Photon Management Research Articles

Exact comprehensive equations for the photon management properties of silicon nanowire.

Unique photon management (PM) properties of silicon nanowire (SiNW) make it an attractive building block for a host of nanowire photonic devices including photodetectors, chemical and gas sensors, waveguides, optical switches, solar cells, and lasers. However, the lack of efficient equations for the quantitative estimation of the SiNW’s PM properties limits the rational design of such devices. Herein, we establish comprehensive equations to evaluate several important performance features for the PM properties of SiNW, based on theoretical simulations. Firstly, the relationships between the resonant wavelengths (RW), where SiNW can harvest light most effectively, and the size of SiNW are formulized. Then, equations for the light-harvesting efficiency at RW, which determines the single-frequency performance limit of SiNW-based photonic devices, are established. Finally, equations for the light-harvesting efficiency of SiNW in full-spectrum, which are of great significance in photovoltaics, are established. Furthermore, using these equations, we have derived four extra formulas to estimate the optimal size of SiNW in light-harvesting. These equations can reproduce majority of the reported experimental and theoretical results with only ~5% error deviations. Our study fills up a gap in quantitatively predicting the SiNW’s PM properties, which will contribute significantly to its practical applications.

Stretchable Hexagonal Diffraction Gratings as Optical Diffusers for In Situ Tunable Broadband Photon Management

A stretchable transmissive hexagonal diffraction grating, which has the potential to act as an optical diffuser, is demonstrated. Leveraging the simplicity of the self‐assembly fabrication process, the photon manipulation capability of polystyrene nanosphere arrays, and elastomeric properties of polydimethylsiloxane, the proposed device is capable of reproducible in situ tuning of both diffraction efficiency and spectral range. While being able to achieve maximum diffraction efficiencies of about 80%, the device displays highly efficient and broadband light diffusion fairly independent of incident light polarization and angle of incidence. Due to its efficient and tunable diffraction capabilities, one potential application of the reported device can be broadband photon management in solar cells and photodetectors by significant increase of the light path length inside the light‐absorbing thin films of these devices. As a proof of concept, the proposed optical diffuser is utilized for light absorption enhancement in colloidal quantum dot semiconductor thin films. The demonstrated devices enable integration of cheap and widely used materials with simple cost‐effective fabrication for photon management in optics, photonics, and optoelectronics.

(Invited) Photon Managements By Employing Nanostructures for Light Emission Diodes

It is of current interest to develop the photon management with nanostructures since the ability to suppress the reflection and light trapping over a broad range of wavelengths and incident angles plays an important role in the performance of light-emitting diodes (LEDs). Superior light-extraction characteristics of nanostructures for LEDs are discussed in this talk. These advantages are mainly attributed to the subwavelength dimensions of the nanowires, which makes the nanostructures behave like an effective homogeneous medium with continuous gradient of refraction index, significantly reducing the reflection through destructive interferences. The relation between the geometrical configurations of nanostructures and the light-harvesting characteristics is discussed. We also demonstrated their applications in solar cells and photodetectors. This research paves the way to optimize the nanostructured optoelectronic devices with efficient light management by controlling structure profile of nanostructures.

Concurrent improvement in optical and electrical characteristics by using inverted pyramidal array structures toward efficient Si heterojunction solar cells

The Si heterojunction (SHJ) solar cell is presently the most popular design in the crystalline Si (c-Si) photovoltaics due to the high open-circuit voltages (VOC). Photon management by surface structuring techniques to control the light entering the devices is critical for boosting cell efficiency although it usually comes with the VOC loss caused by severe surface recombination. For the first time, the periodic inverted pyramid (IP) structure fabricated by photolithography and anisotropic etching processes was employed for SHJ solar cells, demonstrating concurrent improvement in optical and electrical characteristics (i.e., short-circuit current density (JSC) and VOC). Periodic IP structures show superior light-harvesting properties as most of the incident rays bounce three times on the walls of the IPs but only twice between conventional random upright pyramids (UPs). The high minority carrier lifetime of the IP structures after a-Si:H passivation results in an enhanced VOC by 28mV, showing improved carrier collection efficiency due to the superior passivation of the IP structure over the random UP structures. The superior antireflective (AR) ability and passivation results demonstrate that the IP structure has the potential to replace conventional UP structures to further boost the efficiency in solar cell applications.

