DENSITY FUNCTION THEORY STUDY OF DIFFERENT DYE MOLECULES AND TIO 2 DOPED BROOKITE SURFACES FOR FOR APPLICATION IN DYE-SENSITIZED SOLAR CELL

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1 DENSITY FUNCTION THEORY STUDY OF DIFFERENT DYE MOLECULES AND TIO 2 DOPED BROOKITE SURFACES FOR FOR APPLICATION IN DYE-SENSITIZED SOLAR CELL Eric N Maluta 1, 2, Steve R Dima 1, 2, Steve Ranwaha 1, Ife Elebeleye 1, Rapela R Maphanga 2,3 1 Department of Physics, University of Venda, P/Bag x 5050, Thohoyandou, National Institute for Theoretical Physics (NITheP), Gauteng, South Africa 3 CSIR Modelling and Digital Science, PO Box 395 Pretoria 0001, Pretoria, South Africa.

2 OUTLINE Background Introduction to DSCCs Methodology Results and Discussions Conclusions Acknowledgements

3 BACKGROUND Silicon solar cells High efficiency and stability, on the market. Problem: Expensive, heavy, rigid, not yet compatible with real low cost technology. Uses high cost technology DSSCs Affordable since production costs are low Flexible Relatively high photon to energy conversion efficiency. Problem: Low efficiency

4 Introduction to DSCCs 1. Conductive supporting glass 2. TiO 2 nanoparticles (nanotubes). 3. A dye (Sensitizer), which act as the light absorber. 4. A electrolyte (redox couple (I - /I 3- )), located in the space between the dye and the counter electrode. 5. Counter electrode Configuration of a dye sensitized solar cell

LIGHT ABSORPTION IN DSSCs Light absorption in dye sensitized solar cell The dye absorbs photons of light and become excited and inject an electron into the conduction band of TiO 2 semiconductor.

5 LIGHT ABSORPTION IN DSSCs Light absorption in dye sensitized solar cell The dye absorbs photons of light and become excited and inject an electron into the conduction band of TiO 2 semiconductor. The electron move by diffusion through the porous TiO 2 layer to the conductive support and passes the external load to reach the counter electrode. The positive charges resulting from injection process is transferred to tri-iodide to yield iodide. The iodide reduces the oxidized dye S + to its original state. S.

6 Two important aspects of improving DSSCs efficiency Photon absorption and Electron Transport Two main components Dye molecules ( organic and inorganic) and TiO 2 as a semiconductor The optimizations of these two parameters can enhance the DSSCs efficiency. Polyene-diphenylaniline dye (D5) and (D7) adsorbed on Brookite TiO 2 Surface Hagberg, D.P, Edvinsson T, Marinado T, Boschloo G, Hagfeldt A and Sun L, 2006, Chem. Commun.,

Why Dye molecules and TiO 2 Semiconductor TiO 2 nanoparticles film (layer) and the dye sensitizer loaded on its surface are the most important parts of the DSSCs structure for

For optimum utilization in solar energy, the requirement is the reduction of TiO 2 band gap or increase the absorption of the dye molecules to NIR, so that the absorption

7 Why Dye molecules and TiO 2 Semiconductor TiO 2 nanoparticles film (layer) and the dye sensitizer loaded on its surface are the most important parts of the DSSCs structure for photon absorption and transport. For optimum utilization in solar energy, the requirement is the reduction of TiO 2 band gap or increase the absorption of the dye molecules to NIR, so that the absorption properties match well with solar spectra. Low dye uptake High dye uptake Low current generation and overall conversion efficiency High current generation and overall conversion efficiency Which region of the solar spectrum does the dye molecules absorb. Giovannetti R, Zannotti M, Alibabaei L, Ferraro S, Int. J Photoenergy 2014

