‘Blocky’ Donor-Acceptor Polymers Containing Selenophene, Benzodithiophene and Thienothiophene for Improved Molecular Ordering

Controlling the phase-separation behavior and achieving an ideal morphology has turned into one of the most important challenges in the field of polymer electronics. In this study we report a straightforward route to „blocky‟ copolymers that incorporates selenophene into a benzodithiophene (BDT)-thienothiophene (TT) donor-acceptor system for improved molecular ordering. The blocky structure preserves the optical properties of the parent polymers, which is different than an analogue employing purely statistical sequence. Peak force quantitative nanomechanical mode atomic force microscopy reveals a more ordered network-like morphology in blocky polymer:PC71BM films. However the photovoltaic properties of blocky polymers are still lower than the physical mixtures of the two parent polymers. This blocky copolymers approach can be applied to many other polymerization methods to prepare many new types of blocky D-A polymers. As such, it could be a new tool for tuning the polymer crystallinity, and eventually achieving controllable solid-state morphology for polymer electronic applications.


Introduction
)carbonyl]thieno [3,4-b]thiophenediyl}) (PTB7), 3 which reach champion power conversion efficiencies of 7.2% [4][5][6] , 5.8% 2 , and 9.2% 7 , respectively.For all of the above materials the solubilizing side-chains were optimized to obtain the best photovoltaic performances.Yet these examples are few in number relative to the vast reported structures that do not reach acceptable performance yet have "ideal" or near ideal optical properties and HOMO-LUMO energy levels.While side-chain engineering has been the most important method for tuning the polymer solubility and solid-state properties, finding the optimal morphology is almost a trialand-error process and requires major synthetic efforts.This is due to the extreme difficulty in predicting solid-state morphology.Controlling phase-separation with an electron acceptor and achieving an ideal morphology has indeed turned into one of the most important challenges in the field.
Crystalline polymer domains play an important role in improving charge separation, charge carrier mobility, and device stability. 8,9However many D-A polymers have low crystallinity.For instance, Grazing Incidence Wide-angle X-ray Scattering (GIWAXS) analysis shows that poly [4,8- ethylhexanoyl)-thieno [3,4-b]thiophene-)-2-6-diyl)] (PBDTTT-C-T) has no clear π-π stacking reflection, highlighting the very low crystallinity of this polymer. 10On the other hand, recent studies have showed introducing heavy atoms into the polymer backbone can increase the tendency of the polymer to form better ordered phases, leading better charge carrier mobility. 11- 21But doing so may also decrease the solubility and increase the difficulty of polymer purification and processing. 22,23Therefore it would be interesting to develop polymers that contain a D-A structure with some incorporation of heavy atoms and ideally achieve a block-like sequence as a means to improve the molecular ordering of polymer.
In this study we report a straightforward route to blocky copolymers that incorporate selenophene into a benzodithiophene (BDT)-thienothiophene (TT) donor-acceptor system.
Because the HOMO energy level of D-A polymer is mostly localized at donor moieties,

Polymer Chemistry Accepted Manuscript
substituting TT with selenophene retains the delocalization of HOMO level over the polymer.
Meanwhile, formation of blocks preserves the optical properties of the parent polymers, which is different to an analogue employing statistical sequence.The photovoltaic properties of blocky polymers are compared together with the statistical polymer and physical mixtures of the parent polymers, which give us a clearer idea of the phase-separation behavior of the blocky polymer in films that also contain fullerene-acceptors.

