Improving Solar Cell Efficiency Using Photonic Band-Gap Materials

ARTICLE IN PRESS

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,Ã, Hwang Leea, Irina Puscasuc, Martin Prallec, Lucia Florescua,b, David Z. Tingb, Jonathan P. Dowlinga

aDepartment of Physics and Astronomy, Hearne Institute for Theoretical Physics, Louisiana State University, 202 Nicholson Hall, Baton Rouge, LA 70803, USA bJet Propulsion Laboratory, California Institute of Technology, Mail Stop T1714 106, 4800 Oak Grove Drive, Pasadena, CA 91109, USA cIon 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 coupling between the absorber and the cell (Fig. 1). However, any approach to solar-cell efficiency improve- Photovoltaic (PV) solar energy conversion systems (or ment that does not address this fundamental wavelength- solar cells) are the most widely used power systems. band mismatch, can achieve at most around 30% efficiency However, these devices suffer of very low conversion [1]. Moreover, this can be achieved only for concentrated efficiency. This is due to the wavelength mismatch between radiation, which requires an additional optical device, the narrow wavelength band associated with the semicon- which is not desirable in applications where the mass is a ductor energy gap and the broad band of the (blackbody) critical concern. emission curve of the Sun. The power loss is associated This article outlines novel approaches to the design of with both long-wavelength photons that do not have highly efficient solar cells using photonic band-gap (PBG) enough energy to excite electron–hole pairs across the materials [2,3]. These are a new class of periodic materials energy gap (leading to a 24% loss in silicon, for instance) that allow precise control of all electromagnetic wave and short-wavelength photons that excite pairs with energy properties [4–6]. A PBG occurs in a periodic dielectric or above the gap, which thereby waste the extra kinetic energy metallic media, similarly to the electronic band gap in as heat (giving a 32% loss in silicon). The efficiency of the semiconductor crystals. In the spectral range of the PBG, thermophotovoltaic (TPV) system may be increased by the electromagnetic radiation light cannot propagate. The recycling the photons with frequency larger than the solar ability to tailor the properties of the electromagnetic cell band-gap frequency, by using a spectrally dependent radiation in a prescribed manner through the engineering of the photonic dispersion relation enables the design of systems that accurately control the emission and absorp- ÃCorresponding author. Jet Propulsion Laboratory, California Institute of Technology, Mail Stop T1714 106, 4800 Oak Grove Drive, Pasadena, tion of light. This gives rises to new phenomena including CA 91109, USA. the inhibition and enhancement of the spontaneous E-mail address: [email protected] (M. Florescu). emission [3], strong localization of light [2], formation of

0927-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2007.05.001 ARTICLE IN PRESS 1600 M. Florescu et al. / Solar Energy Materials & Solar Cells 91 (2007) 1599–1610

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 ð273:4Þ. 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 Fig. 1. Schematic of a TPV energy conversion scheme. An intermediate higher temperature than a blackbody. This is solely due to absorber is heated by the Sun’s thermal radiation. The photovoltaic (PV) cell is illuminated by radiation from emitter transmitted by a filter. 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 atom–photon bound states [7], quantum interference heated photonic crystal emitter achieves thermal equili- effects in spontaneous emission [8], single atom and brium at a higher temperature than would otherwise be collective atomic switching behavior by coherent resonant possible. These facts suggests the possibility to leverage the pumping, and atomic inversion without fluctuations [9]. funneling properties of photonic crystals to improve the These remarkable phenomena have attracted a consider- spectral coupling of an emitter into the acceptance band of able interest for important technological applications, such a PV cell. 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 pro- cesses. 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, mid- infrared 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 Fig. 2. Spectral funneling of the thermal radiation by photonic crystals. emitters, it is possible to achieve dramatic modifications By designing a photonic band gap in prescribed frequency region of the of Planck’s blackbody radiation spectrum [15,21].In photonic crystal emission spectrum, the structure becomes unable to the PBG spectral range the thermal emission of radiation radiate at these frequencies and the corresponding energy is re-radiated in the allowed spectral range. As a consequence, the intensity of the is strongly suppressed, whereas for specific frequencies in blackbody emission at these frequencies increases, and the photonic the allowed photonic bands, that correspond to transmis- crystal emitter radiates the same power as it would a blackbody sion resonances of the photonic crystal, the thermal maintained at a higher temperature. ARTICLE IN PRESS M. Florescu et al. / Solar Energy Materials & Solar Cells 91 (2007) 1599–1610 1601

