Charge‐Carrier Dynamics and Mobilities in Formamidinium Lead Mixed‐Halide Perovskites

Abstract

by at least four orders of magnitude. [ 17,18 ] MAPbI 3 also appears to exhibit only shallow trap-levels and although the grain boundaries have recently been shown to induce nonradiative decay, [ 19 ] regions only a few tens of nm away from the grain boundaries appear to be unaffected. [ 20,21 ] Furthermore, low Urbach energies, which are extracted from near band edge optical absorption measurements and serve as a benchmark for crystalline phase disorder, indicate low disorder and sharp band edges for lead tri-halide perovskites (15-23 meV). [ 22,23 ] All of these properties contribute to high open-circuit voltages, long charge-carrier lifetimes, and micrometer diffusion lengths, which are crucial for planar-hetero junction photovoltaics. [ 1,24 ] Nonetheless, these parameters are also expected to depend on the infl uence of crystallization condition on perovskite morphology in fabricated fi lms. [ 25 ] A distinct benefi t of organic-inorganic perovskite materials (e.g., over silicon) is that their bandgap can be tuned relatively easily with chemical composition, allowing attractive coloration and multijunction or tandem cell designs. For example, changing the metal cation at the M site from Pb 2+ to the less toxic Sn 2+ to form CH 3 NH 3 SnI 3 shifts the optical bandgap from 1.55 to 1.3 eV into the range of the "ideal" single-junction solar cell bandgap between 1.1 and 1.4 eV. [ 26,27 ] However, stability issues arising from the oxidation of tin have so far prevented widespread use. Alternatively, tuning the size of the A site cation has been proven to change optical and electronic properties of the perovskite and to signifi cantly infl uence solar cell performance. [ 28 ] Replacing MA in MAPbI 3 by the larger cation formamidinium HC(NH 2 ) 2 + (FA) was found to decrease the bandgap from 1.57 to 1.48 eV, [29][30][31] yield long photoluminescence (PL) lifetimes, high PCEs, [ 28 ] and lower recombination and device hysteresis. [ 32 ] Highest PCEs of 20.1% have been reported [12][13][14] to date for solar cells based on FAPbI 3 making this an attractive system to explore. In addition, the gradual replacement of the MA cation by FA through the fi lm was shown to create a mixed cation-lead-iodide PSC allowing for energetic gradients. [ 33 ] However, mixing of the halide component in the perovskite offers the fi nest tuning of the optical properties of the perovskite fi lm. Here, the mixed organic lead iodide/bromide system has recently gained strong interest for application in PSCs. [ 11,28 ] By changing the ratio between bromide and iodide (at the X site anion), the bandgap can be tailored between 1.55 eV (MAPbI 3 ) and 2.3 eV (MAPbBr 3 ), which results in the coverage of much of the visible spectrum and paves the way for the development of tandem solar cells. [ 11 ] In addition to MAPb(Br y I 1-y ) 3 , its formamidinium relative FAPb(Br y I 1-y ) 3 has been explored. [ 28 ] Most fractional mixtures of FAPb(Br y I 1-y ) 3 were found to be crystalline, with the exception of the region between y = 0.3 and Recent year...

The Emergence of the Mixed Perovskites and Their Applications as Solar Cells

“…This phenomenon was also documented in other reports. [166,173] Moreover, this photo-induced process was reversible: when relaxed in the dark, the film reverted back to its original single-phase state. Combining multiscale modeling with nanoscale imaging, Bischak [173] et al explored the origin of reversible phase separation.…”

“…Based on the FAPb(Br y I 1−y ) 3 , Herz's group systematically investigated this issue. [165][166][167] In general, there are three types of charge-carrier recombination within a semiconductor: Shockley-Read-Hall recombination, radiative recombination and Auger recombination. The Shockley-Read-Hall recombination rate reflects the defect density in a semiconductor material.…”

“…Results indicated that the radiative recombination increased monotonically with the content of Br and it may be attributed to the modified band structure. [166] In addition, Auger recombination involves three carriers: an electron recombines with a hole and gives energy to a third carrier. The energy and momentum required in the three bodies to finish the process is quite strict, which depends on the electronic band structure within the polycrystalline PVSK thin film.…”

Perovskite-Inspired Photovoltaic Materials: Toward Best Practices in Materials Characterization and Calculations

“…While one of the aims of exploring PIMs is to find non-toxic materials, it should not be assumed that lead-free or thallium-free compounds are non-toxic. For example, Sn 2+ is generally considered less toxic than Pb 2+ , 218 and some Sn-based compounds, such as SnS, 219 are indeed not hazardous. However, other compounds, such as SnCl 2 , have the same oral toxicity classification as PbI 2 .…”

A mixed-cation lead mixed-halide perovskite absorber for tandem solar cells

“…Furthermore, as iodide is substituted with bromide, a crystal phase transition occurs from a trigonal to a cubic structure; in compositions near the transition, the material is unable to crystallize, resulting in an apparently "amorphous" phase with high levels of energetic disorder and unexpectedly low absorption. These compositions additionally have much lower charge-carrier mobilities in the range of 1 cm 2 V -1 s -1 , in comparison to > 20 cm 2 V -1 s -1 , in the neat iodide perovskite (17). For tandem applications, these problems arise at the Br composition needed to form the desired top-cell band gap of ~1.7 to 1.8 eV.…”