シリコンベース新材料を用いた薄膜結晶太陽電池を目指して

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1 シリコンベース新材料を用いた薄膜結晶太陽電池を目指して 筑波大数理物質科学研究科電子 物理工学専攻 JST-PRESTO 末益 崇 BaSi 2 c=1.158nm b=0.680nm a=0.892nm Jan. 25, 2010 日本板硝子工学助成会 1

2 太陽電池の分類 単結晶シリコン 材料による分類 シリコン系 結晶シリコン アモルファス 多結晶シリコン 微結晶シリコン 太陽電池 化合物系 III-V 族 (GaAs) CIGS 系 (CuInSe 2 ) 多接合へテロ接合型 (HIT) シリサイド系結晶 CdTe 有機系 有機半導体 色素増感 動作原理による分類 厚みによる分類 pn 接合型太陽電池色素増感太陽電池結晶シリコン太陽電池薄膜太陽電池 殆どの太陽電池は この型です μm < 10μm 接合数による分類 単接合型太陽電池 多接合型太陽電池 市販品の殆どは この型です 2

3 太陽電池の現状 太陽電池用の半導体材料 (Si が 95%) 60% 25% 単結晶 Si 多結晶 Si Si リボン 4% 非晶質 Si 7% 4% Others Si の利点 資源が豊富 成熟した技術 Si の欠点 光吸収係数が小さい 厚い太陽電池 Siの大量消費 (50μmは必要) 安定確保が困難 禁制帯幅 Eg が小さい ( 理想値 Eg=1.4eV vs Eg Si =1.1eV 将来進むべき方向 資源の豊富な元素で構成される 高効率 & 薄膜太陽電池 3

4 光のエネルギーを吸収するとは? ( 光のエネルギー )= (E 2 E 1 ) エネルギー E 2 E 1 ( 光のエネルギー )< (E 2 E 1 ) 光は吸収されない ( 光のエネルギー ) > (E 2 E 1 ) 余分なエネルギーが無駄になる E 2 E 2 E 1 E 1 4

5 太陽光のスペクトル AM 1 1 θ AM 0 地表 AM 0 1 cosθ Pho oton flux de ensity (e ev -1 cm -2 s -1 ) Pho oton flux (ev -1 cm -2 s -1 ) K Black-body radiation Phon energy (ev) AM1.5 (100mW/cm 2 ) Eg Si =1.1eV J SC = q 3.5 E g F( E) de Photon energy(ev) 5

6 Ph hoton flux desity (cm -2 s -1 ) 太陽電池の出力 Enegy gap (ev) Photocur rrent dens sity (ma/c cm 2 ) エネルギー変換効率 : Egで決まる eV が太陽電池に適す η J photo (ma/cm 2 ) qe g 100(mW/cm 2 ) 0.8 = 28% Si 6

7 BaSi 2 の特徴 目標 : Si 系薄膜結晶太陽電池 on SiO 2 Si-based crystalline thin-film solar cells on SiO 2 禁制帯幅が太陽電池に相応しい Eg: 1.3 ev (BaSi 2 )~1.4 ev (Ba 0.5 Sr 0.5 Si 2 ) suitable for solar cells High efficiency solar cell Nakamura,..,Suemasu, APL 81 (2002) Morita,, Suemasu, JJAP 45 (2006) L390. c=1.158nm158nm (Theory) Imai and Watanabe, TSF 515 (2007) 光吸収係数が大きい Very large optical absorption coefficient α α~ 10 5 cm -1 at 1.5 ev More than 100 times larger than crystalline Si Thin-film solar cell Morita,..,Suemasu, TSF 508 (2006) 363. (Theory) D. B. Migas et al, PSS (b) 244 (2007) b=0.680nm Conversion efficiency a=0.892nm 資源が豊富 Si & Ba abundant chemical elements Clarke number: Si(2), Ba(14), Sr(15) cf. CIGS: Cu(26), In(66), Ga(35), S(16), Se(69) Si 面にエピタキシャル成長可能 Epitaxial growth possible on a surface High-quality crystal growth Inomata,..,Suemasu, JJAP 43 (2004) 4155, L478, L771. Ba 0.5 Sr 0.5 Si 2 7