High performance thin film solar cells on plastic substrates with nanostructure-enhanced flexibility

Plastic substrates possess conspicuous advantages for flexible thin film solar cell applications due to their superior flexibility and light weight characteristics. However, there are several challenges of using plastic substrates for high performance thin film solar cells since they usually have low melting/softening temperature and high coefficient of thermal expansion (CTE). In this work, we demonstrated a low cost process to achieve regular nanocone arrays on polyimide (PI) substrates which have unique photon management property and excellent mechanical flexibility. To leverage these benefits of the nanocone substrates, flexible amorphous Si solar cells were fabricated on the structures. Intriguingly, it was discovered that properly designed nanocones can significantly improve solar cell device performance via light management. And the fabrication yield of properly designed nanocone solar cells is much higher than that of planar devices. In addition, the nanocone plastic solar cells possess much improved bendability and robustness verified by both experiment and mechanical modeling, showing unique stress release mechanism originated from three-dimensional nanostructure design. This property is of practical significance for flexible electronics not limited to solar cells.

Coherent Control of Photocurrent in a Strongly Scattering Photoelectrochemical System

A fundamental issue that limits the efficiency of many photoelectrochemical systems is that the photon absorption length is typically much longer than the electron diffusion length. Various photon management schemes have been developed to enhance light absorption; one simple approach is to use randomly scattering media to enable broadband and wide-angle enhancement. However, such systems are often opaque, making it difficult to probe photoinduced processes. Here we use wave interference effects to modify the spatial distribution of light inside a highly scattering dye-sensitized solar cell to control photon absorption in a space-dependent manner. By shaping the incident wavefront of a laser beam, we enhance or suppress photocurrent by increasing or decreasing light concentration on the front side of the mesoporous photoanode where the collection efficiency of photoelectrons is maximal. Enhanced light absorption is achieved by reducing reflection through the open boundary of the photoanode via destructive interference, leading to a factor of 2 increase in photocurrent. This approach opens the door to probing and manipulating photoelectrochemical processes in specific regions inside nominally opaque media.

Photelectrodes: High‐Efficiency InP‐Based Photocathode for Hydrogen Production by Interface Energetics Design and Photon Management (Adv. Funct. Mater. 5/2016)

Photelectrodes: High‐Efficiency InP‐Based Photocathode for Hydrogen Production by Interface Energetics Design and Photon Management (Adv. Funct. Mater. 5/2016)

Binary coded patterns for photon control using necklace problem concept.

Pseudo-disordered structures enable additional design freedom for photon management. However, the optimization and interpretation is challenging when the large number of degrees of freedom encounters computationally intensive electromagnetic simulation method. Here we propose a novel one-dimensional multi-periodic pattern generation method to help us squeeze the disorder design space before performing rigorous calculation, by making use of the periodic attribute of the pattern. Consequently, thanks to the pre-filtered design space, it typically relieves us from computational burden and enables us to 'globally' optimize and study pseudo-disordered patterns. As an example, we show how this approach can be used to comprehensively optimize and systematically analyze generated disorder for broadband light trapping in thin film.

Advanced nanostructured materials and their application for improvement of sun-light harvesting and efficiency of solar cells

This review describes the application of different nanostructured materials in solar cells technology for improvement of sun-light harvesting and their efficiency. Several approaches have recently been proposed to increase the efficiency of solar cells above the theoretical limit which are based on a “photon management” concept that employs such phenomena as: (i) down-conversion, and (ii) surface plasmon resonance effect (iii) decreasing of the loss due to the reflection of the radiation, (iv) increasing of the reflection from the back contact, v) increasing of the effective solar cells surface, etc. The results demonstrate the possibility for to increasing of light harvesting, short circuit current and efficiency by application of nanomaterials in thin film and hetero-junction (HJ) solar cells. The first promising results allow an expectation for application of advanced nanomaterials in the 3d generation solar cells.

Photon management properties of rare-earth (Nd,Yb,Sm)-doped CeO2 films prepared by pulsed laser deposition.

