Study on charge separation and collection for stable mesoscopic sensitized solar cells

    Study on Charge Separation and Collection for stable mesoscopic sensitized solar cells Li Feng (Bachelor of Eng, SJTU) A thesis submitted for the degree of Doctor of Philosophy Department of Material Science and Engineering National University of Singapore 2013-08 ‐ 1 ‐    DECLARATION I hereby declare that this thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. …………………… Li Feng August, 2013 ‐ 2 ‐    ACKNOWLEDGEMENTS At first, I would like to express my deepest thanks and gratitude to my supervisor Dr. WANG QING for his advice and instruction with kindness and wisdom on research as well as on personality in the past four years. Second, my profound thanks must be extended to Dr. James Robert Jennings, as his enthusiasm in research greatly encouraged me. Moreover, thanks to the abundant discussions with and advices from him, my horizon has been broadened significantly, both theoretically and experimentally. Third, I’d like to thank Mr. Julianto Chua and his supervisor Prof. Subodh Mhaisalkar at NTU for their fruitful help in the fabrication of solid-state dye-sensitized solar cells. My heart-felt thanks also go to my friends, Ms. Liu Yeru, Ms. Fan Li, Dr. Sun Lidong, Dr. Md. Anower HOSSAIN, Dr. Xingzhu Wang, Ms. Koh Zhen Yu, Mr. Huang Qizhao, Mr. Shen Chao and Mr. Pan Feng, for their kind help on the study itself, as well as understanding and tolerance of my heavy equipment occupancy. Besides, I’d like to thank my friend Zhang Wei and his supervisor Professor Liu Bin, for the chance to work with their group on solid state dye-sensitized solar cells. Thanks for the scholarship and equipment offered by NUS and fundings (NUS startup grant No. R-284-000-064-133; URC grant No. R-284-000-068-112; NRF CRP grant No. R-284-000-079-592). Lastly, my forever gratitude goes to my parents, parents-in-law and wife for their great love, understanding and support. ‐ 3 ‐    Table of Contents DECLARATION   ‐ 2 ‐  ACKNOWLEDGEMENTS  . ‐ 3 ‐  Summary  . ‐ 7 ‐  List of Symbols and Abbreviations  ‐ 9 ‐  List of Figures and Tables  . ‐ 12 ‐  Chapter 1. Introduction   ‐ 14 ‐  1.1 Renewable Energy  . ‐ 14 ‐  1.2 Basic Concept and Working Principles of DSCs  . ‐ 16 ‐  1.3 Improving the Performance and Stability of DSCs   ‐ 20 ‐  Chapter 2. Models for Charge Transport and Transfer in DSCs   ‐ 23 ‐  2.1 j-V Characteristic, the Origin of Photovoltage and Photocurrent  . ‐ 23 ‐  2.1.1 Origin of Photovoltage  . ‐ 24 ‐  2.1.2 IPCE, Light Harvesting and Electron Injection   ‐ 25 ‐  2.2 Dye regeneration  . ‐ 28 ‐  2.3 Electron Transport and Recombination  . ‐ 30 ‐  2.3.1 Diffusion Model, trap free case   ‐ 32 ‐  2.3.2 Multiple Trapping Model  . ‐ 33 ‐  2.4 Mass transport.   ‐ 35 ‐  Chapter 3. Experiment Methods  . ‐ 37 ‐  3.1 Fabrication of DSCs with liquid electrolyte.  . ‐ 37 ‐  3.2 Characterization Methods  . ‐ 38 ‐  3.2.1 j-V characteristics  . ‐ 38 ‐  3.2.2 IPCE under different conditions.   ‐ 38 ‐  3.2.3 Impedance Spectroscopy.  . ‐ 39 ‐  3.2.4 Transient Absorption Spectroscopy  ‐ 42 ‐  3.2.5 Other Characterization Techniques.  . ‐ 44 ‐  ‐ 4 ‐    Chapter 4. Evolution of Charge Collection / Separation Efficiencies in Dye-sensitized Solar Cells upon Aging   ‐ 46 ‐  4.1 Introduction  . ‐ 46 ‐  4.2 Experiments  . ‐ 47 ‐  4.3 Results and Discussions   ‐ 48 ‐  4.3.1 j-V Characteristics.  . ‐ 48 ‐  4.3.2 Interpretation of Voc evolution  ‐ 50 ‐  4.3.3 Evolution of charge collection efficiency.  . ‐ 53 ‐  4.3.4 Evolution of separation efficiency derived from short-circuit IPCE.  . ‐ 55 ‐  4.3.5 Evolution of separation efficiency derived from OC IPCE.   ‐ 59 ‐  4.4 Conclusions  . ‐ 61 ‐  Chapter 5. Determining the Conductivities of the Two Charge Transport Phases in Solid-State Dye -Sensitized Solar Cells by Impedance Spectroscopy  . ‐ 63 ‐  5.1 Introduction  . ‐ 63 ‐  5.2 Experimental Section  . ‐ 66 ‐  5.2.1 Fabrication of ss-DSCs.  . ‐ 66 ‐  5.2.2 Device Characterization.   ‐ 67 ‐  5.3 Results and Discussion   ‐ 67 ‐  5.4 Conclusions  . ‐ 77 ‐  Chapter 6. Determination of Sensitizer Regeneration Efficiency in Dye-Sensitized Solar Cells ‐ 78 ‐  6.1 Introduction  . ‐ 78 ‐  6.2 Experiments and Methods  . ‐ 80 ‐  6.3 Results and Discussion   ‐ 81 ‐  6.3.1 Determination of regeneration efficiency using transient and steady-state approaches.   ‐ 81 ‐  6.3.2 Dependence of electron concentration on photon flux in regular and inert cells.   ‐ 89 ‐  6.3.3 Dependence of EDR rate constant on electron concentration in inert cells.   ‐ 91 ‐  6.3.4 Regeneration efficiency under working conditions.   ‐ 94 ‐  6.4 Conclusion . ‐ 95 ‐  Chapter 7. Influence of supporting electrolyte concentration on photocurrent of dye-sensitized solar cells employing robust electrolytes.  . ‐ 97 ‐  7.1 Introduction  . ‐ 97 ‐  ‐ 5 ‐    7.2 Experiments  . ‐ 98 ‐  7.3 Results and discussion  . ‐ 99 ‐  7.3.1 j-V Characteristics   ‐ 99 ‐  7.3.2 Mono IPCE and OC IPCE   ‐ 100 ‐  7.3.3 Light harvesting efficiency and charge collection efficiency  . ‐ 101 ‐  7.3.4 Charge separation efficiency   ‐ 103 ‐  7.4 Conclusion  . ‐ 106 ‐  Conclusions and Outlook   ‐ 107 ‐  Appendix  . ‐ 111 ‐  A.1. Derivation of the impedance expression.  . ‐ 111 ‐  A.2. Redundancy Examination.  . ‐ 112 ‐  A.3. Effect of “parallel” overlayer.  . ‐ 114 ‐  A.4. Typical simulations of electron channel.  . ‐ 115 ‐  References  . ‐ 116 ‐  Publications  . ‐ 130 ‐  ‐ 6 ‐    Summary Current-voltage (j-V) characteristics, impedance spectroscopy (IS) and incident photon to current efficiency (IPCE) were monitored for dye-sensitized solar cells (DSCs) employing different electrolytes under a range of different conditions over one month, in order to correlate the evolution of short-circuit current density , open-circuit voltage and power conversion efficiency (PCE) to the energetic and kinetic parameters such as TiO2 conduction band edge ( ) and effective dye regeneration rate constant. Although no standard aging protocol was followed in this study, the ‘pseudo-aging’ test represents the first attempt to de-convolute various kinetic processes upon prolonged aging, and has provided new insights into device operation. To apply IS for studying charge transport in the mesoporous film of solid-state dye-sensitized solar cells (ss-DSCs), it is necessary to determine the relative conductivities of the electron and hole transporting phases, given the equivalent positions of the distributed electron and hole transport resistances ( and ) in the equivalent circuit. Here in-plane transistor-like ss-DSCs employing spiro-OMeTAD as hole conductor were fabricated and characterized with IS. By design, and are no longer in equivalent positions as they were in ss-DSCs with regular solar cell geometry (regular ss-DSCs), providing a means to determine their values independently. Fitting and simulation results of transistor-like devices combined with cross checks against results obtained for regular ss-DSCs clearly showed that significantly larger than is under all conditions studied. Charge transport and transfer in ss-DSCs were then discussed and effective carrier diffusion lengths are calculated. The results suggested that charge collection is limited by an inadequate electron diffusion length ( ) in ss-DSCs, implying the necessity to enhance electron transport or retard recombination in order to improve the performance of ss-DSCs. The experimental methodology proposed here will find important