Figure 3 (a) Solubility of PNTB-Cl and PNTB6-Cl in chloroform after stirring for 24 h; (b) The absorption spectra of polymer pure films under different conditions (c) Schematics of LBL deposition; (d) Illustration of film morphology washed by chloroform and N3 penetration between polymer domains. Reproduced with permission.[16] Copyright 2021, The Royal Society of Chemistry.
Wu et al. [33] further developed a NTI-based polymer PNTB-HD by employing a linear chain in NTI. The PNTB-HD exhibited a stronger aggregation tendency in chloroform solution, which leads to higher hole mobility, better phase separation and favorable morphology of blend films. Therefore, the PNTB-HD:N3-based OSCs exhibited an optimized PCE of 18.15 %, which is much higher
than 16.77 % and 16.28 % for PNTB-2T:N3 and PM6:N3. Importantly, the PNTB-HD: N3 device also exhibited much better thermal stability with 82.9 % of initial efficiency maintained under continuous heating at 65 °C (Figure 4). The single crystal of NTIs with butyl and ethylhexyl substitutions provide some information to understand how the alkyl chains in NTI impact on physical properties of polymers. In NTI-EH, the distances of 2.72 and 2.67 Å between C=O and H-C were observed. However, interactions between C=O with H-C in NTI-Bu are much stronger and their distances of 2.49 and 2.55 Å are much shorter. He groups synthesized two polymers (PNTB-OAc and PNTB-OH) by introducing hydroxy (OH) or acetoxy (OAc) groups into the NTI unit.[69] Compared to the hydroxy group, the acetoxy group makes NTI-OAc have more regular hydrogen bonds and the polymer PNTB-OAc show ordered packing and stronger crystallinity. The PNTB-OAc:N3-based Q-PHJ OSCs achieved a much higher PCE of 16.53 %, while PCE for PNTB-OH is only 6.79 %. Additionally, the PNTB-OAc:N3-based Q-PHJ OSCs also deliver superior photostability and storage stability. He groups introduced a hydroxylated BTIC-OH-δ as a bifunctional layer in Q-PHJ film.[70] The inserted BTIC-OH-δ layer effectively protected the donor layer from the erosion of the chloroform solution, complemented the light absorption and formed cascade energy levels between the initial donor and an acceptor in a Q-PHJ device. The PNTB-HD: N3-based Q-PHJ OSCs treated with a BTIC-OH-δ molecular layer achieved a high PCE of 17.33 %, the corresponding semi-transparent device also achieved a PCE of 13.44 % with AVT of 23.62 %.