Optimal Light Trapping Research Articles

Transforming materials discovery for artificial photosynthesis: High-throughput screening of earth-abundant semiconductors

We present a highly efficient workflow for designing semiconductor structures with specific physical properties, which can be utilized for a range of applications, including photocatalytic water splitting. Our algorithm generates candidate structures composed of earth-abundant elements that exhibit optimal light-trapping, high efficiency in H2 and/or O2 production, and resistance to reduction and oxidation in aqueous media. To achieve this, we use an ionic translation model trained on the Inorganic Crystal Structure Database to predict over 30 000 undiscovered semiconductor compositions. These predictions are then screened for redox stability under hydrogen evolution reaction or oxygen evolution reaction conditions before generating thermodynamically stable crystal structures and calculating accurate bandgap values for the compounds. Our approach results in the identification of dozens of promising semiconductor candidates with ideal properties for artificial photosynthesis, offering significant advancement toward the conversion of sunlight into chemical fuels.

Optimized light trapping of InP-ZnO core–shell nanocone array as high-absorption solar cells

Optimized light trapping of InP-ZnO core–shell nanocone array as high-absorption solar cells

Three-phase co-assembly of compositionally tunable WO3/TiO2 inverse opal photoelectrodes

Heterostructured WO3/TiO2 photonic crystal films in the form of three-dimensional macroporous inverse opals were developed by single-step, three-phase co-assembly of colloidal templates with water soluble precursors enabling simultaneous growth of both metal oxides into a firmly interconnected periodic pore framework whose skeletal walls consisted of uniformly distributed nanoscale type II WO3-TiO2 heterojunctions. A marked size- and composition-dependence of the binary WO3/TiO2 inverse opal properties was evidenced by controlling the W/Ti molar ratio and macropore diameter that allowed Fermi level and photonic band gap engineering according to optical and photoelectron spectroscopies supplemented by electrochemical measurements. Compositional tuning along with the reduction of WO3 nanocrystallite size and the concomitant W5+ defect growth with increasing TiO2 phase content were explored for the optimization of photocurrent generation by WO3/TiO2 inverse opal photoanodes combining reduced charge carrier recombination and optimal slow light trapping. One step co-assembly of mixed metal oxides is concluded as a promising route for the development of heterostructured inverse opal networks with tailored electronic properties and improved solar light harvesting for photo-induced applications.

Visible Light Trapping against Charge Recombination in FeOx-TiO2 Photonic Crystal Photocatalysts.

Tailoring metal oxide photocatalysts in the form of heterostructured photonic crystals has spurred particular interest as an advanced route to simultaneously improve harnessing of solar light and charge separation relying on the combined effect of light trapping by macroporous periodic structures and compositional materials’ modifications. In this work, surface deposition of FeOx nanoclusters on TiO2 photonic crystals is investigated to explore the interplay of slow-photon amplification, visible light absorption, and charge separation in FeOx–TiO2 photocatalytic films. Photonic bandgap engineered TiO2 inverse opals deposited by the convective evaporation-induced co-assembly method were surface modified by successive chemisorption-calcination cycles using Fe(III) acetylacetonate, which allowed the controlled variation of FeOx loading on the photonic films. Low amounts of FeOx nanoclusters on the TiO2 inverse opals resulted in diameter-selective improvements of photocatalytic performance on salicylic acid degradation and photocurrent density under visible light, surpassing similarly modified P25 films. The observed enhancement was related to the combination of optimal light trapping and charge separation induced by the FeOx–TiO2 interfacial coupling. However, an increase of the FeOx loading resulted in severe performance deterioration, particularly prominent under UV-Vis light, attributed to persistent surface recombination via diverse defect d-states.