Polymer synthesis and characterization
The blocky polymers were synthesized through a three-step Stille coupling (Scheme 1).Briefly, donor-acceptor polymer fragments with trimethyltin or Br end-groups were prepared by using an excess of one of the monomer type (either di-halo or di-stanyl coupling partner) and then coupled together in a third polymerization.Polymer fragments incorporating selenophene (PBDTSe-T fragments) with trimethyltin end-groups were synthesized with a benzodithiophene (BDT):selenophene monomer ratio of 1.2:1.After heating to reflux in toluene for 4 hours, the reaction mixture of PBDTSe-T fragments was precipitated into hexane and washed twice to remove excess monomer and low molecular weight fragments.Three complementary fragments (PBDTTT-C-T fragments) with Br end-groups were prepared with varied BDT: thienothiophene monomer ratios (1:1.1, 1:1.15 and 1:1.2 mol:mol), which is in order to produce PBDTTT-C-T fragments with different molecular weights.Each PBDTTT-C-T fragment product was precipitated into methanol, which also eroded any remaining trimethyltin end-groups.The precipitate was then placed in a Soxhlet apparatus and wash with hexanes and extracted with chloroform.Finally, a third polymerization was carried out with equal amounts (mol:mol) of PDBTSe-T and PDBTTT-C-T fragments.The ratio of selenophene and TT monomers in the three blocky polymers are 1:1.59,1:1.16 and 1:0.85, based on molecular weights of their respective PBDTSe-T fragments and PBDTTT-C-T fragment reactants (Table 1).Two parent (PDBTTT-C-T) 24 and a statistical copolymer with a Se and TT monomer ratio of 1:1 (mol:mol) were also synthesized to complete the study.

Polymer Chemistry Accepted Manuscript
All blocky polymers incorporated PBDTSe-T fragment with number average molecular weight (Mn) of 8.0 kDa.Molecular weights of PBDTTT-C-T fragments were 15.3 kDa, 10.9 kDa and 8.0 kDa, for blocky polymer with selenophene:thienothiophene monomer ratio (mol:mol) of 1:1.59, 1:1.16 and 1:0.85, respectively.Fragment coupling and formation of blocky polymers was confirmed by GPC analysis.After the third polymerizations a new elution peak was observed at smaller retention volume in all cases (Figure 1 and S1, Table 1).Polymer molecular weights are roughly doubled compared with fragment reactants, indicating most of polymers are di-block structures.However it is also clear from the GPC that a small amount of unreacted parent polymer or multi-blocks polymer is present.Therefore we use the term "blocky" to describe the structures of polymer studied here.
Absorption spectra were collected for both solutions (Figure 2) and thin films (Figure 3).The absorption profiles of blocky polymers have distinct peaks that correspond to the two parent structures.This is in contrast to the statistical copolymer, which only has one broad absorption band at a longer wavelength.Optical energy gaps were determined by the onset of thin film absorptions (Table 1).The optical properties of two parent structures are preserved in the blocky polymers.
The ordering of blocky and statistical polymers was first investigated with X-ray diffraction (Figure S2 and Figure S3).The broad diffraction peak at 2θ ~ 23.3° may come from Si Wafer with SiO 2 . 25The blocky polymers also have a weak reflection at 2θ ~ 4.5°, corresponding to an interlayer spacing of ~ 20 Å.This peak is more obvious in polymer:PC 71 BM blend films due to the lower background signal.On the other hand the statistical polymer did not produce a clear interlayer spacing reflection.Interestingly, the 1:1.16 blocky polymer shows an additional signal at 2θ ~ 3.2°.This signal may come from different orientations of polymer backbones.

