Kinetics of the Regeneration by Iodide of Dye Sensitizers Adsorbed on Mesoporous Titania

Regeneration of dye sensitizer molecules by reducing species contained in the electrolyte is a key mechanism in liquid dye-sensitized solar cells because it competes kinetically with a detrimental charge recombination process. Kinetics of the reduction by iodide ions of the oxidized states (S+) of two RuII complex dyes and four organic π-conjugated bridged donor−acceptor sensitizers were examined as a function of the electrolyte concentration. Results show that two different cases can be distinguished. A sublinear behavior of the regeneration rate and a plateau value reached at high bulk iodide concentrations were found for N820 ruthenium dye and interpreted as being due to an associative interaction involving the formation of (S+, I−)···I− surface complexes prior to the reaction. On the other hand, feeble reaction rates at low electrolyte concentrations and a superlinear behavior are observed predominantly for the organic dyes, pointing to a repulsive interaction between the dyed surface and iodide anions. At higher iodide bulk concentration, a linear behavior is reached, providing an estimate of a second-order rate constant. A correlation of these two opposite behaviors with the structure of the dye is observed, emphasizing the role of sulfur atoms in the association of I− anions in the dye-sensitized layer. These findings allow for a better understanding of the dye−electrolyte interaction and of the effect of the iodide concentration on the photovoltaic performances of dye-sensitized solar cells.

Electronic and structural modification of titanium dioxide/zinc oxide photoanode for dye-sensitized solar cells

