Molecular Design, Characterization, and Implementation of Organic Semiconducting Oligothiophenes

This work describes new self-assembly strategies and realizes new directions for rational side chain design in organic semiconductors. I examined the synthesis and structure property relations of monomers and dimers of a benchmark organic semiconductor system using a variety of linear alkyl side chains. I observed critical onsets for packing trends based on alkyl side chain length in both monomers and dimers. Monomer systems exhibited a pronounced even-odd effect manifesting directly from side chain length. In the dimer systems, I observed spontaneous bimolecular crystal formation with PC61BM which undergoes an order-to-disorder transition at small side chain lengths. Combining these systematic structure-property studies, I designed and synthesized a set of BTTT dimers with variable substitution of long and short side chains. These systems displayed solid-state packing behaviors unique to their side chain pattern, both in neat films and in blends with fullerene derivatives. As these oligomers provide excellent model systems for their polymer analogues, the translation of the structure-property lessons here will benefit small molecule systems and polymer systems alike. More broadly, this work identifies the utility and application of variable substitution for imparting order and ‘smart’ self-assembly in organic semicondutors or other organic crystalline materials

Impact of morphology on polaron delocalization in a semicrystalline conjugated polymer

We investigate the delocalization of holes in the semicrystalline conjugated polymer poly(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene) (PBTTT) by directly measuring the hyperfine coupling between photogenerated polarons and bound nuclear spins using electron nuclear double resonance spectroscopy. An extrapolation of the corresponding oligomer spectra reveals that charges tend to delocalize over 4.0–4.8 nm with delocalization strongly dependent on molecular order and crystallinity of the PBTTT polymer thin films. Density functional theory calculations of hyperfine couplings confirm that long-range corrected functionals appropriately describe the change in coupling strength with increasing oligomer size and agree well with the experimentally measured polymer limit. Our discussion presents general guidelines illustrating the various pitfalls and opportunities when deducing polaron localization lengths from hyperfine coupling spectra of conjugated polymers

Effect of alkyl chain length on the properties of triphenylamine-based hole transport materials and their performance in perovskite solar cells

A new series of diacetylide-triphenylamine (DATPA) derivatives with five different alkyl chains in the para position, MeO, EtO, nPrO, iPrO and BuO, were synthesised, fully characterised and their function as hole-transport materials in perovskite solar cells (PSC) studied. Their thermal, optical and electrochemical properties were investigated along with their molecular packing and charge transport properties to analyse the influence of different alkyl chains in the solar cell parameters. The shorter alkyl chain facilitates more compact packing structures which enhanced the hole mobilities and reduced recombination. This work suggests that the molecule with the methoxy substituent (MeO) exhibits the best semiconductive properties with a power conversion efficiency of up to 5.63%, an open circuit voltage (Voc) of 0.83 V, a photocurrent density (Jsc) of 10.84 mA cm−2 and a fill factor of 62.3% in perovskite solar cells. Upon replacing the methoxy group with longer alkyl chain substituents without changing the energy levels, there is a decrease in the charge mobility as well as PCE (e.g. 3.29% for BuO-DATPA). The alkyl chain length of semiconductive molecules plays an important role in achieving high performance perovskite solar cells

Wingspan

As a young boy, Ken Boyd \u2761 fell in love with flying. Now retired, he\u27s returned to the skies in his own hand-crafted plane

Rubrene single crystal solar cells and the effect of crystallinity on interfacial recombination

Single crystal studies provide a better understanding of the basic properties of organic photovoltaic devices. Therefore, in this work, rubrene single crystals with a thickness of 250 nm to 1000 nm were used to produce an inverted bilayer organic solar cell. Subsequently, polycrystalline rubrene (orthorhombic, triclinic) and amorphous bilayer solar cells of the same thickness as single crystals were studied to make comparisons across platforms. To investigate how single crystal, polycrystalline (triclinic-orthorhombic) and amorphous forms alter the charge carrier recombination mechanism at the rubrene/PCBM interface, light intensity measurements were carried out. The light intensity dependency of the J(SC), V-OC and FF parameters in organic solar cells with different forms of rubrene was determined. Monomolecular (Shockley Read Hall) recombination is observed in devices employing amorphous and polycrystalline rubrene in addition to bimolecular recombination, whereas the single crystal device is weakly affected by trap assisted SRH recombination due to reduced trap states at the donor acceptor interface. To date, the proposed work is the only systematic study examining transport and interface recombination mechanisms in organic solar cells produced by different structure forms of rubrene.D. A. K. acknowledges the Turkey research fellowship (TUBITAK 2214/A-1059B141501315). K. K. thanks the Turkey Scholarship Council (2214/A-1059B141501316). B. G acknowledges the 1003 -Primary Subjects R;D Funding Program (218M940) and A. L. B. acknowledges the Office of Naval Research (N0001416-1-2612 and N000147-14-1-0053).Office of Naval Research [N0001416-1-2612, N000147-14-1-0053]; Turkey research fellowship [TUBITAK 2214/A-1059B141501315]; Turkey Scholarship Council [2214/A-1059B141501316]; 1003 -Primary Subjects R;D Funding Program [218M940

