Two-dimensional Ruddlesden-Popper halide perovskite solar absorbers with short-chain interlayer spacers

Two-dimensional Ruddlesden-Popper (2DRP) halide perovskites have emerged as promising solar absorbers due to their much-enhanced stability compared with their three-dimensional counterparts, but the light conversion efficiency is limited by their intrinsic optoelectronic properties such as increased exciton binding energy and poor cross-layer carrier transport properties. We herein demonstrate, through first-principles calculations, that the efficiency of 2DRP halide perovskites can be enhanced by adopting the short-chain interlayer spacers among perovskite layers. We adopted short-chain alkylmethylammonium (MA) and formamidinium (FA) organic cations to exchange the regular long-chain butylammonium (BA) and phenethylammonium (PEA) cations in 2DRP halide perovskites with the formula $A_{2}B{\mathrm{Pb}}_2{\mathrm{I}}_7$ ($A=\mathrm{BA}$, PEA, MA, and FA; $B=\mathrm{MA}$ and FA). We find that varying the interlayer spacers results in changed distortion of ${\mathrm{PbI}}_6$ octahedra of the perovskite framework, which in turn influences the stability and electronic structure of 2DRP perovskites. Compared with the long-chain BA/PEA intercalated 2DRP perovskites, the short-chain MA/FA intercalated ones have slightly reduced thermodynamic stability, but their calculated bandgaps are closer to the ideal value for solar cells (1.3–1.6 eV), accompanied with comparable carrier effective masses and optical absorption. More importantly, they demonstrate lower exciton binding energies and two orders of magnitude increased out-of-plane carrier tunneling probability. These factors are expected to further enhance the power conversion efficiency of 2DRP-based solar cells.

Figure 1

(a) Schematic illustration of the two-dimensional Ruddlesden-Popper (2DRP) perovskites ${\left(A\right)}_2\left(B\right){\mathrm{Pb}}_2{\mathrm{I}}_7$ ($A=\mathrm{PEA}$, BA, MA, and FA; $B=\mathrm{MA}$ and FA) structures. FA or MA cations at the $A$ site act as a short-chain interlayer spacer, and PEA or BA act as a long-chain interlayer spacer. The ${\mathrm{PbI}}_6$ octahedral framework is shown in blue. The elements of C, N, and H are in brown, light blue, and pink, respectively. (b) Relationship between calculated decomposition enthalpy $\mathrm{\Delta }H$ and the distribution of Pb-I bond lengths of the 2DRP perovskites.

Figure 2

(a) Relationship between Pb-I-Pb bond angle variance $\left({\sigma }^2\right)$ (blue points) and bandgap (red points). It shows that a larger distortion of ${\mathrm{PbI}}_6$ octahedron leads to a larger bandgap. ${\mathrm{Pb}}_2{\mathrm{I}}_7$ in the labels are omitted for convenience. (b) Crystal structures of $\mathrm{M}{\mathrm{A}}_3{\mathrm{Pb}}_2{\mathrm{I}}_7$ and $\mathrm{F}{\mathrm{A}}_2\mathrm{MA}{\mathrm{Pb}}_2{\mathrm{I}}_7$, with significantly different degrees of structure distortion. The ${\mathrm{PbI}}_6$ octahedra are shown in gray. (c) The schematic of orbital hybridization between Pb cation and I ion near the band edge in two-dimensional Ruddlesden-Popper (2DRP) perovskites. (e) Illustration of crystal orbitals of the valence band maximum (VBM) for a less distorted cell $\left(\mathrm{M}{\mathrm{A}}_3{\mathrm{Pb}}_2{\mathrm{I}}_7\right)$ and a strongly distorted one $\left(\mathrm{F}{\mathrm{A}}_2\mathrm{MA}{\mathrm{Pb}}_2{\mathrm{I}}_7\right)$ derived from the I $5p$ and $\mathrm{Pb}6s$ orbitals. The band-decomposed charge densities of (d) conduction band minimum (CBM) and (f) VBM for $\mathrm{M}{\mathrm{A}}_3{\mathrm{Pb}}_2{\mathrm{I}}_7$ and $\mathrm{F}{\mathrm{A}}_2\mathrm{MA}{\mathrm{Pb}}_2{\mathrm{I}}_7$.

Figure 3

Calculated (a) in-plane electron and hole effective mass, (b) optical absorption spectra with visible light spectra at the background. (c) Theoretical estimated exciton binding energies. Experimental values of $\mathrm{B}{\mathrm{A}}_2\mathrm{MA}{\mathrm{Pb}}_2{\mathrm{I}}_7$ and ${\mathrm{PEA}}_2\mathrm{MA}{\mathrm{Pb}}_2{\mathrm{I}}_7$ are also shown in this figure, and (d) spectroscopic limited maximum efficiency (SLME) based on the improved Shockley-Queisser model of ${\left(A\right)}_2\left(B\right){\mathrm{Pb}}_2{\mathrm{I}}_7$ ($A=\mathrm{PEA}$, BA, MA, and FA; $B=\mathrm{MA}$ and FA). ${\mathrm{Pb}}_2{\mathrm{I}}_7$ in the labels of (a) and (c) are omitted for convenience. Colored shadows in (c) are a guide to the eye.

Figure 4

(a) Demonstration of type one band alignment of the quantum well structure of 2D perovskites. Barrier heights are defined by conduction band (CB)/valence band (VB) offset between the Pb-I layers and organic interlayer spacers. (b) Illustration of interlayer charge transport process. (c) Electron and hole tunneling probability of two-dimensional Ruddlesden-Popper (2DRP) perovskites. ${\mathrm{Pb}}_2{\mathrm{I}}_7$ in the labels are omitted for convenience. (d) Schematic representation of 2DRP perovskites considered as heterostructure composed by M1(Pb-I layer) and M2 (organic spacer).