Dynamics of Intramolecular Energy Hopping in Multi-Bodipy Self-Assembled Metallocyclic Species: A Tool for Probing Subtle Structural Distortions in Solution

The intramolecular excitation energy transfer (EET) processes in a series of fluorescent-unquenched, self-assembled metallocycles consisting of spatially fixed-separated and parallel-aligned Bodipy chromophores, are investigated here by steady-state and femtosecond-fluorescence upconversion measurements in the solution phase. These multi-Bodipy macrocycles, namely, the rhomboid (<b>A1</b>), the tetragon (<b>A2</b>) and the hexagon (<b>A3</b>), are formed via temperature-regulated Pt­(II)–pyridyl coordination and consist, respectively, of two, four, and six Bodipy subunits, which are locked at the corners and aligned with their long molecular axes perpendicular to the rigid polygonal frame formed by the alternating B···Pt­(II) connectivities. Extensive simulations and fits to the experimental fluorescence anisotropy decays <i>r</i>(<i>t</i>) show that EET within the cyclic scaffolds is quite <i>uniform</i> and much <i>faster</i> than the intrinsic decay rate of the Bodipy’s. The equalization of the excitation survival probabilities over time of all chromophores is found to be dependent upon the size of the macrocycle. From the observed dynamics supported by geometry optimization calculations, it is concluded that, in contrast to the model compound <b>A1</b>, in the large macrocycles the perfect parallel orientation of the Bodipy dipoles is lifted through limited out-of-plane distortions of the metallocyclic framework from a planar conformation. Additionally, we show that, as opposed to analogous covalent macrocycles, the survival probability of excitons as well as the degree of symmetry distortion and homogeneity in dipole spacing remains nearly intact as the size of the macrocycle increases from tetragon to hexagon

Excited State and Injection Dynamics of Triphenylamine Sensitizers Containing a Benzothiazole Electron-Accepting Group on TiO₂ and Al₂O₃ Thin Films

The excited state and electron injection dynamics of three new organic sensitizers, comprising a triphenylamine moiety connected by an ethenylene (C−C double-bond) or ethynylene (C−C triple-bond) π-spacer to an electron-withdrawing benzothiazole bearing a cyanoacrylic acid anchoring group, have been studied using a combination of steady-state and femtosecond-resolved spectroscopies. The measurements were carried out for the three dyes in predominantly neutral and completely deprotonated forms in liquid solutions and bound on nanocrystalline TiO₂ and Al₂O₃ thin films. In addition, quantum-chemical calculations were performed to predict absorption spectra of the sensitizers and their corresponding cation radicals. Time-resolved fluorescence (TRF) measurements on TiO₂ indicate that electron injection takes place on a <0.2 ps time scale. Transient electronic absorption (TA) measurements provide evidence for the formation of radical cations not only in dye-sensitized TiO₂ films but also in Al₂O₃ ones. The cation lifetime in Al₂O₃ is significantly shorter compared to TiO₂, indicating a faster recombination of injected electrons with the dye cations. In addition, the ground-state bleach band in dye-sensitized TiO₂ films experiences a gradual red-shift, which is indicative of a transient Stark effect. Finally, femtosecond transient absorption measurements in the IR region point to an ultrafast generation of injected electrons for all dyes. A faster recombination of the injected electrons with the dye cations is observed for the sensitizer decorated with auxiliary electron-donating methoxy groups on the triphenylamine moiety

Surface Modification of ZnO Layers via Hydrogen Plasma Treatment for Efficient Inverted Polymer Solar Cells

Modifications of the ZnO electron extraction layer with low-pressure H plasma treatment increased the efficiency of inverted polymer solar cells (PSCs) based on four different photoactive blends, namely, poly­(3-hexylthiophene):[6,6]-phenyl C₇₁ butyric acid methyl ester (P3HT:PC₇₁BM), P3HT:1′,1″,4′,4″-tetrahydro-di­[1,4]­methano­naphthaleno-[5,6]­ullerene-C₆₀ (P3HT:IC₆₀BA), poly­[(9-(1-octylnonyl)-9H-carbazole-2,7-diyl)-2,5-thio­phenediyl-2,1,3-benzo­thiadiazole-4,7-diyl-2,5-thio­phenediyl]:PC₇₁BM (PCDTBT:PC₇₁BM), and (poly­[[4,8-bis­[(2-ethyl­hexyl)­oxy]­benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithio­phene-2,6-diyl]­[3-fluoro-2-(2-ethyl­hexy)­carbonyl]­thieno­[3,4-<i>b</i>]­thio­phenediyl]]):PC₇₁BM (PTB7:PC₇₁BM), irrespective of the donor:acceptor combination in the photoactive blend. The drastic improvement in device efficiency is dominantly attributable to the reduction in the work function of ZnO followed by a decreased energy barrier for electron extraction from fullerene acceptor. In addition, reduced recombination losses and improved nanomorphology of the photoactive blend in the devices with the H plasma treated ZnO layer were observed, whereas exciton dissociation also improved with hydrogen treatment. As a result, the inverted PSC consisting of the P3HT:PC₇₁BM blend exhibited a high power conversion efficiency (PCE) of 4.4%, the one consisting of the P3HT:IC₆₀BA blend exhibited a PCE of 6.6%, and our champion devices with the PCDTBT:PC₇₁BM and PTB7:PC₇₁BM blends reached high PCEs of 7.4 and 8.0%, respectively

Triazine-Substituted Zinc Porphyrin as an Electron Transport Interfacial Material for Efficiency Enhancement and Degradation Retardation in Planar Perovskite Solar Cells

Motivated by the excellent electron-transfer capability of porphyrin molecules in natural photosynthesis, we introduce here the first application of a porphyrin compound to improve the performance of planar perovskite solar cells. The insertion of a thin layer consisting of a triazine-substituted Zn porphyrin between the TiO₂ electron transport layer and the CH₃NH₃PbI₃ perovskite film significantly augmented electron transfer toward TiO₂ while also sufficiently improved the morphology of the perovskite film. The devices employing porphyrin-modified TiO₂ exhibited a significant increase in the short-circuit current densities and a small increase in the fill factor. As a result, they delivered a maximum power conversion efficiency (PCE) of 16.87% (average 14.33%), which represents a 12% enhancement compared to 15.01% (average 12.53%) of the reference cell. Moreover, the porphyrin-modified cells exhibited improved hysteretic behavior and a higher stabilized power output of 14.40% compared to 10.70% of the reference devices. Importantly, nonencapsulated perovskite solar cells embedding a thin porphyrin interlayer showed an elongated lifetime retaining 86% of the initial PCE after 200 h, while the reference devices exhibited higher efficiency loss due to faster decomposition of CH₃NH₃PbI₃ to PbI₂