Interfacial versus Bulk Properties of Hole-Transporting Materials for Perovskite Solar Cells: Isomeric Triphenylamine-Based Enamines versus Spiro-OMeTAD

Abstract

Here, we report on three new triphenylamine-based enamines synthesized by condensation of an appropriate primary amine with 2,2-diphenylacetaldehyde and characterized by experimental techniques and density functional theory (DFT) computations. Experimental results allow highlighting attractive properties including solid-state ionization potential in the range of 5.33-5.69 eV in solid-state and hole mobilities exceeding 10^-3 cm²/V·s, which are higher than those in spiro-OMeTAD at the same electric fields. DFT-based analysis points to the presence of several conformers close in energy at room temperature. The newly synthesized hole-transporting materials (HTMs) were used in perovskite solar cells and exhibited performances comparable to that of spiro-OMeTAD. The device containing one newly synthesized hole-transporting enamine was characterized by a power conversion efficiency of 18.4%. Our analysis indicates that the perovskite-HTM interface dominates the properties of perovskite solar cells. PL measurements indicate smaller efficiency for perovskite-to-new HTM hole transfer as compared to spiro-OMeTAD. Nevertheless, the comparable power conversion efficiencies and simple synthesis of the new compounds make them attractive candidates for utilization in perovskite solar cells.

Keywords: enamine; hole mobility; perovskite solar cell; spiro-OMeTAD; time of flight; triphenylamine.

Scheme 1. Syntheses of Enamines 1–3

Figure 1

DSC thermograms of enamine 3 (N₂ atmosphere, scan rate 10 °C/min).

Figure 2

(a) Geometry of the lowest energy conformers of compounds 1, 2, and 3. The relative energy of the ground-state conformers with respect to compound 2 are also shown (ω*B97XD/6-31G(d,p) level). (b) Graphical representation of HOMO and LUMO wavefunctions for the lowest energy (GS) conformer for compounds 1–3.

Figure 3

Cyclic voltammograms of dilute solutions of compounds 1–3 in dichloromethane (room temperature) recorded at a sweep rate of 0.1 V/s (a); photoelectron emission spectra (b) of the layers recorded in air; and absorption spectra (c) of 10^–5 M THF solutions and solid films of compounds 1–3.

Figure 4

Experimental and theoretical (ω*B97XD/6-31G(d,p)) spectra for the lowest energy conformer (GS) and for all conformers with energy within 2 kcal/mol (see Figures S3–S5 and S8) of compounds 1 (a), 2 (b) and 3 (c). To generate the theoretical spectra, the TD-DFT stick transitions were convoluted with Gaussians with a full width at half-maximum equal to 20 nm, followed by normalization of the resulting spectrum.

Figure 5

ToF transient curves of compound 1 recorded at different temperatures and a constant electric field of E = 3.8 × 10⁵ V/cm (a); electric field dependencies of hole drift mobilities for the vacuum-deposited layers of enamines 1–3 recorded at room temperature (b); logarithm of the zero-field hole mobility vs (1/T)² for compounds 1–3 (c); and temperature dependences of the field dependencies of hole mobility (β = f(σ̂²)) for compounds 1–3 (d).

Figure 6

(a) Schematic visualization of equilibrium energy diagrams of the studied devices [energy levels of compounds 1–3 were taken from photoelectron emission measurements for their films (Table 2)]. Cross-sectional SEM images of the devices fabricated with (b) spiro-OMeTAD, (c) HTM 1, (d) HTM 2, and (e) HTM 3.

Figure 7

(a) Current density vs voltage (J–V) curves (backward measurements) of the best devices with the HTMs under study: compound 1 40 mM + additives, compound 2 20 mM + additives, compound 3 40 mM + additives, and spiro-OMeTAD 70 mM + additives. Perovskite with 17% Br. Additives are tBP, LiTFSI, and FK209 in 3.3, 0.5, and 0.03 molar ratio. (b) IPCE spectra as a function of wavelength obtained for compound 1 40 mM + additives, compound 2 20 mM + additives, compound 3 40 mM + additives, and spiro-OMeTAD 70 mM + additives as HTM in PSCs, perovskite 17% Br. (c) Maximum power point tracking of PSCs using the HTMs under study. (d) Steady-state PL spectra of only-perovskite and perovskite–HTM films (glass/perovskite/HTM). (e) TRPL decay at 754 nm acquired for all samples when exciting at 550 nm from the HTM side.

Figure 8

PCE evolution during the long-term stability test. The values have been normalized.