Light management in thin-film Si solar cells. Hitoshi Sai

Transcription

1 9th Aug University of Southampton Light management in thin-film Si solar cells Hitoshi Sai Research Center for Photovoltaic Technology (RCPVT) National Institute of Advance Industrial Science & Technology (AIST) Tsukuba, Japan 1

2 Outline 1. Introduction of AIST & RCPVT 2. Issues in light trapping technologies in TFSi solar cells 3. Research 2-1. Rolls of the front/rear textures 2-2. Systematic survey of textures by self-ordered structures 2-3. Flattened light scattering substrate 4. Summary 2

3 Location of AIST Fukushima Tokyo AIST (Tsukuba city) Mt. Fuji 3

4 RCPVT in AIST Director: Prof. M. Kondo c-si R&D for PV materials & devices High η & cost reduction Lower environmental impacts System Thin-Film Si System operation & evaluation Infra technology Technology development International cooperation Impartiality Characterization Characterization & testing Infra structure for industry Dye & thin-film Organic CIGS III-V Researcher : 30 Total member : >150 4

5 Thin-film Si High efficiency multi-junction with stable a-si:h, µc-si:h and µc-sige:h. Matsui, PIP 18, 48 (2010) Smets, JAP (2008) High-mobility (> 100cm 2 /Vs) TCO material: In 2 O 3 :H Koida, JAP 107, (2010) p a-si i n p triple junction) a-si:h µc-si:h µc-si 1-x Ge x :H i µc-si n p µc-si i 1-x Ge x n Effective light trapping structures Sai, APL 93, (2008) Plasma diagnostics for PECVD Nunomura, APL (2009) Quantum efficiency total 27.8 ma/cm 2 a-si:h µc-si 0.8 Ge 0.2 :H 9.1 µc-si:h MHI 150 W (1.1 x 1.4 m 2 ) Wavelength (nm) 5

6 CIGS, III-V High efficiency CIGS Ishizuka, APL (2008) Flexible CIGS Ishizuka, JRSE (2009) III-V quantum dots T. Sugaya APL (2010) T. Sugaya APL (2010) 15.9% CIGS sub-module Mini-band formation by InGaAs QD superlattice 17.7% CIGS flexible cell on Ti foil 6

7 Organic solar cells (film/dye) Organic dye for DSSC Komura, JMC (2009) Ionic liquid for DSSC Wang, Chem.Mat (2009) Thin-film organic PV Yamanari, SOLMAT (2009) Taima, SOLMAT (2009) OCH 3 O * S n * Plant-mimic solar cells 7

8 Characterization irradiance (µw/cm2 /nm) Wide-range High Fidelity Solar Simulator (WHSS) IEC Reference solar radiation Wavelength (nm) Solar Simulator Dejure Standard Primary reference cells Reference module calibration 8

9 Mega-SolarTown 1MW system including various c-si/hit & TFSi PV modules

10 Outline 1. Introduction of AIST & RCPVT 2. Issues in light trapping technologies in thin-film Si solar cells 3. Research 2-1. Rolls of the front/rear textures 2-2. Systematic survey of textures by self-ordered structures 2-3. Flat light scattering substrate 4. Summary 10

11 Light management in TFSi solar cells Light management for: Enhancing light absorption (in-coupling & trapping) Appropriate distribution of VIS/IR light to component cells. Norm. photon number a-si:h µc-si:h Solar irradiation (AM1.5G) Glass AR coatings /structures Wavelength (µm) a-si:h µc-si:h Buffer BSR p i n p i n TCO p-i-n type Transparent window Textures Intermediate reflector Buffer layer & Back reflector Textured TCO e.g., CVD-SnO 2 :F 1µm 11

12 Textures for TFSi solar cells CVD-SnO 2 (Asahi-U) 1 ZnO sputtering + wet etching 3 Ag/Al reactive sputtering 5 1µm 1µm LP(MO)CVD-ZnO 2 ZnO electroplating 4 ZnO(Sputtering) 6 1µm 1µm [1] K. Sato, Rep. Res. Lab. AGC., Ltd. 42 (1992) 129. [2] S. Faÿ, SOLMAT 86 (2005) 385. [3] M. Berginski, JAP 101 (2007) [4] N. Toyoma, Proc. WCPEC3 (2003) [5] A. Takano, JJAP 43 (2004) L227. [6] G. Yue, WCPEC5 (2010). 12

13 Trade-off between Jsc and Voc/FF J SC Trade-off V OC & FF Clacks or porous materials: post-oxidation, shunts Nasuno, JJAP 40 (2001) L303. Matsui, JNCS (2002) Smets, APL 92 (2008) Python, SOLMAT 93 (2009) but we need light trapping. 13

14 Issues 1. Rolls of the front / rear textures in TFSi solar cell. What kind of textures should be used in the front/rear sides? 2. What is the best texture? a-si:h µc-si:h Tandem Characterizing one-side textured cells Systematic survey on texture size by means of self-ordered structures 3. Trade-off between light absorption and deterioration of film quality. Light scattering with a flat surface 14

15 Outline 1. Introduction of AIST & RCPVT 2. Issues in light trapping technologies in TFSi solar cells 3. Research 2-1. Rolls of the front/rear textures 2-2. Systematic survey of textures by self-ordered structures 2-3. Flat light scattering substrate 4. Summary H. Sai et al., JAP 108 (2010)

16 µc-si:h cells with various textures Substrate σ rms (nm) Polished SnO 2 :F or glass < 2 Textured SnO 2 :F (Asahi-U) 30 Sputtered/Etched ZnO:Ga 70 TCO glass(p-i-n) µc-si:h (t = 2µm) Polished by CMP BSR(n-i-p) Sai et al., JAP 108 (2010)

