Active Layer Materials for Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 26

P3HT, and thus, a V oc of 0.84 V has been achieved from P3HDTTT/PCBM-based
device, which is 0.24 V higher than that of P3HT/PCBM-based device [ 20 ].
Another strategy to lower the band gap of rr-P3HT was the synthesis of reg-
ioregular poly(3-hexylselenophene) (rr-P3HS). Ballantyne et al. successfully
completed the synthesis of this polymer and applied it in OPVs [ 21 ]. The optical
band gap (i.e., 1.60 eV) of the resulting polymer was much lower than that of the
P3HT. The cells were fabricated by following similar conditions as used in P3HT-
based devices, and a PCE of 2.7 % was achieved.

2.2.1.2 Polymers with 2,1,3-Benzothiadiazole

2,1,3-Benzothiadiazole (BT) has been widely used as electron deficient building
block in conjugated polymers with D/A structure. This category of polymer donors
has been extensively studied and showed outstanding photovoltaic performances
(the performances of the copolymers are summarized in Table 2.2 ). As known,
thiophene is a typical electron-rich unit with weak aromatic property and hence
thiophene derivatives are broadly used as electron donating building blocks in
conjugated polymers. Several copolymers, based on derivatives of dithiophene and
BT are shown in Scheme 2.6 . It can be seen that the dithiophene derivatives shown
in Scheme 2.6 possess well planar structures, which were confined by the bridge
atoms, like N, C, or Si, or by a planar conjugated unit.
PCPDTBT is the first low band gap polymer which was successfully synthe-
sized and used in PSCs [ 22 , 25 ]. PCPDTBT has strong and broad absorption band
extending to near-infrared region, corresponding to a band gap of 1.50 eV, and this
polymer also exhibited a good hole mobility, 1 9 10 -3 cm 2 /V s by field effect
transistor (FET) [ 23 ]. Initially, the PCE of PCPDTBT/PCBM system was *3.2 %
[ 22 ]. Then, the morphology of the blend was optimized by using diiodooctane
(DIO) or 1,8-dithiol-octane (ODT) as additive during the spin-coating process, and
hence the PCE was improved to [5%[ 26 ]. Since, the solubility of PCBM in DIO

Table 2.2 Properties and devices characteristics for polymers of Schemes 2.6 and 2.7

J sc (mA/cm 2 )

Polymer Eg(opt)

(eV)

HOMO/LUMO
(eV/eV)

V oc (V) FF PCE (%) Refs

PCPDTBT 1.40 -5.30/- 3.57 16.2 0.62 0.55 5.5 [ 26 ]

PSBTBT 1.37 N.A. 17.3 0.57 0.61 5.9 [ 24 ]

PDTPBT 1.43 -4.81/- 3.08 11.9 0.54 0.44 2.8 [ 30 ]

PFDTBT N.A. N.A. 7.70 1.00 0.54 4.2 [ 31 ]

PFSiDTBT 1.86 -5.70/- 3.81 9.40 0.90 0.51 5.4 [ 32 ]

PCDTBT 1.88 -5.50/- 3.60 10.6 0.88 0.66 6.1 [ 33 ]

PDTPDTBT 1.46 -5.00/- 3.43 9.47 0.52 0.44 2.2 [ 34 ]

PCPDTDTBT 1.55 N.A. 8.75 0.60 0.4 2.1 [ 35 ]

PBDTDTBT 1.75 -5.31/- 3.44 10.7 0.92 0.57 5.66 [ 36 ]

PSiDTBT 1.53 -4.99/- 3.17 10.67 0.62 0.52 3.4 [ 37 ]