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Solar Energy Materials & Solar Cells 91 (2007) 1599–1610 www.elsevier.com/locate/solmat
Improving solar cell efficiency using photonic band-gap materials Marian Florescua,b,, Hwang Leea, Irina Puscasuc, Martin Prallec, Lucia Florescua,b, David Z. Tingb, Jonathan P. Dowlinga a
Department of Physics and Astronomy, Hearne Institute for Theoretical Physics, Louisiana State University, 202 Nicholson Hall, Baton Rouge, LA 70803, USA b Jet Propulsion Laboratory, California Institute of Technology, Mail Stop T1714 106, 4800 Oak Grove Drive, Pasadena, CA 91109, USA c Ion Optics Inc., 411 Waverley Oaks Rd. Suite 144, Waltham, MA 02452, USA Received 31 October 2006; received in revised form 2 May 2007; accepted 2 May 2007 Available online 29 June 2007
Abstract The potential of using photonic crystal structures for realizing highly efficient and reliable solar-cell devices is presented. We show that due their ability to modify the spectral and angular characteristics of thermal radiation, photonic crystals emerge as one of the leading candidates for frequency- and angular-selective radiating elements in thermophotovoltaic devices. We show that employing photonic crystal-based angle- and frequency-selective absorbers facilitates a strong enhancement of the conversion efficiency of solar cell devices without using concentrators. r 2007 Elsevier B.V. All rights reserved. Keywords: Photonic band-gap materials; Thermophotovoltaics; Solar cells
1. Introduction Photovoltaic (PV) solar energy conversion systems (or solar cells) are the most widely used power systems. However, these devices suffer of very low conversion efficiency. This is due to the wavelength mismatch between the narrow wavelength band associated with the semiconductor energy gap and the broad band of the (blackbody) emission curve of the Sun. The power loss is associated with both long-wavelength photons that do not have enough energy to excite electron–hole pairs across the energy gap (leading to a 24% loss in silicon, for instance) and short-wavelength photons that excite pairs with energy above the gap, which thereby waste the extra kinetic energy as heat (giving a 32% loss in silicon). The efficiency of the thermophotovoltaic (TPV) system may be increased by recycling the photons with frequency larger than the solar cell band-gap frequency, by using a spectrally dependent Corresponding author. Jet Propulsion Laboratory, California Institute of Technology, Mail Stop T1714 106, 4800 Oak Grove Drive, Pasadena, CA 91109, USA. E-mail address:
[email protected] (M. Florescu).
0927-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2007.05.001
coupling between the absorber and the cell (Fig. 1). However, any approach to solar-cell efficiency improvement that does not address this fundamental wavelengthband mismatch, can achieve at most around 30% efficiency [1]. Moreover, this can be achieved only for concentrated radiation, which requires an additional optical device, which is not desirable in applications where the mass is a critical concern. This article outlines novel approaches to the design of highly efficient solar cells using photonic band-gap (PBG) materials [2,3]. These are a new class of periodic materials that allow precise control of all electromagnetic wave properties [4–6]. A PBG occurs in a periodic dielectric or metallic media, similarly to the electronic band gap in semiconductor crystals. In the spectral range of the PBG, the electromagnetic radiation light cannot propagate. The ability to tailor the properties of the electromagnetic radiation in a prescribed manner through the engineering of the photonic dispersion relation enables the design of systems that accurately control the emission and absorption of light. This gives rises to new phenomena including the inhibition and enhancement of the spontaneous emission [3], strong localization of light [2], formation of
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Fig. 1. Schematic of a TPV energy conversion scheme. An intermediate absorber is heated by the Sun’s thermal radiation. The photovoltaic (PV) cell is illuminated by radiation from emitter transmitted by a filter.
