Optical properties of ZnO nanostructures and application to dye-sensitized solar cells

1. Optical properties of ZnO nanostructures and application to dye-sensitized solar cells Wen-Feng Hsieh (謝文峰) http://www.ieo.nctu.edu.tw/wfhsieh/ Department of Photonics & Institute of Electro-Optical Engineering National Chiao Tung University Department of Electro-Optical Engineering & Institute of Electro-Optical Science and Engineering National Cheng Kung University NCHU 2009/10/16

2. Acknowledgements Supported by National Science Council Collaborators: • Nanowires • Dr. H. C. Hsu, Center for Condensed Matter, NTU • Mr. C.Y. Wu, King Yuan Electronics • Mr. S. Onions Yang (PhD student) • Quantum dots and powders • Dr. C. J. Pan • Dr. K. F. Lin • Mr. W. T. Hsu(LED industry) • DSSC • Mr. H. M. Cheng(PhD student, ITRI) • Mr. W. H. Chiu (PhD student) • Thin films • Dr. C. H. Hsu (NSRRC) • Dr. W. R. Liu • Mr. Y. H. Lee

3. Outline • Introduction • Nanostructures and Quantum Dots • Why ZnO? • Why ZnO Nanostructures for solar cell applications? • Optical Properties of ZnO Nanowires, Quantum Dots and ZnO Powders – Random lasing – Exciton-phonon coupling and biexcitons • Performance of ZnO dye-sensitized solar cells • Crystal Defect Structures on the Physical Properties of ZnO Epitaxial Films • Large Rabi splitting of ZnO quantum well in a microcavity • Summary

4. Quantum Confined Structures (a) Modifying density of states (b) Bandgap Engineering artificial atoms (c) Exciton polariton laser (BEC) Microcavity ZnO/MgO ZnO/Cu2O QW

5. 35μm Bose-Einstein condensation of exciton polaritons Nature, Vol. 443, 28, Sept. (2006) Exciting energy : 1.768 eV AOM AOM (1-μs pulse, T=100μs) Reduce sample heating CdTe QWs @ 19K • The laser beam was carefully shaped into “top hat” intensity profile.→ uniform excitation spot on the sample • Control parameter: excitation power (polariton density)

6. Energy and angle-resolved emission intensity • Below threshld: the emission is broadly distribution in momentum and energy. • Above threshold: the emission comes almost exclusivelyfrom the k||=0 lowest energy state.

7. Occupancy of the ground state • Ground state occupancy:Linear → exponential • Linewidth:Decreasing by a factor of two → increase of the polariton coherenceFurther increase→ interaction between polaritons • Blue shift:shift slightly→ the strong coupling regime • Measuring the ordinary lasing excitation threshold → Being 50 times higher than the condensation threshold.

8. Semiconductor Material Parameters 25 ZnO II-VI 3.37 0.37 2.34 60 High Mott density~ 3.7x1019 cm−3.

9. Characteristic of ZnO • II-VI wide band gap (3.37eV) wurtzite-type semiconductor • Large binding energy of free exciton (60 meV) • Thermally stable at room temperature • Large exciton-phonon interaction • EL from p−i−n junctions has been demonstrated at room-temperature • Lasing from a single wire • Stimulated emission and lasing of random-growth oriented ZnO nanowires • An extremely fast carrier cooling time on the order of 200 fs (200-500 fs for GaN) and hot phonon (LO phonon) cooling time of 1.75 ps (ns in GaAs). GaN (25 meV) ZnSe (22 meV) Tsukazaki, et al., Nature Mater. 4, 42 (2005) P. D. Yang, Science JAP 97, 064315 (2005) & J. Cryst. Growth 287, 189–193 (2006) CKSun, et al., APL 87, 023106 (2005)

10. Why ZnO Quantum dots? • Wannier excitons in ZnO are stable at room temperature, have an extremely large oscillator strength, and emit UV-blue light. • ZnO QDs are interesting as high T single-photon sources for quantum computing and free-space quantum cryptography. • Controlling the Spontaneous Emission Rate of Single QDs in a 2D PhC. PRL 95, 013904 (2005) Triggered SP sources using non-resonant pulsed pumping Decoupled Photon anti-bunching Coupled with cavity mode GaN-QDs @ T < 40K APL 87, 051916 (2005)

11. Dye-Sensitized Solar Cell (DSSC) < Structure & Principle > < Cell reactions > - S(adsorbed) + h  S (adsorbed) - S (adsorbed) S +(adsorbed) + e-(injected) - S +(adsorbed) + A-  S(adsorbed) + A - A(cathode) + e- A-(cathode) (S+/S) e- cb e- VOC e- (A/A-) electrolyte e- (S+/S) vb <Advantages> • Low cost and simple manufacturing • Environmental compatibility • Transparent solar cell e- Adsorbed dye FTO TiO2 Pt e- N3 B. O’Regan and M. Grätzel, Nature, 353, 737 (1991) M. Grätzel, Nature , 414, 338 (2001).

