The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

Thin-film structures can reduce the cost of solar power by using inexpensive substrates and a lower quantity and quality of semiconductor material. However, the resulting short optical path length and minority carrier diffusion length necessitates either a high absorption coefficient or excellent light trapping. Semiconducting nanowire arrays have already been shown to have low reflective losses compared to planar semiconductors, but their light-trapping properties have not been measured. Using optical transmission and photocurrent measurements on thin silicon films, we demonstrate that ordered arrays of silicon nanowires increase the path length of incident solar radiation by up to a factor of 73. This extraordinary light-trapping path length enhancement factor is above the randomized scattering (Lambertian) limit (2n2 ∼ 25 without a back reflector) and is superior to other light-trapping methods. By changing the silicon film thickness and nanowire length, we show that there is a competition between impr...

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Light Trapping in Silicon Nanowire Solar Cells

In this paper, we use the transfer matrix method to calculate the optical absorptance of vertically-aligned silicon nanowire (SiNW) arrays. For fixed filling ratio, significant optical absorption enhancement occurs when the lattice constant is increased from 100 nm to 600 nm. The enhancement arises from an increase in field concentration within the nanowire as well as excitation of guided resonance modes. We quantify the absorption enhancement in terms of ultimate efficiency. Results show that an optimized SiNW array with lattice constant of 600 nm and wire diameter of 540 nm has a 72.4% higher ultimate efficiency than a Si thin film of equal thickness. The enhancement effect can be maintained over a large range of incidence angles.

Optical absorption enhancement in silicon nanowire arrays with a large lattice constant for photovoltaic applications.

We use a rigorous electromagnetic approach to analyze the fundamental limit of light-trapping enhancement in grating structures. This limit can exceed the bulk limit of 4n2, but has significant angular dependency. We explicitly show that 2D gratings provide more enhancement than 1D gratings. We also show the effects of the grating profile’s symmetry on the absorption enhancement limit. Numerical simulations are applied to support the theory. Our findings provide general guidance for the design of grating structures for light-trapping solar cells.

/pdf/fundamental-limit-of-light-trapping-in-grating-structures-1qyuigicd0.pdf

Fundamental limit of light trapping in grating structures.

Nanophotonic light trapping for solar cells is an exciting field that has seen exponential growth in the last few years. There has been a growing appreciation for solar energy as a major solution to the world’s energy problems, and the need to reduce materials costs by the use of thinner solar cells. At the same time, we have the newly developed ability to fabricate controlled structures on the nanoscale quickly and cheaply, and the computational power to optimize the structures and extract physical insights. In this paper, we review the theory of nanophotonic light trapping, with experimental examples given where possible. We focus particularly on periodic structures, since this is where physical understanding is most developed, and where theory and experiment can be most directly compared. We also provide a discussion on the parasitic losses and electrical effects that need to be considered when designing nanophotonic solar cells.

/pdf/nanophotonic-light-trapping-in-solar-cells-2o1od4251g.pdf

Nanophotonic light trapping in solar cells

We utilize photonic crystals to simulate enhanced light-trapping in a-Si:H thin film solar cells A one dimensional photonic crystal or distributed Bragg reflector with alternating dielectric layers acts as low loss backreflector A two dimensional photonic crystal between the absorber layer and the Bragg reflector diffracts light at oblique angles within the absorber The photonic crystal geometry is optimized to obtain maximum absorption The photonic crystal provides lossless diffraction of photons, increasing the photon path length within the absorber layer The simulation predicts significantly enhanced photon harvesting between 600 and 775nm below the band edge, and an absorption increase by more than a factor of 10 near the band edge The optical path length ratio can exceed the classical limit predicted for randomly roughened scattering surfaces at most wavelengths near the band edge The optical modeling is performed with a rigorous scattering matrix approach where Maxwell’s equations are solved 

Photonic crystal enhanced light-trapping in thin film solar cells

We design photonic crystal back reflectors for thin film a-Si:H solar cells that maximize the absorption of photons within the absorber layer. Rigorous scattering matrix (S-matrix) simulation approaches are used where Maxwell's equations are rigorously solved for the layered solar cell structure. Metallic photonic crystals consisting of hole arrays as well as metallic pyramidal arrays are considered. Simulations predict an optimum lattice spacing between 700 and 750 nm and an increase in absorption of ~10%. The metallic photonic crystals strongly diffract light within the absorber layer, and are very effective in harvesting weakly absorbing photons with energies just above the band edge.

Simulation and modelling of photonic and plasmonic crystal back reflectors for efficient light trapping

A major route to improving solar cell efficiencies is by improving light trapping in solar absorber layers. Traditional light trapping schemes involve a textured metallic back reflector that also introduces losses at optical wavelengths. Here we develop alternative light trapping schemes with a-Si:H thin film solar cells, that do not use metallic components, thereby avoiding losses. We utilize low loss one-dimensional photonic crystals as distributed Bragg reflectors (DBR) at the backside of the solar cells. The DBR is constructed with alternating layers of crystalline Si and SiO 2 . Between the DBR and the absorber layer, there is a layer of 2D photonic crystal composed of amorphous silicon and SiO 2 . The 2D photonic crystal layer will diffract light at oblique angles, so that total internal reflection is formed inside the absorber layer. We have achieved enhanced light-trapping in both crystalline and amorphous silicon solar cells at near-infrared wavelengths where absorption lengths are very large. Very high absorption is achieved throughout optical wavelengths. The optical modeling is performed with a rigorous 3 dimensional scattering matrix approach where Maxwelli¯s equations are solved in Fourier space.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S1946427400033972

Enhancing Light-trapping and Efficiency of Solar Cells with Photonic Crystals

We design and fabricate a-Si:H solar cells with plasmonic crystal back reflectors demonstrating enhanced absorption of near infrared photons. Rigorous simulations predict plasmonic nanoparticles on ITO can increase absorption in the near infrared. Rigorous simulations also predict plasmonic crystal back reflectors with a pitch of ∼750 nm to optimize the enhanced absorption. Such periodic back reflectors have been fabricated with photolithography and a-Si:H solar cells have been grown on them by PECVD. The solar cells display ∼9% improvement in short circuit current and external quantum efficiency and a absorption enhancement factor exceeding 8 above 650 nm.

Surface plasmon enhancement of optical absorption of thin film a-Si:H solar cells

Improved light-trapping is a major route to improving solar cell efficiencies. We design a combination of a 2-dimensional photonic crystal and a one-dimensional distributed Bragg reflector as the back reflector for a-Si:H solar cells. This configuration avoids inherent losses associated with textured back-reflectors. The photonic crystals are composed of ITO and can easily serve as a conducting back contact. We have optimized the geometry of the photonic crystal to maximize absorption using rigorous scattering matrix simulations. The photonic crystal provides strong diffraction of red and near-IR wavelengths within the absorber layer and can enhance the absorption by more than a factor of 10 relative to the case without the photonic crystal. The optical path length with the photonic crystal can improve over the limit for a random roughened scattering surface.

Dayu Zhou

Papers

Photonic crystal enhanced light-trapping in thin film solar cells

Simulation and modelling of photonic and plasmonic crystal back reflectors for efficient light trapping

Enhancing Light-trapping and Efficiency of Solar Cells with Photonic Crystals

Surface plasmon enhancement of optical absorption of thin film a-Si:H solar cells

Improved Photon Absorption in a-Si:H Solar Cells using Photonic Crystal Architectures