Functionalization
of organic semiconductors through the attachment
of bulky side groups to the conjugated core has imparted solution
processability to this class of otherwise insoluble materials. A consequence
of this functionalization is that the bulky side groups impact the
solid-state packing of these materials. To examine the importance
of side-group electronic character on accessing the structural phase
space of functionalized materials, germanium was substituted for silicon
in triisopropylsilylethynylpentacene (TIPS-Pn) to produce
triisopropylgermanylethynylpentacene (TIPGe-Pn),
with the TIPGe side group comparable in size to TIPS, but higher in
electron density. We find TIPGe-Pn single crystals exhibit slip-stack,
herringbone, and brickwork packing motifs depending on growth conditions,
a stark contrast to TIPS-Pn, which accesses only the brickwork packing
motif in both single crystals and thin films. Polycrystalline thin
films of TIPGe-Pn exhibit two new, unidentified polymorphs from spin-coating
and postdeposition annealing. Our experiments suggest that access
to the structural phase space is not guided solely by the size of
the side group; the electronic character of the side group in functionalized
compounds also plays a significant role. As such, simple atomistic
substitutions can cause significant differences in the accessible
solid structures.
While typical perovskite solar cells (PSCs) with doped Spiro-OMeTAD as hole transport material (HTM) have shown rapid increase in their power-conversion efficiencies (PCEs), their poor stability remains a big concern...
Intercrystallite
molecular connections are widely recognized to
tremendously impact the macroscopic properties of semicrystalline
polymers. Because it is challenging to directly probe such connections,
theoretical frameworks have been developed to quantify their concentrations
and predict the mechanical properties that result from these connections.
Tie-chain connectivity similarly impacts the electrical properties
in semicrystalline conjugated polymers. Yet, its quantitative impact
has eluded the community. Here, we assess the Huang–Brown model,
a framework commonly used to describe the structural origins of mechanical
properties in polyolefins, to quantitatively elucidate the effect
of tie chains on the electrical properties of a model conjugated polymer.
We found that a critical tie-chain fraction of 10–3 is needed to support macroscopic charge transport, below which intercrystallite
connectivity limits charge transport, and above which intracrystallite
disorder is the bottleneck. Extending the Huang–Brown framework
to conjugated polymers enables the prediction of macroscopic electrical
properties based on experimentally accessible morphological parameters.
Our study implicates the importance of long and rigid polymer chains
for efficient charge transport over device length scales.
Herein, we describe the design and synthesis of a suite of molecules based on a benzodithiophene "universal crystal engineering core". After computationally screening derivatives, a trialkylsilylethynebased crystal engineering strategy was employed to tailor the crystal packing for use as the active material in an organic field-effect transistor. Electronic structure calculations were undertaken to reveal derivatives that exhibit exceptional potential for high-efficiency hole transport. The promising theoretical properties are reflected in the preliminary device results, with the computationally optimized material showing simple solution processing, enhanced stability, and a maximum hole mobility of 1.6 cm 2 V À1 s À1 .
Size exclusion chromatography (SEC) is not well suited for characterizing the molecular weight (MW) and MW distribution of conjugated polymers, especially those that absorb strongly at the detection wavelengths, or those that interact with and adsorb on the walls of SEC columns. We demonstrate diffusion-ordered NMR spectroscopy (DOSY) as a complementary method for characterizing the size and size distribution of conjugated polymers. Starting with four batches of poly(3-hexylthiophene), whose distinct and narrow MW distributions had been fully characterized, as a model system, we establish a power-law relationship between the weight-average MW and the diffusion coefficient measured through DOSY. We extend this approach to characterizing poly[4-( 4,4-dihexadecyl-4H-cyclopenta-[1,2-b:5,4-b′]dithiophen-2-yl)-alt-[1,2,5]thiadiazolo-[3,4-c]pyridine], whose absorption properties preclude its characterization with light scattering based techniques, including SEC. By applying the same power law on the diffusion coefficients obtained by DOSY measurements, we extracted P3HT-equivalent MWs and MW distributions for six different batches of PCDTPT. By circumventing the practical issues in SEC measurements, DOSY shows promise as a versatile complement for determining polymer size.
Most contemporary X‐ray detectors adopt device structures with non/low‐gain energy conversion, such that a fairly thick X‐ray photoconductor or scintillator is required to generate sufficient X‐ray‐induced charges, and thus numerous merits for thin devices, such as mechanical flexibility and high spatial resolution, have to be compromised. This dilemma is overcome by adopting a new high‐gain device concept of a heterojunction X‐ray phototransistor. In contrast to conventional detectors, X‐ray phototransistors allow both electrical gating and photodoping for effective carrier‐density modulation, leading to high photoconductive gain and low noise. As a result, ultrahigh sensitivities of over 105 μC Gyair−1 cm−2 with low detection limit are achieved by just using an ≈50 nm thin photoconductor. The employment of ultrathin photoconductors also endows the detectors with superior flexibility and high imaging resolution. This concept offers great promise in realizing well‐balanced detection performance, mechanical flexibility, integration, and cost for next‐generation X‐ray detectors.
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