General design of self-doped small molecules as efficient hole extraction materials for polymer solar cells†
The development of high performance hole transport materials (HTMs) without a chemical dopant is critical to achieve long-term device durability. The general design of self-doping materials based on a phenolamine structure with strong electronic spin concentration is reported for the first time. A phenol-enhanced self-doped mechanism is also proposed. Compared to their precursors, dimethylphenolamine derivatives, TBP-OH4, TPD-OH4 and Spiro-OH8, displayed much higher spin concentration in their neutral states. Phenol acts as a hole trap in the traditional concept, however, the films of TBP-OH4, TPD-OH4 and Spiro-OH8 exhibited higher conductivities than those of methoxyl precursors. Meanwhile, phenolamine derivatives have good solublility in polar organic solvents and show good solvent resistance in chlorobenzene. Considering the relatively good band alignment, film-formation and solvent resistance against chlorobenzene, Spiro-OH8 and TPD-OH4 exhibited comparable performance with that of PEDOT:PSS-4083. Most importantly, a new generation of self-doped systems based on a phenolamine structure might provide new insight in developing efficient HTMs for organic electronics.