8 他の太陽電池材料との比較 : 特徴は何か? Characteristic points of BaSi 2 compared with other materials BaSi 2 系 8

9 研究項目 Schottky-junction pn-junction Tandem E F 1. 不純物ドーピングによる伝導型 キャリア密度制御 Control of electron and hole concentrations by impurity doping 2. 分光感度特性 Photoresponsivity 3. BaSi 2 /Si トンネル接合の形成 Formation of heavily doped BaSi 2/Si tunnel junction for an electrical contact 4. 太陽電池動作の実証 Demonstration of solar cell 9

10 Growth of BaSi 2 epitaxial films using a template layer Inomata,..,Suemasu, Jpn. J. Appl. Phys. 43 (2004) 4155, L478, L771. Reactive deposition epitaxy Tsub: 550 Ba MBE growth Tsub: 600 Ba Si BaSi 2 template (10nm) Undoped n-basi 2 TEM Pt 0.2 μm ](Log Scale) Intensity [counts] XRD θ-2θ XRD pattern BaSi 2 (200) BaSi (*) 2 (600) BaSi 2 (400) θ [deg] 10

11 Electrical properties of undoped BaSi 2 film Ele ectron den nsity [cm -3 ] 8x x x x x x10 15 Temperature [K] /T [1/K] Morita,, Suemasu, Thin Solid Films 508 (2006) 363. Zintl phase (A a X x ) Si-Si: covalent Ba-Si: ionic cm 2 /Vs] Mobility [ Ba Si Baサイトより Siサイトが置換されやすい Y. Imai et al., Intermetallics 15 (2007) Y Rb Cs Fr Ba La Ac B Al Ga In Si N P As Sb 11

12 Electrical properties of impurity-doped BaSi 2 films Kobayashi,. Suemasu, Thin Solid Films 515 (2007) Kobayashi,..Suemasu, Appl. Phys. Express 1 (2008) Ba BaSi 2 (10nm) (ρ>1000 Ωcm) Si Ba Impurity doped BaSi 2 (ρ>1000 Ωcm) Sb (V) In, Al(III) Elec ctron mobi ility [cm 2 /Vs] Sb-doped n-basi 2 In-doped p-basi Electron concentration [cm -3 ] Ho ole mobilit ty [cm 2 /Vs s] Hole concentration [cm -3 ] 12

13 1. 不純物ドーピングによる伝導型 キャリア密度制御 Control of electron and hole concentrations by impurity doping 2. 分光感度特性 Photoresponsivity 3. BaSi 2 /Si トンネル接合の形成 Formation of heavily doped BaSi 2 /Si tunnel junction for an electrical contact 4. 太陽電池動作の実証 Demonstration of solar cell 13

14 Photoresponse properties of BaSi 2 epitaxial films Stripe-shaped electrode hv 1.5 mm Current(applied voltage 1~10 V) Sb-doped n + -BaSi 2 Matsumoto,., Suemasu, Appl. Phys. Express 2 (2009) hv electron E~10 V/cm L~1 mm undoped n-basi 2 (850nm) BaSi 2 CZ- S( CZ- n= cm -3 n= cm -3 hole band structure Pho otoresponsivity (A/W) Photoresponse spectra of BaSi 2 Photoresponse spectra of Cz-Si RT 1% 2% 1.0V 1.5V 2.0V 2.5V Photon energy (ev) Phot toresponsivity (A/W) CZ-Si, n= cm -3 1 V 3 V Photon energy (ev) 14

15 Analysis of grain size by Electron Backscatter Diffraction Matsumoto,..,Suemasu, Jpn. J. Appl. Phys. (2010) in press. Sample Electron Grain Diffraction Pattern Atomic Plane Diffraction Electron ND mapping Grain size 3~10 μm TD mapping 100 μm 100 μm 15

