Photon Management Research Articles

Full-spectrum photon management of solar cell structures for photovoltaic–thermoelectric hybrid systems

In this paper, a novel ultra-broadband photon management structure is proposed for crystalline silicon thin-film solar cells used in the photovoltaic–thermoelectric hybrid system. Nanostructures are employed on both front and back side. Optical behavior of the structure in ultra-broadband (300–2500nm) are investigated through the Finite Difference Time Domain method. By combing moth-eye and inverted-parabolic surface, a new composite surface structure is proposed for anti-reflection in the ultra-broadband wavelengths. Front metallic nanoparticles, plasmonic back reflector and metallic gratings are studied for light-trapping and the effect of plasmonic back reflector is validated by the experimental data of the external quantum efficiency. The effects of incident angle are discussed for metallic gratings. Numerical computation shows that the incorporation of anti-reflection and light-trapping can obtain high absorption in the solar cell and ensure the rest incident light transmits to the thermoelectric generator efficiently. This work shows potential full-spectrum utilization of solar energy for various photovoltaic devices related with hybrid photovoltaic–thermoelectric systems.

High‐Performance Ta2O5/Al‐Doped Ag Electrode for Resonant Light Harvesting in Efficient Organic Solar Cells

A efficient indium tin oxide (ITO)‐free transparent electrode based on an improved Ag film is designed by introducing small amount of Al during co‐deposition, producing ultrathin and smooth Ag film with low loss. A transparent electrode as thin as 4 nm is achieved by depositing the film on top of Ta2O5 layer, and organic solar cells based on such ultrathin electrode are built, producing power conversion efficiency over 7%. The device efficiency can be optimized by simply tuning Ta2O5 layer thickness external to the organic photovoltaic (OPV) structure to create an optical cavity resonance inside the photoactive layer. Therefore Ta2O5/Al‐doped Ag films function as a high‐performance electrode with high transparency, low resistance, improved photon management capability and mechanical flexibility.

Exploring the effective photon management by InP nanoparticles: Broadband light absorption enhancement of InP/In0.53Ga0.47As/InP thin-film photodetectors

High-index dielectric and semiconductor nanoparticles with the characteristics of low absorption loss and strong scattering have attracted more and more attention for improving performance of thin-film photovoltaic devices. In this paper, we focus our attention on InP nanoparticles and study the influence of the substrate and the geometrical configurations on their scattering properties. We demonstrate that, compared with the InP sphere, the InP cylinder has higher coupling efficiency due to the stronger interactions between the optical mode in the nanoparticle and its induced mirror image in the substrate. Moreover, we propose novel thin-film InGaAs photodetectors integrated with the periodically arranged InP nanoparticles on the substrate. Broadband light absorption enhancement is achieved over the wavelength range between 1.0 μm and 1.7 μm. The highest average absorption enhancement of 59.7% is realized for the photodetector with the optimized cylinder InP nanoparticles. These outstanding characteristics attribute to the preferentially forward scattering of single InP nanoparticle along with the effective coupling of incident light into the guided modes through the collective diffraction effect of InP nanoparticles array.

Thermal Emission Control via Bandgap Engineering in Aperiodically Designed Nanophotonic Devices.

Aperiodic photonic crystals can open up novel routes for more efficient photon management due to increased degrees of freedom in their design along with the unique properties brought about by the long-range aperiodic order as compared to their periodic counterparts. In this work we first describe the fundamental notions underlying the idea of thermal emission/absorption control on the basis of the systematic use of aperiodic multilayer designs in photonic quasicrystals. Then, we illustrate the potential applications of this approach in order to enhance the performance of daytime radiative coolers and solar thermoelectric energy generators.

