Low temperature excitonic spectroscopy and dynamics as a probe of quality in hybrid perovskite thin films

Abstract

We have developed a framework for using temperature dependent static and dynamic photoluminescence (PL) of hybrid organic-inorganic perovskites (PVSKs) to characterize lattice defects in thin films, based on the presence of nanodomains at low temperature. Our high-stability PVSK films are fabricated using a novel continuous liquid interface propagation technique, and in the tetragonal phase (T > 120 K), they exhibit bi-exponential recombination from free charge carriers with an average PL lifetime of ∼200 ns. Below 120 K, the emergence of the orthorhombic phase is accompanied by a reduction in lifetimes by an order of magnitude, which we establish to be the result of a crossover from free carrier to exciton-dominated radiative recombination. Analysis of the PL as a function of excitation power at different temperatures provides direct evidence that the exciton binding energy is different in the two phases, and using these results, we present a theoretical approach to estimate this variable binding energy. Our findings explain this anomalous low temperature behavior for the first time, attributing it to an inherent fundamental property of the hybrid PVSKs that can be used as an effective probe of thin film quality.

Cation Dynamics in Mixed-Cation (MA)x(FA)1–xPbI3 Hybrid Perovskites from Solid-State NMR

“…4 Further, our finding provides a natural explanation of the fact that charge carrier lifetimes in PV perovskites decrease with temperature, a striking property which stands in stark contrast to classic semiconductors, such as Si or GaAs, in which charge recombination is a phonon-driven process and is thus hampered at lower temperatures. [45][46] More specifically, below 120 K charge carrier lifetimes were observed to drop abruptly, by an order of magnitude, consistent with freezing of the whole-body reorientation of the cation, as determined from 2 H spectra in this work. Other work 47 has reported that the fill factor (FF) of a MAPbI3-based cell decreased roughly linearly with temperature, by a factor of 2.6 between 300 and 100 K, and led to a similar trend in the photovoltaic efficiency (PCE) (decrease by a factor of 3 between 300 and 100 K).…”

Tuning Excitonic Properties of Pure and Mixed Halide Perovskite Thin Films via Interfacial Engineering

“…[41,42] Furthermore, annealing for long time periods has been observed to lead to PVSK decomposition due to the formation of PbI 2 . [43,44] Expectedly, these samples show significantly modified properties. The PVSK thin film deposited on ITO glass appears to be optically almost "dark", and PL is progressively enhanced by the nano-structured, single crystalline, and micro-structured ZnO layers [Figure S4(a)], in that order.…”

“…N tot is the total charge carrier density, directly proportional to the excitation power P, implying for free charge carriers and for excitonic recombination. Therefore, by fitting a second order polynomial to the power dependent PL of PVSK at any fixed temperature T [Figure S5], the quadratic and linear coefficients corresponding to free charge carriers and excitons [25,44] at that T can be This article is protected by copyright. All rights reserved.…”

“…extracted, which in turn allow E B and the relative proportion of free and bound charge carriers to be evaluated, as summarized in Figure 5. We use a formalism [44] based on the Saha-Langmuir equation shown in Equation 2.…”

“…is the reduced exciton mass (~ 0. although in SC-ZnO/PVSK there is a non-monotonicity, with a discontinuity arising around the temperature where the structural phase transition occurs. We have calculated E B averaged over the entire temperature range over which PL is collected, but in some instances [44] , E B has shown different values in the two structural phases, with a smaller binding energy in the low T orthorhombic phase. This might explain the increase in free carriers around the transition.…”