In-depth interpretation! Demonstrate that Pb/Sn mixed cobalt minerals are not affected by ion transfer effects like Pb devices
Research highlights:
1. Explore the lead-tin alloy in the mixed formamidine-casium (FA-Cs) perovskite composition used for the bottom gate of FET (field effect transistor) devices, and these devices show near-ideal FET performance with adjustable p-type mobility.
2. For the best mixed composition Cs0.15FA0.85Pb0.5Sn0.5I3, the high hole mobility of 5.4 cm 2 V-1s-1 at room temperature is one of the highest p-type field effect mobility reported by 3D perovskite thin film FET.
3. Through the combination of temperature-dependent transport measurement and biased photoluminescence (PL) microscope, it is proved that the addition of Sn inhibits the ion migration, and these findings are related to the high mobility and operational stability of mixed metal-based perovskite devices.
1、 Problems and challenges faced by field effect transistor (FET) and mixed metal charge transfer theory
Field effect transistor (FET) is a triode device, which can reliably detect the long-distance charge transfer on the dielectric perovskite semiconductor interface. Unlike spectral measurements, field-effect transistors can detect more local transport, thus providing an upper limit for the mobility obtained by semiconductors. Field-effect transistors can make more realistic estimates of long-range transport, including the effects of interface and morphology. Since the first report on halide perovskite type FET in 1999, two-dimensional (2D) perovskite PEA2SnI4 film has been used as a semiconductor channel. Sn-based 2D – 3D mixed perovskites are also used as active materials in perovskite FETs. Recently, carrier mobility of up to 50 cm2 V – 1 s – 1 has been demonstrated in 3D pure Sn-based perovskites 19 and 20. However, there is serious instability associated with the oxidation of Sn2+to Sn4+. As far as Pb-based 3D perovskite is concerned, the composition engineering of site A and the modification of halides have shown that the properties of charge transfer in FETs based on metal halide perovskite have been significantly affected. However, due to the soft nature of halide perovskite, the unexpected ion migration effect has greatly hindered the research on the room temperature charge transfer of more general 3D Pb-containing perovskite, which requires careful optimization of material composition and processing. The moving ions in perovskite shield the applied potential and reduce the grid modulation of carriers at the interface, resulting in obvious low mobility at room temperature and obvious hysteresis and non-ideality in device characteristics. In addition, most 3D perovskite FETs contain methyl ammonium cations (MA+, CH3NH3+), which are known to be inherently thermally unstable, and may introduce additional problems of dipole disorder, thus further reducing carrier mobility near room temperature. These two effects currently hinder the use of FET measurements to study the intrinsic charge transfer behavior in 3D halide perovskite.
2、 Introduction to achievements
Here, Professor Satyaprasad P. Senanayak of the National Institute of Homibaba, India, has produced near-ideal field-effect transistor performance, and the device shows adjustable p-type mobility. The highest hole mobility is 5.4 cm2V – 1 s – 1 at room temperature, which is one of the highest p-type field effect mobility reported by 3D perovskite thin film FET. Through the combination of temperature-dependent transport measurement and biased photoluminescence (PL) microscopy, we proved that the addition of Sn inhibited ion migration, and related these findings to the high mobility and operational stability of mixed metal-based perovskite devices. This work has improved the basic understanding of the charge transport physics of this kind of important low-bandgap mixed metal (Pb/Sn) perovskite. Compared with pure tin perovskite, this kind of perovskite shows better environmental stability.
3、 Results and discussion
Key point 1: FET device performance
The device characteristics of bottom gate and bottom electric shock (BGBC) FET made of different perovskite with different Pb/Sn ratios have been fabricated. The FET made with the more traditional Pb based Cs0.15FA0.85PbI3 composition (hereinafter referred to as CsFAPbI3) shows n-type field effect transmission, with a low field effect mobility of about 10 − 3. When Pb2+is replaced by Sn2+by 25% (Cs0.15FA0.85Pb0.75Sn0.25I3), an obvious transition from n-type to p-type field effect transport is observed, and the hole mobility at room temperature increases to about 0.02. However, the two perovskite components show significant hysteresis. By further increasing the Sn content to 50% (Cs0.15FA0.85Pb0.5Sn0.5I3, hereinafter referred to as CsFAPb0.5Sn0.5I3), the channel current increased by three orders of magnitude, μ FET reaches a maximum value of 5.4 cm2V – 1s – 1. Importantly, the device made of CsFAPb0.5Sn0.5I3 also shows clear hysteresis free output characteristics, and has good linearity and saturation state. The author has carried out a wide range of scanning rate correlation measurements of the transmission characteristics in the temperature range (100 – 300 K) and quantified the hysteresis. These measurement results show that in the Pb-Sn-based FET, the hysteresis phenomenon is strongly suppressed at all scanning rates and temperatures, which further confirms that the ion migration decreases after doping Sn in the perovskite composition.
