As a light-electricity conversion device, light absorption plays a primary role in determining the achievable efficiency of a solar cell. Seeking an effective light trapping method without causing significant impact on the electrical performance of solar cells is an ongoing challenge. In the market, dominant crystalline silicon solar cells, pyramid surface textures are introduced on the front surface to scatter light into the solar cell over a large angular range, increasing the effective path length in the cell [1–3]. However, the micrometer-sized surface textures are not applicable for thin film solar cells because the surface roughness would exceed the film thickness (~300 nm). In addition, the large surface texture area also increases the surface carrier recombination leading to degraded solar cell performance. Recently sub-wavelength light trapping structures, which can induce strong scatting and nanophotonic modes [4–9], have demonstrated great capabilities to trap light offering tremendous opportunities for thin-film solar cells.
Both dielectric and metallic nanostructures have been employed for nanophotonic light trapping however, their operation mechanisms are distinctly different. Specially designed metallic nanostructures can effectively excite surface plasmon resonance (SPR) or surface plasmon polariton (SPP) at the metal-dielectric interface by interacting with the incident light, resulting in a strongly localized and reconstructed electromagnetic field [4, 5]. Plasmonic nanostructures improve the performance of photovoltaic (PV) devices by either guiding or concentrating the incident light. On the other hand, lossless dielectric nanostructures improve solar cell performance by antireflection or photonic modes. In this review, the light-trapping effect from both dielectric and metallic materials will be summarized, their advantages and drawbacks will be analyzed, and the challenges and future perspectives in the field will be provided.
2 Light trapping with plasmonic nanoparticles and nanostructures
Among various proposed plasmonic structures, nanoparticles are highly preferable for PV device applications because of their strong scattering properties, tunable by particle species, size, shape, and dielectric environment and also because of their simple preparation and integration methods that can be readily compatible with the standard solar cell fabrication process without significantly increasing the production cost [10–25]. Metallic nanoparticles that support localized surface plasmons (LSP) in the visible or near-infrared regions can greatly enhance the light path length inside solar cells [12, 16, 26–31]. Metallic nanoparticles and nanostructures can enhance the performance of PV devices based on three main mechanisms: (a) the scattering from the metal particles (far-field effect) and (b) the near-field enhancement from small nanoparticles, which directly enhance the absorbance of the semiconductor in the close vicinity of the nanoparticles due to the enhanced field; (c) In comparison, plasmonic nanostructures can further enhance solar cell performance by coupling light into SPP modes or guided modes from a nanostructured metallic film on the back side of a thin PV absorbing layer [32–35].
Rapid progress has been made in the field of plasmonic solar cells in the past few years. Various nanoparticle and nanostructure fabrication methods have been explored. Until now, plasmonic effects have been investigated in almost all kinds of solar cells, such as crystalline silicon (c-Si)-based solar cells , amorphous silicon (a-Si) solar cells [12, 16], GaAs solar cells , CdSe solar cells , organic solar cells [37, 38], and perovskite solar cells .
Systematic numerical modeling for different light-trapping structures has been conducted toward ultrahigh efficiency and low-cost solar cells [40–42]. Among all the designs, metallic nanoparticles have been the simplest and most commonly used plasmonic nanomaterials in effectively enhancing the solar cell absorption [10–25]. Acting as scatters, these nanoparticles have been integrated on the top or at the back surfaces of solar cells during cell fabrication [9, 12, 16, 26–30]. Both experiments and simulations have proven that after nanoparticle integration, the light absorption in solar cells can be improved particularly in the longer wavelength range near the bandgap of the absorbers, where more light will be scattered due to the plasmonic-enhanced scattering [16, 30, 31].
2.1 Plasmonic scattering enhancement
Plasmonic in-coupling of scattered light from a metallic nanoparticle layer into the absorbing layer has been extensively reported for different types of solar cells [26, 43–45]. In a typical configuration, metallic nanoparticles such as Ag, Au, or Al can be deposited onto the front or back surface of the solar cells, either embedded into the front electrode or electronically isolated by a transparent conductive oxide layer. The absorption enhancement in the active layer can be optimized by adjusting the geometries of the nanoparticles and their surface coverage density depending on the dielectric environment around the nanoparticles.
