Indacenodithiophene-4,9-dione-Based D-A-D Type Dopant-Free Hole Transporting Materials for Environmentally Robust 30% Efficient Indoor Perovskite Solar Cells

To develop low-light responsive materials for perovskite solar cells (PSCs), for the first time, we synthesized s-indaceno[1,2-b:5,6-b’]-dithiophene-4,9-dione core acceptor linked 4-methoxy-N-(4-methoxyphenyl)-N-phenylaniline and N,N-diphenylaniline end-cap donors based on symmetrical donor–acceptor–donor (D-A-D) type two hole-transporting materials (HTMs), TPA-IDT-OMe and TPA-IDT-H. The relative synthetic costs of the two HTMs, TPA-IDT-OMe and TPA-IDT-H, are about one-seventh and one-fifth times cheaper, respectively, compared to the commercially available spiro-OMeTAD. Both the symmetrical D-A-D type HTMs presented good thermal stability, remarkable photophysical and electrochemical properties, along with down-shifted HOMO energy levels, extended π-conjugation, and enhanced π-π stacking in the film state. Moreover, the methoxy group-bearing TPA-IDT-OMe exhibited superior hole extraction and hole mobilities with negligible bimolecular recombination at the perovskite/HTM interfaces due to the suitable band alignment with the perovskite energy levels, a more planar conjugated backbone, and improved intermolecular π-π stacking in the film state, which led to better interaction with the perovskite layer. As a result, the PSCs from dopant-free TPA-IDT-OMe yielded an impressive power conversion efficiency (PCE) value of 30.19%, with high open-circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF) values of 0.89 V, 157.30 μA/cm2, and 70.30%, respectively, under indoor light illumination (1000 lx, 321.6 μW/cm2) which is comparable to the PCE value of the doped spiro-OMeTAD-based devices. Moreover, TPA-IDT-OMe showed remarkable UV stability after 240 h of aging and admissible moisture stability with relative humidity (RH) of (40% ± 5%) at room temperature due to its greater hydrophobicity compared to TPA-IDT-H and the reference spiro-OMeTAD. The poor solubility of TPA-IDT-H illustrates its poor film formation ability, which results in greater bimolecular and trap-assisted recombination and lower hole mobility, leading to a lower PCE value of 27.08% for the undoped TPA-IDT-H-based PSCs. This finding opens a new path to design low-light responsive D-A-D architecture-based small molecular HTMs for efficient and stable indoor perovskite solar cells.