Over the last decade, the use of ternary blend bulk heterojunctions (T-BHJ) has become a promising strategy to increase the performance of organic solar cells (OSC). [1-2] T-BHJ are generally nanocomposite layers using a mixture of two donors with one acceptor aiming an enlargement of light absorption of the OSC and thus higher photocurrent density. However, it was very recently shown that adding a highly ordered donor sensitizer to a binary polymer:fullerene blend generates reduction of charge carrier recombination inside the T-BHJs leading to fill factors up to 77% demonstrating that the T-BHJ concept is very powerful to improve performance of organic solar cells . The underlying mechanism was correlated to effective charge transfer from the disordered host system, i.e. PTB7 or PTB7-Th, onto the highly ordered sensitized Si-PCPDTBT avoiding hole trapping inside the host matrix.
Here, we show that the same phenomena can also be observed in T-BHJ using only amorphous. Indeed, PTB7:PTB7-Th:PC71BM blends show fill factor of 74%, while the binary PTB7 and PTB7-Th devices show much lower FF of 66% and 68%, respectively.  We studied the underlying mechanism by multiple techniques such light-intensity I-V, transient Voc and charge transport measurements, 2D-GIXD and XPS/UPS spectroscopy, STEM-SI microscopy and theoretic modeling. Our results indicate that difference in the enthalpy of mixing between PTB7 and PTB7-Th with the fullerene causes a self-assembly of the ternary blend into a three-phase separated bulk heterojunction in which PTB7-Th phase are in contact with fullerene phases and are strongly mixed, while the PTB7 phases are purified and separated from fullerenes via the PTB7-Th. This leads to efficient cascade reaction and charge screen at the PTB7/PTB7-Th/PC71BM interface. The enthalpy model applied furthermore to the ordered Si-PCPDTBT (T-BHJ) of  demonstrating its generality. We further discuss T-BHJ using non fullerene acceptor such as ITIC to reduce recombination in polymer solar cells.
Ref 1 : T. Ameri et al. Adv. Mater. 2013, 25, 4245–4266
Ref 2 : Q. An, et al. EnergyEnviron.Sci. 2016,9,281—322
Ref 3 : Gasparini et al., Nature Energy 2016, doi:10.1038/nenergy.2016.118).
Ref 4: S. Ben Dkhil et al., manuscript submitted.