CeO2 is a promising material for applications in optoelectronics and photovoltaics due to its large band gap and values of the refractive index and lattice parameters, which are suitable for silicon-based devices. In this study, we show that trivalent Sm, Nd and Yb ions can be successfully inserted and optically activated in CeO2 films grown at a relatively low deposition temperature (400 °C), which is compatible with inorganic photovoltaics. CeO2 thin films can therefore be efficiently functionalized with photon-management properties by doping with trivalent rare earth (RE) ions. Structural and optical analyses provide details of the electronic level structure of the films and of their energy transfer mechanisms. In particular, we give evidence of the existence of an absorption band centered at 350 nm from which energy transfer to rare earth ions occurs. The transfer mechanisms can be completely explained only by considering the spontaneous migration of Ce(3+) ions in CeO2 at a short distance from the RE(3+) ions. The strong absorption cross section of the f-d transitions in Ce(3+) ions efficiently intercepts the UV photons of the solar spectrum and therefore strongly increases the potential of these layers as downshifters and downconverters.

From light trapping to solar energy utilization: A novel photovoltaic–thermoelectric hybrid system to fully utilize solar spectrum

In this paper, the comprehensive photon and thermal management approaches are proposed to increase the full-spectrum solar energy utilization in PV–TE (photovoltaic–thermoelectric) hybrid systems. The bio-inspired moth-eye nanostructured surface is adopted to suppress the reflection for full solar spectrum photons while the enhanced transmission film is used to improve the transmission for photons with energy below the band-gap of PV cells, making the reasonable utilization of solar spectrum energy. The PV–TE hybrid system is studied for both terrestrial and space applications, corresponding to different thermal management approaches. The effects of geometric parameters, incident angle, thermal concentration ratio, and optical concentration ratio on the system performance are discussed. The results show that the PV–TE hybrid system exhibits better performance under both AM1.5 and AM0 illumination without any optical concentration compared to pure PV cells under the same conditions. For an optical concentrated solar PV–TE system, although the efficiency of the system may decrease with the increment of optical concentration ratio, the total power output increases for both terrestrial and space applications. The results indicate that a lower concentration ratio is more suitable for PV–TE hybrid systems in both terrestrial and space applications unless more powerful temperature management tricks are taken.

Efficiency Enhancement of Silicon Heterojunction Solar Cells via Photon Management Using Graphene Quantum Dot as Downconverters.

By employing graphene quantum dots (GQDs), we have achieved a high efficiency of 16.55% in n-type Si heterojunction solar cells. The efficiency enhancement is based on the photon downconversion phenomenon of GQDs to make more photons absorbed in the depletion region for effective carrier separation, leading to the enhanced photovoltaic effect. The short circuit current and the fill factor are increased from 35.31 to 37.47 mA/cm(2) and 70.29% to 72.51%, respectively. The work demonstrated here holds the promise for incorporating graphene-based materials in commercially available solar devices for developing ultrahigh efficiency photovoltaic cells in the future.

High‐Efficiency InP‐Based Photocathode for Hydrogen Production by Interface Energetics Design and Photon Management

The solar energy conversion efficiency of photoelectrochemical (PEC) devices is usually limited by poor interface energetics, limiting the onset potential, and light reflection losses. Here, a three‐pronged approach to obtain excellent performance of an InP‐based photoelectrode for water reduction is presented. First, a buried p–n+ junction is fabricated, which shifts the valence band edge favorably with respect to the hydrogen redox potential. Photoelectron spectroscopy substantiates that the shift of the surface photovoltage is mainly determined by the buried junction. Second, a periodic array of InP nanopillars is created at the surface of the photoelectrode to substantially reduce the optical reflection losses. This device displays an unprecedented photocathodic power‐saved efficiency of 15.8% for single junction water reduction. Third, a thin TiO2 protection layer significantly increases the stability of the InP‐based photoelectrode. Careful design of the interface energetics based on surface photovoltage spectroscopy allows obtaining a PEC cell with stable record performance in water reduction.