We present a design of highly efficient solar cells using coating over a many-wavelength-thick substrate. The PBG materials as intermediary between the Sun and the PV radiation is emitted from the substrate and passes through cells. We predict limiting conversion efficiency of around the passive photonic crystal filter and then is emitted into 60%. We propose two approaches to achieve this. The first vacuum. The absorbance A is given by energy conserva- approach is to couple broadband solar radiation into a tion, namely, A þ T þ R ¼ 1, where R and T are PBG material, engineered to re-emit the solar radiation reflectance and transmittance, respectively. The absorbance into a narrow frequency band corresponding to the is unity if the source is a perfect blackbody. Finding the semiconductor energy gap. In this way, power loss due to absorbance is equivalent to finding the thermal emittance photons of wavelength too much above or below the gap is E, since using Kirchhoff’s second law, the ratio of the eliminated. Another approach is intended to eliminate the thermal emittance to the absorbance is the same, indepen- roadblocks in the design of TPV systems based on non- dent of the nature of the material. Consequently, it is concentrated radiation, and makes use of a photonic possible to then compute E by matrix transfer techniques crystal-based angle-selective absorber. The selective absor- [15,22]. Once E is obtained, multiplication by the Planck ber has the property of absorbing only certain parts of the power spectrum gives the power spectrum of the PBG whole solar spectrum. If the absorber can absorb solar emitter pTHðo; TÞ in terms of the emittance EðoÞ and radiation whose frequency is above the solar cell band-gap blackbody spectrum pBBðo; TÞ, as given by frequency, the TPV efficiency of 45% can be achieved by pTHðo; TÞ¼ ðoÞpBBðo; TÞ. (5) using non-concentrated radiation (maximum of the dashed E curve in Fig. 4). In this case, additional spectral filters are Therefore, the thermal radiant power in a photonic crystal needed in front of the absorber. Here we show a specially can be controlled by altering the thermal emittance. designed photonic crystal that exhibits both angular and spectral selectivity in absorption and emission. Also, 2. Photonic crystal-based solar TPV: concepts and designs experimental studies show that the photonic crystal- enhanced (PCE) infrared emitters enhance the wall plug 2.1. TPV conversion efficiency conversion efficiency of MWIR solar cells relative to blackbody broad band sources. The conversion efficiency of a TPV solar system is determined by both the absorption efficiency of the 1.1. Thermal emission control intermediate absorber and the cell conversion efficiency. Let us first examine the absorption efficiency of the From the foundations of quantum mechanics, it is intermediate absorber. The incident power density is known that atomic oscillators in thermal equilibrium with related to the spectral power density defined as Z photon heat bath at temperature T have an average energy at frequency o given by PS ¼ do_obSðoÞ, (6) _o eðo; TÞ¼ , (1) where expð_o=kBTÞ1 2 F S o where _ is the Dirac constant and kB is the Boltzmann bSðoÞ¼ _ (7) 4p3c2 e o=kBT S 1 constant. The energy density per unit frequency, then, can is the spectral photon flux. Here, TS is the temperature of be written as 5 the Sun and F S is a geometric factor, equal to 2:16 10 p uðo; TÞ¼rðoÞeðo; TÞ, (2) for non-concentrated light (determined by the radius of the where rðoÞ is the electromagnetic density of modes. For Sun and the distance between the Sun and the Earth), and free space, the density of modes has the form p for the full concentration. This leads to the Stefan– Boltzmann’s law 2 FS 2o r ðoÞ¼ , (3) F pc3 P ¼ A sT 4. (8) S p S such that the radiant power then takes he usual form of Planck’s law The intermediate absorber loses its energy by emitting radiation with the rate ½F =psT4 , where the geometric 2 _ A A BB 1 o o factor F is equal to p since the absorber emits in all p ðo; TÞ¼ cuðo; TÞ¼ _ . (4) A 4 2pc3 e o=kBT1 directions. Hence the net gain of the absorber is This suggests that since the density of electromagnetic F F modes is altered in a photonic crystal, the radiant power P ¼ S sT 4 A sT4 . (9) net p S p A can also be altered. The