Photophysical and Electrochemical Characterization of BODIPY-Containing Dyads Comparing the Influence of an A–D–A versus D–A Motif on Excited-State Photophysics

A complete photophysical characterization of organic molecules designed for use as molecular materials is critical in the design and construction of devices such as organic photovoltaics (OPV). The nature of a molecule’s excited state will be altered in molecules employing the same chromophoric units but possessing different molecular architectures. For this reason, we examine the photophysical reactions of two BODIPY-based D–A and A–D–A molecules, where D is the donor and A is the acceptor. A BODIPY (4,4′-difluoro-4-bora-3a,4a-diaza-<i>s</i>-indacene) moiety serves as the A component and is connected through the <i>meso</i> position using a 3-hexylthiophene linker to a <i>N</i>-(2-ethylhexyl)­dithieno­[3,2-<i>b</i>:2′,3′-<i>d</i>]­pyrrole (DTP), which serves as the D component. An A–D–A motif is compared to its corresponding D–A dyad counterpart. We show a potential advantage to the A–D–A motif over the D–A motif in creating longer-lived excited states. Transient absorption (TA) spectroscopy is used to characterize the photophysical evolution of each molecule’s excited state. Global analysis of TA data using singular value decomposition and target analysis is performed to identify decay-associated difference spectra (DADS). The DADS reveal the spectral features associated with charge-transfer excited states that evolve with different dynamics. A–D–A possess slightly longer excited-state lifetimes, 42 ps nonradiative decay, and 4.64 ns radiative decay compared to those of D–A, 24 ps nonradiative decay, and 3.95 ns radiative decay. A longer lived A–D–A component is observed with microsecond lifetimes, representing a small fraction of the total photophyscial product. Steady-state and time-resolved photoluminescence augment the insights from TA, while electrochemistry and spectroelectrochemistry are employed to identify the nature of the excited state. Density functional theory supports the observed electronic and electrochemical properties of the D–A and A–D–A molecules. These results form a complete picture of the electronic and photophysical properties of D–A and A–D–A and provide contextualization for structure–function relationships between molecules and OPV devices

Improved Performances in Polymer BHJ Solar Cells Through Frontier Orbital Tuning of Small Molecule Additives in Ternary Blends

Polymer solar cells fabricated in air under ambient conditions are of significant current interest, because of the implications in practicality of such devices. However, only moderate performance has been obtained for the air-processed devices. Here, we report that enhanced short circuit current density (<i>J</i>_SC) and open circuit voltage (<i>V</i>_OC) in air-processed poly­(3-hexylthiophene) (P3HT)-based solar cells can be obtained by using a series of donor–acceptor dyes as the third component in the device. Power conversion efficiencies up to 4.6% were obtained upon addition of the dyes which are comparable to high-performance P3HT solar cells fabricated in controlled environments. Multilayer planar solar cells containing interlayers of the donor–acceptor dyes, revealed that along with infrared sensitization, an energy level cascade architecture and Förster resonance energy transfer could contribute to the enhanced performance

Crystallinity and Morphology Effects on a Solvent-Processed Solar Cell Using a Triarylamine-Substituted Squaraine

2,4-Bis­[4′-(<i><i>N,N</i></i>-di­(4″-hydroxyphenyl)­amino)-2′,6′-dihydroxyphenyl]­squaraine (Sq-TAA-OH, optical bandgap 1.4 eV, HOMO level −5.3 eV by ultraviolet photoelectron spectroscopy) is used as an active layer material in solution processed, bulk-heterojunction organic photovoltaic cells with configuration ITO/PEDOT:PSS/Sq-TAA-OH:PC₇₁BM/LiF/Al. Power conversion efficiencies (PCEs) up to 4.8% are obtained by a well-reproducible procedure using a mixture of good and poor Sq-TAA-OH solubilizing organic solvents, with diiodooctane (DIO) additive to make a bulk heterojunction layer, followed by thermal annealing, to give optimized <i>V</i>_OC = 0.84–0.86 V, <i>J</i>_SC = 10 mA cm^–2, and FF = 0.53. X-ray diffraction and scattering studies of pristine, pure Sq-TAA-OH solution-cast films show <i>d</i>-spacing features similar to single-crystal packing and spacing. The DIO additive in a good solvent/poor solvent mixture apparently broadens the size distribution of Sq-TAA-OH crystallites in pristine films, but thermal annealing provides a narrower size distribution. Direct X-ray diffraction and scattering morphological studies of “as-fabricated” active layers show improved Sq-TAA-OH/PC₇₁BM phase separation and formation of crystallites, ∼48 nm in size, under conditions that give the best PCE