Effect of Co‐Solvents on the Crystallization and Phase Distribution of Mixed‐Dimensional Perovskites

Abstract

Solution‐processed quasi‐2D perovskites are promising for stable and efficient solar cells because of their superior environmental stability compared to 3D perovskites and tunable optoelectronic properties. Changing the number of inorganic layers (n) sandwiched between the organic spacers allows for tuning of the bandgap. However, narrowing the phase distribution around a specific n‐value is a challenge. In‐situ UV–vis–NIR absorption spectroscopy is used to time‐resolve the crystallization dynamics of quasi‐2D butylammonium‐based (BA) perovskites with <n> = 4, processed from N,N‐dimethylformamide (DMF) in the presence of different co‐solvents. By combining with photoluminescence, transient absorption, and grazing‐incidence wide‐angle X‐ray scattering, the crystallization is correlated to the distribution of phases with different n‐values. Infrared spectroscopy and density functional theory reveal that the phase distribution correlates with perovskite precursor—co‐solvent interaction energies and that stronger interactions shift the phase distribution towards smaller n‐values. Careful tuning of the solvent/co‐solvent ratio provides a more homogeneous phase distribution, with highly oriented perovskite crystals and suppressed formation of n = 1–2 phases, providing a power conversion efficiency for BA2MA3Pb4I13 solar cells that increases from 3.5% when processed from DMF to over 11% and 10% when processed from DMF/dimethyl sulfoxide and DMF/N‐methyl‐2‐pyrrolidone mixtures, respectively.

Roles of Long‐Chain Alkylamine Ligands in Triple‐Halide Perovskites for Efficient NiOx‐Based Inverted Perovskite Solar Cells

“…A weak peak at 652 nm wavelength was observed in both perovskite films with SLP and BSLP (Figure 2c, inset), which are attributed to the formation of 2D perovskites. [ 30,37,38 ] Note that a slight redshift (785−792 nm) of the emission peak for perovskite films treated by ligands is observed, which is attributed to the decrease in bandgap. Figure 2d–f clearly shows the difference of Fermi–level ( E F ) energy and cutoff ( E cutoff ) regions and energy‐level band alignments for three perovskite films.…”

High-performance Ruddlesden–Popper two-dimensional perovskite solar cells via solution processed inorganic charge transport layers

“…3(c), the best performing PSC using NiO x /C 60 : C 70 shows a high PCE of 14.4%, which is an excellent value for (BA) 2 (MA) 3 Pb 4 I 13 based 2D PSCs. [13][14][15][16][17][33][34][35] By reverse scanning, the PCE becomes slightly larger to 14.9%, indicating a small hysteresis of the PSCs. The difference from the J-V performance is mainly induced by the slightly increased FF (75.2%) and V oc (1.05 V) whereas for PEDOT:PSS/PCBM based PSCs shown in Fig.…”

Quasi-2D halide perovskite crystals and their optoelectronic applications

“…98 During the crystal growth process, a low supersaturation condition is required to maintain continuous crystal growth along the vertical direction without any other nucleation. 99 In devices fabrication, the adjustment of supersaturation can be achieved by controlling the rate of solvent evaporation, and more specically, by controlling the additive type 100,101 and quantity, 102 solvent ratio, 103,104 and substrate preheating temperature. 97 As previously discussed in thin lms, there are various coexisting phases with different n values and the proposed energy funnelling effect plays an important role in highly efficient carrier transport, making it difficult to specify the contributions from each phase or other interplay factors such as the energy funnelling effect.…”

“…98 During the crystal growth process, a low supersaturation condition is required to maintain continuous crystal growth along the vertical direction without any other nucleation. 99 In devices fabrication, the adjustment of supersaturation can be achieved by controlling the rate of solvent evaporation, and more specifically, by controlling the additive type 100,101 and quantity, 102 solvent ratio, 103,104 and substrate preheating temperature. 97…”