Optimizing the Thermodynamics and Kinetics of the Triplet-Pair Dissociation in Donor–Acceptor Copolymers for Intramolecular Singlet Fission

Singlet fission (SF) is a two-step process in which a singlet splits into two triplets throughout the so-called correlated triplet-pair (1TT) state. Intramolecular SF (iSF) materials, in particular, have attracted growing interest as they can be easily implemented in single-junction solar cells and boost their power conversion efficiency. Still, the potential of iSF materials such as polymers and oligomers for photovoltaic applications has been partially hindered by their ability to go beyond the 1TT intermediate and generate free triplets, whose mechanism remains poorly understood. In this work, the main aspects governing the 1TT dissociation in donor–acceptor copolymers and the key features that optimize this process are exposed. First, we show that both thermodynamics and kinetics play a crucial role in the intramolecular triplet-pair separation and second, we uncover the inherent flexibility of the donor unit as the fundamental ingredient to optimize them simultaneously. Overall, these results provide a better understanding of the intramolecular 1TT dissociation process and establish a new paradigm for the development of novel iSF active materials.

Solid hybrid dye-sensitized solar cells

Nathalie Rossier-Iten

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.

EPFL2006

La0.5Sr0.5Fe1-yMyO3-[delta] (M = Ti, Ta) perovskite oxides for oxygen separation membranes

Defne Bayraktar

Mixed ionic electronic conducting perovskite materials have been receiving considerable attention for the application of oxygen separation membranes. These membranes have potential to be integrated in industrial processes that require pure oxygen and to provide advantages considering economical and environmental aspects. The used perovskite materials can accommodate a high concentration of disordered oxygen vacancies and provide high oxygen permeation flux in the presence of an oxygen partial pressure gradient. However, the materials with high oxygen flux are observed to have low chemical and mechanical stability, originating from oxygen vacancy formation and reduction of the B-site transition metal ion. Based on the fact that the stability of the materials at reducing pO2 is dependent on the redox stability of the B-site cation, a partial substitution of the B-site transition metal with a more stable cation is considered in this thesis to improve the stability of the base perovskite oxide. The approach used for the identification of the suitable material based on the host composition La0.5Sr0.5FeO3-δ (LSF) was to first explore the B-site substituting elements in terms of their effect on the thermal expansion and the isothermal expansion measured during the atmosphere change from air to argon. The elements Al, Cr, Zr, Ga, Ti, Sn, Ta, In, V, and Mg were used for 10 or 20% substitution of Fe. The isothermal expansion of the host material was decreased by substitution, which was attributed to decreased amount of oxygen vacancy formation as it was evidenced later by TGA of chosen compositions. As a result, the compositions substituted with higher valence ions (particularly, 20% Ti4+, LSFTi2, and 10% Ta5+, LSFTa1) were identified as the least expanding materials in isothermal conditions. The effect of substitution was further characterized extensively including structural, mechanical, and oxygen transport properties. The oxygen vacancy (VO••) concentration changes induced by substitution of Fe were reflected in unit cell sizes of heat-treated samples under argon atmosphere. The increase in the unit cell size was attributed to the isothermal expansion due to B-site reduction and lowered to half in case of substituted LSFTi2 and LSFTa1 compared to the host material LSF. Similar trends were observed for interrelated properties such as the coefficient of thermal expansion, isothermal expansion, and the actual amount of oxygen vacancies formed under argon at 900°C, all of which were higher for LSF. The mechanical properties of LSF, LSFTi2, and LSFTa1 were characterized at room temperature. The Young's modulus and the bending strength of the materials were in the same range (141-147 GPa and ∼120 MPa, respectively) while the fracture toughness of the LSF sample was improved by Ti and especially Ta substitution. The fractography of the samples provided evidence that the several LSF samples showed extended cracks possibly related to residual stresses forming during cooling, due to non-equilibrated oxygen stoichiometry across the sample. The potential step measurements showed that the chemical diffusion and the surface exchange coefficients measured during oxidation of the samples were higher than the ones measured during reduction of the samples. The tendency was reversed at lowered pO2 along with the decrease of both coefficients. The influence of B-site substitution on both coefficients was not substantial. The oxygen permeation flux of LSF, LSFTi2, and LSFTa1 was measured under air/argon gradient on planar membranes. The permeation rates of the membranes with thicknesses close to 1 mm were considered to be limited by surface exchange kinetics at temperatures above 875°C. The permeation rate of LSF was decreased (from 0.2 µmolcm-2s-1) to its third by Ti and Ta additions. The permeation rate of Ti and Ta-substituted samples showed a drop around 875°C, keeping a similar activation energy at lower and higher temperature regions. Oxygen vacancy ordering in the perovskite structure was given as a possible explanation, which provided explanation for the slow equilibrium in the lower temperature region. Tubular membranes of LSF and LSFTi2 (0.47 mm and 0.36 mm thick, respectively) and a planar LSFTa1 membrane were characterized using an air/(Ar-CH4) gradient. The measurements conducted using pure Ar on the lean-side provided the possibility to compare the thickness dependence of the permeation. In agreement with the previous observations, the permeation of the materials at and above 900°C was independent of the thickness, therefore, surface exchange limited. Stability under reducing atmospheres was increased by substitution. The LSF membrane failed shortly after the introduction of CH4 to the system, while LSFTi2 survived 5% CH4 and its oxygen permeation was improved substantially by a factor of 14. LSFTa1 membrane was measured with pure CH4 and the permeation was increased by a factor of 9, reaching 0.7 µmolcm-2s-1 at 900°C. The membrane operated stably over 2000h with pure CH4, 1000h. A slow degradation of 3.7% per 1000h was observed at 1000°C, most probably due to cation migration. The compositions identified as a result of B-site substitution screening were effective in improving the stability of the materials without extensive loss of the oxygen permeation rate. Ta-substituted material was shown to be promising with its long-term stability as a partial oxidation membrane.

EPFL2008

Branched methoxydiphenylamine-substituted fluorene derivatives as hole transporting materials for high-performance perovskite solar cells

Michael Graetzel, Paul Gratia, Taisuke Matsui, Mohammad Khaja Nazeeruddin, Michael Saliba, Simona Urnikaite

Small-molecule hole transporting materials based on methoxydiphenylamine-substituted fluorene fragments were synthesized and incorporated into a perovskite solar cell, which displayed a power conversion efficiency of up to 19.96%, one of the highest conversion efficiencies reported. The investigated hole transporting materials were synthesized in two steps from commercially available and relatively inexpensive starting reagents, resulting in up to fivefold cost reduction of the final product compared with spiro-OMeTAD. Electro-optical and thermoanalytical measurements such as UV/Vis, thin-film conductivity, hole mobility, DSC, TGA, ionization potential and current voltage scans of the full perovskite solar cells have been carried out to characterize the new materials.