Enhanced electro-optical properties of TiO2 Nanocone Arrays for ultrathin GaAs Solar cells

An antireflective coating made by TiO2 nanocone array is designed on an ultrathin multilayer GaAs substrate (the thickness of GaAs is 300 nm). The intrinsic light trapping, carrier recombination and photoconversion are simulated by a complete photoelectric simulation. The results show that the optimized light trapping design can enhance the electromagnetic resonance in active layer and lead to an extraordinary enhancement of both absorption and power conversion capabilities in GaAs solar cell. The short-circuit current density increases by 51.12%, from 12.01 mA/cm2 to 18.15 mA/cm2 and power conversion efficiency increases by 52.83%, from 11.00% to 16.81% compared to the reference benchmark.

Evolutionary design algorithm for optimal light trapping in solar cells

Light scattering at rough interfaces is a standard approach to enhance absorption of light in the absorber layers of solar cells due to light trapping. In this work, an automata optimization algorithm, based on the Constructal Theory, is proposed for the heuristic geometric design to enhance light trapping of silicon solar cell surface textures. Surface textures were investigated computationally for light absorption of a normally incident simulated solar spectrum, under constraints of transverse area of the texture. The photogenerated current density was calculated for textures to estimate light trapping. In the automata optimization, Elemental Constructals, consisting of dielectric material blocks, were assembled on top of the solar cell forming a texture. Three variations of the proposed automata algorithm were tested and their capabilities were determined. The best result for this optimization was a photocurrent J=17.48mA/cm2 for grid periodicity above the wavelength and J=18.31mA/cm2 for grid periodicity in the sub-wavelength range. The evolutionary algorithm was also applied to assemble a periodic-disordered texture from fixed shape gratings to better harness incident light. Different textures with fixed shapes of gratings had been investigated to determine optimal geometric parameters. Optimized triangular, trapezoidal, and rounded-tip textures achieved the best results in this phase and were implemented to assemble the periodic-disordered texture which reached J=19.75mA/cm2 for triangular gratings. Results found with this pristine evolutionary algorithm corroborate its efficiency in finding, practically, geometries that lead to greater light absorption with a minimum amount of dielectric materials.

Nano-fabrication of depth-varying amorphous silicon crescent shell array for light trapping

We report a new structure of depth controllable amorphous silicon (a-Si) crescent shells array, fabricated by the SiO2 monolayer array assisted deposition of a-Si by plasma enhanced chemical vapor deposition and nanosphere lithography, for high-efficiency light trapping applications. The depth of the crescent shell cavity was tailored by selective etching of a-Si layer of the SiO2/a-Si core/shell nanoparticle array with a varied etching time. The morphological changes of the crescent shells were examined by scanning electron microscopy and atomic force microscopy. A simple model is developed to describe the geometrical evolution of the a-Si crescent shells. Spectroscopic measurements and finite difference time domain simulations were conducted to examine the optical performance of the crescent shells. Results show that these nanostructures all have a broadband high efficiency absorption and that the light trapping capability of these crescent shell structures depends on the excitation of depths-regulated optical resonance modes. With an appropriate selection of process parameters, the structure of crescent a-Si shells may be fine-tuned to achieve an optimal light trapping capacity.

Broadband enhancement of dielectric light trapping nanostructure used in ultra-thin solar cells

A dielectric fishnet nanostructure is designed to increase the light trapping capability of ultra-thin solar cells. The complex performance of ultra-thin cells such as the optical response and electrical response are fully quantified in simulation through a complete optoelectronic investigation. The results show that the optimized light trapping nanostructure can enhances the electromagnetic resonance in active layer then lead to extraordinary enhancement of both absorption and light-conversion capabilities in the solar cell. The short-circuit current density increases by 49.46% from 9.40 mA/cm2 to 14.05 mA/cm2 and light-conversion efficiency increases by 51.84% from 9.51% to 14.44% compared to the benchmark, a solar cell with an ITO-GaAs-Ag structure.