atom–photon bound states [7], quantum interference effects in spontaneous emission [8], single atom and collective atomic switching behavior by coherent resonant pumping, and atomic inversion without fluctuations [9]. These remarkable phenomena have attracted a considerable interest for important technological applications, such as low-threshold micro-lasers [10,11], ultra-fast all-optical switches, and micro-transistors [12–14]. The modifications of the spontaneous emission rate of atoms inside the photonic crystal structure determine, in turn, important alterations of thermal radiative processes. Thermal radiation is just spontaneous emission thermally driven and in thermal equilibrium with its material surroundings. In 1999, Cornelius and Dowling suggested the use of PBG materials for the modification of thermal emission [15]. They explored two alternative approaches: a method based on a passive lossless PBG thin-film coating over the absorber, and an approach which uses an active PBG material made out of an absorptive medium. Thermal emission modification has been experimentally demonstrated in 2000, using a thin slab of 3D photonic crystal on a silicon substrate [16]. Pralle et al. demonstrated a thermally excited, narrow-band, midinfrared source using a PBG technique [17]. Recently, researchers at Sandia Labs demonstrated a high-efficiency TPV system using tungsten photonic crystals [18–20]. These studies suggest that by optimizing the coupling of the multi-mode radiation field of a PBG material and a spatially extended collection of atomic or electronic emitters, it is possible to achieve dramatic modifications of Planck’s blackbody radiation spectrum [15,21]. In the PBG spectral range the thermal emission of radiation is strongly suppressed, whereas for specific frequencies in the allowed photonic bands, that correspond to transmission resonances of the photonic crystal, the thermal
emission of radiation is resonantly enhanced up to the black-body limit. The ability of the photonic crystals to funnel the thermal radiation into a prescribed spectral range is illustrated in Fig. 2, which shows a comparison between the intensity emitted by a photonic crystal sample when electrically heated, which reaches a temperature of 420 when the electrically heated with an input power of 135 mW (black curve), and two blackbody systems, one kept at the same temperature as the photonic crystal at the expense of using a higher input power (315 mW) and a second one exposed at the same input power as the photonic crystal sample, but having a lower temperature ð273:4 Þ. We notice in the case of the photonic crystal sample that by eliminating the emission in certain frequency bands (corresponding to the spectral range of the PBG), the emission is enhanced in the spectral region corresponding to the allowed bands and, with the same input power, the photonic crystal reaches a higher temperature than a blackbody. This is solely due to the funneling of the thermal radiation from the forbidden spectral range (the orange area in Fig. 2) into the allowed spectral range (the brown area in Fig. 2). Therefore, the heated photonic crystal emitter achieves thermal equilibrium at a higher temperature than would otherwise be possible. These facts suggests the possibility to leverage the funneling properties of photonic crystals to improve the spectral coupling of an emitter into the acceptance band of a PV cell.
Fig. 2. Spectral funneling of the thermal radiation by photonic crystals. By designing a photonic band gap in prescribed frequency region of the photonic crystal emission spectrum, the structure becomes unable to radiate at these frequencies and the corresponding energy is re-radiated in the allowed spectral range. As a consequence, the intensity of the blackbody emission at these frequencies increases, and the photonic crystal emitter radiates the same power as it would a blackbody maintained at a higher temperature.
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We present a design of highly efficient solar cells using PBG materials as intermediary between the Sun and the PV cells. We predict limiting conversion efficiency of around 60%. We propose two approaches to achieve this. The first approach is to couple broadband solar radiation into a PBG material, engineered to re-emit the solar radiation into a narrow frequency band corresponding to the semiconductor energy gap. In this way, power loss due to photons of wavelength too much above or below the gap is eliminated. Another approach is intended to eliminate the roadblocks in the design of TPV systems based on nonconcentrated radiation, and makes use of a photonic crystal-based angle-selective absorber. The selective absorber has the property of absorbing only certain parts of the whole solar spectrum. If the absorber can absorb solar radiation whose frequency is above the solar cell band-gap frequency, the TPV efficiency of 45% can be achieved by using non-concentrated radiation (maximum of the dashed curve in Fig. 4). In this case, additional spectral filters are needed in front of the absorber. Here we show a specially designed photonic crystal that exhibits both angular and spectral selectivity in absorption and emission. Also, experimental studies show that the photonic crystalenhanced (PCE) infrared emitters enhance the wall plug conversion efficiency of MWIR solar cells relative to blackbody broad band sources. 1.1. Thermal emission control From the foundations of quantum mechanics, it is known that atomic oscillators in thermal equilibrium with photon heat bath at temperature T have an average energy at frequency o given by eðo; TÞ ¼