use in other ss-DSCs. Regeneration of the oxidized dye in DSCs is frequently studied using the transient absorption (TA) technique. However, TA measurements are generally not performed using ‐ 7 ‐    complete DSCs at the maximum power point (MPP) on the j-V curve, and the electron concentration in the nanocrystalline TiO2 films used in these devices is often not well characterized, which may lead to results that are not relevant to actual solar cell operation. In this work, dye regeneration kinetics was studied at the MPP and at open circuit (where interpretation of results is simpler). Using a combination of TA, differential IPCE measurements and IS, the dependence of electron-dye recombination rate and overall sensitizer regeneration efficiency on TiO2 electron concentration was unambiguously demonstrated. The validity of a commonly used approach for determining regeneration efficiency in which the electron-dye recombination rate constant is estimated from TA decays of cells employing a redox-inactive electrolyte solution was also examined. It was found that this widespread practice may be unsuitable for accurate determination of the regeneration rate constant or efficiency. It was further shown that, despite near-quantitative regeneration at short circuit or low photovoltage, PCE was limited by inefficient regeneration in stable DSCs with practically relevant electrolyte solutions using I /I as redox mediator. The effect of a systematic variation of supporting electrolyte (SE) concentration on photocurrent of DSCs using low-volatility electrolytes, which showed good long-term stability and usually possess high viscosity and ionic strength, was examined. A degradation of the j-V characteristics and PCE with increasing SE concentration was observed. The relative importance of electrolyte viscosity and ionic strength in determining charge collection and charge separation yields were discussed. After correction of charge collection losses, it was found that even near open-circuit conditions where mass transport effects can be neglected, charge separation yield is strongly dependent on SE concentration, which was qualitatively consistent with a kinetic electrolyte effect or possibly ionic strength/viscosity related modulation in reorganization energies. The results implied the importance of voltage-dependent dye regeneration yield for solvent-free electrolytes, where ionic strength and viscosity are usually very high. ‐ 8 ‐    List of Symbols and Abbreviations cross section area of the mesoporous film chemical capacitance of the mesoporous film distributed (normalized) chemical capacitance of the mesoporous film thickness of the mesoporous film free electron diffusion coefficient without trapping and detrapping D oxidized dye molecule effective electron diffusion coefficient with trapping and detrapping DSCs Dye-sensitized Solar Cells TiO2 conduction band edge EDR Electron Dye Recombination EER Electron Electrolyte Recombination , quasi Fermi level of electrons in TiO2 , Fermi level of redox couples in the electrolyte frequency FF Fill Factor FTO Fluorine doped Tin-Oxide HOMO highest occupied molecular orbital HTM Hole Transporting Material incident photon flux IL Ionic Liquid IPCE Incident Photon to Current Efficiency IS Impedance Spectroscopy ‐ 9 ‐    j-V current-voltage characteristics short-circuit current density Boltzman constant EDR rate constant EER rate constant pseudo first-order EER rate constant dye regeneration rate constant pseudo first-order dye regeneration rate constant free electron diffusion length effective electron diffusion length effective hole diffusion length LUMO lowest unoccupied molecular orbital MPP Maximum Power Point free electron concentration