The Effect of UV-Irradiation (under Short-Circuit Condition) on Dye-Sensitized Solar Cells Sensitized with a Ru-Complex Dye Functionalized with a (diphenylamino)Styryl-Thiophen Group

A new ruthenium complex, cis-di(thiocyanato)(2,2′-bipyridine-4,4′-dicarboxylic acid)(4,4′-bis(2-(5-(2-(4-diphenylaminophenyl)ethenyl)-thiophen-2-yl)ethenyl)-2,2′-bipyridine)ruthenium(II) (named E322) has been synthesized for use in dye-sensitized solar cells (DSCs). Higher extinction coefficient and a broader absorption compared to the standard Ru-dye, N719, were aimed. DSCs were fabricated with E322, and the efficiency was 0.12% initially. (4.06% for N719, as reference). The efficiency was enhanced to 1.83% by exposing the cell under simulated sunlight containing UV-irradiation at short-circuit condition. The reasons of this enhancement are (1) enhanceing electron injection from sensitizer to TiO2 following a shift toward positive potentials of the conduction band of TiO2 by the adsorption of protons or cations from the sensitizer, or from the redox electrolyte and (2) improving the regeneration reaction of the oxidized dye by the redox electrolyte by the dissolution of aggregated dye from the surface of TiO2 following the treatment

Photoelectrochemical studies of dye-sensitized solar cells using organic dyes

The dye-sensitized solar cell (DSC) is a promising efficient low-cost molecular photovoltaic device. One of the key components in DSCs is the dye, as it is responsible for the capture of sunlight. State-of-the-art DSC devices, based on ruthenium dyes, show record efficiencies of 10-12 %. During the last decade, metal-free organic dyes have been extensively explored as sensitizers for DSC application. The use of organic dyes is particularly attractive as it enables easy structural modifications, due to fairly short synthetic routes and reduced material cost. Novel dye should in addition to the light-harvesting properties also be compatible with the DSC components. In this thesis, a series of new organic dyes are investigated, both when integrated in the DSC device and as individual components. The evaluation methods consisted of different electrochemical and photoelectrochemical techniques. Whereas the light-harvesting properties of the dyes were fairly easily improved, the behavior of the dye integrated in the DSC showed less predictable photovoltaic results. The dye series studied in Papers II and IV revealed that their dye energetics limited vital electron-transfer processes, the dye regeneration (Paper II) and injection quantum yield (Paper IV). Further, in Papers III-VI, it was observed that different dye structures seemed to alter the interfacial electron recombination with the electrolyte. In addition to the dye structure sterics, some organic dyes appear to enhance the interfacial recombination, possibly due to specific dye-redox acceptor interaction (Paper V). The impact of dye sterical modifications versus the use of coadsorbent was explored in Paper VI. The dye layer properties in the presence and absence of various coadsorbents were further investigated in Paper VII. The core of this thesis is the identification of the processes and properties limiting the performance of the DSC device, aiming at an overall understanding of the compatibility between the DSC components and novel organic dyes.QC 2010073

Photoelectrochemical studies of dye-sensitized solar cells using organic dyes

The dye-sensitized solar cell (DSC) is a promising efficient low-cost molecular photovoltaic device. One of the key components in DSCs is the dye, as it is responsible for the capture of sunlight. State-of-the-art DSC devices, based on ruthenium dyes, show record efficiencies of 10-12 %. During the last decade, metal-free organic dyes have been extensively explored as sensitizers for DSC application. The use of organic dyes is particularly attractive as it enables easy structural modifications, due to fairly short synthetic routes and reduced material cost. Novel dye should in addition to the light-harvesting properties also be compatible with the DSC components. In this thesis, a series of new organic dyes are investigated, both when integrated in the DSC device and as individual components. The evaluation methods consisted of different electrochemical and photoelectrochemical techniques. Whereas the light-harvesting properties of the dyes were fairly easily improved, the behavior of the dye integrated in the DSC showed less predictable photovoltaic results. The dye series studied in Papers II and IV revealed that their dye energetics limited vital electron-transfer processes, the dye regeneration (Paper II) and injection quantum yield (Paper IV). Further, in Papers III-VI, it was observed that different dye structures seemed to alter the interfacial electron recombination with the electrolyte. In addition to the dye structure sterics, some organic dyes appear to enhance the interfacial recombination, possibly due to specific dye-redox acceptor interaction (Paper V). The impact of dye sterical modifications versus the use of coadsorbent was explored in Paper VI. The dye layer properties in the presence and absence of various coadsorbents were further investigated in Paper VII. The core of this thesis is the identification of the processes and properties limiting the performance of the DSC device, aiming at an overall understanding of the compatibility between the DSC components and novel organic dyes.QC 2010073

