Projects

Improving Solar Energy Conversion Efficiency with Plasmonic Nanostructures
Abstract: In this project we explore how plasmonic nanostructures can be used to improve the solar light conversion efficiency.
Keywords: Plasmonics, Metamaterials, Photovoltaics
Collaborators: Joshua Pearce (MTU), Paul Bergstrom (MTU), Anand Kulkarni (MTU), Michael Heben (University of Toledo)

Plasmonic Metamaterial Perfect Absorbers

Improvements in optical enhancement for solar photovoltaic (PV) devices using conventional natural materials has begun to show diminishing returns, yet metamaterials, which are rationally designed geometries of optical materials that can be tuned to respond to any region of the electromagnetic spectrum offer an opportunity to continue to improve solar device performance [1]. Historically, periodic structures with sub-wavelength features are used to construct a metamaterial for a given application. Designing and constructing a material in this way a metamaterial possesses optical properties that are not observed in their constituent materials enabling the properties to be determined from structure rather than merely composition. The advent of metamaterials has provided PV device designers among others with unprecedented flexibility in manipulating light and producing new functionalities [2].

Another approach to optical enhancement of PV is to consider plasmonics, which is a rapidly growing field for the application of surface plasmons to device performance improvement. Surface-plasmon-based devices already proposed include not only PV but also light-emitting devices, data storage, biosensors, nano-imaging, waveguides, perfect absorbers, and others [3-7].

Surface plasmons are collective oscillations of surface electrons whereas surface plasmon polariton (SPP) describes a coupled state between a surface plasmon and a photon [8]. On the other hand, localized surface plasmons (LSP) are confined to bounded geometries such as metallic nanoparticles or nanostrips of various topologies [9-11]. The fluctuations of surface charge results in highly localized and significantly enhanced electromagnetic fields in the vicinity of metallic surfaces.

Typically, surface plasmon resonances exhibit a strong relationship to the size, shape and the dielectric properties of the surrounding medium. The resonances of noble metals are mostly in the visible or infrared region of the electromagnetic spectrum, which is the range of interest for PV applications [12].

For the application, we are considering here photons arriving at the metal surface produce surface waves in the form of SPP along the metal-semiconductor interface of the top layer of a PV device. This occurs when the photons interact with the collective oscillations of free electrons in the metal of the absorber. The metallic nanostructures have the ability to maintain SPPs, which provides electromagnetic field confinement and enhancement. This phenomenon has already attracted substantial attention in the PV scientific community, but has yet to be commercialized and scaled.

Perhaps even more promising it is possible to construct plasmonic metamaterial “perfect absorbers” with such surface waves. Metamaterial absorbers are extremely flexible and have already been designed with broadband [13-18], polarization-independent [16-20], and wide-angle [13, 14, 16, 18, 19, 21] optical absorption for a host of applications. Both theoretical work through device simulations [14, 17, 21] and physical fabrication [13, 15, 16, 18-20, 22] of ultrathin metamaterial absorbers with these features have been realized. The theoretical limit of nearly 100% absorbance at the tunable resonant wavelength can be obtained with proposed designs of ultrathin, wide-angle perfect absorber structures for infrared light [21]. A study showed an average measured absorption of 71% from 400–700 nm, which represents a majority of the energy in the solar spectrum, with an ultrathin (260 nm), broadband and polarization independent plasmonic super absorber [18]. In addition, the simulations indicated that the absorption levels could be increased to 85%.

All of these features are critical to maximize the efficiency of PV devices, yet are generally lacking in schemes to optically enhance solar cell design. Even small improvements in optical enhancement in specific cases can have a large effect. For example, improving (enlarging) the acceptance angle for high absorption for a PV device may eliminate the need for mechanical tracking of a PV system, which can have a substantial impact on both the first cost as well as the operating and maintenance costs.

Complex perfect absorbers have more degrees of freedom in terms of impedance-match, polarization-independence, and wide angular reception, which make them superior systems for managing light than simple plasmonic nanoparticle approaches. Thus perfect absorbers are better candidates for enhancing solar PV conversion efficiency.

Despite this promise, perfect absorbers have inherent Ohmic loss (i.e, heating), which loses both the power of the photons but also decreases the efficiency of the next electrical conversion because of resultant higher cell operating temperature.