(Invited) Advanced Photon Management in Printed High-Efficiency Multijunction Solar Cells

High efficiency photovoltaic (PV) cells represent a clean and efficient method to utilize the abundant solar energy in both space and terrestrial applications. Advances in solar cell technology and associated light management systems are the primary determinants of improvements in efficiency. Single junction (SJ) solar cells are already near theoretical efficiency limits defined by thermalization losses and sub-bandgap transparency. Devices that incorporate multiple junctions (i.e. sub-cells) in monolithic stacks, known as multijunction (MJ) cells, provide one of the most attractive routes to ultrahigh efficiency. Over the last decade, increases in the efficiency of MJ cells correspond to nearly 1% (absolute) per year, reaching values that are presently ~44%. Further improvements, however, will require solutions to daunting challenges in achieving lattice-matched or metamorphic epitaxial growth in complex stacks and in maintaining current matched outputs from each of the sub-cells. Here we present materials and strategies to make printed multijunction solar cell structures, to bypass the challenges in conventional multijunction cell design. In the first part, I will talk about printing-based assembly of microscale, quadruple junction, four-terminal solar cells with measured efficiencies of 43.9% at concentrations exceeding 1000 suns, and modules with efficiencies of 36.5%. Secondly, I will introduce an alternative device architecture, which uses low refractive index interface materials for enhanced photon recycling in printed multijunction solar cell stacks. These results establish routes to ultrahigh efficiency cells and modules, with potential to approach thermodynamic efficiency limits and realize large-scale photovoltaic energy production.

An All Solution-Processed Lead Halide Perovskite-BiVO4 Tandem Architecture Delivering STH of 2.5%

We report an all solution-processed tandem water splitting device composed of a BiVO4 photoanode and a single-junction lead halide perovskite solar cell (CH3NH3PbI3). In this configuration, the surface of BiVO4 photoanode is in contact with water for water oxidation reaction, and the lead halide perovskite solar cell is situated behind the BiVO4 photoanode to supply additional photovoltage to drive water reduction reaction through a Pt gauze. This architecture allows efficient solar photon management, with BiVO4 photoanode selectively harvesting high energy photons (< 500 nm) and lead halide perovskite solar cell selectively harvesting low energy photons (500 to 800 nm) in a single-pass excitation. By modifying the surface of BiVO4 photoanode with an earth-abundant cobalt phosphate thin film, we showed that this design is capable of delivering solar to hydrogen conversion efficiency (STH) of 2.5 % under AM 1.5G illumination at neutral pH. Our results demonstrate the potential of using low-cost, single-junction lead halide perovskite solar cells to assist n-type metal oxides to carry out bias-free photocatalytic water splitting. The unique photoanode-photovoltaic design offers opportunities to explore other semiconducting materials with extended visible light absorption ability to further improve the STH.

(Nanocarbons Division Richard E. Smalley Research Award) Nanocarbons for Optoelectronic Applications

Carbon is the key to many technological applications that have become indispensable in our daily life. Altering the periodic binding motifs in networks of sp3-, sp2-, and sp-hybridized C-atoms is the conceptual starting point for a wide palette of carbon allotropes. To this end, the past two decades have served as a test-bed for measuring the physico-chemical properties of low-dimensional carbon with the advent of fullerenes (0D), followed in chronological order by carbon nanotubes (1D), carbon nanohorns, and, most recently, by graphene (2D). These species are now poised for use in wide-ranging applications. Expanding global needs for energy have led to a significant effort to develop alternatives to fossil fuels. While alternative sources for energy are already in use, they comprise a small percentage of the energy demands needed to carry us through the 21st century. No single source will solve the global needs, but the development of photovoltaics has vast potential as a point-of-use power source. Recent work has shown that hybrid photoelectrochemical efforts with a percolation network of photon absorbers coupled to anelectron/hole transporter in combination with advanced photon management are the ideal design for realizing breakthroughs in high photon conversion efficiencies suitable for the catalysis of water. I will report on our efforts regarding a unifying strategy to use the unprecedented charge transfer chemistry of 0D fullerenes, the ballistic conductance of 1D carbon nanotubes, the semiconducting features of carbon nanohorns, and the high mobility of charge carriers in 2D graphene, together in a groundbreaking approach to solving a far-reaching challenge, that is, the efficient use of the abundant light energy around us. Leading

Monolithically interconnected lamellar Cu(In,Ga)Se2 micro solar cells under full white light concentration