in the mesoporous film trapped electron concentration in the mesoporous film total electron concentration in the mesoporous film PCE Power Conversion Efficiency PD photo-diode elementary charge QFL Quasi Fermi Level constant phase element (non-ideal chemical capacitance) distributed (normalized) constant phase element charge transfer (recombination) resistance of the mesoporous film distributed (normalized) charge transfer (recombination) resistance ‐ 10 ‐    References References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 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ACS Nano, 2013. 7(9): p. 8233-8242. 2. Li, F.; Jennings, J. R.; Wang, Q.; Chua, J.; Mathews, N.; Mhaisalkar, S. G.; Moon, S.-J.; Zakeeruddin, S. M.; Grätzel, M., Determining the Conductivities of the Two Charge Transport Phases in Solid-State Dye-Sensitized Solar Cells by Impedance Spectroscopy. The Journal of Physical Chemistry C 2013, 117 (21), 10980-10989. 3. Li, F.; Jennings, J. R.; Mathews, N.; Wang, Q., Evolution of Charge Collection / Separation Efficiencies in Dye-Sensitized Solar Cells Upon Aging: A Case Study. Journal of The Electrochemical Society 2011, 158 (9), B1158-B1163. 4. Jennings, J. R.; Li, F.; Wang, Q., Reliable Determination of Electron Diffusion Length and Charge Separation Efficiency in Dye-Sensitized Solar Cells. The Journal of Physical Chemistry C 2010, 114 (34), 14665-14674. 5. Zhang, W.; Zhu, R.; Li, F.; Wang, Q.; Liu, B., High-Performance Solid-State Organic Dye Sensitized Solar Cells with P3HT as Hole Transporter. The Journal of Physical Chemistry C 2011, 115 (14), 7038-7043. 6. Xingzhu Wang, Jing Yang, Hao Yu, Feng Li, Li Fan, Dawei Zhang, Yeru Liu, Zhen Yu Koh, Jiahong Pan, Lei Yan and Qing Wang, Benzothiazole-cyclopentadithiophene Bridged D-A-π-A Organic Sensitizer with Enhanced NIR Light Absorption for Efficient Dye-sensitized Solar Cells, chem. comn., accepted. Conferences: ‐ 130 ‐    Publications 7. Li, F.; Jennings, J. R.; Wang, Q., Dye Regeneration under Working Conditions in Dye-Sensitized Solar Cell, HOPV 13, Seville, Spain. 8. Li, F.; Jennings, J. R.; Mhaisalkar, S. G.;Wang, Q., Influence of supporting electrolyte concentration on photocurrent of dye-sensitized solar cells employing robust electrolytes, ICMAT 2011, Singapore. 9. Li, F.; Jennings, J. R.; Wang, Q., Quantifying Dye Regeneration Efficiency in Dye-sensitized Solar Cells at Operating Conditions, ICMAT 2013, Singapore. Manuscript in progress: 10. Feng Li, James Robert Jennings, Xingzhu Wang, Li Fan, Zhen Yu Koh, Lei Yan, Hao Yu and Qing Wang, Influence of electrolyte ionic strength and viscosity on recombination and regeneration kinetics in dye-sensitized solar cells, submitted, invited article to The Journal of Physical Chemistry C for Prof. Michael Grätzel Festschrift.   ‐ 131 ‐    [...]... change EER reaction order in total electron concentration charge collection efficiency electron injection efficiency light harversting efficiency dye regeneration efficiency charge separation efficiency λ wavelength free electron lifetime effective electron lifetime angular frequency ‐ 11 ‐    List of Figures and Tables   Figure 1-1 Typical structure of DSCs Figure 1-2 Major charge transfer and transport... motivation and introduction of this work 1.2 Basic Concept and Working Principles of DSCs As mentioned in the previous section, DSCs are a promising challenger to the conventional photovoltaics They can be classified as excitonic solar cells because charge carriers are generated and separated simultaneously across a heterointerface upon excitation.