For the front side-integrated nanoparticles on a-Si solar cells, the enhancement in the photocurrent and efficiency have been achieved by a number of groups [16, 30, 46–48]. For example, both Ag and Au nanoparticles have been demonstrated to be effective in increasing the light path through the plasmonic light scattering, thereby, increasing the solar cell efficiency by around 10% and 8.3%, respectively [16, 48]. For the nanoparticles integrated at the rear side of the solar cells, most of the studies used Ag nanoparticles fabricated by the thermal annealing method, chemical methods, or the laser interference lithography method. V. E. Ferry et al. systematically investigated the rear surface-located Ag nanoparticles fabricated by the laser interference lithography for a-Si solar cells and showed a relative 46% short circuit current (JSC) enhancement due to the plasmonic back scattering . At optimized particle sizes and pitches, a solar cell with an efficiency of 9.6% was achieved with an active layer of only 90 nm in thickness . Recently, nucleated Ag nanoparticle geometry was proposed and demonstrated to be an effective way to achieve broadband light trapping for a-Si thin film solar cells when they were integrated at the rear side of the solar cells by introducing strong scattering with large scattering angles . High energy conversion efficiency of 11% that breaks the nanoplasmonic solar cell efficiency record was recently demonstrated by integrating the nucleated nanoparticles with graphene nanofibers .
From the cost effectiveness point of view, metal nanoparticles with strong plasmonic effect and fabricated with low-cost species, for example, Al and Cu, are always attractive for large-scale practical applications. However, challenges exist in fabricating these low-cost nanoparticles due to their more active chemical properties and additional efforts required in harnessing strong plasmonic resonances at desired wavelengths. To this end, exciting experimental progress has been made by using Al nanoparticles in c-Si solar cells [14, 43, 51]. However, less success has been achieved in a-Si solar cells.
2.2 Plasmonic near-field enhancement
In comparison to the forward or back scattering from the comparative large nanoparticles, small metallic nanoparticles can be applied in the close vicinity to the active layer of solar cells to make use of the strongly enhanced LSP near field to significantly increase the absorption in the active layer around the nanoparticles. There have been a number of publications detailing the enhanced absorbance from the embedded metallic nanoparticles in the active layers of thin film solar cells, particularly in organic solar cells [52–56].
Theoretically embedding metallic nanoparticles in the active layer of thin film solar cells can lead to significant absorption enhancement. For instance, it was predicated that an ideal conversion efficiency of 18% can be achieved by combining an Ag/a-Si nanocomposite layer with an only 20-nm-thick active layer as shown in Figure 1 . Experimentally, special attention needs to be paid due to the consideration from recombination loss caused by the metallic nanoparticles in the active layer. Near-field light concentration has been demonstrated in ultrasmall (a few nanometers) gold nanoparticles integrated in a-Si solar cells between the front electrode and the photoactive layer . Significant enhancements in both the photocurrent of 14.1% and fill factor of 12.3% have been achieved due to the strong plasmonic near-field concentration and the reduced contact resistance, respectively. On the other hand, plasmonic tandem solar cells have also been reported  with the efficiency increased from 5.22% to 6.24%, achieving a 20% enhancement due to the LSP near-field absorption enhancement.
2.3 Plasmonic nanostructures
Although comparatively simple to fabricate and integrate, nanoparticles are limited in harnessing different plasmonic modes, which are useful for light localization and concentration inside the solar cells. To this end, various plasmonic nanostructures have been designed and fabricated to host desired modes at specific or broad wavelengths for PV applications.
