Donor engineering of a benzothiadiazole-based D-A-D-type molecular semiconductor for perovskite solar cells: a theoretical study

Due to the easy formation of compact molecular packing arrangements and the favorable photophysical and electrochemical properties, donor-acceptor-donor (D-A-D)-type small molecule hole-transporting materials (HTMs) have been widely synthesized and researched to improve the efficiency and stability of perovskite solar cells (PSCs). The main approach in recent experiments has been to seek good acceptors, whereas the influence of the electron-donating units has been less reported. In this work, six new benzothiadiazole-based D-A-D-type HTMs are tailored by employing the ethyl-substituted phenoxazine (POZ), phenothiazine (PTZ) and carbazole (CZ) as the donors. To obtain an elementary understanding of new HTMs, the electronic, optical, hole-transporting and interfacial properties are simulated with quantum chemistry methods. The results indicate that all tailored HTMs exhibit suitable energy alignment compared with the band structures of the perovskite, and the continuous highest occupied molecular orbital (HOMO) levels will be helpful for interfacial energy regulation. In comparison with the YN1, the maximum absorption wavelengths of the newly designed HTMs are red-shifted due to the decreased excitation energies from the ground-state to the first singlet excited-state. Importantly, the hole mobilities of all designed HTMs are distinctly higher than the referenced YN1, which is contributed by the better planarity of the molecular skeleton and the easier orbital overlapping between adjacent molecules. The interfacial simulations manifest that the FAPbI3/SM37 system displays a more stable adsorption configuration and greater charge redistributions at the interface compared to YN1, which further promotes the separation of photogenerated electron-hole pairs. Moreover, larger Stokes shifts and better solubility are also acquired for the new HTMs. In summary, our calculations not only propose several potential highly efficient HTMs, but also provide useful insights at the atomic level for the experimental synthesis of new D-A-D-type HTMs.

Efficient hole transport materials based on naphthyridine core designed for application in perovskite solar photovoltaics

Naphthyridine-based compounds with a donor-acceptor-donor (D-A-D) skeleton were considered as hole transport materials (HTMs) for perovskite solar cells (PSCs). The optical characteristics, stability, solubility, Hirshfeld surface analysis, crystal structure, and hole transport properties of the HTMs were studied systematically. The HOMO energies of all HTMs were higher than valence band of CH₃NH₃PbI₃ (MAPbI₃) perovskite signifying naphthyridine-based HTMs had appropriate energy alignments for usage in PSCs. The LUMO level of designed HTMs were higher than MAPbI₃ conduction band ensuring prevention of backward electronic movement from MAPbI₃ to the cathode. The λ_abs^max amounts of all HTMs were close 400 nm, which showed their competition with perovskite was impossible. The 18NP and 26NP HTMs had higher hole mobilities compared to that of the Spiro-OMeTAD. Considering aligned HOMO energies, suitable hole mobilities, satisfactory stability and solubility, 18NP (1,8-Naphthyridine) and 26NP (2,6-Naphthyridine) were introduced as the best HTM materials for PSCs which could replace Spiro-OMeTAD.

A π-extended triphenylamine based dopant-free hole-transporting material for perovskite solar cells via heteroatom substitution

The triphenylamine (TPA) group is an important molecular fragment that has been widely used to design efficient hole-transporting materials (HTMs). However, the applicability of triphenylamine derived HTMs that exhibit low hole mobility and conductivity in commercial perovskite solar cells (PSCs) has been limited. To aid in the development of highly desirable TPA-based HTMs, we utilized a combination of density functional theory (DFT) and Marcus electron transfer theory to investigate the effect of heteroatoms, including boron, carbon, nitrogen, oxygen, silicon, phosphorus, sulfur, germanium, arsenic, and selenium atoms, on the energy levels, optical properties, hole mobility, and interfacial charge transfer behaviors of a series of HTMs. Our computational results revealed that compared with the commonly referenced OMeTPA-TPA molecule, most heteroatoms lead to deeper energy levels. Furthermore, these heteroatom-based HTMs exhibit improved hole mobility due to their more rigid molecular structures. More significantly, these heteroatoms also enhance the interface interaction in perovskite/HTM systems, resulting in a larger internal electric field. Our work represents a new approach that aids in the understanding and designing of more efficient and better performing HTMs, which we hope can be used as a platform to propel the developmental commercialization of these highly desirable PSCs.