Nanophotonic Light Trapping Research Articles

Broadband light absorption enhancement in a–Si:H ultrathin film solar cells with nanophotonic light trapping structures

Light trapping over a wide wavelength range is critical for applications such as thin-film solar cells (TFSCs). Therefore, in this paper, we report a plasmonic light trapping scheme as a solution to overcome the limited absorption of light in hydrogenated amorphous silicon (a-Si:H) thin film solar cells. Here we propose plasmonic nanostructures, which have periodic nanodisks added to the front surface of the cell. These structures combine a variety of photonic effects to reduce reflection, improve light trapping, and increase absorption over a broadband solar spectrum. Initially, to test whether the suggested plasmonic nanoparticles can enhance light-trapping within the absorber layers of the cell, we used the 3D finite difference time domain method to simulate the propagation of electromagnetic waves and the absorbed light in each layer of the cell. Additionally, the obtained optical properties after the FDTD simulation are used to design electrical model simulations using SCAPS software. The results show that the front surface of nanodisk-shaped solar cells with only a 100-nm-thick a-Si:H layer can absorb light at wavelengths ranging from 300 to 800 nm, which is greater than that of flat a-Si:H film devices. With optimized structural parameters, the short circuit current density Jsc could be enhanced by 98 % and 71 % with the addition of Au and Cu nanodisks, respectively. Additionally, the conversion efficiency is improved by 28 % and 26 % for Au and Cu, respectively. The proposed design of the light trapping structure can provide guidelines for ultra-TFSCs with high performance.

Nanopatterned Back-Reflector with Engineered Near-Field/Far-Field Light Scattering for Enhanced Light Trapping in Silicon-Based Multijunction Solar Cells.

Multijunction solar cells provide a path to overcome the efficiency limits of standard silicon solar cells by harvesting a broader range of the solar spectrum more efficiently. However, Si-based multijunction architectures are hindered by incomplete harvesting in the near-infrared (near-IR) spectral range as Si subcells have weak absorption close to the band gap. Here, we introduce an integrated near-field/far-field light trapping scheme to enhance the efficiency of silicon-based multijunction solar cells in the near-IR range. To achieve this, we design a nanopatterned diffractive silver back-reflector featuring a scattering matrix that optimizes trapping of multiply scattered light into a range of diffraction angles. We minimize reflection to the zeroth order and parasitic plasmonic absorption in silver by engineering destructive interference in the patterned back-contact. Numerical and experimental assessment of the optimal design on the performance of single-junction Si TOPCon solar cells highlights an improved external quantum efficiency over a planar back-reflector (+1.52 mA/cm2). Nanopatterned metagrating back-reflectors are fabricated on GaInP/GaInAsP//Si two-terminal triple-junction solar cells via substrate conformal imprint lithography and characterized optically and electronically, demonstrating a power conversion efficiency improvement of +0.9%abs over the planar reference. Overall, our work demonstrates the potential of nanophotonic light trapping for enhancing the efficiency of silicon-based multijunction solar cells, paving the way for more efficient and sustainable solar energy technologies.

Solar Cells: Diffraction‐Grated Perovskite Induced Highly Efficient Solar Cells through Nanophotonic Light Trapping (Adv. Energy Mater. 12/2018)

Solar Cells: Diffraction‐Grated Perovskite Induced Highly Efficient Solar Cells through Nanophotonic Light Trapping (Adv. Energy Mater. 12/2018)

Diffraction‐Grated Perovskite Induced Highly Efficient Solar Cells through Nanophotonic Light Trapping

AbstractAchieving light harvesting is crucial for the efficiency of the solar cell. Constructing optical structures often can benefit from micro‐nanophotonic imprinting. Here, a simple and facile strategy is developed to introduce a large area grating structure into the perovskite‐active layer of a solar cell by utilizing commercial optical discs (CD‐R and DVD‐R) and achieve high photovoltaic performance. The constructed diffraction grating on the perovskite active layer realizes nanophotonic light trapping by diffraction and effectively suppresses carrier recombination. Compared to the pristine perovskite solar cells (PSCs), the diffraction‐grating perovskite devices with DVD obtain higher power conversion efficiency and photocurrent density, which are improved from 16.71% and 21.67 mA cm−2 to 19.71% and 23.11 mA cm−2. Moreover, the stability of the PSCs with diffraction‐grating‐structured perovskite active layer is greatly enhanced. The method can boost photonics merge into the remarkable perovskite materials for various applications.

Broadening of Light Coupling to Waveguide Modes in Solar Cells by Disordered Grating Textures

We investigated the light coupling to waveguide modes in nanophotonic thin-film solar cells exhibiting a tailored disordered grating texture by rigorous optical simulations. Such disordered nanophotonic light trapping concepts have demonstrated enhanced short-circuit current density compared to ordered nanophotonic thin-film solar cells. This observation is commonly explained by a spectral broadening of the resonant light coupling to waveguide modes. In this work, we investigated the origin of this spectral broadening. We identified two basic mechanisms that lead to a spectral broadening of the light coupling to waveguide modes: (1) the broadening of the wave vector transferred by the disordered interface texture and (2) the broadening of the waveguide mode due to the distortion of the wave guiding absorber layer. Depending on the type of disorder, the contribution from each of the mechanisms varies.

Broadband perfect light trapping in the thinnest monolayer graphene-MoS2 photovoltaic cell: the new application of spectrum-splitting structure.