As mentioned in the previous section, it is more challenging to achieve useful solar cell performance enhancement by integrating plasmonic structures on the front side of solar cells because of the light losses from the metallic structures. Therefore, plasmonic back reflectors have been more popularly employed for light management in thin film solar cells, which can be naturally integrated with the metal back reflector in a standard design in the solar cell geometry. The incorporated plasmonic nanostructures can couple light into SPP modes or photonic waveguide modes at the metal/semiconductor interface if the nanostructures are carefully designed. Two types of modes can be effectively harnessed through employing periodically nanostructured plasmonic back reflectors . One is the photonic modes traveling parallel to the absorbing layers in the long wavelength regions due to the coupling of the incident light into the waveguide modes. The other is the SPP mode excited by plasmonics when the shape and size of the metallic nanostructures satisfy the coupling conditions. Near the resonant frequency, the SPP modes travel in parallel to the interface of the metal/semiconductor providing strong near-field localization inside the semiconductor layer, which leads to significantly increased absorption in the semiconductor layer. Tailoring the nanostructure design can allow these two modes to couple to each other, therefore, providing an even higher enhanced absorption in the absorbing layer.
It should be pointed out that even being integrated at the back side of solar cells, the metallic nanostructures still present competitive absorption mechanism with the solar absorber, although in some cases, this can be suppressed if the solar cells are thick enough rendering little remained spectrum in the plasmonic absorption peak of the nanostructure. One way to circumvent the absorption loss from metallic nanostructures is to couple the incident light into both the SPP modes and photonic modes inside the semiconductor layer by tuning the shapes and sizes of the nanostructures. The coupling to the photonic modes is preferred as they do not suffer from significant loss in the metal. It has been demonstrated that a relatively high absorption enhancement can be achieved by incorporating nanostructured plasmonic back reflector into solar cells [19, 49, 57–60]. The most commonly used nanostructures for plasmonic back reflectors for a-Si thin film solar cells are the nanocone  or nanodome  structures, nanocylinder-induced conformal structures [19, 49, 61], and nanovoid structures [57, 58] as shown in Figure 2.
In the nanodome a-Si solar cell design, Ag periodic nanostructures were implemented as the substrate for solar cell as shown in Figure 2A . This nanodome structured solar cell could absorb 94% of the light from 400 nm to 800 nm with an a-Si layer that is only 280 nm in thickness. In comparison, the flat solar cell can only absorb 65% of the light in the same spectrum. The efficiency of the solar cell was enhanced from 4.7% to 5.9% with the nanodome structure, which mainly arose from the increased JSC. The nanodome geometry of the solar cells provides several enhancement mechanisms. First, the gradually increased effective refractive index leads to a better matching with air effectively suppressing the reflection loss on the solar cell surface. Second, the SPP modes coupled into the a-Si layer and the scattered light along the in-plane direction by the Ag nanocone structure of the back electrode increase the light traveling path, providing an additional plasmonic light-trapping mechanism. Similar plasmonic absorption enhancement mechanisms were also proved by V. E. Ferry who demonstrated Ag hemiellipsoidal plasmonic back electrode for a-Si solar cells, as shown in Figure 2B . After the deposition of the a-Si solar cell over the plasmonic back reflector, the conformal hemiellipsoidal shape of the back electrode was transferred to the a-Si layer and the top ITO layer due to the ultrathin thickness (90 nm) of the a-Si solar cell. Experimentally, the performance of the solar cell could be improved significantly from 6.32% to 9.6% with a rounded Ag hemiellipsoidal plasmonic back reflector of 290 nm in diameter and 400 nm in periodicity. The EQE measurement presented a broadband photocurrent enhancement in both the blue and the red regions of the absorption spectrum. The enhanced photocurrent on the blue side was attributed to the top interface nanostructures formed by the conformal deposition of solar cells, which could act as weakly coupled Mie resonators to forward scatter the incident light into the a-Si layer. On the other hand, the enhanced photocurrent in the red part of the spectrum was because of the plasmonic back reflector coupling light to the SPP modes and waveguide modes.
The nanovoid structure, as shown in Figure 2C, is another commonly used plasmonic back reflector. The optical properties of plasmonic nanovoid structures have been investigated extensively both in theory and in experiment [57, 58]. Rich hybrid modes can be excited inside the void structures, which mix the LSP modes with the Fabry-Perot cavity modes and the rim dipole modes associated with the charge buildup at the void rims. On the outer surface of the structure, SPP propagating modes can be also excited. The nanovoid plasmonic back reflector can be fabricated by electrochemical deposition through a template of closely packed self-assembled arrays of nanospheres. It was first demonstrated in organic solar cells in 2011 , achieving an enhancement of 3.5 times at the plasmonic resonant frequency in the EQE measurement and a four times enhancement in the overall efficiency. Significant localized field enhancements led to strong absorption enhancement in the active layer of the solar cell. Enhanced scattering from the textured surface in the void structure was another important factor contributing to the efficiency increase across the interested spectrum.
