Impact of Excess Lead Iodide on the Recombination Kinetics in Metal Halide Perovskites

Fundmental comprehension of light-induced processes in perovskites are still scarce. One active debate surrounds the influence of excess lead iodide (PbI2) on device performance, as well as optoelectronic properties, where both beneficial and detrimental traits have been reported. Here, we study its impact on charge carrier recombination kinetics by simultaneously acquiring the photoluminescence quantum yield and time-resolved photoluminescence as a function of excitation wavelength (450–780 nm). The presence of PbI2 in the perovskite film is identified via a unique spectroscopic signature in the PLQY spectrum. Probing the recombination in the presence and absence of this signature, we detect a radiative bimolecular recombination mechanism induced by PbI2. Spatially resolving the photoluminescence, we determine that this radiative process occurs in a small volume at the PbI2/perovskite interface, which is only active when charge carriers are generated in PbI2, and therefore provide deeper insight into how excess PbI2 may improve the properties of perovskite-based devices

Luminescent Intermediates and Humidity Dependent Room Temperature Conversion of the MAPbI 3 Perovskite Precursor

Preparation of metal halide perovskites under room temperature attracts attention because of energy saving by removing thermal annealing. Room temperature trans formation of spin cast wet films consisting of methylammo nium MA iodide, PbI2, and dimethylformamide toward solid MAPbI3 perovskite proceeds via several intermediate crystal line states and is strongly dependent on ambient humidity. Light transmission and photoluminescence PL microscopy and spectroscopy were used to monitor the growth of crystals and transformation of their properties in time under nitrogen atmosphere at room temperature. Under low humidity, a highly luminescent intermediate phase with low absorption in the visible range appears, with the PL spectra composed of several bands in the range from 600 to 760 nm. We assign these bands to low dimensional nanocrystals and two dimensional inclusions MAPbI3 intermediates, where the exciton confinement shifts the spectrum to higher energies in comparison with the bulk MAPbI3. The intermediate levels of ambient humidity 10 amp; 8722;50 appear to catalyze the conversion of the intermediate phase to MAPbI3. At a high ambient humidity gt;80 , the initially formed MAPbI3 is quickly transformed to the transparent hydrate phase of MAPbI3. The role of ambient water catalyzing the material transformation by competing for Pb coordination with the solvent molecules is discusse

Complex evolution of photoluminescence during phase segregation of MAPb I1 xBrx 3 mixed halide perovskite

Under illumination, the photoluminescence of a mixed-halide perovskite such as MAPb(I1-xBrx)3 is known to undergo a significant intensity enhancement while spectrally shifting to lower energies. The evolution of low energy photoluminescence is attributed to the formation of iodide rich domains due to phase segregation. This process is detrimental for optoelectronic devices however the mechanism is not well-understood. Here we present a real-time study of the photoluminescence evolution in MAPb(I1-xBrx)3 samples during light-induced phase segregation. We show that the evolution of photoluminescence proceeds via several intermediate stages making it more complex than previously suggested. Within the first few seconds of photo-excitation, we found a very rapid formation of a short-lived intense photoluminescence band with a peak energy even lower than the final emission of the fully segregated sample. We propose that this emission comes from small pure iodide nano-domains formed during the initial stage of photo-induced phase-segregation

Excitation wavelength dependence of photoluminescence flickering in degraded MAPbI 3 perovskite and its connection to lead iodide formation

Metal halide perovskite semiconductors often exhibit photoluminescence blinking and flickering when luminescence of individual small nano- or even microcrystals is monitored. The nature of these fluctuations is not well understood but must be related to the presence of metastable non-radiative recombination channels and efficient charge migration in these materials. Here we report on the excitation wavelength dependence of photoluminescence flickering effect in degraded methylammonium lead iodide (MAPbI3) thin films. While the luminescence intensity is temporary stable when excited in the blue region with wavelength shorter than 530 nm, excitation with red light (wavelength longer than 530 nm) results in luminescence flickering. It is hypothesised that the wavelength dependence reflects the excitation energy dependence of the photochemical mechanism that switches non-radiative recombination channels on and off. The effect can also be related to hindered charge carrier diffusion due to their localization in the interfacial layer between MAPbI3 and PbI2 which is formed in the course of degradation

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

Relating Defect Luminescence and Nonradiative Charge Recombination in MAPbI 3 Perovskite Films

Nonradiative losses in semiconductors are related to defects. At cryogenic temperatures, defect-related photoluminescence (PL) at energies lower than the band-edge PL is observed in methylammonium lead triiodide perovskite. We applied multispectral PL imaging to samples prepared by two different procedures and exhibiting 1 order of magnitude different PL quantum yield (PLQY). The high-PLQY sample showed concentration of the emitting defect sites around 1012-1013 cm-3. No correlation between PLQY and the relative intensity of the defect emission was found when micrometer-sized local regions of the same sample were compared. However, a clear positive correlation between the lower PLQY and higher defect emission was observed when two preparation methods were contrasted. Therefore, although the emissive defects are not connected directly with the nonradiative centers and may be spatially separated at the nano scale, chemical processes during the perovskite synthesis promote/prevent formation of both types of defects at the same time

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

Deconvoluting Energy Transport Mechanisms in Metal Halide Perovskites Using CsPbBr3 Nanowires as a Model System

Understanding energy transport in metal halide perovskites is essential to effectively guide further optimization of materials and device designs. However, difficulties to disentangle charge carrier diffusion, photon recycling, and photon transport have led to contradicting reports and uncertainty regarding which mechanism dominates. In this study, monocrystalline CsPbBr3 nanowires serve as 1D model systems to help unravel the respective contribution of energy transport processes in metal halide perovskites. Spatially, temporally, and spectrally resolved photoluminescence PL microscopy reveals characteristic signatures of each transport mechanism from which a robust model describing the PL signal accounting for carrier diffusion, photon propagation, and photon recycling is developed. For the investigated CsPbBr3 nanowires, an ambipolar carrier mobility of amp; 956; 35 cm2 V amp; 8722;1 s amp; 8722;1 is determined, and is found that charge carrier diffusion dominates the energy transport process over photon recycling. Moreover, the general applicability of the developed model is demonstrated on different perovskite compounds by applying it to data provided in previous related reports, from which clarity is gained as to why conflicting reports exist. These findings, therefore, serve as a useful tool to assist future studies aimed at characterizing energy transport mechanisms in semiconductor nanowires using P