Construction of novel Donor-Acceptor type self-assembled molecules together with NiOx for flexible perovskite solar cells

Donor-Acceptor (D-A) type self-assembled molecules (SAMs) are regarded as star organic small molecules that can effectively alleviate the undesired NiO_x/perovskite interface problem and demonstrate exceptional hole extraction and transport ability. However, the structure-performance relationships of donor units or anchoring groups of suitable D-A SAMs have not been unveiled. Here, we try to discuss such structure-performance relationships by designing and synthesizing a series of D-A type SAMs (TQB, DQB, DQC and DQP), which incorporate dibenzofurans-substituted bis(4-methoxyphenyl)amine or methoxy triphenylamine as donor unit, quinoxaline as acceptor unit, and phenylboronic acid or benzoic acid or phenylphosphinic acid as anchoring group. Among them, DQB, a dibenzofurans-substituted phenylboronic acid derivative, has unique advantages in terms of thermal stability, interface contact and hole extraction. For different donors, the relevant test and characterization results make clear that the donor with a larger rigid plane building block in SAMs not only enhances thermal stability and electron-donating capability of the SAMs, but also induces high-quality perovskite films. Furthermore, methoxy on the more electron-rich terminal group better passivates perovskite defects. As an anchoring group of D-A type SAMs, the mildly acidic boronic acid group can form the strongest B–O–Ni bond. It stands to reason that unacceptable interfacial reactions and recombination are suppressed. Consequently, the power conversion efficiency of 17.88 % generated by NiO_x/DQB-based p-i-n flexible perovskite solar cells is superior to that of other devices. Our results offer a reference for the design of SAMs with better performance.