Shiyu Feng

Exploiting Noncovalently Conformational Locking as a Design Strategy for High Performance Fused-Ring Electron Acceptor Used in Polymer Solar Cells

We have developed a kind of novel fused-ring small molecular acceptor, whose planar conformation can be locked by intramolecular noncovalent interaction. The formation of planar supramolecular fused-ring structure by conformation locking can effectively broaden its absorption spectrum, enhance the electron mobility, and reduce the nonradiative energy loss. Polymer solar cells (PSCs) based on this acceptor afforded a power conversion efficiency (PCE) of 9.6%. In contrast, PSCs based on similar acceptor, which cannot form a flat conformation, only gave a PCE of 2.3%. Such design strategy, which can make the synthesis of small molecular acceptor much easier, will be promising in developing a new acceptor for high efficiency polymer solar cells.

Ternary-Blend Polymer Solar Cells Combining Fullerene and Nonfullerene Acceptors to Synergistically Boost the Photovoltaic Performance

A ternary-blend strategy is presented to surmount the shortcomings of both fullerene derivatives and nonfullerene small molecules as acceptors for the first time. The optimal ternary device shows a high power conversion efficiency (PCE) of 10.4%. Moreover, a significant enhancement in PCE (≈35%) relative to both of the binary reference devices, which has never been achieved before in high-efficiency ternary devices, is demonstrated.

4-Alkyl-3,5-difluorophenyl-Substituted Benzodithiophene-Based Wide Band Gap Polymers for High-Efficiency Polymer Solar Cells.

Two novel polymers PTFBDT-BZS and PTFBDT-BZO with 4-alkyl-3,5-difluorophenyl substituted benzodithiophene as the donor unit, benzothiadiazole or benzooxadiazole as the acceptor unit, and thiophene as the spacer have been synthesized and used as donor materials for polymer solar cells (PSCs). These two polymers exhibited wide optical band gaps of about 1.8 eV. PSCs with the blend of PTFBDT-BZS:PC71BM (1:2, by weight) as the active layer fabricated without using any processing additive and any postannealing treatment showed power conversion efficiency (PCE) of 8.24% with an open circuit voltage (Voc) of 0.89 V, a short circuit current (Jsc) of 12.67 mA/cm(2), and a fill factor (FF) of 0.73 under AM 1.5G illumination, indicating that PTFBDT-BZS is a very promising donor polymer for PSCs. The blend of PTFBDT-BZO:PC71BM showed a lower PCE of 5.67% with a Voc of 0.96 V, a Jsc of 9.24 mA/cm(2), and an FF of 0.64. One reason for the lower PCE is probably due to that PTFBDT-BZO has a smaller LUMO offset with PC71BM, which cannot provide enough driving force for charge separation. And another reason is probably due to that PTFBDT-BZO has a lower hole mobility in comparison with PTFBDT-BZS.

Fused-Ring Acceptors with Asymmetric Side Chains for High-Performance Thick-Film Organic Solar Cells

A kind of new fused-ring electron acceptor, IDT-OB, bearing asymmetric side chains, is synthesized for high-efficiency thick-film organic solar cells. The introduction of asymmetric side chains can increase the solubility of acceptor molecules, enable the acceptor molecules to pack closely in a dislocated way, and form favorable phase separation when blended with PBDB-T. As expected, PBDB-T:IDT-OB-based devices exhibit high and balanced hole and electron mobility and give a high power conversion efficiency (PCE) of 10.12%. More importantly, the IDT-OB-based devices are not very sensitive to the film thickness, a PCE of 9.17% can still be obtained even the thickness of active layer is up to 210 nm.

Simultaneous enhancement of the molecular planarity and the solubility of non-fullerene acceptors: effect of aliphatic side-chain substitution on the photovoltaic performance

Three planar nonfullerene acceptors (FTIC-C8C6, FTIC-C6C6 and FTIC-C6C8) comprising a central fluorenedicyclopentathiophene (FT) core and two 2-methylene-(3-(1,1-dicyanomethylene)-indanone) terminal groups are designed and synthesized. The coplanarity of the molecular backbone can be maintained through a locked conformation via intramolecular noncovalent interactions. The solubility of these nonfullerene acceptors is very good because the FT core can bear enough flexible aliphatic side-chain substitutions. Thus, the dilemma of the planarity–solubility tradeoff can be minimized. Through changing the length of the six flexible aliphatic side chains at the central FT core, we can easily adjust the π–π interactions of nonfullerene acceptors and optimize the nanoscale morphology of the photoactive layers. Among these three small molecular acceptors, FTIC-C6C8 based active layers show the best morphology together with the highest electron and hole mobility. These inherent advantages of FTIC-C6C8 guarantee it a high power conversion efficiency of 11.12% when used in non-fullerene polymer solar cells with a wide-bandgap polymer donor PBDB-T. Our results provide an appropriate molecular design strategy for building high-performance nonfullerene acceptors and show that optimizing alkyl-side chains is a very effective way to further improve the photovoltaic performance of devices.

