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Efficient broadband energy absorption based on inverted-pyramid photonic crystal surface and two-dimensional randomly patterned metallic reflector
Applied Energy 172 (2016) 59–65
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Efficient broadband energy absorption based on inverted-pyramid photonic crystal surface and two-dimensional randomly patterned metallic reflector Zhi-Hui Chen a,⇑, Na Qiao a, Yang Wang b, Li Liang a, Yibiao Yang a, Han Ye c, Shaoding Liu a a Key Lab of Advanced Transducers and Intelligent Control System, Ministry of Education and Shanxi Province, College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, China b Key Laboratory of Chemical Biology and Molecular Engineering of Ministry of Education, Institute of Biotechnology, Shanxi University, Taiyuan 030006, China c State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing 100876, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Absorption is enhanced in a
broadband (0.3–9.9 lm) in a novel structure. Absorption is angle-insensitive and polarization-insensitive. Large thickness and period are favored to enhance a broadband absorption. Our structure is experimentally feasible and versatile for applied energy.
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Article history: Received 24 August 2015 Received in revised form 24 March 2016 Accepted 26 March 2016
Keywords: Broadband energy absorption Photonic crystal surface Randomly patterned metallic reflector
a b s t r a c t We propose a hybrid structure containing photonic crystal surface with inverted-pyramid arrays on the top, two-dimensional (2D) patterned metallic reflector with random pyramids’ sizes at the bottom, and a Si film between the two layers, to achieve high-efficiency, broad-band and wide-angle energy absorption. Due to the scattering effect and waveguide mode resonance effect of the structure, the enhanced absorption is close to or even surpasses the Yablonovitch limit in a broad wavelength range (0.3–9.9 lm). What’s more, the high absorption efficiencies are insensitive to the variation of incidence angle (from 0° to 80°) and substantial electric field modes concentrate in the inverted-pyramid arrays and Si film. Comparing to un-patterned film and one-dimensional (1D) patterned film, 2D-patterned film obtains higher absorption efficiency for both s polarization and p polarization. Furthermore, we find that the high absorption band expands to a broader wavelength range with the size of structure increasing. Our multifunctional structure is experimentally feasible and is expected to have a wide application in the areas of energy harvesting, energy conversion, energy conservation and sustainable energy utilization. Ó 2016 Elsevier Ltd. All rights reserved.
⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (Z.-H. Chen). http://dx.doi.org/10.1016/j.apenergy.2016.03.098 0306-2619/Ó 2016 Elsevier Ltd. All rights reserved.
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1. Introduction High performance broadband light absorbers have drawn considerable interest since they show remarkable potential in a wide range of applications in energy harvesting and conversion, for example, the high-efficient broadband light absorption can enhance the photocurrent of photovoltaic (PV) cells [1–3], concentrate solar thermal energy [1], be integrated in a PV-thermal system [4], improve the sensitivity of imaging and sensing [5,6], and realize the near infrared stealth [7], etc. The Yablonovitch limit of absorption is applicable to bulk structures [8], but the absorption efficiency of subwavelength photonic structures with proper interface design could easily overcome this limit. In the past few decades, various subwavelength optical coupling structures, such as nanoparticle imprinted patterns [9], hybrid photonic crystal (PC) structures [10,11] plasmonics nanoparticle [12], metallic gratings [13], nanocone gratings [14] had been proposed aiming to obtain high broadband absorption based on the high-loss materials and the mechanisms of creating scattering, multiple resonance modes, surface plasmon polariton. However, most of these structures acted on either visible wavelength range or infrared wavelength range, the realized high absorption band is not very broad, and the technological process is complicated and expensive. In the present study, a novel hybrid structure is proposed for the first time to simultaneously realize large absorption in both visible and infrared wavelength ranges (0.3–9.9 lm) for both s polarization (with nonzero Ey) and p polarization (with nonzero Ex). The hybrid structure is composed of a Si film, a PC surface with inverted-pyramid arrays on the top of Si (polymer or colloidal quantum dots (QDs) filled in the PC surface) and twodimensional (2D) randomly patterned metallic reflector at the bottom of Si. Our strategy is to use the PC surface (filled invertedpyramid arrays) as an antireflection coating on the top, and use the PC surface together with the 2D patterned metallic reflector to enhance light scattering and light trapping. On the basis of the strategy, optical path length in the Si film is significantly increased and many waveguide mode resonances are formed, thus the lightmatter interaction and optical absorption will be enhanced eventually. Comparing to our previous work [10], high absorption efficiency is achieved for both s polarization and p polarization by using 2D-patterned film in this work. Additionally, the effects of structure size on broadband (0.3–9.9 lm) absorption were studied. It is demonstrated that our structure could provide high-efficiency, broad-band, angle-insensitive and polarization-insensitive light absorption within the wavelength range from 0.3 lm to 9.9 lm, and the high absorption band expands to a broader wavelength range with the size of structure increasing.
