Importance of 2D Conjugated Side Chains of Benzodithiophene-Based Polymers in Controlling Polymer Packing, Interfacial Ordering, and Composition Variations of All-Polymer Solar Cells
Changyeon LeeThota GiridharJoonhyeong ChoiSeonha KimYoungwoong KimTaesu KimWonho LeeHan‐Hee ChoCheng WangHarald AdeBumjoon J. Kim
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We delineate the important role of 2D conjugated alkylthiophene side chains of polymers in manipulating the molecular orientation and ordering at the polymer donor/polymer acceptor (PD/PA) interface as well as the composition variations in the blend active layer of all-polymer solar cells (all-PSCs). To systematically investigate the impact of 2D conjugated side chains on the performance of all-PSCs, we synthesized a series of poly(benzo[1,2-b:4,5-b̀]-dithiophene-thieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) polymer donors with different contents of alkoxy and alkylthiophene side chains, from 0 to 100% (PBDT-TPD (P1, 100% alkoxy side chain), PBDTT0.29-TPD (P2), PBDTT0.59-TPD (P3), PBDTT0.76-TPD (P4), and PBDTT-TPD (P5, 100% alkylthiophene side chain). The P1–P5 polymer donors produced similar PCEs of ∼6% in fullerene-based PSCs. In contrast, for the all-PSC systems, the changes in the side chain composition of the polymers induced a strong increasing trend in the power conversion efficiencies (PCEs), from 2.82% (P1), to 3.16% (P2), to 4.41% (P3), to 5.32% (P4), and to 6.60% (P5). The significant increase in the PCEs of the all-PSCs was attributed mainly to improvements in the short-circuit current density (JSC) and fill factor (FF). The 2D conjugated side chains promoted localized molecular orientation and ordering relative to the PD/PA interfaces and improved domain purity, which led to enhanced exciton dissociation and charge transport characteristics of the all-PSCs. Our observations highlight the advantage of incorporating 2D conjugated side chains into polymer donors for producing high-performance all-PSC systems.Keywords:
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An investigation on the opto-electronic and photovoltaic properties of a pair of alkoxy substituted quinoxaline-based copolymers PTTQx is performed in order to describe the effect of changing the position of alkoxy substituents on the peripheral phenyl rings. The copolymer with meta-positioned alkoxy showed lower HOMO and LUMO levels and a higher Voc of 0.73 V, while the copolymer with para-positioned alkoxy displayed higher HOMO and LUMO levels and lower Voc of 0.60 V when a polymer/PC71BM blend film was used as the active layer in polymer solar cells (PSCs) under AM 1.5 G irradiation (100 mW cm−2). With the good agreement between theoretical calculation and experimental observation, it has been observed that the effect of the substituents depends on the position of the alkoxy group which exhibits a stronger electron donating effect in the para-position than in the meta-position. The resonance electron donating effect of the alkoxy group on the para-position can elevate the HOMO and LUMO levels simultaneously, while this effect is not obviously reflected on the meta-position. Therefore, PTTQx-m exhibits lower HOMO level, higher Voc correspondingly and thereby higher PCE of the PSCs based on it.
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We delineate the important role of 2D conjugated alkylthiophene side chains of polymers in manipulating the molecular orientation and ordering at the polymer donor/polymer acceptor (PD/PA) interface as well as the composition variations in the blend active layer of all-polymer solar cells (all-PSCs). To systematically investigate the impact of 2D conjugated side chains on the performance of all-PSCs, we synthesized a series of poly(benzo[1,2-b:4,5-b̀]-dithiophene-thieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) polymer donors with different contents of alkoxy and alkylthiophene side chains, from 0 to 100% (PBDT-TPD (P1, 100% alkoxy side chain), PBDTT0.29-TPD (P2), PBDTT0.59-TPD (P3), PBDTT0.76-TPD (P4), and PBDTT-TPD (P5, 100% alkylthiophene side chain). The P1–P5 polymer donors produced similar PCEs of ∼6% in fullerene-based PSCs. In contrast, for the all-PSC systems, the changes in the side chain composition of the polymers induced a strong increasing trend in the power conversion efficiencies (PCEs), from 2.82% (P1), to 3.16% (P2), to 4.41% (P3), to 5.32% (P4), and to 6.60% (P5). The significant increase in the PCEs of the all-PSCs was attributed mainly to improvements in the short-circuit current density (JSC) and fill factor (FF). The 2D conjugated side chains promoted localized molecular orientation and ordering relative to the PD/PA interfaces and improved domain purity, which led to enhanced exciton dissociation and charge transport characteristics of the all-PSCs. Our observations highlight the advantage of incorporating 2D conjugated side chains into polymer donors for producing high-performance all-PSC systems.
