Polymeric Photovoltaic Cell Properties

In the fabrication of bulk-heterojunction photovoltaic cell (PVC) devices, copolymers P2, P6, P8, P10, and P12 were used as the donor phase to blend with different ratios of methanofullerene [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as the typical acceptor phase. As described by UV absorptions of FO-PT copolymers in solid films, FO1-PT polymer derivatives (FO:PT=1:1) possessed broader spectral absorption coverages than their FO3-PT polymer analogues (FO:PT=3:1) in the visible ranges between 400 and 800 nm. Due to the benefits of narrower band-gaps and broader visible absorption ranges in FO1-PT polymers with higher PT contents, FO1-PT polymer derivatives were chosen to survey their potentials for PVC applications. In Figure 2.9(a), the HOMO and LUMO levels of FO1-PT polymer derivatives also match those of good hole-transporting materials for PVC devices with an electron-transporting material PCBM. Thus, FO1-PT polymer derivatives were appropriate for the fabrication of PVC devices with a configuration of

ITO/PEDOT:PSS/FO1-PT:PCBM/LiF/Al as shown in Figure 2.9(b). To evaluate the PVC properties of FO-PT copolymers, a composite thin film of FO1-PT:PCBM was prepared by spin-coating a solution of P12 and PCBM (1:4 w/w) in the mixture solution of chlorobenzene and chloroform (1:1 vol.) onto a quartz plate, and its PL spectrum was recorded, as shown in Figure 2.10. Compared with the PL spectrum of pure P12, complete PL quenching was observed as a result of blending P12 with PCBM, which could be attributed to the different kinetics of charge transfer (~10^-14 s) and recombination (~10^-3 s).⁷⁶ The PL quenching property indicates that FO-PT copolymers can be used as proper electron donors in PVC devices.

The I-V characteristics of photovoltaic cell devices with different weight ratios of P12:PCBM = 1:1, 1:2, and 1:4 as an active layer are presented in Figure 2.11, which were measured under AM 1.5 illumination for a calibrated solar simulator with an intensity of 100 mW/cm². The photovoltaic properties obtained from the I-V curves are listed in Table 2.4. The open circuit voltage (Voc), short circuit current (Isc), and fill factor (FF) of the PVC device based on the ratio of P12:PCBM = 1:4 (w/w) were 0.64 V, 2.7 mA/cm², and 29%, respectively, which were all higher than those of PVC devices based on the ratios of P12:PCBM = 1:1 and 1:2 (w/w). Generally, the values of energy conversion efficiency (ECE) in PVC devices are sensitive to the weight ratios of acceptors to donors. In the case of P12/PCBM blends, the best efficiency observed was P12:PCBM = 1:4 (w/w), which was a similar dependence on donor/acceptor weight ratio in some earlier publications.^85,89 In addition, according to the photovoltaic results of these copolymers in Table 2.4, the highest ECE value of 0.51% was obtained from a solar cell device with P12 as an electron donor.

Apparently, comparing the molecular structures of copolymers P2, P6, P8, P10, and P12, the longest conjugation length and the heterocyclic structures (thiophene units)

of P12 could lead to the highest photovoltaic efficiency among these synthesized copolymers. This result indicates that the incorporation of longer conjugation lengths and heterocyclic moieties into conjugated copolymers could make favorable contribution to photovoltaic properties.

Several parameters are suspected to responsible for the low efficiencies in the PVCs, such as the thickness of the film, the disorder of the film morphology, and the large difference of charge-carrier mobility, etc. The lower molecular weights of FO1-PT polymer derivatives resulted in the solid films with thinner thicknesses and the fewer harvested photons from the solar energy. Therefore, the Isc values of the copolymers (listed in Table 2.4) were only 1.30~2.70 mA/cm². For example, the Voc, Isc, FF, and ECE values of P2 were 0.43 V, 1.86 mA/cm², 27%, and 0.22%, respectively, which were not as high as the same polymer published earlier by Cho et al,^85-86 which might be due to the lower molecular weight or the lower fill factor of P2.

In comparison with the photovoltaic results of P2, P6, P8, and P10, the PVC based on P12 showed a much higher Voc value of 0.64 V in Table 2.4, except P4 due to its poor solubility. Generally, the Voc value is related to the difference between the oxidation potential of the donor and the reduction potential of the acceptor (PCBM).⁹⁰ However, compared with the other polymers with lower HOMO levels, the PVC based on P12 possessing the highest Voc value did not follow the previous general regulation in this work. In addition to the above-mentioned influences on PVCs, the deficits of the ECE values in our polymers were mainly caused by low fill factors which indicate lacks of ordered continuity in the polymer/PCBM blends.⁹¹ The disorder of the film morphology also severely affects the charge carrier mobility, which is believed to be the bottleneck for the Isc values.

Although the photovoltaic properties of the copolymers in this work were not the

best results compared with the other low bandgap polymers, the preliminary results of PVC devices made of the newly synthesized polymers were still not optimized.

Further improvements are underway to optimize the PVC devices by the modification of the film morphology, layer thicknesses, postproduction treatment conditions, and the other electron acceptors.

(a)

(b)

Figure 2.9 (a) Energy levels for an ideal donor polymer for PCBM along with donors P1-P12. Dashed lines display the HOMO and LUMO thresholds of an ideal donor polymer between 5.2-3.8 eV for air stability (5.2 eV) and effective charge transfer to PCBM (3.8 eV). (b) Device structure consisting of an 100 nm thick blending active layer (copolymers:PCBM), which was sandwiched between PEDOT:PSS and an aluminum top electrode.

Figure 2.10 PL spectra of P12 film and a blending film of P12/PCBM (1:4 w/w).

-0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 -4

-3 -2 -1 0 1 2

Current density (mA/cm2 )

Cell voltage (V)

(a) (b) (c) (a) (b) (c)

Figure 2.11 I-V curves of the polymer solar cells with different compositions of P12/PCBM (a) 1:1 w/w (square symbols), (b) 1:2 w/w (circle symbols), and (c) 1:4 w/w (star symbols) measured in the dark (dash lines) and under the illumination of AM 1.5, 100mW/cm² (solid lines).

Table 2.4 Photovoltaic Properties of Copolymers with a Solar Cell Device

aMeasured under AM 1.5 irradiation, 100mW/cm.