Photophysical analysis of dual fluorescence and phosphorescence emissions observed for semi-aliphatic polyimides at lower temperatures.

The photoluminescence (PL) properties of four types of blue fluorescent semi-aliphatic polyimides (PIs) derived from aromatic dianhydrides (ODPA, BPDA, HQDEA, and BPADA) and an alicyclic diamine (DCHM) were investigated at temperatures ranging from room temperature (RT, 298 K) to 30 K to analyse the origins of their non-radiative relaxation (NR) processes. These PIs exhibited significant increases in fluorescence (FL) intensity and lifetimes when lowering the temperature, stabilising below 100 K. The PIs containing ether (-O-) linkages showed a shoulder peak at around 500 nm below 150 K, which is attributable to phosphorescence (PH). These results show that the NR deactivation at RT includes three processes: intersystem crossing (ISC) from the excited singlet (S1) to the triplet (T1) state, temperature-dependent NR from the S1 state, which becomes suppressed below around 100 K, and temperature-independent NR. Based on the analyses of the temperature dependences, polymer structures, and quantum chemical analysis of molecular orbitals, we contemplate that the temperature-dependent NR is attributable to the excitation quenching by defect states mediated by excitation migration, and the temperature-independent NR may be caused by the deactivation of the excited state induced by molecular vibrations.

Electric Field-Induced Quenching of MAPbI3 Photoluminescence in PeLED Architecture.

Photoluminescence (PL) measurements are a widely used technique for the investigation of perovskite-based materials and devices. Although electric field-induced PL quenching provides additional useful information, this phenomenon is quite complex and not yet clearly understood. Here, we address the PL quenching of methylammonium lead iodide (MAPbI3) perovskite in a light-emitting diode (PeLED) architecture. We distinguish two quenching mechanisms: (a) indirect quenching by slow irreversible or partially reversible material changes that occur gradually under the applied light and electric field and (b) direct quenching by the influence of the electric field on the charge carrier densities, their spatial distributions, and radiative recombination rates. Direct quenching, observed under the abrupt application of negative voltage, causes a decrease of the PL intensity. However, the PL intensity then partially recovers within tens of milliseconds as mobile ions screen the internal electric field. The screening time increases to hundreds of seconds at low temperatures, indicating activation energies for ion motion of about 80 meV. On the other hand, ultrafast time-resolved PL measurements revealed two main phases of direct quenching: an instantaneous reduction in the radiative carrier recombination rate, which we attribute to the electron and hole displacement within individual perovskite grains, followed by a second phase lasting hundreds of picoseconds, which is due to the charge carrier extraction and spatial separation of electron and hole "clouds" within the entire perovskite layer thickness.

Direct Tracking of Charge Carrier Drift and Extraction from Perovskite Solar Cells by Means of Transient Electroabsorption Spectroscopy.

The best perovskite solar cells currently demonstrate more than 25% efficiencies, yet many fundamental processes that determine the operation of these devices are still not fully understood. In particular, even though the device performance strongly depends on charge carrier transport across the perovskite layer to selective electrodes, information about this process is still very controversial. Here, we investigate charge carrier motion and extraction from an archetypical CH3NH3PbI3 (MAPI) perovskite solar cell. We use the ultrafast electric-field-modulated transient absorption technique, which allows us to evaluate the electric field dynamics from the time-resolved electroabsorption spectra and to visualize the motion of charge carriers with subpicosecond time resolution. We demonstrate that photogenerated holes drift across the mesoporous TiO2/perovskite layer during hundreds of picoseconds. On the other hand, their extraction into the spiro-OMeTAD hole transporting layer lasts for more than 1 nanosecond, suggesting that the hole extraction is limited by the perovskite/spiro-OMeTAD interface rather than by the hole transport through the perovskite layer. Additionally, we use the ultrafast time-resolved fluorescence technique that reveals fluorescence decay during tens of picoseconds, which we attribute to the spatial separation of electrons and holes.

Tuning structural isomers of phenylenediammonium to afford efficient and stable perovskite solar cells and modules.

