Origin of Ionic Inhomogeneity in MAPb IxBr1 x 3 Perovskite Thin Films Revealed by In Situ Spectroscopy during Spin Coating and Annealing

Irradiation induced phase segregation in mixed methylammonium halide perovskite samples such as methylammonium lead bromide iodide, MAPb IxBr1 x 3, is being studied intensively, since it limits the efficiency of wide band gap perovskite solar cells. It has been postulated that this phenomenon depends on the intrinsic ionic in homogeneity in samples induced already during film formation. A deeper understanding of the MAPb IxBr1 x 3 formation processes and the influence of the halide ratio, solvents, and perovskite precursor composition as well as the influence of processing parameters during deposition, e.g. by spin coating and annealing parameters, is still lacking. Here, we use a fiber optic based optical in situ setup to study the formation processes of the MAPb IxBr1 x 3 series on a sub second timescale during spin coating and thermal annealing. In situ UV vis measurements during spin coating reveal the influence of different halide ratios, x, in the precursor solution on the phase crystallizing preferentially. Pure bromide samples directly form a perovskite phase, samples with high iodide content form a solvate intermediate phase and samples with a mixed stoichiometry between 0.1 amp; 8804; x amp; 8804; 0.6 form both. This leads to heterogeneous formation process via two competing reaction pathways, that leads to a heterogeneous mixture of phases, during spin coating and rationalizes the compositional heterogeneity of mixed bromide iodide samples even after annealing

Microscopic insight into the reversibility of photodegradation in MAPbI3 thin films

Whether optoelectronic devices based on metal-halide perovskite semiconductors will become a commercially viable technology will be determined by their intrinsic and operational stability. Recent results indicate there is some reversibility of perovskite degradation in thin films and devices, although mechanistic insight into the processes driving degradation and recovery are still scarce. We here present a comparative spectroscopic study of methylammonium lead iodide (MAPbI3) films having undergone either photo- or thermal degradation under controlled conditions. We confirm that the degradation mechanism pertaining to each type of stress is inherently different. Our results from photoluminescence microscopy measurements paint a spatially, spectrally and temporarily resolved picture showing that, unlike thermally degraded samples, photodegraded samples are in a state that intermittently recovers to luminescent MAPbI3 upon laser excitation. This indicates that rather than irreversibly decomposing, photoinduced degradation leaves MAPbI3 structurally or compositionally intact but induces defects causing non-radiative recombination losses

Alkali salts at Interface modifiers in n i p hybrid perovskite solar cells

After demonstration of a 23 power conversion efficiency, a high operational stability is the next most important scientific and technological challenge in perovskite solar cells PSCs . A potential failure mechanism is tied to a bias induced ion migration, which causes current voltage hysteresis and a decay in the device performance over time. Herein, alkali salts are shown to mitigate hysteresis and stabilize device performance in n i p hybrid planar PSCs. Different alkali salts of potassium chloride, iodide, and nitrate as well as sodium chloride and iodide are deposited from aqueous solution onto the n type contact, based on SnO2, prior to deposition of the perovskite absorber Cs0.05 FA0.83MA0.17 0.95Pb I0.83Br0.17 3. Introduction of potassium based alkali salts suppresses the current voltage hysteresis and stabilizes the operational device stability at the maximum power point. This is attributed to the suppression of hole trapping at the n type selective transport layer SnO2 perovskite interface observed by surface photovoltage spectroscopy, which is interpreted to reduce interfacial recombination and improve charge carrier extraction. The best and most stable performance of 19 is achieved using potassium nitrate as the interface modifier. Devices with higher and more stable performance exhibit substantially lower current transients, analyzed during maximum power point trackin

Role of solution concentration in formation kinetics of bromide perovskite thin films during spin-coating monitored by optical in situ metrology

Optoelectronic devices based on metal halide perovskites continue to show a improved performance, and solution-based coating techniques pave the way for large-area applications. However, not all parameters influencing the thin film formation process of metal halide perovskites are identified and entirely rationalised over their full compositional range, thus hampering optimised thin film fabrication. Furthermore, while the perovskite deposition via spin-coating and annealing is an easily accessible technique, more profound insights into the chemical formation process are still lacking. Varying the precursor solution concentration is commonly used to vary the resulting thin film thickness. This study shows that varying the precursor solution concentration also affects the thin film morphology and optoelectronic quality. Hence, we herein investigate the influence of the precursor solution concentration on the formation process of a pure bromide-based triple cation perovskite (Cs0.05MA0.10FA0.85PbBr3) by fiber-based optical in situ measurement. During the spin-coating process, in situ UV-vis and PL measurements reveal formation kinetics are strongly dependent on the concentration. Furthermore, we identify delayed nucleation and retarded growth kinetics for more concentrated precursor solutions. In addition, we quantify the shifting chemical equilibrium of colloidal pre-coordination in the precursor solution depending on concentration. Namely, colloids are pre-organised to a higher degree and higher-coordination lead-bromide complexes tend to form in more concentrated precursor solutions. Thus, the modified solution chemistry rationalises retarded perovskite formation kinetics and highlights the precursor concentration as an influential and optimisable parameter for solution-based thin film deposition