ability of the photonic crystal to change the spectral In what regards the cell conversion efficiency, assuming properties of the emitted and absorbed electromagnetic that the spectral filter allows only the radiation with radiation can be illustrated considering a photonic crystal frequencies bigger than the gap frequency oG, and the ARTICLE IN PRESS 1602 M. Florescu et al. / Solar Energy Materials & Solar Cells 91 (2007) 1599–1610 recombination loss is all radiative, the open-circuit angle- and frequency-selective absorber. The selective assumption may be used for estimation of the maximum absorber has the property of absorbing only certain parts conversion efficiency. Under this assumption, we have of the whole solar spectrum. If the absorber can absorb _o _o Dm solar radiation of frequency above the solar cell band-gap ¼ , (10) frequency, a TPV efficiency of 45% can be achieved [1] (the k T k T B A B C maximum of the dashed curve in Fig. 4). Again, additional from the generalized Planck’s law [23]. Here, T C is the spectral filters are needed in front of the absorber. We show temperature of the cell and Dm is the chemical potential. that a suitably designed photonic crystal can be used as a The efficiency of an electron–hole pair to generate electrical selective emitter as well as a selective absorber. If, for energy is then given by the chemical potential divided by example, we match the band-edge frequency of the the photon energy as Dm=_o ¼ 1 ½TC=TA, which is the photonic crystal to the semiconductor band-gap frequency, Carnot efficiency. The actual working situation is a slight it is possible to suppress both emission and absorption of deviation from the open-circuit condition, such that this photons of frequency below that of the semiconductor expression of efficiency still holds (Fig. 3). band-gap. Consequently, the photonic crystal sample plays Combining the two contributions, we have the efficiency simultaneously the role of a selective emitter (with respect of the TPV conversion system as with the cell) and a selective absorber (with respect to ! 4 the Sun). F A TA TC In addition to the frequency-selectivity, thermal emission ZPV ¼ 1 4 1 . (11) F S T S TA 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 2.2. Non-concentrated radiation: frequency- and angle- the TPV system. If the solid angle of the emission at the selective absorber Sun side can be made very small, it is possible to achieve the same enhancement of the solar cell efficiency as in An increased efficiency of a TPV system may be obtained devices using concentrators. In other words, just by mainly by recycling of photons of frequency larger than the engineering the emission solid angle, the energy conversion solar cell band-gap frequency by using a spectral filter efficiency can be increased without using concentrators. between the absorber and the cell. The combined system of The radiation concentration in Eq. (11) is mathemati- the absorber and the filter can be called a selective emitter. cally described by the increase of the Sun’s geometric factor However, such a high efficiency can be achieved only for F S. However, a decrease of the absorber’s geometrical concentrated radiation, which requires additional optical factor F A leads to the same effect. Physically, the decrease devices, not desirable for instance for space applications, of the absorber’s F A implies that the emission and where mass is of critical concern. absorption of radiation is confined to a certain range of In order to design a high-efficiency TPV system using directions. Fig. 4 shows the TPV efficiency as a function of non-concentrated radiation, we have introduced an 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 mono- disperse 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 Fig. 3. Schematic of the photonic crystal-based TPV energy conversion. An intermediate absorber is heated by absorbing thermal radiation. The deposition (CVD) using disilane (Si2H6) gas as a precursor. photovoltaic (PV) cell is illuminated by radiation from emitter transmitted After disilane is deposited uniformly in the voids, the by a filter. crystal is heated to 600 C in order to improve the silicon ARTICLE IN PRESS M. Florescu et al. / Solar Energy Materials & Solar Cells 91 (2007) 1599–1610 1603

