Effect Of the organic cation on the optical properties of lead iodine perovskites

Abstract

The effect of 14 organic cations on the optical properties of the lead iodine perovskite is analyzed. The electronic and optical properties are obtained using first principles. The absorption coefficients are split into inter-atomic species components in order to quantify all of the contributions. For energies close to the bandgaps, the main contribution is from the Pb-Pb intra-species transitions. For higher energy this contribution is still important in addition to I-I and Pb-I contributions, and to the 3 and 4 species term. Almost all absorption properties are qualitatively similar. Furthermore, this absorption coefficient splitting also allows the optical characteristics that the substitution of Pb by another element should satisfy to be identified in order to reduce the toxicity because of Pb while maintaining a high absorption capacity. Introduction.Lead iodine perovskites are promising semiconductors for solar energy conversion. The most studied so far is the methylammonium because of its electronic properties, low non-radiative recombination, and its high device efficiencies15-20 % [1,2]. In addition to their efficiencies, the perovskites have attracted interest because of the novel device structures and new perovskite materials [3][4][5][6][7].

Effect of the structure of lead iodine perovskites on the photovoltaic efficiencies

“…The momentum matrix elements can be split [19,20] as where is the inter-species (intra-species if A=B) component coupling the basis set functions on different species atoms A and B. For direct transitions the transition probabily is proportional to .…”

“…Therefore the absorption coefficient can be split as an exact many-species expansion [19,20] with terms no larger than those of 4 species: . The first term involves intra-species (A=B) and inter-species (A≠B) contributions .…”

“…Obviously and . Then if the absorption coefficient is expressed as a many-species expansion [19,20], the currents and efficiencies can also be split into as [14,[37][38][39]. As a consequence the electronic properties for energies close to the bandgap are directly related to the octahedra.…”

“…However, the size of the organic cation can distort the octahedra, and therefore, indirectly influence the electronic properties for energies near the bandgap. Furthermore, as we will show later, the contribution of the organic cation states to the absorption coefficient is appreciable for energies above the bandgap [19,20].…”

Contributions to Optical Properties and Efficiencies of Methyl–Ammonium Lead, Tin, and Germanium Iodide Perovskites

“…The parameters of the structures used in the calculation were taken from reference 25 The optical properties were obtained from the complex dielectric function using the Kramers-Kronig relationships 26 . They can be expressed as an exact many-species expansion 27,28 with terms that involve up to 4 species. It allows the different contributions to the absorption coefficient , where e g is the photon flux density emitted.…”