Effect of Non-fullerene Acceptors’ Side Chains on the Morphology and Photovoltaic Performance of Organic Solar Cells

Three indacenodithieno[3,2-b]thiophene (IT) cored small molecular acceptors (ITIC-SC6, ITIC-SC8, and ITIC-SC2C6) were synthesized, and the influence of side chains on their performances in solar cells was systematically probed. Our investigations have demonstrated the variation of side chains greatly affects the charge dissociation, charge mobility, and morphology of the donor:acceptor blend films. ITIC-SC2C6 with four branched side chains showed improved solubility, which can ensure the polymer donor to form favorable fibrous nanostructure during the drying of the blend film. Consequently, devices based on PBDB-ST:ITIC-SC2C6 demonstrated higher charge mobility, more effective exciton dissociation, and the optimal power conversion efficiency up to 9.16% with an FF of 0.63, a Jsc of 15.81 mA cm-2, and a Voc of 0.92 V. These results reveal that the side chain engineering is a valid way of tuning the morphology of blend films and further improving PCE in polymer solar cells.

Influence of polymer side chains on the photovoltaic performance of non-fullerene organic solar cells

Novel polymers comprising a 3-fluoro-5-alkylthiophenyl benzodithiophene donor unit and a 5-fluoro-6-alkoxy (or alkylthio)-2,1,3-benzothiadiazole (BT) acceptor unit were synthesized. Both POF and PSF possess low HOMO and LUMO energy levels due to the incorporation of fluorine atoms. Additionally, alkoxy and alkylthio substitution on the BT unit also had a great influence on the molecular packing and the energy level of the resulting polymers. The introduction of the alkylthio side chains on the BT unit of PSF led to a significant downshift of the HOMO energy level in comparison to that of POF with an alkoxy substituent due to the weaker electron-donating properties of the sulfur atom than that of oxygen. However, the steric hindrance caused by the large sulfur atoms resulted in reduced planarity of the backbone of PSF, which might influence the charge transport and the morphology of the blend film. As a result, POF based NF-PSCs exhibited a PCE of 7.28%, with a Voc of 0.86 V, a Jsc of 14.9 mA cm−2, and an FF of 0.47, while a low PCE of 1.55% with a Voc of 0.95 V, a Jsc of 5.6 mA cm−2, and an FF of 0.29 was obtained for PSF based non-fullerene polymer solar cells (NF-PSCs). Therefore, the side chain engineering of the donor polymer is crucial for maximizing both Jsc and Voc values to achieve high performance polymer solar cells.

Fused pentacyclic electron acceptors with four cis-arranged alkyl side chains for efficient polymer solar cells

We have synthesized a novel kind of fused pentacyclic small molecule acceptor (IDIDT-C8) with cis-arranged alkyl side chains for the first time. Specifically, four aliphatic groups were introduced on one side of the diindeno[1,2-b:2′,1′-d]thiophene unit to construct the molecular backbone with cis-arranged side chains. Such cis-arranged alkyl side chains can play a significant role in adjusting the crystallinity and molecular packing styles. Therefore, the formation of large molecular aggregations can be suppressed in the photoactive layer. Accordingly, polymer solar cells (PSCs) fabricated with polymer PBDB-T as a donor and IDIDT-C8 as an acceptor show a maximum power conversion efficiency of 10.10% with an FF of 65.9%, a Jsc of 15.81 mA cm−2, and a high Voc of 0.97 V, which are among the highest values for single-junction PSCs with fused five-ring electron acceptors up to now.

Enhance the performance of polymer solar cells via extension of the flanking end groups of fused ring acceptors

Two new fused ring electron acceptors (FREAs) IDT-IC-T and IDT-IC-B with thienyl or phenyl substituents at the terminal INCN unit are synthesized. Theoretical calculations indicate that the two acceptors dominantly favor an intermolecular π-π stacking between the flanking terminal groups. The twist angle between the aryl substituent and INCN unit has a significant influence on the π-π stacking distance of terminal unit. IDT-IC-T with a smaller twist angle has a shorter π-π stacking distance than that of IDT-IC-B with a larger twist angle. In addition, extending the conjugation also affects the blend film morphology. IDT-IC-T and IDT-IC-B based photoactive films show appropriate nanoscale phase separations; whereas, blend films based on the parent compound IDT-IC show large-size acceptor domains. As expected, PBDB-T:IDT-IC-T blend films show higher and more balanced electron and hole mobilities. Moreover, these two acceptors present a good charge-transport connectivity arising from the extended conjugation and the increased intermolecular overlapping. Ultimately, IDT-IC-T demonstrates the highest electron mobility (1.47×10−4 cm2 V−1 s−1) and the best power conversion efficiency (PCE) of 9.43%. As for IDT-IC, which only shows an electron mobility of 7.33×10−5 cm2 V−1 s−1 and a PCE of 5.82%. These findings provide a facile and effective way to improve the photovoltaic performance.

High Efficiency Ternary Polymer Solar Cells based on a Fused Pentacyclic Electron Acceptor

Nonfullerene acceptors (NFAs) with a fused-ring backbone usually tend to form large aggregations in the active layer when blended with polymer donors. One solution is to increase the steric hindrance by introducing bulky side chains onto the planar backbone; however, the enlarged intermolecular distance hampers the effective transport of electrons. Here, we provide a ternary-blend strategy to suppress the aggregation of NFAs but preserve their close π–π stacking in the nanophase. Using this method, a fused pentacyclic electron acceptor IDT-2O, which has fewer fused rings and a simpler synthetic route than commonly used heptacyclic and nonacyclic acceptors, can surprisingly demonstrate a high power conversion efficiency of 10.67%, higher than those of the corresponding binary devices. In such ternary-blend devices, PC71BM was selected as the third component to improve the film morphology and enhance the charge transport ability. As a result, increased short-circuit current and fill factors were achieved in these ternary-blend solar cells, giving rise to boosted photoelectric conversion efficiency.