Enhanced photovoltaics inspired by the fovea centralis

Effects of Solvent Additive and Micro-Patterned Substrate on the Properties of Thin Films Based on P3HT:PC70BM Blends Deposited by MAPLE

Lately, there is a growing interest in organic photovoltaic (OPV) cells due to the organic materials' properties and compatibility with various types of substrates. However, their efficiencies are low relative to the silicon ones; therefore, other ways (i.e., electrode micron/nanostructuring, synthesis of new organic materials, use of additives) to improve their performances are still being sought. In this context, we studied the behavior of the common organic bulk heterojunction (P3HT:PC70BM) deposited by matrix-assisted pulsed laser evaporation (MAPLE) with/without 0.3% of 1,8-diiodooctane (DIO) additive on flat and micro-patterned ITO substrates. The obtained results showed that in the MAPLE process, a small quantity of additive can modify the morphology of the organic films and decrease their roughness. Besides the use of the additive, the micro-patterning of the electrode leads to a greater increase in the absorption of the studied photovoltaic structures. The inferred values of the filling factors for the measured cells in ambient conditions range from 19% for the photovoltaic structures with no additive and without substrate patterning to 27% for the counterpart structures with patterning and a small quantity of additive.

Highly efficient light trapping of clustered silicon nanowires for solar cell applications

Due to their excellent photoelectric performance, nanostructures have attracted considerable attention in research to improve the power conversion efficiency of thin-film solar cells (TFSCs). Furthermore, cylindrical silicon nanowires (Cy-SiNWs) are regarded as promising candidates for a new generation of TFSCs. On this basis, many new nanostructures derived from conventional Cy-SiNWs have been studied extensively, but most of these structures require high manufacturing accuracy because of their complex morphology. In this paper, an ingenious design of clustered silicon nanowires (Cl-SiNWs) is introduced, whose cross section is similar to the flower shape and consists of four arcs with the same radius. Hence, it requires lower manufacturing difficulty compared with nanostructures with curvature variation of the cross-section profile (i.e., elliptic shape, crescent shape, etc.). In this study, the optical and electrical characterizations are numerically investigated using the finite-difference time-domain method. The numerical simulation shows that the optimized Cl-SiNWs achieve an optical ultimate efficiency (η_ul) and circuit current density (J_sc) of 33.66% and 27.54mA/cm², respectively, with an enhancement of 7.3% over conventional Cy-SiNWs. Further, the η_ul and J_sc improve to 42.20% and 34.53mA/cm² by adding the silicon substrate and silver backreflector. More importantly, the η_ul of Cl-SiNWs always obtained a higher value than Cy-SiNWs at a recommended diameter range of 360-560 nm. Therefore, the suggested Cl-SiNWs have exhibited tremendous potential for the future development of low-cost and highly efficient solar cells.

Simulation of optical absorption in conical nanowires

The optical absorptance from arrays of GaAs nanowires (NWs) was examined by the finite element method. Absorptance in cylindrical NWs, frustum nanocones (with base wider than the top) and inverted frustum nanocones (with top wider than the base) was compared. The introduction of higher order HE_1n modes, the red-shift of the HE_1n modes along the NW length due to NW tapering, and the red-shift of the modes due to increase of the overall NW diameter all contribute to a broadening of the absorption spectrum in conical NWs as compared to NWs with a constant diameter. The optical reflectance versus NW top diameter shows a minimum due to a balance between reflectance from the top of the NWs and reflectance from the substrate between NWs. The optimum geometry for photovoltaic energy conversion was determined from the total photocurrent. An optimum photocurrent of 26.5 mAcm^-2 was obtained, corresponding to a conical NW morphology with base diameter of 200 nm, top diameter of 110 nm, and length of 2000 nm. An optimized inverse tapered conical morphology gave comparable performance.

