Tailoring optoelectronic performance through compositional engineering to optimize trap densities in Cs x MA(1− x) PbI3 perovskite nanowires

Organic-inorganic methylammonium lead iodide perovskite (MAPbI3) nanowires (NWs) have attracted significant attention in the realm of optoelectronic devices due to their outstanding optoelectronic properties. However, the persistent challenge of high trap densities has been a limiting factor in realizing their full potential in device performance. To address this challenge, we incorporated cesium (Cs) and systematically investigated the impact of Cs concentration on the trap densities and the optoelectronic characteristics of Cs x MA(1−x)PbI3 NWs. Our findings unveiled an initial reduction in trap densities as Cs+ content increased, with the lowest point occurring at x = 0.2. However, beyond this threshold, trap densities began to rise, eventually surpassing those observed in pure MAPbI3 at x = 0.4. Furthermore, we fabricated single NW photodetectors to assess how Cs+ content influenced optoelectronic properties. The results indicated that Cs+ incorporation led to enhancements in photocurrent and response speed, withthe optimal performance observed at x = 0.2. Our study provides valuable insights into the role of Cs+ incorporation in tailoring the optoelectronic properties of perovskite NWs.

Organic-inorganic methylammonium lead iodide perovskite (MAPbI 3 ) nanowires (NWs) have attracted significant attention in the realm of optoelectronic devices due to their outstanding optoelectronic properties.However, the persistent challenge of high trap densities has been a limiting factor in realizing their full potential in device performance.To address this challenge, we incorporated cesium (Cs) and systematically investigated the impact of Cs concentration on the trap densities and the optoelectronic characteristics of Cs x MA (1−x) PbI 3 NWs.Our findings unveiled an initial reduction in trap densities as Cs + content increased, with the lowest point occurring at x = 0.2.However, beyond this threshold, trap densities began to rise, eventually surpassing those observed in pure MAPbI 3 at x = 0.4.Furthermore, we fabricated single NW photodetectors to assess how Cs + content influenced optoelectronic properties.The results indicated that Cs + incorporation led to enhancements in photocurrent and response speed, with Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
The elevated trap densities exacerbate the capture and recombination of photogenerated carriers, leading to diminished photocurrent and slower response speeds in devices [20,21].Consequently, there is an urgent imperative to develop a straightforward yet effective method for reducing trap densities in MAPbI 3 NWs.Several strategies have been proposed to mitigate trap densities in organic halide perovskites, and among these, the creation of mixed cation perovskites has emerged as a successful approach to reduce trap densities and optimize optoelectronic properties [22][23][24].One effective method involves introducing larger organic cations like formamidinium (FA + ) in place of some of the methylammonium (MA + ), thereby enhancing the optoelectronic characteristics of perovskites [19,[25][26][27].Additionally, it has been demonstrated that the partial substitution of MA + with small inorganic cations such as cesium cation (Cs + ) can alter the interaction between [PbI 6 ] + and I − , leading to crystal lattice shrinkage, reduced atomic vacancies, lower trap densities, and improved optoelectronic properties [28][29][30].Despite the booming strategies and developments in constructing various mixed cation perovskites, our understanding of the optimal concentration of Cs + in MAPbI 3 NWs for achieving the best photoelectric properties remains limited.In other words, it remains unclear whether there exists an optimum Cs + concentration in MAPbI 3 NWs to yield the most favorable optoelectronic characteristics.Thus, there is an urgent need to elucidate how varying concentrations of Cs + impact trap densities and optoelectronic properties.
In this study, we conducted a systematic exploration of the impact of Cs + concentration on both the trap densities and the optoelectronic properties of MAPbI 3 perovskite NWs.Our investigation revealed that Cs + incorporation yielded a substantial reduction in trap densities, accompanied by significant improvements in optoelectronic properties.However, it is noteworthy that an excessive amount of Cs + incorporation led to the increase in trap densities, consequently leading to a degradation in device performance.