8 The TiO 2 becomes active only under UV light with energy greater than the TiO 2 band (3.4eV for brookite phase ). Unfortunately for its application on dye-sensitized solar cells, TiO 2 is inactive under visible light due to its wind band gap. Han YX, Yang CL, Wang MS, Ma XG and Wang, LZ 2015 Sol. Energy Mater. Sol. Cells Zallen R, and Moret MP 2006 Solid State Commun. 137 (3) Ma J.G, Zhang CR, Gong JJ, Wu YZ, Kou SZ, Yang H, Chen YH, Liu ZJ and Chen HS 2015 Mater 8(8)

9 Finite element method (MATLAB) The TiO 2 nanotube is represented by a cylinder penetrating the entire TiO 2 film with a bulk concentration of dye molecules at the right hand end of the nanotube. R 0 Pore radius 10 nm d Pore length 20 μm w Initial spread of dye 0.01d D Dye diffusion coefficient cm 2 s -1 l mol Length of dye molecule 0.5 nm Γ max Maximum pore surface coverage cm -2

where D is the diffusion coefficient, c is the concentration, r is the distance

10 The concentration of dye molecules inside the film can be calculated using Fick s second law for Cartesian (one dimensional) and Cylindrical (two dimensional). where D is the diffusion coefficient, c is the concentration, r is the distance from the centre of the pore and x is the distance along the pore from the extracting electrode

$Surface coverage We consider the molecules flowing onto the surface and being adsorbed onto it les than the desorbed molecules to give a fractional surface coverage, given by where Γ(x, t) is the$

11 Surface coverage We consider the molecules flowing onto the surface and being adsorbed onto it les than the desorbed molecules to give a fractional surface coverage, given by where Γ(x, t) is the surface coverage a distance z from the electron extracting electrode and Γ max is the maximum surface coverage of the TiO 2 nanotube film. If all molecules a distance l mol out of the surface are adsorbed, where l mol is the length of the dye molecules, where k ads and k des are the adsorption and desorption rate of dye molecules respectively.

Results Along the pore the diffusion takes place slower and takes longer than the characteristic time (t z =d 2 /D) Diffusion along the pore is slowed down by

12 Results Along the pore the diffusion takes place slower and takes longer than the characteristic time (t z =d 2 /D) Diffusion along the pore is slowed down by the adsorption of dye molecules at the pore surface This delay is seen more clearly for large k values Concentration along the TiO 2 film Surface concentration

13 Effect of time on the surface concentration Surface concentration on the TiO 2 film for 12 and 24 hours

14 Surface concentration on the TiO 2 film for 36 and 48 hours

15 Time variation of dye uptake on the pore at the second end is the same for different diffusion coefficient. The graph was drawn with the time scaled by t z =d 2 /D, where d is the length of the nanotube and D is the diffusion coefficient. Concentration along the TiO2 film Surface concentration

16 C. Lee, W. Yang, R. G. Parr, Phys. Rev. B, 37 (1988) 785. METHODOLOGY 02 The structures of four dye molecules were built using Gauss view molecular builder interface. All DFT/TD-DFT calculations were executed by Gaussian 09 quantum chemical package. The structural optimization of the ground state of the diphenylaniline dyes in gas phase was performed using the hybrid density functional B3LYP and 6-31G* basis sets. The same basis sets were used to simulate the UV-Vis spectra of the dyes in gas phase. About singlet-singlet transitions was considered in order to account for the excitation energy in the whole of the absorption spectrum. The ground state structures obtained through geometrical optimization imported into a new 3D atomic window and the optical properties were calculated using Material Studio Vulnerability Analysis Methodology Program (VAMP) ( for some of the calculations Segall M, Lindan PJD, Probert M, Pickard C, Hasnip P, Clark S, Payne M, 2002, J Phys Condens Matter 14 (11), 2717

17 METHODOLOGY 02 All calculations was carried out using Cambridge Sequential Total Energy Package (CASTEP) code in Materials Studio The generalized gradient approximation (GGA) in the scheme of Perdew- Burke-Ernzerhof (PBE) for the exchange-correlation functional was considered. The convergence test was performed to determine convergence parameters (energy cut-off and k-points) The Brilloiouin zone k-point sampling was performed using a Monkhorst Pack mesh and the cut-off potential energy of plane waves was set to 650 ev The surfaces were doped by replacing one Ti or O element with Ru element. Segall M, Lindan PJD, Probert M, Pickard C, Hasnip P, Clark S, Payne M, 2002, J Phys Condens Matter 14 (11), 2717

