Luận án tiến sĩ: Understanding the performance determining processes in dye sensitized solar cells

Understanding the Performance Determining Processes in Dye Sensitized Solar Cells by Andras Marton A dissertation submitted to The Johns Hopkins University in conformity with the requirements of the degree of Doctor of Philosophy Baltimore, Maryland June, 2006 UMI Number: 3240768 INFORMATION TO USERS The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleed-through, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion.

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Box 1346 Ann Arbor, MI 48106-1346 Abstract The work presented in this thesis is aimed at optimizing the performance of the dye- sensitized solar cell (DSSC). Chapter 1 presents a brief overview of the history of solar cells and introduces the reader to the components, mechanisms and characterization techniques relevant to the DSSC. Chapter 2 describes the effect pH has on electron injection and collection processes in DSSC’s. Charge collection efficiency, and not the electron injection yield, was related to the TiO, surface protonation-deprotonation equilibria.

In Chapter 3 we examine alternate redox mediators to optimize the regeneration of the oxidized sensitizer while maximizing open circuit voltage (Voc). Bipyrazine based ruthenium sensitizers anchored to SnO; had higher photocurrent efficiencies and cell voltages with the alternative redox mediator (SeCN),/SeCN’ than with the commonly used I,/I. In Chapter 4 the interaction of iodide with a ruthenium ` polypyridyl sensitizer was studied to elucidate the workings of the iodide/tri-iodide redox mediator. The interaction between the sensitizer and iodide in solid phase and in dichloromethane solution was shown to potentially improve solar cell performance.

A more specific interaction between iodide and a zinc tetraphenylporphyrin sensitizer was shown to improve the light harvesting efficiency of the DSSC in Chapter 5. Advisor/Readers: Gerald J. Howard Fairbrother, John P. Toscano ii Acknowledgements First and foremost, I would like to thank Dr.

Gerald Meyer for his guidance, encouragement and patience during my thesis research. I found his deep knowledge and fascination with science to be the greatest motivation a student can receive. Special appreciation goes to my thesis committee Dr. Howard Fairbrother and John Toscano for their time and the feedback they provided.

I would like to thank Dr. David Watson, Dr. Georg Hasselmann and Dr. Jovan Giaimo for all their guidance and friendship.

I appreciate the work of Amy Narducci Sarjeant for the many X-ray structures she collected for me. | I would like to thank Dr. Brian Bergeron, Dr. Arnold Stux and Dr.

Paul Hoertz for their help and ideas during my first few years. Many thanks goes for past and present Meyer group members Shane Ardo, Dr. Laura Bauer, Dr. Nira Birenbaum, Rachael Carlisle, Wei Chen, Chris Clark, Amanda Fond, Robert Freundlich, James Gardner, Gerard Higins, Tamae Ito, Dr.

Feng Liu, Amanda Morris, Ramya Srinivasan, Aaron Staniszewski, Jonathan Stromberg, Diane Wong, Hailong Xia, Dr. Mei Yang for all the great time we spent together. I’m thankful to Dr. Stephen Bransfield and Leslie Machado for their friendship and kindness to me.

Their company brings happiness and comfort to my everyday life. Finally, I would like to thank my parents, my brother, my grandparents, my cousins, aunts and uncles for their love and support. Thank you for understanding my absence from home for so long. Without an amazing family like you none of this would have been possible.

iti Table of Contents ADStYractu.ccccccscccsssssccocssscosees eecce ©@©esoœeeo©eoeeooeeẴẰ©oo009602020606006ee0600060602060606066e96 999909909s960e6e6e6e6e66 lị Acknowledg€Im€InfS.o-oss se s9 S6E16161E56565 88566655 sxssssssseesessee bili Table of COTIÉ€TIÉS.o 85685556sseses IV List of Ta Ì€§. 9989566666854 ssessese VỈ List Of SceImeS. Wi List Of Charts.ccccccsssssscssssssscsssscsescesers H1 sssissnansnssnnsnasesseeosoes Vili -_ List of EÏØUITS. 9 0995685866566566655855585665656esesesssssesse EX Chapter 1: Introduction to Dye-sensitized Solar Cells.2 Composition of a dye-sensitized solar Cell.

--- sàn TT T01 Tàn HT TT HH nguy 6 1.3 Semiconductors in dye-sensitized solar cells: TiOz and SnO¿,.4 Redox Im€iAỂOT.TH HH ThS 12 G1991 01101 1111101901 1.3 Dye-sensitization Mechanisim.4 Characterization of dye-sensitized solar celÏs.5 Optimization of dye-sensitized solar ceÏÏS. -s- s6 G xxx se ssa - 20 IS. 22 Chapter 2: Influence of Surface Protonation on the Sensitization Efficiency of Porphyrin-Derivatized 'TÌO;. Ăn HH ng TT To TH TH HH CC TC ve 31 3.6 Acknowl€gTm€TI.