_o , expð_o=kB TÞ 1
(1)
where _ is the Dirac constant and kB is the Boltzmann constant. The energy density per unit frequency, then, can be written as uðo; TÞ ¼ rðoÞeðo; TÞ,
(2)
where rðoÞ is the electromagnetic density of modes. For free space, the density of modes has the form 2o2 , (3) pc3 such that the radiant power then takes he usual form of Planck’s law
rFS ðoÞ ¼
1 o2 _o pBB ðo; TÞ ¼ cuðo; TÞ ¼ . 3 _o=k B T1 4 2pc e
(4)
This suggests that since the density of electromagnetic modes is altered in a photonic crystal, the radiant power can also be altered. The ability of the photonic crystal to change the spectral properties of the emitted and absorbed electromagnetic radiation can be illustrated considering a photonic crystal
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coating over a many-wavelength-thick substrate. The radiation is emitted from the substrate and passes through the passive photonic crystal filter and then is emitted into vacuum. The absorbance A is given by energy conservation, namely, A þ T þ R ¼ 1, where R and T are reflectance and transmittance, respectively. The absorbance is unity if the source is a perfect blackbody. Finding the absorbance is equivalent to finding the thermal emittance E, since using Kirchhoff’s second law, the ratio of the thermal emittance to the absorbance is the same, independent of the nature of the material. Consequently, it is possible to then compute E by matrix transfer techniques [15,22]. Once E is obtained, multiplication by the Planck power spectrum gives the power spectrum of the PBG emitter pTH ðo; TÞ in terms of the emittance EðoÞ and blackbody spectrum pBB ðo; TÞ, as given by pTH ðo; TÞ ¼ EðoÞpBB ðo; TÞ.
(5)
Therefore, the thermal radiant power in a photonic crystal can be controlled by altering the thermal emittance. 2. Photonic crystal-based solar TPV: concepts and designs 2.1. TPV conversion efficiency The conversion efficiency of a TPV solar system is determined by both the absorption efficiency of the intermediate absorber and the cell conversion efficiency. Let us first examine the absorption efficiency of the intermediate absorber. The incident power density is related to the spectral power density defined as Z PS ¼ do_obS ðoÞ, (6) where FS o2 (7) 4p3 c2 e_o=kB T S 1 is the spectral photon flux. Here, T S is the temperature of the Sun and F S is a geometric factor, equal to 2:16 105 p for non-concentrated light (determined by the radius of the Sun and the distance between the Sun and the Earth), and p for the full concentration. This leads to the Stefan– Boltzmann’s law FA PS ¼ (8) sT 4S . p bS ðoÞ ¼
The intermediate absorber loses its energy by emitting radiation with the rate ½F A =psT 4A , where the geometric factor F A is equal to p since the absorber emits in all directions. Hence the net gain of the absorber is FS FA Pnet ¼ (9) sT 4S sT 4A . p p In what regards the cell conversion efficiency, assuming that the spectral filter allows only the radiation with frequencies bigger than the gap frequency oG , and the
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recombination loss is all radiative, the open-circuit assumption may be used for estimation of the maximum conversion efficiency. Under this assumption, we have _o _o Dm ¼ , kB T A kB T C
(10)
from the generalized Planck’s law [23]. Here, T C is the temperature of the cell and Dm is the chemical potential. The efficiency of an electron–hole pair to generate electrical energy is then given by the chemical potential divided by the photon energy as Dm=_o ¼ 1 ½T C =T A , which is the Carnot efficiency. The actual working situation is a slight deviation from the open-circuit condition, such that this expression of efficiency still holds (Fig. 3). Combining the two contributions, we have the efficiency of the TPV conversion system as ! F A T 4A TC 1 ZPV ¼ 1 . (11) F S T 4S TA 2.2. Non-concentrated radiation: frequency- and angleselective absorber An increased efficiency of a TPV system may be obtained mainly by recycling of photons of frequency larger than the solar cell band-gap frequency by using a spectral filter between the absorber and the cell. The combined system of the absorber and the filter can be called a selective emitter. However, such a high efficiency can be achieved only for concentrated radiation, which requires additional optical devices, not desirable for instance for space applications, where mass is of critical concern. In order to design a high-efficiency TPV system using non-concentrated radiation, we have introduced an
Fig. 3. Schematic of the photonic crystal-based TPV energy conversion. An intermediate absorber is heated by absorbing thermal radiation. The photovoltaic (PV) cell is illuminated by radiation from emitter transmitted by a filter.