18 Results and discussion Table 1. Optimized structural parameters for bulk brookite TiO 2 compared with experimental and previous theoretical results Experimental [ # ] This work Literature [*] Results Deviation (%) Results Deviation (%) a (Å) b (Å) c (Å) # Beltran A, Gracia L and Andres J J. Phys. Chem. B, 110(46) *Meagher E. P and Lager G.A 1979 Mineral

19 Structures (A) (B) (C) Figure 4. Theoretical models. (A) (100), (B) (110) TiO 2, and (C) (210) TiO 2

20 (A) Ru Ti (B) O Representation of a doping method, which is a substitution method.

21 Results and discussion Table 1. Optimized structural parameters for bulk brookite TiO 2 compared with experimental and previous theoretical results Experimental [ # ] This work Literature [*] Results Deviation (%) Results Deviation (%) a (Å) b (Å) c (Å) # Beltran A, Gracia L and Andres J J. Phys. Chem. B, 110(46) *Meagher E. P and Lager G.A 1979 Mineral

22 (110) Surface Electronic Properties

23 Eg = ev (210) Surface Electronic Properties

24 (210) Surface Electronic Properties

25 Calculated optical properties of doped (100) and (210) surface

26 Calculated optical properties of doped (110) surface

Oxygen, Grey balls Represent Carbon and white balls

27 CR1 CROCONATES DYE MOLECULES CR2 Figure Optimized structures of CR1 and CR2 where Red ball represent Oxygen, Grey balls Represent Carbon and white balls represent Hydrogen Chitumalla, R. K., Lim, M., Gao, X., and Jang, J. (2015).

28 Figure Calculated UV-Vis spectrum for CR1 and CR2 dye molecules.

29 Figure Calculated optical absorption for two dye molecules.

30 WAVELENGTH( nm) ABSORPTION LHE LHE(%) WAVELENGTH (nm) ABSORPTIION LHE LHE (%) The light harvesting efficiency of CR1 and CR2 dye molecules.

31 The HOMO-LUMO energy gap of CR1 and CR2 dye molecules. DYE MOLECULE HOMO (ev) LUMO (ev) HOMO-LUMO GAP (ev) CR CR

32 Computed isodensity surfaces for HOMO, LUMO of CR1 and CR2 using B3LYP/6-311+G (d,p) level of theory CR1 HOMO CR1 LUMO CR2 HOMO CR2 LUMO

33 Polyene-Diphenylaniline Organic Chromophores

34 Figure Optimized structure of D5: 3-(5-(4-(diphenylamine) styrl) thiophen-2-yl)-2-cyanoacrylic acid. (Atoms represented according to colour: carbon in grey, nitrogen in blue, sulphur in yellow, oxygen in red and hydrogen in white). D. Kuang, P. Comte, S. M. Zakeeruddin, D. P. Hagberg, K. M. Karlsoon, L. Sun, M. K. Nazeeruddin, M. Gratzel. Solar Energy, 85 (2011) 1189.

35 Figure Optimized structure of D7: 3-(5-bis(4-(diphenylamino)styrl)thiophen-2-yl)-2- cyanoacrylic acid. (Atoms represented according to colour: carbon in grey, nitrogen in blue, sulphur in yellow, oxygen in red and hydrogen in white).

36 Figure Optimized structure of D9: 5-(4-(bis (4-methoxyphenylamino) styrl) thiophen-2-yl)-2- cyanoacrylic acid. (Atoms represented according to colour: carbon in grey, nitrogen in blue, sulphur in yellow, oxygen in red and hydrogen in white).

37 Figure Optimized structure of D11: 3-(5-bis (4.4-dimethoxyphenylamiono) styrl) thiophen-2-yl) -2- cyanoacrylic acid (Atoms represented according to colour: carbon in grey, nitrogen in blue, sulphur in yellow, oxygen in red and hydrogen in white).

