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It is well known that the surface trap states and electronic disorders in the solution-processed CH3 NH3 PbI3 perovskite film affect the solar cell performance significantly and moisture sensitivity of photoactive perovskite material limits its practical applications. Herein, we show the surface modification of a perovskite film with a solution-processable hydrophobic polymer (poly(4-vinylpyridine), PVP), which passivates the undercoordinated lead (Pb) atoms (on the surface of perovskite) by its pyridine Lewis base side chains and thereby eliminates surface-trap states and non-radiative recombination. Moreover, it acts as an electron barrier between the perovskite and hole-transport layer (HTL) to reduce interfacial charge recombination, which led to improvement in open-circuit voltage (Voc ) by 120 to 160 mV whereas the standard cell fabricated in same conditions showed Voc as low as 0.9 V owing to dominating interfacial recombination processes. Consequently, the power conversion efficiency (PCE) increased by 3 to 5 % in the polymer-modified devices (PCE=15 %) with Voc more than 1.05 V and hysteresis-less J-V curves. Advantageously, hydrophobicity of the polymer chain was found to protect the perovskite surface from moisture and improved stability of the non-encapsulated cells, which retained their device performance up to 30 days of exposure to open atmosphere (50 % humidity).

The optical properties of InGaN/GaN multiple-quantum-well (MQW) nanocolumns whose emission wavelength is in the yellow-to-red region (2.1 ∼ 2.3 eV) have been investigated using time-resolved photoluminescence (PL) measurements. The analysis of the PL decay curves using a stretched exponential function shows that the radiative and nonradiative rates in the red region are almost the same as those in the yellow region. Long-lived migrating carriers also contribute the strong PL intensity in the red region. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical properties in a single crystal of the organic-inorganic hybrid perovskite quantum-well material (C4H9NH3)2(CH3NH3)Pb2Br7 have been investigated. This material has double-layer sheets of corner-sharing [PbBr6]4- octahedrons constituting the inorganic well. The estimated longitudinal-transverse (LT) splitting, exciton exchange energy, and biexciton binding energy are about 60 meV, 17 meV and 26 meV respectively. We can also study well-width dependence in angstroms by comparing with (C4H9NH3)2PbBr4 which has single-layer sheets comprising the inorganic well. This is the first time the well-width dependence of excitonic properties in hybrid organic-inorganic quantum materials is investigated. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The coherent dynamics of excitons in InAs quantum dots (QDs) was investigated in the telecommunication wavelength range using a transient four-wave mixing technique. The sample was fabricated on an InP(311)B substrate using strain compensation to control the emission wavelength. This technique also enabled us to fabricate a 150-layer stacked QD structure for obtaining a high S/N in the four-wave mixing measurements, although no high-sensitive heterodyne detection was carried out. The dephasing time and transition dipole moment were precisely estimated from the polarization dependence of signals, taking into account their anisotropic properties. The population lifetimes of the excitons were also measured by using a polarization-dependent pumpprobe technique. A quantitative comparison of these anisotropies demonstrates that in our QDs, non-radiative population relaxation, polarization relaxation and pure dephasing are considerably smaller than the radiative relaxation. A comparison of the results of the four-wave mixing and pump-probe measurements revealed that the pure dephasing could be directly estimated with an accuracy of greater than 0.1 meV by comparing the results of four-wave mixing and pump-probe measurements. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We systematically investigate and compare the structural and optical properties of InGaN nanocolumns (NCs) and InGaN/GaN single quantum structures (SQSs) fabricated on top of GaN NCs as functions of the GaN column diameter. Each InGaN/GaN SQS comprises an In-rich InGaN single quantum dot at the center of a semipolar Ga-rich InGaN single quantum well. The InGaN NCs and InGaN/GaN SQSs qualitatively exhibit similar optical properties. However, the specific diameters at which the photoluminescence spectra change from single-peak emission patterns to double-peak patterns are different. Based on scanning transmission electron microscopy observations of the InGaN layers formed on the GaN NCs with different growth times, a model for the growth of InGaN on GaN NCs is proposed and the origin of the difference in the specific diameters is elucidated. Finally, when QSs are introduced to the InGaN-based NCs, the radiative recombination probability increases while the nonradiative and surface recombination probabilities decrease, thus leading to high emission efficiency.

We have investigated how the morphology and diameter of GaN nanocolumn affect the optical properties of exciton and biexciton. We measured photoluminescence (PL) of excitons in GaN nanocolumns at low temperature and found that the PL spectra depend on the column diameter. Changing the distance between the sample and the collimating lens, it was established that this phenomenon is due to the z -polarized PL radiated from the side surface. This result proves that the nano-scale morphology produces a change in the optical properties. In addition, to elucidate the mechanism causing the increase of the biexciton binding energy observed in a previous experiment, we calculated the exciton binding energies by the variational method and also estimated their sizes. From the calculated results we can expect that the only biexcitons experience spatial confinement effect even when the nanocolumn width is much larger than the exciton Bohr radius. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

During strength measurements on plastic-coated optical glass fibers, the positions of fracture origins were determined by an acoustic technique. For samples other than those in which fracture was initiated at or very near the fixed ends, Weibull plots of the strength data exhibited a single-mode distribution with low variability (m=20 to 25, corresponding to a coefficient of variation of 6.6 to 10.6%).