angle- and frequency-selective absorber. The selective absorber has the property of absorbing only certain parts of the whole solar spectrum. If the absorber can absorb solar radiation of frequency above the solar cell band-gap frequency, a TPV efficiency of 45% can be achieved [1] (the maximum of the dashed curve in Fig. 4). Again, additional spectral filters are needed in front of the absorber. We show that a suitably designed photonic crystal can be used as a selective emitter as well as a selective absorber. If, for example, we match the band-edge frequency of the photonic crystal to the semiconductor band-gap frequency, it is possible to suppress both emission and absorption of photons of frequency below that of the semiconductor band-gap. Consequently, the photonic crystal sample plays simultaneously the role of a selective emitter (with respect with the cell) and a selective absorber (with respect to the Sun). In addition to the frequency-selectivity, thermal emission of the photonic crystal has angular selectivity as well. The control over the angular distribution of the emitted radiation can be extremely for the overall efficiency of the TPV system. If the solid angle of the emission at the Sun side can be made very small, it is possible to achieve the same enhancement of the solar cell efficiency as in devices using concentrators. In other words, just by engineering the emission solid angle, the energy conversion efficiency can be increased without using concentrators. The radiation concentration in Eq. (11) is mathematically described by the increase of the Sun’s geometric factor F S . However, a decrease of the absorber’s geometrical factor F A leads to the same effect. Physically, the decrease of the absorber’s F A implies that the emission and absorption of radiation is confined to a certain range of directions. Fig. 4 shows the TPV efficiency as a function of the absorber temperature assuming F A =F S ¼ 100 (solid curve) and F A =F S ¼ 1000 (dashed curve). The TPV efficiency for 100 reaches up to 68% at about 727 C and 44% at 427 C. Such a narrowing of absorption angle can be realized by exploiting the absorption anisotropy of the photonic crystal. As an illustrative example we consider an inverted opal photonic crystal consisting of FCC structure of air spheres in a solid background of silicon. Inverted opal photonic crystals are ideal for high-quality, large-scale fabrication of PBG materials with band gaps at micron and sub-micron wavelengths [24]. In an optimal configuration, such as the one presented in Fig. 5, the PBG can be as large as almost 10% of the central frequency. Experimentally, an artificial inverted opal can be created starting with monodisperse silica spheres with a diameter around 870 nm. These spheres form a closed-packed FCC lattice by a process of sedimentation in an aqueous solution of ethylene glycol. In the second stage, silicon is grown inside the voids of the opal template by means of chemical vapor deposition (CVD) using disilane (Si2H6) gas as a precursor. After disilane is deposited uniformly in the voids, the crystal is heated to 600 C in order to improve the silicon
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Fig. 4. TPV conversion efficiency as a function of the absorber temperature. The cell temperature is assumed to be 27 C. Solid line is for F A =F S ¼ 100, and the dashed line is for F A =F S ¼ 1000.
pffiffi Fig. 5. A close-packed inverted opal structure ðr=a ¼ ð2Þ=4Þ viewed as sequence of yABCABC yplanes grown along the ½1 1 1 direction. In each plane, the low-dielectric constant spheres (here, for simplicity, we assume air spheres) are embedded in a high-dielectric constant host medium pand ffiffiffi are sitting on a triangular lattice of lattice constant axy ¼ a= 2.
