Organic Photovoltaic Cell Properties

According to the previously described physical properties of P1-P6, these coplanar semiconducting copolymers P1-P6 are suitable for PSC applications. To investigate the potential use of copolymers P1-P6 in PSCs, bulk heterojunction PSC devices with a configuration of ITO/PEDOT:PSS/P1-P6:PCBM (1:1 w/w)/Ca/Al were fabricated from an active layer where copolymers P1-P6 were blended with a complementary fullerene-based electron acceptor (PCBM) in a weight ratio of 1:1 (w/w) initially (and later followed with various weight ratios for the optimum copolymer). The photovoltaic properties of PSC devices containing copolymers P1-P6:PCBM (1:1 w/w) are listed in Table 4.4. Figures 4.8(a) and 4.8(b) illustrate I-V curves and EQE values, respectively, for PSC devices containing copolymers P1-P6:PCBM (1:1 w/w) under monochromatic illumination, where EQE is displayed as a function of wavelength. Due to the minor variations in open circuit voltage (Voc) values (0.822-0.630 V) in P1-P6, Figure 4.8(a) demonstrates the sequence of the best power conversion efficiency (PCE) values for P4, P1, and P2 according to their short circuit current density (Isc) values of 7.70, 6.34, and 5.26 mA/cm², respectively. As shown in Figure 4.8(b), broader EQE curves for P4, P1, and P2 covered almost the entire visible spectrum from 350 to 650 nm with the maximum EQE values of 60%, 50%, and 40%, respectively, which also explained for their high power conversion efficiency (PCE) values over 2.12%. Among these PSC devices containing copolymers P1-P6, the best performance was the PSC device fabricated by P4:PCBM (1:1 w/w) which reached an AM 1.5G power conversion efficiency (PCE) of 2.79%, with a short circuit current density (Isc) of 7.70 mA/cm², an open circuit voltage (Voc) of 0.683 V, and a fill factor (FF) of 0.53.

Due to the requirements of higher charge mobilities and better absorptions of

polymers in PSC devices,¹³¹ the hole mobility values of copolymers P1-P6 were (see Table 4.4) estimated from Equation 1 via space-charge limit current (SCLC) by fabricating a hole-only device.¹³²

3 2

0 /8

9 V L

J = ε ε_rμ (1) where ε₀ε_r is the permittivity of the polymer, μ is the carrier mobility, L is the device thickness. Ideally, Isc was determined by the product of the photoinduced charge carrier density and the charge carrier mobility within the organic semiconductors.²² Surprisingly, the hole mobilities of copolymers P5-P6 were not as high as those of copolymers P1-P4, which was probably due to the lower molecular weights and worse solubilities resulting in inferior film-forming qualities, even though their optical band gaps were smaller than the other copolymers (P1-P4). Thus, due to the relatively lower hole mobilities and less light-harvesting capabilities at the longer absorption wavelength ranges of P3, P5, and P6, their PSC devices showed lower photocurrent values of 4.43, 3.03, and 2.68 mA/cm², respectively, in comparison with those containing P1, P2, and P4. This phenomenon of lower photocurrents further explained the worse EQE values and narrower absorption wavelength regions in the PSC devices containing copolymers P3, P5, and P6, where the EQE values of the visible spectra from 350 to 600 nm were only below 40%. Thus, not only optical properties but also charge transporting properties could be tuned by changing the lengths of oligothienyl and bithiazole-based main-chains. Comparing the FF values in P1-P4 (excluding P5-P6 due to their poor soubilities), the highest values of 53% in PSC devices containing copolymers P1 and P4 were obtained likely due to the more densely packed lamellar sheets in P1 and P4 (with smaller d1 spacing values of 11.9 and 15.1 Å resulting from highly ordered structural packings) than P2 and P3 with a longer d spacing value of 17.3 Å), as proven by XRD patterns previously.

The Voc values covered a rather wide range among the PSC devices containing copolymers P1-P6, which were related to the differences of energy levels between the HOMO levels of the polymers and the LUMO levels of the acceptors.^22,25-26 Therefore, the PSC devices containing copolymers P1, P2, and P3 (with HOMO energy levels of -5.40, -5.51, and -5.55 eV, respectively) showed slight increases of Voc values (0.730, 0.777, and 0.822 V, respectively), which indicated that the insertion of more bithiazole units had some influence on the relationship between the HOMO levels of copolymers and the Voc values of PSC devices. Moreover, followed by increasing the HOMO level of copolymer P4 (from -5.40 to -5.07 V), the Voc value of the PSC device containing P4 was ca. 0.05 V lower than that containing P1, which was due to the insertion of the strong electron-donating thiophene moieties in the molecular structure of P4.

The AFM topographies of polymer blends (P1-P6:PCBM=1:1 w/w) were investigated by the casting films of DCB solutions as shown in Figure 4.9, where the images were obtained in a surface area of 2 × 2 μm² by the tapping mode. The phase image of blended copolymer P4 showed coarse chain-like features across the surface, which were attributed to the domains of the highly stacked polymer chains of P4.In comparison with blended copolymers P1-P3, the solid film of blended copolymer P4 revealed a rather uneven surface with a root mean square (RMS) roughness of 7.3 nm.

The rougher surface of blended copolymer P4 was caused by the better self-assembled stacking between the bithiazole and thiophene units, which enhanced both hole mobility and photocurrent.⁵¹ Furthermore, the solid film of blended copolymer P1 showed the moderate rough surface with a RMS roughness of 5.2 nm.