mal) (n = 1.5) as an example. The gold material is chosen due to its high reflection and absorption at the wavelength of 0.3– 9.9 lm. The width of gold pyramid is in a certain proportion to the unit size (T), and the length of gold pyramid equals to T in x or y direction (detailed structure is shown in Fig. S1 in the Supplementary material). The unit size (T) was set as 1.8 lm initially to study the wavelength range from 0.3 lm to 2.0 lm. The thickness of Si (L) is 2 lm, and the thickness of gold substrate (t) is 100 nm which is thick enough to reflect the light in the wavelength range from 0.3 lm to 9.9 lm. In real fabrication, the inverted-pyramid textures on two sides of the Si film can be fabricated by irradiating the surface with a nanosecond pulsed laser followed by alkali etching [5] or by other various etching techniques [15], and then the polymer and gold film can be deposited on the corresponding patterned sides, respectively. 2.2. Rigorous coupled wave analysis (RCWA) The RCWA method is a useful method to calculate electromagnetic wave propagation in periodic structures. In this work, all numerical simulations are performed by using RCWA [5,16] method in one unit cell of structure. The three dimensional space domain is (x, y, z) = (T/2:T/2, T/2:T/2, Z:+Z) lm, where the Z value changes with T. The wavelength range addressed within our work ranges from 0.3 lm to 9.9 lm. The electric field distributions are got from the monitor in the middle of one unit. The complex refractive indices n and k (representing the real and imaginary parts of refractive indices respectively) of Si and gold (Fig. 1(b)) are dependent on wavelength, which are obtained from Palik’s book [17]. Since the light source is usually very far from the light absorbing devices in real applications, e.g., solar cell, infrared photodetectors, etc., the light source could be considered as a plane wave source when it is arriving at the devices, and the propagation direction of incident light is along the z axis. The wavelength of incoming radiation is a fixed wavelength for every simulation, and the wavelength is scanned from 0.3 lm to 9.9 lm in this work. In every simulation, a plane wave is injected from the top of the structure, the distance between the plane wave source and the top of structure is at least two times of the target wavelength. It can be considered that the source is infinitely far away from the sample and the incoming beam has equal intensity across the studied surface. In the RCWA method, the total transmission and total reflection of light can be calculated, thus the total absorption can be computed according to the law of energy conservation. In the following study, the term ‘‘absorption efficiency” means ‘‘normalized absorbed power”. The absorption efficiencies of s and p polarization are studied when T, L and incidence angle change. 3. Results and discussion
2. Model and method 3.1. Effects of unit size and incidence angle on absorption efficiency 2.1. Proposed structure Fig. 1(a) shows the three-dimensional schematic of one unit of our proposed structure. In this work, the calculations are performed within one unit cell. The inverted right square pyramid arrays on the top are filled by low refractive index (n) polymer, eight inverted-pyramids with random sizes in x and y directions are used in one period to represent the quasi-random distribution of 2D patterned metallic reflector at the bottom, and the Si film is between the two layers. The low refractive index polymer is added to form a gradual variation of the refractive index between air (n = 1.0) and Si (n > 3.4), which could reduce the optical reflection from the air–Si interface. The refractive index varies from 1.3 to 1.7 for different kinds of polymer, we chose PVFM (Polyvinyl for-
In order to optimize the unit size (T) of our structure, the effect of T on absorption efficiency is studied for both p and s polarization when the other parameters are unchanged. Here, light is injected in the normal direction, and the wavelength range from 0.3 lm to 2.0 lm is studied. According to Fig. 2(a) and (b), when light is injected to our structure in the normal direction, either for p or s polarization, high absorption efficiency can be obtained at the wavelength of 0.3– 2.0 lm as T is changing from 1.25 lm to 3.0 lm. The absorption efficiencies reach almost 100% for shorter wavelength (0.3– 0.9 lm). The absorption efficiencies for longer wavelength (0.9– 2.0 lm) are lower than that for shorter wavelength, but the average absorption efficiency for longer wavelength is still above
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Fig. 1. (a) Schematic of one unit of three-dimensional structure. (b) Complex refractive indices of Si and Au with n and k representing the real and imaginary parts, respectively.
Fig. 2. The absorption efficiencies within the wavelength range from 0.3 lm to 2.0 lm for p and s polarization under normal incidence condition. (a and b) T changes from 0.5 lm to 3.0 lm. (c and d) Incidence angle changes from 0° to 80° when T = 1.8 lm.
70%. We expect that the main reason for the above results is that there are more high-order waveguide modes [18–21] for short wavelength than those for longer wavelength when the thickness of film is fixed. Then, we take T = 1.8 lm as an example to investigate the angular performance of our structure. Fig. 2(c) and (d) shows that high absorption efficiencies are insensitive to the variation of incidence angle (from 0° to 80°) for both p and s polarization when wavelength changes from 0.3 lm to 2.0 lm. Since the radiation from sun at sea level is mainly in the wavelength range of 0.3–2.0 lm, our structure could capture most of the injected sun radiation with any incidence angle and polarization. The light absorption by Si material can be converted to electric energy and thermal energy. In details, when the sunlight is injected upon Si material, the electrons present in the valence band will absorb solar energy and be excited to the conduction band, then the free electrons can be extracted to an external circuit, thus electric energy is generated. Meanwhile, the rest solar energy will be transferred to the vibration of Si lattice, thus thermal energy is gener-
ated. Moreover, gold material has good thermal conductivity. Therefore, high light absorption of our structure can increase the photocurrent of Si solar cells, and the gold substrate of our structure can increase the heat dissipation of solar cells.