12. Dye-Sensitized Solar Cell (DSSC) • η of 5 -11% with TiO2 nanoparticles • Surface area of 800 – 1000 cm2 per cm2 • Electron diffusivity: 10-4 cm2/s Poor charge collectors?

13. ZnO in thin film solar cells • Advantages • High mobility (ex ZnO: μe ~200 cm2/Vs; μh~50 cm2/Vs) • Good physical and chemical stability • Transparency • Easy of processing • Non-toxic • Lower in cost

14. Pump 700°C Ar flow 200 sccm O2 flow 2 sccm Catalyst (Au) on substrate Quartz tube Zn+Mg2N3 mixture in alumina boat Fabrication of ZnO Nanowires Thermal Vapor Deposition

15. Sol-gel process of ZnO quantum dots and powders Easy & Quick

16. PLD Experiments TEM 8-Circle diffractometer Growth condition: KrF laser: 248 nm,10Hz,7 J/cm2 ; Target : 5N ZnO target Substrate : c-sapphire ; Growth rate and sample thickness: 0.063 Å/s ; D~0.5-0.8 mm 0.27 Å/s and D~0.21 mm 0.38 Å/s and D~0.3 μm

17. Random Lasing Action in ZnO Nanosaws JAP (2006) & J. of Crystal Growth 287 (2006) 189–193 Room temperature Under 355 nmPulsed Laser Excitation 36 MW/cm2 31 MW/cm2 25 MW/cm2 21 MW/cm2 16 MW/cm2 10 MW/cm2 • Lasing wavelength ~ 390 nm • Lasing linewidth < 0.6 nm • Threshold about 15 MW/cm2 Partially polarized

18. Lasing Mechanism Random Lasing Action in ZnO Nanosaws “Self-formed” laser cavity efficient light scattering in high gain ZnO medium Excitation area depen. of emission spectra of ZnO nanosaws under constant pumping power density “Self-Formed” Laser Cavity H. Cao et al. PRL 1999, 82, 2278

19. Substrate effect on ZnO nanowires Nanotechnology 16, 2882–2886 (2005). ZnO film/Si(111) ZnO film/Sapphire(0001) Epi-film Poly-film In-plane preferred orientation Without in-plane orientation

20. Band gap engineering and stimulated emission of ZnMgO nanowires Zn & Mg in ZnO/MgO core-shell interdiffuse to become ZnMgO after annealed. APPL. PHYS. LETT. 89, 013101 (2006)

21. Acoustic and optical phonon assisted formation of biexcitons Appl. Phys. Lett. 91, 111907 (2007) Temperature dependent PL spectra in ZnO powders Temperature dependent PL Fx 80K Bx • For P < 20 mW, • IBX grows faster than IFX.

22. Integrated emission intensity of biexciton vs. pumping Iexc0.32 10K 0.53 40K 80K 1.86 • Power exponent for biexciton should be close to 2. • Observe anomalous temperature dependent power exponents

23. Emission of exciton-polariton Dispersion of exciton-polariton Power dependent PL 10K Fx 80K Bx E CB • Lower Te for than for • More excitons populate in @ 80K than @ 10K 15 meV n = 2

24. Exciton temperatures (Te) vs. bath temperatures (Tb) close to the lowest optical phonon energy of 12 meV • Increasing Te due to laser heating

25. Temperature dependent biexciton formation Iexc0.32 10K 0.53 40K 80K 1.86 • At low T, the relaxation process involves only emission of acoustic phonons to lower kinetic energy. • At high T, optical phonons participate in to efficiently lower exciton kinetic energy, since Te ~ . E CB 15 meV n = 2

26. HRTEM image of ZnO-QD using 0.06M Zn-Acetate • HRTEM shows good crystal structure with multi-domain in a single dot. (average size: 4.3±0.3nm). Chem. Phys. Lett. 409, 208–211 (2005).