Recently, similar plasmonic nanovoid structured back reflectors fabricated by electrochemically anodizing Al foil have also been applied in a-Si solar cells as shown in Figure 2C. The efficiency of the solar cell with the nanovoids was improved from 5.6% to 7.1%, mainly due to the broadband optical absorption enhancement, which enhanced JSC significantly . Both experimental measurements and numerical calculations suggested that the enhancement was resulted from the efficient scattering of sun light from the textured surface in the short wavelength region and the excitation of the SPP modes, LSP resonances, and the waveguide modes in the long wavelength region.
3 Light trapping with dielectric nanoparticles and nanostructures
To circumvent the inherent losses associated with metallic nanoparticles and nanostructures, dielectric nanoparticles have been employed in solar cells, due to the almost negligible absorption loss in the visible to near-infrared range and broadband scattering, which is beneficial for solar cells. Although their scattering cross sections are less than those of metallic nanoparticles at the resonance wavelengths, the absence of particle absorption leads to comparable light absorption enhancement in the active layer of solar cells. In addition, the refractive indices of the dielectric nanoparticles are normally from 1.5 to 2.0 making them competitive candidates for antireflection components in the solar cell geometry, in particular, when they are incorporated on the top side of solar cells.
3.1 Enhancement from dielectric nanoparticles
Both simulation and experimental work has been extensively reported on dielectric nanoparticles integrated with solar cells [46, 62–65]. For instance, SiO2 nanoparticles were incorporated on the top surface of quantum-well solar cells leading to an increased Jsc of 12.9% and efficiency enhancement of 17% as shown in Figure 3A . The enhancements are attributed to the coupling of the incident light into the lateral propagating modes, with optical confinement provided by the refractive index contrast of the quantum-well layers. In this case, antireflection effect does not play a significant role because the SiO2 nanoparticles were sitting on a 15-nm-thick SiO2 top layer of the solar.
However dielectric nanoparticles can play a significant role in antireflection if they are integrated on substrates with high refractive indices. For example, closely packed TiO2 nanoparticle films with controlled thicknesses were integrated on top of Si solar cells using the spin-coating method . It has been found that the reflectance of the substrate was reduced dramatically from 35% to <15% over the entire absorption wavelength range, leading to a 30% improvement in the photocurrent after the integration of nanoparticles. This was mainly attributed to the gradient index of TiO2, which provide strong antireflection effect for planar Si solar cells.
Recently, a new concept of light trapping in a-Si thin film solar cells through the use of whispering gallery modes (WGMs) generated by the closely packed SiO2 nanoparticles has been proposed to enhance the absorption and the photocurrent [63, 64]. The closely packed SiO2 nanoparticles were integrated on the top surface of standard a-Si solar cells as shown in Figure 3B. Solar cell performance can be improved through the diffractive coupling of light from air to the high-index absorbing layer via the confined resonant WGMs inside the nanospheres. In addition, the periodic distribution of the nanospheres can form a 2D waveguide for light coupling between the adjacent spheres, leading to mode splitting. Another obvious advantage of these closely packed SiO2 nanospheres for PV application is that the spherical geometry allows the acceptance of light from large angles of incidence.
The light-trapping effect of the SiO2 nanospheres on the solar cell was evaluated through calculating the spectral current density generated from the a-Si layer with and without the nanospheres. The overall integrated current density in the a-Si was increased from 12.33 mA/cm2 to 13.77 mA/cm2 after integrating the nanoparticles, which corresponded to an enhancement of 12%. The enhancement in the short wavelength was attributed to the textured antireflection coating induced by the spheres. The discrete enhancement peak was ascribed to the WGM coupling into the a-Si layer. Experimental observation of broadband light trapping enhancement by placing periodic dielectric surface-textured structures embedding closely packed polystyrene (PS) nanospheres on top of a flat ultrathin a-Si absorber (100 nm) was also demonstrated as shown in Figure 3C . By embedding the nanospheres, the absorption of the a-Si layer increased from 23.8% to 39.9% due to the wave guiding of the light inside the PMMA embedding layer and the coupling between free space light to the resonant WGMs.
