A Highly Emissive Surface Layer in Mixed‐Halide Multication Perovskites

Abstract

Mixed‐halide lead perovskites have attracted significant attention in the field of photovoltaics and other optoelectronic applications due to their promising bandgap tunability and device performance. Here, the changes in photoluminescence and photoconductance of solution‐processed triple‐cation mixed‐halide (Cs0.06MA0.15FA0.79)Pb(Br0.4I0.6)3 perovskite films (MA: methylammonium, FA: formamidinium) are studied under solar‐equivalent illumination. It is found that the illumination leads to localized surface sites of iodide‐rich perovskite intermixed with passivating PbI2 material. Time‐ and spectrally resolved photoluminescence measurements reveal that photoexcited charges efficiently transfer to the passivated iodide‐rich perovskite surface layer, leading to high local carrier densities on these sites. The carriers on this surface layer therefore recombine with a high radiative efficiency, with the photoluminescence quantum efficiency of the film under solar excitation densities increasing from 3% to over 45%. At higher excitation densities, nonradiative Auger recombination starts to dominate due to the extremely high concentration of charges on the surface layer. This work reveals new insight into phase segregation of mixed‐halide mixed‐cation perovskites, as well as routes to highly luminescent films by controlling charge density and transfer in novel device structures.

2D/3D Heterostructure for Semitransparent Perovskite Solar Cells with Engineered Bandgap Enables Efficiencies Exceeding 25% in Four‐Terminal Tandems with Silicon and CIGS

“…[ 3,52,69,73,74 ] A common issue is photo‐induced phase segregation and subsequent funneling of charge carriers into iodide‐rich lower bandgap regions, [ 75 ] acting as radiative recombination centers. [ 68,69,74–80 ] In addition, a higher density of defect states [ 41,79,81,82 ] and stronger interfacial recombination (e.g., due to energy level offsets) [ 51,79,83–87 ] have been proposed to increase non‐radiative recombination losses. Finally, the concentration of vacancy defects and strength of phase segregation seem to be directly linked to each other.…”

“…[ 72,74,79,80,88–90 ] All these factors result in a larger open‐circuit‐voltage ( V OC ) deficit (defined by E g /q − V OC ) for wide‐bandgap PSCs (>≈0.5 V) as compared to state‐of‐the‐art low‐bandgap PSCs (≈0.35–0.4 V), negating a linear increase of V OC with increasing bandgap. [ 3,52 ] We would like to stress that the relative contribution of the above mentioned factors to the V OC deficit is a complicated function of the exact perovskite composition, [ 69,72,91 ] film surface nature, [ 78,84,90 ] defect density as well as the charge extraction layers employed. [ 79,86,87 ] One popular approach to reduce the V OC deficit is to deposit a large organic cation on the surface of a 3D perovskite film, which acts as a 2D passivation agent and/or improves the energetic alignment with the charge transport layer.…”

Elucidating and Mitigating Degradation Processes in Perovskite Light‐Emitting Diodes

“…These results at higher carrier densities are consistent with radiative recombination on the bromide-rich layers being able to more effectively compete with processes including downhill energy transfer, charge trapping, and charge quenching by the adjacent contact compared to measurements at lower carrier densities. [33] Figure 2. STEM-EDX maps of a,b) bromine (Br-K α ), and (d, e) iodine (I-L α ) distributions for a pristine device a,d), and a device after 3 h of operation at 2 V b,e).…”

“…Passivation with alkali metals such as potassium have been previously shown to reduce nonradiative losses and ion migration in perovskite devices. [29][30][31][32][33][34][35][36][37][38][39][40] We prepared a series of passivated perovskite thin films by addition of different amounts of potassium iodide solution (in the range x = 0-0. 44 3 , with y = 0-1 being the fraction of bromide out of total halide (with corresponding PL emission peaks spanning 560 to 826 nm, see Figure S7, Supporting Information).…”

Study of transport parameters and defect states in thin film perovskites under different environments − air or vacuum − and after light-soaking

“…Evolution of the band gap with light-soaking was also observed by other groups either from small variations of the absorptance spectra [33] or from Photo Deflection Spectroscopy (PDS) [34]. They have attributed these variations to modifications of the surface of the films by creation of low gap regions and trapping states.…”