crystallization and allow the diffusion of silicon throughout the sample. Finally, the silica template is removed using controlled fluoride-based etching designed to avoid affecting the silicon backbone, and leaving behind a closedpacked FCC lattice of air spheres in a silicon background [24]. Photonic crystals are usually characterized by the dispersion relation (or band structure, representing the relationship between the frequency and the wave-vector) and the photonic density of states (the number of available electromagnetic modes at a specific frequency and at a
location within the photonic crystal) (see Fig. 6), which can be employed to infer their radiative response. In Fig. 7 we plot the angular dependence of the absorption for a fully infiltrated inverted opal structure shown in Fig. 5. Each plot corresponds to different incident angles for a fixed azimuthal angle. Figs. 8 and 9 are the enlarged portions of Fig. 7 at the lower band-edge of the first stop band around oa=2pc ¼ 0:5 and at the higher band-edge of the second stop band around oa=2pc ¼ 0:8, respectively. The absorption is enhanced at the band-edge location. However, in the presence of absorption, as the incident angle changes, the band edge is not so well defined anymore. For some directions there are some tails that enter the gap and the position of the band edge frequency depends on the specific direction. As a result the absorption is enhanced considerably, for a given spectral range, for specific incident directions. This fact opens a novel way to make an angular-selective PBG absorber, which, in turn, enables high-efficiency solar energy conversion without using concentrators. The increase in the energy conversion efficiency is determined by the small angle of thermal emission of the intermediate absorber on the Sun side. For the intermediate absorber, the gain comes from the absorption of solar radiation and the loss is determined by the emission by the absorber. As we decrease the solid angle of the emission by the absorber, we can decrease the loss due to the emission by the absorber, and effectively increase the net gain. The so-called minimal emission refers to a situation where the solid angle of the emission (F A ) is the same as the solid angle extended by the Sun (F S ). Such an angular selective
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Fig. 6. Band structure and corresponding DOS for a close-packed FCC lattice of air spheres in silicon ðSi ¼ 11:9Þ [22].
Fig. 7. Absorption spectra for the photonic crystal of a fully infiltrated inverted opal structure with different incident angles. The position of the band edge moves as the direction changes. More detailed view is depicted in Figs. 8 and 9 for the lower band-edge of the first stop band around oa=ð2pcÞ ¼ 0:5 and the higher band-edge of the second stop band around oa=ð2pcÞ ¼ 0:8, respectively.
absorber, due to the decrease of the radiation loss, gets much hotter than in the conventional case. A larger value of temperature difference between the absorber and the PV cell becomes available, leading to higher Carnot efficiency. For a small solid angle for absorption such that F A =F S ¼ 100, the theoretical limit of TPV conversion efficiency becomes 68% at about 727 1C (see Fig. 4). However, we can see from Fig. 8 that there are absorption peaks at different frequencies. Hence, absorption is enhanced at a different angle of incidence for a different frequency. In other words, the angular selectivity is
restricted to a small frequency range. This effect might be useful when applied in the tandem cell configuration. In realizing an angular selective absorber, the angular selectivity should cover a wide range of frequencies. Otherwise, the absorber emits outside of the desired emission cone with different frequencies. 2.3. Wide-band angular-selective absorber The best possible absorber would absorb radiation at all frequencies, but only inside the solid angle subtended by
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Fig. 8. Absorption spectra at the lower band-edge of the first stop band around from Fig. 7.
Fig. 9. Absorption spectra at the higher band-edge of the first stop band around from Fig. 7.
the Sun. Wide-band angular-selectivity can be realized by designing a photonic crystal absorber such that the absorption is suppressed at all frequencies for all angles except inside the desired cone. In other words, all the frequencies are lying inside the PBG except for one preferred direction. The effect of such an absorber is equivalent to using fully concentrated radiation. Since the angular selectivity requires a sharp cutoff in the emission solid angle, a one-dimensional defect photonic crystal
structure embedded in the above discussed photonic crystal is an excellent candidate for a wide-band angular-selective PBG absorber. We considered a structure with a complete three-dimensional band gap with one-dimensional absorption characteristics. Such a property can be realized with a 3D–2D–3D [25,26] photonic crystal heterostructure as the one depicted in Fig. 10, where a waveguide channel is built in a 2D photonic crystal as a defect mode. The 2D photonic crystal is, in turn, embedded in a 3D photonic
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Fig. 10. Design of a PBG wide-band angular selective absorber. The micro-structure consists of a waveguide channel in a 2D photonic crystal, which is embedded in a 3D photonic crystal. The 1D waveguide is generated by removing one row of rods in the longitudinal direction. The 3D photonic crystal is assumed to be a woodpile structure [28] that presents a photonic band gap of about 20% of the mid-gap frequency. In this example, the 2D photonic crystal consists of square rods of width a2D =a ¼ 0:3. The width and the height of the stacking rods in the woodpile structure are a3D =a ¼ 0:25 and h3D =a ¼ 0:3, respectively, where a is the dielectric lattice constant of the embedding 3D photonic crystal [25,26].
