Publication date: 2nd November 2020
The last decade has seen the most spectacular rise of the halide perovskites with the general formula of ABX3, where X = I, Br or Cl. Their photovoltaic and light emissive properties, almost invariably with B = Pb, have reached superlative levels of performance within this exceptionally short span of time. The A site is known to accommodate methyl ammonium (MA)+, (CH3NH3)+, formamidinium (FA), (CH(NH2)2)+ and Cs+ ions while retaining the perovskite structure. MAPbI3 is possibly the most investigated among the pure systems. Despite the unprecedented rise in the photovoltaic efficiency with MAPbI3 as the active material in solar cells, rapid degradation of the performance arising from multiple contributing factors has been a vexing issue in this field. Through numerous experiments, it has now been established that A-site cationic substitutions, such as FA for MA to form MA1-xFAxPbX3, can increase the stability and the performance of these materials under real-life operating conditions. However, the possible mechanisms behind such improvements or even the microscopic changes brought about by such substitutions are not very well understood at present. We address these issues by first making a comparative study of the pure end-members, MAPbX3 and FAPbX3 to understand their distinctive features at some fundamental aspects. Our results establish that the dielectric properties of the two are very different arising from a strong contribution from nearly freely rotating dipoles present for the MA containing compounds, while being absent for the FA series.1 Quasi-elastic neutron scattering experiments over a wide temperature range also underline the differences in the rotational dynamics of these two organic moieties.2 We then investigated3 the substitutional series, MA1–xFAxPbI3, for a range of compositions (x) and over a wide temperature range to cover all crystallographic forms present to map out the structural phase diagram in the temperature–composition phase space; four crystallographic phases are found to exist for this solid solution series, namely, cubic (Pm-3m), tetragonal (I4/mcm), orthorhombic (Pnma), and large-cell cubic (Im-3). Temperature and frequency dependent dielectric measurements show remarkable dependency of dielectric properties on the specific crystal structures; significantly, it is seen that for the most relevant compositions, the presence of FA ions hinder the nearly-free rotations of the MA units, giving rise to a glassy dipolar state. It is tempting to associate this glassy, locked state of significantly FA doped MA1–xFAxPbI3 with the enhanced stability of similar compositions. We also find evidence of this systematically enhanced stability on doping of FA from our analysis of the temperature dependent photoluminescence from this solid solution with different composition, x.4 We shall present these results to emphasize the structure-property correlation in MA1–xFAxPbI3, touching on the possible origin of the enhanced stability with FA doping of MAPbI3. If time permits, I may briefly discuss the consequences of small amount of Cs doping, forming Csδ(MA1–xFAx)1-δ PbI3 on these properties reported here.4
This work is a result of several collaborations with C De, J. P. Embs, TN Guru Row, A Hossain, BP Kore, A Mohanty, R. Mukhopadhyay, Sharada G, V. K. Sharma, D Swain, and M. Tyagi.
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