Morphology characterization
To further study the solid state morphology of blocky polymers, atomic force microscopy (AFM) images of polymer:PC 71 BM blend films were collected by both tapping mode and peak force quantitative nanomechanical mode.In tapping mode phase images (Figure 4), fiber-like features can be observed for 1:1.59 and 1:1.16 blocky polymer:PC 71 BM films, in contrast with their physical mixture analogues that have feature-less phase images.On the other hand structures that appear in the phase image of 1:0.85 blocky polymer:PC 71 BM film are more aggregated and less ordered, similar to that of the 1:0.85 physical mixture: PC 71 BM film which may due to their higher PBDTSe-T fragment ratios.Peak force quantitative nanomechanical (PF QEM) mode AFM allows one to map the adhesion force between the sample and the AFM tip.This depends on the chemical composition of the sample area and is less affected by surface topology.All the three blocky polymer:PC 71 BM films are networks composed of long and straight fibers (Figure 5).This is in contrast to the more randomly packed networks of parent polymer:PC 71 BM films (Figure S9) or the disordered short features observed in the statistical polymer or physical mixture films (Figure S9  To further investigate the photovoltaic properties of blocky polymers, we also fabricated devices utilizing physical mixtures of the two parent polymers.The ratios of selenophene and thienothiophene monomers in the physical mixtures are 1:1.59,1:1.16 and 1:0.85, corresponding to the ratios of selenophene and thienothiophene monomers in the three blocky analogues.
Devices utilizing physical mixtures have the same V OC values as blocky polymer devices.
However, increasing the amount of PBDTTT-C-T polymer in the physical mixture from 1:0.85 to 1:1.59 increases the J SC from 13.7 ± 0.2 mA/cm 2 to 14.5 ± 0.2 mA/cm 2 , which is in contrast with the consistent J SC values of blocky polymer devices.Mixture devices also have maximum EQE at ~500 nm (Figure S2), similar to blocky devices.At wavelength beyond 700 nm, where neither PBDTSe-T nor PC 71 BM absorb light, the EQE of physical mixture devices are decreased when the amount of PBDTTT-C-T was reduced.This behavior is different than that of blocky polymers, where the EQE at longer wavelength is nearly identical.the FF of physical mixture devices increases to 63~64%, which is higher than the FF of devices utilizing either the blocky or the two parent polymers.As a result better power conversion efficiencies are observed by mixture devices, while devices incorporating 1:1.59 (mol:mol) PBDTSe-T:PBDTTT-C-T mixture exhibited efficiency of 6.8 ± 0.2%.This is even higher than the efficiency of 6.6 ± 0.3% for device utilizing only PBDTTT-C-T polymer.The improved FF may come from more efficient charge dissociation or better charge transport in polymer:PCBM blend.
Measuring photocurrent as a function of applied voltage can show the field-dependent charge dissociation in solar cell devices. 26,27At low applied voltage, devices utilizing PBDTSe-T polymer, PBDTTT-C-T polymer and 1:1.59 blocky polymer have photocurrents that increase linearly with effective voltage, which is due to the direct correlation between the constant diffusion and drift current. 26The photocurrent of the blocky polymers were nearly identical to that of PBDTTT-C-T, indicating similar charge generation rate and dissociation efficiency of bound electron-hole pair.On the other hand the effective photocurrent of physical polymer mixtures was the same as PBDTSe-T at lower applied voltage (V 0 -V <0.1 V).Higher applied voltage increases the charge generation, and the photocurrent saturated at a similar voltage as the blocky polymer.This behavior shows a lower charge carrier dissociation efficiency in the physical mixture devices, which needs stronger electric field to dissociate bound electron-hole pairs at the donor-acceptor interface.Therefore the higher FF of mixture device is most likely from improved charge transport or reduced bimolecular recombination, both of which result from the formation of separated PBDTSe-T phases in mixed film.This facilitate more efficient charge extraction with energy levels cascades. 28,29On the other hand, the different charge dissociation efficiency between blocky and physical mixture cells, and the similarity of their

Summary
We report here a straightforward synthetic route to blocky D-A copolymers consisting selenophene, benzodithiophene, and thienothiophene.This approach can be applied to many other polymerization methods to prepare many new types of blocky D-A polymers.The blocky polymer structure preserves the optical properties of their respective two-component systems.In this case, the difference between blocky polymers and their analogous physical mixtures indicates PBDTSe-T fragments in the blocky polymers do not form separated phases, but form crystalline region containing both fragments.Though more ordered morphologies are observed with blocky polymers, the physical mixture of parent polymers perform better in solar cell devices, which is not well corresponded to the AFM results.The polymer crystallinity can be further modified by changing lengths of different blocks.As such, blocky copolymers could be a new tool for tuning the polymer crystallinity, and eventually achieving controllable solid-state morphology for organic electronic applications.
chemical components indicates PBDTSe-T fragments in the blocky polymers do not form separated phases..

Figure 7
Figure 7 Photocurrent versus applied voltage for devices of PBDTSe-T, PBDTTT-C-T, 1:1.59 blocky polymer and 1:1.59 physical mixture.The open circuit (V=V OC ) and short circuit (V=0) points are marked as stars and crosses, respectively.