AbstractThin film solar cells already benefit from significant material and energy savings. By using photon management, the conversion efficiency and the power density can be enhanced further, including a reduction of material costs. In this work, micrometer‐sized Cu(In,Ga)Se2 (CIGS) thin film solar cells were investigated under concentrated white light illumination (1–50×). The cell design is based on industrially standardized, lamellar shaped solar cells with monolithic interconnects (P‐scribe). In order to characterize the shunt and serial resistance profiles and their impact on the device performance the cell width was reduced stepwise from 1900 to 200 µm and the P1‐scribe thickness was varied between 45 and 320 µm. The results are compared to macroscopic solar cells in standard geometry and dot‐shaped microcells with ring contacts. Under concentrated white light, the maximal conversion efficiency could be increased by more than 3.8% absolute for the lamellar microcells and more than 4.8% absolute in case of dot‐shaped microcells compared to their initial values at 1 sun illumination. The power density could be raised by a factor of 51 and 70, respectively. But apparently, the optimum concentration level and the improvement in performance strongly depend on the chosen cell geometry, the used contact method and the electrical material properties. It turns out, that the widely used industrial thin film solar cell design pattern cannot simply be adapted to prepare micro‐concentrator CIGS solar modules, without significant optimization. Based on the experimental and simulated results, modifications for the cell design are proposed. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Ligand Binding to Distinct Sites on Nanocrystals Affecting Energy and Charge Transfer.

Hybrid optoelectronic devices are attractive because they offer the promise of low-cost, roll-to-roll fabrication. Despite this, energy transfer between organic and inorganic interfaces is not well understood. Device engineering on this class of solution-processed materials generally focuses on replacing the long insulating ligands with short ones. Here, we show that energy and charge transfer between an inorganic nanocrystal (NC) donor and organic molecular acceptor is acutely sensitive to the chemical moiety linking the two species. Our results reveal that the CdS NCs have distinct binding sites for different chemical species because only resonance energy transfer (RET) is observed for the carboxylic-acid-functionalized ligand, while both RET and charge transfer are observed for the amine-functionalized ligand. We observe that the equilibrium constant for this static quenching term increases with decreasing particle size. This finding offers a new approach in the design of hybrid thin films for devices and NC probes based on RET used for imaging, sensing, signal transduction, and photon management.

(Invited) Advanced Photon Management in Printed High-Efficiency Multijunction Solar Cells

Multijunction (MJ) solar cells have the potential to operate across the entire solar spectrum, for ultrahigh efficiencies in solar energy harvesting. We present materials and strategies of using printed MJ solar cell structures to bypass the challenges in conventional multijunction cell design. Firstly, microscale, quadruple junction (InGaP/GaAs/InGaAsNSb//Ge), four-terminal solar cells are fabricated by printing-based assembly with a refractive index matched interface material As2Se3. We obtain solar cells with measured efficiencies of 43.9% at concentrations exceeding 1000 suns, and modules with efficiencies of 36.5%. Secondly, we present a printed MJ cell architecture with low refractive indices as interfaces, providing further enhanced capabilities in photon recycling and photon extraction. Experiments demonstrate that thin-film GaAs devices printed on low-index substrates exhibit improved photon recycling, leading to increased open-circuit voltages, consistent with theoretical predictions. These concepts represent important aspects of design for solar cells that approach thermodynamic efficiency limits for full spectrum operation.

GaN-Based Multiple-Quantum-Well Light-Emitting Diodes Employing Nanotechnology for Photon Management

Nanostructures have been proved to be an efficient way of modifying/improving the performance of GaN-based light-emitting diodes (LEDs). The achievements in photon management include strain relaxation, light extraction enhancement, radiation pattern control, and white-light devices. In this paper, we discuss the impact and the underlying physics of applying nanotechnology on LEDs. A variety of nanostructures are introduced, as well as the fabrication techniques.