3.2. Effect of 2D patterned structure on absorption efficiency As shown in Fig. 2(a) and (b), when T is larger than 1.25 lm, the average absorbed power of our structure exceeds 70% of the initial injected power within the wavelength range from 0.3 lm to 2.0 lm. Then, we chose T = 1.8 lm as an example to study the effect of 2D patterned structure on absorption efficiency. We compare the Yablonovitch limit [10,14] (AYablonovitch = 1–1/(1 + 4n2aL), ‘‘n” is the real part of the refractive index of Si, ‘‘a” is the absorption coefficient, ‘‘L” is the thickness of Si film.) with the absorption efficiencies of s and p polarizations incidence for the structures in Fig. 3(a) and (c) under normal incidence condition. In Fig. 3(a), the structure is an un-patterned flat Si film. In Fig. 3(b), one-
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Fig. 3. (a–c) Three-dimensional structure in one unit: (a) Un-patterned film. (b) 1D-patterened film: V-grooves and gold reflector with random-size grooves in y direction are introduced at the top and bottom of Si film, respectively. (c) 2D-patterened film: inverted-pyramid arrays and gold reflector with random-size pyramids in both x and y directions are introduced at the top and bottom of Si film, respectively. (d and e) Yablonovitch limit (green short dash line) and absorption efficiencies for s and p polarization under normal incidence condition in the structures of (a–c) when k changes from 0.3 lm to 2.0 lm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
dimensional (1D) periodic V-grooves and gold reflector with random-size grooves in y direction are introduced at the top and bottom of Si film, respectively. In Fig. 3(c), inverted-pyramid arrays and gold reflector with random-size pyramids in both x and y directions are introduced at the top and bottom of Si film, respectively. For the structures in Fig. 3(b) and (c), the top grooves and inverted-pyramid arrays are filled by polymer (n = 1.5). Fig. 3(d) shows that the absorption efficiencies of 2D-patterned film are higher than both 1D-patterned film and un-patterned film for s polarization, because electric field for s polarization is along the y axis, the refractive indices of structures in Fig. 3(a) and (b) are invariant in y direction so that light cannot be scattered effectively for s polarization, while the structure in Fig. 3(c) is patterned in both x and y directions, which can enhance the light scattering and absorption for both p polarization and s polarization. When the structure is un-patterned, there are Fabry-Pérot cavity modes between the top surface and gold reflector [22]. These modes lead to more absorption by the gold material at infrared wavelengths. When the structure is 1D patterned, there are some individual guided resonance modes for s polarization between the surface 1D PC and 1D patterned metallic reflector, which introduce some absorption peaks at infrared wavelengths. Fig. 3(e) shows that the absorption efficiencies of both 2D-patterned and 1Dpatterned film are higher than that of un-patterned film for p polarization. The enhanced absorption can be close to or even surpass the Yablonovitch limit in the wavelength range from 0.3 lm to 2.0 lm. In our previous work [10], it is reported that the surface 1D PC and 1D patterned metallic reflector worked together on visible and infrared light absorption for only TM polarization. The waveguide modes between the PC and gold reflector led to large absorption by the Si and gold material. In this work, the absorption enhancement is achieved for both s polarization and p polarization by using 2D-patterned film due to the reduction of incidence reflection from the air–Si interface and the light trapping by
increasing optical path lengths and creating waveguide modes in the film. Besides, our structure is robust for energy application because the metallic reflector was randomly patterned and it is not sensitive to the small perturbation of structure parameters in the real fabrications. This also reduces the difficulty of fabrication and facilitates the large-area fabrication for its application in buildings. 3.3. Effects of T and L on broadband (0.3–9.9lm) absorption We take p polarization under normal incidence condition as an example to study the effects of T and L on broadband (0.3–9.9 lm)
Fig. 4. Yablonovitch limit and p polarization light absorption efficiencies of three 2D patterned structures with different T and L values, when k changes from 0.3 lm to 9.9 lm under normal incidence condition. The green short dash line: Yablonovitch limit for L = 2 lm. The blue line: Yablonovitch limit for L = 10 lm. The yellow line: T = L = 3 lm. The black short dash line: T = L = 6 lm. The pink line: T = L = 10 lm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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absorption. Fig. 4 shows that the high absorption wavelength range depends on both T and L. There are more waveguide modes in short wavelength range than long wavelength range for fixed T and L, and there are more long-wavelength waveguide modes for large T and L than small T and L. The waveguide mode resonance can increase the light-mater interaction in the film, which enhance the light absorption. So with the increase of T and L, the high absorption band for our structure will expand to a broader wavelength range and absorption efficiencies will increase at longer wavelength, while there is no obvious change of absorption efficiencies at shorter wavelength (0.3–2.0 lm). When the values of T and L vary, the drop point of absorption is at around the wavelength which is close to the value of T and L. When T = L = 10 lm, the enhanced absorption is close to the Yablonovitch limit at short wavelength (0.3–0.9 lm) and remains high in a broad wavelength range (0.3–9.9 lm). In order to further understand the relationship between high absorption bandwidth and the values of T and L, we choose T = 1.8 lm and T = 10 lm to investigate the effect of L on absorption efficiency for both s and p polarization, as shown in Fig. 5. Here, light is injected in the normal direction, and the studied wavelength range is from 0.3 lm to 9.9 lm. We find that the high absorption (above 60%) wavelength range is mainly related to T, but not closely related to L. For short wavelength (0.3–0.9 lm), there are many guided modes for both small T (T = 1.8 lm) and large T (T = 10 lm), so the light absorption is high and vary slightly for different T. However, for long wavelength (0.9–9.9 lm), there are less guided modes for small T (T = 1.8 lm) than large T (T = 10 lm), which leads to the obvious difference of absorption for different T. In addition, the high absorption can cover a broad wavelength range (0.3–9.9 lm) for T = 10 lm. These results agree well with those in Fig. 4. Generally, periodic micro/nano structures play a main role in the light absorption enhancement when the target wavelengths are in a narrow band, while random micro/nano structures show superior performance when the target wavelengths are in a broadband range. In previous work, the optimal periodicity (T) for light trapping in the structure with periodic patterned surface is close to the target wavelength [14]. However, in this work, the smallest periodicity (T) for the achievement of broadband light absorption is equal to the target absorption band-
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width and the high absorption band expands to a broader wavelength range with the size of structure increasing. 3.4. Spatial distributions of the electric field In order to further illustrate the reason of light absorption enhancement, the steady state electric field distributions in our structure are studied when T and L are equal to 10 lm. The spatial electric fields (E2 = Ex2 + Ey2 + Ez2) were calculated by the RCWA method. By analyzing the electric field distributions (Fig. 6), we find that substantial electric field modes concentrate in the inverted-pyramid arrays and Si film. Moreover, short-wavelength guided modes primarily exist near the PC surface area, while the long-wavelength guided modes are distributed in both PC surface area and Si film. It indicates that the PC surface primarily works on visible wavelength range, and the patterned metallic reflector mainly works on infrared bands because the Si film has small absorption in the infrared wavelength range. Furthermore, since the metallic reflector is 2D randomly patterned, the field distributions at y–z section will be a little different, but this will not affect our conclusion. The inverted-pyramid arrays on the surface can be filled by colloidal quantum dots (QDs) film to further enhance the total light absorption of our structure [23]. QDs are semiconductor particles with the sizes in nanometer scale (below the Exciton Bohr radius). Due to quantum confinement effect, the energy levels of QD split up, thus QD has similar discontinuous electronic energy levels as those of atomic structure. Since the bandgaps (i.e., energy difference between the top of the valence band and the bottom of the conduction band) of QDs are tunable across a wide range of energy levels by changing the size of QD (the bandgap of QD increases with the size of QD decreasing), broadband solar energy can be absorbed by using different sizes of QDs in solar cells. This property makes QDs good at improving the photovoltaic efficiency of solar cells. Since the light absorption by both QDs and Si materials can be converted to photocurrents, a QD–Si hybrid solar cell with high energy utilization efficiency could be formed by using our structure. In the experiment, surface recombination states over QDs should be taken into account. In order to meliorate the performance of the photonic devices, the fluorescence quantum yields
Fig. 5. The absorption efficiencies of s and p polarization light in 2D patterned structures within the wavelength range from 0.3 lm to 9.9 lm. (a and b) are the absorption efficiencies for T = 1.8 lm when L changes from 1.5 lm to 10 lm. (c and d) are the absorption efficiencies for T = 10 lm when L changes from 6.5 lm to 10 lm.
