Side-Chain Engineering of Donor–Acceptor Conjugated Small Molecules As Dopant-Free Hole-Transport Materials for Efficient Normal Planar Perovskite Solar Cells

Abstract

Simultaneously improving efficiency and stability, which are particularly crucial factors for the commercialization of perovskite solar cells (PSCs), remains a major challenge. For high-efficiency normal PSCs, the development of stable dopant-free hole-transport materials (HTMs) seems imperative. Here, we developed potential donor–acceptor small molecules (BTTI) as HTMs for normal planar PSCs. Through tailoring its alkyl side-chain length as BTTI-C6, BTTI-C8, and BTTI-C12, our results show that upon shortening the side chain of BTTI, the hole mobility, film-forming capability, and resultant device performance were remarkably improved, with the device conversion efficiencies of 19.69% for BTTI-C6, 18.89% for BTTI-C8, and 17.49% for BTTI-C12. Meanwhile, compared to those made with the routine doped Spiro-OMeTAD, devices based on our dopant-free HTMs exhibited significantly improved stability. This work paves the way to the development of effective dopant-free HTMs for high-performance PSCs.

Defect Passivation for Perovskite Solar Cells: from Molecule Design to Device Performance

“…To further improve the efficiency, the donor‐acceptor‐donor (D‐A‐D) type HTM with a high dipole moment and appropriate intermolecular π–π stacking that leads to a higher hole mobility than that of the D‐D type HTM were designed as the surface passivation agents for PSCs [132–134] . Guo and co‐workers synthesized a novel D‐A‐D type dopant‐free HTM called MPA‐BTTI composed of an imide‐functionalized thiophene‐based building block (acceptor) and two end triphenylamine donor units [135] .…”

A Review on Solution‐Processable Dopant‐Free Small Molecules as Hole‐Transporting Materials for Efficient Perovskite Solar Cells

“…Up to now, the reported polycyclic cores can be grouped into four types according to their fused degree, namely tetracyclic, [121][122][123] pentacyclic, [124][125][126][127][128][129][130][131][132] hexacyclic, [133] and heptacyclic cores (Figure 9). [134][135][136] There are two kinds of structures for tetracyclic cores, fluoranthene and benzotrithiophene (BTT).…”

“…In contrast to tetracyclic cores, the pentacyclic ones had a much bigger family of efficient SM-HTMs in hand. [124][125][126][127][128][129][130][131][132] The first efficient HTMs is from Jang et al [124] in 2018, where di(1-benzothieno)[3,2-b:2′,3′-d]pyrrole (DBTP) was chosen as the core and meta-methoxy revised DPA as the arms to form the molecule mDPA-DBTP. The molecule adopted a face-on orientation in the solid state, and together with the high planarity of the fused DBTP core that endowed the molecule with a strengthened intermolecular π-π stacking, a correspondingly very high hole mobility of 6.34 × 10 −4 cm 2 V −1 s −1 was obtained, superior to the value of doped spiro-OMeTAD The TPV spectra and intensity-modulated photocurrent/photovoltage (IMPS/IMVS) analyses confirmed that mDPA-DBTP presented a shorter charge extraction time and a longer charge recombination time than doped spiro-OMeTAD, suggesting better hole extraction and recombination suppression abilities.…”

“…Different from the above pentacyclic cores that are electron-donors, the cores in this paragraph are electron-acceptor units. [129][130][131] In 2019, Guo et al [130] designed two SM-HTMs of this kind noted as MPA-BTI and MPA-BTTI utilizing imidefunctionalized fused units as the cores and TPA as the arm (Figure 8), where the imide-based fused thiophene units worked as the acceptor units and TPA as the donor one. The high planarity of the cores could enable good intermolecular π-π stacking, thus benefiting the charge transportation.…”

Recent Advances in Organic Hole Transporting Materials for Perovskite Solar Cells

“…[ 69 ] Three dipolar small molecules 2,8‐bis(4‐(bis(4‐methoxyphenyl)amino)phenyl)‐5‐alkyl‐4H‐thieno[2′,3′:4,5]thieno[3,2‐c]thieno[2′,3′:4,5]thieno[2,3‐e]azepine‐4,6(5H)‐dione (BTTI) with different alkyl chain lengths were applied in the convenient planar device with a configuration of ITO/SnO 2 /CsFAMA/HTMs/Au. [ 70 ] The results indicated that shorter alkyl chain BTTI‐C 6 is favorable to have better hole mobility, film‐forming ability, leading to an impressive PCE of 19.69%. In contrast, BTTI‐C 8 and BTTI‐C 12 demonstrated efficiencies of 18.89% and 17.49%, respectively.…”