Sequential Cesium Incorporation for Highly Efficient Formamidinium- Cesium Perovskite Solar Cells

Although pure formamidinium iodide perovskite (FAPbI3) possesses an optimal gap for photovoltaics, their poor phase stability limits the long-term operational stability of the devices. A promising approach to enhance their phase stability is to incorporate cesium into FAPbI3. However, state-of-the-art formamidinium-cesium (FA-Cs) iodide perovskites demonstrate much worse efficiency compared with FAPbI3, limited by different crystallization dynamics of formamidinium and cesium, which result in poor composition homogeneity and high trap densities. We develop a novel strategy of crystallization decoupling processes of formamidinium and cesium via a sequential cesium incorporation approach. As such, we obtain highly reproducible and highly efficient solar cells based on FA1-xCsxPbI3 films, with uniform composition distribution and low defect densities. In addition, our cesium-incorporated perovskites demonstrate much enhanced stability compared with FAPbI3, as a result of suppressed ionic migration due to reduced electron-phonon coupling.


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Metal-halide perovskites with superior photophysical properties and low-cost solution process have emerged as promising candidates for different optoelectronic devices, including solar cells, light-emitting diodes, etc(1- 3). For perovskite solar cells (PSCs), different compositions have been attempted for high efficiencies. Among others, formamidinium lead iodide (FAPbI3) have promised great potential, due to their optimal band-gap of ~1.5 eV and excellent thermal stability (4)(5)(6).
However, the photoactive FAPbI3 black phase would easily transform into a non-photoactive yellow δ-FAPbI3 phase at room temperature, especially under humid conditions. The poor phase stability challenges both efficiency and long-term stability of PSCs based on FAPbI3 (7,8). It is generally believed that the phase instability of FAPbI3 perovskites originates from its unsuitable tolerant factor. To address this problem, alloying FA + with MA + /Cs + cations or partially substituting I − with Br − ions have been employed to tune the tolerant factor (9)(10)(11). The resulting mixed-ion FA-based perovskites exhibit improved resistance to phase transition.
However, because of the complex crystallization dynamics of formamidinium and cesium, these pure iodide FA-Cs perovskites suffer from poor composition homogeneity and high defects/traps densities (16,17). The PSCs based on these films are therefore facing relatively low efficiencies.
Especially, strong non-radiative recombination in all reported FA-Cs based PSCs limited the open-circuit voltage (Voc) of the resulting devices (18,19).
Herein, we decouple the crystallization processes of formamidinium and cesium through a sequential Cs incorporation strategy, and achieve highly efficient FA1-xCsxPbI3 (x=0.05-0. 16) perovskites. The ratio of FA and Cs in FA1-xCsxPbI3 can be straightforwardly tuned by 4 introducing different content of Cs on the FA-based perovskite precursor film during the sequential Cs incorporation process. The resulting FA1-xCsxPbI3 perovskites show enhanced phase stability and reduced defects/trap density. As a result, the champion FA0.91Cs0.09PbI3 PSCs yield a record power conversion efficiency (PCE) of 24.7% (certified stabilized 23.8%) with improved Voc and fill factor (FF), which is the highest efficiency for the pure iodide FA1-xCsxPbI3 perovskites. Compared with FAPbI3, the FA0.91Cs0.09PbI3 perovskite shows reduced electronphonon coupling and lattice fluctuations, contributing to the excellent operational stability of the FA0.91Cs0.09PbI3 based PSCs.  (Table S1). The corresponding FA1-xCsxPbI3 perovskite films are noted as x=0.05, x=0.09, x=0. 16.