Dielectric nanospheres cannot only be integrated on the top surface of solar cells for forward scattering but also in the middle of the absorbing layer to enhance the absorbance by the lateral scattering. Recently, a new solar cell concept with embedded SiO2 nanospheres within the semiconductor layer was proposed theoretically . FDTD simulations of SiO2 nanospheres embedded in a 1-μm c-Si film with a 75-nm Si3N4 antireflection coating were conducted. The integrated absorption enhancement of the incident light across the visible AM 1.5 spectrum was on the order of 5–10% compared to a normal solar cell without the nanoparticles. The enhancement arose from the scattered fields from the periodic nanospheres, which result in the lateral modes due to the constructive and destructive interferences.
Experimentally, SiO2 nanospheres have also been integrated in the a-Si/μ-Si tandem solar cells between the top a-Si cell and the bottom μ-Si cell as an advanced intermediate reflector layer (IRL) for light management [68, 70]. The functions of the IRL are to reflect back the visible light into the top a-Si cell and to transmit the near-infrared light into the bottom μ-Si solar cell [71, 72]. Replacing the normal IRL with SiO2 nanospheres can solve the challenge of reduced light transmittance for the bottom cell by effective forward scattering of the near-infrared light. The cross-sectional profile of the solar cell with the SiO2 nanosphere IRL is shown in Figure 3D. The nanometer-sized light scatters were placed at the interface between the top and bottom cells. The incident near-infrared portion of light was forward scattered from the nanosphere scatters (ray 1), and trapped by the subsequent internal reflection from the bottom cell (ray 2), which resulted in the enhanced near-infrared absorbance of the bottom cell. The incident red light was reflected by the scatterers (ray 3), improving the red response of the top cell, which can be confirmed by the EQE measurement. The photo current density of the top cell was increased from 10.1 mA/cm2 to 11.0 mA/cm2 due to the Fresnel reflection of the red light from the scatterers. The photo current density of the bottom cell was increased from 6.3 mA/cm2 to 7.3 mA/cm2. The total efficiency of the solar cell was enhanced by 16%, from 7.2% to 7.9% after incorporating the SiO2 nanosphere light scatterers.
3.2 Enhancement from dielectric nanostructures
Compared to dielectric nanoparticles, innovative dielectric nanostructure design and fabrication provide more flexibility to introduce useful optical modes to enhance PV device performance. For example, nanostructures can be directly formed in the absorbing layer allowing the excitation of strong optical resonances, which can significantly improve the light absorbance in the solar cell beyond the conventional limit compared to a flat absorber. As shown in Figure 4A, the InP nanowire array thin film solar cell with a p-i-n junction is a good example of employing nanostructures to enhance light absorption. Light absorption exceeding the ray optics limit was demonstrated leading to a 13.8% efficiency .
In addition to the nanowire solar cells, systematic simulations have been conducted on the potential benefits from nanostructuring on both sides of ultrathin c-Si solar cell as shown in the inset of Figure 4B . High light absorbance can be obtained through using high-aspect ratio dense nanocone arrays on the front surface as an effective antireflection structure and low-aspect ratio sparse nanocone arrays at the back surface as a grating to couple light to the guided resonances in the absorber. The optimized structures with an absorbing layer of only 2 μm in thickness can result in significant absorption enhancement close to the Yablonovitch limit (Figure 4C) . The double-sided nanocone-textured structures were also demonstrated experimentally , as shown in Figure 4B. Outstanding light trapping effect was achieved. For a 6.8-μm-thick solar cell with only the front side nanocone array, a Jsc of 19.1 mA/cm2 was realized leading to an impressive overall solar power conversion efficiency of 6.2%. A similar nanocone array structure was also employed for a-Si solar cells as shown in Figure 4D . Broadband absorption enhancement in the solar cell through the whole wavelength range was achieved compared to the flat reference solar cell due to the dramatically reduced reflection from the cone structure providing a gradient refractive index change from the air to the a-Si substrate.