Organic chromophore-sensitized ZnO solar cells: Electrolyte-dependent dye desorption and band-edge shifts

An org. chromophore D5 (3-(5-(4-(diphenylamino)styryl)thiophene-2-yl)-2-cyanoacrylic acid) was tested as a sensitizer in photoelectrochem. mesoporous ZnO solar cells. Using thin (∼3 μm) mesoporous ZnO electrodes, high incident photon-to-current conversion efficiencies of up to 70% were obtained, while power conversion efficiencies up to 2.4% were found in simulated sunlight (100 mW cm-2). Long dye adsorption times (16 h) could be used without aggregation or pptn. of the dye. The compn. of the iodide/triiodide-based electrolyte was found to be crucial in optimization of the ZnO-based dye-sensitized solar cell. A high concn. of Li+ ions was found to be shift the ZnO conduction band edge to more neg. potential, whereas opposite behavior is found for mesoporous TiO2 cells. It was also found to be detrimental for solar cell performance and stability. Electrolyte-dependent and photoinduced dye desorption from the ZnO electrode was identified as a major stability problem in D5-sensitized ZnO solar cells

A novel organic chromophore for dye-sensitized nanostructured solar cells

A novel and efficient polyene-diphenylaniline dye for dye-sensitized solar cells was synthesized. The dye has a short synthesis route and is readily adsorbed on TiO2 under a variety of dye-bath conditions. The overall solar-to-energy conversion efficiency is over 5% in the preliminary tests, in comparison with the conventional N719 dye which gives 6% under the same conditions. The dye is designed for future use also in solid state devices, with triarylamine based hole conductors

Structural Modification of Organic Dyes for Efficient Coadsorbent-Free Dye-Sensitized Solar Cells

Three triphenylamine-based org. sensitizers with different electron-donating substituents (butoxyl chains or dimethylamine groups) were examd. to investigate the effect of bulky alkoxy donor substituents on the photovoltaic performances of dye-sensitized solar cells in the presence and absence of the coadsorbent chenodeoxycholic acid (CDCA) in dye-bath solns. The study showed that, using the D29 dye without bulky alkoxy substituents, the power conversion efficiency of dye-sensitized solar cell was significantly increased by ∼84% in the presence of CDCA as compared to that in the absence of CDCA addn. during the sensitization. However, the photovoltaic performance of D35-dye-sensitized solar cell having four bulky butoxyl substituents was not dependent on CDCA at all, probably due to the inherent structural nature of the D35 mol. The dye-sensitized solar cell based on the D37 sensitizer with only two bulky butoxyl chains displayed an expected medium performance as compared to D29 and D35. The inclusion of bulky alkoxy electron-donating substituents in dye mols. for efficient dye-sensitized solar cells suppressed the electron recombination and reduced the interactions between dye mols. This emphasizes the importance of designing novel dyes including functional groups that incorporate the properties normally needed from an external coadsorbent. The development of a coadsorbent-free system is in particular important for the future economization and simplification of the dye-sensitized solar cell assembly process

Effect of Anchoring Group on Electron Injection and Recombination Dynamics in Organic Dye-Sensitized Solar Cells

In the field of dye-sensitized solar cells the no. of different sensitizing dyes is increasing rapidly. To produce low-cost dyes, much work is being directed toward synthesizing all-org., Ru-free dyes with high extinction coeffs. and broad absorption bands with large solar spectrum overlap. One of the best dyes, the polyene-diphenylaniline dye D5L2A1, has a rather blue absorption with an IPCE onset at ∼650 nm, but it still has an energy conversion efficiency of almost 6%. To increase the overlap with the solar spectrum, the cyanoacrylic acid anchoring group was changed to rhodanine-3-acetic acid in the complex D5L2A3. This gave an IPCE onset at ∼750 nm, but unfortunately, it also decreased the overall efficiency to a modest 1.7%. By femtosecond transient absorption, the electron injection into TiO2 for the 2 dyes are ultrafast and indistinguishable with the time resoln. (<200 fs). However, charge recombination is also ultrafast, with different fractions of a ∼500 fs component for the 2 dyes. Yet, the fraction of the faster decay component is larger for D5L2A3 than for D5L2A1. An interpretation of changing the anchoring group is presented. Probably a lack of electron d. on the binding oxygens of the D5L2A3 LUMO, due to the rhodanine group, promotes a higher probability for electron injection to short-lived surface trap states compared to the situation for the fully-conjugated D5L2A1