Optical and structural measurements of perovskite films indicate that Cs has successfully been incorporated into the lattice of FAPbI3 perovskites. Fig. 1A shows the ultraviolet-visible (UVvis) spectra of FA1-xCsxPbI3 perovskite films, in which the absorption edges of FA1-xCsxPbI3 perovskites strongly depend on the amount of Cs incorporation. When x increases from 0 to 0.16, the absorption edge of FA1-xCsxPbI3 perovskites gradually blue-shifts from 816 nm to 802 nm, and the corresponding photoluminescence (PL) peaks shift from 809 nm to 797 nm. The X-ray diffraction (XRD) measurements are carried out to investigate the crystal structure evolution of FA1-xCsxPbI3 perovskites (Fig. 1B). All the FA1-xCsxPbI3 perovskites exhibit stronger peak intensity than the pure FAPbI3 film, suggesting that sequential Cs incorporation enhances the overall crystallinity of perovskite films. The inset image of Fig. 1B shows that the (110) peak between 13.8°-14.1° shifts to a higher degree, indicating that Cs are incorporated into the perovskite lattice. The tolerance factor of FA1-xCsxPbI3 perovskites also reduced compared with pure FA perovskite, which expected to stabilize perovskite structure ( fig. S1).
The Cs incorporation also significantly improves the film morphologies (Fig. 1C). All FA1-xCsxPbI3 films show enlarged and pinhole-free grains compared to the FAPbI3 film, which shows coarse grains and pinholes. As shown in the cross-sectional images, FA1-xCsxPbI3 perovskite 6 films (550 to 600 nm) are composed of micrometer-sized grains comparable to the film thickness, benefiting efficient charge extraction. performance, we then chose the case of FA0.91Cs0.09PbI3 for detailed investigations on Cs incorporation and its role on film and device properties. 7 X-ray photoelectron spectroscopy (XPS) spectra are conducted to explore the effect of sequential Cs incorporation on the chemical composition of perovskite films. All core-level peaks are assigned to Cs, Pb, I, C, and N ( Fig. 2A, 2B, and fig. S3). Fig. 2A S3). These results further confirm that Cs has been successfully incorporated into the FAPbI3 perovskite lattice to form FA0.91Cs0.09PbI3 perovskite.
Since the distribution of Cs in FA1-xCsxPbI3 perovskite has a significant effect on both the phase stability and traps/defects (11,20), we proceed to investigate the distribution of Cs in the resulting FA0.91Cs0.09PbI3 perovskite. The energy dispersive spectroscopy (EDS) mapping and time-of-flight secondary ion mass spectrometry (ToF-SIMS) establish that the incorporated Cs homogenously distributes in the surface and bulk of FA0.91Cs0.09PbI3 perovskite ( Fig. 2C and fig.   S4). In addition, other ions, including FA + , Pb 2+ and I − , are also uniformly distributed throughout the FA0.91Cs0.01PbI3 perovskite film ( fig. S5). Such uniform distribution of Cs ions is beneficial to improve the phase stability and reduce defect concentration. (Fig. 2C).  . S7). In addition, the time-resolved PL (TRPL) spectra in Fig. 2D show that the PL lifetime (τ) of FA0.91Cs0.09PbI3 is much longer (413.84 ns) than that of FAPbI3 (165.49 ns).