38 D5 Bond length D7 Bond length D9 Bond length D11 Bond length / / / / / / / / Angles Angles Angles Angles / / / / / / Dihedral Dihedral Dihedral Dihedral / / / / Table 2. Selected bond lengths (nm), bond angles ( ) and dihedral ( ) of D5, D7, D9 and D11 dyes. Dihedral or contained by two plane face

39 The calculated geometrical properties of D5 dye are in close agreement with those reported in the literature. Similar geometrical characteristics resulting from similar geometry of the dye structures were observed for D5, D7, D9 and D11. The distance between the C atom in carboxyl and N atom in aniline were 1.435, 1.435, and nm for D5, D7, D9 and D11, respectively Z. Cai-Rong, L. Zi-Jiang, Y. Chen, H. Chen, W. You-Zhi, Y. Li-Hua, J. Mol. Struct., 899 (2009) 86.

40 Figure Calculated frontier molecular orbitals using B3LYP/6-31G* theory and isodensity surfaces of HOMO and LUMO of D5, D7, D9 and D11 dyes.

41 HOMO in D5 is more delocalized over the entire molecular region whereas the HOMOs in D7, D9 and D11 are mainly distributed in the electron donor molecules functional group but extend to linker groups in D7. However, the HOMO- LUMO gaps are 2.27eV, 2.72eV, 1.51eV and 1.53eV for D5, D7, D9 and D11 respectively The LUMOs in D5, D7, D9 and D11 are localized over the cyanoacrylic acid anchor and the linker groups although they extend to some of the polyenediphenyl in D11 dye. Different electronic density distribution between HOMO and LUMO results in intermolecular charge separation between the donor and the acceptor group of the sensitizer when excited f

42 0,9 0,8 D5 D7 D9 D11 f 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0, Wavelenght (nm) Figure Simulated absorption spectra of D5, D7, D9 and D11 dyes where f is the absorption also called the oscillator strength of the dye associated with the maximum absorption of the dye U. Mehmood, I. A. Hussein, K. Harrabi, S. Ahmed, J. Adv. Mater. Sci. Eng., (2015) 1. L. Schmidt-Mende, U. Bach, R. Humphry-Baker, T. Horiuchi, H. Miura, S. Ito, M. Gratzel, Ersch Adv. Mater., 17 (2005) 813.

43 Dyes λ max E (ev) ƒ LHE D D D D Table 3. Computed maximum absorption (λ max ), excitation energy (ev), oscillator strength (ƒ) and light harvesting efficiency (LHE) of the dyes. From the calculations in Table 3, D9 and D11 shows the highest light harvesting efficiencies, also, from the simulated absorption spectra in Figure 6, D11 shows more spectral red shift absorption than D5, D7 and D9. This explains the highest current-voltage characteristics observed for D11 as reported by Kuang s electrochemical experiment

44 Conclusions In this work, four polyenediphenyl based photosensitizers (D5, D7, D9 and D11) were studied computationally, using DFT and TD- DFT methods. The D5 and D7 dyes showed absorption in the visible region while D9 and D11 showed spectral red shift absorption owing to their reduced HOMO-LUMO energy gap. It was also observed that the D9 and D11 dyes with methoxy groups in their donor moiety showed a broader peak and improved red spectra response than D5 and D7 dyes without methoxy group. The D9 and D11 sensitizers with two diphenylaniline donor moiety showed the highest light harvesting efficiencies. The low HOMO-LUMO energy gap, absorption spectra and high light harvesting efficiency suggest that the addition of the methoxy groups to the donor moiety of D9 and D11 is responsible for their improved optical performance. The results suggested that the dyes are promising sensitizers for DSSCs application.

45 ACKNOWLEDGEMENTS University of Venda (Department of Physics) Centre for High Performance Computing (CHPC) National Institute for Theoretical Physics (NITheP) NRF Thuthuka Program

46 Thank you

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