On the interplay of cell thickness and optimum period of silicon thin‐film solar cells: light trapping and plasmonic losses

AbstractLight trapping and photon management in honeycomb‐textured microcrystalline silicon solar cells are investigated experimentally and by modeling of the manufacturing process and the optical wave propagation. The solar cells on honeycomb‐textured substrates exhibit short circuit current densities exceeding 30 mA/cm2 and energy conversion efficiencies of up to 11.0%. By controlling the fabrication process, the period and height of the honeycomb‐textured substrates are varied. The influence of the honeycomb substrate morphology on the interfaces of the individual solar cell layers and the quantum efficiency is determined. The optical wave propagation is calculated using 3D finite difference time domain simulations. A very good agreement between the optical simulation and experimental results is obtained. Strategies are discussed on how to increase the short circuit current density beyond 30 mA/cm2. In particular, the influence of plasmonic losses of the textured silver (Ag) reflector on the short circuit current and quantum efficiency of the solar cell is discussed. Finally, solar cell structures with reduced plasmonic losses are proposed. Copyright © 2015 John Wiley & Sons, Ltd.

A Multi‐optical Collector of Sunlight Employing Luminescent Materials and Photonic Nanostructures

A multifunctional, semitransparent photovoltaic device is proposed for harvesting sunlight over a tunable spectral range extending from the UV to the IR. Self‐assembled monolayers of nanospheres applied to luminescent concentrators provide a photon management architecture, which relies on luminescence and diffraction of light. The light diffraction is tuned by changing the sphere diameter to match the transparency region of the fluorophores. The importance of each optical mechanism is inferred from a systematic experimental investigation of the external quantum efficiency of fabricated devices, and from the calculation of the resulting photocurrent under the AM 1.5 solar spectrum. Compared to the conventional luminescent concentrator, relative photocurrent improvements are shown between 50% and 500% depending on the spectral properties of the device components. It has been demonstrated how to tailor the photovoltaic performances, the color and the degree of transparency of the device to provide a versatile photovoltaic unit for sustainable building‐integrated applications.

Hybrid Metal-Semiconductor Nanostructure for Ultrahigh Optical Absorption and Low Electrical Resistance at Optoelectronic Interfaces.

Engineered optoelectronic surfaces must control both the flow of light and the flow of electrons at an interface; however, nanostructures for photon and electron management have typically been studied and optimized separately. In this work, we unify these concepts in a new hybrid metal-semiconductor surface that offers both strong light absorption and high electrical conductivity. We use metal-assisted chemical etching to nanostructure the surface of a silicon wafer, creating an array of silicon nanopillars protruding through holes in a gold film. When coated with a silicon nitride anti-reflection layer, we observe broad-band absorption of up to 97% in this structure, which is remarkable considering that metal covers 60% of the top surface. We use optical simulations to show that Mie-like resonances in the nanopillars funnel light around the metal layer and into the substrate, rendering the metal nearly transparent to the incoming light. Our results show that, across a wide parameter space, hybrid metal-semiconductor surfaces with absorption above 90% and sheet resistance below 20 Ω/□ are realizable, suggesting a new paradigm wherein transparent electrodes and photon management textures are designed and fabricated together to create high-performance optoelectronic interfaces.

High Performance Ultrathin GaAs Solar Cells Enabled with Heterogeneously Integrated Dielectric Periodic Nanostructures.

Due to their favorable materials properties including direct bandgap and high electron mobilities, epitaxially grown III-V compound semiconductors such as gallium arsenide (GaAs) provide unmatched performance over silicon in solar energy harvesting. Nonetheless, their large-scale deployment in terrestrial photovoltaics remains challenging mainly due to the high cost of growing device quality epitaxial materials. In this regard, reducing the thickness of constituent active materials under appropriate light management schemes is a conceptually viable option to lower the cost of GaAs solar cells. Here, we present a type of high efficiency, ultrathin GaAs solar cell that incorporates bifacial photon management enabled by techniques of transfer printing to maximize the absorption and photovoltaic performance without compromising the optimized electronic configuration of planar devices. Nanoimprint lithography and dry etching of titanium dioxide (TiO2) deposited directly on the window layer of GaAs solar cells formed hexagonal arrays of nanoscale posts that serve as lossless photonic nanostructures for antireflection, diffraction, and light trapping in conjunction with a co-integrated rear-surface reflector. Systematic studies on optical and electrical properties and photovoltaic performance in experiments, as well as numerical modeling, quantitatively describe the optimal design rules for ultrathin, nanostructured GaAs solar cells and their integrated modules.