Organic halide salt passivation is considered to be an essential strategy to reduce defects in state-of-the-art perovskite solar cells (PSCs). This strategy, however, suffers from the inevitable formation of in-plane favored two-dimensional (2D) perovskite layers with impaired charge transport, especially under thermal conditions, impeding photovoltaic performance and device scale-up. To overcome this limitation, we studied the energy barrier of 2D perovskite formation from ortho-, meta- and para-isomers of (phenylene)di(ethylammonium) iodide (PDEAI2) that were designed for tailored defect passivation. Treatment with the most sterically hindered ortho-isomer not only prevents the formation of surficial 2D perovskite film, even at elevated temperatures, but also maximizes the passivation effect on both shallow- and deep-level defects. The ensuing PSCs achieve an efficiency of 23.9% with long-term operational stability (over 1000 h). Importantly, a record efficiency of 21.4% for the perovskite module with an active area of 26 cm2 was achieved.

Ultrafast Spectroscopic Analysis of Pressure-Induced Variations of Excited-State Energy and Intramolecular Proton Transfer in Semi-Aliphatic Polyimide Films.

The relationship between the photoexcitation dynamics and the structures of semi-aliphatic polyimides (3H-PIs) was investigated using ultrafast fluorescent emission spectroscopy at atmospheric and increased pressures of up to 4 GPa. The 3H-PI films exhibited prominent fluorescence with extremely large Stokes shifts (Δν > 10 000 cm-1) through an excited-state intramolecular proton transfer (ESIPT) induced by keto-enol tautomerism at the isolated dianhydride moiety. The incorporation of bulky -CH3 and -CF3 side groups at the diamine moiety of the PIs increased the quantum yields of the ESIPT fluorescence owing to an enhanced interchain free volume. In addition, 3H-PI films emitted another fluorescence at shorter wavelengths originating from closely packed polyimide (PI) chains (in aggregated forms), which was mediated through a Förster resonance energy transfer (FRET) from an isolated enol form into aggregated forms. The FRET process became more dominant than the ESIPT process at higher pressures owing to an enhancement of the FRET efficiency caused by the increased dipole-dipole interactions associated with a densification of the PI chain packing. The efficiency of the FRET rapidly increased by applying pressure up to 1 GPa owing to an effective compression of the interchain free volume and additionally gradually increased at higher pressures owing to structural and/or conformational changes in the main chains.

Doped but Stable: Spirobisacridine Hole Transporting Materials for Hysteresis-Free and Stable Perovskite Solar Cells.

Four spirobisacridine (SBA) hole-transporting materials were synthesized and employed in perovskite solar cells (PSCs). The molecules bear electronically inert alkyl chains of different length and bulkiness, attached to in-plane N atoms of nearly orthogonal spiro-connected acridines. Di-p-methoxyphenylamine (DMPA) substituents tailored to the central SBA-platform define electronic properties of the materials mimicking the structure of the benchmark 2,2',7,7'-tetrakis(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (spiro-MeOTAD), while the alkyl pending groups affect molecular packing in thin films and affect the long-term performance of PSCs. Devices with SBA-based hole transporting layers (HTL) attain efficiencies on par with spiro-MeOTAD. More importantly, solar cells with the new HTMs are hysteresis-free and demonstrate good operational stability, despite being doped as spiro-MeOTAD. The best performing MeSBA-DMPA retained 88% of the initial efficiency after a 1000 h aging test under constant illumination. The results clearly demonstrate that SBA-based compounds are potent candidates for a design of new HTMs for PSCs with improved longevity.

Electroluminescence Dynamics in Perovskite Solar Cells Reveals Giant Overshoot Effect.

High performance of both photovoltaic and electroluminescent devices requires low nonradiative recombination losses. In perovskites, such loses strongly depend on the carrier traps related to the mobile ions and vacancies, causing I- V hysteresis of solar cells and influencing the performance of other optoelectronic devices, such as photodetectors and LEDs. To address the dynamics of the mobile ions, here we investigate electroluminescence time evolution in perovskite solar cells under constant and pulsed voltage conditions. We propose a model, accounting for the spatial ion accumulation and explaining the complex electroluminescence dynamics both on fast (microseconds) and slow (seconds) time scales. We demonstrate the appearance of a high-intensity short electroluminescence peak (overshoot pulse) immediately after termination of the electrical pulse. The generation of a giant overshoot pulse suggests a simple way to achieve high pulsed luminescence intensity with a low current density, which opens new prospects toward optical gain and implementation of electrically pumped lasers.

