Triarylamine-based crosslinked hole-transporting material with an ionic dopant for high-performance PEDOT:PSS-free polymer solar cells

A triarylamine-based material DVTPD containing two styryl groups has been developed. Upon isothermal heating at 180 °C for 30 min, DVTPD can be thermally cross-linked to form a solvent-resistant layer to realize the fabrication of solution-processed multilayer devices. The crosslinked DVTPD (denoted as X-DVTPD) layer possesses not only hole-collecting ability (HOMO = -5.3 eV) but also electron-blocking capability (LUMO = -2.2 eV). By incorporation of an ionic dopant, 4-isopropyl-4′-methyldiphenyliodonium tetrakis(pentafluorophenylborate) (DPITPFB), into the X-DVTPD material (1:10 in wt%), a favourable morphology of the dopant/matrix layer was formed and the hole-mobility is significantly improved by three orders of magnitude compared to its non-doped state. This DPITPFB:X-DVTPD (1:10 in wt%) layer was employed as the hole-transporting layer to fabricate polymer solar cell devices (PSCs). The E_HOMO of the polymer in the active layer relative to the E_HOMO of the X-DVTPD (-5.3 eV) governs the hole transportation highly associated with the device performance. The higher-lying E_HOMO (-5.0 eV) of P3HT causes a large energy barrier for the hole transportation at the interface, leading to an unsatisfactory efficiency. The E_HOMO level of the PTB7 copolymer (-5.15 eV) is closer to -5.3 eV. As a result, the PTB7-based device can achieve 80% of the efficiency obtained from the corresponding PEDOT:PSS-based device. Furthermore, the PBDCPDTFBT copolymer has the same E_HOMO (-5.3 eV) with X-DVTPD. Consequently, the PBDCPDTFBT-based device showed a comparable efficiency of 5.3% to the corresponding PEDOT:PSS-based device. More importantly, PNDTDTFBT having the lowest-lying E_HOMO of -5.4 eV exhibits superior performance with a high PCE of 6.64%, outperforming its reference PEDOT:PSS-based device. This simple and useful hole-transporting system integrating the crosslinking and doping strategies to replace PEDOT:PSS can be widely used in solution-processed organic electronic devices.