A facile synthesis of silicon nanowires/micropillars structure using lithography and metal-assisted chemical etching method

In this paper, a new fabrication method for silicon nanowires/micropillars structure is proposed. This method is using lithography and metal-assisted chemical etching to fabricate the structure of silicon nanowires/micropillars. In the experiment, to obtain the optimal light trapping properties of silicon nanowires/micropillars structure, the optimal light trapping properties of silicon nanowires were systemically investigated, including etching concentration, etching time and etching temperature. And then, a large-area uniform, controllable and optimal light trapping silicon nanowires/micropillars structure was fabricated using lithography and metal-assisted chemical etching method. This controllable and facile method for silicon nanowires/micropillars structure will expand its application in the field of optoelectronic devices and solar cells.

Scalable simple liquid deposition techniques for the enhancement of light absorption in thin films: Distributed Bragg reflectors coupled to 1D nanoimprinted textures

Light trapping within a light absorbing medium is a key to highly efficient thin film solar cells. We propose a large-scale procedure based on materials with low absorption for the fabrication of combined Distributed Bragg Reflector (DBR) and grating light trapping structures. Using Rigorous Coupled Wave Analysis (RCWA) numerical simulations we designed a combined DBR and 1D grating structure allowing to significantly improve the absorption in a aSi:H film as thin as 100 nm. The optimized light trapping structure was fabricated. The enhancement of light absorption in thin aSi:H film was experimentally proven and discussed quantitatively with respect to the theoretical expectations.

A Two‐Resonance Tapping Cavity for an Optimal Light Trapping in Thin‐Film Solar Cells

An optimal photon absorption in thin film photovoltaic technologies can only be reached by effectively trapping the light in the absorber layer provided a considerable portion of the photons is rejected or scattered out of such layer. Here, a new optical cavity is proposed that can be made to have a resonant character at two different nonharmonic frequencies when adjusting the materials or geometry configurations of the partially transmitting cavity layers. Specific configurations are found where a reminiscence of such two fundamental resonances coexists leading to a broadband light trapping. When a PTB7‐Th:PC71BM organic cell is integrated within such cavity, a power conversion efficiency of 11.1% is measured. This study also demonstrates that when materials alternative to organics are used in the photoactive cell layer, a similar cavity can be implemented to also obtain the largest light absorption possible. Indeed, when it is applied to perovskite cells, an external quantum efficiency is predicted that closely matches its corresponding internal one for a broad wavelength range.

Towards the efficiency limits of silicon solar cells: How thin is too thin?

It is currently possible to fabricate crystalline silicon solar cells with the absorber thickness ranging from a few hundreds of micrometres (conventional wafer-based cells) to devices as thin as 1μm. In this work, we use a model single-junction solar cell to calculate the limits of energy conversion efficiency and estimate the optimal absorber thickness. We have found that the limiting efficiency for cells in the thickness range between 40 and 500μm is very similar and close to 29%. In this regard, we argue that decreasing the thickness below around 40μm is counter-productive, as it significantly reduces the maximum achievable efficiency, even when an optimal light trapping is implemented. We analyse the roles of incomplete light trapping and extrinsic (bulk and surface) recombination mechanisms. For a reasonably high material quality, consistent with present-day fabrication techniques, the optimal thickness is always higher than a few tens of micrometres. We identify incomplete light trapping and parasitic losses as a major roadblock for improving the efficiency upon the current record of 25.6% for silicon solar cells. Finally, considering the main parameters that impact the solar cell performance, we quantify the requirements for achieving a given efficiency, which helps us to establish a proper design strategy for high efficiency silicon solar cells.

New strategy to promote conversion efficiency using high-index nanostructures in thin-film solar cells.

Nano-scaled metallic or dielectric structures may provide various ways to trap light into thin-film solar cells for improving the conversion efficiency. In most schemes, the textured active layers are involved into light trapping structures that can provide perfect optical benefits but also bring undesirable degradation of electrical performance. Here we propose a novel approach to design high-performance thin-film solar cells. In our strategy, a flat active layer is adopted for avoiding electrical degradation, and an optimization algorithm is applied to seek for an optimized light trapping structure for the best optical benefit. As an example, we show that the efficiency of a flat a-Si:H thin-film solar cell can be promoted close to the certified highest value. It is also pointed out that, by choosing appropriate dielectric materials with high refractive index (>3) and high transmissivity in wavelength region of 350 nm–800 nm, the conversion efficiency of solar cells can be further enhanced.

