Understanding the design principles of organic sensitizers for highly efficient dye sensitized solar cells

Abstract

The conversion of solar to electrical power has presented itself as a solution to meet the growing energy demand and replace our current fossil fuel dependence, which causes dramatic damages to the environment. At the heart of this challenge lies the performance optimization of photovoltaic (PV) technologies, but also rendering their manufacture cost-effective. Dye sensitized solar cells (DSSCs) have attracted significant attention over the past decades because of their low-cost, facile manufacturing processes and wide range of very peculiar applications. Notably, DSSCs can power portable electronic devices under low ambient light or be used as semi-transparent cells for building integrated photovoltaics(BIPV). These attractive features are the consequence of the deviceâs unique architecture. A layer of mesoporous titanium dioxide (TiO2) is coated with a colored light absorber in contact with a redox active electrolyte, squeezed between a counter electrode.

Dye design is an important step in developing high efficiency devices as each of the dye structural feature will influence its opto-electrochemical properties. Modern era dyes are constructed according to the Donor-Ï-Acceptor(D-Ï-A) pattern, where the donor would be an electron rich moiety and the acceptor an electron poor one. The D-Ï-A pattern was improved over the years, by adding an auxiliary electron accepting unit between the donor and the Ï-bridge to yield the new D-A-Ï-A pattern.

The aim of this thesis was to understand the principles that dictate the design of efficient sensitizers. The efforts were focused on organic dyes combined with the newly developed high photo-voltage Cu(II/I) based electrolyte.

Photoanode/Electrolyte Interface Stability in Aqueous Dye-Sensitized Solar Cells

Guido Viscardi

O'Regan and Gratzel conceived a dye-sensitized solar cell (DSSC), which operates as an artificial photosynthetic system converting solar light into electricity. Nevertheless, it is only recently that the scientific community is focusing on truly aqueous DSSCs that use water as electrolyte, thus avoiding any flammable and toxic organic solvents. The most critical aspect of these devices is the photoanode/electrolyte interface. Indeed, it is necessary to select dyes that chemisorb onto the semiconductor layer while allowing the aqueous electrolyte to thoroughly impregnate the mesostructure of the electrode; at the same time, it is necessary to avoid the anchoring unit of the dye to be hydrolyzed by water, thus causing the detachment of the sensitizer from the active material particles. This requires a thorough analysis of the structure-property relationships, in particular the evaluation of the chemical formula, stability, and performances of the dyes on the electrode surface in truly aqueous environment. Herein, we investigate nine different sensitizers, selected based on their different properties in terms of light absorption, chemical structure, and hydrophobic/hydrophilic nature. Wettability, resistance to dye desorption, and spectral variations of sensitized photoanodes, along with the photovoltaic performance of the resulting devices, are systematically investigated to identify some useful guidelines to choose and design suitable dyes of different colors for truly aqueous DSSCs.

Wiley-V C H Verlag Gmbh2017

Novel organic dyes for enhanced red to near IR response in dye sensitised solar cells

The aim of this work was to develop new, organic dyes, capable of harvesting photons in the red to near-IR region of the spectrum. A series of squaraine and tricarbocyanine dyes were produced and characterised and their optical and redox properties studied. The results of initial photovoltaic tests, followed by optimisations of cell performance, have also been presented. These clearly demonstrate the effectiveness of these dyes in solar cells. A total of 12 new squaraine dyes were successfully synthesised, seven of which were tested in "dye-sensitised" solar cells. The efficiencies ranged from 1.2%-3.7%, for the best dye [B1]. It was concluded that enhanced performance could be achieved in the presence of chenodeoxycholic acid (CDCA) and tetrabutylpyridine (TBP). These additives both help to prevent aggregation and increase the cell potential, limiting charge collection losses and enhancing performance. Three new tricarbocyanine dyes were also synthesised, two of which were used in solar cells. Unfortunately, due to the interaction of [B12] with the electrolyte, the dye proved to be unstable, as both the absorption maxima in UV-vis and fluorescence emission wavelengths were shifted to higher energies. Thus, the red response of the dye was significantly reduced and photovoltaic performance hindered. A limited photo-response of [B14] was also seen, although this dye appeared to be very stable in the presence of the electrolyte. To deduce the possible reasons for the poor performance of these dyes, their optical and electrochemical properties were investigated. Thus, cyclic voltametry was employed to identify the redox potentials of the dyes and their charge injection and recombination dynamics were studied through nanosecond time-resolved transient laser spectroscopy. It was concluded that the dyes theoretically possess the ideal characteristics for solar cell function and that the problem of [B14] lay in the purity of the dye. [B14] proved to be very difficult to purify as the dye was unstable in solution and decomposed on the columns employed in chromatography. Fortunately the dye appeared to be stable in the presence of an electrolyte when adsorbed on TiO2. The dye gave an IPCE of 12% between 800 – 900nm, which is particularly impressive, especially if the purity of the dye is considered. A new method of purification must be found however, if photovoltaic performance is to be enhanced. Finally, the effects of a compact blocking layer on device performance were studied. It was clearly demonstrated that a compact layer was essential. This was investigated and a relationship between the structure of the dye and the necessity of the blocking layer was established.

EPFL2006

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

Strategies to Optimizing Dye-Sensitized Solar Cells

Sophie Wenger

EPFL2010

Novel organic dyes for enhanced red to near IR response in dye sensitised solar cells

EPFL2006