However, increasing the numbers of bithiazole units with alkyl side-chains in P2 and P3, the surfaces of polymer blends (P2 and P3) showed RMS roughnesses of 3.3 and 2.1 nm, respectively. The smoother surfaces of blended copolymers P2 and P3

compared with that of blended copolymer P1 indicated that more side chains of copolymers P2 and P3 would disrupt the polymer crystallization in the polymer blends and led to lower photocurrents. It is worthy to mention that the solid films of blended copolymers P5 and P6 showed rather rough surfaces, but the large values of RMS roughnesses (6.9 and 9.3 nm) were contributed from the aggregation of polymer chains due to their poor solubilities, which reduced the interfaces between donor (copolymers) and acceptor (PCBM) significantly. Owing to the unfavorable morphologies for charge transport offered by poor solubilities, the PSC devices based on P5 and P6 gave relatively low current densities (Isc) as shown in Table 4.

Therefore, excluding P5 and P6, the blended copolymers (P1-P4:PCBM=1:1 w/w) have the same order of PCE values as those of root mean square (RMS) roughnesses in AFM, i.e., P4 (7.3 nm) > P1 (5.2 nm) > P2 (3.3 nm) > P3 (2.1 nm).

Since the best performance of PSC devices fabricated by polymer blends P1-P6:PCBM (1:1 w/w) was made of P4, current-voltage characteristics of PSC devices as a function of blended copolymer P4:PCBM in various weight compositions are shown in Figure 4.10 and Table 4.5. The optimum photovoltaic performance with the maximum PCE value of 3.04% (Voc = 0.70 V, Isc = 8.00 mA/cm², FF = 53.7%) was obtained in the PSC device having a weight ratio of P4:PCBM=1:2. Using lower weight ratios of PCBM in blended copolymers P4:PCBM (P4:PCBM=1:0.5 and 1:1 w/w) led to reductions in the Isc values due to the inefficient charge separation and electron transporting properties, resulting in the lower PCE results.^131b However, loading larger weight ratios of PCBM in blended copolymers P4:PCBM (1:3 and 1:4 w/w) also reduced the Isc and PCE values, which could be probably attributed to the increased aggregation of PCBM so as to influence the separation of charges.

Furthermore, an unbalanced charge transporting property was introduced due to the

large PCBM ratio. Hence, both Isc and PCE values decreased with larger PCBM molar ratios of 1:3 and 1:4 (w/w) because of the two reasons described here.^133a Therefore, the most efficient PSC device with the maximum PCE value of 3.04% was established by the blended copolymer P4 with a weight ratio of P4/PCBM=1:2 in this report, which has a similar result as the PSC devices containing thiophene-based polymers.^67,133b

Table 4.4 Photovoltaic Properties of PSC Devices Containing an Active Layer of P1-P6:PCBM = 1:1 (w/w) with the Configuration of

ITO/PEDOT:PSS/Polymer:PCBM/Ca/Al^a weight ratio of P1-P6:PCBM = 1:1.

Table 4.5 Photovoltaic Parameters^a for Bulk-Heterojunction PSC Devices Containing Different Weight Ratios of Blended Copolymer P4:PCBM Weight ratios of blended

a PSC devices with the configuration of ITO/PEDOT:PSS/Polymer:PCBM/Ca/Al were measured under AM 1.5 irradiation, 100 mW/cm².

(a)

Figure 4.8 (a) I-V curves (under simulated AM 1.5 solar irradiation) and (b) EQE wavelength dependencies of PSC devices with an active layer of blended copolymers P1-P6:PCBM (1:1 w/w).

(a) (b)

(c) (d)

(e) (f)

Figure 4.9 AFM images for solid films of blended copolymers (a) P1, (b) P2, (c) P3, (d) P4, (e) P5, and (f) P6 with PCBM (1:1 w/w) as-cast from DCB solutions.

-0.2 0.0 0.2 0.4 0.6 0.8 -10

-8 -6 -4 -2 0

P4:PCBM=1:0.5 P4:PCBM=1:4 P4:PCBM=1:3 P4:PCBM=1:1 P4:PCBM=1:2

Current density

(

)

Voltage (V)

Figure 4.10 I-V curves of PSC devices containing an active layer of P4:PCBM (w/w) with different weight ratios under simulated AM 1.5 solar irradiation.

4.4 Conclusions

Using the concept of incorporating electron-withdrawing groups in the donor-acceptor conjugated copolymers, we have successfully synthesized six cyclopentadithiophene-bithiazole-based copolymers (P1-P6) employing oligo(bithiazole), bithiazole-oligo(thiophene), and diarylene-cyanovinylene-bithiazole groups by palladium (0)-catalyzed Stille coupling reactions. The band gaps and HOMO/LUMO levels of these resulting copolymers can be finely tuned as demonstrated in the exploration of optical absorption and electrochemical properties.

In powder X-ray diffraction (XRD) measurements, these copolymers exhibited obvious diffraction features indicating highly ordered π-π stacking in the solid state.

mobilities of 3.3 - 5.6×10^-4 cm²V^-1s^-1 and good processabilities for PSC applications.

A preliminary PSC device based on the blended copolymer P4:PCBM=1:2 (w/w) had the maximum power conversion efficiency (PCE) value up to 3.04%, which gave the best photovoltaic performance with the values of Isc = 8.00 mA/cm², FF = 53.7%, and Voc = 0.70 V as well as a peak EQE value of 60% under simulated AM1.5 solar illumination. These copolymers demonstrate a novel family of conjugated copolymers along the path toward achieving low cost PSC applications. Currently, deeper investigation for better photovoltaic properties is underway to further optimize the PSC performance.