Design of Anodic Aluminum Oxide Rear Surface Plasmonic Heterostructures for Light Trapping in Thin Silicon Solar Cells

A metal-dielectric heterostructure that provides the combined capability of light trapping and surface passivation is reported. The light-trapping layer employs a porous aluminum anodic oxide (AAO) with metal nanoparticles formed in the pores on the rear surface of a thin crystalline silicon solar cell. Numerical finite-difference time domain (FDTD) simulations were performed to determine the pore diameter and spacing that would result in optimal light trapping for this metal-dielectric heterostructure. For a 2.5-μm-thick crystalline silicon device, the optimal pore diameter and spacing were determined to be ~250 and ~450 nm, respectively. These conditions resulted in an enhancement of the simulated photocurrent by ~12.6% compared with a device in which the heterostructure was replaced with a homogenous aluminum oxide layer. Simulations also confirmed that the thickness of an underlying dielectric layer should be minimized to 10-20 nm, with the AAO barrier layer being maintained as thin as possible. Finally, it was shown that replacement of silver by aluminum in the pores resulted in a reduction in the photocurrent of 6.3% and would necessitate much larger pore spacing that is difficult to achieve experimentally and would result in thicker AAO barrier layers, which are undesirable.

Thermodynamics of light management in photovoltaic devices

This paper elaborates a comprehensive theory of the thermodynamics of light management in solar cells explicitly considering imperfect light trapping, parasitic absorption and nonradiative recombination losses. A quantitative description of the entropic losses that reduce the open-circuit voltage and the energy conversion efficiency from the radiative limit towards realistic situations is presented. The theory embraces the fundamental limits for idealized solar cell devices given by the Yablonovitch limit and the Shockley-Queisser limit. We discriminate between reversible and irreversible entropic loss processes for four fundamental light management concepts: (i) conventional light trapping as an integral part of the device, (ii) geometric concentration of incident light, (iii) angular restriction of incoming and outgoing light, and (iv) light concentration by luminescent solar collectors. Based on this discrimination, a comprehensive discussion of the interplay between the loss processes and light management is presented. As part of this analysis, a new figure of merit for efficient light trapping in solar cells is introduced as well as an example of a deterministic light trapping concept which induces almost optimal light trapping.

Statistical Analysis of Surface Nanopatterned Thin Film Solar Cells Obtained by Inverse Optimization

This work is a statistical study of the broadband light absorption in thin film solar cells, enhanced by metallic surface nanotexturing. We consider optimum grating structures on the surface of amorphous silicon solar cells obtained by inverse optimization, and study the joint statistics of the resulting absorption enhancement/spectra in the presence of time and structural variants, such as fabrication error and year around changes in the solar irradiance, as well as the angle of incident. We adopt yearly data for solar irradiation at individual hours. In conjunction with the data for light absorption spectra at various incident angles and random samples of the fabrication error vector, we evaluate the real world performance of optimized solar cells. The resulting conclusions serve as a sensitivity/time analysis for better understanding the limits of performance and robustness of thin film cells and optimal light trapping mechanisms.

Highly efficient light-trapping structure design inspired by natural evolution.

Recent advances in nanophotonic light trapping open up the new gateway to enhance the absorption of solar energy beyond the so called Yablonovitch Limit. It addresses the urgent needs in developing low cost thin-film solar photovoltaic technologies. However, current design strategy mainly relies on the parametric approach that is subject to the predefined topological design concepts based on physical intuition. Incapable of dealing with the topological variation severely constrains the design of optimal light trapping structure. Inspired by natural evolution process, here we report a design framework driven by topology optimization based on genetic algorithms to achieve a highly efficient light trapping structure. It has been demonstrated that the optimal light trapping structures obtained in this study exhibit more than 3-fold increase over the Yablonovitch Limit with the broadband absorption efficiency of 48.1%, beyond the reach of intuitive designs.