Characteristics and Photovoltaic Applications of Au-Doped ZnO-Sm Nanoparticle Films

Au-doped ZnO-samarium nitrate (Sm) nanoparticles with fixed concentrations of Sm (1 wt %) and various concentrations of Au (0.0, 0.5, 1.0 and 1.5 wt %) were prepared and used as photoelectrodes to enhance the photovoltaic efficiency of dye-sensitized solar cells (DSSCs). The cell fabricated with 1.5 wt % of Au-doped ZnO-Sm nanoparticles film achieved an optimal efficiency of 4.35%, which is about 76% higher than that of 0.0 wt % of Au-doped ZnO-Sm-based cell (2.47%). This increase might be due to the formation of a blocking layer at the ZnO-Sm/Au interface, which inhibits the recombination of electrons. This increase may also be attributed to the addition of rare-earth ions in ZnO to enhance the non-absorbable wavelength region of light via up/down-conversion of near-infrared and ultraviolet radiations to visible emission and reduce the recombination loss of electron in the cell. The efficiency of cells may be increased by the blocking layer and up/down-conversion process and thus promote the overall performance of the cells. This work indicates that Au-doped ZnO-Sm nanoparticle films have the potential to increase the performance of DSSCs.

Inspection of Biomimicry Approaches as an Alternative to Address Climate-Related Energy Building Challenges: A Framework for Application in Panama

In the Panama context, energy consumption in the building sector is mostly related to the conditioning of indoor spaces for cooling and lighting. Different nature strategies can be mimic to strongly impact these two aspects in the building sector, such as the ones presented here. A comprehensive analysis regarding literature related to biomimicry-based approaches destined to improve buildings designs is presented here. This analysis is driven by the increasing energy regulations demands to meet future local goals and to propose a framework for applications in Panama. Such biomimicry-based approaches have been further analyzed and evaluated to propose the incorporation of organism-based design for three of the most climate types found in Panama. Consequently, a SWOT analysis helped realized the potential that biomimicry-based approaches might have in improving the odds of in meeting the local and global regulations demands. The need for multidisciplinary collaboration to accomplish biomimicry-based-designed buildings, brings an increment in the competitivity regarding more trained human-assets, widening the standard-construction-sector thinking. Finally, the analysis presented here can serve as the foundation for further technical assessment, via numerical and experimental means.

Broadband solar absorption with silicon metamaterials driven by strong proximity effects

Absorption of the solar radiation over a wide spectral range is of utmost importance to applications related to the harvesting of solar energy. We numerically demonstrate broadband solar absorption enhancement employing a metamaterial in the form of arrays composed of subwavelength silicon truncated inverted cones, henceforth referred to as light funnel (LF) arrays. We show that the broadband absorption efficiency of an unoptimized LF array is 36% greater compared with an optically-maximized NP array. We show that photon trapping in LF arrays is motivated by proximity effects related to the optical overlap between LFs. We make the distinction between two types of optical overlap: weak overlap in which the coupling between the sparse array modes and the impinging illumination increases with array densification, and strong overlap where the array densification introduces new highly absorbing modes. We show that in nanopillar (NP) arrays the optical intensity inside the NPs decreases upon densification and the overall increase in absorptivity is due to increase in filling ratio (as was also shown by others), while the densification of LF arrays increases the optical intensity inside the individual LF and with the concurrent increase in filling ratio concludes light trapping much superior to that of NP arrays. Light trapping governed by strong proximity effects was not reported to date, and we believe it is an important paradigm for miniaturized lab-on-chip technologies.

Geometry-driven carrier extraction enhancement in photovoltaic cells based on arrays of subwavelength light funnels

Texturing the front surface of thin film photovoltaic cells with ordered or disordered arrangements of subwavelength structures is beneficial in terms of efficient light harvesting as well as efficient carrier extraction. Previous studies demonstrated efficient broadband absorption of solar radiation with surface arrays of subwavelength inverted cones (light funnels - LFs). In the current work, we use three-dimensional finite-difference time-domain electromagnetic calculations as well as three-dimensional device calculations to examine carrier extraction from photovoltaic cells that are composed of LF arrays on top of underlying substrates. For the selected geometry under examination, we show a broadband absorption enhancement of 14% for the LF photovoltaic cell compared with a cell based on the respective optically optimized nanopillar arrays. However, we show that the nominal power conversion efficiency is 60% higher in the LF cell which is due to the enhancement of both open-circuit voltage and short-circuit current. The higher open-circuit voltage in the LF cell is due to the higher injection of photocarriers, and the higher short-circuit current is a result of the unique LF geometry that supports efficient carrier extraction due to the naturally occurring gradients of the quasi-Fermi levels and minority carrier conductivity that allow for enhanced contact selectivity. We believe that this work paves the way towards a new approach for carrier collection in photonic devices for energy applications.

