Paul E. Hopkinson

Iodine Migration and its Effect on Hysteresis in Perovskite Solar Cells.

The migration and accumulation of iodide ions create a modulation of the respective interfacial barriers causing the hysteresis in solar cells based on methylammonium lead iodide perovskites. Iodide ions are identified as the migrating species by measuring temperature dependent current-transients and photoelectron spectroscopy. The involved changes in the built-in potential due to ion migration are directly measured by electroabsorption spectroscopy.

The effect of tuning the microstructure of TIPS-tetraazapentacene on the performance of solution processed thin film transistors

We report a comprehensive study of the symmetrical 6,13-bis(triisopropylsilylethynyl)tetraazapentacene (TIPS-TAP) used as an electron transporting material in organic field-effect transistors. We study the optical, electronic, structural and morphological properties of thin films of TIPS-TAP as deposited by spin-coating and zone-casting techniques. Depending on the solution processing conditions and procedures we find a variety of microstructures for TIPS-TAP ranging from highly polycrystalline to well-aligned crystalline films. Field-effect transistors are fabricated in two different architectures to evaluate the charge transport properties of TIPS-TAP in such films, and bias-stress experiments reveal a good electric stability of TIPS-TAP. The extracted electron mobilities vary over several orders of magnitude depending on the resulting morphology of the active layer reaching a maximum of 0.42 cm2 V−1 s−1 for uniaxial aligned crystallites in zone-cast transistors.

Effect of Injection Layer Sub-Bandgap States on Electron Injection in Organic Light-Emitting Diodes

It is generally considered that the injection of charges into an active layer of an organic light-emitting diode (OLED) is solely determined by the energetic injection barrier formed at the device interfaces. Here, we demonstrate that the density of surface states of the electron-injecting ZnO layer has a profound effect on both the charge injection and the overall performance of the OLED device. Introducing a dopant into ZnO reduces both the energy depth and density of surface states without altering the position of the energy levels-thus, the magnitude of the injection barrier formed at the organic/ZnO interface remains unchanged. Changes observed in the density of surface states result in an improved electron injection and enhanced luminescence of the device. We implemented a numerical simulation, modeling the effects of energetics and the density of surface states on the electron injection, demonstrating that both contributions should be considered when choosing the appropriate injection layer.

Simultaneous enhancement in open circuit voltage and short circuit current of hybrid organic–inorganic photovoltaics by inorganic interfacial modification

One of the key advantages of metal oxide/polymer organic–inorganic hybrid photovoltaic devices is the possibility to control the photo-induced charge separation efficiency by interfacial modification. While a large variety of organic modifiers have been investigated, inorganic modification layers remain largely unexplored. Here, we investigate the model poly(3-hexathiophene)/ZnO system and show that by introducing a caesium carbonate interlayer, a simultaneous increase in all photovoltaic performance parameters can be achieved. While improved energy level alignment results in a significant increase in the open circuit voltage, the suppression of interfacial bound charge pairs formation causes a reduction in interfacial recombination losses and an increase in short circuit current. The overall power conversion efficiency is enhanced twelve fold, demonstrating the significant potential of inorganic modifiers for improving the performance of hybrid photovoltaic devices.