Mapping Electronic Structure: Single-Crystal, Optoelectronic, and Electrochemical Studies of D–A–D′ HTM Systems

Utilizing spiro-OMeTAD as a hole transport material (HTM) in perovskite solar cells (PSCs) has demonstrated an excellent device efficiency. However, the increasing commercial cost of spiro-OMeTAD highlights the need for gainful alternatives. This study used a donor–acceptor–donor′ (D–A–D′) architecture to develop two carbazole-based compounds, MK01 and MK01-alkyl. The acceptor (A) unit is a cyanopyridone or cyanopyridine ring, while the donor (D) and donor′ (D′) units are fluorene and N-ethylcarbazole rings, respectively. These compounds were synthesized from readily available starting materials at an estimated cost of $12.90/g for MK01 and $25.82/g for MK01-alkyl, substantially lower than the $400/g cost of spiro-OMeTAD. Photophysical studies revealed absorption maxima (λabs) at 386 nm (MK01) and 352 nm (MK01-alkyl), with emission maxima (λem) at 556 nm (MK01) and 520 nm (MK01-alkyl), respectively, in solution. The absolute quantum yields of MK01 and MK01-alkyl were 19.55% and 71.50%, respectively, in DMSO, while in the solid state, they were 1.84% and 15.50%. Thermogravimetric analysis showed a 5% weight loss at 401 °C for MK01 and 378 °C for MK01-alkyl, and the corresponding glass transition temperatures (Tg’s) were 173 and 119 °C, respectively. Electrochemical analyses showed that their HOMO energy levels were compatible with perovskite valence bands and that LUMO levels were sufficient for electron-blocking functionality. Further, all these experimental findings were well supported by DFT calculations. Moreover, MK01-alkyl exhibited hole mobility (3.5 × 10–5 cm2 V–1 s–1), while MK01 films showed minimal charge transport and no detectable photoresponse.