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.

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 pffiffi enter the gap and the position of the band edge frequency Fig. 5. A close-packed inverted opal structure ðr=a ¼ ð2Þ=4Þ viewed as depends on the specific direction. As a result the absorption sequence of yABCABC yplanes grown along the ½111 direction. In is enhanced considerably, for a given spectral range, for each plane, the low-dielectric constant spheres (here, for simplicity, we assume air spheres) are embedded in a high-dielectric constant host specific incident directions. This fact opens a novel way to make an angular-selective PBG absorber, which, in turn, mediump andffiffiffi are sitting on a triangular lattice of lattice constant axy ¼ a= 2. enables high-efficiency solar energy conversion without using concentrators. The increase in the energy conversion efficiency is crystallization and allow the diffusion of silicon through- determined by the small angle of thermal emission of the out the sample. Finally, the silica template is removed using intermediate absorber on the Sun side. For the intermedi- controlled fluoride-based etching designed to avoid affect- ate absorber, the gain comes from the absorption of solar ing the silicon backbone, and leaving behind a closed- radiation and the loss is determined by the emission by the packed FCC lattice of air spheres in a silicon background absorber. As we decrease the solid angle of the emission by [24]. Photonic crystals are usually characterized by the the absorber, we can decrease the loss due to the emission dispersion relation (or band structure, representing the by the absorber, and effectively increase the net gain. The relationship between the frequency and the wave-vector) so-called minimal emission refers to a situation where the and the photonic density of states (the number of available solid angle of the emission (F A) is the same as the solid electromagnetic modes at a specific frequency and at a angle extended by the Sun (F S). Such an angular selective ARTICLE IN PRESS 1604 M. Florescu et al. / Solar Energy Materials & Solar Cells 91 (2007) 1599–1610

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=ð2pcÞ¼0:5 and the higher band-edge of the second stop band around oa=ð2pcÞ¼0:8, respectively. absorber, due to the decrease of the radiation loss, gets restricted to a small frequency range. This effect might be much hotter than in the conventional case. A larger value useful when applied in the tandem cell configuration. In of temperature difference between the absorber and the PV realizing an angular selective absorber, the angular cell becomes available, leading to higher Carnot efficiency. selectivity should cover a wide range of frequencies. For a small solid angle for absorption such that Otherwise, the absorber emits outside of the desired F A=F S ¼ 100, the theoretical limit of TPV conversion emission cone with different frequencies. efficiency becomes 68% at about 727 1C (see Fig. 4). However, we can see from Fig. 8 that there are absorption 2.3. Wide-band angular-selective absorber peaks at different frequencies. Hence, absorption is enhanced at a different angle of incidence for a different The best possible absorber would absorb radiation at all frequency. In other words, the angular selectivity is frequencies, but only inside the solid angle subtended by ARTICLE IN PRESS M. Florescu et al. / Solar Energy Materials & Solar Cells 91 (2007) 1599–1610 1605

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 structure embedded in the above discussed photonic crystal designing a photonic crystal absorber such that the is an excellent candidate for a wide-band angular-selective absorption is suppressed at all frequencies for all angles PBG absorber. We considered a structure with a complete except inside the desired cone. In other words, all the three-dimensional band gap with one-dimensional absorp- frequencies are lying inside the PBG except for one tion characteristics. Such a property can be realized with a preferred direction. The effect of such an absorber is 3D–2D–3D [25,26] photonic crystal heterostructure as the equivalent to using fully concentrated radiation. Since the one depicted in Fig. 10, where a waveguide channel is built angular selectivity requires a sharp cutoff in the emission in a 2D photonic crystal as a defect mode. The 2D solid angle, a one-dimensional defect photonic crystal photonic crystal is, in turn, embedded in a 3D photonic ARTICLE IN PRESS 1606 M. Florescu et al. / Solar Energy Materials & Solar Cells 91 (2007) 1599–1610

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 sub- gap generated by the waveguide channel has a true one- dimensional 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 Fig. 10. Design of a PBG wide-band angular selective absorber. The relatively broad spectral range. Thus, the photonic crystal micro-structure consists of a waveguide channel in a 2D photonic crystal, heterostructure will operate as a frequency- and angle- which is embedded in a 3D photonic crystal. The 1D waveguide is selective absorber. generated by removing one row of rods in the longitudinal direction. The Furthermore, we envisage a hybrid scheme for 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 intermediate absorber as depicted in Fig. 12. It consists of a this example, the 2D photonic crystal consists of square rods of width 3D–2D–3D photonic crystal architecture acting as a a2D=a ¼ 0:3. The width and the height of the stacking rods in the woodpile frequency and angle-selective absorber on the Sun side, structure are a3D=a ¼ 0:25 and h3D=a ¼ 0:3, respectively, where a is the and a 3D photonic crystal structure acting as a frequency- dielectric lattice constant of the embedding 3D photonic crystal [25,26]. selective 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