Preparation of CsxMA (1−x) PbI 3 NWs
To prepare Cs x MA (1−x) PbI 3 NWs, we first created a precursor solution with a concentration of 0.04 M.This solution was formulated by dissolving CsI, MAI, and PbI 2 in a molar ratio of x:(1−x):1 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.6 and 0.8), all within 2.5 ml of dimethylformamide (DMF).Then, a drop of the precursor was dropped on a clean Si/SiO 2 substrate placed on a holder in a container with isopropyl alcohol (IPA) injected in it.Afterward, the container was sealed and heated by a hot plate at 90 • C for 60 min to form the intermediate phase NWs.Finally, the intermediate phase NWs were heated at 100 • C for 20 min to transfer to the α phase NWs.

Device fabrication and characterization
The single NW photodetectors were prepared by deposition of 50 nm thick Au through a shadow mask.The photoelectric properties of the photodetector and space-charge-limitedcurrent (SCLC) technique were carried out at room temperature by using a semiconductor parameter analyzer (Keithley 4200A-SCS) combined with a probe station.The response speed of the devices was measured using a digital oscilloscope (Keysight DSOX1102G) coupled with a low-noise current preamplifier (Stanford Research System SR570).

Results and discussion
We employed a straightforward and time-efficient lowtemperature solution-based method, as previously detailed in our publication, to grow Cs x MA (1−x) PbI 3 NWs [15].In brief, a precursor solution with a predetermined Cs/MA ratio was deposited onto a Si/SiO 2 substrate and subsequently subjected to heating to facilitate the formation of perovskite NWs.A comprehensive description of the NWs growth process is provided in the experimental section.For the investigation into the influence of Cs + concentration on the properties of the perovskite NWs, we prepared NWs with Cs/MA ratios ranging from x = 0 -0.4. Figure 1 presents SEM images of the Cs x MA (1−x) PbI 3 NWs.Across all samples, the NWs display lengths spanning several micrometers.Within the insets of individual NW SEM images, we observed that the diameter of pure MAPbI 3 NWs is approximately 900 nm.Conversely, in the mixed cation Cs x MA (1−x) PbI 3 samples, the diameter reduces to around 600 nm due to the lattice shrinkage induced by Cs + [29,30].Furthermore, obvious agglomerates are observed in samples with x = 0.3 and 0.4, marked by yellow frames.An enlarged SEM image (figure 1(f)) offers evidence that these agglomerates consist of short NWs.To scrutinize the composition of these NWs, we conducted an EDS analysis, the results of which are presented in table 1.The I:Pb ratio obtained from EDS smaller than 3:1 is mainly due to the unstable nature of the organic perovskite [18,31].From the EDS mapping results (figure S1), it can be observed that all the elements are distributed homogeneously.It is evident that the Cs + concentrations closely align with the intended molar concentrations in the precursors in the Cs 0.1 MA 0.9 PbI 3 and Cs 0.2 MA 0.8 PbI 3 samples.However, in samples with higher x values of 0.3 and 0.4, the Cs + concentration falls below the designed amount, attributed to the formation of CsPbI 3 NWs, which consumes Cs + from the precursors.Additionally, the composition of the short NWs within the agglomerates is primarily CsPbI 3 .Similar results are also observed in the x = 0.6 and 0.8 samples as shown in figure S2.
To explore the impact of chemical composition on the structure of perovskite NWs, we performed XRD analysis on Cs x MA (1−x) PbI 3 NWs, with the results depicted in figure 2. Samples without Cs + or with low Cs + concentrations (x = 0, 0.1, 0.2) exhibit distinct diffraction peaks associated with the α-phase perovskite.In contrast, samples with higher Cs + content (x = 0.3 and 0.4) reveal additional characteristic diffraction peaks corresponding to the δ-phase CsPbI 3 (figure 2(a)).This observation aligns with the SEM findings, confirming the presence of short NWs composed of CsPbI 3 in samples with elevated Cs + content.Furthermore, upon closer examination of the (110) plane within the 13.6 • -14.8 • range in figure 2(b), it becomes evident that the diffraction angle gradually shifts toward higher angles with increasing Cs + content.This shift suggests a reduction in the lattice parameters, consistent with the Bragg equation: 2dsinθ = nλ.The incorporation of Cs + is thus responsible for lattice shrinkage, attributed to the smaller size of Cs + compared to MA + .The Goldschmidt tolerance factors (t effective ) for the Cs x MA 1−x PbI 3 nanowires are calculated by the following formula [32]: where r effective is the effective A-site ionic radius, x and (1−x) is the molar ratio of Cs + to MA + , and r Cs + , r MA + , r Pb 2 + , r I -are the ionic radii of Cs, MA, Pb, and I elements, respectively.The calculated tolerance factors are 0.912, 0.905, 0.901, 0.897 and 0.894 for the x = 0, 0.1, 0.2, 0.3, and 0.4 samples, respectively.To show the crystalline quality of the nanowires, we extracted the full-width at half-maximum (FWHM) of the (110) peaks from the XRD results.It can be observed that the FWHM decreases first and then increases.When x = 0.2, the FWHM is the smallest and the crystal quality is the highest.FWHM increases when the x value exceeding 0.2, indicating that the crystal quality decreased.Similar findings have been reported in the case of Cs-doped MAPbBr 3 single crystals [29].
