Brian C. O’Regan

Ionic transport in hybrid lead iodide perovskite solar cells

Solar cells based on organic–inorganic halide perovskites have recently shown rapidly rising power conversion efficiencies, but exhibit unusual behaviour such as current–voltage hysteresis and a low-frequency giant dielectric response. Ionic transport has been suggested to be an important factor contributing to these effects; however, the chemical origin of this transport and the mobile species are unclear. Here, the activation energies for ionic migration in methylammonium lead iodide (CH3NH3PbI3) are derived from first principles, and are compared with kinetic data extracted from the current–voltage response of a perovskite-based solar cell. We identify the microscopic transport mechanisms, and find facile vacancy-assisted migration of iodide ions with an activation energy of 0.6 eV, in good agreement with the kinetic measurements. The results of this combined computational and experimental study suggest that hybrid halide perovskites are mixed ionic–electronic conductors, a finding that has major implications for solar cell device architectures.

Experimental determination of the rate law for charge carrier decay in a polythiophene: Fullerene solar cell

We use transient photovoltage and differential charging experiments, complemented by transient absorption data, to determine charge carrier lifetimes and densities in a poly(3-hexylthiophene): methanofullerene solar cell at Voc as a function of white light-bias intensity. For a typical device, the charge carrier decay dynamics are observed to exhibit an approximately third order dependence on charge density (dn∕dt∝n3).

Recombination Dynamics as a Key Determinant of Open Circuit Voltage in Organic Bulk Heterojunction Solar Cells: A Comparison of Four Different Donor Polymers

[*] Prof. J. R. Durrant, Mr. A. Maurano, Dr. R. Hamilton, Dr. C. G. Shuttle, Dr. B. O’Regan, Dr. W. Zhang, Prof. I. McCulloch Departments of Chemistry, Imperial College London South Kensington SW7 2AZ (United Kingdom) E-mail: j.durrant@imperial.ac.uk Dr. A. M. Ballantyne, Prof. J. Nelson Departments of Physics, Imperial College London South Kensington SW7 2AZ (United Kingdom) Dr. H. Azimi, Dr. M. Morana, Prof. C. J. Brabec Konarka Austria, Altenbergerstrasse 69 A-4040 Linz (Austria) Dr. H. Azimi Christian Doppler Laboratory for Surface Optics Johannes Kepler University Linz (Austria)

Structure/Function Relationships in Dyes for Solar Energy Conversion: A Two-Atom Change in Dye Structure and the Mechanism for Its Effect on Cell Voltage

Recombination between injected electrons and iodine limits the photovoltage in dye-sensitized solar cells (DSSCs). We have recently suggested that many new dye molecules, intended to improve DSSCs, can accelerate this reaction, negating the expected improvement (J. Am. Chem. Soc. 2008, 130, 2907). Here we study two dyes with only a two-atom change in the structure, yet which give different V(oc)s. Using a range of measurements we show conclusively that the change in V(oc) is due solely to the increase in the recombination rate. From the structure of the dyes, and literature values for iodine binding of similar compounds, we find that it is very likely that the change in V(oc) is due solely to the difference in iodine binding at the site of the two-atom change. Using the large amount of literature on iodine complexation, we suggest structures for dyes that might show improved V(oc).

Charge extraction analysis of charge carrier densities in a polythiophene/fullerene solar cell: Analysis of the origin of the device dark current

We demonstrate the use of a simple charge extraction measurement to determine the charge carrier densities n in annealed poly(3-hexylthiophene):methanofullerene solar cells under operating conditions. By applying charge extraction to the device under forward bias in the dark (Jdark), we find Jdark∝n2.6. This dependence on charge density is the same as that we find for bimolecular recombination losses observed in such devices under irradiation at open circuit, suggesting that the dark current originates from bimolecular recombination at the polymer/fullerene interface.

The effect of molecular aggregates over the interfacial charge transfer processes on dye sensitized solar cells

The electron transfer reaction between the photoinjected electrons in the nanocrystalline TiO2 mesoporous sensitized films and the oxidized electrolyte in dye sensitized solar cells (DSSC) plays a major role on the device efficiency. In this communication we show that, although the presence of molecular aggregates on the free base porphyrin DSSC limits the device photocurrent response under illumination, they form an effective hydrophobic barrier against the oxidized electrolyte impeding fast back-electron transfer kinetics. Therefore, their drawback can be overcome by designing dyes with peripheral moieties that prevent the formation of the aggregates and are able to achieve efficiencies as high as 3.2% under full sun.

Performance enhancement of solution processed perovskite solar cells incorporating functionalized silica nanoparticles

High efficiency, solution processed organic–inorganic trihalide perovskite solar cells are now a reality, meaning that perovskite photovoltaics have the potential to challenge more established photovoltaic technologies. To date, some of the most efficient solution processed perovskite solar cells feature a pre-deposited Al2O3 scaffold and we have shown in a previous communication, that it is possible to make efficient devices by co-depositing the Al2O3 nanoparticles with the perovskite precursor solution. In this work, we have substituted the alumina nanoparticles with 3-aminopropyl (3-oxobutanoic acid) functionalized silica nanoparticles (f-SiO2). We observe performance enhancements in planar heterojunction (PHJ) devices made with up to 0.75 wt% f-SiO2 nanoparticles present in the precursor solution, yielding power conversion efficiencies (PCE) of up to 12.4%, compared to the maximum PCE of 10.5% in the equivalent PHJ devices made without f-SiO2 nanoparticles. The performance enhancement arises in part from an average increase to VOC by up to 50 mV when the nanoparticles are present in the precursor solution and is attributed to substrate passivation within pinholes formed in the perovskite film during processing.

Identifying recombination mechanisms through materials development in perovskite solar cells

Through materials and device developments and by using measurements such as transient photovoltage decay and impedance spectroscopy we have begun to identify recombination mechanisms in perovskite solar cells. At the forefront of our developments is a transparent, indium free, cathode which allows measurements to be made whilst illuminating from both the photoanode side and the cathode side of the device. Recombination is consistently faster when illuminated from the cathode side and we conclude that in this case, as charge carriers are generated closer to the perovskite/SPIRO-OMeTAD interface, interfacial recombination is a significant contributor to voltage losses within the device.