1D defect in the 3D PBG can support a single waveguide mode, which experiences a sharp cutoff in the gap of a 3D photonic crystal, as shown in Fig. 11. In this case, the subgap generated by the waveguide channel has a true onedimensional character, since there is only one direction available for wave propagation. The sharp cut-off of the guided mode at the Brillouin zone boundary gives rise to a low-group velocity do=dk ! 0, which combined with the one-dimensional character of the system leads to a divergent density of states (DOS): rðoÞ / dk=do ! 1. For an infinite structure, there is a physical square-root singularity in the photonic DOS near the cutoff of the waveguide modes [27]. For a finite structure, the divergence is removed by the finite-size effects, but the strong variation with frequency of the photonic DOS remains. The dispersion relation for the PBG hetero-structure described in Fig. 10 presented in Fig. 11 indicates that unidirectional light absorption can be achieved for a relatively broad spectral range. Thus, the photonic crystal heterostructure will operate as a frequency- and angleselective absorber. Furthermore, we envisage a hybrid scheme for the intermediate absorber as depicted in Fig. 12. It consists of a 3D–2D–3D photonic crystal architecture acting as a frequency and angle-selective absorber on the Sun side, and a 3D photonic crystal structure acting as a frequencyselective emitter on the cell side. The absorber and the emitter systems are in thermal contact and reach thermal equilibrium. We note that the absorber and emitter systems are macroscopic objects that exchange energy not only by radiative means, but also through direct thermal contact (exchange of vibrational excitations or phonons). As a result of the thermal contact, the photonic-crystal emitter reaches the same temperature as the absorber, and the thermal energy absorbed is subsequently funneled into a narrow spectral range by the PBG emitter. We also point out, that due to the angle-selective character of the absorber system, the thermal equilibrium temperature of the whole device it will be much higher than the one of a conventional absorber system. 3. Experimental demonstration of PCE infrared emitters for efficient TPV applications
Fig. 11. Schematic dispersion relation of the PBG hetero-structure described in Fig. 10 for propagation along the waveguide direction (w ¼ o=2pc). By removing one row of rods, the linear defect supports a single waveguide mode. By appropriately choosing unit cell size, the mode will experience a sharp cutoff in the spectral region around the desired frequency [25].
crystal. The electromagnetic field is confined vertically by the 3D structure and in-plane by the stop gap of the 2D photonic crystal. By tuning the characteristics of the microstructure (geometry and index of refraction contrast), the
The development of a solar-cell device based on the frequency and angular control of the emission and absorption of thermal radiation in a photonic crystal is a complex and laborious process. Such a device will incorporate all the advances in current solar-cell technology, and then selectively replace components in the solarcell architecture by their photonic crystal engineered counterparts. Similar to the conventional solar cell design, the incident solar power will be absorbed by an absorber system. As we have shown in the previous section, the absorber system may consist of a photonic crystal architecture, engineered such that it has an enhanced absorption coefficient along the direction of the incident
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Fig. 12. A hybrid scheme for the intermediate absorber in TPV solar energy conversion. The inverted opal structure is used for the frequency-selective emitter on the PV cell side. On the other hand, the 1D waveguide in a 3D woodpile structure provides the angular-selective absorber.
