Another important fundamental issue for the PSC field is how
to control the morphology of polymer:fullerene blends to achieve
the best PSC performance. There is likely more than one nearoptimum
PSC morphology. The famous PTB7 family donor
polymers enabled one type of the near-optimum PSC morphology,
as high external quantum efficiencies (EQEsB80%) have
been reported for PTB7-based cells11. However, the PTB7-based
PSC materials and devices have certain limitations. Besides the
sensitivity of the choice of fullerenes, another important
limitation for PTB7 family polymers is that they cannot
perform well when relatively thick active layers (B300 nm) are
used in the PSC device. Thick-film PSCs are important for the
industrial application of PSCs, and thick films should also further
increase the absorption strength of the solar cell and thus cell
efficiency. The reason why PTB7 does not perform well in
thick-film PSCs is partially owing to the relatively low hole
transport ability (space charge limited current (SCLC) mobility
B6104 cm2 V1 s1; ref. 17) related to the low crystallinity
of the PTB7 polymer. There has been also evidence that high
purity of the polymer domain may be an important factor to
achieve efficient thick-film PSCs14,23,24. The PTB7-based
materials systems are characterized by relatively impure
polymer domains25, which could be a reason why these
polymers do not perform well in thick-film PSCs.