Chapter 5 Supramolecular Assembly of H-Bonded Side-Chain Polymers
5.3.5 Photovoltaic Cell Properties
The design and syntheses of the H-bonded side-chain polymers P1/S1-P1/S4 and
P2/S1-P2/S4 is to utilize new narrow band-gap H-donor dyes self-assembled with side-chain conjugated H-acceptor polymers into supramolecular polymeric structures for the PSC applications. To investigate the potential use of H-bonded complexes in PSCs, bulk heterojunction PSC devices with a configuration of ITO/PEDOT:PSS/H-bonded polymer complexes:PCBM (1:1 w/w)/Ca/Al were fabricated from an active layer where H-bonded complexes 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 H-bonded polymer complex). The PSC devices were measured under AM 1.5 illumination for a
calibrated solar simulator with an intensity of 100 mW/cm2. The preliminarily photovoltaic properties are summarized in Table 5.4, and the typical I-V characteristics of all PSC devices are shown in Figure 5.10. Under the white-light illumination, the short circuit current density (Isc), open circuit voltage (Voc), fill factor (FF), and power conversion efficiency (PCE) values of the PSC devices composed of H-bonded polymer complexes were in the range of 0.42-3.17 mA/cm2, 0.38-0.59 V, and 24-34%, 0.06-0.50%, respectively.
The photovoltaic properties of the PSC devices containing H-bonded polymer complexes P1/S1-P1/S4 and P2/S1-P2/S4 were dependent on the solubility and film-forming quality of the H-bonded complexes. However, the PCE values of H-bonded complexes P2/S1-P2/S4 containing H-acceptor copolymer P2 were apparently smaller than those of P1/S1-P1/S4, respectively, because the 1:1 molar ratio of pyridyl and acid units of fully H-bonded P2/S1-P2/S4 would reduce the content of low-band-gap dyes complexed with H-acceptor copolymer P2 bearing 50 mol% of pyridyl units. As shown in Table 5.4, both series of H-bonded complexes (P1/S1-P1/S4 and P2/S1-P2/S4) containing electron donors of end-capping triphenylamine dyes (S1and S3) had better PCE values than those containing end-capping carbazole dyes (S2 and S4), respectively. It might be owing to the larger aggregations of end-capping carbazole dyes (S2 and S4) to reduce the PCE values,
which were confirmed by the redder shifted maximum absorption wavelengths (Δλabs
= λabs,solid - λabs,solution) in solid films of end-capping carbazole dyes (Δλabs = 16 nm for
S2 and Δλabs = 30 nm for S4) than those of end-capping triphenylamine dyes (Δλabs = 11 nm for S1 and Δλabs = 23 nm for S3), respectively. Among the PSC devices containing H-bonded polymer complexes, those composed of H-donor dye S3 , i.e.,
P1/S3 and P2/S3, had the best photovoltaic performance with enhanced Isc values in the corresponding H-bonded complexes P1/S1-P1/S4 and P2/S1-P2/S4, respectively, which might be due to the promoted solubility and better film-forming capability of
S3. Ideally, the Isc values were determined by the product of the photoinduced charge carrier densities and the charge carrier mobilities within the organic semiconductors.108b Thus, it can be recognized that the better results of Isc and FF in the PSC device containing P1/S3 and P2/S3 were obtained likely due to the well-balanced charge flow and less significant recombination loss113,129 originated from the highly order structural packing of alkyl side chains. However, the relatively low Isc and FF values in the PSC devices containing P1/S4 and P2/S4 were poorly understood at this time, but it might be related to the largest aggregation of S4 (with the reddest shifted maximum absorption wavelengths in solid, Δλabs = 30 nm) and geminate charge recombination at the interface due to stable charge-transfer states, which limited the values of the photocurrents.130 Though Voc values were related to
the differences between the HOMO energy levels of the polymers and the LUMO energy levels of the acceptors,131 but it was not noticeably varied among the PSC devices containing H-bonded complexes. In addition, the I-V curves and photovoltaic properties of dyes S1-S4 without complexation with the polymers are illustrated in Table 5.5, which can be compared with the PCE values shown in Table 5.4. In general, the H-bonded polymer complexes containing H-acceptor polymers (P1 and P2) have higher PCE values for the organic solar cells, even though the corresponding dye contents of S1-S4:PCBM = 1:1 (w/w) without complexation with polymers in the active layer were almost doubled than those of H-bonded polymer complexes (H-bonded polymer complexes:PCBM = 1:1 w/w). Hence, H-acceptor polymers (P1 and P2) do really improve and facilitate the fabrication of solar cells. In order to demonstrate the contribution of supramolecular structures in H-bonded polymer complexes, one more PSC device containing an active layer of physical blend P1/S1P (without H-bonds) has been fabricated to compare their photovoltaic properties with those of H-bonded polymer complex P1/S1. Compared with H-bonded polymer complex P1/S1 (PCE = 0.28%), physical blend P1/S1P (without H-bonds) has a smaller PCE value (0.16%) in Table 5.6. The larger aggregations of the acid-protected dye S1P occurred in the polymer blend P1/S1P due to the lack of H-bonding interactions, which also can be confirmed by the red-shifted (33 nm) absorption of
polymer blend P1/S1P in contrast to that of H-bonded polymer complex P1/S1 in solid films.
