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

Abstract

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 P 21/n, triclinic P1̅, and monoclinic C 2/ 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.

Advancements in organic small molecule hole-transporting materials for perovskite solar cells: past and future

“…220 Wang and team (2019) investigated three conjugated quasilinear segments of uorene, built with two bis(4-methoxyphenyl)amino groups for structurally simple molecular semiconductors Z1, Z2, and Z3 at the end. 221 In addition, MPA-BTTI-based devices show better thermal and light stability and retain 90% of the initial PCE aer 500 h of continuous illumination. At the thermal stress of 80 C for 800 h of constant atmosphere, it shows 94% of actual PCE.…”

Molecular interactions and functionalities of an organic additive in a perovskite semiconducting device: a case study towards high performance solar cells

“…As the bias voltage increased, the current went into a trap-lling region with a slope >>2, followed by a quadratic SCLC region. The trap density (n t ) could be estimated from the critical turning voltage, and the trap-lled limit voltage (V TFL ), based on relation (5), 58,59 n…”

Recent Advances in Organic Hole Transporting Materials for Perovskite Solar Cells

“…Three quasi‐linear segments are end‐capped with two bis(4‐methoxyphenyl)amino groups Z1, Z2, and Z3 and used in PSC to clarify space group effect of HTM on charge hopping mechanism and surface morphology. [ 32 ] Morphology of the HTMs spiro‐OMeTAD and Z2 shown in SEM are very smooth, whereas the surfaces of Z1 and Z3 display rough morphology because of semicrystallinity behavior. Among them, Z3 displays the highest PCE 20.84% along with a V OC of 1.115 V, J SC of 25.10 mA cm −2 , and a fill factor (FF) of 0.75, whereas HTM Z2 with the deepest HOMO energy level gave very low PCE of 15.40% due to the inferior hole extraction.…”