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Fabrication of anti-reflection structure on protective layer of solar cells by hot-embossing method. Sol Energ Mat Sol Cells ;93 8 —7. Optimization of anti-reflection moth-eye structures for use in crystalline silicon solar cells. Prog Photovoltaics Res Appl ;19 2 — Enhancement of the light conversion efficiency of silicon solar cells by using nanoimprint anti-reflection layer. Sol Energ Mat Sol Cells ;94 3 — Nanotextured silica surfaces with robust superhydrophobicity and omnidirectional broadband supertransmissivity.

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How Graphene is taking Solar Cells to the next level

Asahi Glass Co. Nelson J. The physics of solar cells. London: Imperial College Press; Texturing industrial multicrystalline silicon solar cells. Sol Energy ;76 1 — 3 — Fundamental limit of nanophotonic light trapping in solar cells. Photonic light-trapping versus Lambertian limits in thin film silicon solar cells with 1D and 2D periodic patterns. Opt Express ;20 Suppl 2 : A— Mokkapati S, Catchpole KR. Nanophotonic light trapping in solar cells.

J Appl Phys ; 10 Fundamental limit of light trapping in grating structures. Opt Express ;18 53 :A— Han SE, Chen G. Toward the Lambertian limit of light trapping in thin nanostructured silicon solar cells.

Three Dimensional Solar Cells Based on Optical Confinement Geometries by Yuan Li (Paperback, 2012)

Nano Letters ;10 11 —6. Effect of dielectric Bragg grating nanostructuring on dye sensitized solar cells. Opt Express ;20 Suppl 6 Nov :A— A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature ; — The renaissance of dye-sensitized solar cells. Nat Photonics ;6 3 —9.

Novel nanostructures for next generation dye-sensitized solar cells. Energy Environ Sci ;5 9 — Combined front and back diffraction gratings for broad band light trapping in thin film solar cell. Opt Express ;20 Suppl 5 May :A— Absorption enhancement in ultrathin crystalline silicon solar cells with antireflection and light-trapping nanocone gratings.

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User Account Log in Register Help. Search Close Advanced Search Help. My Content 1 Recently viewed 1 Nanostructures for pho Add Note. Show Summary Details. More options …. Editor-in-Chief: Sorger, Volker. In co-publication with Science Wise Publishing. Open Access. Online ISSN See all formats and pricing Online. Prices are subject to change without notice. Prices do not include postage and handling if applicable. Highly integrated optical phased arrays: photonic integrated circuits for optical beam shaping and beam steering by Heck, Martijn J. Volume 7 Issue 12 Dec , pp.

Volume 6 Issue 6 Nov , pp. Volume 5 Issue 4 Oct , pp. Volume 4 Issue 4 Dec , pp. Volume 3 Issue 6 Dec , pp. By relating the radiative recombination rate to the intrinsic carrier density n i via a bulk three-dimensional radiative recombination coefficient B 3D , the radiative dark current in an optically thin absorber has previously been expressed in terms of the physical absorber layer thickness W p — e.

Equation 6. However, radiative emissions from quantum-confined structures can be more appropriately described in terms of a two-dimensional radiative recombination coefficient B 2D [ 26 ]. In particular, the rate of radiative recombination in a quantum-confined layer is proportional to the product of the electron and hole densities within the quantum well. In the limit of evenly emitting wells in which the effective carrier densities are the same within each well, the radiative current density generated by a multiple quantum well structure can be expressed as:.

For any given effective well energy, equation 7 implies that the radiative component of the dark current will scale with the number of wells, independent of well thickness. Normalized photoluminescence spectra a and dark current-voltage measurements b from a set of high-voltage InGaAs quantum well solar cell structures, all emitting at approximately 1.

Three Dimensional Solar Cells Based on Optical Confinement Geometries - Semantic Scholar

In Figure 5 , the reverse saturation current density values inferred from both the dark diode current and photoluminescence measurements are compared to calculations using Equations 6 and 7. Also shown is the expected variation in the radiative dark current in the 3D and 2D regimes using Equations 6 and 7. The application of light trapping structures to thin-film devices provides a means to both further suppress the radiative dark current via photon recycling and increase the current output via enhanced optical path lengths within the thin absorber structure.

Figure 6 summarizes the current-voltage characteristics of a simple example of an optically-thin absorber structure employing a reflective back contact [ 6 ]. Record-low dark current characteristics for a GaAs-based device have resulted in an ultra-high open circuit voltage V oc of 1. Equation 6 can be employed to estimate the expected impact of enhanced optical path length on the radiative dark current of a GaAs-based device.

