The Relationship between Chemical Flexibility and Nanoscale Charge Collection in Hybrid Halide Perovskites

Abstract

Hybrid organometal halide perovskites are known for their excellent optoelectronic functionality as well as their wide-ranging chemical flexibility. The composition of hybrid perovskite devices has trended toward increasing complexity as fine-tuned properties are pursued, including multielement mixing on the constituents A and B and halide sites. However, this tunability presents potential challenges for charge extraction in functional devices. Poor consistency and repeatability between devices may arise due to variations in composition and microstructure. Within a single device, spatial heterogeneity in composition and phase segregation may limit the device from achieving its performance potential. This review details how the nanoscale elemental distribution and charge collection in hybrid perovskite materials evolve as chemical complexity increases, highlighting recent results using nondestructive operando synchrotron-based X-ray nanoprobe techniques. The results reveal a strong link between local chemistry and charge collection that must be controlled to develop robust, high-performance hybrid perovskite materials for optoelectronic devices.applications including solar cells, [1,2] lightemitting diodes, [3] lasers, [4] and photodetectors. [5] The exceptional minority carrier diffusion lengths in these materials [6,7] lead to nearly 100% internal quantum efficiency [8] which results in high charge-carrier collection efficiency and high external luminescence efficiency in electron-photon conversion devices. [9] Particularly in the field of photovoltaics (PV), their extraordinary material properties [10][11][12] have enabled perovskite solar absorbers to achieve large improvements in device performance in the past 8 years. The perovskite crystal structure is shown in Figure 1a, where the A-site is CH 3 NH 3 + (MA, methylammonium), the B-site is Pb 2+ , and the X-site is I − following the general perovskite formula ABX 3. After demonstration of a device with 3.8% power conversion efficiency (PCE) by Miyasaka and co-workers in a dye-sensitized solar cell architecture using CH 3 NH 3 PbI 3 , [13] a breakthrough in perovskite photovoltaics occurred in 2012 when the first allsolid-state hybrid perovskite devices were shown by Kim et al., [14] improving the chemical stability of the perovskite and enabling device performance to exceed beyond 9% using CH 3 NH 3 PbI 3 perovskites. With intense investigation of perovskite material properties from research groups all over the world, including bandgap engineering by halide mixing [15,16] and device optimization using A-site mixing, [9,17] the record PCE of hybrid perovskite solar cells reached 22.7% in 2017 after achieving 22.1% PCE in 2015 [18] using a mixture of formamidinium lead iodide (CH(NH 2 ) 2 PbI 3 ) with 5% loading of methylammonium lead bromide (CH 3 NH 3 PbBr 3 ) chemistry. [19,20] Surpassing 22% PCE by leveraging the chemical flexibility of the perovskite structure brought hybrid perovskite solar cells on par in efficiency with most polycrystalline solar absor...

Imaging and Mapping Characterization Tools for Perovskite Solar Cells

“…[111][112][113] Heterogeneous distribution was found in the maps (Figure 13A), showing a molar ratio from 2.25 to 2.85, with average measured stoichiometry of 2.6 and standard deviation of 0.09; substoichiometry was observed, since a ratio of 3 was expected. [88] It was found that current collection increased as the molar ratio reached the stoichiometric value. Furthermore, when observing the XBIC current collection map a comparable heterogeneity is observed.…”

“…Specifically, synchrotron-based XRF uses X-rays with 10 ppm sensitivity at a spatial resolution of 1 µm, [87] bulk penetration, while reducing the risk of sample damage or degradation. [80,88] These photons have a characteristic energy based on the element that was excited, therefore, it is the signal measured by the detector in the XRF instrument that will determine the constituent elements. [80] This way, a blast of energy is released which will be characteristic of a specific element.…”

“…This subject is important to understand the limiting mechanisms in PSCs due to hysteresis. The process can be slightly reverted if a field is induced in the opposite direction, however, for large voltages (e.g., 2 V), the effects seem to be mostly irreversible in the timeframe studied by Luo et al [88] The combination of the quantitative analysis of ion migration with local optoelectronic characterization provided insights to the fundamental operation of PSCs, especially in the context of mixed halide perovskites that now constitute the highest performing PSCs. XRF was selected as measuring technique due to its advantages to detect heavy metals in the bulk with a high spatial resolution without affecting the elemental distribution in the sample, and due to its operando features.…”

“…[88] MAPbBr 3 PSCs were measured using X-ray beam with 250 nm spot size at APS beamline 2-ID-D under helium flow. [88] MAPbBr 3 PSCs were measured using X-ray beam with 250 nm spot size at APS beamline 2-ID-D under helium flow.…”

“…[88] MAPbBr 3 PSCs were measured using X-ray beam with 250 nm spot size at APS beamline 2-ID-D under helium flow. [88] The heterogeneity observed in the XRF maps for this study is relevant due to a large range and standard deviation of the Br:Pb ratios. [111][112][113] Heterogeneous distribution was found in the maps (Figure 13A), showing a molar ratio from 2.25 to 2.85, with average measured stoichiometry of 2.6 and standard deviation of 0.09; substoichiometry was observed, since a ratio of 3 was expected.…”