Enhanced PL intensity and improved PL lifetime indicate that non-radiative recombination is  Benefiting from these advantages of crystallization decoupling engineering, the resulting FA0.91Cs0.09PbI3 shows much enhanced device performance. Fig. 3A . S9). The most striking difference is the Voc, which increases from 1.09 V in FAPbI3 to 1.18V in FA0.91Cs0.09PbI3. The incident photon to electron conversion efficiency (IPCE) (Fig. 3B) is similar for both devices, with a high value over 90% in the wavelength range of 450~650 nm. The short-circuit current density (Jsc) of the FA0.91Cs0.09PbI3 device is slightly decreased compared with the FAPbI3 device, mainly due to slight increase of the bandgap upon Cs incorporation. Fig. 3C compares the PV parameters of FAPbI3-and FA0.91Cs0.09PbI3-based PSCs for 18 devices respectively, indicating that Cs incorporation also improves the device reproducibility. In addition, the FA0.91Cs0.09PbI3 based PSCs exhibit a smaller hysteresis ( fig. S9 and S10), resulting in a stabilized output power of 24.4% (Fig. 3D), a new record for the pure iodide FA1-xCsxPbI3 based PSCs. We also fabricated large-area PSCs based on these Cs-incorporated perovskite films. The champion FA0.91Cs0.09PbI3 device, fabricated on 2.5-cm by 2.5-cm substrates with an effective cell area of 1 cm 2 (Fig. 3E), display a PCE of 22.4%, which is far higher than that of FAPbI3 devices (~19.9%). 10 The significantly enhanced Voc of the FA0.91Cs0.09PbI3 device is mainly due to suppressed nonradiative recombination, which can be quantified by measuring the external quantum efficiency of electroluminescence (EQEEL) values (21). As shown in Fig. 3F, at the injection current densities corresponding to Jsc, the EQEEL value of the FA0.91Cs0.09PbI3 device is 6.38%, while that of the FAPbI3 device is 0.16%. We calculate the voltage losses due to non-radiative recombination (ΔVoc,non-rad) based on the formula (22): In addition to improved PV performance, the FA0.91Cs0.09PbI3 device also shows significantly enhanced stability. We firstly measure the shelf life by storing the unencapsulated devices in dark at 25 °C and 20% relative humidity. Fig. 4A shows that the PCE of the FAPbI3 device decreases by about 30% after 3,000 h aging, whereas the FA0.91Cs0.09PbI3 device shows a degradation of only 10% over 4,500 h aging. We then investigate the long-term operational stability of the PSCs by aging the unencapsulated devices under a nitrogen atmosphere, using maximum power point (MPP) tracking under simulated 1-sun conditions. As shown in Figure   4B, the FA0.91Cs0.09PbI3 based PSCs retains over 90% of the initial PCE while the FAPbI3 device maintains only 60% PCE after 1000 h continuous illumination ( fig. S13). 12 A main reason for enhanced stability PSCs is attributed to suppressed ionic migration. In Fig. 4C and 4D, we compare the I − ions distribution at the ~300 nm depth of the perovskite layer for  (23,24). The electron-phonon interaction is dominated by high energy LO phonons in the high-temperature region, where the measured FWHM data could be fitted by the Boson model (Fig. 4C, fig. S14, and Table S2). Compared with FAPbI3, both electron-LO phonon coupling coefficient (ΓLO) and LO phonon energy (hω) in the FA0.91Cs0.09PbI3 are significantly reduced, indicating that the fluctuation of the PbI6 octahedra cage in FA0.91Cs0.09PbI3 is associated with much smaller energies upon the Cs sequential incorporation. This is consistent with the previous theoretical investigations, which indicate that mixed A-site cations could reduce the lattice fluctuations in halide perovskites (25). As such, the suppressed lattice fluctuations and electron-phonon coupling in FA0.91Cs0.09PbI3 rationalize suppressed ionic migration and hence enhanced stability in FA0.91Cs0.09PbI3 PSCs.
In summary, we successfully develop a novel sequential Cs incorporation strategy to tackle the critical challenge on different crystallization dynamics of different cations in developing FA1-xCsxPbI3 perovskite PSCs. The resulting pure iodide FA1-xCsxPbI3 perovskites show uniform 13 composition distribution and reduced defects/traps density. Compared with FAPbI3, the FA0.91Cs0.09PbI3 exhibits reduced electron-phonon coupling and lattice fluctuations, minimizing ion migration and hence enhancing the stability. As such, we have been able to achieve highly stable PSCs with a high efficiency of 24.7%, a record for FA1-xCsxPbI3 PSCs. This work opens up new possibilities to develop high-quality mixed cation perovskites, presenting a milestone towards the development of highly efficient and highly stable perovskites for various applications, including solar cells, light-emitting diodes and lasers.