High photovoltage in perovskite solar cells: New physical insights from the ultrafast transient absorption spectroscopy

To understand the cause of the high open circuit photovoltage (VOC) achieved by todays' state of the art perovskite solar cells (PSCs), we examine formamidinium lead bromide CH(NH2)(2)PbBr3 films by ultrafast transient absorption spectroscopy (TAS). By using TiO2 and spiro-OMeTAD as charge extraction layers, the devices based on the CH(NH2)(2)PbBr3 films yield VOC as high as 1.5 V ascertaining their high quality. TAS establish that the presence of charge extraction layers has very little influences on the nature of a negative band at 535 nm corresponding to the bleaching of the absorption band edge and two positive bands in the CH(NH2)(2)PbBr3 films. Therefore, we contend that the V-OC in PSC is predominantly determined by the quasi Fermi level splitting within the perovskite layer. (C) 2017 Published by Elsevier B. V

Toward An Understanding Of The Retinal Chromophore In Rhodopsin Mimics

Recently, a rhodopsin protein mimic was constructed by combining mutants of the cellular retinoic acid binding protein II (CRABPII) with an all-trans retinal chromophore. Here, we present a combine computational quantum mechanics/molecular mechanics (QM/MM) and experimental ultrafast kinetic study of CRABPII. We employ the QM/MM models to study the absorption (lambda(a)(max)), fluorescence (lambda(f)(max)), and reactivity of a CRABPII triple mutant incorporating the all-trans protonated chromophore (PSB-KLE-CRABPII). We also study the spectroscopy of the same mutant incorporating the unprotonated chromophore and of another double mutant incorporating the neutral unbound retinal molecule held inside the pocket. Finally, for PSB-KLE-CRABPII, stationary fluorescence spectroscopy and ultrafast transient absorption spectroscopy resolved two different evolving excited state populations which were computationally assigned to distinct locally excited and charge-transfer species. This last species is shown to evolve along reaction paths describing a facile isomerization of the biologically relevant 11-cis and 13-cis double bonds. This work represents a first exploratory attempt to model and study these artificial protein systems. It also indicates directions for improving the QM/MM models so that they could be more effectively used to assist the bottom-up design of genetically encodable probes and actuators employing the retinal chromophore

Exploiting optical nonlinearities for group delay dispersion compensation in femtosecond optical parametric oscillators

A wavelength tunable femtosecond optical parametric oscillator pumped by the second harmonic of a Yb: KGW solid state oscillator was investigated. The intracavity group delay dispersion was positive, and soliton condition was satisfied by introducing negative nonlinearity from cascaded quadratic nonlinearity (CQN). Two different approaches were investigated - CQN induced by the same amplifying nonlinear crystal or CQN induced by an additional second harmonic generating nonlinear crystal inside the same resonator. The second crystal was shown to correct the resonator misalignment induced by the rotation of the amplifying crystal as the wavelength was tuned in the range of 770-970 nm. It simultaneously compensated positive resonator GDD offsets of +/- 1000 fs(2) with +/- 5% SHG power losses, simulating a method for compensation of GDD ripples in a broadband mirror

Cascaded nonlinearity induced spatial domain effects in a high power femtosecond optical parametric oscillator /

We have investigated the effect of cascaded optical nonlinearity on the spatial beam properties of a femtosecond optical parametric oscillator (OPO). The OPO was operated with a tunable phase mismatch by varying the angle of the nonlinear crystal. The cascaded nonlinearity induced self-focusing and defocusing changed resonator's stability and impacted mode properties. With tuning of a phase mismatch, the calculated parabolic part of cascaded nonlinearity lens focal length changes from f similar to 30 mm (D similar to 33 m(-1) at Delta theta similar to -0.5 degrees) to infinity and back to f similar to -110 mm (D similar to -9 m(-1) at Delta theta similar to 0.9 degrees) in the LBO nonlinear crystal. Such high power nonlinear lenses in a cavity operated near its stability limit promoted the generation of axially asymmetric or pass-to-pass unstable resonator modes. It was shown that phase mismatched optical parametric oscillation changes the physical character of the resonator from linear to ring-like with two nonlinear crystals having two different focusing powers. Calculations showed that the QCN induced spatial nonlinear phase should lead to severe longitudinal chromatic aberrations for broad spectrum pulses. A numerical simulation in XYZ spatial domain and calculations using ABCD matrix approach confirmed the physical mechanisms underlying the experimental results and allowed for the interpretation of the observed effects

Unveiling the excited state energy transfer pathways in peridinin-chlorophyll α-protein by ultrafast multi-pulse transient absorption spectroscopy

Time-resolved multi-pulse methods were applied to investigate the excited state dynamics, the interstate couplings, and the excited state energy transfer pathways between the light-harvesting pigments in peridinin-chlorophyll a-protein (PCP). The utilized pump-dump-probe techniques are based on perturbation of the regular PCP energy transfer pathway. The PCP complexes were initially excited with an ultrashort pulse, resonant to the S₀ → S₂ transition of the carotenoid peridinin. A portion of the peridinin-based emissive intramolecular charge transfer (ICT) state was then depopulated by applying an ultrashort NIR pulse that perturbed the interaction between S₁ and ICT states and the energy flow from the carotenoids to the chlorophylls. The presented data indicate that the peridinin S₁ and ICT states are spectrally distinct and coexist in an excited state equilibrium in the PCP complex. Moreover, numeric analysis of the experimental data asserts ICT → Chl-α as the main energy transfer pathway in the photoexcited PCP systems.11 page(s

Synthesis, crystal structures, and laser flash photolysis of 3-nitro-7a,15-methanonaphtho[1',2':6,7][1,3]oxazepino[3,2-a]indole derivatives

The condensation of 1-substituted 9,9a-dihydro-1H-imidazo[1,2-a]indol-2(3H)-ones with 2- hydroxy-6-nitro-1-naphthaldehyde afforded 1′-carbamoylmethyl-8-nitrospiro[benzo[f]chromene- 3,2′-indole] derivatives, which underwent intramolecular cyclisation to derivatives of 3-nitro- 7a,15-methanonaphtho[1′,2′:6,7][1,3]oxazepino[3,2-a]indole upon treatment with a strong base. Laser excitation of the obtained uncoloured molecules of trans- and cis-3-nitro-7a,15-methanonaphtho[ 1′,2′:6,7][1,3]oxazepino[3,2-a]indole induced the formation of short-lived photogenerated species, which absorb in the visible spectrum and thermally revert to the ground state on a nanosecond time scale

Supercontinuum generation by co-filamentation of two color femtosecond laser pulses

In this paper, we experimentally investigate supercontinuum generation via collinear two-color filamentation in sapphire crystal, by launching two femtosecond pulses at fundamental (1030 nm) and second harmonic (515 nm) wavelengths from an amplified Yb:KGW laser. By changing the time delay between the incident pulses, we observe dramatic changes in the supercontinuum spectrum, transmitted energy, position of the nonlinear focus and intensity distribution along the filament-induced luminescence traces. In particular, we show that at some delays the two pump wavelengths can assist each other in generating supercontinuum, whilst at other delays large portions of the supercontinuum spectrum are completely extinguished. The transition between supercontinuum generation and its extinction occurs within a very short (20 fs) span of the delay times, despite the fact that the pump pulses are 220 fs long. We propose that the observed non-trivial spectral dynamics can be interpreted by a mechanism, where co-propagating two pump pulses perturb the nonlinear refractive properties of the medium via Kerr effect and generation of free electron plasma thereby affecting pulse splitting and pulse front steepening, which are the key players in the process of supercontinuum generation in a normally dispersive medium