Molecular design of metal-free D–π-A substituted sensitizers for dye-sensitized solar cells

Novel donor-p-acceptor (D--A) dyes, coded as SD-1, SD-2 and SD-5, were designed and synthesized for dye-sensitized solar cells. All these dyes comprised the same donor and acceptor units while the oligo thiophene units were introduced as linkers between the donor and acceptor units. The photophysical and electrochemical properties of the dyes were investigated by UV/vis spectrometry and cyclic voltammetry. The dyes were subsequently tested as sensitizers in dye-sensitized solar cells. Photovoltaic performance of the device with SD-2 dye showed maximum monochromatic incident photon-to-current efficiency (IPCE) of 82% and an over all conversion efficiency of 6.2% under full sunlight (AM 1.5G, 100 mW cm^2) irradiation.

D-π-A Sensitizers for Dye-Sensitized Solar Cells: Linear vs Branched Oligothiophenes

Amaresh Mishra, Mingkui Wang, Sophie Wenger, Shaik Mohammed Zakeeruddin

Two donor-π-acceptor (D-π-A) dyes, coded as L-3T-DPA 1 and B-5T-DPA 2, were synthesized for application in dye-sensitized solar cells. These D-π-A sensitizers use diphenylamine as donor, an oligothiophene as π-bridge, and cyanoacrylic acid as an acceptor group that can be anchored to the surface of TiO2. While the two dyes comprise the same donor and acceptor units, the bridging oligothiophene is linear in one case and branched in the other case. Photophysical and electrochemical properties of the dyes were investigated by UV−vis spectrometry and cyclic voltammetry. The dyes were subsequently implemented as sensitizers in dye-sensitized solar cells. Photovoltaic devices with dye 1 showed a maximum monochromatic incident photon to current efficiency (IPCE) of 80% and an overall conversion efficiency of 6.8% under full sunlight (AM 1.5G, 100 mW cm−2) irradiation. The photovoltaic performance of branched dye 2 was lower because of less dye loading on the TiO2 surface. The dyes were also tested in ionic liquid and solid-state devices and showed good efficiencies. Long-term stability measurements were performed over 1000 h at full sunlight and at 60 °C in ionic liquid devices. Branched dye 2 thereby showed excellent stability retaining 96% of its initial efficiency, while linear dye 1 retained 73% after 1000 h of irradiation.

American Chemical Society2010

Investigation on Functionalized Ruthenium-Based Sensitizers to Enhance Performance and Robustness of Dye-Sensitized Solar Cells

Nuttapol Pootrakulchote

It has been increasingly aware to the world today that reserves of fossil fuels are limited and their use has serious environmental side effects. Encouraged by this realization was the evolution of the use of cleaner alternative energy, among which Dye-sensitized solar cells (DSCs) are potentially attractive candidates for the lower cost of producing devices which convert an abundant amount of energy from the sun into electricity. Dye-sensitizer in DSCs plays a crucial role as the chlorophyll in plants; to harvest solar light and transfer the energy via electron transfer to a suitable material (TiO2 in this case) to produce electricity. The topic of interest for this thesis is to further enhance the photovoltaic performance and the robustness of DSCs by tuning the optical properties of the dye-sensitizer (Ruthenium complex, in this case) using several strategies including an extension of the π-conjugation system, an introduction of antenna molecules and a modification of the Ru-complex structure. This work focuses on the DSC device fabrication and photovoltaic characterization in order to investigate more insight into structure-property-device performance relationship. New benchmarks for high performance DSCs with ruthenium complex sensitizers with π-extension in their ancillary ligands were presented. The overall conversion efficiency of 9.6% and 8.5% have been achieved with Ru-based sensitizer containing ethylenedioxythiophene, using low-volatile electrolyte and solvent-free electrolyte, respectively. The Rusensitizer functionalized with hexylthio-bithiophene unit exhibited a conversion efficiency of 9.4% with low-volatile electrolyte. All these devices showed good stability under prolonged light soaking at 60 °C. Extending π-conjugation of the anchoring ligand with thiophene units in monoleptic Ru-sensitizer also yields an impressive conversion efficiency of 6.1% using 3-µm-thin mesoporous TiO2 film in corporate with low-volatile electrolyte. DSC devices based on ruthenium sensitizers functionalized with thienothiophene- and EDOT-conjugated bridge, together with carbazole moiety on the ancillary ligands were found efficient with conversion efficiencies of 9.4% and 9.6%, respectively, in presence of a volatile electrolyte. The carbazole-functionalized ruthenium-based DCSs also performed excellently in the stability test using a low-volatile electrolyte. Furthermore, the Ru-complexes synthesized by click-chemistry in association with triazole-derivative moieties were successfully used as DCS sensitizers. DSC devices sensitized with these dyes provided the overall conversion efficiency close to 10% with volatile electrolyte. Further studies with solvent-free electrolyte showed notable device stability under extending full sunlight intensity at 60 °C. The results presented here provide a fertile base for further investigation, which will focus on improving the spectral response of ruthenium dye-sensitizer to full sunlight by searching for new strategies to modify the sensitizer with more efficient functional groups. The target is to reach higher conversion efficiency of DSC devices while retaining their stability under standard reporting conditions.

EPFL2012

Solid-State Sensitized Heterojunction Solar Cells

Soo-Jin Moon

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.