Fig. 1 FET characteristics of Pb-Sn perovskite film
In order to further optimize the device performance, the author also discussed the influence of the change of A site on the charge transfer behavior of perovskite, and kept the composition of B site at Pb0.5Sn0.5. However, it is observed that the best A-bit component with the highest mobility and non-lag transfer characteristics is Cs0.15FA0.85, which was adopted in the early stage of this work.
Key point 2: electronic structure calculation
In order to understand the transport trend of the composition change of the B site, the author calculated its electronic structure using the first principle calculation, and found that the mixing of B metal led to the enhancement of the density of states mainly at the edge of the valence band.
Figure 2 Atomic origin of high-mobility p-type transport in hybrid Pb-Sn devices
With the increase of Sn concentration, the energy of valence band maximum (VBM) also moves upward, which is expected to improve hole injection. In addition, with the increase of Sn content, the reduced effective mass (me * mh */(me *+mh *)) decreases monotonously, which is consistent with the early observation of magneto-optical measurement 37, where me * and mh * represent the mass of electron and hole effect, respectively. This is due to the increase of the Sn fraction in the calculation system Cs0.125FA0.875Pb (1 – x) SnxI3, and the systematic enhancement of the average B – I – B bond angle, which leads to the crystal structure from slightly twisted tetragonal phase to less twisted cubic phase. As the linearity of B – I – B angle increases, the s orbital of B atom and the 5p orbital of I enhance their spatial overlap, and enhance the dispersion in VBM state, thus reducing the effective mass of holes.
Key point 3: defect characterization and transverse electron migration
Fig. 3 Chemical analysis of defects in mixed Pb-Sn perovskite film
In order to determine the relative abundance of Sn2+defects and Sn4+defects in the deposited film, it is manufactured on the same substrate (without Au electrode) used for manufacturing FET. All perovskite films show two characteristic peaks, indicating that Sn4+and Sn2+coexist. The estimation of the relative abundance of the two oxidized Sn4+/Sn2+states shows that the composition of 50% Sn has the lowest ratio of 0.06. Interestingly, 25% of Sn composition shows a maximum ratio of 3.74, even higher than 100% of Sn (1.35).
Fig. 4 Transverse electron migration in perovskite
In order to visualize the biased in-plane ion migration in these mixed perovskite films, the PL mapping results on the transverse devices are shown in Figure 4. Regardless of the composition of perovskite, the unbiased device shows uniform PL intensity in the whole channel. Then the device is subjected to multiple polarization cycles (30V, 30s). After bias, the PL intensity distribution in the channel region of CsFAPbI3 changes constantly, especially leading to the inhibition of PL characteristics near the positive bias electrode, which can be related to the halide migration during bias. However, similar measurements of the transverse devices made of CsFASnI3, CsFAPb0.25Sn0.75I3 and CsFAPa0.5Sn0.5I3 did not show significant changes in the PL distribution in the channel region, indicating that in the perovskite containing Sn, the bias voltage induced transverse ion migration was significantly inhibited. This is completely consistent with the outstanding charge transfer behavior described above. Interestingly, the PL intensity on the negative electrode increases monotonously with the bias voltage. In addition, the photobrightening process of the negative electrode at the bias voltage is specific to the composition containing Sn, and seems to be reversible when the bias voltage is kept for a long time in the dark. These figures show the obvious characteristics of the electrochemical process, and considering that this process mainly occurs on the negative electrode, the author believes that this enhancement indicates the reversible local electrochemical reduction of Sn4+to Sn2+.
4、 Summary
Finally, the author proposed a mechanism to explain the effect of inhibiting ion migration observed in the composition of mixed metal perovskite. In the case of lead perovskite field-effect transistors, ion migration (mainly halides) shields the positive gate potential of field-induced carriers, thus reducing their apparent mobility near room temperature. On the other hand, the application of negative gate potential in mixed Pb-Sn perovskite field-effect tube leads to the accumulation of positive charge ions (A+, B2+) on the interface, and its mobility is often lower than the halide defect in pure Pb-based perovskite. In addition, these defects are compensated by the presence of a large number of negatively charged Sn vacancies (VSn2 -, resulting in doping), resulting in the possible existence of two types of neutral ion defect complexes at the charge transport interface: (2A+- VSn2 -) or (B2+- VSn2 -). This minimizes the ion shielding of the gate potential, thus enabling the detection of the intrinsic activation transport behavior of the remaining shallow electron well. In addition, the optimized CsFA composition does not have the dipole disorder induced by MA+cations, which also helps to enhance the charge transport near room temperature
5、 References
Senanayak, S.P., Dey, K., Shivanna, R. et al. Charge transport in mixed metal halide perovskite semiconductors. Nat. Mater. (2023).
Doi：10.1038/s41563-022-01448-2