Active Layer Materials for Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 31

Scheme 2.10 Synthesis of three kinds of substituted BDTs

eight BDT-based polymers, as shown in Scheme 2.11 , to investigate the correla-
tion among conjugated backbones, band gaps, and molecular energy levels [ 59 ].
Furthermore, the correlation between HOMO levels of those polymers and the V oc
values of the devices based on them were also studied in this work. It can be seen
that V oc of the devices was directly proportional to the offset between HOMO
level of the electron donor material and LUMO level of PCBM. Although, the
polymer PBDTTPZ exhibited strong and broad absorption band, but V oc of the
PBDTTPZ/PCBM-based device was only 0.2 V due to the high-lying HOMO level
of the polymer, meaning that the balance between band gap and HOMO level
would be of great importance to the molecular structure design Fig. 2.2 .
In the mean time, more and more BDT-based polymers were designed, syn-
thesized, and applied in PSCs. The BDT derivatives were copolymerized with
many kinds of building blocks, like derivatives of thiophene, BT and the likes,

Scheme 2.11 Eight BDT-based polymers with identical BDT moieties