Photon Management Research Articles

Nanoscale photon management in silicon solar cells

Light absorption in a photovoltaic device becomes critical as the thickness of an absorber layer is decreased to reduce cost. To enhance light absorption, photon management at the nanoscale has been studied because conventional methods, which are based on micrometer-sized structure, do not work well for thinner solar cells. This article reviews recent progress in photon management on the nanoscale for increasing light absorption in Si solar cells. The methodology for the absorption enhancement will be discussed, followed by advances in nanofabrication techniques that make the methodology a scalable and viable solution. The authors conclude with a discussion of the challenge of photon management schemes and future directions for light trapping in ultra-thin Si solar cells.

Photon management in two-dimensional disordered media

Elaborating reliable and versatile strategies for efficient light coupling between free space and thin films is of crucial importance for new technologies in energy efficiency. Nanostructured materials have opened unprecedented opportunities for light management, notably in thin-film solar cells. Efficient coherent light trapping has been accomplished through the careful design of plasmonic nanoparticles and gratings, resonant dielectric particles and photonic crystals. Alternative approaches have used randomly textured surfaces as strong light diffusers to benefit from their broadband and wide-angle properties. Here, we propose a new strategy for photon management in thin films that combines both advantages of an efficient trapping due to coherent optical effects and broadband/wide-angle properties due to disorder. Our approach consists of the excitation of electromagnetic modes formed by multiple light scattering and wave interference in two-dimensional random media. We show, by numerical calculations, that the spectral and angular responses of thin films containing disordered photonic patterns are intimately related to the in-plane light transport process and can be tuned through structural correlations. Our findings, which are applicable to all waves, are particularly suited for improving the absorption efficiency of thin-film solar cells and can provide a new approach for high-extraction-efficiency light-emitting diodes.

Improving the Light Trapping Efficiency of Plasmonic Polymer Solar Cells through Photon Management

In this study, we have explored how light trapping efficiency can be enhanced by using gold nanoparticles (Au NPs) of various sizes and shapes on the front of polymer solar cells (PSCs) with the active layer—blends of poly(3-hexyl thiophene) and [6,6]-phenyl-C61-butyric acid methyl ester. The light-concentrating behavior was enhanced after we had incorporated gold nanospheres or nanorods into the anodic buffer layer [based on poly(3,4-ethylenedioxythiophene):polystyrenesulfonate] to trigger various localized surface plasmon resonance (LSPR) bands. Comparison of the optical characteristics and the performance of the PSCs prepared with and without Au NPs, and we found that the UV–vis and wavelength-dependent photoluminescent spectral data corroborated with the device performance due to the photon management by considering the light scattering and LSPR effects at the active layer. The presence of Au NPs increased the power conversion efficiency to approximately 4.3% (an enhancement of 24%).

Sandwiching intermediate reflectors in tandem solar cells for improved photon management

In tandem solar cells, intermediate reflectors are employed to increase light absorption in the top cell. Thus far, the use of photonic crystals for this purpose was not optimal since side-lobes in the reflection spectrum reduced the absorption in the bottom cell. To compensate this reduction, the bottom cell thickness had to be excessively increased; nullifying the main advantage of thin-film solar cells. Here, we suggest to solve this issue by reducing the impedance mismatch between photonic crystal and bottom cell using anti-reflection layers. The concept is even validated for solar cells comprising random textures.

Light- and Energy-Harvesting Scheme Employing the Nanoscale Photon Management in the Solar Cells and the Photodetectors

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Self-organized 3-D Nanostructures for Photon Management and Cost-effective Photovoltaics

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Mechanistic modeling of vertebrate spatial contrast sensitivity and acuity at low luminance

The validity of the Barten theoretical model for describing the vertebrate spatial contrast sensitivity function (CSF) and acuity at scotopic light levels has been examined. Although this model (which has its basis in signal modulation transfer theory) can successfully describe vertebrate CSF, and its relation to underlying visual neurophysiology at photopic light levels, significant discrepancies between theory and experimental data have been found at scotopic levels. It is shown that in order to describe scotopic CSF, the theory must be modified to account for important mechanistic changes, which occur as cone vision switches to rod vision. These changes are divided into photon management factors [changes in optical performance (for a dilated pupil), quantum efficiency, receptor sampling] and neural factors (changes in spatial integration area, neural noise, and lateral inhibition in the retina). Predictions of both scotopic CSF and acuity obtained from the modified theory were found to be in good agreement with experimental values obtained from the human, macaque, cat, and owl monkey. The last two species have rod densities particularly suited for scotopic conditions.

