Active Layer Materials for Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 28

(1.7 eV) and deeper HOMO level (-5.3 eV) compared to PDTPBT. Although,
band gap and HOMO level of PBDTBT are suitable for solar cell and V oc over
0.8 V has been recorded, the photovoltaic performance of PBDTBT/PCBM blend
system has not been well optimized, and PCE of this polymer was still below 2 %.
In order to develop more efficient photovoltaic materials, thiophene units were
introduced to the 4- and 7-positions of BT, and an important derivative named as
DTBT was designed and synthesized. DTBT-based conjugated polymers have
been well investigated as photovoltaic materials, and seven representatives are
shown in Scheme 2.7 . The basic information of these polymers is listed in
Table 2.2 . It can be seen that the band gaps and as well the HOMO and LUMO
levels of polymers were influenced by the electron donating properties of the
building blocks. For example, in those seven building blocks, DTP is more elec-
tron-rich than the others, and hence the polymer PDTPDTBT exhibited much
lower band gap and higher HOMO level than the rest [ 38 ]; fluorene unit is the
weakest electron donating effect compared to the other building blocks, and
therefore, its HOMO level reached -5.5 eV, the lowest one in these DTBT-based
polymers [ 39 ]. All of these DTBT polymers exhibited very potential photovoltaic
properties, and 5-7 % of PCEs have been achieved based on them and some
interesting phenomena have been reveals. For instance, V oc values of PFDTBT
[ 39 ], PCDTBT [ 40 , 41 ], and PFSiDTBT [ 42 , 43 ]—based devices were 1, 0.9 and
0.88 V, respectively, which are among the highest values in OPV cells.

2.2.1.3 Pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) Derivatives

DPP and its derivatives, usually have strong absorption bands in the visible range.
Thiophene-based DPP derivatives have well-confined conjugated structures, and
exhibit good charge-carrier mobilities for both holes and electrons. One of the first

Scheme 2.7 Several representatives of DTBT-based polymers with D/A structure