss the associated antireflection and scattering mechanisms. The combined surface textures are achieved by uniformly depositing a layer of indium-tin-oxide nanowhiskers on passivated, micro-grooved silicon solar cells using electron-beam evaporation. The nanowhiskers facilitate optical transmission in the near-infrared, which is optically equivalent to a stack of two dielectric thin-films with step- and graded- refractive index profiles. The ITO nanowhiskers provide broadband anti-reflective properties (R

Convergence speed and local minimum issue have been the major issues for inverse lithography. In this paper, we propose an inverse algorithm that employs an iterative gradient-descent method to improve convergence and reduce the Edge Placement Error (EPE). The algorithm employs a constrained gradient-based optimization to attain the fast converging speed, while a cross-weighting technique is introduced to overcome the local minimum trapping.

We had demonstrated several novel methods to improve the luminescence efficiency of the GaN-based light emitting diodes (LEDs). The high-aspect-ratio GaN nanorods, formed by spun nano-spheres and inductively coupled plasma (ICP) etching, contributed to an enhancement in light output power and better light directionality. Nevertheless, the etching process would affect the electrical properties. We then attempt to rough the surface by synthesizing ZnO nanorods in liquid solution at room temperature. The LEDs with ZnO nanorods enjoyed high extraction efficiency and comparable electric performance than that without nanorods. At third part, we fabricate a high efficiency GaN-based LED by regrowth on SiO2 nanorod patterned sapphire substrate. It could improve the light extraction and internal efficiencies simultaneously.

We propose an inversion calculation method based on a simple "pixel-flipping" approach. The simple method features innovative wavefront-expansion and wavefront-based damping techniques in order to obtain accentuated corrections near the drawn pattern. The method is first employed to be a stand-alone optical proximity correction solution that directly calculates the corrected masks with acceptable contours and image contrast. In addition, a model-based pre-OPC flow, where the initial sizing of drawn patterns and surrounding sub-resolution assist features (SRAF) are simultaneously generated in a single iteration using this inversion calculation is also proposed to minimize technology-transition risks and costs. A mask simplification technique based on the central moments is introduced in order to snap the corrections into 45 degree and axis-aligned line segments. This approach allows achieving optimized corrections while minimizing the impact to the existing and validated correction flow.

The conventional segment-based OPC approach has been applied successfully for many CMOS generations and is currently favored. However, Inverse lithography technology (ILT) is a promising candidate for next-generation optical proximity correction (OPC). Still, there are issues that need to be thoroughly addressed and further optimized. In this work, we propose a model-based pre-OPC flow where the sizing of drawn patterns and placement of surrounding sub-resolution assist features (SRAF) are simultaneously generated in a single iteration using an ILT method. The complex patterns can then be simplified for a conventional OPC solution.

We have made a GaN-based single nanopillar with a diameter of 300nm using the focused ion beam (FIB) technique. The micro-photoluminescence (µ-PL) from the embedded GaN/InGaN multi-quantum wells reveals a blue shift of 68.3 meV in energy. In order to explain the spectrum shift, we have developed a valence force field model to study the strain relaxation mechanism in a single GaN-based nanopillar structure. The strain distribution and strain induced polarization effect inside the multiple quantum wells is added to our self-consistent Poisson, drift-diffusion, and Schrodinger solver to study the spectrum shift of µ-PL.