solar radiation. The emitter system consists of a different photonic crystal architecture, engineered such that it presents an enhanced emission coefficient for a spectral range that matches the photocell semiconductor band gap. In our experimental study, we have focused only on a specific problem: the improvement of the emitter efficiency using a photonic crystal-based emitter system. In order to simplify the analysis, we have used a low-temperature TPV energy conversion experimental set-up. In TPV, an incandescent radiator illuminates a PV cell that converts radiant heat to electrical energy. These systems are an attractive long-term power source since, in principle, they can achieve conversion efficiencies considerably higher than the 6–8% capabilities of current thermoelectric generators. The crucial problem for this technology is matching the emission spectrum of the radiator to the band gap of the photocell. Approaches to this problem include radiators with strong (ionic) emission lines and reflective filters between the radiator and photocell. In this work we explore an alternate radiator concept a PCE narrow band incandescent emitter tuned for peak emission near the band gap of the photocell. Our results show the feasibility of fabricating rugged emitters with tunable wavelengths through control of surface geometry. Furthermore, when radiation from this photonic crystal was shown onto a midIR PV device, we observe significant wall-plug efficiency improvements (45%) relative to a broad blackbody source. We have developed a photonic crystal structure that acts as a selective emitter, preferentially emitting light in a narrow band when heated. With a narrow emission line, yet broader than current technology, these materials can emit greater energy in the selective band at lower temperatures. In Fig. 13 we show a scanning electron micro-graph of the PCE emitter surface. The initial development of PCE emitter surfaces is presented in
Ref. [29]. The PCE technology was then integrated with MEMS technology, yielding discrete silicon-based narrowband infrared light sources as shown in Fig. 13. The fabrication of this device uses traditional photo-lithography processing. More specifically the photonic crystal is fabricated by depositing metal onto an oxide-coated silicon wafer. Using photo-lithography, the photonic crystal holes are patterned onto the surface, and then using dry etching techniques (reactive ion etching), the holes are drilled into the substrate. When heated the surface emits a narrow peak of infrared light with a center wavelength commensurate with photonic crystal lattice spacing (in this case 4:2 mm), as shown in Fig. 14. Peak wavelength and width do not change with temperature variation. The peak of the emission curve will lie on the blackbody emission curve for the measured sample temperature. The most important point for application to TPV is that infrared emission at short and long wavelengths relative to the central peak are dramatically suppressed. Out-of-band emission is emissivity limited to 10% of the blackbody curve at the same temperature. As a result, these emitters have demonstrated unprecedented infrared emission efficiency with total wall plug efficiencies approaching 20% in band. This should translate to improved TPV efficiency and it was this experiment that was the focus of this feasibility study. In order to develop an efficient TPV system, it is necessary to first select the most efficient solar cell device and then tune the emission spectrum of the hot source to the optimum conversion efficiency wavelength of that solar cell. Here, we optimized the solar-cell device to already available PCE emitters. We selected a PV device based on HgCdTe (MCT) manufactured by Vigo Inc. This device was doped with Zn to maximize efficiency from 4 to 5 mm in the infrared. As a solar cell, this device would be very inefficient, but it was chosen to demonstrate the potential
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Fig. 13. PCE MEMS infrared emitter vacuum packaged in a standard lead less chip carrier. The device shows unparalleled wall plug efficiency (larger 10%) in the mid infrared spectrum (MWIR 3–5 mm). This is two orders of magnitude more efficient than IR light emitting diodes highlighting the advantages of photonic crystal emitter enhancement.
Fig. 14. Infrared emission spectrum of the MEMS PCE emitter driven at 0.130 mW of input power. Inband the emission reaches the blackbody emission, but out of band the emission is suppressed dramatically. The blackbody spectrum (red) is at the same temperature as the PCE. The spikes in emission below 2 mm are noise in the measurement.
Fig. 15. Spectral responsivity of the photovoltaic detector. Above 6 mm the solar cell is not longer effective. The wavelength at peak responsivity is between 4–4:5 mm, well aligned with the peak emission of the PCE emitter shown in Fig. 14.
