Dongjun Xie

Fine‐Tuning of Molecular Packing and Energy Level through Methyl Substitution Enabling Excellent Small Molecule Acceptors for Nonfullerene Polymer Solar Cells with Efficiency up to 12.54%

A novel small molecule acceptor MeIC with a methylated end-capping group is developed. Compared to unmethylated counterparts (ITCPTC), MeIC exhibits a higher lowest unoccupied molecular orbital (LUMO) level value, tighter molecular packing, better crystallites quality, and stronger absorption in the range of 520-740 nm. The MeIC-based polymer solar cells (PSCs) with J71 as donor, achieve high power conversion efficiency (PCE), up to 12.54% with a short-circuit current (JSC ) of 18.41 mA cm-2 , significantly higher than that of the device based on J71:ITCPTC (11.63% with a JSC of 17.52 mA cm-2 ). The higher JSC of the PSC based on J71:MeIC can be attributed to more balanced μh /μe , higher charge dissociation and charge collection efficiency, better molecular packing, and more proper phase separation features as indicated by grazing incident X-ray diffraction and resonant soft X-ray scattering results. It is worth mentioning that the as-cast PSCs based on MeIC also yield a high PCE of 11.26%, which is among the highest value for the as-cast nonfullerene PSCs so far. Such a small modification that leads to so significant an improvement of the photovoltaic performance is a quite exciting finding, shining a light on the molecular design of the nonfullerene acceptors.

Thienobenzene-fused perylene bisimide as a non-fullerene acceptor for organic solar cells with a high open-circuit voltage and power conversion efficiency

Perylene bisimide (PBI) based molecules have recently attracted tremendous interest as acceptors in non-fullerene organic solar cells. However, most PBI-based acceptors possess deep LUMO energy levels (−3.9 ∼ −4.0 eV) and show an open-circuit voltage (Voc) below 0.90 V, thus limiting the improvement of device efficiency. Here, we report two novel ring-fused PBI dimers, SdiPBI-BT and diPBI-BT, with thienobenzene fused to the bay region of the PBI subunits. Conventional bulk-heterojunction (BHJ) solar cells based on SdiPBI-BT show a power conversion efficiency (PCE) of 6.71% with a high Voc value of 0.95 V, a short-circuit current density (Jsc) of 10.31 mA cm−2 and a high fill factor (FF) of 68.7%. Devices based on diPBI-BT show a PCE of 5.84% with a high Voc value of 0.99 V. These results demonstrate that ring-fused PBI derivatives are promising materials for non-fullerene cells.

Optimized Fibril Network Morphology by Precise Side-Chain Engineering to Achieve High-Performance Bulk-Heterojunction Organic Solar Cells

A polymer fibril assembly can dictate the morphology framework, in forming a network structure, which is highly advantageous in bulk heterojunction (BHJ) organic solar cells (OSCs). A fundamental understanding of how to manipulate such a fibril assembly and its influence on the BHJ morphology and device performance is crucially important. Here, a series of donor-acceptor polymers, PBT1-O, PBT1-S, and PBT1-C, is used to systematically investigate the relationship between molecular structure, morphology, and photovoltaic performance. The subtle atom change in side chains is found to have profound effect on regulating electronic structure and self-assembly of conjugated polymers. Compared with PBT1-O and PBT1-S, PBT1-C-based OSCs show much higher photovoltaic performance with a record fill factor (FF) of 80.5%, due to the formation of optimal interpenetrating network morphology. Such a fibril network strategy is further extended to nonfullerene OSCs using a small-molecular acceptor, which shows a high efficiency of 12.7% and an FF of 78.5%. The results indicate the formation of well-defined fibrillar structure is a promising approach to achieving a favorable morphology in BHJ OSCs.

