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

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.

EPFL2011

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

High-voltage (1.8 V) tandem solar cell system using a GaAs/AlXGa(1-X)As graded solar cell and dye-sensitised solar cells with organic dyes having different absorption spectra

Pascal Comte, Michael Graetzel, Paul Liska, Péter Péchy, Jun Ho Yum

Stacked multijunction (tandem) solar cells have been prepared by mechanically stacking dye-sensitised solar cells (DSCs) and a GaAs/AlXGa(1-X)As graded solar cell (GGC) as the top and bottom cells, respectively. Three organic dyes with different absorption spectra (D131, D102 and D205) were used in the DSCs, in order to match the photocurrent density between the DSC and the GGC. Tuning the absorption range of the DSC by choosing an appropriate dye, increased the overall photovoltaic conversion efficiency due to the optimal utilisation of the solar spectrum in the individual cells. The open circuit photovoltages (V-OC) of the GGC and the DSC with D131 were 1.11 V and 0.76 V, respectively, resulting in a V-OC of 1.85 V and a photovoltaic conversion efficiency of 7.63% for the tandem cell. Although the overall conversion efficiency has not exceeded that of the GGC (7.66%), these tandem cells provide adequate V-OC values for water splitting applications. (C) 2011 Elsevier Ltd. All rights reserved.