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Fig. 6. Electric field distributions (k1 = 0.4 lm, k2 = 1.2 lm, k3 = 3.0 lm and k2 = 8.0 lm) in one unit cell (x–z section in the center of one unit) of the structure with both T and L expanding to 10 lm under normal incidence. (a–d) and (a0 –d0 ) are for p polarization and s polarization, respectively.
of QD need to be improved by passivating QD with coated shells (e.g., ZnS layers) for suppressing the surface recombination states over QD [24,25]. These good properties above make our structure versatile in the areas of energy conversion, energy conservation and sustainable energy utilization. For example, our structure could be used in the building integrated photovoltaic thermal (BIPVT) system [26,27] for improving energy utilization efficiency (Fig. 7). The visible-light absorption by Si material and QDs can be converted to electric energy and thermal energy as previously mentioned in Sections 3.1 and 3.4, and the broadband light absorption by gold material can be converted to thermal energy. Then, the thermal energy above can be transferred to air/water flowing through the gold film. Thus, our structure based BIPVT system can simultaneously provide electric and thermal energy for the buildings. The advantage of our structure in BIPVT system is that our structure with high-efficiency, broadband, angle-insensitive and polarization-insensitive absorption can not only absorb visible light and near-infrared radiation from sun at sea level but also absorb the mid-infrared radiation from nearby industry, animals, etc., which could make our structure based BIPVT system provide
more thermal energy for the buildings than the common BIPVT system. 4. Conclusion In this work, we have proposed an easy-processing and robust photonic structure which comprises a Si film sandwiched by a photonic crystal (PC) surface on the top and a 2D patterned metallic reflector at the bottom. Colloidal quantum dots (QDs) can be filled in the inverted-pyramid arrays of the PC surface. Based on a systematic study of the structure, it is demonstrated that our structure could provide high-efficiency, broad-band, angle-insensitive and polarization-insensitive light absorption within the wavelength range from 0.3 lm to 9.9 lm, and the high absorption band expands to a broader wavelength range with the size of structure increasing. Such good properties make our structure versatile in the areas of energy conversion, energy conservation and sustainable energy utilization. For example, our structure is useful in building integrated photovoltaic thermal (BIPVT) system. The advantages of our structure are listed as following: (1) Our structure can simultaneously provide electric and thermal energy for buildings because it can absorb broadband radiation including visible light and broadband infrared radiation; (2) our structure can provide a framework for a QD–Si hybrid solar cell with high energy utilization efficiency, since the light absorption by both QDs and Si materials can be converted to photocurrents; (3) our structure is useful in increasing the heat dissipation of solar cells because gold material has good thermal conductivity; (4) our structure is robust for energy application because the metallic reflector was randomly patterned and it is not sensitive to the small perturbation of structure parameters in the real fabrications. This also reduces the difficulty of fabrication and facilitates the large-area fabrication for its application in buildings. The work will be interesting for the researchers and engineers working on the field of applied energy, which can bridge the gaps between research, development and implementation. Acknowledgments
Fig. 7. Conceptual framework for applying our photonic structure in BIPVT systems.
This work was supported by the National Natural Science Foundation of China (61307069, 61575139, 61575138, 61401035,
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61505135), the Natural Science Foundation of Shanxi Province, China (2013021017-3, 2015021013), the Specialized Research Fund for the Doctoral Program of Higher Education, China (20131402120018), the Program for the Top Young Academic Leaders and the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi Province, China, the Excellent Young Scientist Foundation and Qualified Personnel Foundation of Taiyuan University of Technology, China (2014YQ012, tyut-rc201203b), the Open Fund of IPOC (BUPT), China (IPOC2013A001), the Open Fund of Top priority disciplines in colleges and universities in Zhejiang Province, China (2015KF20). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apenergy.2016. 03.098. References [1] Yan J, Chou S-K, Desideri U, Lee D-J. Transition of clean energy systems and technologies towards a sustainable future (Part I). Appl Energy 2015;160:619–22. [2] Brongersma ML, Cui Y, Fan S. Light management for photovoltaics using highindex nanostructures. Nat Mater 2014;13:451–60. [3] De Rossi F, Pontecorvo T, Brown TM. Characterization of photovoltaic devices for indoor light harvesting and customization of flexible dye solar cells to deliver superior efficiency under artificial lighting. Appl Energy 2015;156:413–22. [4] Mojiri A, Stanley C, Rodriguez-Sanchez D, Everett V, Blakers A, Rosengarten G. A spectral-splitting PV–thermal volumetric solar receiver. Appl Energy 2016;169:63–71. [5] Chen Z-H, Wang Y, Yang YB, Qiao N, Wang YC, Yu ZY. Enhanced normaldirection excitation and emission of dual-emitting quantum dots on a cascaded photonic crystal surface. Nanoscale 2014;6:14708–15. [6] Chen Z-H, Hellström S, Yu Z-Y, Qiu M, Fu Y. Time-resolved photocurrents in quantum well/dot infrared photodetectors with different optical coupling structures. Appl Phys Lett 2012;100:043502. [7] Ning R, Bao J, Jiao Z, Xu Y. Omnidirectional polarization-insensitive tunable absorption in graphene metamaterial of nanodisk structure. J Appl Phys 2015;118:203101. [8] Yablonovitch E, Cody GD. Intensity enhancement in textured optical sheets for solar cells. IEEE Trans Electron Dev 1982;29:300–5.