Comparison of periodic light-trapping structures in thin crystalline silicon solar cells

Material costs may be reduced and electrical properties improved by utilizing thinner solar cells. Light trapping makes it possible to reduce wafer thickness without compromising optical absorption in a silicon solar cell. In this work we present a comprehensive comparison of the light-trapping properties of various bi-periodic structures with a square lattice. The geometries that we have investigated are cylinders, cones, inverted pyramids, dimples (half-spheres), and three more advanced structures, which we have called the roof mosaic, rose, and zigzag structure. Through simulations performed with a 20 μm thick Si cell, we have optimized the geometry of each structure for light trapping, investigated the performance at oblique angles of incidence, and computed efficiencies for the different diffraction orders for the optimized structures. We find that the lattice periods that give optimal light trapping are comparable for all structures, but that the light-trapping ability varies considerably between the structures. A far-field analysis reveals that the superior light-trapping structures exhibit a lower symmetry in their diffraction patterns. The best result is obtained for the zigzag structure with a simulated photo-generated current Jph of 37.3 mA/cm2, a light-trapping efficiency comparable to that of Lambertian light-trapping.

Understanding of photocurrent enhancement in real thin film solar cells: towards optimal one-dimensional gratings

Despite the progress in the engineering of structures to enhance photocurrent in thin film solar cells, there are few comprehensive studies which provide general and intuitive insight into the problem of light trapping. Also, lack of theoretical propositions which are consistent with fabrication is an issue to be improved. We investigate a real thin film solar cell with almost conformal layers grown on a 1D grating metallic back-reflector both experimentally and theoretically. Photocurrent increase is observed as an outcome of guided mode excitation in both theory and experiment by obtaining the external quantum efficiency of the cell for different angles of incidence and in both polarization directions. Finally, the effect of geometrical parameters on the short circuit current density of the device is investigated by considering different substrate shapes that are compatible with solar cell fabrication. Based on our simulations, among the investigated shapes, triangular gratings with a very sharp slope in one side, so called sawtooth gratings, are the most promising 1D gratings for optimal light trapping.

On the development of single and multijunction solar cells with hot-wire CVD deposited active layers

We present an overview of the scientific challenges and achievements during the development of thin film silicon based single and multijunction solar cells with hot-wire chemical vapor deposition (HWCVD) of the active silicon layers. The highlights discussed include the development of Ag/ZnO coatings with a proper roughness and morphology for optimal light trapping in single and multijunction thin film silicon solar cells, studies of the structural defects created by a rough substrate surface and their influence on the performance of nc-Si:H n–i–p single junction solar cells, and studies of the phase change during the growth of nc-Si:H by HWCVD and the use of a ‘reverse’ H2 profiling technique to achieve nc-Si:H single junction n–i–p cells with high performance. Thus far, the best AM1.5 efficiency reached for n–i–p cells on stainless-steel with HWCVD i-layers is 8.6% for single junction nc-Si:H solar cells and 10.9% for triple junction solar cells. The opportunities for further improvement of cell efficiency are also discussed. We conclude that the uniqueness of HWCVD and of the i-layers deposited with this technique require some adjustments in the strategy for optimization of single or multijunction solar cells, such as using a reverse H2 profiling technique for the deposition of nc-Si:H i-layers. However, the output performance of solar cells with HWCVD deposited i-layers is close to those with i-layers deposited by other techniques. The difference between the best nc-Si:H n–i–p cells obtained so far in our lab and the reported best n–i–p cells with PECVD i-layers can be mainly attributed to the differences in the rough substrates and to the use of rather thin i-layers.