Direct measurement of the exciton binding energy and effective masses for charge carriers in organic–inorganic tri-halide perovskites

Abstract

Solar cells based on the organic-inorganic tri-halide perovskite family of materials have shown significant progress recently, o ering the prospect of low-cost solar energy from devices that are very simple to process. Fundamental to understanding the operation of these devices is the exciton binding energy, which has proved both di cult to measure directly and controversial. We demonstrate that by using very high magnetic fields it is possible to make an accurate and direct spectroscopic measurement of the exciton binding energy, which we find to be only 16 meV at low temperatures, over three times smaller than has been previously assumed. In the room-temperature phase we show that the binding energy falls to even smaller values of only a few millielectronvolts, which explains their excellent device performance as being due to spontaneous freecarrier generation following light absorption. Additionally, we determine the excitonic reduced e ective mass to be 0.104m e (where m e is the electron mass), significantly smaller than previously estimated experimentally but in good agreement with recent calculations. Our work provides crucial information about the photophysics of these materials, which will in turn allow improved optoelectronic device operation and better understanding of their electronic properties.T he recent rapid development of perovskite solar cells is revolutionizing the photovoltaic research field, with the latest certified power conversion efficiencies reaching over 20% (ref. 1). Initially developed from the concept of the nanostructured excitonic solar cell where there is no requirement for longrange charge or exciton diffusion 1-8 , it has now become clear that owing to the remarkable properties of the inorganic-organic perovskite family of materials ABX 3 (A = CH 3 NH 3 + ; B = Pb 2+ ; and X = Cl − , I − and/or Br − ) these cells 2-10 are capable of operating in a comparable configuration and with comparable performance to the best inorganic semiconductors 7,9,10 , where the solid absorber layer is sandwiched between n-and p-type charge selective contacts in a planar heterojunction configuration 7,8 . Despite this success, several fundamental properties of the organic lead tri-halide perovskites remain controversial and poorly known. In particular the binding energy of the excitons (R * ), bound electron-hole pairs that are the primary photoexcited species created in the absorption process, is vital to understanding the way that the cells function. The operating mechanisms depend on what fraction of excitons dissociate in the bulk material, giving rise to free-charge transport, or what fraction need to be dissociated at heterojunctions within the cells. Knowledge of the true exciton binding energy is also crucial for interpreting spectroscopic measurements based on these materials, such as time-resolved spectroscopy. Values for R * reported in the literature cover a broad range from 2 to 55 meV (refs 11-17), with the larger values being initially adopted and a growing number of reports suggesti...

Breathing bands due to molecular order in CH3NH3PbI3

“…Frost et al estimated the binding energy of excitons in this system to be about 0.7 meV -too small to affect significantly the photovoltaic performance [16]. This is in contradiction with experimental evidences that indicate a value in the range 6-55 meV [5,37,[67][68][69][70][71][72]. Solutions of the Bethe-Salpeter equation for a single molecular orientation revealed huge exciton binding energies of 153 meV for the cubic phase [39] and 40 meV for the tetragonal phase [32].…”

Research Update: Relativistic origin of slow electron-hole recombination in hybrid halide perovskite solar cells

“…This approximation misses the Wannier excitons below the band edge, but they are very shallow. 27,28 If the potential is local,…”

Unveiling the Nature of Charge Carrier Interactions by Electroabsorption Spectroscopy: An Illustration with Lead-Halide Perovskites

“…Conversely, the electronic polarizability of covalent solids generally results in a high electric susceptibility and an efficient screening of the Coulomb interactions between carriers. Now, numerous studies have yielded values of the exciton binding energy in perovskites and, although results show a great variability, they all converge to surprisingly low values of 2-60 meV for CH 3 NH 3 PbI 3 and up to 150 meV for CH 3 NH 3 PbBr 3 , [17][18][19][20][21][22][23] indicating that mostly free carriers are generated at room temperature. In addition, it has been shown that the precise character of the photogenerated carriers, as well as the ratio of free carriers to bound electron-hole pairs are highly dependent upon the nanostructuring of the material.…”