The incorporation of Cs + also has a significant impact on the optical properties of the mixed-cation perovskite.In figure 3(a), we observe the UV-Vis absorption spectra of Cs x MA (1−x) PbI 3 NWs.Notably, these spectra reveal a consistent blue shift in the absorption edges as the value of x increases, indicating that Cs + incorporation leads to a widening of the bandgap.This shift is further confirmed by the corresponding bandgap values extracted from the (αhν) 2 -(hν) curve (figure 3(b)), which demonstrate an increase from 1.54 eV to 1.61 eV with the ascending Cs + content.A similar blue shift, attributed to the presence of Cs + , is also evident in the normalized PL spectra, as depicted in the inset of figure 3(c), in agreement with the UV-Vis observations.It is worth noting that the PL intensities of Cs x MA (1−x) PbI 3 NWs exhibit a consistent increase as Cs + concentrations rise from x = 0 -0.2 (figure 3(c)).However, these PL intensities steadily decrease as the x value surpasses 0.2 and reaches 0.4.This observed trend is similar to the patterns observed in the XRD intensities.The reduction in PL intensities is believed to be associated with the increased trap densities resulting from the excessive incorporation of Cs + [33,34].
To assess the trap densities within Cs x MA (1−x) PbI 3 NWs, we conducted SCLC measurements in a dark environment     using single NW devices for all the samples with similar diameter, as depicted in figures 4(a) and (b).As illustrated in figure 4(c), the current-voltage (I-V) response exhibited a linear behavior at lower voltages, followed by a rapid nonlinear increase as it entered the regime known as the trap-filled limit (TFL), characterized by the onset voltage V TFL .In this TFL region, all available trap states were saturated with injected carriers.Assuming the trap density is dominated by the volume trap states, the V TFL value serves as the basis for calculating the trap densities through the following equation [35]: where N t is the trap densities, l is the length of the NW, e is the elementary charge, ε is the dielectric constant of perovskites, and ε 0 is the vacuum permittivity.It is known that the current-voltage hysteresis in the organic perovskite is a common phenomenon due to the fast ion migration.To minimize the impact of hysteresis on the properties during the measurement, we performed a single I-V sweep with fast sweeping rate to obtain the I-V curves for all the samples.As depicted in figure 4(c), the trap densities within Cs x MA (1−x) PbI 3 NWs demonstrates an initial decline with increasing Cs + content, reaching its minimum value of 4.58 × 10 11 cm −3 at x = 0.2.However, as Cs + content continues to rise, the trap densities begin to increase.Remarkably, in the x = 0.4, 0.6 and 0.8 samples (figure S2), the trap densities surpasse that of pure MAPbI 3 , signifying that excessive Cs + leads to an elevation in trap states.To estimate the effect of surface trap states, we passivated the nanowires by deposition of SiO 2 on the devices.It is found that the trap density decreased slight after SiO 2 passivation (figure S3), which indicated that the surface trap density is relatively small compared with the volume trap density.These findings, derived from the SCLC analysis, align consistently with the results obtained from XRD and PL measurements, thus confirming that Cs 0.2 MA 0.8 PbI 3 NWs exhibit both high crystal quality and PL intensity.
The notably low trap densities offer substantial advantages by reducing the recombination and release of photogenerated carriers.This characteristic proves particularly beneficial for enhancing the optoelectronic properties of perovskite devices.
To gain deeper insights into the influence of Cs + content on the optoelectronic properties of perovskite devices, we conducted a further analysis of single NW photodetectors (figure 5).The dynamic current-time photoresponses of Cs x MA (1−x) PbI 3 NWs photodetectors were recorded at 1 V under an incident light intensity of 1 mW cm −2 from a 655 nm unfocused laser, as depicted in figure 5(a).It is clearly observable that the inclusion of Cs + in the perovskite NWs leads to a substantial increase in photocurrent compared to NWs without Cs + incorporation.As the Cs + content varies, the photocurrent initially rises but subsequently declines.The maximum photocurrent, reaching 480 pA, is achieved within the x = 0.2 sample.Remarkably, the on/off ratio for the Cs 0.2 MA 0.8 PbI 3 NW photodetector is almost 48 times higher than that of pure MAPbI 3 NW.Moreover, alongside the enhanced photocurrent, the device's response speed also improves with Cs + incorporation.Figures 5(b)-(f) illustrate the response speed of the Cs x MA (1−x) PbI 3 NW photodetectors.Response speed encompasses both the rise and decay times (τ r and τ d ), defined as the time taken for the photocurrent to increase from 10% to 90% of its peak value and vice versa [36].Notably, the response speed of the x = 0.1, 0.2, and 0.3 Cs x MA (1−x) PbI 3 NW photodetectors surpasses that of pure MAPbI 3 NW, with the Cs 0.2 MA 0.8 PbI 3 photodetector exhibiting the swiftest rise and decay times.This emphasizes that Cs + incorporation effectively enhances the response speed.However, it is essential to note that the response speed of the Cs 0.4 MA 0.6 PbI 3 photodetector becomes slower than that of pure MAPbI 3 .This indicates that excessive Cs + incorporation hinders the response speed.This phenomenon arises because excessive Cs + incorporation augments the trap densities within the perovskite, subsequently prolonging carrier release and recombination times [20].It should be noted that the response speed of the nanowire devices is slower than that of their thin film and bulk counterparts.This is mainly due to the large surface-to-volume ratio and reduced screening, impurities and defects possibly act as trap centers in nanowires, which prolongs the carrier lifetime [37].Moreover, to understand the efficiency of photo conversion in these devices, we calculated the quantum efficiency (EQE) of different x value devices, as shown in figure S4.It was observed that the incorporation of Cs + in perovskite NWs resulted in a significant increase in EQE compared to NWs without Cs + incorporation.With the change of Cs + content, EQE first increased and then decreased, and the ability to convert optical signals into electrical signals was strongest at x = 0.2.

