Synthesis of ZnO Nano/Microspheres and Development of Organic Solar Cells. Gon Namkoong ARC-Old Dominion University

Transcription

1 Synthesis of ZnO Nano/Microspheres and Development of Organic Solar Cells Gon Namkoong ARC-Old Dominion University

2 Outline Synthesis of ZnO nano/microspheres Control of ZnO morphologies Effect of structure direct agents on ZnO morphologies Control of uniformity, distribution, and size of ZnO spheres Organic solar cells Recombination process of organic solar cells Degradation mechanisms of organic solar cells Simulation of 3D organic morphologies Power of Words

3 Outline Synthesis of ZnO nano/microspheres Control of ZnO morphologies Effect of structure direct agents on ZnO morphologies Control of uniformity, distribution, and size of ZnO spheres Organic solar cells Recombination process of organic solar cells Degradation mechanisms of organic solar cells Simulation of 3D organic morphologies Power of His Words

4 ZnO morphologies Unique optical, electrical, and structural properties Many applications Missing morphology ZnO sphere Materialtoday, Vol 7, p 26 (2004)

5 ZnO structures Noncentrosymetric ZnO structure Nuclear Instruments and Methods in Physics Research Section B. Vol. 281, pp 77 (2012) a=3.247 Å b=5.207 Å b a 1.6

6 ZnO structures (Cont.) ZnO has a noncentrosymmetric crystal structure Scientific reports, Vol 2, pp 587

7 ZnO structures (Cont.) ZnO has a noncentrosymmetric crystal structure Strong spontaneous polarization Scientific reports, Vol 2, pp 587

8 ZnO structures (Cont.) ZnO has a noncentrosymmetric crystal structure Strong spontaneous polarization Scientific reports, Vol 2, pp 587 Surface charge on the (0001) plane

9 Hydrothermal synthesis of ZnO Autoclave reactor Zinc acetate Zn(( CH3COO) 2 2H2O) Ammonia hydroxide NH 4 OH

10 Hydrothermal synthesis of ZnO 2 Zn(NH 3 ) Zn cation Zn anion 2 Zn(NH 3 ) 4 2 Zn(OH 3 ) 4 Scientific reports, Vol 2, pp 587

11 Hydrothermal synthesis of ZnO Preferred growth of ZnO Nature nanotechnology, Vol. 6, pp 103 (2011)

12 Novel approach for ZnO spheres 1. Control of cation species adjustment of ph ZnO polar surface preferential growth Namkoong et al, Thin Solid Films, Vol. 534, pp 76 (2013)

13 ZnO with different ph values Zinc acetate

14 Novel approach for ZnO spheres 2. Passivate the polar surface using SDA Structure directing agents (SDA) Urea and ethanol Control of Zn(NH 3 ) 2+ 4 nucleation species Urea Spherical shape SDA will passivate ZnO that suppress further nucleation Suppression of (0001) growth

15 Effect of SDA on ZnO morphologies Urea ethanol

16 Temporal evolution of ZnO

17 Temporal evolution of ZnO

18 Temporal evolution of ZnO

19 Temporal evolution of ZnO SDA

20 Control of size and distribution of ZnO spheres Urea Urea Ethanol ethanol Urea (1):Ethanol (1) Urea (1):Ethanol (1.25)

21 Xray diffraction measurement (b) Intensity (arb. units) ZnO (100) (a) (002) ZnO (101) ZnO (102) ZnO (110) ZnO (103) ZnO (200) (b)

22 Intensity (a.u.) Confocal PL of ZnO spheres Confocal PL ZnO ZnO121-b Laser : DPSS 355nm Grating : 150 g/mm-500 nm Blz Int.time : 500 ms Temp. : RT Peak at : 387 nm 5μm Wavelength (nm) No defects are observed

23 Summary ZnO polar surface was responsible for preferential growth Structure directing agents (SDA) effectively passivated the ZnO polar surface, leading to balanced vertical and lateral growth rate Careful combination of SDA allowed for the control of both size and size distribution of ZnO spheres