Distance and Driving Force Dependencies of Electron Injection and Recombination Dynamics in Organic Dye-Sensitized Solar Cells

A series of dyes based on a triphenylamine donor and a rhodanine acetic acid anchor/acceptor for solar cell application has been studied with regards to electron injection and recombination kinetics using femtosecond transient absorption. The series contains three dyes, with estd. electron transfer distances ranging from 17.2 to 11.0 Å, and which have shown significant differences in energy conversion efficiencies. The injection and recombination kinetics were studied in the near-IR region where electrons in the conduction band of the TiO2 are suggested to absorb. For all dyes, the injection rate is larger than (200 fs)-1 which implies a quant. injection efficiency. Surprisingly, the subsequent recombination reaction has a rate that increases with increasing linker length. On the other hand, this behavior is consistent with the concomitant decrease in driving force for this series of dyes. Moreover, the lifetimes show exponential distance dependence when cor. for driving force and reorganization energy, which indicates a superexchange interaction between the electrons in TiO2 and the radical cations of the dyes. A dependence on probe wavelength of the attenuation factor was found, giving a β value of 0.38 Å-1 at 940 nm and 0.49 Å-1 at 1040 nm. The difference is suggested to be due to the difference in electronic coupling between fully sepd. dye cations and injected electrons vs. geminate electron-hole pairs. Addn. of tert-butylpyridine, which from previous work is known to cause a substantial drop in the incident photon-to-current-efficiency values for the studied dyes, was found to decrease the amt. of long-lived electrons in the TiO2 without affecting the injection rate

Influence of π-Conjugation Units in Organic Dyes for Dye-Sensitized Solar Cells

Two org. dyes with the general structure donor-conjugated chain-acceptor (D-π-A) were studied as sensitizers for nanocryst. TiO2 solar cells. The electron donor and acceptor groups were pyrrolidine and cyano acrylic acid, resp. The conjugated chain of 2-cyano-3-{5-[2-(4-pyrrolidin-1-ylphenyl)vinyl]thiophen-2-yl}acrylic acid contains one Ph ring and a thiophene unit and is therefore denoted PT, while for 2-cyano-3-{5-[2-(5-pyrrolidin-1-ylthiophen-2-yl)vinyl]thiophen-2-yl}acrylic acid the Ph ring is replaced by a 2nd thiophene unit (TT). Solar-to-elec. energy conversion efficiencies under simulated AM 1.5 irradn. (1000 W m-2) of 2.3% were obtained for solar cells based on PT but of <0.05% for those based on TT. The reasons for the dramatic difference of the efficiencies were analyzed. Photoinduced absorption measurements revealed that the TT dye was not properly regenerated by redox electrolyte after electron injection. This sluggish regeneration is probably due to the 0.3 V less pos. HOMO level for TT dye compared to the PT dye, resulting in a lower driving force for regeneration of the oxidized dye by iodide in the electrolyte. Regeneration of the oxidized TT dye and electron injection from the excited TT dye may be poor due to formation of dye aggregates/complexes, as FTIR measurements show an excess of not properly and/or unidentate bound TT dye mols. instead of bidentate bound PT dye mols. The results highlight that small structural change of dyes results in significant changes in redox energies and binding features, affecting dramatically the performance of these dyes in dye-sensitized solar cells

Preventing Dye Aggregation on ZnO by Adding Water in the Dye-Sensitization Process

ZnO based dye-sensitized solar cells have been studied using N719 and Z-907 as sensitizing dyes, with and without adding water to the dye soln. The solar cells have been characterized using photoelec. measurements and the interface between the dye and the ZnO surface has been studied using photoelectron spectroscopy. It was shown that water in the dye soln. greatly reduces surface dye aggregation and thereby enhances the solar cell performance for N719. For Z-907 where no sign of dye aggregation could be found, the presence of water had minor effect on the surface structure and solar cell performance