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Efficient broadband energy absorption based on inverted-pyramid photonic crystal surface and two-dimensional randomly patterned metallic reflector Zhi-Hui Chen a,⇑, Na Qiao a, Yang Wang b, Li Liang a, Yibiao Yang a, Han Ye c, Shaoding Liu a a Key Lab of Advanced Transducers and Intelligent Control System, Ministry of Education and Shanxi Province, College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, China b Key Laboratory of Chemical Biology and Molecular Engineering of Ministry of Education, Institute of Biotechnology, Shanxi University, Taiyuan 030006, China c State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing 100876, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Absorption is enhanced in a
broadband (0.3–9.9 lm) in a novel structure. Absorption is angle-insensitive and polarization-insensitive. Large thickness and period are favored to enhance a broadband absorption. Our structure is experimentally feasible and versatile for applied energy.
a r t i c l e
i n f o
Article history: Received 24 August 2015 Received in revised form 24 March 2016 Accepted 26 March 2016
Keywords: Broadband energy absorption Photonic crystal surface Randomly patterned metallic reflector
a b s t r a c t We propose a hybrid structure containing photonic crystal surface with inverted-pyramid arrays on the top, two-dimensional (2D) patterned metallic reflector with random pyramids’ sizes at the bottom, and a Si film between the two layers, to achieve high-efficiency, broad-band and wide-angle energy absorption. Due to the scattering effect and waveguide mode resonance effect of the structure, the enhanced absorption is close to or even surpasses the Yablonovitch limit in a broad wavelength range (0.3–9.9 lm). What’s more, the high absorption efficiencies are insensitive to the variation of incidence angle (from 0° to 80°) and substantial electric field modes concentrate in the inverted-pyramid arrays and Si film. Comparing to un-patterned film and one-dimensional (1D) patterned film, 2D-patterned film obtains higher absorption efficiency for both s polarization and p polarization. Furthermore, we find that the high absorption band expands to a broader wavelength range with the size of structure increasing. Our multifunctional structure is experimentally feasible and is expected to have a wide application in the areas of energy harvesting, energy conversion, energy conservation and sustainable energy utilization. Ó 2016 Elsevier Ltd. All rights reserved.
⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (Z.-H. Chen). http://dx.doi.org/10.1016/j.apenergy.2016.03.098 0306-2619/Ó 2016 Elsevier Ltd. All rights reserved.
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1. Introduction High performance broadband light absorbers have drawn considerable interest since they show remarkable potential in a wide range of applications in energy harvesting and conversion, for example, the high-efficient broadband light absorption can enhance the photocurrent of photovoltaic (PV) cells [1–3], concentrate solar thermal energy [1], be integrated in a PV-thermal system [4], improve the sensitivity of imaging and sensing [5,6], and realize the near infrared stealth [7], etc. The Yablonovitch limit of absorption is applicable to bulk structures [8], but the absorption efficiency of subwavelength photonic structures with proper interface design could easily overcome this limit. In the past few decades, various subwavelength optical coupling structures, such as nanoparticle imprinted patterns [9], hybrid photonic crystal (PC) structures [10,11] plasmonics nanoparticle [12], metallic gratings [13], nanocone gratings [14] had been proposed aiming to obtain high broadband absorption based on the high-loss materials and the mechanisms of creating scattering, multiple resonance modes, surface plasmon polariton. However, most of these structures acted on either visible wavelength range or infrared wavelength range, the realized high absorption band is not very broad, and the technological process is complicated and expensive. In the present study, a novel hybrid structure is proposed for the first time to simultaneously realize large absorption in both visible and infrared wavelength ranges (0.3–9.9 lm) for both s polarization (with nonzero Ey) and p polarization (with nonzero Ex). The hybrid structure is composed of a Si film, a PC surface with inverted-pyramid arrays on the top of Si (polymer or colloidal quantum dots (QDs) filled in the PC surface) and twodimensional (2D) randomly patterned metallic reflector at the bottom of Si. Our strategy is to use the PC surface (filled invertedpyramid arrays) as an antireflection coating on the top, and use the PC surface together with the 2D patterned metallic reflector to enhance light scattering and light trapping. On the basis of the strategy, optical path length in the Si film is significantly increased and many waveguide mode resonances are formed, thus the lightmatter interaction and optical absorption will be enhanced eventually. Comparing to our previous work [10], high absorption efficiency is achieved for both s polarization and p polarization by using 2D-patterned film in this work. Additionally, the effects of structure size on broadband (0.3–9.9 lm) absorption were studied. It is demonstrated that our structure could provide high-efficiency, broad-band, angle-insensitive and polarization-insensitive light absorption within the wavelength range from 0.3 lm to 9.9 lm, and the high absorption band expands to a broader wavelength range with the size of structure increasing.
