Synergetic Alkoxy Side-Chain and Chlorine-Contained End Group Strategy toward High Performance Ultra-Narrow Bandgap Small Molecule Acceptors

Ultra-narrow bandgap(ultra-NBG)small molecule acceptors(SMAs)show great potential in organic solar cells(OSCs)due to the extended near-infrared(NIR)absorption.In this work,a synergetic alkoxy side-chain and chlorine-contained end group strategy is employed to achieve A-DA'D-A type ultra-NBG SMAs by introducing alkoxy chains with oxygen atom at the second position into the thiopheneβposition as well as replacing the F atoms with Cl atoms in the end group.As a result,the heptacyclic BZO-4F shows a redshifted absorption onset(960 nm)compared with Y11(932 nm)without oxygen atoms in the side chains.Then,the fluorinated end groups are substituted with the chlorinated ones to synthesize BZO-4Cl.The absorption onset of BZO-4Cl is further redshifted to 990 nm,corresponding to an optical ultra-NBG of 1.25 eV.When blending with the polymer donor PBDB-T,the binary devices based on PBDB-T:BZO-4F and PBDB-T:BZO-4Cl deliver power conversion efficiencies(PCEs)over 12%.Furthermore,ternary devices with the addition of BZ4F-O-1 into PBDB-T:BZO-4Cl system achieve the optimal PCE of 15.51%.This work proposes a synergetic alkoxy side-chain and chlorine-contained end group strategy to achieve A-DA'D-A type ultra-NBG SMAs,which is important for future molecular design.

A-π-A structured non-fullerene acceptors for stable organic solar cells with efficiency over 17%

With the development of photovoltaic materials, especially the small molecule acceptors(SMAs), organic solar cells(OSCs)have made breakthroughs in power conversion efficiencies(PCEs). However, the stability of high-performance OSCs remains a critical challenge for future technological applications. To tackle the inherent instability of SMA materials under the ambient conditions, much effort has been made to improve OSCs stability, including device modification and new materials design. Here we proposed a new electron acceptor design strategy and developed a “quasi-macromolecule”(QM) with an A-π-A structure,where the functionalized π-bridge is used as a linker between two SMAs(A), to improve the long-term stability without deteriorating device efficiencies. Such type of QMs enables excellent synthetic flexibility to modulate their optical/electrochemical properties, crystallization and aggregation behaviors by changing the A and π units. Moreover, QMs possess a unique long conjugated backbone combining high molecular weight over 3.5 k Da with high purity. Compared with the corresponding SMA BTP-4F-OD(Y6-OD), the devices based on newly synthesized A-π-A type acceptors QM1 and QM2 could exhibit better device stability and more promising PCEs of 17.05% and 16.36%, respectively. This kind of “molecular-framework”(A-π-A)structure provides a new design strategy for developing high-efficiency and-stability photovoltaic materials.