Optimized design of silicon thin film solar cells with silicon nanogratings

To increase the performances of thin film solar cells, we show how nanophotonics concepts can be used to silicon nanogratings into effective photon management layers for solar cells. This is accomplished by patterning the silicon nanogratings present on thin film solar cell that support optical resonances. These resonances can be exploited to concentrate randomly polarized sunlight or to effectively couple it to guided and diffracted modes. Our optimization reveals that light absorption of thin film solar cell with silicon nanogratings is enhanced in randomly polarized sunlight. The simulated short-circuit current density (Jsc) over the solar spectrum shows an up to 127% enhancement when comparing with bare thin film cell. The average Jsc enhancement under an unpolarized illumination is almost immune to the incident angle ranging from −40° to 40°. It is interesting that Jsc enhancement of normal incident is not the maximum. The advantages of film solar cell with silicon nanogratings over plasmonic solar cell will be discussed.

Wavelength-selective plasmonics for enhanced cultivation of microalgae

Optimal photon management is a key challenge for photobioreactor design, since light gradients and varying spectral sensitivities between organisms result in uneven illumination and unused photons. This paper demonstrates wavelength specific scattering from plasmonic nano-patterned surfaces as a means of addressing the challenge of photon management in photobioreactors. Modular photobioreactors were constructed with different reflective substrates including arrays of plasmonic nanodisks, broadband reflectors, and untreated glass. It was found that the growth rate of cyanobacterium S. elongatus in photobioreactors equipped with a plasmonic substrate (R623 nm ∼ 35%) was enhanced by 6.5% compared to photobioreactors equipped with untreated glass. Furthermore, plasmonic reflectors showed a normalized power efficiency improvement of 52% over broadband reflectors. Wavelength-specific reflection from plasmonic reflectors increases the flux of useful light to cultures without sacrificing the full spectrum.

All Solution-Processed Lead Halide Perovskite-BiVO4 Tandem Assembly for Photolytic Solar Fuels Production

The quest for economic, large-scale hydrogen production has motivated the search for new materials and device designs capable of splitting water using only energy from the sun. Here we introduce an all solution-processed tandem water splitting assembly composed of a BiVO4 photoanode and a single-junction CH3NH3PbI3 hybrid perovskite solar cell. This unique configuration allows efficient solar photon management, with the metal oxide photoanode selectively harvesting high energy visible photons, and the underlying perovskite solar cell capturing lower energy visible-near IR wavelengths in a single-pass excitation. Operating without external bias under standard AM 1.5G illumination, the photoanode-photovoltaic architecture, in conjunction with an earth-abundant cobalt phosphate catalyst, exhibits a solar-to-hydrogen conversion efficiency of 2.5% at neutral pH. The design of low-cost tandem water splitting assemblies employing single-junction hybrid perovskite materials establishes a potentially promising new frontier for solar water splitting research.

Perfect solar absorber based on nanocone structured surface for high-efficiency solar thermoelectric generators

Nanostructured surface is a promising photon management strategy to tune spectrum in design of the selective solar absorber. In this paper, we propose a nanocone structured surface as a perfect solar absorber in application of the solar thermoelectric generators (STEGs). The trade-off between the solar absorption and the mid-infrared emission is obtained to maximize the STEG efficiency. The effects of the geometric parameters, thermal concentration, incident angle and polarized state as well as the lattice arrangement are systematically investigated. The results show that the STEGs equipped with our proposed selective solar absorber can achieve a peak efficiency of 6.53% under AM1.5G condition (no optical concentration). Furthermore, the selective solar absorber exhibits insensitive behavior to the incident angle and polarization angle as well. This means that the proposed selective solar absorber can utilize solar energy as much as possible and be generally suitable in equipping the STEGs without optical concentration.

Repurposing Blu-ray movie discs as quasi-random nanoimprinting templates for photon management.

Quasi-random nanostructures have attracted significant interests for photon management purposes. To optimize such patterns, typically very expensive fabrication processes are needed to create the pre-designed, subwavelength nanostructures. While quasi-random photonic nanostructures are abundant in nature (for example, in structural coloration), interestingly, they also exist in Blu-ray movie discs, an already mass-produced consumer product. Here we uncover that Blu-ray disc patterns are surprisingly well suited for light-trapping applications. While the algorithms in the Blu-ray industrial standard were developed with the intention of optimizing data compression and error tolerance, they have also created quasi-random arrangement of islands and pits on the final media discs that are nearly optimized for photon management over the solar spectrum, regardless of the information stored on the discs. As a proof-of-concept, imprinting polymer solar cells with the Blu-ray patterns indeed increases their efficiencies. Simulation suggests that Blu-ray patterns could be broadly applied for solar cells made of other materials.