Enhanced fluorescence of phthalimide compounds induced by the incorporation of electron-donating alicyclic amino groups.

Due to their high thermal and environmental stability, polyimides (PIs) are one of the most attractive candidates for novel highly fluorescent polymers, though photophysical studies of PIs are challenging owing to their poor solubility in common solvents. To overcome these problems, we have synthesized and examined a series of low molecular weight model imide compounds: substituted N-cyclohexylphthalimides with alicyclic amino groups at the 3 or 4-positions of the benzene rings (x-NHPIs). Their photophysical properties were systematically investigated by steady-state UV/Visible absorption, fluorescence, and time-resolved fluorescence techniques. In solution, unsubstituted N-cyclohexylphthalimide (NHPI) showed almost no emission, while x-NHPIs exhibited enhanced fluorescence emission depending on the solvent polarity. Analysis of the solvatochromism of the x-NHPIs via Lippert-Mataga plots indicated the generation of large dipole moments in the excited singlet states originating from the intramolecular charge-transfer (ICT) states. The significant difference in the fluorescence quantum yields (Φ) between the 3-substituted (3Pi and 3Pyr) and 4-substituted NHPIs (4Pi and 4Pyr) strongly suggests that the former form a twisted ICT (TICT) state, whereas the latter form a planar ICT (PICT) state when excited. 4-Substituted NHPIs also show high fluorescence yields in the crystalline state. A particularly large Φ value was obtained for the 4Pi crystal, which we explain by the large intermolecular distances and the arrangement of molecules minimizing intermolecular interactions as well as the small non-radiative deactivation rate. These facts clearly demonstrate that the introduction of an alicyclic amino group into NHPI at the 4-position enhances the fluorescence quantum yields significantly, which suggests a new pathway for the development of novel, highly fluorescent PIs.

Twisted Intramolecular Charge Transfer States in Trinary Star-Shaped Triphenylamine-Based Compounds.

Excited state dynamics of trinary star-shaped dendritic compounds with triphenylamine arms and different cores were studied by means of time-resolved fluorescence and transient absorption. Under optical excitation, nonpolar C3 symmetry molecules form polar excited states localized on one of the molecular substituents. Conformational excited state stabilization of molecules with an electron-accepting core causes a formation of twisted internal charge transfer (TICT) states in polar solvents. A low transition dipole moment from TICT state to the ground state causes very weak fluorescence of those compounds and strong dependence on the solvent polarity. The compound formed from the triphenylamine central core and identical arms also experiences excited state twisting, however, weakly sensitive to the solvent polarity.

Hindered Amine Light Stabilizers Increase the Stability of Methylammonium Lead Iodide Perovskite Against Light and Oxygen.

Methylammonium lead iodide perovskite (MAPI) is a promising material for highly efficient photovoltaic devices. However, it suffers from photooxidation, which imposes strict requirements for its protection from oxygen during processing and operation. A hindered amine light stabilizer (HALS) has been found to exert a stabilization effect on methylammonium iodide (MAI) and MAPI against photooxidation. The HALS prevents the degradation of MAI by inhibiting the oxidation of iodide to iodine. Chemical modification of HALS allows its incorporation in MAPI films, which extends the resistivity of MAPI against photodegradation in ambient air from a couple of hours to several days, while causing no significant changes in key properties, such as optical absorption and charge transport. These results represent an important advance in the stabilization of MAPI against decomposition and demonstrate for the first time that antioxidants improve the stability of MAPI.

Influence of Blend Ratio and Processing Additive on Free Carrier Yield and Mobility in PTB7:PC71BM Photovoltaic Solar Cells.

Charge separation and extraction dynamics were investigated in high-performance bulk heterojunction solar cells made from the polymer PTB7 and the soluble fullerene PC71BM on a broad time scale from subpicosecond to microseconds using ultrafast optical probing of carrier drift and the integral-mode photocurrent measurements. We show that the short circuit current is determined by the separation of charge pairs into free carriers, which is strongly influenced by blend composition. This separation is found to be efficient in fullerene-rich blends where a high electron mobility of >0.1 cm2 V-1 s-1 is observed in the first 10 ps after excitation. Morphology optimization using the solvent additive 1,8-diiodooctane (DIO) doubles the charge pair separation efficiency and the short-circuit current. Carrier extraction at low internal electric field is slightly faster from the cells prepared with DIO, which can reduce recombination losses and enhance a fill factor.

