A triphenylamine dye model for the study of intramolecular energy transfer and charge transfer in dye-sensitized solar cells

A novel dye (2TPA-R), contg. two triphenylamine (TPA) units connected by a vinyl group and rhodanine-3-acetic acid as the electron acceptor, is designed and synthesized successfully to reveal the working principles of org. dye in dye-sensitized solar cells (DSSCs). 2TPA and TPA-R, which consist of two TPA units connected by vinyl and a TPA unit linked with rhodanine-3-acetic acid, resp., are also synthesized as refs. to study the intramol. energy transfer (EnT) and charge transfer (ICT) processes of 2TPA-R in CH2Cl2 soln. and on a TiO2 surface. The results suggest that the intramol. EnT and ICT processes show a pos. effect on the performance of DSSCs. However, the flexible structure and less-adsorbed amt. of dye on TiO2 may make it difficult to improve the efficiency of DSSCs. This study on intramol. EnT and ICT processes acts as a guide for the design and synthesis of efficient org. dyes in the future.

Strategies to Optimizing Dye-Sensitized Solar Cells

Sophie Wenger

Dye-sensitized solar cells (DSCs) constitute a novel class of hybrid organic-inorganic solar cells. At the heart of the device is a mesoporous film of titanium dioxide (TiO2) nanoparticles, which are coated with a monolayer of dye sensitive to the visible region of the solar spectrum. The role of the dye is similar to the role of chlorophyll in plants; it harvests solar light and transfers the energy via electron transfer to a suitable material (here TiO2) to produce electricity — as opposed to chemical energy in plants. DSCs are fabricated of abundant and cheap materials using inexpensive processes (e.g. screen-printing) and are likely to be a significant contributor to the future commercial photovoltaic technology portfolio. The work conducted during this thesis aimed at optimizing the DSC using three different strategies: The use of versatile organic sensitizers for stable and efficient DSCs, the study of tandem device architectures in combination with other solar cells to harvest a larger fraction of the solar spectrum, and the development of a validated optoelectric model of the DSC. Organic donor-π-acceptor dyes are an interesting alternative to the standard metal-organic complexes used in DSCs. Efficient photovoltaic conversion and stable performance could be demonstrated with three classes of donor systems, namely diphenylamine, difluorenylaminophenyl, and π-extended tetrathiafulvalene. The highest conversion efficiencies were obtained with a difluorenylaminophenyl donor system (η = 8.3 % with a volatile electrolyte and η = 7.6 % with a solvent-free ionic liquid, which was a new record for organic dyes at the time of publication). Surprisingly, efficient regeneration of the oxidized dye by the I-/I3- redox mediator was found with the π-extended tetrathiafulvalene system, even though the thermodynamic driving force was as low as 150 mV. So far driving forces of 300-500 mV had been regarded as necessary for efficient regeneration of the dye cation. Also, important structure-property relationships pertaining to the recombination of electrons with the electrolyte and to the stability of the device could be identified (i.e. effect of linear vs. branched structure, linker length, and moieties used). The power conversion efficiency of solar cells can be extended beyond the limit for a single cell (∼ 30 %) by using multiple cells with different optical gaps in a tandem device. DSCs and chalcopyrite Cu(In,Ga)Se2 (CIGS) solar cells have complementary optical gaps and are thus suitable systems for integration in a tandem device. It was shown that a monolithic DSC/CIGS tandem device has the potential for increased efficiency over a mechanically stacked device due to increased light transmission to the bottom cell, and a monolithic DSC/CIGS device with an initial efficiency of η = 12.2 % was demonstrated. The degradation of the devices — induced by the corrosion of the CIGS cell in contact with the I-/I3- redox mediator — could be retarded with a protective thin conformal ZnO/TiO2 oxide layer coated on the CIGS cell by atomic layer deposition. Finally, an experimentally validated optical and electrical model of the DSC has been developed to assist the optimization process, which is predominantly conducted by empirical means in the DSC research community. The optical model allows to accurately calculate the internal quantum efficiency of devices, i.e. the ratio of extracted electrons to absorbed photons by the dye, a crucial and so far difficult to determine characteristic. Intrinsic parameters — like injection efficiency, electron diffusion length, or distribution of trap states in the TiO2 — can be extracted from experimental steady-state and time-dependent data with the electric model. The model allows to make a comprehensive and quantitative loss analysis of the different optical and electric loss channels in the DSC. The model has been implemented with a graphical user interface for straightforward usage. All three optimization strategies — organic dyes, tandem architecture, and device modeling — developed during this thesis make a valuable contribution to the development and commercialization of inexpensive and high efficiency DSCs. They enable a comprehensive view of the system and pave the way for a systematic analysis and reduction of losses, which has been the ultimate route to success for several established photovoltaic technologies.

