Molecular Engineering Strategies for the Design and Synthesis of New Organic Photovoltaic Materials

Dramatic improvements in organic photovoltaic device efficiency can be obtained by optimizing spectral absorbance and frontier molecular orbital (FMO) energies, increasing solid state exciton/charge mobility, and utilizing p-/n-type nanoarchitecture. Combining all of these properties into a new material presents a considerable synthetic challenge because potential commercial applications require materials that are high-performance and inexpensive. Thus, it is advantageous to design new materials using a versatile, modular synthetic approach that allows each design criterion to be engineered individually, in a synthetically efficient manner. Several strategies were successfully pursued using simple interchangeable electron donor and acceptor components as functional modules, which provided various donor-acceptor chromophores in a synthetically straightforward manner. This approach provided broad functional tunability to the range of materials produced and, as a result, various molecular engineering requirements were systematically addressed. In some cases, these materials were utilized in photovoltaic devices as p-type active layers or redox enhancement additives. In these cases, competitive power conversion efficiencies were obtained or test device performance was considerably enhanced by comparison to control devices. Fluorenone, fluorenylidene-malononitrile, and squaric acid were utilized as electron acceptor modules, and electron donor module strength was varied using a styrene-based and several di- and triarylamine-based components. One strategy, published in Phys. Chem. Chem. Phys., (Chapter 2, DOI-10.1039/C2CP41813D) is to fix the donor-acceptor lowest unoccupied molecular orbital energy using the synthetically versatile fluorenone module. Fluorenone was chosen because of its ready availability and synthetic versatility, and its multiple functionalization sites allow for selective FMO engineering. Extrapolation of this approach was published in J. Phys. Chem. A. (Chapter 3, DOI-10.1021/jp407854r), describing various fluorenylidene-malononitrile derivatives. Chemical oxidation of fluorenone-based triarylamines to produce stable radicals was published in Tetrahedron Letters (Chapter 4, DOI-10.1016/j.tetlet.2012.10.060). Fluorenone derivatives applied as dye sensitized solar cell redox system enhancement additives was described in RSC Advances (Chapter 5, DOI-10.1039/C3RA40986D). Development of new, functionalized, squaraine-based materials was described in J. Phys. Chem. C. (Chapter 6, DOI-10.1021/jp410362d) and was extrapolated for use in single-heterojunction photovoltaic cells having 4.8% maximum power conversion efficiency. The fundamental insights provided by these findings will be valuable for developing new high-performance photovoltaic materials in the future

Towards the continuous-flow polymer-supported dioxolanation of 4-chlorobenzaldehyde using a vicinal diol-functionalized macroporous monolithic polymer column

This thesis documents the design and synthesis of a grafted macroporous monolithic polymer support for the continuous-flow solid-phase separation of solution-phase aldehydes. Poly(styrene-co-divinylbenzene) (PS-DVB) columns were synthesized within HPLC guard column inserts and grafted with poly(glycidyl methacrylate-co-ethylene dimethacrylate) (PGMA-EDMA). Highly-crosslinked PS-DVB columns (1:1 styrene: DVB) were synthesized with 65 % internal pore volume, occupied by a lightly-crosslinked PGMA-EDMA gel-type graft copolymer (1 wt % EDMA with respect to GMA) leading to an epoxide-functionalized bulk polymer. Polymer-supported epoxide moieties were hydrolyzed via Bronsted acid catalysis, yielding polymer-supported vicinal diol functionalities. Aldehyde immobilization via cyclic acetal formation was attempted under various reaction conditions to no avail

Solution-Processed Photovoltaics with a 3,6-Bis(diarylamino)fluoren-9-ylidene Malononitrile

3,6-Bis­(<i>N,N</i>-dianisylamino)-fluoren-9-ylidene malononitrile (FMBDAA36) was used as an electron donor material in solution-processed organic photovoltaic devices with configuration ITO/PEDOT:PSS/(1:3­[w/w] FMBDAA36:PC₇₁BM)/LiF/Al to give power conversion efficiencies up to 4.1% with open circuit voltage <i>V</i>_OC = 0.89 V, short circuit current <i>J</i>_SC = 10.35 mA cm^–2, and fill factor FF = 44.8%. Conductive atomic force microscopy of the active layer showed granular separation of regions exhibiting easy versus difficult hole transport, consistent with bulk heterojunction type phase separation of FMBDAA36 and PC₇₁BM, respectively. Single-crystal X-ray diffraction analysis showed pure FMBDAA36 to form columnar π-stacks with a 3.3 Å intermolecular spacing

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