Unraveling the Structure–Property Relationship of Molecular Hole-Transporting Materials for Perovskite Solar Cells

Clarifying the structural basis and
microscopic mechanism lying
behind electronic properties of molecular semiconductors is of paramount
importance in further material design to enhance the performance of
perovskite solar cells. In this paper, three conjugated quasilinear
segments of 9,9-dimethyl-9H-fluorene, 9,9-dimethyl-2,7-diphenyl-9H-fluorene, and 2,6-diphenyldithieno­[3,2-b:2′,3′-d]­thiophene are end-capped
with two bis­(4-methoxyphenyl)­amino groups for structurally simple
molecular semiconductors Z1, Z2, and Z3, which crystallize in the
monoclinic P21/n, triclinic P1̅, and monoclinic C2/c space
groups, respectively. The modes and energies of intermolecular noncovalent
interactions in various closely packed dimers extracted from single
crystals are computed based on the quantum theory of atoms in molecules
and energy decomposition analysis. Transfer integrals, reorganization
energies, and center-of-mass distances in these dimers as well as
band structures of single crystals are also calculated to define the
theoretical limit of hole transport and microscopic transport pictures.
Joint X-ray diffraction and space-charge-limiting current measurements
on solution-deposited films suggest the dominant role of crystallinity
in thin-film hole mobility. Photoelectron spectroscopy and photoluminescence
measurements show that an enhanced interfacial interaction between
the perovskite and Z3 could attenuate the adverse impact of reducing
the energetic driving force of hole extraction. Our comparative studies
show that the molecular semiconductor Z3 with a properly aligned highest
occupied molecular orbital energy level and a high thin-film mobility
can be employed for efficient perovskite solar cells, achieving a
good power conversion efficiency of 20.84%, which is even higher than
that of 20.42% for the spiro-OMeTAD control.