M A T E R I A L S F O R T O M O R R O W ' S I N N O V A T I V E A N D S U S T A I N A B L E I N D U S T R Y

S C I E N T I F I C H I G H L I G H T S

6 0 H I G H L I G H T S 2 0 2 3 I

X-ray experiments show the importance of chemical precision in organic electronics

Structural defects in polymeric semiconductors for electronics can arise from synthesis methods, yet their impact on device efficiency remains unclear. A combination of experiments, including X-ray diffraction, and theoretical simulations was used to investigate the effects of homocoupling defects on the properties of a model polymer:fullerene mixture.

Organic semiconducting polymers are commonly used in optoelectronic devices such as organic light- emitting diodes, photovoltaics (OPVs), transistors, or photodetectors (OPDs), and also applied in healthcare and (photo)catalysis. In the field of organic electronics, mixtures are often used to introduce new functionalities that do not exist in the individual components, thereby realising unprecedented material and device properties. A notable example is the combination of the (semi)crystalline polymer PBTTT and the methanofullerene PC61BM. Intercalation of the fullerene bucky balls in between the polymer side chains gives rise to a molecular compound with distinct features. This nanoscale mixing of the two components has, for instance, been used to achieve enhanced detectivities in cavity-based photodetectors.

State-of-the-art conjugated polymers are traditionally synthesised using cross-coupling polymerisation, which alternates electron-rich and electron-deficient building blocks throughout the polymer backbone. However, these synthesis methods can introduce homocoupling defects (i.e., monomer units coupling to themselves rather than to the complementary functionalised comonomer). Homocoupling has been shown to result in compromised device (notably solar cell) efficiencies for some polymers, yet others still performed excellently, hence it remains unclear whether and how such structural defects affect material and device properties.

In this work, homocoupling defects were investigated in the model polymer PBTTT-(OR)2, a novel derivative of PBTTT. Homocoupling-containing PBTTT-(OR)2 (made via Stille polymerisation) was compared with homocoupling- free PBTTT(OR)2 synthesised via an alternative oxidative homopolymerisation strategy to circumvent homocoupling. High-resolution scanning tunnelling microscopy was used to identify and quantify the defects and confirm the absence of homocoupling defects for the alternative synthetic approach. The two polymer batches were then compared as pristine materials and in mixtures with PC61BM by a complementary combination of rapid heat-cool differential scanning calorimetry and temperature-resolved synchrotron X-ray diffraction (XRD),

Fig. 42: Temperature-resolved synchrotron SAXS and WAXS patterns as a function of q for a) defect-free PBTTT-(OR)2 and a ) defect- free PBTTT-(OR)2 mixed with PC61BM upon heating at 50 K min−1 from room temperature to 320°C after slow cooling at 20 K min−1

from 335 to −50°C. The dotted white line in a ) highlights the track of a weak reflection, tentatively associated with PBTTT-(OR)2 stacks that are only partially intercalated with PC61BM. b) Values for db, dcs, and dπ at 25°C and 150°C for defect-free PBTTT-(OR)2 and defect-free PBTTT-(OR)2 mixed with PC61BM, obtained from the SAXS and WAXS experiments and from theoretical calculations.

c) SAXS patterns at room temperature for the polymers in pure form (red) and mixed with PC61BM (blue) for defect-free PBTTT- (OR)2 (upper panel, full lines) and benchmark PBTTT (lower panel, dotted lines), and for defective (Stille) PBTTT-(OR)2 (middle panel).

The d-spacings corresponding to the q-values of the peak maxima (marked with short vertical lines) are indicated.