16 Formation of (111)-oriented Si layers on SiO 2 by Al-induced crystallization O. Nast et al., Appl. Phys. Lett. 73 (1998) BaSi 2 BaSi 2 Al(100nm) SiO 2 sub. Vacuum evaporation breaking the vacuum to form a native Al oxide (111)-oriented Si SiO 2 holder sample a-si(100nm) Al(100nm) SiO 2 sub. Annealing at 500 o C for 10 h in dry N 2 RF magnetron sputtering L-N 2 Al poly-si SiO 2 sub. 5 mm 5 mm 16

17 Growth of polycrystalline BaSi 2 films on AIC-Si/SiO 2 Tsukada,..Suemasu, JC J. Cryst. tgrowth th311 (2009) a-si(100 nm) Al(100 nm) Anneal 500, 10 h Al poly-si (111) oriented Si EBSD(ND) SiO 2 SiO 2 SiO 2 50μm Au/Cr striped electrodes 1.5 mm MBE substrates Undoped n-basi 2 Undoped n-basi 2 BaSi 2 template ~300 nm(~10 16 cm -3 ) ~300 nm(~10 16 cm -3 ) (111)-oriented Si (111)-oriented Si (111)-oriented Si (111)-oriented Si SiO 2 Sub SiO 2 Sub SiO 2 Sub SiO 2 Sub RDE growth Tsub: 550 MBE growth Tsub: 500 Evaporation Au/Cr electrodes 17

18 Photoresponse properties of polycrystalline BaSi 2 films on SiO 2 hv Tsukada,..Suemasu, Appl. Phys. Express 2 (2009) Al striped electrodes 1.5 mm Current (applied voltage 1~5 V) n-basi 2 (300 nm) (111)-oriented Si SiO μm Pho otoresponsiv vity (A/W) % 4% 6% 8% RT 2 V 3 V 4 V 5 V 1 V 2V (AIC-Si 10) Photon energy (ev) 18

19 1. 不純物ドーピングによる伝導型 キャリア密度制御 Control of electron and hole concentrations by impurity doping 2. 分光感度特性 Photoresponsivity 3. BaSi 2 /Si トンネル接合の形成 Formation of heavily doped BaSi 2 /Si tunnel junction for an electrical contact 4. 太陽電池動作の実証 Demonstration of solar cell 19

20 Band diagrams of BaSi 2 /Si structure Suemasu et al., Jpn. J. Appl. Phys. 45 (2006) L519. Vacuum level χ Si =4.0eV χ BaSi2 =3.3eV ΔE C =0.7eV Eg BaSi 2 =1.3eV Eg Si =1.1eV ΔE V =0.5eV n-si/n-basi n + /p + 2 tunnel junction p-si/p-basi 2 n-si ΔE C n-basi BSi 2 p-si p-basi 2 ΔE V 20

21 Formation of n + -BaSi 2 /p + -Si tunnel junction by MBE Ba Ba Si Sb BaSi 2 template Sb-doped n + -BaSi 2 ~ cm -3 BaSi 2 template template d B-doped p + -Si(70nm) B-doped p + -Si(70nm) ~ cm -3 B-doped p + -Si(70nm) p- p- p cm cm cm -3 XRD In ntensity (cou unts) d=0 nm 10 3 Si(222) (301) d=1 nm (600) (200) (400) θ (deg) AFM images of BaSi 2 template d=1 nm 2 nm 10 nm 3 μm template p + -Si p- 21

22 Dependence of I-V characteristics on template layer thickness n + -BaSi t / 2 /template/p + -Si n-basi 2 /p-si Sb-doped n + -BaSi 2 ~ cm -3 template B-doped p + -Si(70nm) 20 p cm -3 d ~ cm -3 1nm 2 nm 10 nm current d=2nm 20 n + -BaSi 2 template p + -Si p- Sb-doped n + -BaSi 2 Undoped n--basi 2 template p cm nm 240 nm Current density (A/ /cm 2 ) d=1nm J =20A/cm 2 at 0.5 V d=10nm Voltage (V) Current de ensity (A/ /cm 2 ) n + BaSi 2 /p + Si d=1nm n BaSi 2 /p Si Voltage (V) 22

