Dynamics of Charge Distribution in Sandwich-Type Light-Emitting Electrochemical Cells Probed by the Stark Effect

A sandwich-type structure composed of thin films of functional materials is the dominant architecture of organic optoelectronic devices. During device operation, injected or inside-generated electrical charges arrange in a specific spatial distribution. This distribution affects the electric field profile throughout the layers and has a fundamental impact on charge transport and recombination/separation processes. However, probing of the charge distribution is challenging because the hidden active layers are experimentally not directly accessible. Here, we study the temporal evolution of organic salt-based light-emitting electrochemical cells and demonstrate that electroabsorption spectroscopy in combination with electrical capacitance measurements enables the determination of the distribution of the injected, uncompensated electronic charge inside these devices. For constant-voltage operating conditions and over a period of hours, the Stark effect signal intensity and the capacitance increase steadily, but to a different extent. We demonstrate that this difference sensitively depends on the position and distribution width of injected mobile electrons. Estimates show a substantial spreading over the active layer of the injected electron density with time, screening the electric field behind the charge peak

Charge Carrier Generation and Extraction in Hybrid Polymer/Quantum Dot Solar Cells

Here we investigate charge carrier generation and extraction processes in hybrid polymer/nanocrystal solar cells by means of time-resolved optical and photoelectrical techniques. We addressed the role of both poly­(3-hexylthiophene) and colloidal arenethiolate-capped PbS quantum dots, which constitute the hybrid composite nanomaterial, in the photoinduced processes most relevant to device operation by changing the compositional ratio and applying chemical and thermal postdeposition treatments. The carrier generation processes were found to be wavelength-dependent: excitons generated in the polymer domains led to long-lived weakly bound charge pairs upon electron transfer to PbS nanocrystals; whereas charge carrier generation in the nanocrystal domains is highly efficient, although effective separation required the application of external electric field. Overall, charge carrier generation was found efficient and almost independent of the strength of applied electric field; therefore, competition between separation of electron–hole pairs into free carriers and geminate recombination is the major factor limiting the fill factor of nanocomposite-based solar cells. Device efficiency improvements thus require faster interfacial charge transfer processes, which are deeply related to the refinement of nanocrystal surface chemistry

Polyimide and Imide Compound Exhibiting Bright Red Fluorescence with Very Large Stokes Shifts via Excited-State Intramolecular Proton Transfer II. Ultrafast Proton Transfer Dynamics in the Excited State

A novel polyimide (PI) and imide compound emitting prominent reddish-orange fluorescence under excitation by UV light were prepared based on 3-hydroxy­pyromellitic dianhydride (PHDA), and their fluorescence properties were examined. The steady-state fluorescence spectrum of a PI film displayed an emission band at 590 nm with a very large Stokes shift (ν = 10 448 cm^–1) via the excited-state intramolecular proton transfer (ESIPT), while the time-resolved fluorescence spectrum showed a rapid decay of the emission band of the enol form at around 400 nm within 15 ps. Transient absorption measurements showed an induced absorption and stimulated emission of the keto form with a time constant of ca. 3.0 ps, implying that ESIPT occurs on this time scale. Consequently, introduction of a hydroxy group into the pyromellitic moiety of PIs and imide compounds led to the long-wavelength ESIPT emission applicable to spectral converters having high thermal, mechanical, and environmental stabilities

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 <i>C</i>₃ 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