A growing trend has been to incorporate electron- withdrawing substituents into the polymer structure, which in many cases have led to dramatic enhancements in solar cell performance.11, 21, 22, 65 It has already been demonstrated that they can effectively lower the HOMO and LUMO levels.66 However, researchers have yet to determine why these substituents, especially the fluorine atom, seem to have a good effect on the hole mobility, morphology, and charge dissociation of the polymer. The following section will categorize examples based on substituent location (on the electron-deficient acceptor moiety or the electron-rich donor moiety) and attempt to survey how photovoltaic properties are impacted.
3.1. Substitution on the Electron-Deficient Acceptor Moiety
Polymer backbones substituted with fluorines on the most electron-deficient unit have received widespread attention for their exceptional performance in solar cells. Three of the top polymers achieving over 7% efficiency contain the benzodithiophene (BnDT) unit copolymerized with a fluorinated acceptor moiety such as thienothiophene (TT),11 benzotriazole (TAZ),22 and benzothiadiazole (BT).21 Table 3 lists the photovoltaic properties compared with their nonfluorinated counterparts, and as can be seen, fluorinating the acceptor moiety seems to lead to better photovoltaic properties all around.
Table 3. Photovoltaic properties of high-performing fluorinated polymers and their nonfluorinated counterparts.
The PTB polymer series was the first to draw attention to incorporating fluorine into DONOR polymers and thus will be the main focus in this section because many studies have already been conducted on this series. Fluorine was originally introduced to the 3-position of the TT moiety as a second electron-withdrawing group (the first being the ester alkyl group) to further lower the HOMO level and therefore enhance Voc.65, 68 Studies on PTB have shown that fluorine only lowers the HOMO level by –0.15 eV (PTB9 vs. PTB7) while the Voc improves from only 0.60 to 0.74 V.11, 67
In an attempt to further optimize the HOMO level of PTB polymers, attention was turned toward other electron- withdrawing substituents. Table 4 shows the various methods in which TT has been modified. Interestingly, when TT was substituted with only a ketone (entry 3), the HOMO level was brought down to –5.12 eV, indicating that a ketone has a comparable electronic impact on PTB as do an ester and fluorine combined.69 When a ketone and fluorine were used in conjunction along with an alkyl chain on the BnDT unit (entry 4), the HOMO level significantly lowered to –5.34 eV.70 However, further attempts to use the even more electron-withdrawing sulfonyl again yielded a HOMO level of only –5.12 eV (entry 5).71 When Ikai and co-workers72 employed phenyl ester pendants 4-fluorophenyl and 4-(trifluoromethyl)phenyl, deep HOMO levels of –5.39 eV and –5.60 eV were observed (entries 6 and 7). However, polymers exhibited rather low mobilities (2.8 × 10−5 and 1.4 × 10−5 cm2 V−1 s−1, respectively), most likely due to the lack of a side chain on TT and the extremely bulky 2-octyldodecyloxy solubilizing chain that was needed on the BnDT unit.72
Table 4. Various methods of modifying thienothiophene and resulting photovoltaic properties.
In an effort to remove reliance on external substituents, a nitrogen atom was introduced into TT (entry 8), thereby changing the unit to the more electron-deficient thienothiazole (TTz), which can also stabilize its quinoid form.73 Initial results for PBnDT-TTz showed a higher efficiency of 2.5% compared with its direct TT analog, but the HOMO level of this TTz-based polymer was still not quite low enough. Just recently, Yu and co-workers67 reported selenium-based derivatives of their PTB series. The resulting –5.05 eV HOMO level of PBSe1 (entry 9) was similarly high as its sulfur-based analog (entry 1).
Of the various electron-withdrawing groups used, fluorine appears to be one of the most promising because it not only lowers the HOMO level but appears to improve morphology. PTB7, which achieved a previously record-breaking 7.4% efficiency,11 has demonstrated a very favorable morphology. The “zig-zag” shape of PTB's backbone is credited with being responsible for its face-on orientation, which allows for maximal contact with the electrode.74 Furthermore, a grazing incidence wide-angle X-ray scattering (GIWAXS) study proposes that within the active layer, a hierarchy exists ranging from PTB7 nanocrystallites > interpenetrating regions of polymer and fullerene > PCBM nanocrystallites (Figure 8).75 The PTB7 crystalline aggregates are believed to be responsible for the high photocurrent observed because its crystallinity not only reduces charge transfer energy, but also is similar in size to exciton diffusion lengths (4–20 nm). Thus, when an exciton is generated within a PTB7 nanocrystallite, the process toward dissociating charges is greatly facilitated (inset of Figure 8). Whether or not this proposed morphology is inherent to PTB polymers or due to fluorine has yet to be determined. Thus, other fluorinated systems, especially their morphology, should be further investigated.
Figure 8. Diagrammatic hypothesis of the hierarchical nanomorphologies in the PTB7/PCBM active layer. Reproduced with permission.75 Copyright 2011, American Chemical Society.
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Similar improvements in morphology are observed in the benzothiadiazole and benzotriazole-based polymers, both of which were fabricated without the use of additives.21, 22 When compared with their nonfluorinated counterparts, the x-ray diffraction (XRD) data of PBnDT–DTffBT and PBnDT–FTAZ both show larger d-spacing values: 18.1 versus 17.7 Å for benzothiadiazole polymers and 18.7 versus 17.8 Å for benzotriazole polymers. It is likely that the repulsive nature of the fluorine atoms is keeping PCBM further away during electron-transfer reactions, possibly enhancing electron-hole charge-transfer complex separation and slowing down processes such as charge recombination. However, additional studies beyond XRD are needed to accurately elucidate the behavior between fluorinated polymers with PCBM.
