Influence of the Donor Size in D-pi-A Organic Dyes for Dye-Sensitized Solar Cells

We report two new molecularly engineered push-pull dyes, i.e., YA421 and YA422, based on substituted quinoxaline as a pi-conjugating linker and bulky-indoline moiety as donor and compared with reported IQ4 dye. Benefitting from increased steric hindrance with the introduction of bis(2,4-dihexyloxy)benzene substitution on the quinoxaline, the electron recombination between redox electrolyte and the TiO2 surface is reduced, especially in redox electrolyte employing Co(II/III) complexes as redox shuttles. It was found that the open circuit photovoltages of IQ4, YA421, and YA422 devices with cobalt-based electrolyte are higher than those with iodide/triiodide electrolyte by 34, 62, and 135 mV, respectively. Moreover, the cells employing graphene nanoplatelets on top of gold spattered film as a counter electrode (CE) show lower charge-transfer resistance compared to platinum as a CE. Consequently, YA422 devices deliver the best power conversion efficiency due to higher fill factor, reaching 10.65% at AM 1.5 simulated sunlight. Electrochemical impedance spectroscopy and transient absorption spectroscopy analysis were performed to understand the electrolyte influence on the device performances with different counter electrode materials and donor structures of donor-pi-acceptor dyes. Laser flash photolysis experiments indicate that even though the dye regeneration of YA422 is slower than that of the other two dyes, the slower back electron transfer of YA422 contributes to the higher device performance.

High-conversion-efficiency organic dye-sensitized solar cells: molecular engineering on D-A-pi-A featured organic indoline dyes

Michael Graetzel, Magdalena Anna Marszalek, Yinghui Wu, Shaik Mohammed Zakeeruddin, Qixing Zhang

This paper reports a new D-A-pi-A organic dye WS-9, which is derived from the known dye WS-2 by incorporating an n-hexylthiophene unit into the pi-conjugation. Due to the presence of a strong electron-withdrawing benzothiadiazole unit in the pi-bridge, the specific D-A-pi-A organic dyes show more complicated electronic transition absorption bands than traditional D-pi-A dyes. The origins of the absorption bands in D-A-pi-A organic dyes are analysed by density functional theory (DFT). The calculated results in combination with the deprotonation experiments suggest that the spectral response range of D-A-pi-A organic dyes is superior to that of D-pi-A ones. When employed in dye-sensitized solar cells (DSSCs), the two dyes show a large difference in aggregation behaviour. It was found that WS-2 forms the unfavourable aggregates more easily. High performance of WS-2 strongly depends on the coadsorbent and suitable dye bath solvent. In contrast, WS-9 shows strong anti-aggregation ability, and always exhibits high performance regardless of the coadsorbent and dye bath solvent. Transient photovoltage and photocurrent decay experiments as well as electrochemical impedance spectroscopy indicate that the injected electron lifetime and charge recombination resistance are largely increased due to the introduction of a hexylthiophene unit, resulting in the high photovoltage based on WS-9. The optimized power conversion efficiency of WS-9 reaches 9.04% with high photocurrent (18.00 mA cm(-2)) and photovoltage (696 mV). The accelerating dye photo-stability was tested upon light irradiation of a dye-adsorbed TiO2 film in the absence of redox electrolyte, and a WS-9-based DSSC device with ionic liquid redox electrolyte. These results suggest that the structural engineering of organic dyes is important for highly efficient photovoltaic performance of solar cells, and our research will pave a novel way to design new efficient D-A-pi-A organic dye sensitizers.

Royal Society of Chemistry2012

New pyrido[3,4-b]pyrazine-based sensitizers for efficient and stable dye-sensitized solar cells

Pascal Comte, Thomas Moehl, Jacques-Edouard Moser, Mateusz Barnaba Wielopolski, Jiabao Yang, Shaik Mohammed Zakeeruddin

A series of new pyrido[3,4-b]pyrazine-based organic sensitizers (PP-I and APP-I–IV) containing different donors and p-spacers have been synthesized and employed in dye-sensitized solar cells (DSSCs). The absorption spectra properties of dyes are analysed by density functional theory (DFT). The calculated results in combination with the experiments suggest that the absorption characteristics and excited state features will mainly be dominated by charge transfer transitions from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) and to higher LUMO orbitals. Furthermore, attaching the octyloxy groups significantly extends the π-conjugation of the donor in APP- IV, which raises the HOMO energy and facilitates its oxidation. As a consequence, APP-IV exhibits the lowest HOMO–LUMO energy gap among all dyes, which, in turn, corresponds well with the red shift of the absorption spectra. Transient photovoltage and photocurrent decay experiments as well as electrochemical impedance spectroscopy indicate that the electron lifetime and charge recombination resistance are increased due to the introduction of octyloxy chains on the donor unit, resulting in the high photovoltage based on APP-IV. It was found that APP-IV based DSSCs with liquid electrolyte display the highest power conversion efficiency (PCE) of 7.12%. Importantly, a PCE of 6.20% has been achieved for APP-IV based DSSCs with ionic-liquid electrolytes and retained 97% of the initial value after continuous light soaking for 1000 h at 60 C. This renders these pyrido[3,4-b]pyrazine-based sensitizers quite promising candidates for highly efficient and stable DSSCs.

Royal Soc Chemistry2014

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.