Since the best performance of PSC device (with the highest PCE value in Table 5.4) was fabricated by the blend of H-bonded polymer complex P1/S3:PCBM (1:1 w/w), the current-voltage characteristics of PSC devices as a function of the weight ratio in H-bonded complex and PCBM were surveyed, and their photovoltaic properties are shown in Figure 5.11 and Table 5.7. The optimum photovoltaic performance with the maximum PCE value of 0.50% (Isc = 3.17 mA/cm2, Voc = 0.47 V, FF = 34%) was obtained in the PSC device having a weight ratio of
P1/S3:PCBM=1:1. Using lower weight ratios of PCBM in blended H-bonded polymer
complex P1/S3:PCBM (2:1 w/w) led to the reduction in Isc values due to the inefficient charge separation and electron transporting properties by the possibly increased aggregation of H-bonded complex P1/S3, resulting in the lower PCE results.132 However, loading larger weight ratios of PCBM in blended H-bonded polymer complex P1/S3:PCBM (1:2 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 affect the separation of charges. Moreover, an unbalanced charge transporting property would be introduced due to the large PCBM ratio. Hence, both Isc and PCE values decreased with larger PCBM molar ratios of 1:2 and 1:4 (w/w) because of the
two reasons described here.133 Overall, the PSC device fabricated by H-bonded polymer complexes P1/S3:PCBM (1:1 w/w) reached the highest power conversion efficiency (PCE) of 0.50%, with a short circuit current density (Isc) of 3.17 mA/cm2, an open circuit voltage (Voc) of 0.47 V, and a fill factor (FF) of 0.34.
Table 5.4 Photovoltaic Properties of PSC Devices Containing an Active Layer of H-Bonded Polymer Complexes:PCBM = 1:1 (w/w) with a Device Configuration
of ITO/PEDOT:PSS/H-Bonded Polymer Complexes:PCBM/Ca/Ala Active layerb
a Measured under AM 1.5 irradiation, 100 mW/cm2.
b H-Bonded Polymer Complexes:PCBM = with the fixed weight ratio of 1:1 (w/w).
Table 5.5 Photovoltaic Properties of PSC Devices Containing an Active Layer of Dyes S1-S4:PCBM = 1:1 (w/w) with a Device Configuration of ITO/PEDOT:PSS/
Dyes S1-S4:PCBM/Ca/Ala
a Measured under AM 1.5 irradiation, 100 mW/cm2.
b Dyes S1-S4:PCBM = with the fixed weight ratio of 1:1 (w/w).
Table 5.6 Photovoltaic Properties of PSC Devices Containing an Active Layer of Mixture:PCBM = 1:1 (w/w) with a Device Configuration of ITO/PEDOT:PSS/
Mixture:PCBM/Ca/Ala
a Measured under AM 1.5 irradiation, 100 mW/cm2.
b P1/S1 and P1/S1P:PCBM = with the fixed weight ratio of 1:1 (w/w).
Table 5.7 Photovoltaic Parametersfor Bulk-Heterojunction PSC Devices Containing Different Weight Ratios of Blended H-Bonded Polymer Complex
P1/S3:PCBMa
a PSC devices with the configuration of ITO/PEDOT:PSS/H-Bonded Polymer Complex P1/S3:PCBM/Ca/Al containing an active blended layer composed of various weight ratios of H-Bonded Polymer Complex P1/S3 and PCBM were measured under AM 1.5 irradiation, 100 mW/cm2.
(a)
(b)
Figure 5.10 I-V curves (under simulated AM 1.5 solar irradiation) dependencies of PSC devices with an active layer of blended (a) H-bonded polymer complexes P1/S1-P1/S4:PCBM (1:1 w/w) and (b) H-bonded polymer complexes P2/S1-P2/S4:PCBM (1:1 w/w).
Figure 5.11 I-V curves of PSC devices containing an active layer of H-bonded polymer complex P1/S3:PCBM (w/w) with different weight ratios under simulated AM 1.5 solar irradiation.
