Interfacial Charge Recombination and Alternative Chromophores Relevant to Dye-Sensitized Solar Cells

charge recombination;interfacial electron transfer;dye-sensitized solar cells;alternative sensitizers;vitamin b12;cobalamin;phlorin;Chemistry

Brigham, Erinn C.Meyer, Gerald J. ;

Johns Hopkins University

关键词: charge recombination;    interfacial electron transfer;    dye-sensitized solar cells;    alternative sensitizers;    vitamin b12;    cobalamin;    phlorin;    Chemistry;   

Others  :  https://jscholarship.library.jhu.edu/bitstream/handle/1774.2/39540/BRIGHAM-DISSERTATION-2015.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: JOHNS HOPKINS DSpace Repository

【 摘 要 】

The need for cleaner and cheaper electricity and fuel drives this research into the fundamental electron transfer processes in dye-sensitized solar cell (DSSC) model systems. This thesis comprises the elucidation of the mechanism of interfacial charge recombination and the investigation of new, cheaper chromophores for DSSCs. The introduction explains the operating principles of DSSCs and covers specific topics that are useful in understanding the chapters that follow.Chapter 2 explores interfacial charge recombination between electrons in nanocrystalline TiO2 films and photo-oxidized molecular sensitizers anchored to the TiO2. The order of the reaction in both electrons and oxidized sensitizers is established, allowing the rate law for recombination, rate = k*[TiO2(e-)]n[S+]m to be written for the first time with experimental support for m = n = 1. The second order rate constant, k, was measured as 1.8 (±0.1) x 1012 Γ-1s-1 at high [S+]. A mechanism for recombination based on the data presented and previous reports is proposed. A pre-equilibrium where the hole and electron diffuse within their respective volumes until an encounter complex is formed, where upon electron transfer can occur. Because of this, the diffusion, the electron transfer or both could control the rate depending upon the relative rates of diffusion and electron transfer.Chapter 3 is an investigation of the extent to which each of the processes, diffusion and electron transfer, control the rate of recombination with a variety of forward applied biases. Applying a forward bias is one way to increase the rate of diffusion and increase the electron concentration, minimizing the contribution of diffusion to the overall rate of recombination. In order to test whether the electron transfer even had an influence on the rate, a series of sensitizers with a range of reduction potentials was tested. This change in driving force is expected to produce a difference in rate constants for electron transfer, according to the Marcus equation. In order to choose wavelengths to monitor charge recombination, the excited state and charge separate state difference spectra of Ru(dcb)2(CN)2, the Black Dye, N719, and YS12 were collected and are given herein. The kinetic data support driving force dependence with applied potentials of -200, -300, and -350 mV, but at -400 and -500 mV, the behavior changed. The reasons for this change is unclear, but it does seem that 1) electron transfer does contribute to the rate of recombination to oxidized sensitizers at potentials relevant to operating DSSCs and 2) there is a driving force dependence, although at very negative potentials other factors may become more influential.Chapter 4 reports on the excited state electron transfer after light excitation of cob(I)alamin anchored to TiO2. This is an important discovery on a fundamental level because excited state electron transfer has never before been observed from any cobalamin species. The reduction potentials of the cob(I)alamin are not suitable for application to traditional DSSCs, it does offer insight into the nature of the excited states that most efficiently inject into TiO2, and it offers the opportunity for future study of charge recombination in the Marcus normal region under conditions of very negative applied potential, which emphasizes the contribution of the electron transfer to the rate.Chapter 5 reports on a potential new organic, highly asymmetric porphyrinoid sensitizer called a phlorin. The sensitizer was evaluated with incident photon to current efficiency (IPCE) measurements, transient absorption in solution and anchored to TiO2, and spectroelectrochemical measurement of the density of states in free energy of the phlorin reduction and oxidation, in addition to the TiO2 acceptor states. It is proposed that an equilibrium exists between the electrons in the TiO2 and the reduced phlorin, which has a very positive reduction potential. This equilibrium is at least part of the reason why the sensitizer has very low IPCE. In sum, it does not appear to be a promising sensitizer, but the large color changes that are possible in a fairly small potential range might suggest it for use in electrochromic devices.