The Effect of Decomposed PbI2 on Microscopic Mechanisms of Scattering in CH3NH3PbI3 Films

Hybrid organic-inorganic perovskites (HOIPs) exhibit long electronic carrier diffusion length, high optical absorption coefficient, and impressive photovoltaic device performance. At the core of any optoelectronic device lie the charge transport properties, especially the microscopic mechanism of scattering, which must efficiently affect the device function. In this work, CH3NH3PbI3 (MAPbI3) films were fabricated by a vapor solution reaction method. Temperature-dependent Hall measurements were introduced to investigate the scattering mechanism in MAPbI3 films. Two kinds of temperature-mobility behaviors were identified in different thermal treatment MAPbI3 films, indicating different scattering mechanisms during the charge transport process in films. We found that the scattering mechanisms in MAPbI3 films were mainly influenced by the decomposed PbI2 components, which could be easily generated at the perovskite grain boundaries (GBs) by releasing the organic species after annealing at a proper temperature. The passivation effects of PbI2 in MAPbI3 films were investigated and further discussed with emphasis on the scattering mechanism in the charge transport process.


Background
Hybrid organic-inorganic perovskite (HOIP) materials have recently emerged as highly efficient optoelectronic materials and are being intensively investigated and developed for photovoltaics, photo-detections, light-emitting diodes, and laser devices [1][2][3][4][5][6]. The perovskite solar cells have gradually emerged in the center of photovoltaic filed because of their power conversion efficiency achieving over 20% during the past 8 years, as well as their cost-effective and scalable processability [7][8][9][10][11][12][13][14]. The investigations on HOIP materials (e.g., CH 3 NH 3 PbX 3 , X = Cl, Br, I) have revealed their great potentials for photovoltaic applications due to optimum band gap, high absorption coefficient, high carrier mobility, and diffusion length on the order of hundreds of nanometers to micrometers in films [15][16][17][18][19]. At the core of any optoelectronic devices lie the electronic properties, especially the scattering mechanism governing charge transport process. There have been many works allowing us to understand HOIP charge transport characteristics. It is apparent that the carrier mobilities of HOIP materials, which are only within the scope of 1~10 cm 2 /V s [20][21][22], are usually limited by the scattering mechanism. The T −1.3 to T −1.6 dependence of the mobilities on temperature have been observed by several groups, which are close to the T −1.5 dependence usually assumed for the scattering of acoustic phonon [23,24]. Furthermore, the scattering from grain boundaries (GBs) on charge transport in HOIPs remains unclear. The impacts of GBs with different studies usually reach conflicting conclusions. Edri et al. found a barrier in potential across the GBs in the dark, which could be reduced during the illumination [25]. Yun et al. also revealed the generation of a very small photo-voltage at GBs, but the reduced photoluminescence efficiency was found due to a deep trapping at GBs [26]. From the above introduction, we can know that although HOIP device efficiencies have increased rapidly, an understanding of the charge transport mechanisms of these materials and their underlying physical mechanisms is only starting to carry out.
In this work, the vapor solution reaction method was employed to construct compact and uniform CH 3 NH 3 PbI 3 (MAPbI 3 ) films. The microscopic mechanism of scattering during the charge transport process in MAPbI 3 films was evaluated via temperature-dependent Hall measurements. Two different behaviors of temperature-dependent Hall mobilities could be identified in the MAPbI 3 films before and after thermal annealing. It is confirmed that the decomposed PbI 2 located at the GBs, which is usually converted from MAPbI 3 upon thermal annealing at a proper temperature, plays an important role in the charge transport process in MAPbI 3 films. The different scattering mechanisms combining the microstructure of MAPbI 3 films were discussed, and the possible physical mechanisms were further proposed.

Methods
MAPbI 3 films were fabricated by the vapor solution reaction method as our previous works [27,28]. The PbI 2 powder (purchased from Xi'an Polymer Light Technology Corp, 99.99%.) was first dissolved in the N,Ndimethylformamide (DMF, Aladdin, 99.9%) solvent with a concentration of 1 mol/mL and stirred at 70°C for 3 h. Then, the PbI 2 film was coated on the substrates by spin-coating with a speed of 4000 rpm, 30 s, and annealed at 70°C for 10 min. The PbI 2 films and MAI powder were separately placed in two different zones of the tubular furnace equipment with a vacuum system. After pumping for 10 min, the MAI power and PbI 2 films are heated to 180°C and 140°C, respectively, and kept these temperatures for more than 100 min. Finally, the pre-prepared MAPbI 3 films with darkened color were annealed at different temperatures (100°C, 120°C, and 145°C) for 1 h, after being washed with isopropanol. All the procedures were carried out in the open air with a humidity of~45%.