5.4 Conclusions
In conclusion, novel supramolecular side-chain polymers (i.e., H-bonded polymer complexes) were constructed by complexation of pyridyl H-acceptor polymers with low-band-gap H-donor dyes in a molar ratio of 1:1 for pyridyl and acid units, which would have much more uptaken loads of photovoltaic dyes in the supramolecular polymeric structures compared with the normal polymer blends. Due to the lack of supramolecular interactions, the larger aggregations of the acid-protected dyes occurred in the polymer blends, and thus a polymer blend (without H-bonds) containing conjugated H-acceptor polymer P1 and acid-protected dye S1P illustrated an obvious reduction in the PCE value in contrast to the
0.0 0.1 0.2 0.3 0.4 0.5
and P2) were utilized to control the mesomorphic, photophysical, and photovoltaic properties effectively by the concept of supramolecular architecture. The supramolecular architectures of H-bonded side-chain polymers were also confirmed by FTIR and XRD measurements. The nematic phase was observed by the introduction of various H-donor dyes and H-acceptor polymers with corresponding supramolecular side-chain structures. In addition, compared with H-donor dyes, the optical properties demonstrated that blue-shifted absorptions occurred in these H-bonded complexes as the H-donor dyes were complexed with H-acceptor polymers.
Thus, the electrochemical properties of H-bonded complexes were adjusted by introducing electron-withdrawing cyano groups and electron-donating amine groups of H-donor dyes to the H-bonded complexes, which could reduce the HOMO energy levels and increase the LUMO energy levels of the H-bonded side-chain polymers, and thus to have narrower band-gaps than H-acceptor polymers. Due to the reduced content of low-band-gap dyes complexed with H-acceptor copolymer P2, the PCE values of H-bonded complexes P2/S1-P2/S4 containing H-acceptor copolymer P2 were apparently smaller than those of P1/S1-P1/S4, respectively. Preliminary PSC devices based on these H-bonded polymer complex P1/S3 blended with PCBM acceptors (1:1 w/w) had the power conversion efficiency up to 0.50%, which gave the best performance with the values of Isc = 3.17 mA/cm2, Voc = 0.47 V, and FF = 34%.
Chapter 6 Conclusion
In conclusion, first, H-donors (asymmetric mono-functional H-donors and symmetric bi-functional H-donor) and H-acceptor polymers were utilized to control the mesomorphic and photoluminescent properties effectively by the concept of supramolecular architecture. The H-acceptor copolymers were composed of different molar ratios of pendent N-vinylcarbazole units and light-emitting H-acceptor groups randomly to increase the glass transition temperatures and to reduce the π-π stacking of the conjugated H-acceptor chormorphores in the copolymers as well as in their H-bonded polymer complexes. The supramolecular architectures of H-bonded side-chain/cross-linking polymers were also confirmed by FTIR and XRD measurements. They have distinct mesomorphism and phase transition temperatures related to their supramolecular structures with different nonlinearities and rigidities.
The mesomorphic properties were changed from the nematic phase to the smectic C phase by the introduction of H-bonds to the supramolecular polymers, and then shifted to the nematic and non-mesogenic phases by various H-donor acids and H-acceptor copolymers with corresponding supramolecular side-chain/cross-linking structures. In addition, the emission color of light-emitting H-acceptor polymers can be tuned by their surrounding non-emitting H-donors. Redder shifts in PL emissions
were observed in the H-bonded supramolecules with H-donors having smaller pKa values.
Second, the mesomorphic and photoluminescent properties of H-bonded copolymers and homopolymer complexes are affected by the isomeric H-donors with different acid-substituted (para-, meta-, and ortho-) positions. These H-bonded complexes can generate supramolecular architectures either by self-H-bonded copolymers or by π-conjugated H-acceptor homopolymers blended with H-donor homopolymers. The supramolecular structures have the nematic and smectic C phases that are related to their bent and linear H-bonded structures, respectively. The photoluminescent properties of self-H-bonded copolymers, as well as the H-bonded homopolymer complexes, can be tuned by the isomeric H-donor moieties, and red-shifted PL emissions are expected in the H-bonded structures. Supramolecular architectures that contain H-bonds in self-H-bonded copolymers P1-P3 and H-bonded homopolymer complexes PBT1/P7-P9 are further confirmed by FT-IR spectroscopy and XRD measurements. Furthermore, the H-bonding interactions between acid-modified gold nanoparticles (AuSCOOH) and acid-protected copolymer P4 affect the fluorescence quenching more effectively, when compared with fluorescence titrations of acid-free-modified gold nanoparticles (AuSC10). Moreover, in contrast to the self-H-bonded copolymer P1, the acid-protected copolymer P4 more readily
captures acid-modified gold nanoparticles (AuSCOOH) in the suparmolecular assembly of nanocomposites. The H-bonding interactions between the pyridyl H-acceptor (from the copolymers) and the acid H-donor units (from both nanoparticles and copolymers) can explain the similarities in fluorescence quenching effects on both copolymers P1 and P4. Various nanocomposites containing two kinds of fluorescent copolymer counterparts (self-H-bonded copolymer P1 and acid-protected copolymer P4) and surface-modified nanoparticles (acid-modified
AuSCOOH and acid-free-modified AuSC10) were developed to display distinct aggregation phenomena in TEM images. The pyridyl H-acceptor units of copolymer
P1 would not only bind with its own para-benzoic acid groups but also with the other
proton donor surfactants from AuSCOOH nanoparticles. Overall, this study is the first to explore the suparmolecular assembly behavior of nanocomposites between fluorescent copolymers and surface-functionalized gold nanoparticles via both PL quenching phenomena and TEM morphologies. Based on the fluorescence quenching and recovery of gold nanocomposites, further chemosensor and biosensor applications of this study can be developed in the near future.
