High Efficiency Triple-Junction Solar Cells Employing Biomimetic Antireflective Structures

High Efficiency Triple-Junction Solar Cells Employing Biomimetic Antireflective Structures M.Y. Chiu, C.-H. Chang, F.-Y. Chang, and Peichen Yu, Green Photonics Laboratory Department of Photonics National Chiao-Tung University, Hsinchu, Taiwan http://www.ieo.nctu.edu.tw/gpl/

Outline Introduction Biomimetics Moth-eye principle Device Fabrication Process Polystyrene nanosphere lithography Optical and Photovoltaic Characteristics Reflectance Engineering via RCWA* Summary *RCWA: rigorous coupled-wave analysis ERATO Symposium, Tohoku Univ. Japan 2011/2/16 2/16

Biomimetics Self-cleaning abilities of a lotus leave: http://spie.org/x33323.xml?articleid=x33323 ERATO Symposium, Tohoku Univ. Japan 2011/2/16 3/16

Biomimetics Colors of butterfly wings: Man-made polymer photonic crystals http://www.imtek.de/ http://www.science.org.au/ ERATO Symposium, Tohoku Univ. Japan 2011/2/16 4/16

Biomimetic Antireflective Structures The moth-eye principle : broadband and omni-directional AR http://tywkiwdbi.blogspot.com/1608/11/scanning-electron-micrographs.html Si Polymer ERATO Symposium, Tohoku Univ. Japan 2011/2/16 5/16

Moth-eye principle Sub-wavelength structure (SWS) λ n a i r n air Graded-index semiconductor n s n eff ERATO Symposium, Tohoku Univ. Japan 2011/2/16 6/16

air Introduction Graded refractive index profile n air n Multi-layer ARC: Material selection for different refractive indices Thermal constant mismatch that change mechanical and optical properties semiconductor n s Biomimetic ARC: Single layer SWS Mechanically and optically robust and durable Profile control possible *ARC: antireflective coating ERATO Symposium, Tohoku Univ. Japan 2011/2/16 7/16

Triple-junction solar cell I ng ap G aa s Ge Power conversion Efficiency ~40% ERATO Symposium, Tohoku Univ. Japan 2011/2/16 8/16

Triple-junction solar cell with SWS Ga 0.5 In 0.5 P/GaAs/Ge Triple-junction solar cell Broadband absorption (300nm ~1800nm) Very thin thickness (~ a few micrometers) InGaP GaAs λ Ge Surface Recombination =>SWS fabricated on SiNx passivation layer Current Matching => Reflectance engineering ERATO Symposium, Tohoku Univ. Japan 2011/2/16 9/16

Polystyrene Nanosphere Lithography Requirements for substrate Hydrophilic surface Homogeneous chemical property Flat and clean surface Spin Coating: 1.Scan speed 2.PS solution concentration Poly Styrene (PS) sphere Substrate 4 wafer 10 μm ERATO Symposium, Tohoku Univ. Japan 2011/2/16 10/16

Profile Control via RIE Sacrificial mask for reactive ion etching (RIE) P o l y s t y r e n e SiN x (n~1.8) T J w a f e r T J w a f e r T J w a f e r 1-step etching 2-step etching 200 1 μ m nm 200 nm ~100 nm-thick SiN x was kept for passivation ERATO Symposium, Tohoku Univ. Japan 2011/2/16 11 /16

Optical Characterization Reflectance spectra (measured by an integrating sphere) Reflectiveity (%) 1 0 0 80 60 40 20 0 SL-ARC SWS AM1.5D 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0 1 6 0 0 0. 0 W a v e l e n g t h ( n m ) 1. 5 1. 0 0. 5 Irradiance (Wm -2 nm - 1 ) SWS shows a much flatter spectrum, particularly in UV and IR. Reflectance of SWS can be further improved by choosing a passivation material with a higher refractive index than SiNx, ~1.8. SWS designed to enhance the spectral response of the current limited junction ERATO Symposium, Tohoku Univ. Japan 2011/2/16 12/16

Device Fabrication Flow 3J wafer pattern ohmic GaAs 1um SiN x deposition top cell: Ga 0. 5 In 0. 5 P middle cell : GaAs front contact cap GaAs metal evaporation RIE etching for SWS spin on PS spheres bo t to m c el l : G e rear contact ERATO Symposium, Tohoku Univ. Japan 2011/2/16 13/16

Device Characterization Current-Voltage measurement No ARC SL-ARC SWS Current Density (ma/cm 2 ) 1 4 1 2 1 0 8 6 4 2 S W S S L - A R C no ARC 0 0.0 0.5 1.0 1.5 2.0 2.5 V o l t a g e ( V ) AR condition w/o ARC SL ARC SWS V o c ( V ) 2.51 2.48 2.52 Jsc (ma/cm 2 ) 9.36 11.37 11.62 F F( % ) 84.98 86.42 86.42 Efficiency (%) 19.93 24.41 25.26 Jsc is increased by 24.2% and 2.2%, compared to those without ARC and with SLARC, respectively. ERATO Symposium, Tohoku Univ. Japan 2011/2/16 14/16

Rigorous Coupled Wave Analysis Modeling parameters SWS AR passivation Window layer Top cell 7x7 hexagonal SiN x parabola array Periodicity~600 nm Height ~ 900nm SiN x 100 nm 50% AlInP 50 nm 50%GaInP 500 nm 1. A l 0.5 In 0.5 P and Ga 0.5 In 0.5 P n,k mismatch. 2. Only top cell is included. ERATO Symposium, Tohoku Univ. Japan 2011/2/16 15/16

Summary We have successfully fabricated SiN x -based SWS for a Ga 0.5 In 0.5 P/GaAs/Ge triple-junction solar cell utilizing the polystyrene nanosphere lithography. PCE and Jsc of triple-junction solar cell were enhanced due to the absorption improvement of the GaAs mid-cell. The angular response of photocurrent nearly follows the cosine law and demonstrates the omnidirectionality of SWS. An RCWA approach enables the reflectance engineering for Jsc optimization of tandem cells with the SWS. ERATO Symposium, Tohoku Univ. Japan 2011/2/16 16/16

Thanks for your attention!! ERATO Symposium, Tohoku Univ. Japan at NCTU 17/16