Background-free electric field-induced second harmonic generation with interdigitated combs of electrodes.

The electric field-induced second harmonic (EFISH) generation is a powerful tool for the investigation of optical nonlinearities, material polarization, internal electric fields, and other properties of photonic materials and devices. A conventional generation of the second harmonics (SH) in materials with the disturbed centrosymmetry causes a field-independent background to EFISH and limits its applications. Here we suggest and analyze the application of the interdigitated combs of electrodes for EFISH generation in thin films. Interdigitated electrodes form an optical transmission amplitude diffraction grating. Phase matching of the EFISH radiation creates unusual diffraction fringes with the zero intensity along the zeroth order direction and with the diffraction angles different from diffraction angles of incident fundamental laser radiation and its second harmonics. It enables a simple geometrical separation of the EFISH signal from a conventional SH background, simplifies the sample preparation, and provides additional experimental possibilities. We demonstrate applicability of the suggested technique for characterization of submicrometer thickness organic films of transparent and resonantly interacting polymers and of their mixtures.

Dissociation of Charge Transfer States and Carrier Separation in Bilayer Organic Solar Cells: A Time-Resolved Electroabsorption Spectroscopy Study.

Ultrafast optical probing of the electric field by means of Stark effect in planar heterojunction cyanine dye/fullerene organic solar cells enables one to directly monitor the dynamics of free electron formation during the dissociation of interfacial charge transfer (CT) states. Motions of electrons and holes is scrutinized separately by selectively probing the Stark shift dynamics at selected wavelengths. It is shown that only charge pairs with an effective electron-hole separation distance of less than 4 nm are created during the dissociation of Frenkel excitons. Dissociation of the coulombically bound charge pairs is identified as the major rate-limiting step for charge carriers' generation. Interfacial CT states split into free charges on the time-scale of tens to hundreds of picoseconds, mainly by electron escape from the Coulomb potential over a barrier that is lowered by the electric field. The motion of holes in the small molecule donor material during the charge separation time is found to be insignificant.

Charge carrier generation and transport in different stoichiometry APFO3:PC61BM solar cells.

In this paper we studied carrier drift dynamics in APFO3:PC61BM solar cells of varied stoichiometry (2:1, 1:1, and 1:4 APFO3:PC61BM) over a wide time range, from subpicoseconds to microseconds with a combination of ultrafast optical electric field probing and conventional transient integrated photocurrent techniques. Carrier drift and extraction dynamics are strongly stoichiometry dependent: the speed of electron or hole drift increases with higher concentration of PC61BM or polymer, respectively. The electron extraction from a sample with 80% PC61BM takes place during hundreds of picoseconds, but slows down to sub-microseconds in a sample with 33% PC61BM. The hole extraction is less stoichiometry dependent: it varies form sub-nanoseconds to tens of nanoseconds when the PC61BM concentration changes from 33% to 80%. The electron extraction rate correlates with the conversion efficiency of solar cells, leading to the conclusion that fast electron motion is essential for efficient charge carrier separation preventing their geminate recombination.

Carrier motion in as-spun and annealed P3HT:PCBM blends revealed by ultrafast optical electric field probing and Monte Carlo simulations.

Charge transport dynamics in solar cell devices based on as-spun and annealed P3HT:PCBM films are compared using ultrafast time-resolved optical probing of the electric field by means of field-induced second harmonic generation. The results show that charge carriers drift about twice as far during the first 3 ns after photogeneration in a device where the active layer has been thermally annealed. The carrier dynamics were modelled using Monte-Carlo simulations and good agreement between experimental and simulated drift dynamics was obtained using identical model parameters for both cells, but with different average PCBM and polymer domain sizes. The calculations suggest that small domain sizes in as-spun samples limit the carrier separation distance disabling their escape from geminate recombination.

Poling induced mass transport in thin polymer films.

