Publication date: 14th January 2021
Halide perovskites perform remarkably in optoelectronic devices [1,2]. This class of materials exhibits a considerable defect tolerance, with long charge carrier lifetimes in spite of large concentrations of defects, which has been related to exotic phenomena such as polarons, Rashba effect and photon recycling [3,4]. However, their performance is still not properly understood given they exhibit compositional and structural heterogeneity in addition to large concentrations of deep charge carrier traps in well localized spatial clusters [5,6]. In this talk, we resolve this paradox revealing that compositional disorder of the perovskite can induce carrier funneling away from deep trap states, improving performance.
We use a series of multimodal microscopy techniques based on synchrotron nanoprobe and optical spectroscopic techniques, including hyperspectral and transient absorption microscopy. We map the local chemical and structural heterogeneity of device relevant FA0.79MA0.16Cs0.05Pb(I0.83Br0.17)3 (FA=formamidinium, MA=methylammonium) perovskite samples on the nanoscale using nano X-ray fluorescence and diffraction. We spatially correlate these measurements with local values of quasi-Fermi level splitting and Urbach energy, which are important device metrics, as well as ultrafast carrier dynamics from transient absorption microscopy. We show that local quasi-Fermi level hotspots are caused by carriers funneling onto local regions with low electronic disorder. Importantly, this funneling mechanism collects charges over micrometers and thus sequesters carriers away from trap clusters associated with higher electronic disorder. The visualization of this nanoscale landscape provides an explanation for the purported defect tolerance in these materials and may open an avenue for a new class of intrinsically disordered but high performing materials.
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