«cv 1H10 11T TT HH HH HH nu Cu nh nu gu gà 57 3.ccscesccsssctscecccessesscssscsscesscessessscssesaeecssseucensesseecesuscsassssvesesessceesenses 58 Chapter 3: Dye-Sensitized SnO; Electrodes with Iodide and. Pseudohalide Redox Mediators .-- 0555 se sec<sessssssseeesessssss OD 3. «ch Hà TH HT TH ng TT TH TH ve 67 3. HH HH HT TT.

TT TH Ho TH Co ch ng gà 69 3. - - ‹ccn191 11116 1101301 1 ng ng nrườ te 87 3. - ch nHY TH TT ng ng gệp 91 Chapter 4: Static and Dynamic Quenching of Ru() Polypyridyl Excited States by Lodide. HH HT TH HT TH ng 3g 97 4.

ng ng To TH Ti cớ 123 4.6 Acknowledg€Ime€ïnt. --- óc 9919013011011 11T ng HH Tư Tu ch ng 124 4. 125 Chapter 5: Axial Halide Ligation to Surface Attached Zinc Porphyrins: Implications in Dye Sensitized Solar CelÌs. - 9T HT TH TH Tu TH HC cu ni 127 5.- HH H901 HT HT nu Cu gu cớ 128 `.

TH TT nu HH TH CC ky 140 5. 143 Appendix 1: Insights into Dye-Sensitization of Planar TiO;: Evidence for Involvement of a Protonated Surface Stafe.---- c0 01910101 T1 TT TT TH HH kg 147 A1.3 Results and Discussion. 92101 0111110 1101 1g HT Cu TH cuc 153 AL1. Án" 001110100 1n HH TH HH và 154 Appendix 2: Evidence for Static Quenching of MLCT Excited States by TOdide m.2 Results and 1SCUSSIOï\.

TT TH 01 0 1T TT ng nh nh nu ng gu ngu 164 A2.-- 0G G180889 2z 165 Curriculum VỈ(ae. - ng ng HH TT TH HH E915 51511 167 Má List of Tables Table 2. Summary of porphyrin energetiCs. Ground-state absorption maxima Amax.abs, Steady-state photoluminescence maxima Amax.em, SUrface adduct formation constant Kạa, and saturation surface Oe A ce 72 Table 3.

Absorbed photon-to-current conversion efficiencies (APCEs) measured at the absorption maximum for each sensitizer with ~ 1 mW/cm2 irradiance. Open circuit voltage (Voc) measured at the absorption maximum for each sensitizer with ~ 1 mW/cm? irradiance. Short circuit photocurrent densitiesš se); open circuit voltages (Voc), and fill factors (ff) obtained under ~ 11.1 mW/cm’ of 488 nm illumination using (SeCN)2/SeCN’ as the redox mediator. The ground-state absorbance was ~ 0.2 at ABS MM.

The log (7;„) divided by the log (Irradiance), and Vọ. divided by log (27s). Irradiance measurements were performed with 488 nm illumination and (SeCN)2/SeCN’ as the redox rn€di8fOT. -- óc - ch HH kg eg 80 Table 4.

Stern-Volmer quenching constants for Ru(bpy)2(deeb)”*" by iodide as a function Ofionic strength. Crystalparameters for Ru(bpy)2(deeb)(D)2 and Ru(bpy)2(deeb)(PF¢)2. Shiftin absorption maxima, equilibrium constant and extinction coefficient for the addition of halide to SE209 on surface andin propylene carbonate. Summary of sensitizer energetics.ccccescesssssssecssesesessecsessssscsecsesaseneese 150 vi List of Schemes Scheme 1.

Processes involved in the dye sensitization of wide band gap semiconductors in a regenerative solar Cell .-- 5 5 sex crsecs 16 Scheme 2. Mechanism of pH-dependent charge collection efficiency. (a) At basic pH, electron injection is followed by rapid charge recombination. (b) At acidic pH, electron injection is followed by transport through the film.

_ May accompany electron fTATISPOFFẲ.-G- G0 HH net 54 Scheme 3. The conduction band-edge energy of single crystal SnO; (Ecg at pH = 1) is shown on the left hand side with the ground- and excited-state reduction potentials of the ruthenium sensitizers, and the reduction potentials of the mediators in acetonitrile on the right. All potentials are vs. Proposed mechanism consistent with experimental observation following the photoexciation of the [Ru(bpy)z(đeeb),(Ï)¿].--- 5 55 5< << c<s << ces 122 vii List of Charts Chart 2.