27. Controllable Size of ZnOnanoparticles by varying concentration of Zn-Acetate • XRD shows good agreement with the HRTEM result of 0.06M (~4.3±0.3nm). • Decreasing ZnO particle size as Zn acetate was decreased. Chem. Phys. Lett. 409, 208–211 (2005). Average Sizes are estimated by Scherrer formula: • λ:0.1542 nm • B:FWHM of diffraction peak • θ: Bragg diffraction angle

28. PL spectra of ZnO QDs • Small green emission due to surface states. • It increases with decreasing size. Function of excitation intensity I ~ L1.3 Evidence of no laser heating by varying pumping from 1.1 to 23 mW/cm2.

29. PL and absorption spectra of ZnO QDs 4.68 nm • Both absorption and PL are blue shifted as decreasing the particle size. • Exhibiting quantum size effect K. F. Lin, et al., Chem. Phys. Lett. 409, 208 (2005)

30. Micro-Raman Spectra Excitation wavelength 515 nm (below ZnO bandgap) 3.5 nm No laser heating • No shift of the E2(high) as the laser power has been varied almost an order of magnitude from 1.5 to 12 mW (3.5 nm in diameter). • The Raman shift mainly results from spatial confinement of optical phonon. Phonon frequency shift depends on size of QDs --- not resulting from localization to defects APPL. PHYS. LETT. 88, 263117 (2006).

31. Reduction of exciton-LO phonon interaction with diminishing ZnO QDs Size dependent PL Temperature dependent PL Photon energy ↑ • Blue shift of FX with less apparent phonon replica as D • Red shift of NBE as T ↑ To be appeared in APL (2007).

32. Size dependent Eb and Fx-phonon coupling Fit with to get exciton binding energy (Eb) and Bohr radius (aB) via aB2 = ħ2/(2m*Eb) Fit with to get exciton-phonon coupling coeff. Eb = 60 meV 67 meV 0.19 87 meV 0.21 0.40 a0 = 0.59 132 meV • Eb ↑ and a0 ↓ as shrinking in particle size

33. 0.7 a a / 0 QD 0 powders a a / B QD B powders 0.6 0.5 Ratio 0.4 0.3 0.2 4 6 8 10 12 Particle size (nm) Correlation of exciton radius with exciton-phonon coupling strength • Reduction of aB makes the exciton less polar to reduce the Fröhlich interaction as reducing particle size.

34. Formation of Branched ZnO Nanowires from Solvothermal Method and Dye-Sensitized Solar Cells Applications Jsc and h of the branched ZnO nanowire DSCs are twice higher than the bare ZnO nanowire ones. J. of Phys. Chem. C (2008).

35. Efficient electron transport in tetrapod-like ZnO metal-free DSCs D149 Tetrapod powders

36. Obstacles for ZnOepitaxy on Si • Larger lattice mismatch between ZnO and Si (~-15%) • Larger thermal mismatch between ZnO and Si • thermal stress  the formation of cracks. [ ZnO=4.75x10-6K-1, Si=2.56x10-6K-1 Wang et al. , J. Cryst. Growth,284, 459 (2005) • The easy oxidation of the Si surface during initial growth • formation of an amorphous SiO2 layer deteriorating the quality of ZnO grown on Si ( ) CRC Handbook of Thermophysical and Thermochemical Data, ISBN-13: 978-0849301971 • ※Solution:grow a lattice matched oxide buffer layer on Si prior to SiO2 film growth

37. ZnO/g-Al2O3/Si(111) Epitaxy A B In-plane: A,B-type domain existed with60orotation: A-typedomaindominant Epitaxial relation : Strain state: Tensile strain along in-plane; compressive strain along surface normal

38. TEM HRTEM near interface SAED ZnO g-Al2O3 Si No intermediate layer on interface ZnO/g-Al2O3 and g-Al2O3 /Si Consist with XRD results

39. Growth temp. vs. XRD • The typical growth condition is at 300oC. • Smallest tilt angle Δθ (0.34o)ZnO (0002) ; largest twist angle ΔΦ (3.85o) [ZnO/c-sapphire(small Δθ and ΔΦ, Δθ~0.048o, ΔΦ~0.57o)] • Opposite trend of Δθ/ΔΦ => quite different from ZnO/C-sapphire and ZnO/Y2O3/Si(111) with same variation trend of Δθ/ΔΦ