mal) (n = 1.5) as an example. The gold material is chosen due to its high reflection and absorption at the wavelength of 0.3– 9.9 lm. The width of gold pyramid is in a certain proportion to the unit size (T), and the length of gold pyramid equals to T in x or y direction (detailed structure is shown in Fig. S1 in the Supplementary material). The unit size (T) was set as 1.8 lm initially to study the wavelength range from 0.3 lm to 2.0 lm. The thickness of Si (L) is 2 lm, and the thickness of gold substrate (t) is 100 nm which is thick enough to reflect the light in the wavelength range from 0.3 lm to 9.9 lm. In real fabrication, the inverted-pyramid textures on two sides of the Si film can be fabricated by irradiating the surface with a nanosecond pulsed laser followed by alkali etching [5] or by other various etching techniques [15], and then the polymer and gold film can be deposited on the corresponding patterned sides, respectively. 2.2. Rigorous coupled wave analysis (RCWA) The RCWA method is a useful method to calculate electromagnetic wave propagation in periodic structures. In this work, all numerical simulations are performed by using RCWA [5,16] method in one unit cell of structure. The three dimensional space domain is (x, y, z) = (T/2:T/2, T/2:T/2, Z:+Z) lm, where the Z value changes with T. The wavelength range addressed within our work ranges from 0.3 lm to 9.9 lm. The electric field distributions are got from the monitor in the middle of one unit. The complex refractive indices n and k (representing the real and imaginary parts of refractive indices respectively) of Si and gold (Fig. 1(b)) are dependent on wavelength, which are obtained from Palik’s book [17]. Since the light source is usually very far from the light absorbing devices in real applications, e.g., solar cell, infrared photodetectors, etc., the light source could be considered as a plane wave source when it is arriving at the devices, and the propagation direction of incident light is along the z axis. The wavelength of incoming radiation is a fixed wavelength for every simulation, and the wavelength is scanned from 0.3 lm to 9.9 lm in this work. In every simulation, a plane wave is injected from the top of the structure, the distance between the plane wave source and the top of structure is at least two times of the target wavelength. It can be considered that the source is infinitely far away from the sample and the incoming beam has equal intensity across the studied surface. In the RCWA method, the total transmission and total reflection of light can be calculated, thus the total absorption can be computed according to the law of energy conservation. In the following study, the term ‘‘absorption efficiency” means ‘‘normalized absorbed power”. The absorption efficiencies of s and p polarization are studied when T, L and incidence angle change. 3. Results and discussion
2. Model and method 3.1. Effects of unit size and incidence angle on absorption efficiency 2.1. Proposed structure Fig. 1(a) shows the three-dimensional schematic of one unit of our proposed structure. In this work, the calculations are performed within one unit cell. The inverted right square pyramid arrays on the top are filled by low refractive index (n) polymer, eight inverted-pyramids with random sizes in x and y directions are used in one period to represent the quasi-random distribution of 2D patterned metallic reflector at the bottom, and the Si film is between the two layers. The low refractive index polymer is added to form a gradual variation of the refractive index between air (n = 1.0) and Si (n > 3.4), which could reduce the optical reflection from the air–Si interface. The refractive index varies from 1.3 to 1.7 for different kinds of polymer, we chose PVFM (Polyvinyl for-
In order to optimize the unit size (T) of our structure, the effect of T on absorption efficiency is studied for both p and s polarization when the other parameters are unchanged. Here, light is injected in the normal direction, and the wavelength range from 0.3 lm to 2.0 lm is studied. According to Fig. 2(a) and (b), when light is injected to our structure in the normal direction, either for p or s polarization, high absorption efficiency can be obtained at the wavelength of 0.3– 2.0 lm as T is changing from 1.25 lm to 3.0 lm. The absorption efficiencies reach almost 100% for shorter wavelength (0.3– 0.9 lm). The absorption efficiencies for longer wavelength (0.9– 2.0 lm) are lower than that for shorter wavelength, but the average absorption efficiency for longer wavelength is still above
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Fig. 1. (a) Schematic of one unit of three-dimensional structure. (b) Complex refractive indices of Si and Au with n and k representing the real and imaginary parts, respectively.
Fig. 2. The absorption efficiencies within the wavelength range from 0.3 lm to 2.0 lm for p and s polarization under normal incidence condition. (a and b) T changes from 0.5 lm to 3.0 lm. (c and d) Incidence angle changes from 0° to 80° when T = 1.8 lm.
70%. We expect that the main reason for the above results is that there are more high-order waveguide modes [18–21] for short wavelength than those for longer wavelength when the thickness of film is fixed. Then, we take T = 1.8 lm as an example to investigate the angular performance of our structure. Fig. 2(c) and (d) shows that high absorption efficiencies are insensitive to the variation of incidence angle (from 0° to 80°) for both p and s polarization when wavelength changes from 0.3 lm to 2.0 lm. Since the radiation from sun at sea level is mainly in the wavelength range of 0.3–2.0 lm, our structure could capture most of the injected sun radiation with any incidence angle and polarization. The light absorption by Si material can be converted to electric energy and thermal energy. In details, when the sunlight is injected upon Si material, the electrons present in the valence band will absorb solar energy and be excited to the conduction band, then the free electrons can be extracted to an external circuit, thus electric energy is generated. Meanwhile, the rest solar energy will be transferred to the vibration of Si lattice, thus thermal energy is gener-
ated. Moreover, gold material has good thermal conductivity. Therefore, high light absorption of our structure can increase the photocurrent of Si solar cells, and the gold substrate of our structure can increase the heat dissipation of solar cells.