Reducing Ohmic Losses in Plasmonic Solar Cells

The geometric skin depth (i.e., depth at which the current density is reduced to 1/e of its value at the surface of the conducting material) in a structured conductor such as metallic nanostructures (e.g., split-ring resonators, strips, etc.) is much larger than in the bulk form of the same conductor [23]. However, in the limit the nanostructure approaches bulk conductor, the geometric skin depth is significantly reduced and approaches its bulk value. More importantly, this skin depth reduction can be exploited as an important resource to minimize Ohmic losses in metallic nanostructures despite higher metallic filling ratio. We call this technique the “Bulk Skin Depth Technique (BSDT).” Previously, we applied this technique to reduce Ohmic losses in metamaterials [23] and to design three-dimensionally isotropic negative index metamaterial [24] As part of this project, we have applied the BSDT to perfect absorbers to turn them into functional solar cell devices and used the indium gallium nitride (InxGa1-xN) as an example (see Fig. 1) [25].

Figure 1 Absorbance in different parts of the absorber achieved by the BSDT process followed by the gradual incorporation of the true complex permittivity of In0.54Ga0.46 such that imaginary parts are (a) 0.1, (b) 0.4, (c) 0.8, and (d) 1.23. The total absorbance in plain In0.54Ga0.46N layer of the same thickness (i.e., d = 14 nm) is also shown for comparison using the experimental complex permittivity data [25] (e) z-component (i.e., out of page) of the magnetic field (A/m) inside the perfect absorber consisting of thin semiconductor layer sandwiched between metal strip and ground plate, (f) x-component (i.e., parallel to the interfaces) of the current density (A/m2), and (g) the power loss density (W/m3) distribution for the near-perfect absorber in (d). Significant portion of the power (i.e., over 85%) is absorbed inside the In0.54Ga0.46N layer.

If we reduce the geometric skin depth in the metallic portions of the metamaterial absorbers (either perfect or near-perfect), we can push the resonant currents induced by solar radiation closer to the semiconductor layers in PV devices. This brings the following important advantages: (1) Optical absorption in the metallic layers (i.e., Ohmic loss) is shifted into semiconductors, which increases useful optical absorption for solar energy conversion. (2) Joule heating is minimized. (3) Because the current is tightly confined to metallic surfaces facing the semiconductor layers, additional interconnects or contacts can be made possible [24] without destroying or short-circuiting the absorber.

In order for a plasmonic PV cell to reach the full potential of the technology the designed cell must maximize the absorption within the semiconductor region. Simultaneously, absorption must be minimized everywhere else including the metallic regions over the cell. This balancing act of absorption must be accomplished over the entire solar spectrum (normally AM1.5, which is ASTM G-173) and ideally over all incidence angles. In order for the BSDT to function, it first requires identification of resonant modes of the metamaterial absorber that contributes to optical absorption. Then the nanostructure of the metal must be geometrically customized as determined by the underlying resonant currents.

Hydrogenated Amorphous Silicon Solar Photovoltaic Cells Despite the material, sustainability, economic and technical benefits of thin-film solar photovoltaic (PV) devices [26-28], conventional crystalline silicon (c-Si) modules dominate the market [29]. The cost of c-Si PV has fallen to the point that the balance of systems (BOS) and thus the efficiency of the modules plays a major role in the levelized cost of electricity for solar [30]. There is thus a clear need to improve the efficiency of thin-film devices further [31].

Thin-film hydrogenated amorphous silicon (a-Si:H) solar photovoltaic (PV) cells have the fastest energy payback time of any Si-based PV cell [32]. Amorphous silicon thin-film solar cells with conversion efficiencies over 10% have been reported [33]; however, there is an increased interest to further improve the efficiency and reduce the cost simultaneously for broader commercialization and lower levelized costs of solar electricity [30]. The greatest technological challenge encountered by a-Si:H PV is light-induced degradation of performance known as the Staebler-Wronski effect (SWE) [34], which is reversible with thermal annealing. SWE is associated with the formation of defect states in the bandgap from exposure to sunlight, which causes a decrease in a-Si:H PV conversion efficiency [35]. It is now clear that SWE is caused by an increased density of multiple types of defect states, which reach saturation in device quality materials known as the degraded steady state (DSS) after approximately 100 h of 1 sun light illumination [36-38]. SWE has been studied in detail for decades and several engineering techniques have been used to minimize its impact including various forms of optical enhancement [39] (OE) and even in-situ annealing in photovoltaic thermal (PVT) hybrid systems, [40, 41] but it has not been eliminated. Therefore, SWE limits the thickness of the intrinsic (i-a-Si:H) layer, and hence the overall absorption capacity of an a-Si:H PV cell.