Because photon emissions are omnidirectional in nature, the OPL enhancements assumed in Figure 7 represent angle averaged values. Enhancements in the OPL due to reflections off the front and back surfaces of the device can result in the re-absorption of emitted photons and thus a significant increase of self-absorption effects, particularly in thicker absorber structures.

Illuminated current-voltage characteristics from a small area 0. An optical photograph of the flexible test cells is shown inset.

Three Dimensional Solar Cells Based on Optical Confinement Geometries (Springer Theses)

Projected dependence of the radiative saturation dark current as a function of GaAs absorber layer thickness assuming four different structures with varying degrees of light trapping, resulting in optical path length enhancements of 1x, 4x, 16x, and 24x. For thin absorbers, such as the test device summarized in Figure 6 , the impact of light trapping on radiative dark current is relatively small. However, the impact of light trapping on the short circuit current of thin-absorber structures can be quite significant. Figure 8 highlights the dependence of the short circuit current density on both the physical absorber layer thickness and the effective optical thickness due to enhancements in the optical path length.

The OPL enhancements shown in Figure 8 , unlike Figure 7 , are not angle averaged but instead describe the average path length of normal incident photons. For these calculations, the GaAs absorption coefficient was modeled using a piecewise continuous function described in Miller et al. Higher energy photons are assumed to be absorbed in a wider energy gap matrix surrounding the GaAs layer, providing The effective thickness in Equation 5 was then assumed to be the product of the physical thickness and the OPL factor. As seen in Figure 7 , the application of light trapping structures which can enhance the optical path length of incident photons is projected to have a significant impact on the current output of thin absorber structures, but has minimal impact on thicker absorber structures.

Projected dependence of the uncoated short circuit current as a function of GaAs absorber layer thickness under AM 1. Also shown is the short circuit current density measured on the optically-thin GaAs test structure characterized in Figure 6 and described in more detail in reference [6]. In the last section, we saw that light trapping is not a particularly effective means to reduce the radiative dark current of optically-thin absorber structures. However, recent experimental work indicates that it may be possible to reduce radiative recombination in thin absorber structures by manipulating the compositional profile of quantum well absorbers [ 19 ].

In particular, a compositional step-grade design has been experimentally observed to enhance the performance of high-voltage InGaAs quantum well solar cells by reducing the overall diode dark current. A comparison of square and step-graded well structures with varying well thickness but comparable well emission energy suggests a 2x reduction in the radiative recombination coefficient.


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Theoretically, we will show that reducing either the Urbach tail or the refractive index environment can result in a notable reduction in the radiative dark current of optically-thin structures. In addition, non-equilibrium effects, partially hot-carrier effects, can lead to even more substantial reductions in the radiative dark current. By embedding narrow energy-gap wells within a wide energy-gap matrix, quantum well solar cells seek to harness a wide spectrum of photons at high voltages in a single-junction device.

Quantum well solar cells have the potential to deliver ultra-high efficiency over a wide range of operating conditions, avoiding the limitations of current matching inherent in multi-junction devices. Over the years, quantum well solar cells have been fabricated using a variety of different material systems, and the basic concept has been extended to include quantum dot absorber structures [ 27 - 29 ].

Clear enhancements in the infrared spectral response have been experimentally observed in both quantum well and quantum dot solar cells. Recently, GaAs-based quantum well solar cells with a novel material structure which minimizes non-radiative recombination have also achieved record-high open circuit voltages, in some cases exceeding 1 V at one-sun bias levels [ 19 , 30 ].

In this section, we detail the additional performance benefits resulting from the use of compositionally step-graded InGaAs well designs. Figure 9 compares external quantum efficiency as derived from measured photoluminescence PL spectra and dark diode current-voltage characteristics from a single square InGaAs well photovoltaic device to a similar structure employing a compositionally step-graded well design. In each structure the well is placed within the junction depletion region, as photogenerated carriers can then escape from the well via field-assisted thermionic emission [ 25 ].

A comparison of the simplified band structures of the square and step-graded wells is illustrated in Figure The indium content of the square and step-graded well structures compared in Figure 9 has been tuned to yield a nearly identical peak PL energy of approximately 1. As a result, the forward emission and carrier collection characteristics are quite similar. These results imply that the use of a composition step-graded profile in the quantum well results in a 2x reduction in the radiative recombination coefficient. There are several possible mechanisms by which a step-graded well profile or other device designs may reduce the radiative recombination coefficient, and thus enhance the limiting operating voltage of photovoltaic devices.

