Sze-Ming Fu

An optimized surface plasmon photovoltaic structure using energy transfer between discrete nano-particles.

Surface plasmon enhancement has been proposed as a way to achieve higher absorption for thin-film photovoltaics, where surface plasmon polariton(SPP) and localized surface plasmon (LSP) are shown to provide dense near field and far field light scattering. Here it is shown that controlled far-field light scattering can be achieved using successive coupling between surface plasmonic (SP) nano-particles. Through genetic algorithm (GA) optimization, energy transfer between discrete nano-particles (ETDNP) is identified, which enhances solar cell efficiency. The optimized energy transfer structure acts like lumped-element transmission line and can properly alter the direction of photon flow. Increased in-plane component of wavevector is thus achieved and photon path length is extended. In addition, Wood-Rayleigh anomaly, at which transmission minimum occurs, is avoided through GA optimization. Optimized energy transfer structure provides 46.95% improvement over baseline planar cell. It achieves larger angular scattering capability compared to conventional surface plasmon polariton back reflector structure and index-guided structure due to SP energy transfer through mode coupling. Via SP mediated energy transfer, an alternative way to control the light flow inside thin-film is proposed, which can be more efficient than conventional index-guided mode using total internal reflection (TIR).

Aperiodic and randomized dielectric mirrors: alternatives to metallic back reflectors for solar cells

Dielectric mirrors have recently emerged for solar cells due to the advantages of lower cost, lower temperature processing, higher throughput, and zero plasmonic absorption as compared to conventional metallic counterparts. Nonetheless, in the past, efforts for incorporating dielectric mirrors into photovoltaics were not successful due to limited bandwidth and insufficient light scattering that prevented their wide usage. In this work, it is shown that the key for ultra-broadband dielectric mirrors is aperiodicity, or randomization. In addition, it has been proven that dielectric mirrors can be widely applicable to thin-film and thick wafer-based solar cells to provide for light trapping comparable to conventional metallic back reflectors at their respective optimal geometries. Finally, the near-field angular emission plot of Poynting vectors is conducted, and it further confirms the superior light-scattering property of dielectric mirrors, especially for diffuse medium reflectors, despite the absence of surface plasmon excitation. The preliminary experimental results also confirm the high feasibility of dielectric mirrors for photovoltaics.

Lithographically fabricable, optimized three-dimensional solar cell random structure

The optimized random reflector is highly preferred for solar cells due to its superiority over an un-optimized totally random surface and its potential to exceed the Lambertian limit. There are some obstacles to overcome for realizing optimized random reflectors, including the feasibility from the process viewpoint and the intensive computational demand for large-scale random reflector design. Here a binary random grating is proposed which can be easily fabricated using common lithographic techniques. By using a global optimization algorithm and three-dimensional (3D) EMW simulation, the solar cell structure with 4 4 quasi-random binary grating can provide 23% higher integrated absorbance than its periodic grating counterpart and 103.5% higher integrated absorbance than a planar cell, approaching the Lambertian limit. Broad-band transmission improvement at short wavelength and a broad-band waveguiding effect at long wavelength is observed for the optimized 3D geometry. Additionally, the optimized random grating surpasses the periodic grating at all incident angles. The absorbance of the large-scale, fully optimized binary pattern can potentially exceed the Lambertian limit while its computational demand is shown to be manageable.

The versatile designs and optimizations for cylindrical TiO2-based scatterers for solar cell anti-reflection coatings.

The anti-reflection coating(ARC) based on dielectric nano-particles has been recently proposed as a new way to achieve the low reflectance required for solar cell front surfaces. In this scenario, the Mie modes associated with the dielectric nano-particles are utilized to facilitate photon forward scattering. In this work, versatile designs together with systematically optimized geometry are examined, for the ARCs based on dielectric scatterers. It is found that the Si3N4-TiO2 or SiO2-TiO2 stack is capable of providing low reflectance while maintaining a flat and passivated ARC-semiconductor interface which can be beneficial for reduced interface recombination and prevent V(OC) degradation associated with topography on the active materials. It is also confirmed that the plasmonic nano-particles placed at the front side of solar cells is not a preferred scheme, even with thorough geometrical optimization. At the ultimate design based on mixed graded index(GI) Mie-scattering, the averaged reflectance can be as low as 0.25%. Such a low reflectance is currently only achievable by ultra-long silicon nano-tips, but silicon nano-tips introduce severe surface recombination. On the other hand, the mixed GI Mie design preserves a flat and passivated ARC-silicon interface, with total thickness reduced to 279.8 nm, much thinner than 1.6 μm for silicon nanotips. In addition, the light trapping capability of mixed GI Mie design is much better than silicon nanotips. In fact, when compared to the state-of-art TiO2 light trapping anti-reflection coating, the mixed GI Mie design provides same light trapping capability while providing much lower reflectance.