24 Outline Synthesis of ZnO nano/microspheres Control of ZnO morphologies Effect of structure direct agents on ZnO morphologies Control of uniformity, distribution, and size of ZnO spheres Synthesis of ZnO nano/microspheres Organic solar cells Recombination process of organic solar cells Degradation mechanisms of organic solar cells Simulation of 3D organic morphologies Power of His Words

25 Current state-of-the art solar cells Efficiency Thin Film PV (CIGS, a-si, CdTe.) Semiconductor PV (GaAs, Si, GaN.) New generation PV (Organic, Inorganic, ) Cost

26 Absorption (arb. units) Organic solar cells Absorption of polymer Transparent solar cells Transparent PTB Wavelength (nm)

27 Fabrication of organic solar cell Polymer PCBM fullerene OMe O Donor PC 71 BM Acceptor

28 PCDTBT:PCBM solar cells - + Heeger et al, Nature photonics, Vol 3, pp 297 (2009)

29 Recombination processes Exciton generation photons exciton - Light 5 n-contact p-contact + exciton =~ nanoseconds

30 Recombination processes Heterojunction bipolar exciton =~10 ns Hetero-interface Dissociation center Nanoscale morphologies photons Light bound e/h - Exciton diffusion length L exciton ~10nm D 5 n-contact exciton p-contact +

31 Recombination processes photons Light 5 free electron - n-contact p-contact free hole +

32 Recombination processes Langevin recombination R k k r ( np 2 r n i ) q ( μ ) n μp ε photons exciton Light bound e/h free electron n-contact p-contact + + Namkoong et al, Organic electronics, Vol. 14, pp 74 (2013)

33 Degradation of organic solar cells Lifetime > 20 years Lifetime < 6 years

34 Degradation mechanisms of organic solar cells Degradation processes of PPV(polyphenylene vinylene) polymer Light (hv) phenylene phenylene Photo-oxidation scission phenylene Reducing charge transport efficiency Creating defects and trap centers

35 Influence of photo-oxidation on charge transport trapped Photo-oxidation X trapped

36 Degradation of organic solar cells Polymers PCBM OMe O PTB7 PC 71 BM PTB7 PCBM

37 Normalized Efficiency Degradation of organic solar cells (a) PTB7/PC 71 BM With TiO x Without TiO X Time (Days) ~25% ~50%

38 Role of TiO 2 for organic solar cells TiO2 Redox process +O 2 CO2 (gas) H2O(gas) Sol-Gel processed TiO x

39 Normalized Efficiency Effect of sealing of organic solar cells on degradation Al/glass TiO 2 PTB7:PCBM/TiOx Organic blends PEDOT:PSS ITO/glass Sealed, in glove box Optical adhesive 32% 0.6 Sealed, in air 41% Unsealed, in air PTB7/PC 71 BM/TiO x With sealant glass in Glove box With sealant glass in air Without sealant glass in air Time (Days) 99%

40 Absorption (arb.units) Absorption (arb.units) Comparison of UV-VIS absorption Sealed PTB7/PCBM in air (c) Not sealed PTB7/PCBM in air PTB7/PC 71 BM/TiO x With sealant glass in air As cast After 10 days After 20 days PTB7/PC 71 BM/TiO x Without sealant glass in air As cast After 10 days After 20 days Wavelength (nm) Wavelength (nm)

41 Absorption (arb. units) Absorption (arb.units) Absorption of PTB7 and PCBM (c) Not sealed PTB7/PCBM in air PTB7 PTB7/PC 71 BM/TiO x Without sealant glass in air As cast After 10 days After 20 days O OR PTB7 PCBM S F S OR R= 2-ethylhexyl OMe S O S n OR S n P3HT PCBM PTB Wavelength (nm) PC 71 BM Wavelength (nm)

42 Degradation mechanism for organic solar cells Sealed PTB7/PCBM in air PTB7 PCBM

43 Degradation mechanism for organic solar cells Not sealed PTB7/PCBM in air PTB7 Defective PCBM defect states Namkoong et al, unpublished work (2014)

44 Summary Degradation of organic solar cells is due to chemical degradation in the presence of oxygen Longer exposure to oxygen will create many defects and trap centers that will force organic solar cells to reduce lifetime The degradation of organic solar cells is governed by the degradation of PCBM rather than organic polymer