EPFL2010

Linear and Time-Resolved THz Spectroscopy of Photonic and Charge Transporting Systems

Jan Cornelius Brauer

Photo-induced charge separation and charge transport are fundamental processes in many energy conversion techniques, where light is converted into chemical energy (PEC), into electrical energy (solar cells) or in the opposite sense if electrical energy is converted into light (LEDs). In all cited examples, charge carriers have either to be transported towards an interface where they can undergo radiative recombination, trigger a chemical reaction, or they have to be evacuated from the interface as free charge carriers to be able to perform work in an external electrical circuit. In this work the charge generation dynamics and transport is studied using THz time domain spectroscopy and complimentary techniques on systems finding application in dye sensitized solar cells and PEC cells for water splitting. The functioning of a dye sensitized solar cell is based on the kinetic competition between several reactions, i.e. electron injection, dye-regeneration, electron back- reaction and charge extraction. Accordingly the rates of all of these reactions have to be optimized in order to optimize the cell performance. This requires a good understanding of the underlying mechanism of these processes. In photochemical cells, the encountered processes are in a way similar. After initial excitation, the generated charges have to be separated and have to reach either the interface to the electrolyte or the external circuit before being annihilated by recombining. Knowing the diffusion length and carrier lifetime in a specific material might therefore be helpful to optimize the film morphology, i. e. the aspect ratio. Chapter 1 gives an overview over the functioning of DSCs and PEC cells and summarizes some theoretical aspects of the processes encountered in these devices. Chapter 2 is devoted to the description of the experimental tools used and built in this work. In chapter 3, charge transport in a liquid molecular hole conductor, 10- methylphenoxazine, is investigated. The transport is rationalized using a model taking into account ionic diffusion and intermolecular hole transfer, i. e. hopping. It is found that the conduction occurs principally by hopping, as the HTM cation seems to be mainly present in an ion-paired form with the anion from the doping agent. Hopping of charge requires molecular vibration to bring acceptor and donor molecule in electronic resonance. Two vibrational modes of the HTM cation and neutral molecules that might be coupled to charge transport are observed. In chapter 4, electron injection from a dye into the conduction band of TiO2 films is investigated by monitoring the appearance of the free electron. This signal is compared to the signal arising from direct UV excitation of the TiO2. In addition to previous studies done in the group, the effect of dye aggregation leading to decelerated electron injection is investigated. Comparing the THz signal amplitude to the injected charge carrier concentration, the THz mobility in TiO2 films was found to be 0.1-0.15 cm2V-1s-1 and constant over the investigated charge carrier concentrations. Furthermore the influence of sintering temperature of the TiO2 film has been addressed in a preliminary study. In chapter 5, the influence of liquid electrolytes, of a solid state hole conducting material and of the commonly used additives Li+ and tBP on the charge generation dynamics have been investigated. While in this work the presence of an electrolyte/HTM has not shown any influence on the electron injection dynamics, the generation of the free electron seems to be strongly dependent on the molecular surrounding. This discrepancy is discussed in terms of a coulombic bound electron- hole pair as an intermediate state in the electron injection dynamics. Chapter 6 contains a study on the charge carrier dynamics in cuprous oxide, being used as material for photoelectrochemical water splitting. The wavelength and fluence dependence on the charge carrier lifetime, mobility and diffusion length was studied. The strong dependence of the charge carrier lifetime is discussed in terms of trapping mediated recombination.

EPFL2012

Photoinduced Ultrafast Dynamics of Coumarin 343 Sensitized p-Type-Nanostructured NiO Films

Ulf Anders Hagfeldt

Photoinduced electron transfer from the valence band of nanocryst. NiO, a p-type semiconductor, to an excited bound dye, Coumarin 343, and the subsequent recombination have been measured by femtosecond transient absorbance spectroscopy probing with white light. It was found that both processes are nonexponential. The photoinduced electron transfer from the semiconductor to the excited bound dye has an ultrafast component (∼200 fs), which is comparable to the time consts. measured for photoinduced electron injection in C343-TiO2 colloid solns. The process is very efficient and constitutes the main path of deactivation of the excited dye. Back electron transfer is also remarkably fast, with the main part of the recombination process happening with a time const. of ∼20 ps. Dye-sensitized nanostructured p-type semiconductors are attractive materials due to their potential use as photocathodes in dye-sensitized solar cells and solid electrolytes in solid-state dye-sensitized solar cells. To the authors knowledge, this is the first time that the photoinduced electron-transfer kinetics of a sensitized p-type semiconductor has been studied.