of PCE TPV. Initial characterization of the photonic crystal emitter device was carried out using a calibrated blackbody reference on a Nicolet Nexus 670 FTIR, equipped with an external emission port, following procedures outlined elsewhere [30]. The spectral characteristics are shown in Fig. 14. The integrated power in the 2–6 mm band is 30 mW of infrared light, resulting in a wall plug efficiency of 23.5%. The spectral characteristics of the Vigo middle wavelength infrared (MWIR) PV detector were provided by the manufacture and shown in Fig. 15. When the emission spectrum is multiplied by the responsivity we get the response curve for both the blackbody and the PCE emitter shown in Fig. 16. Integrating and multiplying by detector area we get 9.8 and 13.4 mV for the PCE emitter and the blackbody, respectively. This agrees well with the measured value of output voltage from the detector of 9.6 and 12.8 mV for the PCE emitter and a
blackbody (with equal aperture), respectively. Solar cell output power was then measured by varying the resistive load and measuring the current and voltage. This is shown in Fig. 17. Converting to output power, we find that for the blackbody we measure 0:56 mW and the PCE emitter yields 0:32 mW (Fig. 18) . The absolute magnitude of these numbers is very low, which is expected because this solar cell device is very inefficient. However, we can compare the relative efficiencies of the PCE emitter and the blackbody reference. Interestingly, the PCE emitter only requires 130 mW of input power verses the blackbody (normalized for area), which requires 315 mW. Therefore, the total wall plug conversion efficiency of the PCE emitter is 2:46 106 and only 1:68 106 for the blackbody. The PCE emitter resulted in a net improvement of 46% over the broadband blackbody source. The improved performance of the PCE emitter is derived from the narrow emission band. Instead
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Fig. 16. The spectral response curve for the system. The red curve is the blackbody and the black is the PCE emitter.
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Fig. 18. Power versus voltage for the TPV solar cell setup. Peak power of 0.32 and 0.56 mW are observed the PCE emitter (black) and blackbody (red), respectively.
Therefore, much of the blackbody emission profile can be converted to electricity. In contrast, higher efficiency solar cells, like polysilicon, have much narrower spectral bandwidth. For these detectors, the narrow emission from photonic crystal surfaces will be dramatically enhanced. We anticipate two to three times improvements are possible with photonic crystal enhancement. 4. Conclusions
Fig. 17. Current–Voltage characteristics for emitter-solar cell system. The total output power is higher for the blackbody (in red) versus the PCE emitter (black) as evidenced by the higher red curve. However, the PCE emitter required significantly less input power.
of emitting photons at all wavelengths, thereby wasting optical power in spectral bands where the solar cell cannot convert them, the PCE emitter concentrates all of the optical power into a narrow band commensurate with the absorption wavelength of the solar cell. As equal power is dumped into the PCE emitter, it achieves a higher temperature than the blackbody and yields more output in the spectral band of interest. This effect will be greatly enhanced at higher emitter temperatures where radiative power dominates. The efficiency enhancement of photonic crystal emitters can be compounded with a narrow spectral absorption solar cell. The Vigo MCT PV device has a very broad spectral responsivity (Fig. 15) from 0.8 to 5:5 mm wavelength.
We have shown that the ability to control the thermal emission and absorption of radiation in a photonic crystal enables the realization of high-efficiency solar cells. We have combined predictive modeling, micro-fabrication, and optical measurements to provide a basis for understanding and controlling the thermal emission and absorption of radiation in complex photonic structures and to design novel solar cell devices. We have demonstrated that the thermal emission in photonic crystal is characterized by spectral- and angular selectivity. The spectral selectivity plays an important role in eliminating wavelength-band mismatch between the semiconductor energy gap and blackbody emission, affecting the efficiency of solar cells, and may lead to a significant increase in the solar cell efficiency. On the other hand, the use of angle-selective absorbers based on suitably designed photonic crystal structures opens new avenues for realizing high-efficiency TPV systems without concentrators. The current state-of-the-art thermal shields use multilayer devices or textured surfaces to reduce the impact of the thermal radiation on thermal sensitive devices. They offer little control over frequency, and, typically, require mechanical operations to achieve a limited control over the angular distribution of the absorbed radiation. The hybrid photonic crystal architecture we have proposed are frequency- and angle-selective and allows a precise control
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of the absorption and emission of thermal radiation. By infiltration such structures with liquid crystals and by the application of an external electric field, the resulting device can be tuned without the use of any mechanical operation. On the other hand, the capability to control the thermal emission and absorption of radiation in a photonic crystal may also find many technological applications in the field of thermally pumped optical devices such as lasers and tunable infrared emitters. Acknowledgments Part of this work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under a grant from the National Aeronautics and Space Administration. MF, HL, and JPD would like to acknowledge the Hearne Institute for Theoretical Physics, the Disruptive Technologies Office, the Army Research Office, and the Louisiana State University Board of Regents LINK Program for support. References [1] [2] [3] [4]
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