Biomimetic omnidirectional broadband structured surface for photon management in photovoltaic–thermoelectric hybrid systems

An advanced photon management approach is developed for a photovoltaic–thermoelectric hybrid system to increase the solar energy utilization efficiency. A biomimetic parabolic-shaped structure on the Si film and a thin SiO2 film at the bottom of the Si film as back-antireflection coating improves the absorptance of photons with wavelengths of 0.3–1.1μm, which can generate electron–hole pairs. Photons with wavelengths of 1.1–2.5μm transmit through the structured surface and are absorbed by the thermoelectric modules, which improves the conversion efficiency of the entire hybrid system. The structured surface with graded refractive index can effectively harvest photons with wavelengths of 0.3–2.5μm. The effects of the structural parameters and the incident angle on the spectral features of the structured surface are analyzed. The results indicate that the structured surface provides high omnidirectional absorptance for wavelengths of 0.3–1.1μm and high transmittance for wavelengths of 1.1–2.5μm as well as favorable polarization-insensitive features over a good incident angle range as a good candidate for the photovoltaic–thermoelectric hybrid systems.

Efficient photon management with Ag nanoparticles coated TiO2 nanowire clusters for photodetector application

TiO2 nanowire clusters (NWCs) covered with Ag nanoparticles (NPs) were synthesized on Si substrates using GLAD technique. Ag NPs/TiO2 NWCs showed enhanced optical absorption all over the spectrum compared to bare TiO2 NWCs. The leakage current of the TiO2 NWCs device (−10.8 μA) was reduced by ∼9 × 103 times for Ag NPs/TiO2 NWCs detector (−1.2 nA) at −1 V applied bias. Ag NPs/TiO2 NWCs detector exhibited maximum photoresponsivity ∼2.4 times (−2 V) as compared to the bare TiO2 NWCs detector. Using 10 K photocapacitance measurement, a sharp peak at 355 nm (∼3.5 eV) was detected due to the main band transition. The other sub band gap defects were observed within 410–490 nm along with a broad hump between 595–700 nm. The light dependent capacitance-time (I-T) characteristic of the Ag/TiO2 NWCs detector shows rise-time (T r ) of 9.6 s and decay time (T d ) of 10 s.

Structural, optical and electrical properties of Nd-doped SnO2 thin films fabricated by reactive magnetron sputtering for solar cell devices

The use of photon conversion layers is an interesting way to improve the overall efficiency of solar cells. Herein we report on Nd-doped SnO2 thin films with photon management property inserted further into CIGS based solar cells. The functionalized layers were deposited by reactive magnetron sputtering whose structural, optical and electrical properties were tuned by varying the deposition temperature. Careful analysis of the structure using XRD and XPS showed that the tetragonal rutile SnO2 phase can be obtained at a deposition temperature as low as 100°C. Transparency was found to be as high as 90% for all layers while the absorption edge is found to increase when increasing the deposition temperature up to 300°C. The photoluminescence measurements under 325nm UV laser excitation showed that 100°C is needed for the optical activation of the rare earth. Despite the small amounts of Nd (around 0.62at%), intense and narrow emission bands have been collected in the Near Infrared Region (NIR) which are characteristics of Nd3+ ions whose the ionic state was confirmed by the 3d XPS core levels. Thus, the emission spectra cover a good part of the spectrum useful to the solar cell. Photoluminescence excitation spectroscopy experiments were also carried out on Nd:SnO2 samples to get insights on the energy transfer. By exciting in the deep UV from 250 to 400nm intense Nd emission was collected giving an experimental evidence of the down-shifting process through a resonant energy transfer from the SnO2 host matrix to the Nd3+ ions. Hall Effect measurements showed that the n-type character and good conductivity of the Nd doped SnO2 films can be correlated to the highest optical activity of Nd in the matrix. An optimal condition is found for the Nd-doped SnO2 film grown at 300°C for which the highest PL and the best electrical data were measured. Finally, we show that the implementation of such optimized Nd–SnO2 films on CIGS based solar cells serving as a transparent conducting oxide and a down shifting converter results in the best power conversion efficiency.