First Principles Design of Dye Molecules with Ullazine Donor for Dye Sensitized Solar Cells

Michael Graetzel, Yunfei Jiao, Wanli Ma, Sheng Meng, Mohammad Khaja Nazeeruddin
American Chemical Society, 2013
Article

Résumé

We design a series of metal-free donor-pi-bridge molecules (denoted VB0-VB4) based on a new donor group-ullazine donor-as sensitizers for dye sensitized solar cell (DSSC) applications. Density functional theory (DFT) and time-dependent DFT calculations reveal that the physical properties of dyes, including spectral response, light harvesting efficiency, and electron injection rate, are systematically improved by combining ullazine donor to a series of length changing pi bridges. Dye VB2 is the best candidate thanks to its outstanding performance on key parameters and achieving a balance between competing factors. Compared to two other series of molecules-L and M dyes, which differ from VB dyes by only the donor group-VB dyes have the largest light harvesting efficiency and the largest number of electrons injected to the conduction band of TiO2. These results suggest that the ullazine group can serve as an excellent donor for future DSSC applications.

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Geometrical and energetical structural changes in organic dyes for dye-sensitized solar cells probed using photoelectron spectroscopy and DFT

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The effects of alkoxy chain length in triarylamine based donor acceptor organic dyes are investigated with respect to the electronic and molecular surface structures on the performance of solar cells and the electron lifetime. The dyes were investigated when adsorbed on TiO2 in a configuration that can be used for dye sensitized solar cells (DSCs). Specifically, the two dyes D35 and D45 were compared using photoelectron spectroscopy (PES) and density functional theory (DFT) calculations. The differences in solar cell characteristics when longer alkoxy chains are introduced in the dye donor unit are attributed to geometrical changes in dye packing while only minor differences were observed in the electronic structure. A higher dye load was observed for D45 on TiO2. However, D35 based solar cells result in higher photocurrent although the dye load is lower. This is explained by different geometrical structures of the dyes on the surface.

Royal Soc Chemistry2016

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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

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Dye-sensitized solar cells (DSCs) are considered as an emerging technology in order to replace conventional silicon solar cells or thin film solar cells such as amorphous silicon, CIGS, and CdTe. Liquid electrolytes containing iodide/triiodide redox couple have a durability problem due to the corrosion of metal contacts. In order to improve the long-term stability of DSC device it is important to find an alternate efficient redox couple. In search of this we are using 2,2',7,7'-tetrakis-(N,N-di-methoxyphenylamine)- 9,9'-spirobifluorene (spiro-OMeTAD) as a hole transport material for solid-state dye-sensitized solar cells (SSDSCs). In comparison to the liquid electrolytes the efficiencies of SSDSCs are inferior, they are around only 30% of the efficiencies obtained with the liquid electrolytes. In optimizing the device performance and stability of SSDSCs, various light harvesting systems are employed to enhance a photovoltaic performance and investigated their properties in SSDSCs. In SSDSCs we use thin TiO2 films to avoid the pore-filling problem of HTM. Hence it is critical to use high molar extinction coefficient dyes with an efficient light harvesting capability for SSDSCs. Representative ruthenium sensitizers such as N719 or Z907 have shown good and stable performances in liquid electrolyte-based DSCs. However, their performances are low in SSDSCs due to insufficient light harvesting in thin mesoporous TiO2 films. A new family of heteroleptic polypyridyl ruthenium sensitizers having thiophene units was employed to increase the light harvesting capabilities and their applicability in SSDSCs. These new dyes could improve the absorbed photon-to-current conversion efficiencies as well as power conversion efficiencies due to their high molar extinction coefficients. The thiophene units of the ancillary ligands not only enhanced molar extinction coefficients but also augmented electron lifetime in the devices. In general, ruthenium sensitizers possess lower molar extinction coefficients compared to organic dyes. In order to increase the molar extinction coefficients and bathochromic shift in the absorption spectra of organic dyes, we applied donor-acceptor concept in organic dyes with different π-conjugation bridges. Consequently, we achieved 6 % power conversion efficiency at AM 1.5G solar irradiation (100 mW/cm2) in a solid-state dye-sensitized solar cell. Transient photovoltage and photocurrent decay measurements showed that the enhanced performance of this device was ascribe to higher charge collection efficiency over a wider potential range. We also examined near infrared absorbing dyes and they could be employed to different device architectures such as tandem cells, Förster Resonance Energy Transfer, or co-sensitization to substantiate panchromatic response. Another interesting type of sensitizers is semiconductor or quantum dots due to their unique properties. However, the efficiency of the semiconductor-sensitized solar cells was only 1-2 % range. Recently, much improved efficiencies were reported with Sb2S3-sensitized cells using different hole conductors. The Sb2S3-sensitized cells with spiro-OMeTAD demonstrated a very high incident photon-to-conversion efficiency (IPCE) of 90 %. This excellent result shows that the semiconductor sensitizers are promising candidate as light absorbers for SSDSCs. Most of the standard ruthenium and organic dyes have the limited absorption in near infrared region of the solar spectrum. Porphyrin sensitizers possess strong absorptions in the visible and near infrared region and they have good chemical, photochemical and thermal stability. However, the power conversion efficiency of SSDSCs devices using a novel D-π-A porphyrin we reached only 1.6 %. In order to improve a cell performance, porphyrin was co-sensitized with an organic dye to increase light harvesting capability in the green wavelength region as well as to reduce the dye aggregation. Instead of spiro-OMeTAD a polymer hole conductor was applied, which had intense spectral response in the visible region. Interestingly, in this system the polymer hole conductor showed dual functions, as a light absorber and a hole transporter. This hybrid solar cell exhibited a clear panchromatic response and improved the power conversion efficiency of device compared to the cell with spiro-OMeTAD.

EPFL2011

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Royal Society of Chemistry2012