Determination of the exciton binding energy and effective masses for methylammonium and formamidinium lead tri-halide perovskite semiconductors

Abstract

The family of organic-inorganic halide perovskite materials has generated tremendous interest in the field of photovoltaics due to their high power conversion efficiencies. There has been intensive development of cells based on the archetypal methylammonium (MA) and recently introduced formamidinium (FA) materials, however, there is still considerable controversy over their fundamental electronic properties. Two of the most important parameters are the binding energy of the exciton (R * ) and its reduced effective mass µ. Here we present extensive magneto optical studies of Cl assisted grown MAPbI 3 as well as MAPbBr 3 and the FA based materials FAPbI 3 and FAPbBr 3 . We fit the excitonic states as a hydrogenic atom in magnetic field and the Landau levels for free carriers to give R * and µ. The values of the exciton binding energy are in the range 14 -25 meV in the low temperature phase and fall considerably at higher temperatures for the tri-iodides, consistent with free carrier behaviour in all devices made from these materials. Both R * and µ increase approximately proportionally to the band gap, and the mass values, 0.09-0.117 m 0 , are consistent with a simple k.p perturbation approach to the band structure which can be generalized to predict values for the effective mass and binding energy for other members of this perovskite family of materials. Broader contextThe recent development of organic/inorganic perovskite semiconductors has had a dramatic impact on the field of thin-film solar cells leading to efficiencies of over 20% in materials such as MAPbI 3 . We have recently shown that significant factors contributing to their remarkable performance are the small excitonic reduced effective mass and the small exciton binding energy. Expanding the perovskite family to materials with a range of different band gaps is opening up the potential of this materials system for a range of different applications, including the design of tandem PV cells and other optoelectronic components such as lasers and light emitting diodes. Knowledge of basic materials parameters such as the effective masses of the charge carriers is vital to the use and design of devices using perovskites. Here we show that the effective masses remain small in this system up to band gaps as high as 2.3 eV, as found in MAPbBr 3 and the exciton binding energy in this system is also much smaller than previously reported. This suggests that sophisticated devices using large band gap perovskites can be expected to show free carrier behaviour at room temperature and will make device design significantly easier. Exchanging the organic cations (Methyammonium or Formamodimium) is also shown to have very little effect on their bandstructure, suggesting that they can be used interchangeably to enhance device performance and stability. IntroductionOrganic-inorganic hybrid materials, in particular the tri-halide perovskites, have been driving a rapid series of breakthroughs in the field of photovoltaic (PV) devices with conversion efficiencies already up t...

Temperature-dependent optical spectra of single-crystal (CH3NH3)PbBr3 cleaved in ultrahigh vacuum

“…Moreover, the related exciton binding energy Fig. 7 (c), is in agreement with the value of 25 ± 5 meV reported from low-temperature magnetoabsorption [23].…”

“…For example, exciton binding energies between 15 and 84 meV [23,[37][38][39][40][41][42] have been reported for (CH 3 NH 3 )PbBr 3 . These variations can be expected to be reduced by studying single crystals [43,44], opening the opportunity to approach the intrinsic photophysics of OIPS.…”

High Open‐Circuit Voltages in Tin‐Rich Low‐Bandgap Perovskite‐Based Planar Heterojunction Photovoltaics

“…[12][13][14][15][16][17] The vast majority of research conducted in this area has focused on "first generation" MAPbX 3 -based perovskites, [8,18] mainly due to the excellent tunability of their optoelectronic properties. [18,19] Characteristics such as high mobility, [20,21] high bimolecular recombination rate for charge carriers, [10] and low exciton binding energy [22][23][24] have resulted in the demonstration of high-performance films and devices for PV, LED, [9,12,[25][26][27] FET, [21] and lasing applications. [10,28] One potential drawback of the MAPbX 3 semiconductor family, however, is its relatively wide bandgap (3.2-1.6 eV), which dramatically limits its sensitivity in the near-IR (NIR) to mid-IR region of the solar spectrum.…”

Hybrid Perovskites: Effective Crystal Growth for Optoelectronic Applications

“…Common Requirements [46,208] Copyright 2016, Nature Magneto-optical absorption spectroscopy [69] Polycrystalline 3D 22 Magneto-optical absorption spectroscopy [71] Polycrystalline 3D 2 Spectroscopic ellipsometry [66] Polycrystalline 3D 13 Optical absorption [72] Polycrystalline 3D 7 Electroabsorption Spectroscopy [8] Polycrystalline 3D 12 Magneto-optical absorption spectroscopy [73] CH3NH3PbBr3 Microcrystalline 3D 84 X-ray spectroscopy/Temperature dependent PL [74] Nanoparticle 3D 320 X-ray spectroscopy [74] Polycrystalline 3D 76 Magnetoabsorption [71] Polycrystalline 3D 40 Optical absorption [72] CH3NH3PbI3-xClx Polycrystalline 3D 55 Optical absorption [67] Polycrystalline 3D 10 Magneto-optical absorption spectroscopy [73] CH(NH2)2PbI3 Polycrystalline 3D 10 Magneto-optical absorption spectroscopy [73] CH(NH2)2PbBr3 Polycrystalline 3D 24 Magneto-optical absorption spectroscopy [73] (C9H19NH3)2PbI4 n/a 2D > 330 Temperature dependent PL [75] (C10H21NH3)2PbI4 Single crystal 2D 370 Optical absorption [76] (C6H5C2H4NH3)2PbI4 Polycrystalline 2D 350 Optical absorption [77] (NH2C(I)=NH2)3PbI5 n/a 1D > 410 Optical absorption [75] (CH3NH3)4PbI6•2H2O n/a 0D 545 Optical absorption [75] Eb: Exciton binding energy, PL: Photoluminescence Reproduced with permission. [98] Copyright 2014, Nature Publishing Group.…”