Zhenghui Luo

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.

Boosting reverse intersystem crossing by increasing donors in triarylboron/phenoxazine hybrids: TADF emitters for high-performance solution-processed OLEDs

Three triarylboron-based TADF emitters are developed by integrating electron-donating phenoxazine units and electron-accepting triarylboron units. Employing these TADF emitters in the solution-processed organic light-emitting diodes achieves a maximum external quantum efficiency of 13.9% and slight efficiency roll-off.

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.

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%.

Pyran-annulated perylene diimide derivatives as non-fullerene acceptors for high performance organic solar cells

There has been growing interest in the effectual strategy of constructing non-fullerene acceptors for organic solar cells that may overcome the defect of traditional fullerene-based acceptors. Herein, two novel push–pull (acceptor–donor–acceptor) type small-molecule acceptors, that is, TPA-PDI2 and TPA-PDI3, with triphenylamine (TPA) as the core unit and pyran-annulated perylene diimides (Py-PDIs) as peripheral groups are designed and synthesized for non-fullerene organic solar cells (OSCs). After device optimization, OSCs based on TPA-PDI3 demonstrate good device performance with a power conversion efficiency (PCE) as high as 5.84%, surpassing the TPA-PDI2-based counterparts fabricated under identical conditions (1.314% PCE). The high efficiency for TPA-PDI3 can be attributed to the complementary absorption spectra with the donor material (PBDB-T), balanced carrier transport and favorable morphologies. To the best of our knowledge, this PCE of 5.84% is among the highest values based on ethynyl-functionalized TPA-shaped non-fullerene acceptors so far.

Use of two structurally similar small molecular acceptors enabling ternary organic solar cells with high efficiencies and fill factors

Ternary blends have shown great potential to increase the power conversion efficiency (PCE) of organic solar cells (OSCs). In this work, we studied a ternary OSC system with a donor polymer (PM6) and two structurally similar non-fullerene acceptors (named ITCPTC and MeIC). Although these two small molecular acceptors (SMAs) exhibit similar absorption spectra, they introduce a surprising synergistic effect on tuning the domain size and crystallinity of the OSC blend. More specifically, MeIC is a SMA with strong crystallinity, which results in excessive phase segregation and large domain size for the PM6:MeIC binary blend. By adding a structurally similar and less crystalline SMA (ITCPTC) into the binary blend, the domain size and morphology of the blend are much improved without sacrificing the electron mobility of the blend. As a result, the optimal blend ratio of PM6 : ITCPTC : MeIC (1 : 0.4 : 0.6) led to an impressive FF of 78.2% and PCE of 14.13%, which are the highest values reported for ternary non-fullerene OSCs reported to date.