Homocoupling Defects of a Small Donor Molecule for Organic Photovoltaics: Quantification of the Eutectic State Diagram by Rapid Heat–Cool Differential Scanning Calorimetry

The conjugated small donor molecule p-DTS(FBTTh 2) 2, leading to efficient bulk heterojunction organic photovoltaic devices with PC 71BM, shows a eutectic phase behavior in combination with its homocoupled analogue. Rapid heat-cool differential scanning calorimetry allows us to construct the state diagram based on (i) the endset of melting of p-DTS(FBTTh 2) 2:homocoupled p-DTS(FBTTh 2) 2 mixtures of different compositions, and (ii) partial integration of the melting trajectory of these mixtures by calculating the fraction of the molten pure excess phase as a function of temperature. These quantitative experimental results are supported by a theoretical calculation model based on Schröder-van Laar equations extended with an interaction function representing the interactions between the mixture's components in the liquid state. The eutectic composition around 30 wt % of homocoupled p-DTS(FBTTh 2) 2 shows the lowest crystallinity (order) of all mixtures together with a maximum loss of photovoltaic efficiency.