45 Simulation of organic morphologies

46 Simulation of organic morphologies AFM image of organic surface (b) Semiconductor surface

47 Simulation of organic morphologies Polymer: Fullerene 1 : 1 O OR 7 days mixing S F O OMe S OR R= 2-ethylhexyl S S n OR S n P3HT PC 71 BM PTB7

48 Y Axis Title J(mA/cm 2 ) Effect of uniform morphologes Light absorption JV characteristics days days 1 day 3 days day mixing days mixing X Axis Title V(Volt)

49 Phase separation of organics Spinodal decomposition Composition Spatial coordinate Binodal decomposition Composition Spatial coordinate

50 Phase separation dg dh TdS Free energy G: Gibbs free energy H: Enthalpy S: Entropy 0 C A Cx C B 1 Composition of polymer

51 Spontaneous process Enthalpy (H) defines system energy. Entropy (S) measures disorders of systems Reactants Enthalpy (DH)<0 Products DS>0 dg dh TdS < 0

52 Phase separation Polymer: Fullerene 1 : 1 dg dh TdS Free energy G: Gibbs free energy H: Enthalpy S: Entropy ds>0 dg < 0 Spontaneous process dg 0 nonspontaneous process 0 C A Cx C B 1 Composition of polymer

53 Flory-Huggins/Allen-Cahn Flory-Huggins type of free energy Free energy f t RT v site C A CB ( ln C A ln CB ABC ACB) m m A Allen-Cahn Equation C 2 2 B f 2 ( M k C) C 0 C A Cx C B 1 Composition of polymer C: concentration a: solution parameter M: diffusivity of the phase k: gradient energy coefficient

54 Numerical simulation of partial differential equations Finite different method 2 2 x f t f ( 2 ) D D O f f f x f i i i '' '' '' '' '' n n n n n n n n n f f f f f f f f f f b c c b c a b c a b c a b Not suitable for higher order differential equation Memory issues Large truncated errors Convergence issues

55 Finite different vs. Spectral method Finite different method Spectral method M. Mehra et al, Comparison between different numerical methods for discretization of PDEs.

56 Spectral methods : interpolating function 1.5 Polynomial polynomial fitting 1.5 Trigonometric fitting trigonometric fitting f(x) f(x) max error = max error = k (x) polynomials ) ikx k ( x e Fourier spectral method e ikx cos( kx) isin( kx)

57 1D Allen-Cahn equation 3 2 u u x u t u 3 2 k k k k u u u ik t u ) ( u j u k FFT ) ( ) / ) ( ( ) ( ) / ( h ik u h u u k n n k n k ifft(u) u Inverse FFT k n n k n k n k n k u u u ik h u u ) ( ) (

58 3D Allen-Cahn Equations ( C) k C f M t C 2 2 ) ln ln ( B A AB B B B A A A site C C C m C C m C v RT f Flory-Huggins type of free energy Allen-Cahn Equation ( C) k C f M t C ) ( ) cos( x k i D FFT

59 Simulated organic morphologies

60 Summary Spectral method has been used to numerically solve higher order differential equations Flory-Huggins and Allen-Cahn equations were used to simulate 3D organic morphologies

61 Outline Synthesis of ZnO nano/microspheres Control of ZnO morphologies Effect of structure direct agents on ZnO morphologies Control of uniformity, distribution, and size of ZnO spheres Synthesis of ZnO nano/microspheres Organic solar cells Recombination process of organic solar cells Degradation mechanisms of organic solar cells Simulation of 3D organic morphologies Power of Words

62 Experiment of the power of words

63 Words 1 In the beginning was the Word, and the Word was with God, and the Word was God. John 1:1 1 In the beginning God created the heavens and the earth. 3 And God said, Let there be light, and there was light. Genesis 1:1,3. 12 For the word of God is alive and active. Sharper than any double-edged sword. Hebrew 4:12

64 Idiom and proverb Korean proverb Birds hear what is said by day, and rats hear what is said by night

65 Prove Cooler (dense air) Warmer Warmer Ground Birds hear what is said by day Cooler (dense air) Ground Rats hear what is said by night

66 Conclusion Research History

67 Conclusion Re-search His story