3.2. Effect of 2D patterned structure on absorption efficiency As shown in Fig. 2(a) and (b), when T is larger than 1.25 lm, the average absorbed power of our structure exceeds 70% of the initial injected power within the wavelength range from 0.3 lm to 2.0 lm. Then, we chose T = 1.8 lm as an example to study the effect of 2D patterned structure on absorption efficiency. We compare the Yablonovitch limit [10,14] (AYablonovitch = 1–1/(1 + 4n2aL), ‘‘n” is the real part of the refractive index of Si, ‘‘a” is the absorption coefficient, ‘‘L” is the thickness of Si film.) with the absorption efficiencies of s and p polarizations incidence for the structures in Fig. 3(a) and (c) under normal incidence condition. In Fig. 3(a), the structure is an un-patterned flat Si film. In Fig. 3(b), one-
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Fig. 3. (a–c) Three-dimensional structure in one unit: (a) Un-patterned film. (b) 1D-patterened film: V-grooves and gold reflector with random-size grooves in y direction are introduced at the top and bottom of Si film, respectively. (c) 2D-patterened film: inverted-pyramid arrays and gold reflector with random-size pyramids in both x and y directions are introduced at the top and bottom of Si film, respectively. (d and e) Yablonovitch limit (green short dash line) and absorption efficiencies for s and p polarization under normal incidence condition in the structures of (a–c) when k changes from 0.3 lm to 2.0 lm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
dimensional (1D) periodic V-grooves and gold reflector with random-size grooves in y direction are introduced at the top and bottom of Si film, respectively. In Fig. 3(c), inverted-pyramid arrays and gold reflector with random-size pyramids in both x and y directions are introduced at the top and bottom of Si film, respectively. For the structures in Fig. 3(b) and (c), the top grooves and inverted-pyramid arrays are filled by polymer (n = 1.5). Fig. 3(d) shows that the absorption efficiencies of 2D-patterned film are higher than both 1D-patterned film and un-patterned film for s polarization, because electric field for s polarization is along the y axis, the refractive indices of structures in Fig. 3(a) and (b) are invariant in y direction so that light cannot be scattered effectively for s polarization, while the structure in Fig. 3(c) is patterned in both x and y directions, which can enhance the light scattering and absorption for both p polarization and s polarization. When the structure is un-patterned, there are Fabry-Pérot cavity modes between the top surface and gold reflector [22]. These modes lead to more absorption by the gold material at infrared wavelengths. When the structure is 1D patterned, there are some individual guided resonance modes for s polarization between the surface 1D PC and 1D patterned metallic reflector, which introduce some absorption peaks at infrared wavelengths. Fig. 3(e) shows that the absorption efficiencies of both 2D-patterned and 1Dpatterned film are higher than that of un-patterned film for p polarization. The enhanced absorption can be close to or even surpass the Yablonovitch limit in the wavelength range from 0.3 lm to 2.0 lm. In our previous work [10], it is reported that the surface 1D PC and 1D patterned metallic reflector worked together on visible and infrared light absorption for only TM polarization. The waveguide modes between the PC and gold reflector led to large absorption by the Si and gold material. In this work, the absorption enhancement is achieved for both s polarization and p polarization by using 2D-patterned film due to the reduction of incidence reflection from the air–Si interface and the light trapping by
increasing optical path lengths and creating waveguide modes in the film. Besides, our structure is robust for energy application because the metallic reflector was randomly patterned and it is not sensitive to the small perturbation of structure parameters in the real fabrications. This also reduces the difficulty of fabrication and facilitates the large-area fabrication for its application in buildings. 3.3. Effects of T and L on broadband (0.3–9.9lm) absorption We take p polarization under normal incidence condition as an example to study the effects of T and L on broadband (0.3–9.9 lm)
Fig. 4. Yablonovitch limit and p polarization light absorption efficiencies of three 2D patterned structures with different T and L values, when k changes from 0.3 lm to 9.9 lm under normal incidence condition. The green short dash line: Yablonovitch limit for L = 2 lm. The blue line: Yablonovitch limit for L = 10 lm. The yellow line: T = L = 3 lm. The black short dash line: T = L = 6 lm. The pink line: T = L = 10 lm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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absorption. Fig. 4 shows that the high absorption wavelength range depends on both T and L. There are more waveguide modes in short wavelength range than long wavelength range for fixed T and L, and there are more long-wavelength waveguide modes for large T and L than small T and L. The waveguide mode resonance can increase the light-mater interaction in the film, which enhance the light absorption. So with the increase of T and L, the high absorption band for our structure will expand to a broader wavelength range and absorption efficiencies will increase at longer wavelength, while there is no obvious change of absorption efficiencies at shorter wavelength (0.3–2.0 lm). When the values of T and L vary, the drop point of absorption is at around the wavelength which is close to the value of T and L. When T = L = 10 lm, the enhanced absorption is close to the Yablonovitch limit at short wavelength (0.3–0.9 lm) and remains high in a broad wavelength range (0.3–9.9 lm). In order to further understand the relationship between high absorption bandwidth and the values of T and L, we choose T = 1.8 lm and T = 10 lm to investigate the effect of L on absorption efficiency for both s and p polarization, as shown in Fig. 5. Here, light is injected in the normal direction, and the studied wavelength range is from 0.3 lm to 9.9 lm. We find that the high absorption (above 60%) wavelength range is mainly related to T, but not closely related to L. For short wavelength (0.3–0.9 lm), there are many guided modes for both small T (T = 1.8 lm) and large T (T = 10 lm), so the light absorption is high and vary slightly for different T. However, for long wavelength (0.9–9.9 lm), there are less guided modes for small T (T = 1.8 lm) than large T (T = 10 lm), which leads to the obvious difference of absorption for different T. In addition, the high absorption can cover a broad wavelength range (0.3–9.9 lm) for T = 10 lm. These results agree well with those in Fig. 4. Generally, periodic micro/nano structures play a main role in the light absorption enhancement when the target wavelengths are in a narrow band, while random micro/nano structures show superior performance when the target wavelengths are in a broadband range. In previous work, the optimal periodicity (T) for light trapping in the structure with periodic patterned surface is close to the target wavelength [14]. However, in this work, the smallest periodicity (T) for the achievement of broadband light absorption is equal to the target absorption band-
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width and the high absorption band expands to a broader wavelength range with the size of structure increasing. 3.4. Spatial distributions of the electric field In order to further illustrate the reason of light absorption enhancement, the steady state electric field distributions in our structure are studied when T and L are equal to 10 lm. The spatial electric fields (E2 = Ex2 + Ey2 + Ez2) were calculated by the RCWA method. By analyzing the electric field distributions (Fig. 6), we find that substantial electric field modes concentrate in the inverted-pyramid arrays and Si film. Moreover, short-wavelength guided modes primarily exist near the PC surface area, while the long-wavelength guided modes are distributed in both PC surface area and Si film. It indicates that the PC surface primarily works on visible wavelength range, and the patterned metallic reflector mainly works on infrared bands because the Si film has small absorption in the infrared wavelength range. Furthermore, since the metallic reflector is 2D randomly patterned, the field distributions at y–z section will be a little different, but this will not affect our conclusion. The inverted-pyramid arrays on the surface can be filled by colloidal quantum dots (QDs) film to further enhance the total light absorption of our structure [23]. QDs are semiconductor particles with the sizes in nanometer scale (below the Exciton Bohr radius). Due to quantum confinement effect, the energy levels of QD split up, thus QD has similar discontinuous electronic energy levels as those of atomic structure. Since the bandgaps (i.e., energy difference between the top of the valence band and the bottom of the conduction band) of QDs are tunable across a wide range of energy levels by changing the size of QD (the bandgap of QD increases with the size of QD decreasing), broadband solar energy can be absorbed by using different sizes of QDs in solar cells. This property makes QDs good at improving the photovoltaic efficiency of solar cells. Since the light absorption by both QDs and Si materials can be converted to photocurrents, a QD–Si hybrid solar cell with high energy utilization efficiency could be formed by using our structure. In the experiment, surface recombination states over QDs should be taken into account. In order to meliorate the performance of the photonic devices, the fluorescence quantum yields
Fig. 5. The absorption efficiencies of s and p polarization light in 2D patterned structures within the wavelength range from 0.3 lm to 9.9 lm. (a and b) are the absorption efficiencies for T = 1.8 lm when L changes from 1.5 lm to 10 lm. (c and d) are the absorption efficiencies for T = 10 lm when L changes from 6.5 lm to 10 lm.