The microstructures of MAPbI 3 films were measured by using X-ray diffraction (XRD) (model: MXP-III, Bruker Inc.) and scanning electron microscopy (SEM) (model: S-3400 N-II, Hitachi Inc.). The fluorescence decay curves from time-resolved photoluminescence (TRPL) measurements were recorded by a fluorescence spectrophotometer based on the time-correlated single photon counting (model: FLS920, Edinburgh Inc.). Temperature-dependent Hall measurements (model: LakeShore 8400 series, LakeShore Inc.) were performed with coplanar configurations by using Al electrodes prepared by thermal evaporation technique. Hall mobilities could be obtained from Hall effect measurements, carried out in a standard van der Pauw configuration by using an electromagnet with a magnetic induction of 0.6 T. All the temperature dependence measurements were taken during heating in the temperature range from 300 to 350 K with a step of 10 K in argon ambient.

Results and Discussion
The morphology evolution of MAPbI 3 films was firstly investigated by XRD measurement. The XRD patterns for the MAPbI 3 films before and after annealing are shown in Fig. 1. It can be clearly seen that the samples before annealing and after annealing at 120°C are composed of pure perovskite phase, which reveals the MAPbI 3 characteristic peaks at 14.04°, 28.42°, and 43.08°c orresponding to the (110), (220), and (330) planes of MAPbI 3 , respectively [29]. However, it is found that the sample after annealing at 145°C is not pure MAPbI 3 film. A new characteristic diffraction peak at 12.56°appears in the corresponding film, which could be observed by the (001) planes of PbI 2 . There have been a lot of previous reports suggesting that a structural conversion from MAPbI 3 to PbI 2 occurs mostly in MAPbI 3 films upon thermal annealing [30][31][32]. According to these reports, we believe that MAPbI 3 films could be decomposed via heating above 145°C in this work, where CH 3 NH 3 I species escape from MAPbI 3 film to form the PbI 2 phase. This indicates the weakly bonded nature between the organic and inorganic species in MAPbI 3 films [33].
The SEM images further gave a deep insight into the morphology evolution of MAPbI 3 films. In Fig. 2a-c, all the films present a compact and conformal structure. However, an amount of newly formed species occurring at GBs is emerged in the MAPbI 3 film annealed at 145°C, which shows relatively bright contrast compared to the adjacent grains. These newly formed species have been reported previously in similar works where they were speculated to be PbI 2 components, which is consistent with the conservation of PbI 2 signals in the corresponding XRD pattern as we discussed before [33]. Fig. 1 The XRD pattern for MAPbI 3 films before and after annealing at 120°C and 145°C From these findings, we can conclude that a compositional change could occur in the MAPbI 3 film annealed above 145°C. By releasing the organic species during annealing, PbI 2 components are decomposed and parts of them are located at the perovskite GBs according to both XRD and SEM results.
An understanding of the charge transport properties in MAPbI 3 films is highly important as the mobility usually dominates device performance. In this work, Hall mobilities of all the MAPbI 3 films were measured as shown in Fig. 3. At room temperature, Hall mobilities are around 0.6~1 cm 2 /V s for the unannealed, 100°Cand 120°C-annealed MAPbI 3 films, which are consistent with the previous reports [20,34]. However, increased Hall mobility reaching to 5 cm 2 /V s is found in the 145°C-annealed MAPbI 3 film, which is nearly one order of magnitude higher than that of the unannealed one. As we know, mobility is usually influenced by the dominant scattering mechanism governing the charge transport process. Such increased Hall mobility probably reflects a reduction of scattering during the charge transport process in the 145°C-annealed MAPbI 3 film. Yang et al. once investigated the surfaces and GBs in MAPbI 3 films via scanning Kelvin probe microscopy (SKPM), which is used to determine the surface potential difference between GBs and inner grains in films. It was found that the MAPbI 3 film without thermal annealing exhibited a higher surface potential at the GBs than that at the bulk, which was usually reported in the previous works [35][36][37]. In contrast, the surface potential at the GBs was obviously reduced after annealing at 150°C. They considered that the decreasing of surface potential resulted from the passivation effect of newly formed PbI 2 phases, which could suppress the barrier of GBs to some extent thus reduced the scattering from GBs [33,38]. Therefore, with the decomposed PbI 2 occurring after annealing at 145°C in this work, the increased Hall mobility can be attributed to the passivation effect of the decomposed PbI 2 at GBs. As the Hall measurements characterize the charge transport property of the entire film, it is inferred that the decomposed PbI 2 passivates the GBs and reduces the energy barrier between grain domains, facilitating the charge transportation in the plane direction [39].