Light trapping by direction-dependent light transmission in front-surface photonic nanostructures

Front-surface photonic nanostructures contribute both reduced reflection loss and light trapping to increase optical absorption in solar cells. We investigated the effect of sub-wavelength photonic nanostructures on light trapping in thin-film crystalline silicon solar cells. We clarified that, even though the angle distribution of scattered light is small compared with that in the case of Lambertian scattering, light trapping enhances with increasing depth of the surface photonic nanostructure, which originates from the direction-dependent light transmission in the surface photonic nanostructure. Our findings indicate that this direction-dependent light transmission contributes to the advanced photon management in thin-film solar cells.

Dielectric Coating‐Induced Absorption Enhancement in Si Nanowire Junctions

The efficient photon management in nanowires coupled with tunable absorption can provide a significant advancement in the current solar cell technology. Adding a shell on the nanowire (NW) offers an extra degree of freedom for tailoring the NW absorption. Calculations are performed to study the shell‐induced changes in the core absorption of an ideal Si nanocylinder after enclosing it with conformal dielectrics of different thicknesses and permittivities. The calculation shows that coating the Si nanowires with nonabsorbing or weakly absorbing dielectric shells can significantly improve the mean spectral absorption efficiency of the Si core for optimized thickness and extinction. Experimental data are brought from a neighboring system consisting of individual NW junctions sandwiched between an oxidized planar Si substrate and a top‐deposited oxide layer. The measured absorption spectra show a good agreement with the theoretical spectra, with a mean spectral absorption enhancement after oxide coating as high as 5.7 in axial junctions, exceeding its theoretical counterpart that is limited to ≈1.5. Coating the nanowires with an optimized oxide shell provides a new approach to significantly enhance the light harvesting in nanowires and will lead to more efficient photovoltaics.

Enhanced light absorption in nanotextured amorphous thin-film silicon caused by femtosecond-laser materials processing

Efficient thin-film solar cells balance the reduced absorption occurring in thin absorber layers by means of various photon management strategies that often involve randomly nanotextured interfaces. We report on broadband absorption enhancement in nanotextured amorphous silicon processed by femtosecond laser materials processing. As identified by micro-Raman spectroscopy and surface profilometry, the absorption of a single femtosecond amplifier laser pulse (30fs, 795nm, 75mJcm−2) creates a thin nanotextured micro-crystalline surface layer. Optical microscopy in transmission and reflection geometry reveals a broadband absorption enhancement in the visual spectrum range for the nanotextured area. Scattered light spectroscopy in combination with spectral interferometry indicates that light trapping for about 100fs is achieved in the femtosecond-laser processed amorphous silicon area and thus is responsible for the observed enhanced absorption and locally enhanced Raman yields. Thus fs-laser materials processing offers an interesting pathway towards advanced photon management in amorphous silicon based thin-film solar cells.

Generation of hot carriers for photon management in future photovoltaics

The most important limitation for the efficiency of photovoltaic energy conversion is related to the mismatch between the broadband character of the solar radiation and the spectral sensitivity of solar cells. Large losses appear at both sides of the solar spectrum. Low-energy photons are not absorbed and escape conversion, whereas high-energy photons can only be utilized partially. Here, we discuss how the loss related to low-energy photons can be reduced by spectral conversion, making use of active layers of Si nanocrystals in an SiO2 matrix. As a possible solution, we consider generation of hot carriers upon sequential intraband absorption of two or more low-energy photons and present experimental data in support of this. Configurations are proposed in which efficient spectral conversion could be achieved, and their feasibilities are discussed. We also address the fast emission from Er3+ ions, purposefully introduced into the active layer, which can be used to trace the successful generation of hot carriers, and in that way serve to evaluate specific excitation schemes.