Approaching the Yablonovitch limit with free-floating arrays of subwavelength trumpet non-imaging light concentrators driven by extraordinary low transmission

Metamaterials based on arrays of subwavelength dielectric structures have recently proved to be a viable research tool towards the realization of various photonic devices. In the current study we introduce a new approach towards efficient light trapping and broadband absorption of solar radiation based on silicon surface arrays composed of subwavelength trumpet non-imaging light concentrators (henceforth, trumpet arrays). In geometrical optics, a three-dimensional trumpet non-imaging light concentrator is a hyperboloid of revolution with an ideal light concentration ratio. We use finite-difference time-domain electromagnetic calculations to examine the optical response of an infinite cubic-tiled substrate-less silicon trumpet array under normal illumination. The absorptivity spectra of trumpet arrays are characterized by strong absorption peaks, some of which are just below the Yablonovitch limit. The enhanced light trapping is attributed solely to the efficient occupation of the array Mie modes, and we show absorption enhancement at near infrared that is an order of magnitude higher than that of the optimized nanopillar (NP) arrays. We show superior broadband absorption of solar radiation in trumpet arrays (with unoptimized geometry) compared with that of the optimized NP arrays (∼26% enhancement). The higher optical absorption in the trumpet array is governed by low transmissivity, in contrast to the NP array in which the absorption is governed by low reflectivity. Finally, we show that low reflectivity in trumpet arrays is governed by modal excitation at the upper part of the trumpets (which is also supported by the weak dependency of the reflectivity on the array height), whereas the transmissivity is governed by modal excitation at the lower part of the trumpets.

Efficient light trapping and broadband absorption of the solar spectrum in nanopillar arrays decorated with deep-subwavelength sidewall features

Silicon nanopillar (NP) arrays are known to exhibit efficient light trapping and broadband absorption of solar radiation. In this study, we consider the effect of deep subwavelength sidewall scalloping (DSSS) on the broadband absorption of the arrays. Practically, the formation of DSSS is a side effect of top-down dry etching of NP arrays of several microns height. We use finite-difference time-domain electromagnetic calculations to show that the presence of DSSS can result in efficient excitation of optical modes in both the arrays and the underlying substrates. We demonstrate a broadband absorption enhancement of >10% in a DSSS-NP array with an underlying substrate. Finally, we use device calculations to examine the effect of DSSS on the electrical performance of a photovoltaic cell, as the main concern is the degradation of the open-circuit voltage due to surface recombination (DSSS results in higher surface-to-volume ratio). We show that the effect of DSSS on open-circuit voltage is negligible. Finally, deep-subwavelength sidewall features offer a new, interesting photon management strategy towards absorption enhancement.

Light Trapping with Silicon Light Funnel Arrays

Silicon light funnels are three-dimensional subwavelength structures in the shape of inverted cones with respect to the incoming illumination. Light funnel (LF) arrays can serve as efficient absorbing layers on account of their light trapping capabilities, which are associated with the presence of high-density complex Mie modes. Specifically, light funnel arrays exhibit broadband absorption enhancement of the solar spectrum. In the current study, we numerically explore the optical coupling between surface light funnel arrays and the underlying substrates. We show that the absorption in the LF array-substrate complex is higher than the absorption in LF arrays of the same height (~10% increase). This, we suggest, implies that a LF array serves as an efficient surface element that imparts additional momentum components to the impinging illumination, and hence optically excites the substrate by near-field light concentration, excitation of traveling guided modes in the substrate, and mode hybridization.

