Solid hybrid dye-sensitized solar cells

Dye-sensitized solar cells (DSC), introduced by O'Regan and Grätzel in 1991, are a low cost alternative to conventional silicon photovoltaic cells, the latter requiring extremely pure starting materials and sophisticated production procedures. DSC's, based on an inorganic wide band-gap semiconductor (TiO2) coated by a ruthenium polypyridyl complex as dye, have been studied and improved over the last decade and have reached a considerable solar to electric conversion efficiency of 11 % over the standard air mass (AM) 1.5 spectrum (100 mWcm-2). Traditionally, a liquid electrolyte redox system is used to regenerate the photo-excitated dye and carry current through the cell. Practical advantages have been gained by replacing the liquid electrolyte with an organic solid hole-transporting material (HTM). These type of cells exhibit a record efficiency of 4 % with 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD) as the HTM. Clearly this lags behind the performance features of liquid electrolyte dye-sensitized solar cells, as the research of the solid-state solar cells (SSD) is still in its infancy. The objective of the present work is to study the SSD performance limitations emerging from the cells and to find strategies for addressing these problems. Interfacial charge recombination is a significant loss mechanism in DSC. This is particularly true for solid-state devices, as the solid HTM is less efficient in screening the internal fields, which assist recombination. The primary, one electron-process, charge recombination occurs between injected electrons in the TiO2 and holes in the oxidized HTM. In this work, a strategy is developed to minimize interfacial charge recombination by using guanidinium carboxylic acid molecules as co-adsorbents to the dye molecules on the TiO2 nanocrystals. The charge-recombination rate of the injected electrons in the TiO2 with the holes in the HTM was retarded by an order of magnitude. The open-circuit voltage was also significantly improved by 14%. We attribute these effects to be due to a "space-filling" on the TiO2 surface and "columbic-screening" of the electrons in the TiO2 from the holes in the HTM. The dipolar nature of the co-adsorbent molecules is also likely to contribute to improvements in open-circuit voltage by further offsetting the energy levels between the n-type semiconductor and the HTM. In order to decrease the recombination of electrons, from the semiconductor, with holes in the HTM, the hole concentration must be reduced. For this reason, new hole-conductors, based on triphenyldiamine, with high hole-mobility have been investigated, avoiding the high degree of oxidation otherwise required in the standard HTM (less hole concentration). The 1,3,5-tris(N-(1-naphtyl)-N-[4-(1-naphthylphenylamino)-phenyl]-amino)-benzene (TTADB), having a very high hole-mobility, exhibits an extremely large open-circuit voltage on flat TiO2 films compared to the Spiro (30% higher). The 4,4',4''-tris(N-3-methylphenyl)-N-phenylamino)-triphenylamine (p-MTDATA) showed an improvement of 35% in the current density for very thin TiO2 films (0.5 µm). However these TPD molecules do not fill the nanocrystalline layer. Another nanoporous filling method has to be found for these HTM candidates to enable them to compete with the Spiro. In order to understand the weaknesses of the materials and interfaces during the aging process of solid DSC's, their long-term stability was studied under constant illumination. The experiments showed a destructive decrease of the shunt resistance and a rapid loss of the oxidized form of the HTM, Spiro-OMeTAD, during constant 0.75 sun illumination. Meanwhile, the short-circuit current of the cells increased during the first five hours of illumination. This phenomenon has already been observed and was attributed to a massive reduction of the Schottky barrier size, at the anode interface. This resulted in the generation of surface states and in a "pinning" of the Fermi level in the semiconductor at this junction. After the first 5 hours, however, the degradation of the solid cells is much more apparent than in the absence of UV light, where cells stabilized after approximately 20 hours with a loss of short circuit current of ∼ 50%. Strategies towards flexible solid-state DSC were investigated. For the flexible cell construction a metal foil was used as substrate and a semi-transparent gold layer as counter electrode, which allowed light transmission through back illumination.

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

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

A Material Approach to Dye-Sensitized Solar Cells

Philippe Pierre Labouchère

With growing concerns on future energy supplies, solar energy appears as an energy source whose potential remains to be tapped at a large scale. In the last two decades, dye-sensitized solar cells (DSCs) have been considered as a competitive means to convert solar energy into electricity thanks to their low manufacturing and environmental costs, simple fabrication and high efficiency. Indeed, record efficiencies as high as 13% have been obtained for liquid DSCs and recently, solid-state DSCs using solid hole transport materials as well as perovskite materials as sensitizers have showed efficiencies above 17%. From a physical point of view, charge carrier generation in a DSC takes place in a photoactive light harvester chemisorbed on a mesoscopic semiconductor oxide. Due to the excellent electronic overlap, the exciton separation occurs by the injection of the excited electrons and holes from the dye into the conduction band of the semiconductor and redox electrolyte, respectively. A high surface area mesoscopic photoanode is necessary to ensure a high-density bulk heterojunction and efficient light harvesting in the visible part of the solar spectrum due to effective dye loading. This implies that the DSC has a large, heterogeneous interface through which photogenerated electrons may be intercepted as a result of the slow electron transport. The latter is governed by an ambipolar diffusion mechanism controlled by trap-limited hopping through a long path to the transparent conductive electrode. It is however hindered by the low electron mobility in anatase TiO2 nanoparticles and the multiple grain boundaries in the mesoporous film. The work conducted during this thesis aimed at developing new architectures for the electrodes of DSCs using notably atomic layer deposition (ALD) and inverse opal host-guest structures with different semiconducting metal oxides. First, using a self-assembled polymeric template and ALD, we fabricated a porous ZnO inverse opal backbone. Advantages are numerous: a high electronic mobility of ZnO, a precise control of the metal oxide deposition using ALD and in situ growth of ZnO nanowires at low temperatures. Besides increasing the surface area available for photoactive dye molecules, single-crystal nanowires offer a defect-free path for photoinjected electrons. However, because of the relative instability of ZnO in acidic environments, a TiO2 overlayer was deposited by ALD. This proved to increase the recombination resistance of photogenerated electrons and increased their lifetime in the porous film. This host-guest approach was further transposed to three related fields in photo-electrochemistry using different ALD precursors. For water splitting, a niobium-doped tin oxide inverse opal backbone was constructed, which served as a host for hematite deposited by atmospheric pressure chemical vapour deposition or ALD. In the field of plasmonics, a TiO2 inverse opal host was made to act as a scaffold for silver nanoparticles. For solid-state DSCs, we strived to achieve an ultra-thin ZnO inverse opal, which acted as a host for perovskite crystals. Finally, we studied the effect of very thin TiO2 and ZnO overlayers deposited by ALD on top of mesoporous ZnO and TiO2, respectively. We were able to accurately monitor the thickness of overlayers and study the effects on photovoltaic parameters such as transport and recombination rates of photogenerated electrons, photocurrent, photovoltage, fill factor and efficiency.

EPFL2014