Recently, spherical nanoshell nanocrystalline (nc-Si) structures have been proposed to improve the broadband absorbance based on the excited low-Q WGMs as shown in Figure 4E . The geometry of the nc-Si solar cell was designed into nanoshell structures providing a 20-fold enhancement in the absorbance of the active layer compared to a flat reference cell. This nanoshell nc-Si solar cell could couple the incident light into the WGMs inside the spherical shells and induce a profitable recirculation of light inside the shell-shaped absorbers, leading to the significant increase in light propagation path in the active layer. As a result the amount of material required to realize comparatively strong light absorbance can be significantly reduced. By optimizing the shell thickness and the inner shell radius, Si nanoshell resonances with relatively low Q values could be excited not only allowing efficient light coupling to the resonant modes but also spectrally broadening the resonant absorption peaks to allow broadband enhancement. Theoretically, the electrical performance of a p-i-n solar cell was calculated. A solar cell with a nc-Si shell of 80 nm in thickness and 225 nm in outer radius could yield an efficiency of 8.1%, which is comparable to that of a flat solar cell with an active layer thickness of 1.5 μm.
4 Conclusions and perspectives
This review provides a snapshot of the exciting development of nanophotonic silicon solar cells. The fast advancements on nanofabrication and nanotechnology, in particular, the increased understanding of fundamental light-mater interactions at the nanometer scale offers a new revenue for us to engineer PV devices from the fundamental light-management point of view, significantly different from the conventional semiconductor engineering approaches. New device architectures based on both metallic and dielectric nanoparticles and nanostructures have been proposed and demonstrated. Improved device performance has been realized. Exciting sciences at the nanometer scale are being harnessed injecting new vitality into the PV research field. New solutions are provided, which has the great potential to address the long-existing challenges in the PV field and lead to viable high-efficiency technology solutions for the solar industry.
To this end, it is expected that the development in the following directions may lead to major scientific and technological breakthroughs. First, the capability of manipulating light at the nanometer scale allows one to target the fundamental efficiency limit of PV devices . For example, innovative nanostructures have been proposed with the aim to address the previously regarded insurmountable challenges of thermodynamic losses. It is expected that ultrahigh efficiency solar cells far exceeding the Shockley-Queisser limit can be realized [7, 77]. Second, effective light management may lead to the development of ultrathin PV devices. Nanophotonics approaches has demonstrated the possibilities of achieving light management approaching or exceeding the Yablonovitch limit, for example, using double-sided nanotextures  or combining nanotextures with plasmonic nanoparticles . As a result, less active material is required to achieve a similar efficiency resulting in ultrathin solar cells that are only a small fraction in thickness of the commercial solar cells [79, 80], solving the high-cost challenge of the current PV technology. This benefit may offset the extra cost and complexity caused by integrating nanostructures in solar cells. Third, a systematic approach is required to synergically evaluate both the optical and electrical properties of a solar device. Fundamentally, nanophotonics approaches can enable significant enhancement in the PV device performance. But practically, the achievable performance is compromised by the electrical losses. Simultaneously considering both the optical and electrical aspects may lead to better performing devices that are more practical for solar industry to adapt. Fourth, developing cost-effective nanophotonics technology, including low-cost materials, fabrication, and integration methods, holds the key for large-scale deployment of nanophotonics solar cells. Most of the plasmonic materials now still focus on using noble metal, which are impractical for full-scale devices. The recent development in employing low-cost Al and Cu nanoparticles in solar cells is encouraging . The performance enhancement has been impressive. In the meantime, printable nanostructure fabrication and integration represent a promising approach for solar industry, which are both low cost and scalable . In the end, from the system point of view, the development of high performance, stable, and integratable energy storage devices is critical for solar energy to become a reliable energy source that is competitive with other main stream sources.
Baohua Jia acknowledges the support from the Australian Research Council through the Discovery Projects (DP150102972 and DP140100849).
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About the article
Published Online: 2015-08-07
Published in Print: 2015-08-01