Recent advances in plasmonics [1, 3, 4, 25, 42-45] provide a new technique to improve the optical enhancement in a-Si:H PV devices and further reduce the negative effects of SWE. Employing plasmonic nanostructures in PV is garnering broad interest [1] because resonant plasmonic nanostructures are capable of producing optical enhancement in absorption by supporting mechanism like Fabry-Perot resonance, guided modes, localized surface plasmon resonance (LSPR), or increased scattering.

Their application has also been studied for PV design, which can result in broadband, polarization independent, and wide angle absorption for ultrathin active absorbing layers (<100 nm) [3, 5, 12, 13, 18, 46-49]. Three major geometries with associated enhancement mechanisms have been proposed: (i) plasmonic nanostructures on top, (ii) plasmonics nanostructures embedded into the active layer, or (iii) textured back contact [42, 50]. For the latter, it has been reported that depositing a-Si:H on such textured or patterned surfaces results in higher defects density and hence reduces open circuit voltage (Voc), short circuit current density (Jsc), and thus the conversion efficiency [51, 52]. Therefore, top surface texturing or front nano-patterned resonant metallic nanostructures coupled with the active semiconductor seems to be a more promising option for optical absorption enhancement in the active layer while minimizing the defect density and at the same time facilitating the feasibility of commercial production of high efficiency a-Si:H PV devices. Massiot, et al. [49] have theoretically demonstrated broadband absorption in their ultrathin (<100 nm) a-Si:H solar cell using sub-wavelength nano-patterned horizontal nanowires, which also managed to reduce the effect of SWE in a-Si:H. However, their design presents a number of practical limitations. First, plasmonic enhancement is a near field enhancement and, therefore, the strong electric field enhancement due to surface plasmons or guided modes tends to be strongly localized near the metal/semiconductor interface (or even the buffer layer). In the case of commercial a-Si:H cells, it is the top p-a-Si:H and contact layer made from indium tin oxide (ITO), which absorbs enhanced near field more strongly [49]. Second, the effects of absorption in the highly defective doped regions of p and n-a-Si:H layers are also not considered in most of the available literature. This is a fundamental omission, since the p-i-n device was developed so that the relatively low-defect density i-layer minimizes losses from the electron-hole pair generation in the higher defect density doped layers [53]. These doped layers have a substantial thickness by surface plasmonic standards as they are usually 15–25 nm thick, and hence can account for a significant amount of the “enhanced” optical absorption while not making a major contribution to carrier collection. The ability of device designers to compress the doped layer thicknesses further is limited by the design requirement to generate a suitable electric field to separate the solar photo-generated charge carriers. The optical absorption in the p or n a-Si:H layer is largely converted into heat via recombination of electron hole pairs rather than charge carrier separation [54].

Plasmonic Hydrogenated Amorphous Silicon Solar Cells

As part of this project, we have studied polarization independent improved light trapping in commercial thin film hydrogenated amorphous silicon (a-Si:H) solar photovoltaic cells using a three-dimensional silver array of multi-resonant nano-disk structures embedded in a silicon nitride anti-reflection coating to enhance optical absorption in the intrinsic layer (i-a-Si:H) for the visible spectrum for any polarization angle [55]. Predicted total optical enhancement (OE) in absorption in the i-a-Si:H for AM-1.5 solar spectrum is 18.51% as compared to the reference, and producing a 19.65% improvement in short-circuit current density (JSC) over 11.7 mA/cm2 for a reference cell. The JSC in the nano-disk patterned solar cell (NDPSC) was found to be higher than the commercial reference structure for any incident angle (see Fig. 2).

Figure 2 Plot for the short circuit current density in NDPSC and reference solar cell, for both TE and TM polarizations as a function of angle of incidence from normal. The short circuit current density in the NDPSC for both TE and TM polarizations is similar and always higher than the short circuit current density of reference solar cell at all angles (except a negligible difference near 80 degrees) and it converges to that of reference cell at 80 degrees.

The NDPSC has a multi-resonant optical response for the visible spectrum and the associated mechanism for OE in i-a-Si:H layer is excitation of Fabry-Perot resonance facilitated by surface plasmon resonances. The detrimental Staebler-Wronski effect in a-Si:H solar cell can be minimized by the additional OE in the NDPSC and self-annealing of defect states by additional heat generation, thus likely improving the overall stabilized characteristics of a-Si:H solar cells.