Addressing carrier extraction from optically-optimized nanopillar arrays for thin-film photovoltaics

Decorating the top surface of silicon solar cells with nanopillar arrays of subwavelength periodicity is a promising path toward low-cost thin-film photovoltaics with enhanced solar radiation absorption due to the inherent light trapping capabilities of nanopillar arrays. Common practice and knowledge for the efficient carrier extraction from the excited nanopillars is the formation of ultra-shallow radial p-n junctions that provide both short carrier collection lengths, and also ensure that the volume of the photo inactive emitter is as small as possible. In the current manuscript, both finite-difference time-domain simulations and three-dimensional device simulations are used to examine carrier extraction from nanopillar arrays that are geometrically optimized in terms of array periodicity and nanopillar diameter to provide maximum absorption of the solar spectrum. The discussion is limited to nanopillars with heights of 2 μm in line with what is currently available with leading top-down fabrication technologies for the formation of nanopillars with smooth sidewalls and radial uniformity. The examination considers both radial and axial homojunctions for various junction depths. It is shown that, contrary to common practice and knowledge, the ultra-shallow junctions are detrimental to the photovoltaic performance of such systems while the radial configuration with a junction depth of ∼50 nm is the most efficient. Furthermore, the open circuit voltage is highest for axial junctions with a junction depth of 100 nm. Also, it is shown that the axial junction is preferable in the low dopant concentration regime and that overall, the axial junction is less sensitive to variations in junction depth.

Self-Catalyzed Growth of Vertically Aligned InN Nanorods by Metal-Organic Vapor Phase Epitaxy

Vertically aligned hexagonal InN nanorods were grown mask-free by conventional metal-organic vapor phase epitaxy without any foreign catalyst. The In droplets on top of the nanorods indicate a self-catalytic vapor-liquid-solid growth mode. A systematic study on important growth parameters has been carried out for the optimization of nanorod morphology. The nanorod N-polarity, induced by high temperature nitridation of the sapphire substrate, is necessary to achieve vertical growth. Hydrogen, usually inapplicable during InN growth due to formation of metallic indium, and silane are needed to enhance the aspect ratio and to reduce parasitic deposition beside the nanorods on the sapphire surface. The results reveal many similarities between InN and GaN nanorod growth showing that the process despite the large difference in growth temperature is similar. Transmission electron microscopy, spatially resolved energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy have been performed to analyze the structural properties. Spatially resolved cathodoluminescence investigations are carried out to verify the optical activity of the InN nanorods. The InN nanorods are expected to be the material of choice for high-efficiency hot carrier solar cells.

Funnel-shaped silicon nanowire for highly efficient light trapping

In this Letter, funnel-shaped silicon nanowires (SiNWs) are newly introduced for highly efficient light trapping. The proposed designs of nanowires are inspired by the funnel shape, which enhances the light trapping with reduced reflections in the wavelength range from 300 to 1100 nm. Composed of both cylindrical and conical units, the funnel nanowires increase the number of leaky mode resonances, yielding an absorption enhancement relative to a uniform nanowire array. The optical properties of the suggested nanowires have been numerically investigated using the 3D finite difference time domain (FDTD) method and compared to cylindrical and conical counterparts. The structural geometrical parameters are studied to maximize the ultimate efficiency and hence the short-circuit current. Carefully engineered structure geometry is shown to yield improved light absorption useful for solar cell applications. The proposed funnel-shaped SiNWs offer an ultimate efficiency of 41.8%, with an enhancement of 54.8% relative to conventional cylindrical SiNWs. Additionally, short-circuit current of 34.2  mA/cm2 is achieved using the suggested design.

Observation of strongly enhanced photoluminescence from inverted cone-shaped silicon nanostructures. [corrected]

Silicon nanowires (SiNWs) attached to a wafer substrate are converted to inversely tapered silicon nanocones (SiNCs). After excitation with visible light, individual SiNCs show a 200-fold enhanced integral band-to-band luminescence as compared to a straight SiNW reference. Furthermore, the reverse taper is responsible for multifold emission peaks in addition to the relatively broad near-infrared (NIR) luminescence spectrum. A thorough numerical mode analysis reveals that unlike a SiNW the inverted SiNC sustains a multitude of leaky whispering gallery modes. The modes are unique to this geometry and they are characterized by a relatively high quality factor (Q ~ 1300) and a low mode volume (0.2 < (λ/n_eff)³ < 4). In addition they show a vertical out coupling of the optically excited NIR luminescence with a numerical aperture as low as 0.22. Estimated Purcell factors F_p ∝ Q/V_m of these modes can explain the enhanced luminescence in individual emission peaks as compared to the SiNW reference. Investigating the relation between the SiNC geometry and the mode formation leads to simple design rules that permit to control the number and wavelength of the hosted modes and therefore the luminescent emission peaks.