Aravind Kumar Chandiran

Dye-sensitized solar cells (DSC) are a new class of molecular photovoltaics that mimics the natural photosynthesis, for the direct conversion of sunlight into electricity. A typical DSC is a sandwich of a dye sensitized nanoparticle TiO2 film and a catalyst coated counter electrode, with a redox electrolyte filling the space in between. The high internal surface area of the mesoporous TiO2 film ensures higher dye uptake for an efficient light harvesting. Following the light illumination, an electron from the highest occupied molecular orbitals (HOMO) of the dye is excited to its lowest unoccupied molecular orbitals (LUMO). Due to an effective electronic orbital coupling between dye and TiO2, the excited electrons are injected into TiO2. The oxidized dye is regenerated by the electron donor present in the electrolyte. The circuit is completed by the transfer of photogenerated electrons in TiO2 via an external load, to counter electrode regenerating the oxidized electrolyte species. In a conventional DSC, the transport rate of the photogenerated electrons in TiO2 competes with the parasitic electron recombination, as these two processes occur at a similar ms time domain. This thesis work is aimed at the fundamental understanding of these electron transfer processes at sensitized photoanodes that were subject to different bulk and surface electronic structural modifications. The implications were shown to be of profound interest for the fabrication of high efficiency and low cost dye-sensitized solar cells. In the first part of the thesis, the bulk electronic structure of the anatase TiO2 nanoparticles is modified by the incorporation of different concentrations of aliovalent cations (Nb5+, Ga3+, Y3+) in the crystal lattice. The cationic doping/substitution in the nanoparticles is achieved by hydrothermal process. The modified nanoparticles retained the parent anatase crystal structure of TiO2. The transparent dye-sensitized solar cells made with these modified films, using our standard heteroleptic metal complex sensitizers and iodide/triiodide redox electrolyte, displayed enhanced photovoltaic performances, at certain added concentrations. Charge extraction measurements in DSC, reveals a formation of deeper trap states below the conduction band of titanium dioxide when Nb ion is incorporated into the TiO2 lattice. On the other hand, Ga3+ and Y3+ did not significantly affect the trap states distribution compared to the reference device. The modification in the trap states and the oxygen stoichiometry due to these cation inclusions significantly influenced the electron recombination and transport rates. These properties are investigated in detail using transient decay techniques. Following the study on the electronic structure of the bulk TiO2, the influence of the surface modification of the TiO2 nanoparticles on the electron transfer dynamics is investigated. The kinetics of electron recombination from TiO2 to the single electron redox shuttles is fast, leading to a significant loss in the open-circuit potential (VOC) of devices. The emergence of cobalt complexes as a potential electrolyte motivated us to investigate the role of surface passivation of the titanium dioxide nanoparticles. Insulating sub-nanometer oxide tunneling layers deposited by atomic layer deposition (ALD) are known to block the electron recombination, thereby, leading to an increase in the VOC and the collection efficiency of the solar cell. A general perception in the DSC community is that any insulating oxide layer can block the recombination. However, in this work, it is unraveled that, just the insulating property of oxides is not sufficient. In addition, the properties such as the conduction band position and the oxidation state of the insulating oxide, the electronic structural modification induced to the underlying TiO2 mesoporous film, modification of surface charges (isoelectric point) and charge of the electrolyte species have to be considered. A complete photovoltaic study has been done by depositing different cycles (by ALD) of four insulating oxides (Ga2O3, ZrO2, Nb2O5 and Ta2O5) and their recombination characteristics, surface electronic properties, transport rate and injection dynamics are investigated with a standard organic dye and Co2+/Co3+ redox mediator. A comparison is made with the conventional iodide/triiodide electrolyte. The second part of the thesis is focused at the development of new type of photoanodes for effective electron transport to increase the charge collection efficiency. We present a way of utilizing thin and conformal overlayer of titanium dioxide or zinc oxide on an insulating mesoporous template as a photoanode for dye-sensitized solar cells. Different thicknesses of TiO2 ranging from 1 to 15 nm are deposited on the surface of the template by ALD. This systematic study helps unraveling the minimum critical thickness of the TiO2 overlayer required to transport the photogenerated electrons efficiently. A merely 6 nm thick TiO2 film on a 3 μm mesoporous insulating substrate is shown to transport 8 mA/cm2 of photocurrent density along with ≈900 mV of open-circuit potential, when using our standard donor-π-acceptor sensitizer and Co(bipyridine)3 redox mediator. The three dimensional conformal shell on the insulating core also exhibits one order of magnitude higher transport rate compared to the conventional TiO2 nanoparticles. Similar nanostructures were made by depositing ZnO on an insulating template and the electron transfer properties are compared with titania. We also developed ALD recipe for the deposition of crystalline TiO2 or ZnO at low temperatures (< 200 °C) on arbitrary mesoporous insulating templates, considering their prospects of making photoanodes on flexible plastic substrates. The experience gained about the deposition of conformal and pin-hole free TiO2 layer in the previous section is extended for the high efficiency solid-state mesoscopic solar cells based on CH3NH3PbI3 perovskite absorbers. We show that a 2 nm as-deposited ALD overlayer on a mesoporous TiO2 film, printed on a bare conducting glass, is sufficient to arrest the recombination process that lead to a power conversion efficiency of 11.5 %. A detailed investigation on the electron back reaction is presented using electrochemical impedance spectroscopy.

EPFL2013

Enhanced-Light-Harvesting Amphiphilic Ruthenium Dye for Efficient Solid-State Dye-Sensitized Solar Cells

Michael Graetzel, Carole Graetzel, Robin Humphry-Baker, Florian Le Formal, Soo-Jin Moon, Mingkui Wang, Peng Wang, Shaik Mohammed Zakeeruddin

A ruthenium sensitizer (coded C101, NaRu (4,4'-bis(5-hexylthiophen-2-yl)-2,2'-bipyridine) (4-carboxylic acid-4'-caboxylate-2,2'-bipyridine) (NCS)(2)) containing a hexylthiophene-conjugated bipyridyl group as an ancillary ligand is presented for use in solid-state dye-sensitized solar cells (SSDSCs). The high molar. extinction coefficient of this dye is advantageous compared to the widely used Z907 dye, (NaRu (4-carboxylic acid-4'-carboxylate) (4,4'-dinonyl-2,2'-bipyridine) (NCS)(2)). In combination with an organic hole-transporting material (spiro-MeOTAD, 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine) 9, 9'-spirobifluorene), the C101 sensitizer exhibits an excellent power-conversion efficiency of 4.5% under AM 1.5 solar (100 mW cm(-2)) irradiation in a SSDSC. From electronic-absorption, transient-photovoltage-decay, and impedance measurements it is inferred that extending the pi-conjugation of spectator ligands induces an enhanced light harvesting and retards the charge recombination, thus favoring the photovoltaic performance of a SSDSC.