17 µc-si:h cells (t ~ 2µm) Polished As-dep. flat moderate rough µc-si:h Natural texture flat moderate rough Flat Asahi-U Textured ZnO 17

18 J-V characteristics type ID J (ma/cm 2 ) η (%) A B p-i-n C D E F G H n-i-p I J K J QE Good (reasonable) performance even after CMP! L J SC 18

19 EQE (p-i-n) Texture: front & rear EQE enhanced in the whole wavelength region. 19

20 EQE (n-i-p) Texture: front & rear EQE enhanced in the whole wavelength region. 20

21 Light trapping in the NIR J ph (NIR) = q EQE(λ) I solar (λ) dλ ( nm) AM1.5G Both can contribute, but rear > front AR-effect by front Design/consider the rear texture as a result of Si deposition even in p-i-n. Texturing in IMR should be effective for light trapping in the top cell. n-i-p is a good structure to survey the effect of textures. 21

22 Outline 1. Introduction of AIST & RCPVT 2. Issues in light trapping technologies in TFSi solar cells 3. Research 2-1. Rolls of the front/rear textures 2-2. Systematic survey of textures by self-ordered structures 2-3. Flat light scattering substrate 4. Summary H. Sai et al., APL (2008) H. Sai et al., JAP (2009) H. Sai et al., SOLMAT (2011) 22

23 Size control: Anodic oxidation Electrochemical oxidation of Al Porous Al 2 O 3 with honeycomb structure Self-ordered dimple pattern on Al Period Voltage Electrolyte Al sheet Al 2 O 3 Barrier layer Al Controllability in size Interpore distance (Period) Pore Cell Porous layer Average period [µm] Electrolyte type Citric 0.4 Maric Phosphoric 0.2 Oxalic(HA) Oxalic Sulfuric Applied voltage [V] A.P. Li et al., JAP 84 (1998) W. Lee et al., Nature Mater. 5 (2006) 741. S.Z. Chu et al., JECS 153 (2006) B

24 Dimple patterns on Al substrates Λ = 0.1 µm (40V, (COOH)2) 0.3 µm (120V, (COOH)2) 1µm 1µm 0.6 µm (270V, C6H8O7) 0.9 µm (370V, C6H8O7) 1µm 1µm 0.45 µm (195V, H3PO4) 1µm Textured SnO2 (Asahi-U) 1µm 24

25 Angular resolved reflection measurement 1.0 ~ 1.2 µm 0.3 ~ 0.35 µm n air ~ 1.0 n Si ~3.5 IR in Si Violet/bule in air λ = µm Reflection intensity (arb.u.) λ = µm, θ i = 5 O Substrate/Ag/ZnO Flat Asahi-U Al, Λ = 0.1 µm Al, Λ = 0.3 µm Al, Λ = 0.9 µm 1E Angle, θ (deg.) 25

26 µc-si:h solar cell Structure :n-i-p Method :PECVD ITO (75nm) ZnO (40nm) Ag Λ Ag n,i,p-µc-si:h (t i = 1µm) Al i-layer deposition t i :1 µm Temp. :180 C Press. :1.5 torr Power :40 mw/cm 2 Gas :SiH 4 /(SiH 4 +H 2 ) = 2.7% Area Substrate :1 cm 2 (active area) :patterned Al, Asahi-U, Flat glass I-V (AM1.5G 100mW/cm 2 ) QE 26

27 Spectral response External quantum efficiency Enhancement of IR response by increasing Λ H. Sai et al., APL (2008) H. Sai et al., JAP (2009) Flat (J SC =18.6mA/cm 2 ) Asahi-U(22.5) Λ=0.1µm(19.6) Wavelength [µm] Λ=0.3µm(21.0) Λ=0.45µm(22.8) Λ=0.9µm(24.3) 27

28 J ph (QE) v.s. Period (t i = 1 µm) Maximum J ph is obtained at Λ ~ 1 µm (1 µm < Λ<?). J SC > 25 ma/cm 2 is within reach. J ph (ma/cm 2 ) V b = 0 V (J SC ) t i = 1 µm, A= 1 cm Average period, (µm) For tandem, multi-periodic structures(?) 28

29 Outline 1. Introduction of AIST & RCPVT 2. Issues in light trapping technologies in TFSi solar cells 3. Research 2-1. Rolls of the front/rear textures 2-2. Systematic survey of textures by self-ordered structures 2-3. Flat light scattering substrate 4. Summary H. Sai et al., APL (2011). 29

30 Flattened Light-Scattering Substrates (FLiSS) n A << n B ~ n active Decoupling the optical interface & the growth interface, or complex material of large n with a (reasonably) flat surface 30

31 Experiment ZnO grating FLiSS Ag µc-si:h, 1µm ITO p i n Ag Si wafer ZnO(low n) dead a-si(high n) A = 1 cm 2 (active area) Type of substrate RMS (nm) Flat < 10 Grating 100 FLiSS (flattened) < 3 Conventional 35 H. Sai et al., APL (2011) 31

32 J-V characteristics 32

33 EQE EQE Flat Ref. texture Grating Flattened Current density (ma/cm 2 ) λ > 750 nm Wavelength (nm) 1.0 Flat Grating Ref.tex. FLiSS Application to tandem cells 33

34 Summary In thin-film silicon cells, light management is an crucial issue to improve the conversion efficiency (from 12% to 14%). Texturing is the most common method for enhancing the optical path length in a device. The contributions of the front and rear textures are different. For µc- Si:H cells, a texture with a feature size of ~ 1 micron is suited. A flat substrate with refractive-index distribution in-plane improves light absorption and QE without deterioration of V OC and FF. There are other options to be explored : plasmonics, photonic back reflectors Thank you for your kind attention. 34