Exploiting Nanoroughness on Holographically Patterned Three‐Dimensional Photonic Crystals

AbstractThe fabrication of three‐dimensional (3D) diamond photonic crystals with controllable nanoroughness (≤120 nm) on the surface from epoxy‐functionalized cyclohexyl polyhedral oligomeric silsesquioxanes (POSS) is reported. The nanoroughness is generated on the 3D network due to microphase separation of the polymer chain segments in a nonsolvent during the rinsing step in holographic lithography process. The degree of roughness can be tuned by the crosslinking density of the polymer network, which is dependent on the loading of photoacid generators, the exposure dosage, and the choice of developer and rinsing solvent. Because the nanoroughness size is small, it does not affect the photonic band gap position of the photonic crystal in the infrared region. The combination of periodic microstructure and nanoroughness, however, offers new opportunities to realize superhydrophobicity and enhanced dye adsorption in addition to the photon management in the 3D photonic crystal.

Simultaneous enhanced photon capture and carrier generation in Si solar cells using Ge quantum dot photonic nanocrystals

We propose a novel solar cell structure with photonic nanocrystals coupled to quantum dots (QDs) for advanced management of photons and carriers. The photonic nanocrystals at the surface create an extra interaction between the photons and the QDs, which promotes light trapping. Photo-generated carriers can be efficiently transported by preparing vertically aligned QDs with electronic coupling. Implementation of the proposed structure was realized in crystalline Si solar cells with Ge QDs by development of a simple and practical formation method based on a wet chemical process without any lithography techniques. The wet process utilizes a periodically modulated etching rate induced by self-organized Ge QDs. The effectiveness of the proposed solar cell was demonstrated by the marked increase of the absolute conversion efficiency when compared with the control crystalline Si solar cells. It is found that light trapping by the photonic nanocrystals has a larger contribution to the efficiency improvement than the contributions from the carrier transport of the vertically aligned QDs.

Nanophotonic light trapping with patterned transparent conductive oxides

Transparent conductive oxides (TCOs) play a crucial role in solar cells by efficiently transmitting sunlight and extracting photo-generated charge. Here, we show how nanophotonics concepts can be used to transform TCO films into effective photon management layers for solar cells. This is accomplished by patterning the TCO layer present on virtually every thin-film solar cell into an array of subwavelength beams that support optical (Mie) resonances. These resonances can be exploited to concentrate randomly polarized sunlight or to effectively couple it to guided and diffracted modes. We first demonstrate these concepts with a model system consisting of a patterned TCO layer on a thin silicon (Si) film and outline a design methodology for high-performance, TCO-based light trapping coatings. We then show that the short circuit current density from a 300 nm thick amorphous silicon (a-Si) cell with an optimized TCO anti-reflection coating can be enhanced from 19.9 mA/cm2 to 21.1 mA/cm2, out of a possible 26.0 mA/cm2, by using an optimized nanobeam array. The key differences and advantages over plasmonic light trapping layers will be discussed.

(Invited) Photon Management with Si Nanocrystals

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Photon management with core-shell nanowire structures

Antireflective Si/oxide core-shell nanowire arrays (NWAs) were fabricated by galvanic etching and subsequent annealing process. The excellent light-harvesting characteristics of the core-shell NWAs, such as broadband working ranges, omnidirectionality, and polarization-insensitivity, ascribed to the smooth index transition from air to the substrates, have been demonstrated. By tuning core-shell volume ratios, we obtained enhanced light trapping regions implemented in either the planar Si underneath NWAs or the core regions of NWAs, greatly benefiting the geometry design of planar and radial p-n junction cell structures, respectively. This photon management scheme indicates the potential use in nanostructured photovoltaic applications.

Enhanced photon absorption and carrier generation in nanowire solar cells

Overall performance of a thin film solar cell is determined by the efficiency of converting photons to electrons through light absorption, carrier generation, and carrier collection. Recently, photon management has emerged as a powerful tool to further boost this conversion efficiency. Here we propose a novel nanograting solar cell design that achieves enhanced broadband light absorption and carrier generation in conjunction with the reduced use of active and non-earth-abundant materials. A test using this design for the short circuit current density in CuInxGa(1-x)Se2 (CIGS) thin film solar cells shows up to 250% enhancement when compared to the bare thin film cells. In addition, placing metal strips on top of the nanograting to act as the top electrode reduces the use of non-earth-abundant materials that is normally used as the transparent conducting materials. This novel solar cell design has the potential to become a new solar cell platform technology for various thin film solar cell systems.