The development of a solar-cell device based on the frequency and angular control of the emission and Fig. 11. Schematic dispersion relation of the PBG hetero-structure absorption of thermal radiation in a photonic crystal is a described in Fig. 10 for propagation along the waveguide direction complex and laborious process. Such a device will (w ¼ o=2pc). By removing one row of rods, the linear defect supports a incorporate all the advances in current solar-cell technol- single waveguide mode. By appropriately choosing unit cell size, the mode ogy, and then selectively replace components in the solar- will experience a sharp cutoff in the spectral region around the desired frequency [25]. cell architecture by their photonic crystal engineered counterparts. Similar to the conventional solar cell design, the incident solar power will be absorbed by an absorber crystal. The electromagnetic field is confined vertically by system. As we have shown in the previous section, the the 3D structure and in-plane by the stop gap of the 2D absorber system may consist of a photonic crystal photonic crystal. By tuning the characteristics of the micro- architecture, engineered such that it has an enhanced structure (geometry and index of refraction contrast), the absorption coefficient along the direction of the incident ARTICLE IN PRESS M. Florescu et al. / Solar Energy Materials & Solar Cells 91 (2007) 1599–1610 1607

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 Ref. [29]. The PCE technology was then integrated with photonic crystal architecture, engineered such that it MEMS technology, yielding discrete silicon-based narrow- presents an enhanced emission coefficient for a spectral band infrared light sources as shown in Fig. 13. The range that matches the photocell semiconductor band gap. fabrication of this device uses traditional photo-lithogra- In our experimental study, we have focused only on a phy processing. More specifically the photonic crystal is specific problem: the improvement of the emitter efficiency fabricated by depositing metal onto an oxide-coated silicon using a photonic crystal-based emitter system. In order to wafer. Using photo-lithography, the photonic crystal holes simplify the analysis, we have used a low-temperature TPV are patterned onto the surface, and then using dry etching energy conversion experimental set-up. In TPV, an techniques (reactive ion etching), the holes are drilled into incandescent radiator illuminates a PV cell that converts the substrate. When heated the surface emits a narrow peak radiant heat to electrical energy. These systems are an of infrared light with a center wavelength commensurate attractive long-term power source since, in principle, they with photonic crystal lattice spacing (in this case 4:2 mm), as can achieve conversion efficiencies considerably higher shown in Fig. 14. Peak wavelength and width do not than the 6–8% capabilities of current thermoelectric change with temperature variation. The peak of the generators. The crucial problem for this technology is emission curve will lie on the blackbody emission curve matching the emission spectrum of the radiator to the band for the measured sample temperature. The most important gap of the photocell. Approaches to this problem include point for application to TPV is that infrared emission at radiators with strong (ionic) emission lines and reflective short and long wavelengths relative to the central peak are filters between the radiator and photocell. In this work we dramatically suppressed. Out-of-band emission is emissiv- explore an alternate radiator concept a PCE narrow band ity limited to 10% of the blackbody curve at the same incandescent emitter tuned for peak emission near the band temperature. As a result, these emitters have demonstrated gap of the photocell. Our results show the feasibility of unprecedented infrared emission efficiency with total wall fabricating rugged emitters with tunable wavelengths plug efficiencies approaching 20% in band. This should through control of surface geometry. Furthermore, when translate to improved TPV efficiency and it was this radiation from this photonic crystal was shown onto a mid- experiment that was the focus of this feasibility study. IR PV device, we observe significant wall-plug efficiency In order to develop an efficient TPV system, it is improvements (45%) relative to a broad blackbody source. necessary to first select the most efficient solar cell device We have developed a photonic crystal structure that acts and then tune the emission spectrum of the hot source to as a selective emitter, preferentially emitting light in a the optimum conversion efficiency wavelength of that solar narrow band when heated. With a narrow emission line, cell. Here, we optimized the solar-cell device to already yet broader than current technology, these materials can available PCE emitters. We selected a PV device based on emit greater energy in the selective band at lower HgCdTe (MCT) manufactured by Vigo Inc. This device temperatures. In Fig. 13 we show a scanning electron was doped with Zn to maximize efficiency from 4 to 5 mmin micro-graph of the PCE emitter surface. The initial the infrared. As a solar cell, this device would be very development of PCE emitter surfaces is presented in inefficient, but it was chosen to demonstrate the potential ARTICLE IN PRESS 1608 M. Florescu et al. / Solar Energy Materials & Solar Cells 91 (2007) 1599–1610