23 Photoresponsivity of BaSi 2 layers on tunnel junction Saito,..Suemasu, Appl. Phys. Express (2010) in press. Tunnel junction Current flow normal to the sample plane n + -BaSi 2 Undoped BaSi 2 (360nm) n + -BaSi 2 p + -Si hv p-, cm -3 Photo oresponsivit ty (A/W) Photo oresponsivit ty (A/W) % % 15% Reverse 1 V 2 V 3 V 4 V Photon energy (ev) % % % Forward 1 V 3V 4 V 5 V 23

24 Wet chemical etching of BaSi 2 layers on Si 1mm φ Saito,..Suemasu, Suemasu Jpn. J. Appl. Phys. 48 (2009) Au/Cr SiO 2 BaSi 2 HCL+H 2 O HF+H 2 O BaSi 2 HCL 5% 15s 2% 10s 1% 180s 0.5% 120s HF 5% 15s 2% 15s 2% 15s 2% 15s (5,5% 30s) (0.5,0.5% 60s) (0.5,1.5% 60s) (1.5,0.5% 60s) 24

25 1. 不純物ドーピングによる伝導型 キャリア密度制御 Control of electron and hole concentrations by impurity doping 2. 分光感度特性 Photoresponsivity 特願 , US2009/ 特願 , PCT/WO2009/ 特願 , 3. BaSi 2 /Si トンネル接合の形成 Formation of heavily doped BaSi 2 /Si tunnel junction for an electrical contact 4. 太陽電池動作の実証 Demonstration of solar cell Schottky-junction pn-junction Tandem E F 25

26 Schottky-barrier diode n-basi 2 /CoSi 2 sunlight ~0.5μm n + -BaSi 2 φ B ~1.6eV ドリフト n-basi 2 E (undoped) F CoSi 2 CoSi 2 /n-basi 2 /n + -BaSi 2 p + -Si (111) Eg=1.3eV Epitaxial growth of BaSi 2 /CoSi 2 / structure Suemasu et al., J. Cryst. Growth 310 (2008) Ichikawa,, Suemasu, Appl. Surf. Sci. 254 (2008) Si Co Si Ba Ba CoSi 2 (30nm) Si(10nm) CoSi 2 BaSi 2 ( 22nm) CoSi 2 BaSi 2 ( 240nm) CoSi 2 26

27 Characterization of n-basi 2 /CoSi 2 Schottky diode XRD In ntensity (co ounts) θ-2θ XRD pattern Curre ent density (A A/cm 2 ) BaSi BaSi 2 2 (200) (400) CoSi 2 (222) BaSi 2 * θ (deg) AM1.5, 1Sun J sc =11.3mA/cm 2 V oc = V FF=0.72 V η=2.9% Voltage (V) TEM (600) BaSi 2 CoSi 2 5nm Si Si[11-2] 27

28 まとめ 不純物ドーピングによる伝導型 キャリア密度制御 Control of electron and hole concentrations by impurity doping 分光感度特性 BaSi 2 /Si トンネル接合の形成 n-type: Sb( cm -3 ), As p-type: In( cm -3 ), Al, Cu Photoresponsivity Photocurrent increases sharply for photons greater than 1.25 ev (~Eg). R~75mA/W at 2.3 ev. Formation of heavily doped BaSi 2 /Si tunnel junction for an electrical contact Wet chemical etching 太陽電池動作の実証 Demonstration ti of solar cell Schottky-barrier diode CoSi 2 /n-basi 2, η~2-3% n + -BaSi 2 /p + -Si, J =21A/cm 2 at 0.5 V 28

29 今後の展開 Future plan 1. pn homojunction solar cell p-ba 1-x Sr x Si 2 undoped n-ba 1-x Sr x Si 2 表面電極 p + -Si p-si E F 構造が単純 SiO 2 基板を利用 pn n + -Ba 1-x Sr x Si 反射防止膜接合 2 Ba 1-x Sr x Si 2 薄膜多結晶 (~1μm) トンネル接合 Si 薄膜多結晶 (~0.1μm) ZnO:Al SiO 2 基板 <111> 配向 大粒径 超平坦高品質薄膜結晶 効率 20% 超を目指せる & 多接合への展開 2. pn heterojunction solar cell 29