Alleviating nanostructural phase impurities enhances the optoelectronic properties, device performance and stability of cesium-formamidinium metal–halide perovskites

The technique of alloying FA+ with Cs+ is often used to promote structural stabilization of the desirable α-FAPbI3 phase in halide perovskite devices. However, the precise mechanisms by which these alloying approaches improve the optoelectronic quality and enhance the stability have remained elusive. In this study, we advance that understanding by investigating the effect of cationic alloying in CsxFA1−xPbI3 perovskite thin-films and solar-cell devices. Selected-area electron diffraction patterns combined with microwave conductivity measurements reveal that fine Cs+ tuning (Cs0.15FA0.85PbI3) leads to a minimization of stacking faults and an increase in the photoconductivity of the perovskite films. Ultra-sensitive external quantum efficiency, kelvin-probe force microscopy and photoluminescence quantum yield measurements demonstrate similar Urbach energy values, comparable surface potential fluctuations and marginal impact on radiative emission yields, respectively, irrespective of Cs content. Despite this, these nanoscopic defects appear to have a detrimental impact on inter-grains’/domains’ carrier transport, as evidenced by conductive-atomic force microscopy and corroborated by drastically reduced solar cell performance. Importantly, encapsulated Cs0.15FA0.85PbI3 devices show robust operational stability retaining 85% of the initial steady-state power conversion efficiency for 1400 hours under continuous 1 sun illumination at 35 °C, in open-circuit conditions. Our findings provide nuance to the famous defect tolerance of halide perovskites while providing solid evidence about the detrimental impact of these subtle structural imperfections on the long-term operational stability.

Supplementary information Note1.The quantification of the {111} C SFs and δ-CsPbI 3 domains is based on counting the diffracting domains, that solely appear with a dark black contrast in the field of view of the BF micrographs.As halide-perovskites degrade quickly under the electron beam, the specimen was never tilted to change the diffraction contrast and therefore any BF image was recorded as initially viewed.Furthermore, any domain that appears to be grey (not in full diffraction conditions) was never counted in our analysis of the {111} C SFs and δ-CsPbI 3 domains.Within the field of view of each micrograph for all samples, any diffracting domain having the characteristic striped contrast was counted as the {111} C SFs while the δ-CsPbI 3 phase is based on the number of indexed domains that are found to be in this phase.According to this methodology, no SFs were found in all the [011] C zone axes of the cubic phases of Cs 0.15 FA 0.85 PbI 3 samples among multiple experiments.However, for the sake of clarity and precision, we cannot exclude the probability about the presence of a random SF domain that we were not able to identify/index as it was not in diffraction conditions or within our field of view.
Electronic Supplementary Material (ESI) for Energy & Environmental Science.This journal is © The Royal Society of Chemistry 2024 Supplementary information Note2.The ionic conductivity is obtained by impedance spectroscopy measurements of ITO/Perovskite/Cu.The conductivity between two parallel plates is defined as where is the resistance, is the thickness of the layer, and is the area.The geometric    capacity is given by where is the material's dielectric constant and the vacuum permittivity.Together with the   0 RC time constant, the conductivity can be expressed as  After a few hundred seconds, all the mobile ions have moved, and the current can be attributed to the electronic conductivity.To avoid internal electric field contributions, for example, due to the different work functions of the electrodes, we took the average current measured after applying a positive and a negative voltage bias, i.e., .As a bias, we use  = ( +  +  - )/2 0.2 V, which is in the approximate linear regime of the JV curve.
The concentration of mobile ions is estimated by integrating the current response after applying a bias, subtracting the contribution by electronic conductivity.With increasing bias, the concentration of mobile ions increases until it levels off close to 0.8 V (Fig. 2b).Note that we limit the bias to 0.8 V as higher biases sometimes irreversibly degrade the device.As a result, the obtained mobile ion density corresponds to a lower bound of mobile ions.We find that the concentration of mobile ions extracted at a bias of 0.8 V remains approximately constant with increasing Cs content (Supplementary Fig. 12).This indicates that the ion conductivity change observed in impedance spectroscopy is related to a change in the mobility of mobile ions.Lett. 6, 1942Lett. 6, -1969Lett. 6, (2021)).
0 where is the frequency at which migration occurs.The electric modulus was used to   determine and , as shown in Fig. 1a.The advantage of the electric modulus is that it    suppresses signals with low impedance, such as interface polarization, that can overlay effects caused by long-range conduction (see Fig. 1b) 1 .

Figure 1 .
Figure 1.a) The real and imaginary part of the modulus and b) the impedance of Cs 0.05 FA 0.95 PbI 3 measured by impedance spectroscopy.At low frequencies, interfacial polarization due to the accumulation of ions at the interfaces dominates the impedance, which is suppressed in the modulus.and are obtained by the imaginary and the real part of the    modulus, respectively.

Figure 2 .
Figure 2. a) Current transient of Cs 0.05 FA 0.95 PbI 3 measured after applying a bias of 0.2 V.The initial decay is related to the redistribution of mobile ions within the perovskite.The steady-

Table S1 .
Goldschmidt tolerance factor (t) values of the Cs x FA 1-x PbI 3 perovskite films.The ionic radii calculations are based on ref.2

Table S2 .
Urbach energy at room temperature (RT) as obtained from apparent Urbach energy An, Y. et al.Structural Stability of Formamidinium-and Cesium-Based Halide Perovskites.ACS Energy