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 Fig. 15. Spectral responsivity of the photovoltaic detector. Above 6 mm emission, but out of band the emission is suppressed dramatically. The the solar cell is not longer effective. The wavelength at peak responsivity is blackbody spectrum (red) is at the same temperature as the PCE. The between 4–4:5 mm, well aligned with the peak emission of the PCE emitter spikes in emission below 2 mm are noise in the measurement. shown in Fig. 14. of PCE TPV. Initial characterization of the photonic blackbody (with equal aperture), respectively. Solar cell crystal emitter device was carried out using a calibrated output power was then measured by varying the resistive blackbody reference on a Nicolet Nexus 670 FTIR, load and measuring the current and voltage. This is shown equipped with an external emission port, following in Fig. 17. Converting to output power, we find that for the procedures outlined elsewhere [30]. The spectral character- blackbody we measure 0:56 mW and the PCE emitter yields istics are shown in Fig. 14. The integrated power in the 0:32 mW (Fig. 18) . The absolute magnitude of these 2–6 mm band is 30 mW of infrared light, resulting in a wall numbers is very low, which is expected because this solar plug efficiency of 23.5%. The spectral characteristics of the cell device is very inefficient. However, we can compare the Vigo middle wavelength infrared (MWIR) PV detector relative efficiencies of the PCE emitter and the blackbody were provided by the manufacture and shown in Fig. 15. reference. Interestingly, the PCE emitter only requires When the emission spectrum is multiplied by the respon- 130 mW of input power verses the blackbody (normalized sivity we get the response curve for both the blackbody and for area), which requires 315 mW. Therefore, the total wall the PCE emitter shown in Fig. 16. Integrating and plug conversion efficiency of the PCE emitter is 2:46 106 multiplying by detector area we get 9.8 and 13.4 mV for and only 1:68 106for the blackbody. The PCE emitter the PCE emitter and the blackbody, respectively. This resulted in a net improvement of 46% over the broadband agrees well with the measured value of output voltage from blackbody source. The improved performance of the PCE the detector of 9.6 and 12.8 mV for the PCE emitter and a emitter is derived from the narrow emission band. Instead ARTICLE IN PRESS M. Florescu et al. / Solar Energy Materials & Solar Cells 91 (2007) 1599–1610 1609

Fig. 16. The spectral response curve for the system. The red curve is the Fig. 18. Power versus voltage for the TPV solar cell setup. Peak power of blackbody and the black is the PCE emitter. 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 band- width. 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

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 Fig. 17. Current–Voltage characteristics for emitter-solar cell system. The total output power is higher for the blackbody (in red) versus the PCE novel solar cell devices. We have demonstrated that the emitter (black) as evidenced by the higher red curve. However, the PCE thermal emission in photonic crystal is characterized by emitter required significantly less input power. spectral- and angular selectivity. The spectral selectivity plays an important role in eliminating wavelength-band mismatch between the semiconductor energy gap and of emitting photons at all wavelengths, thereby wasting blackbody emission, affecting the efficiency of solar cells, optical power in spectral bands where the solar cell cannot and may lead to a significant increase in the solar cell convert them, the PCE emitter concentrates all of the efficiency. On the other hand, the use of angle-selective optical power into a narrow band commensurate with the absorbers based on suitably designed photonic crystal absorption wavelength of the solar cell. As equal power is structures opens new avenues for realizing high-efficiency dumped into the PCE emitter, it achieves a higher TPV systems without concentrators. temperature than the blackbody and yields more output The current state-of-the-art thermal shields use multi- in the spectral band of interest. This effect will be greatly layer devices or textured surfaces to reduce the impact of enhanced at higher emitter temperatures where radiative the thermal radiation on thermal sensitive devices. They power dominates. offer little control over frequency, and, typically, require The efficiency enhancement of photonic crystal emitters mechanical operations to achieve a limited control over the can be compounded with a narrow spectral absorption angular distribution of the absorbed radiation. The hybrid solar cell. The Vigo MCT PV device has a very broad photonic crystal architecture we have proposed are spectral responsivity (Fig. 15)from0.8to5:5 mm wavelength. frequency- and angle-selective and allows a precise control ARTICLE IN PRESS 1610 M. Florescu et al. / Solar Energy Materials & Solar Cells 91 (2007) 1599–1610 of the absorption and emission of thermal radiation. By [5] J.W. Haus, G. 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