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 experi- mentally 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.