2008-11-19, Plank, Natalie O. V., Snaith, Henry J., Ducati, Caterina, Bendall, James S., Schmidt-Mende, Lukas, Welland, Mark E.

We report a hydrothermal synthesis method for MgO shell coatings directly onto the surface of ZnO nanowire arrays. The entire process can be carried out below 100 °C. The MgO shells are produced by the addition of 10 mM magnesium nitrate with 0.2 M sodium hydroxide in water, resulting in a shell thickness of up to 8 nm, verified by high resolution transmission electron microscopy. The viability of the MgO layer as a functional element of optoelectronic devices was tested on solid-state organic hole-transporter based dye-sensitized solar cells. Incorporation of the MgO shell into the solar cell resulted in substantive efficiency improvements of over 400% in comparison to the pristine ZnO nanowire based photovoltaics, indicating that electrons can efficiently tunnel through the 'insulating' MgO shell.

2007-01-21, Burke, Anthony, Schmidt-Mende, Lukas, Ito, Seigo, Grätzel, Michael

A squaraine dye incorporating two carboxylic acid attaching groups has been synthesised and used successfully in both liquid and solid-state solar cells, with solar energy to electricity conversion efficiencies (η) under AM 1.5 G irradiation (100 mW cm⁻²) of 3.7 and 1.5% and short-circuit current densities (J_scs) of 8.6 and 4.2 mA cm⁻², with open-circuit voltages (V_oc) of 591 and 681 mV and fill factors (FF) of 73 and 53%, respectively.

2007, Campbell, Wayne M., Jolley, Kenneth W., Wagner, Pawel, Wagner, Klaudia, Walsh, Penny J., Gordon, Keith C., Schmidt-Mende, Lukas, Nazeeruddin, Mohammad K., Wang, Qing, Grätzel, Michael, Officer, David L.

In TiO₂-based dye-sensitized nanocrystalline solar cells, efficiencies of up to 11% have been obtained using Ru dyes, but the limited availability of these dyes together with their undesirable environmental impact have led to the search for cheaper and safer organic-based dyes. In this Letter, we report the synthesis, electronic, and photovoltaic properties of novel green porphyrin sensitizers. All six porphyrin dyes give solar cell efficiencies of ≥5%, but the best performing dye under standard global AM 1.5 solar conditions gives a short circuit photocurrent density (j_sc) of 14.0 ± 0.20 mA/cm², an open circuit voltage of 680 ± 30 mV, and a fill factor of 0.74, corresponding to an overall conversion efficiency of 7.1%, which, for porphyrin-based sensitizers, is unprecedented. This same dye gives an efficiency of 3.6% in a solid-state cell with spiro-MeOTAD as the hole transporting component, comparable to solid-state cells incorporating the best performing ruthenium dyes.

2006, Snaith, Henry J., Schmidt-Mende, Lukas, Grätzel, Michael, Chiesa, Marco

2008, Plank, Natalie O. V., Welland, Mark E., MacManus-Driscoll, Judith L., Schmidt-Mende, Lukas

2007, Schmidt-Mende, Lukas, MacManus-Driscoll, Judith L.

2006-12-20, Kroeze, Jessica E., Hirata, Narukuni, Koops, Sara, Nazeeruddin, Mohammad Khaja, Schmidt-Mende, Lukas, Grätzel, Michael, Durrant, James R.

The optimization of interfacial charge transfer is crucial to the design of dye-sensitized solar cells. In this paper we address the dynamics of the charge separation and recombination in liquid-electrolyte and solid-state cells employing a series of amphiphilic ruthenium dyes with varying hydrocarbon chain lengths, acting as an insulating barrier for electron−hole recombination. Dynamics of electron injection, monitored by time-resolved emission spectroscopy, and of charge recombination and regeneration, monitored by transient optical absorption spectroscopy, are correlated with device performance. We find that increasing dye alkyl chain length results in slower charge recombination dynamics to both the dye cation and the redox electrolyte or solid-state hole conductor (spiro-OMeTAD). These slower recombination dynamics are however paralleled by reduced rates for both electron injection into the TiO₂ electrode and dye regeneration by the I-/I₃- redox couple or spiro-OMeTAD. Kinetic competition between electron recombination with dye cations and dye ground state regeneration by the iodide electrolyte is found to be a key factor for liquid electrolyte cells, with optimum device performance being obtained when the dye regeneration is just fast enough to compete with electron−hole recombination. These results are discussed in terms of the minimization of kinetic redundancy in solid-state and liquid-electrolyte dye-sensitized photovoltaic devices.

2008, Musselman, Kevin P., Mulholland, Gregory J., Robinson, Adam P., Schmidt-Mende, Lukas, MacManus-Driscoll, Judith L.

2007, Snaith, Henry J., Schmidt-Mende, Lukas

This Progress Report highlights recent developments in dye-sensitized solar cells composed of both liquid electrolytes and solid-state hole transport materials. The authors discuss and review the present understanding of and recent developments in the operational processes, such as charge generation, transport, recombination, and charge collection. Also, the merits and challenges of alternative device approaches are discussed, including extremely thin absorber cells, devices containing inorganic p-type hole-transporters and non-TiO₂ mesoporous metal-oxide electrodes employed in dye-sensitized solar cells.

2006, Kroeze, Jessica E., Hirata, Narukuni, Schmidt-Mende, Lukas, Orizu, Charles, Ogier, Simon D., Carr, Kathryn, Grätzel, Michael, Durrant, James R.

Solid-state dye-sensitized solar cells employing a solid organic hole-transport material (HTM) are currently under intensive investigation, since they offer a number of practical advantages over liquid-electrolyte junction devices. Of particular importance to the design of such devices is the control of interfacial charge transfer. In this paper, the factors that determine the yield of hole transfer at the dye/HTM interface and its correlation with solid-state-cell performance are identified. To this end, a series of novel triarylamine type oligomers, varying in molecular weight and mobility, are studied. Transient absorption spectroscopy is used to determine hole-transfer yields and pore-penetration characteristics. No correlation between hole mobility and cell performance is observed. However, it is found that the photocurrent is directly proportional to the hole-transfer yield. This hole-transfer yield depends on the extent of pore penetration in the dye-sensitized film as well as on the thermodynamic driving force ΔG_dye–HTM for interfacial charge transfer. Future design of alternative solid-state HTMs should focus on the optimization of pore-filling properties and the control of interfacial energetics rather than on increasing material hole mobilities.