[11, 12] Thus the created excitons are dissociated before... with the IS and differential IPCE Lastly, it must be noted here as the focus of my work is the charge separation and collection in mesoscopic systems Little effort has been done to actually optimize the PCE of the devices, and this may be addressed in the future, which is included in the conclusion and outlook section with more details ‐ 22 ‐    Chapter 2 Chapter 2 Models for Charge Transport and Transfer... to expect the interaction between them during diffusion, which is known as ambipolar diffusion and the ambipolar diffusion coefficient of electrons is: / where is the ambipolar diffusion coefficient, electrons and counter ions, and In DSCs under typical conditions, and thus between (2.16) / and are the concentration of are the corresponding diffusion coefficients [98-100] (∼3 × 1020 cm-3) is much higher... DSCs Figure 4-3 Evolution of thermodynamic and kinetic parameters obtained from IS Figure 4-4 Evolution of and Figure 4-5 Normal and 1-Sun IPCE of fresh and aged DSCs; predicted and measured ; mono IPCE of the fresh cells and corresponding differential IPCE at 627 nm Figure 4-6 Evolution of upon aging, determined from normal IPCE and 1-Sun IPCE Figure 4-7 OC IPCE, and / versus for selected aging times;... discussed in Chapter 6 2.2 Dye regeneration As mentioned previously, DSCs can be considered as excitonic solar cells For a complete charge separation process, a “hole injection” procedure coupled with the electron injection is naturally expected This “hole injection” process in DSCs is termed as dye regeneration (refer to Fig 2-2), as the oxidized dye molecules are reduced and return to the original ground... basis for the diffusion oriented charge transport is the high concentration of counter ions in the electrolyte (c.a 0.1~1 M, much larger than and the total electron concentration away from the electrons ( , as will be discussed later.) only nanometers dybye length of electrons), which effectively screen the negative charge in the TiO2 With such short distances, it is reasonable to expect the interaction... external contact and mechanical support for the electrode; and in order to improve interconnection among particles and adhesion to the substrate, nanoparticle electrodes are often sintered and heat treated, whereas nanorod and nanotube based electrodes can be grown directly and continuously on appropriate substrates [16, 17] Given the unique nature of DSCs (photons harvested by sensitizers), semiconductors... be elaborated Under illumination, sensitizers are excited by photons with certain energy [excitation, route (1)] and become oxidized after injecting electrons into the vacant electronic states (conduction band states) of semiconductor film [electron injection, route (2)], which usually occurs on a sub-picosecond timescale [49] Whereas it must be admitted that relaxation of excited dye molecules [route... 2.9-2.11 only describes the regeneration using I /I ‐ 28 ‐    and do not Chapter 2 apply to other redox mediators such cobalt complexes, ferrocene derivatives and HTM.[36, 41, 80, 81] Based on the reaction mechanisms and the continuity equations of free electrons and oxidized dye molecules, , which is independent of I , can be derived [51]: I (2.12) I where and are the rate constants for dye regeneration . ‐1‐    Study on Charge Separation and Collection for stable mesoscopic sensitized solar cells Li Feng (Bachelor of Eng, SJTU) A thesis submitted for the degree of Doctor. Absorption Spectroscopy ‐  42‐ 3.2.5 Other Characterization Techniques. ‐  44‐ ‐5‐  Chapter 4. Evolution of Charge Collection / Separation Efficiencies in Dye -sensitized Solar Cells upon. transient and steady-state approaches. ‐81‐ 6.3.2 Dependence of electron concentration on photon flux in regular and inert cells.  ‐ 89‐ 6.3.3 Dependence of EDR rate constant on electron concentration