In this study we report investigation of the polymer film morphology modifications during their corona poling for fabrication of nonlinear optically (NLO) active materials. We demonstrate that at certain poling conditions surface and spatial inhomogeneities in the poled area of the sample appear. Densities of the inhomogeinities depend on the strength of the poling field, the sample temperature during the poling, and the prepoling conditions. Optimization of the poling conditions directed toward avoiding surface modifications enables us to increase the overall observable effective nonlinearity of the sample up to 10 times. To investigate, understand, and eventually explain the formation of the spatial and surface structure inhomogeinites in the poled material we have used optical, second harmonic, and scanning electron microscope measurements, as well as the conductivity measurements of the thin films. We present results of poled polymer host-guest films where (dimethylamino)benzylidene-1,3-indandione and low dipole moment 2,2',2″-(4,4',4″-nitrilotribenzylidene)triindan-1,3-dione were used as guests in poly(methyl methacrylate), polystyrol, and polysulfone matrixes doped at 10 wt %.

Relaxation pathways of excited N-(triphenylmethyl)salicylidenimine in solutions.

Excited state relaxation of N-(triphenylmethyl)-salicylidenimine (MS1) in protic and aprotic solvents has been investigated by means of absorption pump-probe spectroscopy with femtosecond time resolution and fluorescence spectroscopy with picosecond time resolution. Short-lived excited states and long-lived photoproducts have been identified from the differential absorption spectra. Excited states and photoproducts were different under excitation of enol-closed and cis-keto tautomers. As a result, the commonly accepted excited state relaxation model of aromatic anils, which assumes an ultrafast transformation of excited enol-closed tautomers into cis-keto tautomers, has been modified. Performed quantum chemical calculations suggest that hydrogen-bonded ethanol molecules facilitate formation of cis-keto tautomers and are responsible for their different relaxation pathways in comparison with relaxation of excited enol-closed tauromers. Fluorescence decay on a nanosecond time scale was attributed to aggregated MS1 molecules.

Exciton annihilation and energy transfer in self-assembled peptide-porphyrin complexes depends on peptide secondary structure.

We used picosecond transient absorption and fluorescence lifetime spectroscopy to study singlet exciton annihilation and depolarization in self-assembled aggregates of meso-tetra(4-sulfonatophenyl)porphine (TPPS(4)) and a synthetic 22-residue polypeptide. The polypeptide was designed and previously shown to bind three TPPS(4) monomers via electrostatic interactions between the sulfonate groups and cationic lysine residues. Additionally, the peptide induces formation of TPPS(4) J-aggregates in acidic solutions when the peptide secondary structure is disordered. In neutral solutions, the peptide adopts an α-helical secondary structure that can bind TPPS(4) with high affinity but J-aggregate formation is inhibited. Detailed analysis of excitation-power dependent transient absorption kinetics was used to obtain rate constants describing the energy transfer between TPPS(4) molecules in an aggregate under acidic and neutral conditions. Independently, such analysis was confirmed by picosecond fluorescence emission depolarization measurements. We find that energy transfer between TPPS(4) monomers in a peptide-TPPS(4) complex is more than 30 times faster in acidic aqueous solution than in neutral solutions (9 vs 279 ps). This result was attributed to a conformational change of the peptide backbone from disordered at low pH to α-helical at neutral pH and suggests a new approach to control intermolecular energy transfer with possible applications in fluorescent sensors or biomimetic light harvesting antennas.

Spectral dependence of energy transfer in wild-type peripheral light-harvesting complexes of photosynthetic bacteria.

The precise position of the upper exciton component and relevant vibronic transitions of the B850 ring in peripheral light-harvesting complexes from purple photosynthetic bacteria are important values for determining the exciton bandwidth and electronic structure of the B850 ring. To determine the presence of these components in wild-type LH2 complexes the pump-probe femtosecond transient spectra obtained with excitation into the 730-840 nm spectral range are analyzed. We show that at excitation wavelengths less than 780 nm B850 absorption bands are present and that, in accordance with exciton theory, these bands peak further in the blue when the lowest optically allowed transition is more red-shifted.

Bifurcation suppression for stability improvement in Nd:YVO4 regenerative amplifier.

Continuously pumped regenerative amplifiers are subject to energy instability at high pulse repetition rates due to period doubling bifurcation. Theoretical and experimental data are presented, in order to differentiate and understand instability effects in Nd:YVO4 regenerative amplifier, and possible techniques for performance optimization are analyzed. An increase in the seed pulse energy is demonstrated to improve amplification dynamics. Addition of a preamplifier is shown as an efficient way to achieve seed energy high enough to provide stable operation at repetition rates up to 200 kHz with average output power near the theoretical limit.