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Rational design and synthesis of polymeric semiconductors is critical to the development of polymer solar cells (PSCs). In this work, a new series of benzodithiophene–difuranylbenzooxadiazole-based donor–acceptor co-polymers—namely, PBDT-DFBO, PBDTT-DFBO, and PBDTF-DFBO, with various side groups—have been developed for bulk-heterojunction PSCs. These side-group substituents provide the opportunity to tailor the opto-electrical properties of the polymers. In addition, we show that the reduction of the bandgap of polymers and the enhancement of charge mobility in the devices can be accomplished concurrently by substituting the alkylthienyl side group with its furan counterpart. In the preliminary investigation, one could obtain PSCs with a power conversion efficiency (PCE) of 2.1% for PBDT-DFBO with an alkoxyl side chain, 2.2% for PBDTT-DFBO with an alkylthienyl side group, and 3.0% for PBDTF-DFBO with an alkylfuranyl side group. Further optimizing the performance of the devices was conducted via a simple solvent treatment. The PSCs based on PBDTF-DFBO:PC71BM could even achieve 7.0% PCE, which exhibited an enhancement of 130%. To the best of our knowledge, the value of 7.0% is the highest efficiency for furan-containing PSCs to date and is also comparable with the hitherto reported highest efficiency of the single junction PSCs. Through a combination of testing charge transport by the space-charge limited current (SCLC) model and examining the morphology by atomic force microscopy (AFM), we found that the effects of solvent treatment on the improved performance originate from higher and more balanced charge transport and the formation of fiberlike interpenetrating morphologies, which are beneficial to the increase of short-circuit current density (Jsc) and fill factor (FF). This work demonstrates a good example for tuning absorption range, energy level, charge transport, and photovoltaic properties of the polymers by side-chain engineering and the solvent treatment can offer a simple and effective method to improve the efficiency of PSCs.
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The molecular packing structures of two conjugated polymers based on alkoxy naphthalene, one with cyano-substituents and one without, have been investigated to determine the effects of electron-withdrawing cyano-groups on the performance of bulk-heterojunction solar cells. The substituted cyano-groups facilitate the self-assembly of the polymer chains, and the cyano-substituted polymer:PC71BM blend exhibits enhanced exciton dissociation to PC71BM. Moreover, the electron-withdrawing cyano-groups lower the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the conjugated polymer, which leads to a higher open circuit voltage (V(OC)) and a lower energy loss during electron transfer from the donor to the acceptor. A bulk-heterojunction device fabricated with the cyano-substituted polymer:PC71BM blend has a higher V(OC) (0.89 V), a higher fill factor (FF) (51.4%), and a lower short circuit current (J(SC)) (7.4 mA/cm(2)) than that of the noncyano-substituted polymer:PC71BM blend under AM 1.5G illumination with an intensity of 100 mW cm(-2). Thus, the cyano-substitution of conjugated polymers may be an effective strategy for optimizing the domain size and crystallinity of the polymer:PC71BM blend, and for increasing V(OC) by tuning the HOMO and LUMO energy levels of the conjugated polymer.
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The ternary approach using a smaller band gap acceptor as the near-infrared (NIR) absorber to increase the short-circuit current density (Jsc) usually decreases the open-circuit voltage (Voc). In this contribution, we report a small-molecule acceptor, IN-4F, which has a reduced band gap and a higher LUMO level than IT-4F, hence enabling the concurrent increase in the Jsc and Voc when using IT-4F as the acceptor guest of the host binary of PM6:IT-4F. IN-4F was judiciously designed by fusing benzodithiophene (BDT) and thieno[2′,3′:4,5]thieno to make a larger π-system so as to upshift the LUMO level and reduce the optical band gap and, meanwhile, by substituting the BDT-4,8 positions with trialkylsilylthiophene chains to downshift the HOMO level to match the deep HOMO of PM6. Again, the structural similarity between IN-4F and IT-4F makes the nanoscaled homogeneous fine film morphology and the π–π stacking patterns both well kept; hence, the fill factor (FF) is well maintained. The IN-4F-based binary solar cell shows 13.1% efficiency, and its ternary solar cell blended with IT-4F supplies 14.9% efficiency. Again, the use of IN-4F as the guest acceptor of the PM6:Y6 system enables the increase in Voc due to its higher LUMO level, the increase in Jsc because of the increase in charge mobilities, and the maintenance of FF, affording 16.3% efficiency. This work demonstrates that the π-system extending and the trialkylsilylthiophene chain substitution can be an effective strategy to synthesize a nonfullerene acceptor guest to realize a ternary material system, which enables to increase Voc from its entanglement with Jsc (an issue of the current material approach).
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Abstract A systematic study of the mesomorphic properties of three series of copper(II) complexes based on β-diketonate ligands containing branched side chains is reported. These disc-like compounds have four, six and eight flexible alkoxy side chains appended to the central core, in which two or four side chains were substituted by bulkier secondary alkoxy groups: 1-methylbutyloxy R ' = C5(2°) or 1-methylheptyloxy R ' = C8(2°). The mesomorphic results indicated that at least eight side chains are required to form stable columnar mesophases; other compounds with four or six side chains are not mesogenic regardless of the combination of the carbon length on the alkoxy or secondary alkoxy groups of the side chains. The compounds 3 with shorter R ' = C5(2°) side chains were all non-mesogenic regardless of the carbon length of three alkoxy side chains (R = C8, C10, C12) used. However, when the longer 1-methylheptyloxy side chain R ' = C8(2°) was substituted, the compounds 3b-3e with various alkoxy groups (R = C6, C7, C8, C10, C12) exhibited columnar phases. The mesophases were characterized and identified as columnar hexagonal phases (Colh), as expected, by thermal analysis and optical polarized microscopy. The presence of the introduced secondary alkoxy groups apparently appeared to influence the formation of columnar phases. The clearing points were relatively lower than other similar copper(II) compounds not substituted by secondary alkoxy side chains.
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