Third, supramolecular nanocomposites containing two series fluorescent polymers with different pyridyl content, lateral methyl-substituted polymers
PBT1-PBT3 and lateral methoxyl-substituted polymers PBOT1-PBOT3, and donor
nanoparticles were developed to display distinct aggregation phenomena in TEM images. H-acceptor polymers PBT1-PBOT3 would not only display highly aggregation but also well dispersion with the proton donor surfactants from nanoparticles AuSCOOH. Furthermore, the H-bonding interactions between acid-modified gold nanoparticles (AuSCOOH) and lateral methoxyl-substituted polymer PBOT1 affect the fluorescence quenching more effectively, when compared with fluorescence titrations of acid-free-modified gold nanoparticles (AuSC10).
Moreover, in contrast to the lateral methyl-substituted polymer PBT1, lateral methoxyl-substituted polymer PBOT1 more readily captures acid-modified gold nanoparticles (AuSCOOH) in the suparmolecular assembly of nanocomposites. The H-bonding interactions between the pyridyl H-acceptor (from the polymers) and the acid H-donor units (from both nanoparticles and polymers) can explain the similarities in fluorescence quenching effects on both polymers PBOT1 and PBT1.
Moreover, we established the exponential equation to predict Stern-Volmer constant in varous pyridyl units of polymers according to these experimental information.
Various nanocomposites containing two kinds of fluorescent polymer counterparts (methoxyl-substituted polymer PBOT1 and methyl-substituted polymer PBT1) and surface-modified nanoparticles (acid-modified AuSCOOH and acid-free-modified
AuSC10) were developed to display distinct aggregation phenomena in TEM images.
Based on the fluorescence quenching and recovery of gold nanocomposites, further chemosensor and biosensor applications of this study can be developed in the near future.
Finally, novel supramolecular side-chain polymers (i.e., H-bonded polymer complexes) were constructed by complexation of pyridyl H-acceptor polymers with low-band-gap H-donor dyes in a molar ratio of 1:1 for pyridyl and acid units, which would have much more uptaken loads of photovoltaic dyes in the supramolecular polymeric structures compared with the normal polymer blends. Due to the lack of supramolecular interactions, the larger aggregations of the acid-protected dyes occurred in the polymer blends, and thus a polymer blend (without H-bonds) containing conjugated H-acceptor polymer P1 and acid-protected dye S1P illustrated an obvious reduction in the PCE value in contrast to the supramolecular analogue
P1/S1. H-donor dyes (S1-S4) and H-acceptor polymers (P1 and P2) were utilized to control the mesomorphic, photophysical, and photovoltaic properties effectively by the concept of supramolecular architecture. The supramolecular architectures of H-bonded side-chain polymers were also confirmed by FTIR and XRD measurements.
The nematic phase was observed by the introduction of various H-donor dyes and H-acceptor polymers with corresponding supramolecular side-chain structures. In addition, compared with H-donor dyes, the optical properties demonstrated that
blue-shifted absorptions occurred in theseH-bonded complexes as the H-donor dyes were complexed with H-acceptor polymers. Thus, the electrochemical properties of H-bonded complexes were adjusted by introducing electron-withdrawing cyano groups and electron-donating amine groups of H-donor dyes to the H-bonded complexes, which could reduce the HOMO energy levels and increase the LUMO energy levels of the H-bonded side-chain polymers, and thus to have narrower band-gaps than H-acceptor polymers. Due to the reduced content of low-band-gap dyes complexed with H-acceptor copolymer P2, the PCE values of H-bonded complexes P2/S1-P2/S4 containing H-acceptor copolymer P2 were apparently smaller than those of P1/S1-P1/S4, respectively. Preliminary PSC devices based on these H-bonded polymer complex P1/S3 blended with PCBM acceptors (1:1 w/w) had the power conversion efficiency up to 0.50%, which gave the best performance with the values of Isc = 3.17 mA/cm2, Voc = 0.47 V, and FF = 34%.
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