An ultra-efficient energy transfer beyond plasmonic light scattering

The energy transfer between nano-particles is of great importance for, solar cells, light-emitting diodes, nano-particle waveguides, and other photonic devices. This study shows through novel design and algorithm optimization, the energy transfer efficiency between plasmonic and dielectric nano-particles can be greatly improved. Using versatile designs including core-shell wrapping, supercells and dielectric mediated plasmonic scattering, 0.05 dB/μm attenuation can be achieved, which is 20-fold reduction over the baseline plasmonic nano-particle chain, and 8-fold reduction over the baseline dielectric nano-particle chain. In addition, it is also found that the dielectric nano-particle chains can actually be more efficient than the plasmonic ones, at their respective optimized geometry. The underlying physics is that although plasmonic nano-particles provide stronger coupling and field emission, the effect of plasmonic absorption loss is actually more dominant resulting in high attenuation. Finally, the group velocity for all design schemes proposed in this work is shown to be maintained above 0.4c, and it is found that the geometry optimization for transmission also boosts the group velocity.

A unified mathematical framework for intermediate band solar cells

The modeling of intermediate band solar cell has been developed since 90s and continued effort is made to facilitate the realization of this novel device. Two formulation has been used to model the generation recombination rate of IBSC including conventionally available modified-Shockley-Reed-Hall formulation or later proposed IBSC formulation (Luque and Marti, PRL 78 5014). This paper proves that these two formulations are actually mathematically equivalent and actually one can be derived from the other. A unified mathematical framework can thus be established and the conventional drift-diffusion model can thus be employed for modeling novel IBSC with the inclusion of new model for intermediate band carrier transport. The debate whether the addition of impurity atoms would decrease the efficiency by shorter recombination lifetime or increase the efficiency by more absorption is studied, and results confirm that the efficient removal of photo-generated carriers from valence and conduction bands and solar concentration is the key to the success of subbandgap photovoltaics.

Toward ultimate nanophotonic light trapping using pattern-designed quasi-guided mode excitations

In this work, a shape-optimized periodic pattern design is employed to boost the short circuit current of solar cells. A decent result of an additional 16.1% enhancement in short circuit current is achieved by solely pattern-wise optimization, compared to the baseline structure that is already under full parameter optimization. The underlying physics is that the shape-optimized pattern leads to optimal quasi-guided mode excitations. As a result of the pattern design, a single strongly confined quasi-guided mode is replaced with several weakly confined modes, to cover a broader spectral range. Previous works of optimized periodic gratings result in gradually varied grating heights and require grayscale lithography leading to high process complexity. Using randomized pattern for isotropic Lambertian light trapping, on the other hand, leads to an overly large simulation domain. The proposed pattern design methodology achieves the optimal balance between the slow-light enhancement strength and the enhancement spectral range for nanophotonic light trapping using quasi-guided modes.

Lithographically-definable solar cell random reflector using genetic algorithm optimization

Randomly textured Lambertian surface provides broad band cosine emission and thus is suitable for photovoltaic application. Nonetheless, variation of efficiency and non-optimized nature of randomly textured devices are undesirable. Here it is shown that using genetic algorithm, a 4×4 binary quasi-random grating can provide 23% higher absorption than 2D periodic grating and 103.5% higher than planar cells, approaching Lambertian limit. The improvement is attributed to broad band transmission for high energy photon and broad band waveguiding effect for low energy photons. Large scale fully-optimized binary grating can potentially surpass Lambertian limit due to its optimized nature and should be employed for future thin-film photovoltaic devices to reduce film thickness and cost.

Experimental and theoretical study of low-cost hydrothermally grown nanowire silicon solar cell

The low cost and high absorbance is the two most important considerations for thin-film silicon photovoltaics. In this work, aluminum doped zinc oxide(AZO) nanowire is grown by low cost hydrothermal method and nano-structured amorphous silicon(α-Si) solar cell is realized on these AZO nanowire substrate. The experimental result shows superior light trapping property compared to conventional planar structure with 18.83% improvement in photocurrent. Simulation result shows the required thickness for full absorption for high aspect ratio AZO nanowire solar cell is ~0.3 m, much thinner than planar type solar cell. The calculation reveals that solar cell absorbance increases with the nanowire length and packing density, indicating material volume is the first order effect for nanowire solar cells. The proposed low-cost AZO nanowire array amorphous silicon solar cell is very promising for future nanostructured silicon photovoltaics.

Optimization of plasmonic cavity-resonant multijunction cells

Abstract. Improving spectral photon harvesting is important for thin-film multijunction cells. We show that efficient spectral flux management can be achieved using genetic algorithm-optimized surface plasmon (SP) cavity-resonant type multijunction cells. We also observe that the excitation of the SP quasi-guided mode, Fabry–Perot mode, and SP polariton significantly enhance the photocurrent of multijunction cells. Two types of cavity structures are investigated. For the optimized SP intermediate reflector and bottom-grating cavity, the resonant cavity mode efficiently increases the long-wavelength absorption in the bottom cell by 63.27%, resulting in reduced absorbance asymmetry between the top and the bottom cells. Accordingly, the matched integrated absorbance is increased by 14.92%. For the optimized SP top- and bottom-grating (TBG) cavity, the integrated absorbance and current matching are improved due to the higher transmission through the solar cell front surface and the excitation of the quasi-guided mode with a more localized field in the bottom cell. The matched integrated absorbance is improved by 85.68% for the TBG cavity.