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Small molecule organic semiconductors (OSCs) suffer from their uncontrolled nucleation and growth during solution processing limiting their functionality in electronic devices. In this work, we present a new method based on dip-coating a blend consisting of OSC and insulating polymer to control the crystallization of the active film for organic field-effect transistors. A small fraction of amorphous poly(methyl methacrylate) (PMMA) efficiently improves the crystallization of dip-coated small molecule OSCs, α,ω-dihexylquaterthiophene (DH4T) and diketopyrrolopyrrole-sexithiophene (DPP6T). The maximum charge carrier mobilities of dip-coated OSC:PMMA films are significantly higher than drop-cast blend ones and comparable with OSC single crystals. The high charge carrier mobility originates from a continuous alignment of the crystalline films and stratified OSC and PMMA layers. The improved crystallization is attributed to two mechanisms: firstly, the polymer binder leads to a viscosity gradient at the meniscus during dip-coating, facilitating the draw of solute and thus mass transport. Secondly, the polymer binder solidifies at the bottom layer, reducing the nucleation barrier height of small molecule OSC. Our findings demonstrate that a small fraction of a polymer binder during dip-coating efficiently balances the mass transport during Max Planck Institute for Polymer Research -Author's Manuscript 2 solution processing and improves the crystallization as well as the electronic properties of small molecule OSC films. Crystallization of small molecular organic semiconductors (OSCs) during meniscusguided coating was enhanced by blending with a minor amount of an insulating polymer binder. The polymer binder increased mass transport and formed a bottom layer reducing the nucleation barrier height for OSC crystallization. The aligned crystalline films with stratified OSC/polymer layers contribute to a high charge transport in field effect transistors.

Well‐defined small molecule (SM) donors can be used as alternatives to π‐conjugated polymers in bulk‐heterojunction (BHJ) solar cells with fullerene acceptors (e.g., PC61/71BM). Taking advantage of their synthetic tunability, combinations of various donor and acceptor motifs can lead to a wide range of optical, electronic, and self‐assembling properties that, in turn, may impact material performance in BHJ solar cells. In this report, it is shown that changing the sequence of donor and acceptor units along the π‐extended backbone of benzo[1,2‐b:4,5‐b′]dithiophene–6,7‐difluoroquinoxaline SM donors critically impacts (i) molecular packing, (ii) propensity to order and preferential aggregate orientations in thin‐films, and (iii) charge transport in BHJ solar cells. In these systems (SM1‐3), it is found that 6,7‐difluoroquinoxaline ([2F]Q) motifs directly appended to the central benzo[1,2‐b:4,5‐b′]dithiophene (BDT) unit yield a lower‐bandgap analogue (SM1) with favorable molecular packing and aggregation patterns in thin films, and optimized BHJ solar cell efficiencies of ≈6.6%. 1H‐1H DQ‐SQ NMR analyses indicate that SM1 and its counterpart with [2F]Q motifs substituted as end‐group SM3 possess distinct self‐assembly patterns, correlating with the significant charge transport and BHJ device efficiency differences observed for the two analogous SM donors (avg. 6.3% vs 2.0%, respectively).

End-group-directed molecular self-assembly can be an effective strategy in the design of "organic electronics" amenable to specific morphologies and charge transport patterns for device applications. In this study, we report on the design, self-ordering, and transistor characteristics of an analogous set of pyrene-functionalized diketopyrrolopyrroles (DPP; namely, SM1−3) obtained by "successive incorporation" of DPP motifs. The well-defined pyrenesubstituted DPP analogues are systematically examined in correlation of (i) the number of incorporated DPP motifs in the π-extended main-chain and (ii) the solution-processing conditions employed for the thin film formation. Solvent vapor enhanced drop-casting (SVED) from chloroform (CHCl 3 ) and tetrahydrofuran (THF) are found to promote very distinct, long-range morphologies of SM1−3. During THF-mediated SVED, the growth of a one-dimensional fiber of SM1−3 originates from the formation of initial aggregates in THF. In particular, extending the π-conjugation of the DPP core, and, concurrently, the number of alkyl side-chains involved, is found to mitigate the long-range self-assembly of SM1−3 and, in turn, to lower crystal size and fiber length. Field-effect transistors based on SM3 exhibit ambipolar behavior, demonstrating the relevance of pyrene-functionalized diketopyrrolopyrroles in the design and development of solution-processed ambipolar small-molecule semiconductors.

In particular, in recent work, we showed that the development of BHJ morphologies between donor and acceptor components, [20] material processing, and optimization of thin-film morphologies via postprocessing treatments [21] is not independent of the substitution pattern and functional groups appended to SM donors. As a result, the rational design of SM donors with gradually improved BHJ solar cell efficiencies must concurrently address: (i) bandgap tuning and optimization of spectral absorption (inherent to the SM main chain) and (ii) pendant-group substitution promoting structural order and mediating morphological effects.In this contribution, we report on the rational pendant-group substitution and concurrent bandgap reduction in benzo[1,2-b:4,5-b′]dithiophene-6,7-difluoroquinoxaline SM donors, raising BHJ solar cell efficiencies from ≈6 to >8% with PC 71 BM, and we provide a detailed understanding of the role of the selective substitutions on material and BHJ device performance improvements. Taking advantage of the synthetic modularity of electrondeficient quinoxaline motifs, we show that the introduction of acrylate functions on the pyrazine ring is an effective approach to significantly increasing the electron affinity of the acceptor unit, in turn narrowing the optical gap of the SM donor. Importantly, our device examinations show that this discrete synthetic modification does not alter the propensity of benzo[1,2-b:4,5-b′]dithiophene-6,7-difluoroquinoxaline SM donors to order and form favorable thin-film BHJ morphologies with PCBM; carrier extraction in optimized BHJ active layers proceeds with notably suppressed geminate and bimolecular recombination, while the high open-circuit voltage (V OC ) of the BHJ solar cells is maintained to ≈1 V.In the design of symmetrical SM donors alternating donor and acceptor motifs (DADAD) setting their optical gap, two avenues have prevailed thus far: (i) swapping the donor [acceptor] units for more electron-rich [-deficient] motifs-albeit with distinctly different molecular structure [14,22] -and (ii) increasing [reducing] their conjugation length by replicating [removing] DA motifs [14] or, more commonly, by varying the number of rings involved in the main chain. [1,17] In either cases, the synthetic modifications generally impact SM self-assembly in thin films, solution-processing and device optimization procedures, [18,23] Solution-processable small molecule (SM) donors are promising alternatives to their polymer counterparts in bulk-heterojunction (BHJ) solar cells. While SM donors with favorable spectral absorption, self-assembly patterns, optimum thin-film morphologies, and high carrier mobilities in optimized donor-acceptor blends are required to further BHJ device efficiencies, material structure governs each one of those attributes. As a result, the rational design of SM donors with gradually improved BHJ solar cell efficiencies must concurrently address: (i) bandgap tuning and optimization of spectral absorption (inherent to the SM main chain) and (ii) pendant...