Morphological Evolution and Singlet Fission in Aqueous Suspensions of TIPS-Pentacene Nanoparticles

We report the observation of singlet fission in aqueous suspensions of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn) nanoparticles (NPs) synthesized using the reprecipitation method. By altering the synthesis conditions we are able to fabricate NPs which evolve from a system of poorly coupled to highly coupled chromophores. This morphological evolution can also be suppressed for a period of several months. Absorption spectra confirm that the particles evolve over time, displaying increased intermolecular interaction, if the initial reaction conditions seeded a polycrystalline sample. We correlate these differences in morphologies to different rates of singlet state decay, where higher intermolecular interaction drives a more rapid rate of decay. Ultrafast time-resolved photoluminescence spectroscopy confirms a short first excited singlet state lifetime (<2 ps), and transient absorption spectroscopy is used to probe the generation of triplets. We find that NPs with greater interchromophore coupling are less efficient at singlet fission. This is surprising and contrasts with previous reports of fission in TIPS-Pn. It is suggested that the slow morphological evolution used to generate highly coupled chromophores also introduces singlet exciton traps. We observe a persistent singlet signal in transient absorption measurements and a long-lived fluorescence anisotropy component, supporting this hypothesis. As such, it is clear that both long-range and short-range order play significant roles in the efficacy of singlet fission. A rapid initial fluorescence polarization dephasing is also observed (<1 ps), suggesting that excitons rapidly migrate over crystalline grain boundaries or within amorphous regions

Lessons learnt from spatially resolved electro- and photoluminescence imaging: Interfacial delamination in CH3NH3PbI3 planar perovskite solar cells upon illumination

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.The influence of illumination on the long-term performance of planar structured perovskite solar cells (PSCs) is investigated using fast and spatially resolved luminescence imaging. The authors analyze the effect of illuminated current density-voltage (J-V) and light-soaking measurements on pristine PSCs by providing visual evidence for the spatial inhomogeneous evolution of device performance. Regions that are exposed to light initially produce stronger electroluminescence signals than surrounding unilluminated regions, mainly due to a lower contact resistance and, possibly, higher charge collection efficiency. Over a period of several days, however, these initially illuminated regions appear to degrade more quickly despite the device being stored in a dark, moisture- and oxygen-free environment. Using transmission electron microscopy, this accelerated degradation is attributed to delamination between the perovskite and the titanium dioxide (TiO2) layer. An ion migration mechanism is proposed for this delamination process, which is in accordance with previous current-voltage hysteresis observations. These results provide evidence for the intrinsic instability of CH3NH3PbI3-based devices under illumination and have major implications for the design of PSCs from the standpoint of long-term performance and stability

Extended hot carrier lifetimes observed in bulk In<inf>0.265±0.02</inf>Ga<inf>0.735</inf>N under high-density photoexcitation

We have investigated the ultrafast carrier dynamics in a 1 μm bulk In0.265Ga0.735N thin film grown using energetic neutral atom-beam lithography/epitaxy molecular beam epitaxy. Cathodoluminescence and X-ray diffraction experiments are used to observe the existence of indium-rich domains in the sample. These domains give rise to a second carrier population and bi-exponential carrier cooling is observed with characteristic lifetimes of 1.6 and 14 ps at a carrier density of 1.3 × 1016cm-3. A combination of band-filling, screening, and hot-phonon effects gives rise to a two-fold enhanced mono-exponential cooling rate of 28 ps at a carrier density of 8.4 × 1018cm-3. This is the longest carrier thermalization time observed in bulk InGaN alloys to date