Ming-Hsuan Kao

Multi-functional stacked light-trapping structure for stabilizing and boosting solar-electricity efficiency of hydrogenated amorphous silicon solar cells

A sandwiched light-trapping electrode structure, which consists of a capping aluminum-doped ZnO (AZO) layer, dispersed plasmonic Au-nanoparticles (Au-NPs), and a micro-structured transparent conductive substrate, is employed to stabilize and boost the conversion-efficiency of hydrogenated amorphous silicon (a-Si:H) solar cells. The conformal AZO ultrathin layer (5 nm) smoothened the Au-NP-dispersed electrode surface, thereby reducing defects across the AZO/a-Si:H interface and resulting in a high resistance to photo-degradation in the ultraviolet-blue photoresponse band. With the plasmonic light-trapping structure, the cell has a high conversion-efficiency of 10.1% and the photo-degradation is as small as 7%.

Modeling and Optimization of Sub-Wavelength Grating Nanostructures on Cu(In,Ga)Se2 Solar Cell

In this study, an optical simulation of Cu(In,Ga)Se2 (CIGS) solar cells by the rigorous coupled-wave analysis (RCWA) method is carried out to investigate the effects of surface morphology on the light absorption and power conversion efficiencies. Various sub-wavelength grating (SWG) nanostructures of periodic ZnO:Al (AZO) on CIGS solar cells were discussed in detail. SWG nanostructures were used as efficient antireflection layers. From the simulation results, AZO structures with nipple arrays effectively suppress the Fresnel reflection compared with nanorod- and cone-shaped AZO structures. The optimized reflectance decreased from 8.44 to 3.02% and the efficiency increased from 14.92 to 16.11% accordingly. The remarkable enhancement in light harvesting is attributed to the gradient refractive index profile between the AZO nanostructures and air.

The external light trapping for perovskite solar cells using nanoimprinted polymer metamaterial patterns

In this work, we develop the process flow for nano-imprinted metamaterial patterns for perovskite solar cell light trapping. Nanoimprint can be scaled for large-area photovoltaics at low cost, compared to conventional lithography techniques. While conventional grating is normally underneath the solar cell active layers, the degraded electrical characteristic can be a problem. On the other hand, the external light trapping separates the electrical and optical designs and, therefore, it is more promising for complex photonic management without sacrificing the diode characteristics. We demonstrate the concept by perovskite solar cells with metamaterial patterns. Imprinting a deeper metamaterial pattern is suggested to enhance the far field effect for further efficiency improvement.

A sandwiched buffer layer enabling pulsed ultraviolet- and visible-laser annealings for direct fabricating poly-Si field-effect transistors on the polyimide

A sandwiched buffer layer of SiO2/Al/SiO2 enables ultraviolet-laser crystallization and visible-laser activation for direct fabrication of a poly-Si flexible field-effect-transistor (fFET) on polyimides. The buffer layer can produce heat accumulation and laser reflection from the Al/SiO2 interface to facilitate grain growth and contact resistance reduction of poly-Si without damaging the polyimide substrate. The feature size of poly-Si fFET has shrunk to 400 nm via laser annealing, with the on/off current-ratio exceeding 5 × 106 and a subthreshold swing of 190 mV/dec. Moreover, the transfer characteristics of fFET by tension stress can be maintained until the bending radius reaches over 15 mm.

The external light trapping using down-conversion polymer and diffuse trench reflectors

External light trapping has been proposed as an alternative to internal light trapping. The advantage is that the electrical and optical characteristics can be designed separately. This enhances the degree of improvement that nano-photonics can contribute to the solar cell efficiency and JSC. In this work, we use diffuse trench reflectors with down-conversion polymer to demonstrate the concepts of a low-cost, widely applicable scheme for the external light trap. A >50% enhancement of the JSC can be observed with proper designs and configurations. The proposed external light trap can be applied to nearly all thin-film solar cell technologies since the external optical components do not affect the electrical diode characteristic of the solar cells. The efficient external light trap is attributed to the high reflectance of the disuse mirror and its wide-angle diffractions, optical confinement due to the trench reflector, and the additional short-wavelength spectral enhancement by the down-conversion mechanism.

Photonic density of state (PDOS) enhancement by brillouin zone engineering

The material absorption can be enhanced by increased photonic density of state(PDOS). Here we demonstrate that through the design in the Brillouin zone of nanophotonic light trapping structures, the PDOS can be effectively increased. The theoretical work using rigorously coupled wave calculation shows the increased spectral absorption and the optimal quasiguided mode excitations in terms of their quality factors and the number of excited modes. The experimental work is currently ongoing and the preliminary result confirms enhanced short circuit current if the PDOS is properly adjusted. Unlike random structures, the proposed Brillouin zone engineered structures are more feasible from modeling and device process viewpoints.

Non-reciprocal meta-surfaces for nanophtonic light trapping: Optical isolators versus solar cells

Nano-photonic concept has been shown to be very effective for boosting solar cell conversion efficiency. In this work, the novel concepts of non-reciprocality and chiral meta-surface are demonstrated to be very useful for photovoltaics. The asymmetric chiral patterns are designed and optimized using genetic algorithm. The enhanced optical absorption is observed and it is attributed to the non-reciprocal nature of the solar cell front meta-surface. The asymmetric solar cell front surface enhances the anti-reflection and, at the same time, forbids the photon escape once it enters the devices. This concept is very similar to optical isolators where one-way transmission is intentionally designed for enhanced optical diode effect. Preliminary experimental results are supplied to confirm the superiority of asymmetric chiral design. The ongoing experimental effort is adjusting the PECVD deposition conditions to boost the efficiency on patterned substrates.

Reflectance reduction in large-area nanostructured c-Si solar cells using dry etching method

The enhanced photoelectric conversion is demonstrated in nanostructured photovoltaics using colloidal lithography and reactive-ion-etching (RIE) techniques. From the reflectance spectroscopy, trapezoid-cone arrays (TCAs) Si with SiNx passivation layer effectively suppress the Fresnel reflection in the wavelength range from 400 nm to 1000 nm. The power conversion shows the TCAs Si solar cell with 120 nm thickness of SiNx passivation layer achieves 13.736%, which is 8.87% and 2.56% enhancement compared to the conventional KOH-textured photovoltaics and TCAs with 80-nm-thick SiNx, respectively. An optical simulation based on RCWA describes the optimized shape of nanostructure to further reduce reflectance for maximum light absorption.

Fabrication and simulation of antireflective nanostructures on c-Si solar cells

The enhanced photoelectric conversion is demonstrated in nanostructured photovoltaics using colloidal lithography and reactive-ion-etching (RIE) techniques. From the reflectance spectroscopy, trapezoid-cone arrays (TCAs) Si with SiNx passivation layer effectively suppress the reflection in the wavelength range from 400 nm to 1000 nm. The power conversion shows the TCAs Si solar cell with 120 nm thickness of SiNx passivation layer achieves 13.736%, which is 8.87% and 2.56% enhancement compared to the conventional KOH-textured photovoltaics and TCAs with 80-nm-thick SiNx, respectively. An optical simulation based on RCWA describes the optimized shape of nano structure to further reduce reflectance for maximum light absorption.