This then begs the question: is there a certain fluorine concentration that leads to optimum interactions with PCBM? Jen and co-workers76 examined nonfluoro-, monofluoro-, and difluoro-substituted benzothiadiazole polymers PIDT–BT, PIDT–FBT, and PIDT–DFBT (Table 5). As expected, the HOMO energy levels lowered and Voc increased with increasing fluorine concentration on the benzothiadiazole acceptor moiety. However, other properties such as Jsc, FF (fill factor), and hole mobility were roughly similar for all three polymers. Given that this is just one specific series, it would be interesting to see similar studies conducted on other systems. Such studies would gauge the influence of fluorine concentration on how polymers pack with fullerenes and the effect on charge recombination (geminate and bimolecular) to give further insight on charge transfer processes with PCBM.
Table 5. Photovoltaic properties of PIDT-BT, PIDT-FBT, PIDT-DFBT.
3.2. Substitution on the Electron-Rich Donor Moiety
Not all fluorine substitutions appear to be beneficial. When Yu and co-workers77 fluorinated the BnDT donor moiety (Scheme 1), solar cells performed poorly compared with PTB7.77 Similar to the previous strategy, fluorinating the BnDT unit was intended to fine-tune the HOMO level of PTB polymers. The resulting HOMO levels of PTBF2 and PTBF3 were indeed lowered by –0.26 and –0.33 eV, respectively. However, transmission electron micrographs (TEM) of the polymer–PCBM films for PTBF2 and PTBF3 revealed noncontinuous networks with large phase domains on the order of 50–200 nm (Figure 9), encouraging charge recombination and leading to dramatic decreases in Voc, FF, and efficiency. In addition to the difficulty of synthesizing the fluorinated BnDT unit, PTBF2 and PTBF3 were observed to be unstable. The fluorines on BnDT pull electron density away from the TT moiety, concentrating it on the 4- and 6-positions of TT, making the polymer vulnerable to singlet oxygen attack.
Figure 9. TEM images of polymer/PC71BM blend films prepared from dichlorobenzene solvent: PTBF0 (a), PTBF1 (b), PTBF2 (c), and PTBF3 (d). Scale bar = 200 nm. Reproduced with permission.77 Copyright 2011, American Chemical Society.
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3.3. Substituent Location
The improvement or decline in morphology of DONOR polymers is most likely related the location of the fluorine(s), more specifically which moiety is fluorinated. When the most electron-deficient unit is fluorinated (such as TT,11, 74 benzothiadiazole,21 or benzotriazole),22 the fluorines seem to keep PCBM at a distance creating phase domains (≈10–20 nm) that favor charge separation. It is unclear if this is a property inherent to these specific polymer systems because this favorable polymer–PCBM interaction is not observed when the electron-rich unit (BnDT) is fluorinated.77 From an electronic standpoint, this is in agreement with the weak donor-strong acceptor approach.78 The “weak donor” should be kept electron-rich and the “strong acceptor” should be as electron- deficient as possible.
In addition, a recent report by the Yu group suggests that electron-withdrawing groups should be placed such that the resulting local dipole moments do not cancel each other out based on their study of PTBF2 and PBB3.79 PTBF2 contains two opposing fluorines on the BnDT unit while PBB3 contains two adjacent TT units trans to another. In both cases, the internal dipole moment is greatly reduced according to calculations. Similar to PTB7, polymer PBB3 exhibits a good thin-film morphology, a high hole mobility, and even lower band gap (Table 6). Despite these favorable characteristics, PBB3 shows a comparatively low Jsc and thus efficiency of only 2.04%, suggesting that other factors need to be considered. Yu et al. propose that the minimized dipole moment in PTBF2 and PBB3 prevents the excited state from polarizing, leading to faster charge recombination and ultimately low power conversion efficiencies.
Table 6. Photovoltaic properties of PTB7, PTBF2, and PBB3.
The influence of fluorine on hole mobility, morphology, and other photovoltaic properties has yet to be quantified or correlated. Yu and co-workers65 suggest that there appears to be increased interaction between electron-rich aromatic rings and electron-deficient fluorinated aromatic rings. This is consistent with findings that fluorinated and nonfluorinated rings stack co-facially rather than in herringbone fashion as observed in traditional benzene rings.80, 81 Matsuo and co-workers82 have recently demonstrated that Ar—F··Ar—H and CH··F interactions help facilitate face-to-face stacking in FPPT compared with that in PPT (Scheme 2 and Figure 10), which leads to a hole mobility two orders of magnitude greater in FPPT.82 Although this study was done on small molecules for organic thin-film transistors, an analogous study in the context of DONOR polymers for solar cells would certainly be beneficial to further understand the interesting behavior of these fluorines. For example, would it be beneficial to have a 1:1 ratio of fluorinated to nonfluorinated rings? Would it be favorable for the donor and fluorinated acceptor moieties to be similarly shaped? More studies focused on the physical chemistry and device physics of carefully crafted systems are needed to elucidate fluorinated polymer–PCBM interactions and how morphology, hole mobility, local dipole moments, and charge recombination are affected.
Figure 10. Molecular design and concept for the enhancement of π−π stacking between neighboring charge transporting units by the introduction of Ar and FAr substituents. Reproduced with permission.82 Copyright 2011, American Chemical Society.
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