Conclusion
In summary, this study focused on the systematic investigation of the impact of Cs + incorporation on the trap densities and the optoelectronic properties of Cs x MA (1−x) PbI 3 mixed cation perovskite NWs.It was found that Cs + incorporation led to significant trap densities reduction.However, excessive Cs + incorporation resulted in an increase in trap densities.The reduction in trap densities, as observed in Cs 0.2 MA 0.8 PbI 3 NWs, was associated with improved crystal quality and photoluminescence intensity.These findings were further supported by SCLC measurements, demonstrating that Cs 0.2 MA 0.8 PbI 3 NWs exhibited the lowest trap densities among the studied compositions.In terms of optoelectronic performance, Cs 0.2 MA 0.8 PbI 3 NW-based photodetectors displayed significantly enhanced photocurrents and response speeds compared to pure MAPbI 3 .However, it is crucial to note that excessive Cs + incorporation, as seen in Cs 0.4 MA 0.6 PbI 3 , negatively impacted response speed due to increased trap densities.This work sheds light on the intricate relationship between Cs + incorporation and the properties of mixed cation perovskite NWs, providing valuable insights into optimizing the optoelectronic performance of these materials.

Figure 1 .
Figure 1.(a)-(e) SEM images of CsxMA (1−x) PbI 3 NWs with x value ranging from 0 to 0.4, the insets are the enlarged individual NWs.(f) The enlarged NWs marked by yellow frames in (d) and (e).

Figure 4 .
Figure 4.The schematic (a) and the optic image (b) of the single NW device.(c) I-V responses of CsxMA (1−x) PbI 3 NWs devices measured in dark.