Computational Verification of So-Called Perovskite Solar Cells as PbI₆⁴⁻-Aligned Solar Cells

Abstract

Effective sensitizing components in so-called perovskite solar cells (PSC) are lead hexaiodide (PbI 6 4− ) salts of PbI 6 4− (MeNH 3 + ) n (n = 2∼4). Density-functional-theory-based molecular modeling (DFT/MM) of X-ray crystalline structure of PbI 6 4− /MeNH 3 + salt (FOLLIB) verifies that the packing unit of FOLLIB has UV/Vis absorption spectrum at λmax = 424 nm, giving pale yellow color as complementary color. DFT/MM of the horizontal component in the FOLLIB gives narrow energy gap of 0.3 eV, verifying remarkable semiconducting property through tight alignments of PbI 6 4− components coupled with MeNH 3 + . DFT/MM of the central PbI 6 4− /MeNH 3 + components verifies that the central component has UV/Vis absorption spectra with respective λmax = 570 nm, λmax = 762 nm and λmax = 945 nm, and plays an essential role as panchromatic sensitizers. In addition, their equilibrium geometric structures show slightly hypsochromic UV/Vis absorption spectra at respective λmax = 486 nm, λmax = 560 nm, and λmax = 563 nm as results of migration of MeNH 3 + close to PbI 6 4− . DFT/MM also verifies that PbI 6 4− components align tightly to nanocrystalline TiO 2 (nc-TiO 2 ) and to spiro-OMeTAD in PSC through electron density induced by van der Waals interaction. Miyasaka and his group reported the first lead halide-sensitizers for photovoltaic cells, comprising the solid-state lead trihalides with perovskite structures.1 Since then, the name "perovskite solar cell (PSC)" has been introduced to most photovoltaic research and developments using methylammonium lead trihalides [MeNH 3 PbX 3 , (X = I, Br, and Cl)] as sensitizing chemicals.2 On the other hand, it was reported in 1987 that drop-wise addition of a concentrated aqueous solution of Pb(NO 3 ) 2 to an aqueous solution containing excess MeNH 3 + I − yields a black precipitate of MeNH 3 PbI 3 , and that the solutions of MeNH 3 PbI 3 gives pale yellow crystalline solid that is composed of lead hexaiodide (PbI 6 4− ), MeNH 3 + and H 2 O. 3 In addition, the crystalline structure was analyzed by X-ray crystallography and the packing unit of the crystal is now coded as FOLLIB in Cambridge Structural Data (CSD). We understand that lead hexaiodide ion (PbI 6 4− ) are coordinated with methylammonium ion (MeNH 3 + ), changed into dark colored components in polar solvent, and predict that PbI 6 4− should play an essential role in efficient and effective molecular-structuring in perovskite-type solar cells, e.g., nanocrytalline TiO 2 (nc-TiO 2 )/MeNH 3 PbI 3 /spiro-OMeTAD-based PSC. Our density-functional-theory-based molecular modeling (DFT/MM) verifies essential roles of molecular alignment of PbI 6 4− components in electronic and photonic properties, i.e., high electron diffusion and diffusion length 4 and panchromatic light harvesting characteristics toward so-called PSC, i.e., PbI 6 4− -aligned solar cells. Computational MethodsWe perform DFT-based molecular modeling (DFT/MM), equivalent to the quantum mechanics/molecular mechanics (QM/MM) method, by using the B3LYP exchange-corr...

Molecular-Orbital-Based Verification of Water Photo-Splitting on Pt-Loaded TiO2: Effective Formation of HOOH and Reduced Platinum Cluster as Precursors of O2and H2

“…As the model of Pt-loaded nc-TiO 2 catalyst, van der Waals and Coulomb interaction-based complex of [HOTi 9 O 18 H] with [Pt 6 ] and [HO -&H 2 O] is successfully molecular-modeled, and the molecular orbital energy structure rationalizes that the effective watersplitting catalysis should occur via effective electron transfer from HOMO of [HO -&H 2 O] to LUMO of [Pt 6 ] when the catalyst is photo-energized. DFT/MM also verifies successive oxidation of HOOH to 3 O 2 and photo-regeneration of HOOH via radical anion of O 2…”

Validity of density-functional-theory-based molecular modeling for UV/visible spectroscopy and rationale of panchromatic PbI6 4−(MeNH3 +)4-structured molecular solar cells

“…The electron transfer gaps ΔE t (ELUMO-EHOMO) and ΔE t [ELUMO (+1)-ELUMO] are measures of the semiconducting electron transfer capability at the ground and excited states (electron accepted states, i.e., radical anion states) of molecules and molecular aggregates. [3][4][5] The chemical reactivities of the molecular species have been successfully investigated in terms of the change in electron density of the frontier orbitals upon electron addition=extraction, i.e., the Fukui function. [26][27][28][29] The semiconducting carrier mobility or electrical conductivity of the PSC materials has been reported.…”

“…The structural change may come from a kind of strain caused by crystalline matrixes. Considering that there is no such strain at amorphous states, the three-dimensional symmetrical structure of EQG( 1 As mentioned in the preceding papers, [3][4][5] the energy transfer gap between LUMO and LUMO+ is a measure of the semiconducting molecular transfer under solar light irradiance. Photogenerated electrons convert LUMO and LUMO+1 into HOMO (SOMO) and LUMO of the resulting radical anion.…”

“…Density-functional-theory (DFT) 1,2) -based molecular modeling is a novel approach for verifying and predicting the molecular-orbital-based electronic properties of molecular aggregates in which the constituent molecules are aligned by van der Waals and Coulomb interactions (vdW&Clmb). [3][4][5] Recently, Persson and colleagues reported that DFT-based molecular modeling is a useful tool for predicting the electronic and energetic properties of crystalline materials in which crystalline structures play decisive roles in functionalities such as solar light conversion efficiency. 6) Molecules in crystalline lattice matrixes are aligned via vdW&Clmb.…”

“…The diffusion length of solar cells is defined by the electron lifetime and electron diffusion coefficient of the solar cell devices being affected by electron leakage at molecular heterojunctions. In the preceding paper, 3) we successfully attempted to verify the high efficiency of PbI ) 4 -aligned solar cells. 18,19)…”