Light Trapping Concepts for Photon Management in Solar Cells

AbstractPhoton management is a key component in the development of efficient solar cells. Especially light-trapping concepts have a high potential to realize enhanced efficiencies. Here, we give an overview over several light trapping concepts for photon management in solar cells. These include basic as well as advanced light-trapping concepts. The theoretical limits of light path enhancement of the different concepts are given and experimental work on these topics is presented. The potential of 3D photonic crystals is discussed in the context of the corresponding approaches as well.

Efficiency enhancement of screen-printed multicrystalline silicon solar cells by integrating gold nanoparticles via a dip coating process

Multicrystalline silicon solar cells play an increasingly important role in the world photovoltaic market. Boosting the comparatively low energy conversion efficiency of multicrystalline silicon solar cells is of great academic and industrial significance. In this paper, Au nanoparticles of an optimized size, synthesized by the iterative seeding method, were integrated onto industrially available surface-textured multicrystalline silicon solar cells via a dip coating method. Enhanced performance of the light absorption, the external quantum efficiency and the energy conversion efficiency were consistently demonstrated, resulting from the light scattering by the sized-tailored Au nanoparticles placed on the front surface of the solar cells, particularly in the spectral range from 800 to 1200 nm, an enhancement of the external quantum efficiency by more than 11% near λ = 1150 nm and the short-circuit current by 0.93% were both observed. As a result, an increase in the energy conversion efficiency up to 1.97% under the standard testing conditions (25°C, global air mass 1.5 spectrum, 1000 Wm−2) was achieved. This study opens new perspectives for plasmonic nanoparticle applications for photon management in multicrystalline silicon solar cells.

Photon management in La2BaZnO5:Tm3+, Yb3+ and La2BaZnO5:Pr3+, Yb3+ by two step cross-relaxation and energy transfer

A luminescence study from UV to NIR of La2BaZnO5:Tm3+, Yb3+ and La2BaZnO5:Pr3+, Yb3+ is reported. Emission spectra of La2BaZnO5:Tm3+, Yb3+ reveal an unexpected high intensity of the Tm3+3F4→3H6 emission at 5000cm−1 upon excitation in the Tm3+1G4 level. This unusual efficiency is explained by two successive cross-relaxation steps between Tm3+ ions, resulting in potential emission of three NIR photons for each absorbed visible photon. Emission and decay spectra of La2BaZnO5:Pr3+, Yb3+ give evidence for a Pr3+ (3P0→1G4) to Yb3+ (2F7/2→2F5/2) energy transfer. The Pr3+ (1G4→3H4) to Yb3+ (2F7/2→2F5/2) energy transfer could not be derived from our data.

Diffractive Backside Structures via Nanoimprint Lithography

For decreasing thicknesses of wafer based silicon solar cells, photon management structures to maintain high quantum efficiencies will gain importance. Diffractive gratings on the wafer back side can be designed to achieve very high path length enhancements, especially for weakly absorbed infrared radiation. This technologically demanding concept has to be realised using processes with upscaling potential. Therefore, we present a fabrication process for producing photonic structures in silicon based on interference lithography and nanoimprint lithography (NIL).We realised linear as well as crossed gratings of different depths, which were etched into the wafer back side. Polarisation dependent reflection measurements were made to get information about potential absorption enhancement as well as the occurrence of parasitic absorption in the metal reflector. This is conducted for a PECVD silicon oxide buffer layer between grating and reflector as well as a spin coated silicon oxide layer. Besides these optical characterisations, we further investigated the electrical properties of the back surface, where we applied a concept in which electrical and optical properties are decoupled. This is realised by a layer stack on the wafer back side, consisting of a thin Al2O3 passivation and a doped amorphous silicon layer.

Approaching the Lambertian Limit in Randomly Textured Thin-Film Solar Cells

ABSTRACTThe Lambertian limit represents a benchmark for the enhancement of the effective path length in solar cells, which is important as soon as the absorption length exceeds the absorber thickness. In previous publications it has been shown that either extremely thick or extremely thin solar cells can be driven close to this limit by exploiting up to date photon management. In this contribution we show that the Lambertian limit can also be achieved with thin-film solar cells based on amorphous silicon for practically relevant absorber thicknesses. Departing from superstrates, which are currently incorporated into state-of-the-art thin-film solar cells, we show that their topology has simply to be downscaled to typical feature sizes of about 100 nm in order to achieve this goal. By systematically studying the impact of the modulation height and the lateral feature sizes of the incorporated textures and of the absorber thickness we are able to deduce intuitive guidelines how to approach the Lambertian limit in randomly textured thin-film solar cells.

Nanorod photon management in nitride-based devices

Nanorod photon management in nitride-based devices

3D photonic crystals for photon management in solar cells

3D photonic crystals for photon management in solar cells