Compact Roll-to-Roll Coater for in Situ X‑ray Diffraction Characterization of Organic Electronics Printing

We describe a compact roll-to-roll (R2R) coater that is capable of tracking the crystallization process of semiconducting polymers during solution printing using X-ray scattering at synchrotron beamlines. An improved understanding of the morphology evolution during the solution-processing of organic semiconductor materials during R2R coating processes is necessary to bridge the gap between “lab” and “fab”. The instrument consists of a vacuum chuck to hold the flexible plastic substrate uniformly flat for grazing incidence X-ray scattering. The time resolution of the drying process that is achievable can be tuned by controlling two independent motor speeds, namely, the speed of the moving flexible substrate and the speed of the printer head moving in the opposite direction. With this novel design, we are able to achieve a wide range of drying time resolutions, from tens of milliseconds to seconds. This allows examination of the crystallization process over either fast or slow drying processes depending on coating conditions. Using regioregular poly­(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) inks based on two different solvents as a model system, we demonstrate the capability of our in situ R2R printing tool by observing two distinct crystallization processes for inks drying from the solvents with different boiling points (evaporation rates). We also observed delayed on-set point for the crystallization of P3HT polymer in the 1:1 P3HT/PCBM BHJ blend, and the inhibited crystallization of the P3HT during the late stage of the drying process

Tuning the Morphology of Solution-Sheared P3HT:PCBM Films

Organic bulk heterojunction (BHJ) solar cells are a promising alternative for future clean-energy applications. However, to become attractive for consumer applications, such as wearable, flexible, or semitransparent power-generating electronics, they need to be manufactured by high-throughput, low-cost, large-area-capable printing techniques. However, most research reported on BHJ solar cells is conducted using spin coating, a single batch fabrication method, thus limiting the reported results to the research lab. In this work, we investigate the morphology of solution-sheared films for BHJ solar cell applications, using the widely studied model blend P3HT:PCBM. Solution shearing is a coating technique that is upscalable to industrial manufacturing processes and has demonstrated to yield record performance organic field-effect transistors. Using grazing incident small-angle X-ray scattering, grazing incident wide-angle X-ray scattering, and UV–vis spectroscopy, we investigate the influence of solvent, film drying time, and substrate temperature on P3HT aggregation, conjugation length, crystallite orientation, and PCBM domain size. One important finding of this study is that, in contrast to spin-coated films, the P3HT molecular orientation can be controlled by the substrate chemistry, with PEDOT:PSS substrates yielding face-on orientation at the substrate–film interface, an orientation highly favorable for organic solar cells

Molecular Doping of a High Mobility Diketopyrrolopyrrole–Dithienylthieno[3,2‑<i>b</i>]thiophene Donor–Acceptor Copolymer with F6TCNNQ

Herein we present a molecular doping of a high mobility diketopyrrolopyrrole–dithienylthieno­[3,2-<i>b</i>]­thiophene donor–acceptor copolymer poly­[3,6-(dithiophene-2-yl)-2,5-di­(6-dodecyl­octadecyl)­pyrrolo­[3,4-<i>c</i>]­pyrrole-1,4-dione-<i>alt</i>-thieno­[3,2-<i>b</i>]­thiophene], PDPP­(6-DO)₂TT, with the electron-deficient compound hexafluoro­tetracyano­naphthoquino­dimethane (F6TCNNQ). Despite a slightly negative HOMO_donor–LUMO_acceptor offset of −0.12 eV which may suggest a reduced driving force for the charge transfer (CT), a partial charge CT was experimentally observed in PDPP­(6-DO)₂TT:F6TCNNQ by absorption, vibrational, and electron paramagnetic resonance spectroscopies and predicted by density functional theory calculations. Despite the modest CT, PDPP­(6-DO)₂TT:F6TCNNQ films possess unexpectedly high conductivities up to 2 S/cm (comparable with the conductivities of the benchmark doped polymer system P3HT:F4TCNQ having a large positive offset). The observation of the high conductivity in doped PDPP­(6-DO)₂TT films can be explained by a high hole mobility in PDPP­(6-DO)₂TT blends which compensates a lowered (relatively to P3HT:F4TCNQ) concentration of free charge carriers. We also show that F6TCNNQ-doped P3HT, the system which has not been reported so far to the best of our knowledge, exhibits a conductivity up to 7 S/cm, which exceeds the conductivity of the benchmark P3HT:F4TCNQ system