Yeru Liu

An organic redox mediator for dye-sensitized solar cells with near unity quantum efficiency

The organic redox mediator tetramethylformaminium disulfide/tetramethylthiourea was evaluated in dye-sensitized nanocrystalline TiO2 solar cells, as an alternative to the conventional I3−/I− redox couple. Devices were optimized by judicious variation of electrolyte composition and selection of sensitizing dye. The best performance of the dye-sensitized solar cells incorporating this redox mediator was achieved by using a metal-free indoline-based sensitizer (D131). It was found that conventional ruthenium based sensitizers (e.g. Z907) exhibited inferior performance, possibly as a result of an insufficient driving force for sensitizer regeneration. A near unity incident photon-to-collected-electron conversion efficiency was achieved at low light intensity for optimized devices. The overall light-to-electric power conversion efficiency under AM 1.5 1 Sun illumination reached 3.88%. This represents an increase of ca. 25% compared with previously reported DSCs using this redox mediator. Factors limiting cell performance were further investigated using transient absorption spectroscopy and electrochemical impedance spectroscopy.

Heterogeneous electron transfer from dye-sensitized nanocrystalline TiO2 to [Co(bpy)3]3+: insights gained from impedance spectroscopy.

Dye-sensitized solar cells (DSCs) employing the [Co(bpy)3](3+/2+) redox mediator have recently attained efficiencies in excess of 12%, increasing the attractiveness of DSCs as an alternative to conventional photovoltaics. Heterogeneous electron transfer from dye-sensitized nanocrystalline TiO2 to [Co(bpy)3](3+) ions in solution, a process known as recombination in the context of DSC operation, is an important loss mechanism in these solar cells. Here, we employ impedance spectroscopy over a range of temperatures to characterize electron storage, transport, and recombination in efficient DSCs based on the [Co(bpy)3](3+/2+) redox mediator, with either the amphiphillic ruthenium sensitizer Z907 or the state-of-the-art organic sensitizer Y123. The temperature dependence of the electron-transport resistance indicates that transport occurs via states at energies lower than commonly assumed for the TiO2 conduction band edge. We show that a non-exponential dependence of capacitance, transport resistance, and recombination resistance on photovoltage can be interpreted as evidence for partial unpinning of the TiO2 energy levels. We also find that the nature of the sensitizing dye determines the predominant recombination route: via the conduction band for Y123 and via band gap states for Z907, which is the main reason for the superior performance of Y123. The different mechanisms appear to arise from changes in electronic coupling between TiO2 donor states and [Co(bpy)3](3+) acceptor states, as opposed to changes in the density of TiO2 states or their energetic matching with the acceptor-state distribution. These findings have implications for modeling heterogeneous electron transfer at dye-sensitized semiconductor-solution interfaces in general and for the optimization of DSCs.

Synthesis of perylene dyes with multiple triphenylamine substituents.

Perylene monoimide (PMI) was brominated to give tetra- and tribrominated molecules, which underwent a Suzuki coupling reaction with 4-(diphenylamino)phenylboronic acid to give PMI derivatives. The photophysical and electrochemical properties of the synthesized compounds were investigated, and theoretical calculations were performed. Single crystals of tetrasubstituted PMI were grown and studied in detail. The structure-property relationships were examined to reveal the effect of the position and number of substituents on the perylene core unit. All molecules showed a broad absorption up to 750 nm. Corresponding anhydrides of PMIs were used for fabrication of dye-sensitized solar cells. The molecule with four triphenylamine units on perylene monoanhydride showed the highest power conversion efficiency.

Significant performance improvement in dye-sensitized solar cells employing cobalt(III/II) tris-bipyridyl redox mediators by co-grafting alkyl phosphonic acids with a ruthenium sensitizer

Efficiencies of up to 8.5% for dye-sensitized solar cells employing a ruthenium dye with a cobalt complex redox mediator have been achieved, by using octadecylphosphonic acid (OPA) as a coadsorbent. This success is due to improved electron injection and reduced recombination.

Efficient Dye-Sensitized Solar Cells Using a Tetramethylthiourea Redox Mediator

An organic redox couple tetramethylthiourea/tetramethylformaminium disulfide (TMTU/TMFDS(2+) ) is evaluated in dye-sensitized solar cells in conjunction with a series of indoline and ruthenium-based dyes. Of these, devices with indoline dye D205 show the best performance, with an optimized power conversion efficiency of 7.6 % under AM 1.5G 1 sun illumination. Charge collection and injection are highly efficient in all TMTU-based DSCs studied. Regeneration of indoline dyes is highly efficient, whereas regeneration of ruthenium dyes by TMTU is less efficient, accounting for their inferior performance. Impedance spectroscopy results reveal that using an optimized TMTU/TMFDS(2+) electrolyte solution, the TiO2 conduction band edge is 300-400 meV lower than when an optimized I3 (-) /I(-) electrolyte is used. The would-be loss in open-circuit voltage caused by the downward conduction band shift is mostly compensated by approximately the 200 meV lower redox level of the TMTU/TMFDS(2+) electrolyte and up to 1000 times slower recombination rates. This makes TMTU/TMFDS(2+) a promising redox couple in the development of highly efficient solar energy conversion devices.

Low-cost and flexible poly(3,4-ethylenedioxythiophene) based counter electrodes for efficient energy conversion in dye-sensitized solar cells

A roll-to-roll slot die coating process was used to deposit PEDOT:PSS on PET and the resulting composite films were evaluated for use as counter electrodes in dye-sensitized solar cells (DSCs). The effect of depositing an additional layer of electropolymerized PEDOT onto the flat PEDOT/PET electrodes was also studied. Counter electrodes and complete DSCs were characterized by steady-state current–voltage, electrochemical impedance spectroscopy and IPCE measurements. Evidence that the PEDOT-based counter electrode can modify the electrolyte solution composition, probably by increasing the proton concentration, is presented. The overall cell performance under 1 Sun illumination is improved by addition of an electropolymerized layer of PEDOT to flat PEDOT/PET counter electrodes, but both remain inferior to platinized FTO. Under 0.2 Sun illumination, devices employing counter electrodes with electropolymerized PEDOT on a PEDOT/PET substrate (named as EP/PEDOT/PET) perform almost similarly to those with platinized FTO counter electrodes, suggesting that EP/PEDOT/PET electrodes may be useful in DSCs designed for low light (e.g. indoor) operation, especially considering the low costs of PEDOT and the roll-to-roll and electropolymerization processes.

Synthesis of multi-donor dyes and influence of molecular design on dye-sensitized solar cells

Multi-donor incorporated organic dyes were designed and synthesized and their structure–property relationship was investigated in dye-sensitized solar cells (DSCs). Five N,N-disubstituted aniline donor groups along with carbazole and thiophene secondary donors were combined into the (D)n–π–A design with cyanoacrylic acid as the acceptor. The dyes showed broad absorption bands with absorption maxima in the range of 470–485 nm and optical band gap around 1.94–2.37 eV. Our results indicate that incorporation of bulky groups and bent-type architecture helps to improve the performance of DSCs.