The light absorption of a monolayer graphene-molybdenum disulfide photovoltaic (GM-PV) cell in a wedge-shaped microcavity with a spectrum-splitting structure is investigated theoretically. The GM-PV cell, which is three times thinner than the traditional photovoltaic cell, exhibits up to 98% light absorptance in a wide wavelength range. This rate exceeds the fundamental limit of nanophotonic light trapping in solar cells. The effects of defect layer thickness, GM-PV cell position in the microcavity, incident angle, and lens aberration on the light absorptance of the GM-PV cell are explored. Despite these effects, the GM-PV cell can still achieve at least 90% light absorptance with the current technology. Our proposal provides different methods to design light-trapping structures and apply spectrum-splitting systems.

Disorder Improves Light Absorption in Thin Film Silicon Solar Cells with Hybrid Light Trapping Structure

We present a systematic simulation study on the impact of disorder in thin film silicon solar cells with hybrid light trapping structure. For the periodical structures introducing certain randomness in some parameters, the nanophotonic light trapping effect is demonstrated to be superior to their periodic counterparts. The nanophotonic light trapping effect can be associated with the increased modes induced by the structural disorders. Our study is a systematic proof that certain disorder is conceptually an advantage for nanophotonic light trapping concepts in thin film solar cells. The result is relevant to the large field of research on nanophotonic light trapping which currently investigates and prototypes a number of new concepts including disordered periodic and quasiperiodic textures. The random effect on the shape of the pattern (position, height, and radius) investigated in this paper could be a good approach to estimate the influence of experimental inaccuracies for periodic or quasi-periodic structures.

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.

Thermodynamic limits of nanophotonic light trapping in thin film silicon solar cells

We have extended an earlier thermodynamic treatment of light-trapping in lattice-textured solar cells to higher absorptances. This treatment is used to calculate the quantum efficiency spectra and short-circuit current densities JSC for thin-film silicon solar cells with ideal lattice textures. An optimal triangular lattice period of 900 nm yields a calculated JSC that is 2 mA/cm2 larger than for ideal random textures in a 1000 nm thick cell. We compare the calculations to recent experiments with periodically textured cells. While the experimental cells give JSC values that are comparable to the best cells with conventional textures, they do not show the features associated with the prediction of higher JSC. We discuss the role of imperfections in the periodic texturing, and suggest that cells used with solar tracking may realize the predicted JSC improvement.

Disorder improves nanophotonic light trapping in thin-film solar cells

We present a systematic experimental study on the impact of disorder in advanced nanophotonic light-trapping concepts of thin-film solar cells. Thin-film solar cells made of hydrogenated amorphous silicon were prepared on imprint-textured glass superstrates. For periodically textured superstrates of periods below 500 nm, the nanophotonic light-trapping effect is already superior to state-of-the-art randomly textured front contacts. The nanophotonic light-trapping effect can be associated to light coupling to leaky waveguide modes causing resonances in the external quantum efficiency of only a few nanometer widths for wavelengths longer than 500 nm. With increasing disorder of the nanotextured front contact, these resonances broaden and their relative altitude decreases. Moreover, overall the external quantum efficiency, i.e., the light-trapping effect, increases incrementally with increasing disorder. Thereby, our study is a systematic experimental proof that disorder is conceptually an advantage for nanophotonic light-trapping concepts employing grating couplers in thin-film solar cells. The result is relevant for the large field of research on nanophotonic light trapping in thin-film solar cells which currently investigates and prototypes a number of new concepts including disordered periodic and quasi periodic textures.

Optimal design of nano-scale surface light trapping structures for enhancing light absorption in thin film photovoltaics

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.

Optimization of periodic nanostructures for enhanced light-trapping in ultra-thin photovoltaics

Nanophotonic light trapping offers a promising approach to increased efficiency in thin-film organic photovoltaics. In this paper, an extension of the direct-binary-search algorithm was adopted to optimize dielectric nanophotonic structures for increasing power output of ultra-thin organic solar cells. The optimized devices were comprised of an absorber layer sandwiched between two patterned, transparent, conducting cladding layers. Light trapping in such devices with an absorber thickness of only 10nm increases power output by a factor of 16 when compared to a flat reference device. We further show that even under oblique illumination with angles ranging from 0 to 60 degrees, such a device could produce over 7 times more power compared to a flat reference device. Finally, we also performed a spectral and parametric analysis of the optimized design, and show that the increase is primarily due to guided-mode resonances. Our simulations indicate that this new design approach has the potential to significantly increase the performance of ultra-thin solar cells in realistic scenarios.

Nanophotonic light trapping in 3-dimensional thin-film silicon architectures

Emerging low cost and large area periodic texturing methods promote the fabrication of complex absorber structures for thin film silicon solar cells. We present a comprehensive numerical analysis of a 2 μm square periodic polycrystalline silicon absorber architecture designed in our laboratories. Simulations are performed on the basis of a precise finite element reconstruction of the experimentally realized silicon structure. In contrast to many other publications, superstrate light trapping effects are included in our model. Excellent agreement to measured absorptance spectra is obtained. For the inclusion of the absorber into a standard single junction cell layout, we show that light trapping close to the Yablonovitch limit can be realized, but is usually strongly damped by parasitic absorption.

Nanophotonic light trapping in solar cells

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.

Plasmonics and nanophotonics for photovoltaics

Abstract