Atomistic origins of high-performance in hybrid halide perovskite solar cells

Abstract

The performance of organometallic perovskite solar cells has rapidly surpassed that of both conventional dye-sensitized and organic photovoltaics. High-power conversion efficiency can be realized in both mesoporous and thin-film device architectures. We address the origin of this success in the context of the materials chemistry and physics of the bulk perovskite as described by electronic structure calculations. In addition to the basic optoelectronic properties essential for an efficient photovoltaic device (spectrally suitable band gap, high optical absorption, low carrier effective masses), the materials are structurally and compositionally flexible. As we show, hybrid perovskites exhibit spontaneous electric polarization; we also suggest ways in which this can be tuned through judicious choice of the organic cation. The presence of ferroelectric domains will result in internal junctions that may aid separation of photoexcited electron and hole pairs, and reduction of recombination through segregation of charge carriers. The combination of high dielectric constant and low effective mass promotes both Wannier-Mott exciton separation and effective ionization of donor and acceptor defects. The photoferroic effect could be exploited in nanostructured films to generate a higher open circuit voltage and may contribute to the current-voltage hysteresis observed in perovskite solar cells.

Figure 1

Schematic perovskite crystal structure of MAPbI₃ (a), and the possible orientations of molecular dipoles within the lattice (b). Note that MA has an associated molecular dipole of 2.3 D, a fundamental difference compared to the spherical cation symmetry in inorganic perovskites such as CsSnI₃.

Figure 2

A significant increase in dipole moment (from 2.3 to 6.6 D, by a vacuum calculation with B3LYP/6-31G*) is achieved through increasing the degree of methyl-fluorination.

Figure 3

Valence band ionization potentials of MAPbI₃ with respect to the vacuum level. Calculations were performed on the nonpolar (110) surface with slab thickness of 25 Å and a vacuum thickness of 15 Å. The Kohn–Sham eigenvalues (PBEsol) are corrected by the bulk quasi-particle energies (ΔE = 0.2 eV). Values for the energetically similar inorganic thin-film absorber Cu₂ZnSnS₄ and Si are shown for comparison.

Figure 4

Possible decomposition pathway of hybrid halide perovskites in the presence of water. A water molecule, a, is required to initiate the process with the decomposition being driven by the phase changes of both hydrogen iodide, (b, soluble in water) and the methylammonia (c, volatile and soluble in water). This pathway results in the formation of a yellow solid, which corresponds the experimentally observed PbI₂, d.

Figure 5

Schematic of the 1D built-in potential (upper panels) and associated 2D electron and hole separation pathways (lower panels) in (a,d) a macroscopic p–n junction; (b,e) a single domain ferroelectric thin-film; (c,f) a multidomain ferroelectric thin-film. In the multidomain ferroelectric, which we propose for hybrid perovskites, electrons will move along minima in the potential, while holes will move along maxima (i.e., antiphase boundaries).