On The Importance of Chemical Precision in Organic Electronics – Fullerene Intercalation in Perfectly Alternating Conjugated Polymers and Implications on Device Performance

The true structure of alternating conjugated polymers – the state-of-the-art materials for a number of organic electronics technologies – often deviates from the idealized picture. The presence of structural defects is inherently linked to the applied cross-coupling polymerizations (notably Stille) and varies according to the experimental conditions. Nevertheless, many polymers still perform excellently in the envisaged applications, which raises the question if one should even care about these defects. Here, this question is addressed by comparing a traditionally prepared alkoxy-PBTTT polymer to a newly synthesized homocoupling-free variant, as verified by scanning tunneling microscopy (STM). We (for the first time) quantify the amount of homocoupling by STM and shed new light on the actual distribution of these defects in the polymer material. Additionally, we demonstrate that matrix-assisted laser desorption-ionization - time of flight (MALDI-ToF) mass spectrometry only shows a fraction of the homocoupled species and hence gives an incomplete picture. Further, it is shown through a combination of experimental techniques (and supported by calculations) that these defects hinder fullerene intercalation, whereas this returns for the homocoupling-free polymer and then affords stronger intermolecular charge-transfer interactions. This demonstrates that molecular defects may (strongly) impact material and blend properties and calls for increased attention for defect-free materials.