Structures of the porphyrin sensitizers, 1: M = Zn(II), X = CO;H; 2: M= 2H, X = CO2H; 3: M =Pt(1), X = CO2H; 4: M = Zn(II), X = PO¿H;. Structures of the porphyrin sensitizers, 1: M = Zn(II), 2: M = Ho, and 3: M "x7. 146 viii List of Figures Figure 1. Structure of cis-di(thiocyanato)bis(2,2’-bipyridyl-4,4’-dicarboxylic acid)ruthenium(I]) sensitizer.

Commonly referred to as N3 dye. Potential distribution for n-type semiconductors. Latimer type diagram for [/I;. All potentials are measured in aqueous solution vs NHHE.

HH HH TH Tàn TT TH TH TH HH HH HH 13 Figure 1. Current-voltage curve for N3 sensitized TiO. electrode immersed in an electrolyte comprised of 0.6 M 3-butyl-1-methyl imidazolium hexaforophosphate, 0.05 M I, in 1:1 valeronitrile:acetonitrile. Quasi monochromatic radiation centered at 500 nm was used.

Scanning electron micrographs of TiO, films on FTO. Images a-c are low- surface-area TiO. films prepared by oxidation of sputter-deposited titanium films at deposition times of 4 min (a), 7. Image d is a nanocrystalline film from the sol-gel pTD.-- óc HH ng cee 33 Figure 2.

Equilibrium binding data for 1 on low-surface-area TiO, (a) and nanocrystalline T1O› (b). The insets show plots of [1]/J vs [1] and fits to the Langmuir adsorption isotherm. For low-surface-area TiOo, Kaa = 10° M? and 7;max = 4. For nanocrystalline TiO, Kag = 101 MT and 7;max = 9.

Superimposed on the plots of 7`vs concentration are fits to the data based on the calculated values Of Kag and Ï mạx.- nàn HH1 1111111 xprrke 35 Figure 2. a) Mott-Schottky plots for an underivatized low-surface-area TiO film with frequency of 100 Hz and peak to peak voltage oscillation of 3 mV. Superimposed are linear fits of the data. b) TiO> flat-band potential (Erg) vs electrolyte pH for an underivatized low-surface-area TiO, film.

Erg was estimated from the onset potentials of bandgap photocurrent (a, slope = 68 + 3 mV/pH) and from the x-intercepts of Mott-Schottky plots (A, slope = 59 + 2 mV/pH). (a) Photocurrent action spectra of 2-derivatized low-surface-area TiO film at electrolyte pH’s of 2. (b) Photocurrent action spectra of 1-derivatized low-surface-area TiO, film at electrolyte pH’s of 2. IPCE = incident photon-to-current efficiency.

Superimposed on the photoaction spectra are absorptance spectra of the films (—). Inset to Fig. 4a: Plots of Soret band photocurrent vs electrolyte pH for 1(m), 2(®), 3(A), and 4(¥). Transient absorbance difference (AA) spectra of 1-modified nanocrystalline TiO, (8) and 1-modified nanocrystalline ZrO, (4) films, 10 ns after 417 nm excitation.

7¡ = 3 x 10° mol/cm” on both films. The films were immersed in 0.1 M Na2B,Q;7 at pH 2. Superimposed on the spectrum of 1-modified TiO) is a simulated spectrum (---) based on the sum of contributions from the ° (r,7*) excited state and the oxidized pOTPHTiH. - G1 HH ng ng gu 43 ix Figure 2.

Transient absorbance difference (AA) traces at 380 nm (a), 430 nm (b), and 700 nm (c) for a 1-modified nanocrystalline TiO. film immersed in 0.1 M Na;B4O; at pH’s of 2. The pump wavelength WAS 417 TIM. G0 0 TH Tu TH TT 01081 00011 0111011100118 550 46 Figure 2.

pH Dependence of the normalized absorbance difference (AAo/AAo max), 10 ns after pulsed laser excitation, at 380 nm (m), 430 nm (®), and 700 nm( A) for a 1- modified nanocrystalline TiO, film. Superimposed on the data is the pH dependence of Soret band sensitized photocurrent for a 1-modified low-surface-area TiO? film (O). Normalized ground-state absorbance and steady-state photoluminescence vs wavelength of Ru(deeb)(bpy)2(PF¢)2 (solid line), Ru(deeb)2(dpp)(PFs)2 (dashed line), Ru(deeb)›(bpz) (PF¢)2 (dotted line), on SnO; in argon purged acetonitrile SOLUION. A Langmuir binding isotherm model of Ru(deeb)(bpy)2(PF¢)2 (squares), Ru(deeb)2(dpp)(PFe)2 (circles), and Ru(deeb)2(bpz)(PF¢)2 (diamonds), applied to the SnO surface.

Absorbance values were recorded after 24 hours of soaking.