Active Layer Materials for Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 29

organic electronic applications for DPP-based polymers was in ambipolar organic
FETs. A DPP-based polymer having a mobility of 0.1 cm 2 V -1 s -1 for both holes
and electrons were reported by Winnewisser et al. [ 44 ]. On the other hand, the
DPP core can be synthesized easily with good yield (see Scheme 2.8 ). Therefore,
DPP-based polymers and small molecular materials attracted much attention and
more and more DPP-based photovoltaic materials have been developed in the
recent years.
A large number of low band gap polymers have been synthesized since 2008,
with the DPP unit and some of them are shown in Scheme 2.8 and their basic
properties are listed in Table 2.3 . The homopolymer (PDPP) was synthesized
through Yamamoto coupling polymerization [ 45 ]. The band gap of this polymer is
very low (1.24 eV). Moreover, the polymer is stable under ambient condition due
to its low-lying HOMO level. However, the PCE of PDPP/PCBM-based solar cell
was only 0.3 %. One of the main reasons given by the authors is the energy
difference between the LUMO of the donor and the LUMO of the acceptor, which
is only 0.18 eV [ 46 , 47 ]. To extend the conjugation of the backbone, a third
thiophene was added as shown in PTDPP. The polymerization was performed via a
Suzuki cross-coupling polymerization, using 2,5-thiophenebis(boronic ester) as
monomer [ 48 ]. The band gap of PTDPP was *1.30 eV, and a PCE of 4.7 % was
recorded (Scheme 2.9 ).
DPP was also copolymerized with the other commonly used conjugated com-
ponents. It is worthy to mention that in comparison with DTBT-based polymers,
DPP-based polymers possess much lower band gaps. For example, band gap of
PFDPP and PCZDPP are 1.77 and 1.57 eV [ 45 , 49 ], respectively, which are lower
than their DTBT-based counterparts (PFDTBT and PCDTBT). Not only DPP-

Scheme 2.8 Synthesis of the

DPP core

Table 2.3 Polymer properties and device characteristics for the DPP-based polymers

J sc (mA/cm 2 )

Polymer Eg(opt)

(eV)

HOMO/LUMO
(eV/eV)

V oc (V) FF PCE (%) Refs

PTDPP 1.30 -5.17/- 3.16 11.8 0.65 0.60 4.7 [ 48 ]

PFDPP 1.77 -5.43/- 3.67 2.41 0.91 0.41 0.9 [ 45 ]

PCPDTDPP 1.39 -5.25/- 3.74 5.73 0.61 0.49 1.7 [ 45 ]

PPDTDPP 1.40 -5.10/- 3.40 11.3 0.61 0.58 4.0 [ 50 ]

PPPDPP 1.53 -5.35/- 3.53 10.8 0.80 0.65 5.5 [ 51 ]

PDPP 1.24 -5.29/- 3.99 0.76 0.64 0.58 0.3 [ 45 ]

PCZDPP 1.57 -5.44/- 3.92 8.6 0.80 0.47 3.2 [ 49 ]

PFGeDPP 1.63 -5.38/- 3.70 4.10 0.76 0.62 1.5 [ 52 ]

PTPDDPP 1.13 -4.90/- 3.63 14.9 0.38 0.48 2.7 [ 53 ]

PBDTDPP 1.43 -5.15/- 3.69 6.72 0.74 0.56 2.8 [ 54 ]