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Fig. 6. Electric field distributions (k1 = 0.4 lm, k2 = 1.2 lm, k3 = 3.0 lm and k2 = 8.0 lm) in one unit cell (x–z section in the center of one unit) of the structure with both T and L expanding to 10 lm under normal incidence. (a–d) and (a0 –d0 ) are for p polarization and s polarization, respectively.
of QD need to be improved by passivating QD with coated shells (e.g., ZnS layers) for suppressing the surface recombination states over QD [24,25]. These good properties above make our structure versatile in the areas of energy conversion, energy conservation and sustainable energy utilization. For example, our structure could be used in the building integrated photovoltaic thermal (BIPVT) system [26,27] for improving energy utilization efficiency (Fig. 7). The visible-light absorption by Si material and QDs can be converted to electric energy and thermal energy as previously mentioned in Sections 3.1 and 3.4, and the broadband light absorption by gold material can be converted to thermal energy. Then, the thermal energy above can be transferred to air/water flowing through the gold film. Thus, our structure based BIPVT system can simultaneously provide electric and thermal energy for the buildings. The advantage of our structure in BIPVT system is that our structure with high-efficiency, broadband, angle-insensitive and polarization-insensitive absorption can not only absorb visible light and near-infrared radiation from sun at sea level but also absorb the mid-infrared radiation from nearby industry, animals, etc., which could make our structure based BIPVT system provide
more thermal energy for the buildings than the common BIPVT system. 4. Conclusion In this work, we have proposed an easy-processing and robust photonic structure which comprises a Si film sandwiched by a photonic crystal (PC) surface on the top and a 2D patterned metallic reflector at the bottom. Colloidal quantum dots (QDs) can be filled in the inverted-pyramid arrays of the PC surface. Based on a systematic study of the structure, it is demonstrated that our structure could provide high-efficiency, broad-band, angle-insensitive and polarization-insensitive light absorption within the wavelength range from 0.3 lm to 9.9 lm, and the high absorption band expands to a broader wavelength range with the size of structure increasing. Such good properties make our structure versatile in the areas of energy conversion, energy conservation and sustainable energy utilization. For example, our structure is useful in building integrated photovoltaic thermal (BIPVT) system. The advantages of our structure are listed as following: (1) Our structure can simultaneously provide electric and thermal energy for buildings because it can absorb broadband radiation including visible light and broadband infrared radiation; (2) our structure can provide a framework for a QD–Si hybrid solar cell with high energy utilization efficiency, since the light absorption by both QDs and Si materials can be converted to photocurrents; (3) our structure is useful in increasing the heat dissipation of solar cells because gold material has good thermal conductivity; (4) our structure is robust for energy application because the metallic reflector was randomly patterned and it is not sensitive to the small perturbation of structure parameters in the real fabrications. This also reduces the difficulty of fabrication and facilitates the large-area fabrication for its application in buildings. The work will be interesting for the researchers and engineers working on the field of applied energy, which can bridge the gaps between research, development and implementation. Acknowledgments
Fig. 7. Conceptual framework for applying our photonic structure in BIPVT systems.
This work was supported by the National Natural Science Foundation of China (61307069, 61575139, 61575138, 61401035,
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61505135), the Natural Science Foundation of Shanxi Province, China (2013021017-3, 2015021013), the Specialized Research Fund for the Doctoral Program of Higher Education, China (20131402120018), the Program for the Top Young Academic Leaders and the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi Province, China, the Excellent Young Scientist Foundation and Qualified Personnel Foundation of Taiyuan University of Technology, China (2014YQ012, tyut-rc201203b), the Open Fund of IPOC (BUPT), China (IPOC2013A001), the Open Fund of Top priority disciplines in colleges and universities in Zhejiang Province, China (2015KF20). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apenergy.2016. 03.098. References [1] Yan J, Chou S-K, Desideri U, Lee D-J. Transition of clean energy systems and technologies towards a sustainable future (Part I). Appl Energy 2015;160:619–22. [2] Brongersma ML, Cui Y, Fan S. Light management for photovoltaics using highindex nanostructures. Nat Mater 2014;13:451–60. [3] De Rossi F, Pontecorvo T, Brown TM. Characterization of photovoltaic devices for indoor light harvesting and customization of flexible dye solar cells to deliver superior efficiency under artificial lighting. Appl Energy 2015;156:413–22. [4] Mojiri A, Stanley C, Rodriguez-Sanchez D, Everett V, Blakers A, Rosengarten G. A spectral-splitting PV–thermal volumetric solar receiver. Appl Energy 2016;169:63–71. [5] Chen Z-H, Wang Y, Yang YB, Qiao N, Wang YC, Yu ZY. Enhanced normaldirection excitation and emission of dual-emitting quantum dots on a cascaded photonic crystal surface. Nanoscale 2014;6:14708–15. [6] Chen Z-H, Hellström S, Yu Z-Y, Qiu M, Fu Y. Time-resolved photocurrents in quantum well/dot infrared photodetectors with different optical coupling structures. Appl Phys Lett 2012;100:043502. [7] Ning R, Bao J, Jiao Z, Xu Y. Omnidirectional polarization-insensitive tunable absorption in graphene metamaterial of nanodisk structure. J Appl Phys 2015;118:203101. [8] Yablonovitch E, Cody GD. Intensity enhancement in textured optical sheets for solar cells. IEEE Trans Electron Dev 1982;29:300–5.
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