This page contains a Flash digital edition of a book.
research  review Solar cell anti-reflection coatings


reach new heights A multilayer coating realizes omnidirectional,broadband antireflection


A TEAM OF RESEARCHERS from Rensselaer Polytechnic Institute (RPI), Magnolia Solar, Inc. and Pohang University of Science and Technology have demonstrated a novel multilayer antireflection (AR) coating utilizing tailored- and low-refractive index technology.


This novel multilayer AR coating exceeds the performance of the widely employed double-layer AR (DLAR) coating on state- of-the-art inverted metamorphic triple- junction solar cells. According to the team, the multijunction solar cell gains over 4 percent in efficiency when the industry- standard DLAR coating is replaced with an optimized four-layer AR coating.


“The measured reflectance reduction and omnidirectional photovoltaic performance enhancement of the four-layer AR coating are to our knowledge, the largest ever reported in the literature of solar cell devices,” writes the team in its paper.


Considering that the solar spectrum is an intrinsically broadband spectrum, such broadband characteristics of the AR coating are undoubtedly beneficial for high power conversion efficiency. Furthermore, omnidirectional AR characteristics have become important for the rapidly expanding terrestrial application of solar cells. This is because solar irradiance


Xing Yan. This is because alumina shows no absorption over a wider band than silica, and almost the same tailorability as nanoporous silica. According to the photocurrent measurements performed by the team, DLAR coating cannot compete with four-layer AR coating at normal incidence. When DLAR coating improves the short-circuit current density (JSC) over an uncoated triple-junction solar cell by 27.5 percent, four-layer AR coating improves by 31.6 percent.


Four-layer AR coating beats DLAR coating at all angle-of-incidences on a state-of-the-art inverted metamorphic triple-junction solar cell


received by non-tracking solar cells in terrestrial applications usually has a wide range of incident angles. Both broadband and omnidirectional AR characteristics are attainable by four-layer AR coatings, as demonstrated by the RPI-led team.


The excellent broadband and omnidirectional AR characteristics of the four-layer AR coating are achieved through refractive index matching at multiple layer interfaces. By using tailored and low- refractive index nanoporous silica layers, the team has greatly reduced the refractive index contrast at the semiconductor / AR coating / air interfaces. Through a multilayer design methodology powered by a genetic algorithm optimisation, favourable antireflective properties over a specified wavelength range and angle-of- incidence range were found. Two porous layers of the four-layer AR coating were fabricated by oblique-angle deposition of bulk silica thereby resulting in films with refractive indices of 1.32 and 1.11. This is less than the refractive index of silica. The two dense layers lying below nanoporous silica were fabricated by co-deposition of silica / titanium dioxide using sputtering.


Carefully designed four-layer step- graded thin film realizes favourable antireflective interference of light for wavelength range of 350 nm to 1600 nm, and angle-of-incidence range of 0 degree to 80 degree


Although silica is chosen for the implementation of nanoporous layers of the four-layer AR coating, other oxides, such as alumina, may be explored in order to capture the entire spectrum of solar irradiance, according to team member


At a grazing angle of incidence, four-layer AR coating significantly exceeds DLAR coating in AR performance. The JSC improvement of four-layer AR coating at 80° is 53.3 percent, whereas the JSC improvement of DLAR coating is only 5.3 percent. The angle-of-incidence (0° - 80°) averaged photocurrent enhancement of the four-layer AR coating amounts to 34.4 percent, compared to 25.3 percent for DLAR coating.


How does this tailored- and low-refractive index AR coating technology compare with continuously graded AR coatings, such as biomimetic antireflective subwavelength structures? According to the team, their multilayer AR coating technology may even outperform subwavelength structures, in several aspects. Due to a layer-by-layer deposition process, the structure profile of a multilayer AR coating can be precisely controlled.


“The tailorability and optimization of such a customizable approach readily lends itself to the incorporation of the AR coating design into solar cell device structures for application-specific requirements,” writes the RPI-led team in their paper.


According to Jaehee Cho and Fred Schubert, it will be of interest to investigate the viability of applying this novel AR coating on surface-textured devices in the future. They will also investigate innovative fabrication methods for depositing low- refractive index AR coatings on curved surfaces, such as a hemispherical lens.


X. Yan et. al. Adv. Funct. Mater. 23 583 (2013)


March 2013 www.compoundsemiconductor.net 69


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116  |  Page 117  |  Page 118  |  Page 119  |  Page 120  |  Page 121  |  Page 122  |  Page 123  |  Page 124  |  Page 125  |  Page 126  |  Page 127  |  Page 128  |  Page 129  |  Page 130  |  Page 131  |  Page 132  |  Page 133  |  Page 134  |  Page 135  |  Page 136  |  Page 137  |  Page 138  |  Page 139  |  Page 140  |  Page 141  |  Page 142  |  Page 143