Side-chain effects on energy-level modulation and device performance of organic semiconductor acceptors in organic solar cells

Two new non-fullerene acceptors, IDTC and IDTO, were designed and synthesized for the application in organic solar cells (OSCs). Compared with IDTC, the introduction of electron-donating alkoxy groups of IDTO leads to a higher LUMO level with a slightly blue-shifted absorption. Using the polymer PBDB-T as donor and the two small molecules as acceptors in the conventional device structure, the IDTC-based OSC exhibits a power conversion efficiency (PCE) of 9.35% with an open-circuit voltage (VOC) of 0.917 V, a short-circuit current density (JSC) of 16.56 mA cm-2, and a fill factor (FF) of 61.61%. For the OSC based on IDTO, a higher PCE of 10.02% with a VOC of 0.943 V, a JSC of 16.25 mA cm-2, and an FF of 65.41% are obtained. The more balanced μe/μh, evident aggregation, and phase separation contribute to the higher FF for the device based on IDTO. The increased JSC for the device based on PBDB-T:IDTC can be attributed to the red-shifted and stronger absorption of the PBDB-T:IDTC blend film. These results indicate fine-tuning the electronic energy and absorption of non-fullerene acceptors is feasible to improve the performance of OSCs.

A three-dimensional thiophene-annulated perylene bisimide as a fullerene-free acceptor for a high performance polymer solar cell with the highest PCE of 8.28% and a VOC over 1.0 V

A new propeller-shaped small-molecule acceptor of BPT-S with S-annulated perylene bisimide (PBI) as peripheral groups was designed and synthesized. Compared to the unannulated counterpart (BPT), BPT-S exhibits a blue-shift absorption spectrum, stronger absorption in the region of 400–510 nm, higher LUMO energy level and twisted molecular geometry. The BPT-S-based device with PDBT-T1 as the donor achieved a power conversion efficiency (PCE) as high as 8.28% with an impressively high open-circuit voltage (VOC) of 1.02 V, a near 16% enhancement in PCE with respect to the BPT-based control device (7.16%). The high photovoltaic performance for the BPT-S-based device can be attributed to its relatively high-lying LUMO level, complementary absorption spectra with the donor material, favorable morphology and balanced carrier transport. To the best of our knowledge, a PCE of 8.28% is the highest value for the device based on S-annulated PBIs as acceptors reported so far. This work indicates the great potential of the 3D S-annulated PBI-based acceptors for tandem (or multi-junction) organic solar cells due to the proper absorption spectra and high photovoltaic performance.

Asymmetrical Ladder‐Type Donor‐Induced Polar Small Molecule Acceptor to Promote Fill Factors Approaching 77% for High‐Performance Nonfullerene Polymer Solar Cells

In this work, an effectual strategy of constructing polar small molecule acceptors (SMAs) to promote fill factor (FF) of nonfullerene polymer solar cells (PSCs) is first reported. Three asymmetrical SMAs of IDT6CN, IDT6CN-Th, and IDT6CN-M, which own large dipole moments, are designed and synthesized. The PSCs based on three polar SMAs exhibit apparently higher FFs compared with their symmetrical analogues. The asymmetrical design strategy accompanied with side chain and end group engineering makes IDT6CN-Th- and IDT6CN-M-based nonfullerene PSCs achieve high power conversion efficiency with FFs approaching 77%.

Revealing the new potential of an indandione unit for constructing efficient yellow thermally activated delayed fluorescence emitters with short emissive lifetimes

Simultaneously accomplishing a high efficiency and a slow efficiency roll-off at practical luminance levels remains challenging for thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). In this study, for the first time, we reveal the new potential of an indandione unit featuring double carbonyl moieties, which is widely used in organic solar cells, as an electron-accepting core for constructing efficient TADF emitters. As a proof of concept, two TADF emitters, 5PXZ-PIDO and 5,6PXZ-PIDO, are developed by connecting an indandione (IDO) core with electron-donating phenoxazine (PXZ) units via phenylene π-bridges. Impressively, both emitters exhibit a distinct TADF nature with short DF lifetimes of ∼2 μs, and display relatively high photoluminescence quantum yields (ΦPLs) of over 70%. More importantly, a yellow OLED based on 5,6PXZ-PIDO achieves a high external quantum efficiency of 14.2% and an ultra-slow efficiency roll-off of 16.0% at a practical luminance of 1000 cd m−2, which is outstanding among previously reported carbonyl-based TADF emitters. This finding unlocks the huge potential of indandione-based molecules as TADF emitters for high-performance OLEDs.