40. PL spectra vs. growth temperature -- variation of DLE and NBE Grown @ 300oC Grown @ 200oC • Sharp near band edge emission (NBE) • Low deep level emission (DLE) • Least FWHM of NBE but the largest IDLE/INBE at 300oC • Opposite variation trend of IDEL/NBE and NBE performance NBE Grown @ 400oC DLE • correlated with XRD results

41. Optical property vs. types of TDs Liuet al. J. Phys. D: Appl. Phys. 41 065105 (2008) Screw TDs affects the FWHM of NBE. Edge TDs affects the DLE emission. No clear correlation after exchanging abscissas!

42. Post annealing Δθ(0002)=0.34o Δφ(10-11)=1.7o Δθ(0002)=0.24o Δφ(10-11)=1.3o Annealing 100min @800oC under oxygen atmosphere @ 1atm ZnO thickness ~270nm • After annealing under oxygen atmosphere: • Lower DLE • DLE mainly comes from the existence of oxygen vacancy • Better NBE performance • Improved structural quality

43. Summary • We have successfully grown high-quality ZnO epitaxial films by PLD on Si(111) substrates with a thin γ-Al2O3 buffer layer and there exist two (111)-oriented γ-Al2O3 domains rotated 60° from each other against surface normal. • The sample with the least FWHM of θ-rocking curve of ZnO(0002) is 0.34o and the thickness of ZnO is 270 nm only; however, there exists the largest Δf(1.77o) of ZnO . • The post annealing under pure oxygen atmosphere at 1 atm improved the DLE, which mainly comes from oxygen vacancies, and the structural quality was also improved. Crystalline structure ΔΦ (twisting) Δθ (tilting) Optical property Ratio (IDLE/INBE) NBE FWHM TDs Edge Screw

44. Y2O3 Si Y2O3 ZnO/Y2O3/Si(111) B A 9.6 nm (Å-1) In-plane: two Y2O3 domains coexist with 60orotation B-type (111) domain A-type (111) domain c b a dominant b a Epitaxial relation : Lattice mismatch ~ -15.07% with B-type dominant a b c

45. XRD vs. Growth temperature (Å) (Å) (Å) (Å) • The optimal growth condition: 400oC • Small Δθ and ΔΦ (Δθ~0.048o, Δ Φ~0.57o ). • Tensile strain along in-plane; compressive strain along surface normal (different from that of ZnO grown on C-sapphire)

46. Interfacial structure High-quality ZnO epi-film on Si(111) W.-R. Liu et al., Crystal Growth and Design (2009) Atomic arrangement of O sub-lattice in Y2O3 (111) planes O vacancy 3.75Å 10.606Å O atom Y2O3 3.75Å The least mosaic distribution is 0.27o The least twist angle is 0.51o ZnO 3.258Å

47. 6 6 7 7 7 6 TEM ZnO 0.511 nm ZnO ~1.2 nm 0.309 nm Y2O3 Fourier filtered image Y2O3 ┴ ┴ ┴ ┴ ┴ ┴ 0.309 nm Domain- match epitaxy (DME) (periodic misfit dislocation) Si Residual strain ~ -0.68% W.-R. Liu et al., Crystal Growth and Design (accepted)

48. Photoluminescence NBE (FXA=3.296eV) DLE (2.2eV) • Low deep-level emission (DLE) at room temperature • The FWHM of NBE ≈ 100 meV • The narrow FWHM of NBE ≈ 9.12 meV at 13K Good optical quality (PL ) for epi-ZnO/Y2O3/Si(111)

49. Temperature dependent PL At low-temperature the DoXA dominant. • Above 100K, free • exciton dominates • Above 160K DoX • vanishes. • Above 80K TES • vanishes. • The binding energy of exciton : 56.6 ±6.5 meV

50. Summary • The epitaxial relationship with • A domain matching epitaxy (DME) of ZnO is comprehended with the 8-unit ZnO matching the 7-unit Y2O3 along direction via periodic misfit dislocation resulting in -0.68% mismatchand small residual tensile strain. • The PL spectra show low deep-level and strong near-band-edge emission (3.295 eV) with FWHM of ~100 meV at RTand narrow FWHM of 9.1 meV at LT. • The XRD,TEM and PL results show high-quality crystal structure and optical property of ZnO epi-films grown on Si(111) using Y2O3 buffer layer.