Limitations of Ultra-thin Transparent Conducting Oxides for Plasmonic Solar Cells

To fully exploit the potential benefits offered by plasmonic-based devices, TCOs with high transmittance (low loss) and low enough resistivity are to be used as device top contacts in our design [55]. However, for current transparent conducting oxides (TCOs) to be successfully integrated into the novel proposed plasmonic enhanced PV devices, ultra-thin TCOs films are required [45]. For example, our simulations showed a 19.65 % increase (see Fig. 2) in short circuit current (JSC) for nanocylinder patterned solar cell (NCPSC) in which the ITO layer thickness was kept at 10 nm to minimize the parasitic Ohmic losses and simultaneously act as a buffer layer while helping to tune the resonance for maximum absorption [45]. TCOs such as the most established indium tin oxide (ITO), aluminum-doped zinc oxide (AZO) and zinc oxide (ZnO) are standard integral materials in current thin-film solar PV devices [55, 56-58]. Bulk material properties for common TCOs including ITO have been well researched and documented for different processing conditions and substrates [55, 56, 59–63]; however, this is not the case for ultra-thin TCOs. The few exceptions include Sychkova et al. [64], who reported both optical and electrical properties of 9–80 nm ITO films deposited by pulsed DC sputtering varied with thickness and showed a general increase in resistivity with decrease in film thickness [64]. Other notable studies on ultra-thin ITO films using various deposition techniques include the following: Chen et al. who used filtered cathodic vacuum arc (FCVA) to deposit 30–50 nm on heated quartz and Si substrates [65]; Tseng and Lo, who used DC magnetron sputter for 34.71–71.64 nm ITO film on PET (polyethylene terephthalate) [66]; Kim et al. who used RF magnetron sputter for films between 40 and 280 nm deposited on PMMA substrate heated at 70 C [67]; Alam and Cameron, who used sol–gel process for 50–250 nm film deposited on titanium dioxide film [60]; and Betz et al. who used planar DC magnetron sputtering for 50, 100 and 300 nm films on glass substrates [68]. The results from these few thin TCO studies reveal a pattern in which resistivity increases rapidly as film thickness decreases from 50 to 10 nm.

The electrical properties of ITO thin films depend on the preparation method, the deposition parameters used for a given deposition technique and the subsequent heat treatments. Key factors for the low resistivity have not been clearly documented because of the complex structure of the unit cell of crystalline In2O3 formed by 80 atoms and the complex nature of the conducting mechanisms in polycrystalline films [69]. The issue is further complicated by the large number of processing parameters, even for a single technique.

As part of this project, to probe these challenges and to determine if ITO, AZO and ZnO are viable candidate materials for use in plasmonic-enhanced thin-film PV devices, we performed numerical sensitivity analysis on TCO thickness (10–50 nm) versus optical absorption in the i-a-Si:H layer of nano-disk patterned thin-film a-Si:H solar cells (NDPSC) using COMSOL Multiphysics RF module v4.3b. We used these simulation results to guide the experimental work which investigated both optical and electrical properties of ultra-thin films simultaneously deposited on both glass and silicon substrates. The effects of deposition and post-processing parameters on material properties of ITO, AZO and ZnO ultra-thin TCOs were probed and the suitability of TCOs for integration into plasmonic-enhanced thin-film solar PV devices was assessed. From these results some of the limitations of thin TCOs for plasmonic optical enhancement of thin-film PV were identified [70].

Figure 3 FESEM images for (a) 10nm ITO on glass, (b) 10nm ITO on silicon (with oxide spacer), (c) 20nm ITO on glass and (d) 20nm ITO on silicon (with oxide spacer).

Ultra-thin TCOs present a number of challenges for use as thin top contacts on plasmonic-enhanced PV devices. First, both ultra-thin TCO optical and electrical parameters differ greatly from those of thicker (bulk) films deposited under the same conditions. Second, they are delicate due to their thickness, requiring very long annealing times to prevent film cracking. The reactive gases (usually oxygen or hydrogen) require careful monitoring to avoid over-oxidizing or over-reducing the film as it impacts their stoichiometry. There is a trade-off between conductivity and transparency of the deposited films.

Acknowledgment

This project is currently funded at Michigan Tech by the National Science Foundation under grant CBET-1235750 and supported by ThinSilicon Corporation, Mountain View, CA. Our collaborators include Joshua Pearce, Paul Bergstrom, and Anand Kulkarni at Michigan Tech, Michael Heben at University of Toledo.

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