Correlating Crystal Thickness, Surface Morphology, and Charge Transport in Pristine and Doped Rubrene Single Crystals

The relationship between charge transport and surface morphology is investigated by utilizing rubrene single crystals of varying thicknesses. In the case of pristine crystals, the surface conductivities decrease exponentially as the crystal thickness increases until ∼4 μm, beyond which the surface conductivity saturates. Investigation of the surface morphology using optical and atomic force microscopy reveals that thicker crystals have a higher number of molecular steps, increasing the overall surface roughness compared with thin crystals. The density of molecular steps as a surface trap is further quantified with the subthreshold slope of rubrene air-gap transistors. This thickness-dependent surface conductivity is rationalized by a shift from in-plane to out-of-plane transport governed by surface roughness. The surface transport is disrupted by roughening of the crystal surface and becomes limited by the slower vertical crystallographic axis on molecular step edges. Separately, we investigate surface-doping of rubrene crystals by using fluoroalkyltrichrolosilane and observe a different mechanism for charge transport which is independent of surface roughness. This work demonstrates that the correlation between crystal thickness, surface morphology, and charge transport must be taken into account when measuring organic single crystals. Considering the fact that these molecular steps are universally observed on organic/inorganic and single/polycrystals, we believe that our findings can be widely applied to improve charge transport understanding

Breaking the Bimolecular Crystal: The Effect of Side-Chain Length on Oligothiophene/Fullerene Intercalation

Polymer/fullerene bimolecular crystal formation has been investigated using a variety of conjugated polymers and fullerenes to understand the design rules that influence donor–acceptor interaction. Modifications of the polymer by varying the substitution side-chain position, density, and branching have demonstrated the importance of the “pocket” dimensions (free volume between side chains where the fullerene resides) for controlling intercalation. Yet the effect of pocket height has not been systematically explored because of the solubility limitations in polymers. In this report, we present an experimental investigation into the effect of the pocket height by synthesizing poly­[2,5-bis­(3-<i>alkyl</i>thiophen-2-yl)­thieno­[3,2-<i>b</i>]­thiophene] dimers with varied side chain lengths and track the morphological changes of the dimer/fullerene blends using grazing-incidence X-ray scattering, thermal measurements, and photoluminescence quenching. We identify two regimes: (1) oligomers with side chains greater than or equal to heptyl (C7) form bimolecular crystals and (2) oligomers with less than or equal to hexyl (C6) form amorphous blends. This work provides the first observation of an order-to-disorder transition mediated by side-chain length in donor-fullerene intercalated blends

Poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2‑<i>b</i>]thiophene] Oligomer Single-Crystal Nanowires from Supercritical Solution and Their Anisotropic Exciton Dynamics

Supercritical fluids, exhibiting a combination of liquid-like solvation power and gas-like diffusivity, are a relatively unexplored medium for processing and crystallization of oligomer and polymeric semiconductors whose optoelectronic properties critically depend on the microstructure. Here we report oligomer crystallization from the polymer organic semiconductor, poly­[2,5-bis­(3-dodecylthiophen-2-yl)­thieno­[3,2-<i>b</i>]­thiophene] (PBTTT) in supercritical hexane, yielding needle-like single crystals up to several microns in length. We characterize the crystals’ photophysical properties by time- and polarization-resolved photoluminescence (TPRPL) spectroscopy. These techniques reveal two-dimensional interchromophore coupling facilitated by the high degree of π-stacking order within the crystal. Furthermore, the crystals obtained from supercritical fluid were found to be similar photophysically as the crystallites found in solution-cast thin films and distinct from solution-grown crystals that exhibited spectroscopic signatures indicative of different packing geometries