For example, any shifts in the absorption profile, and in particular the sub-band gap e. Urbach tail region, can impact radiative emissions. Figure 11 compares the calculated reverse saturation current density assuming two different Urbach tail energies. As in earlier sections, the absorption spectrum is modeled using a piecewise continuous function [ 21 ]. The absorption spectrum is then used to generate an external quantum efficiency spectrum, which is in turn used to calculate the radiative dark current based upon detailed balance concepts — e.

Equations 5 and 1. Reducing the activation energy which describes the sub-bandgap absorption profile results in a reduction in the radiative dark current, but more so in thicker absorbers. Altering in the Urbach tail absorption characteristics may thus provide some benefits, but seems unlikely to account for the 2x reduction in the radiative recombination B-coefficient observed in thin step-graded well structures.

Estimated external quantum efficiency a and dark current-voltage measurements b from two high-voltage InGaAs quantum well structures, one employing a square well and the other a compositionally step-graded well. The external quantum efficiency was estimated from PL measurements assuming a reciprocity relationship between spectral response characteristics and luminescent emissions in PV and LED devices [12].

Simplified band structures of the a square and b step-graded well structures employed in the Figure 9 comparison, illustrating the field-assisted photogenerated carrier escape processes. Restricting the angular range of emissions provides another mechanism for reducing radiative dark current. The most direct means of restricting the range of angular emissions is to alter the refractive index environment in which the absorber layer is embedded. Figure 12 a summarizes the calculated radiative saturation dark current for several different refractive index values.

Reducing the refractive index of the material above and below the absorber layer can effectively reduce the angular emissions, and lowering the refractive index from 3. However, the changes in effective value of the refractive index in the step-graded structure are not expected to be this large. Step-graded structures could also potentially alter the strain profile in the well, and strain in quantum wells has been found to result in a non-isotropic radiation profile that may reduce overall radiative recombination losses [ 31 ].

The non-isotropic radiation profile resulting from strain is in many ways similar to that resulting from a reduction in the refractive index of the barrier material, and while potentially beneficial, would seem unlikely to account for the 2x reduction in dark current observed in step-graded structures. Step-graded structures may also provide a means of minimizing the overall recombination losses in quantum well solar cells.

Faster escape rates can potentially be obtained by employing a step-graded compositional profile to allow photogenerated carriers to readily hop out of the InGaAs well [ 19 ], as illustrated in Figure Enhanced extraction of hot carriers from the absorber region of a photovoltaic device has been suggested as a potential mechanism for reducing radiation losses and increasing efficiency [ 8 ]. Hot carrier effects can result in a large reduction in the radiative recombination, potentially reducing the B-coefficient by many orders of magnitude — see Figure 12 b.

Even a small effective carrier temperature difference of less than 1 kT is projected to result in more than a 2x reduction in the radiative dark current.


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Hot carrier effects can potentially be further enhanced by optimizing device design and employing optical concentration [ 8 ]. In the previous section, we summarized how the operating voltage of nano-enhanced absorbers can be enhanced by suppressing the radiative dark current. In this section we will focus on the application of three dimensional quantum dot nanostructures in photovoltaics to increase the current output. Luque et al. The enhancement in efficiency is due to increased infrared light absorption via optical up-conversion.

Figure 13 shows the band diagram of a PV cell with an intermediate band. This configuration will enable the absorption of two additional sub-bandgap photons in addition to one above bandgap photon. With proper design, the ultimate open circuit voltage should not be affected by the insertion of the intermediate band.

Instead, the open circuit voltage will be equal to the separation between valance and conduction band quasi Fermi levels of the wider bandgap host material, independent of the intermediate band material, i. Based on the IBSC theory, a maximum efficiency is possible when the host material bandgap is 1. Semiconductor quantum dots QDs are perhaps the best choice to create an intermediate band in a single-junction solar cell due to the inherent tunability of their shape, size, and quantum confinement properties. For an IBSC to work properly, the QD system being used must satisfy certain conditions in terms of bandgaps and band alignments [ 32 ].

In type I QDs, both electrons in the conduction band and holes in the valence band are confined. But in GaSb QDs the high effective mass of holes puts the hole energy levels close to each other and making it difficult to achieve an intermediate band. Levy et al. Schematic showing the bands involved in an intermediate band photovoltaic cell. The intermediate band is located between the conduction and valence bands of a barrier material. The open circuit voltage of this ideal cell is equal to the separation between the conduction and valence band quasi Fermi levels EFC and EFV.

There are three transitions — valence band to conduction band, valence band to intermediate band, and intermediate band to conduction band — that contribute to the photocurrent. A typical QD, due to its small size, has a small absorption cross section for incident photons. Due to this fact, a large number of QD layers need to be stacked together to provide sufficient sub-bandgap photon absorption.