Rational Design of Molecular Hole-Transporting Materials for Perovskite Solar Cells: Direct versus Inverted Device Configurations

Abstract

Due to a still limited understanding of the reasons making 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD) the state-of-the-art hole-transporting material (HTM) for emerging photovoltaic applications, the molecular tailoring of organic components for perovskite solar cells (PSCs) lacks in solid design criteria. Charge delocalization in radical cationic states can undoubtedly be considered as one of the essential prerequisites for an HTM, but this aspect has been investigated to a relatively minor extent. In marked contrast with the 3-D structure of Spiro-OMeTAD, truxene-based HTMs Trux1 and Trux2 have been employed for the first time in PSCs fabricated with a direct (n-i-p) or inverted (p-i-n) architecture, exhibiting a peculiar behavior with respect to the referential HTM. Notwithstanding the efficient hole extraction from the perovskite layer exhibited by Trux1 and Trux2 in direct configuration devices, their photovoltaic performances were detrimentally affected by their poor hole transport. Conversely, an outstanding improvement of the photovoltaic performances in dopant-free inverted configuration devices compared to Spiro-OMeTAD was recorded, ascribable to the use of thinner HTM layers. The rationalization of the photovoltaic performances exhibited by different configuration devices discussed in this paper can provide new and unexpected prospects for engineering the interface between the active layer of perovskite-based solar cells and the hole transporters.

Facilely Synthesized spiro[fluorene‐9,9′‐phenanthren‐10′‐one] in Donor–Acceptor–Donor Hole‐Transporting Materials for Perovskite Solar Cells

“…[1] PSCs have since achieved rapid increase in PCE from 3.8 %t oo ver 20 %, [2][3][4][5][6] owing to the perovskite active layer,w hich exhibits severalb enefits like broad absorptivity, long electron-hole diffusion length, [7] efficient charge-transport capability, [8][9][10] and low charge-recombination rate. [44,45] Considerable efforts have also been devoted to investigating suitable HTMs by replacing the spiro-based core with novel spiro-based HTMs like spiro-acridine-fluorene, [46,47] spiro-bi[thieno [3,4-b] [1,4]dioxepine], [48] dispiro-oxepine, [49] spiro[fluorene-9,9'-xanthene], [6,[50][51][52][53] and spiro-phenylpyrazole-fluorine. [2,3,5] As Spiro-OMeTAD exhibits al ow recombination rate and efficient hole-transport mobility,t he overall devicep erformance is improved when the materiali si ncluded.…”

“…[12][13][14] To overcome these issues, numerouss tudies have focusedo nl ow-costp roduction without as piro-based linkage in the HTM, mainly using acene, [15] carbazole, [16][17][18][19][20][21][22][23] indole, [24][25][26] phenothiazine, [27] phenoxazine, [28] thiophene, [29][30][31][32][33][34][35] triarylamine, [36][37][38][39] triazateuxene, [40][41][42][43] and truxene. [44,45] Considerable efforts have also been devoted to investigating suitable HTMs by replacing the spiro-based core with novel spiro-based HTMs like spiro-acridine-fluorene, [46,47] spiro-bi[thieno [3,4-b] [1,4]dioxepine], [48] dispiro-oxepine, [49] spiro[fluorene-9,9'-xanthene], [6,[50][51][52][53] and spiro-phenylpyrazole-fluorine. [54] We ...…”

Design of Cone‐Shaped Hole Transporting Material Organic Structures for Perovskite Solar Cells Applications

“…The HOMO energy levels of H1 (‐5.23 eV) and H2 (‐5.24 eV) were determined from the E 1/2 of the HTM/HTM + couple. E 1/2 , HOMO and LUMO values calculated for S‐OMeTAD match well with literature . Electron rich center of phenothiazine moiety (H2) shows lower oxidation potential (higher energy) compared to H1 which tells that H2 could possibly exhibit fast interfacial hole transfer kinetics.…”

“…[a] CV of the E 1/2 of first redox potential of HTM molecules. [b] HOMO can be calculated by E HOMO =‐[(E 1/2 ox ‐ E 1/2 Fc/Fc + ) + 5.1] eV . Ferrocene was used as internal standard in each experiment.…”

Triindolo‐Truxene Derivatives: Design, Synthesis, and Fine‐Tuning of Electronic Properties and Molecular Assembly through Molecular Engineering

“…[24][25][26] To alter their electronic properties, variouss ubstituents, such as alkyl groups,f luorine atoms, and cyano groups,h ave been introduced into their molecular backbones. [24,27] Truxene, aw ell-performing and widely used building block in light-emitting [28][29][30][31][32] and hole-transporting [32][33][34][35][36][37][38] materials, features adjustable properties through facile functionaliza-tion. [39][40] Truxene has six aromaticc arbon atoms in the 2-, 3-, 7-, 8-, 12-, and 13-positions of the three fluorene subunits ( Figure 1) that can be functionalized by bromination, [41] acetylation, [42] nitration, [43] and so forth.…”