3.3. Photovoltaic properties
Fig. 4(a) shows the JeV curves for the optimized BHJ OPVs using
poly(NTD-TPA) or poly(CR-TPA), and Table 2 lists the average device
characteristics of four devices, including the short-circuit current
density Jsc, open-circuit voltage Voc, fill factor (FF), and PCE under
illumination with 100 mW cm2 of AM 1.5G. BHJ OPVs using
poly(NTD-TPA) always showed higher PCEs than these using
poly(CR-TPA). The PCEs of the BHJ OPVs using poly(NTD-TPA) andpoly(CR-TPA) were both decreased by making the BHJ layer thicker.
The decrease was caused mainly by a decrease in the FF, which is
ascribed to relatively low hole mobilities in amorphous polymers
compared to crystalline polymers or increased photocurrent
recombination in the thicker BHJ layer [24,41,42]. The optimized
BHJ OPV cell using poly(NTD-TPA) had higher IPCEs than the cell
using poly(CR-TPA), as shown in Fig. 4(b), because the absorbance
of the poly(NTD-TPA):PC70BM at wavelengths of 450e650 nm
exceeded that of poly(CR-TPA):PC70BM, as shown in Fig. 2(b). To
accentuate the absorption peak at the wavelength of 553 nm due to
intramolecular CT from TPA donors to NTD acceptors in the
poly(NTD-TPA) [Fig. 2(a)], BHJ OPVs with PC60BM that has not absorption
at visible wavelengths were also fabricated, characterized,
and summarized in Fig. S1 and Table S1 in Supporting Information.
The enhanced absorption of AM 1.5G solar-simulated light led to
the higher Jsc in the BHJ OPVs based on poly(NTD-TPA) than in those
based on poly(CR-TPA).