Wiley-Blackwell2010

Interfacial engineering for competitive Dye-Sensitized Solar Cells production

Natalie Flores Diaz

Renewable energy deployment and implementation of a circular economy will play a crucial role in decreasing CO2 emissions to meet the target from the Paris agreement. Dye-sensitized solar cells (DSSCs) can be designed in various colors and transparency for building-integrated photovoltaics (BIPV). This emerging technology has gained significant attention for its remarkably high power conversion efficiency (PCE) under indoor lighting up to 34%. To improve the performance of DSSCs, various approaches to study and reduce the interfacial electron recombination from the conduction band of the semiconductor to the electrolyte were implemented in this thesis. One of the approaches was the design of a tandem electrolyte containing two redox species, the radical AZADO (AZ+/0) and the [Co(bpy)3]2+/3+ complex. DSSCs with tandem electrolytes showed increased open-circuit voltage (VOC) up to 1009 mV and enhanced PCE of 9.15%. The VOC enhancement was determined to result from the higher redox potential of (AZ+/0) and the slower electron recombination rate. These findings demonstrate the advantages of employing tandem electrolytes to improve the VOC and overall performance of DSSCs. To decrease the solvent's volatility and the interfacial electron recombination in cobalt-based electrolytes, hyperbranched networks (HB) were designed through a click thiol-ene reaction between a thiol-siloxane and an acrylate monomer. The formed polymers were denoted HB1 (thiol-siloxane and PEGMA) and HB2 (thiol-siloxane and PEGMA). The addition of 20% of HB polymers to a cobalt-based electrolyte retained up to 98% of their initial weight until 150°C. The devices employing 10%(v/v) of HB1 attained the highest PCE under AM1.5 of 8.52%, comparable to 9.36% for the cobalt reference electrolyte. All the HB-electrolytes presented higher VOC values than the cobalt reference due to an upward shift in the CB of TiO2 and longer electron lifetimes. It was determined that the HB-polymers reduce the electron recombination and enhance the photovoltaic performance under low light intensity leading to a PCE of 23.19% under 1000 lux compared to 20.37% for the Co-reference. We then developed a new dye, R7, with a strong electron-donating PAH core and BPT2 as the central building block and a bulkier HF donor to decrease electron recombination at the interface TiO2/electrolyte. We found that in the presence of a copper electrolyte, the dye R7 outperformed devices with R6. EIS revealed that the bulky HF-donor introduction effectively suppressed the electron recombination between the TiO2 surface and the redox shuttle. Additionally, we employed a co-sensitized system of R7+Y123 to reach the outstanding parameters of JSC of 16.15 mAcm-2, VOC of 1035 mV, FF of 0.76 to achieve a top PCE of 12.7%. To the best of our knowledge, this is the highest for copper-based DSSCs using a blue photosensitizer. Lastly, the interfacial phenomena in DSSCs with carbon counter electrodes (CCEs) and copper-based electrolytes were studied to determine the feasibility of the production of monolithic DSSCs with copper-based hole-transporting materials. It was found that the electron transport in devices with CCEs is in the same order of magnitude as those with PEDOT. However, the devices with CCEs presented a decrease of 2 orders of magnitude in the recombination resistance, resulting in decreased VOC and JSC, leading to a PCE of 17% compared to 24.5% for PEDOT devices under 1000 lux illumination.