In order to further study the passivation effect of decomposed PbI 2 locating at GBs in MAPbI 3 films, temperature-dependent Hall mobilities were introduced to investigate the scattering mechanism in the MAPbI 3 films before and after annealing. Hall mobilitiestemperature behaviors within the temperature range from 300 to 350 K are shown in Fig. 4a. It is clearly seen that the mobility is increased with temperature for the un-and 120°C-annealed MAPbI 3 films. As we know, the GBs in the films with grain sizes on the micrometer scale play an important role in the charge transport process and the carrier mobility is limited by the potential energy barriers at GBs [40]. Such GBs with a large number of defects can trap the carriers and form the electrically charged states, which impede the motion of carriers from one crystallite to another and thus reduce the mobility [41]. With increasing the temperature, the carriers gain the kinetic energy to overcome the potential barriers and the carrier mobility can be increased accordingly [42]. Consequently, it is indicated that a GB scattering governs the charge transport process [43]. Seto et al. established the theoretical model to describe the GB scattering process and Hall mobility μ 0 shows the thermally activated behavior as below: where k B is the Boltzmann's constant, μ 0 is the exponential prefactor, and E B is the activation energy which corresponds to the potential energy barrier height [44]. The relationship between ln μ H and 1000/T is given within the temperature from 300 to 350 K as shown in Fig. 4b while the potential barrier height E B of GBs can be deduced from the slope of the linear fitting. It can be found that the potential barrier height E B of GBs is about 208 meV for the unannealed MAPbI 3 film and slightly reduces to 147 meV after annealing at 120°C, which is almost in accordance with the previous reports [45]. However, after annealing at 145°C, the MAPbI 3 film where the decomposed PbI 2 locating at the GBs exhibits a different temperature-dependent behavior. It is interesting to find that the mobility is decreased with the temperature increasing, which finally exhibits a T −2.0 dependence. Such close to T −1.5 dependence is usually assumed for the acoustic phonon scattering [23,24]. It thus appears that the charge transport process in the 145°C-annealed MAPbI 3 film is no longer dominated by the GB scattering, of which the acoustic phonon scattering would be instead in the charge transport process. Therefore, we could convince that the decomposed PbI 2 locating at the GBs acts as a passivation layer between the grains and suppresses the potential barrier of GBs, thus leading to the change of scattering mechanism in the charge transport process from GB scattering to acoustic phonon scattering.
Furthermore, the TRPL decay was employed and performed on the MAPbI 3 films before and after thermal annealing, and the emission lifetime could be obtained by fitting the fluorescence emission decay spectra using the exponential function. The corresponding fluorescence emission lifetime would reflect the charge recombination behavior in the corresponding MAPbI 3 films. Figure 5 shows the TRPL decay spectra, and Table 1 displays the corresponding lifetime of MAPbI3 films. The fluorescence emission decay curves are fitted with two-component exponential decay which exhibits the same scale of lifetime to the reported PL decay in MAPbI 3 films [46]. The fast decay component, τ 1 , might come from the surface or interface charge recombination lifetime, and the long decay component, τ 2 , could be attributed to the bulk charge recombination lifetime [47,48]. It is found that the long decay component τ 2 shows little variation in the MAPbI 3 films before and after thermal annealing. However, the fast decay component τ 1 is increased from 1.39 ns for unannealed sample to 6.05 ns for 145°C-annealed one, proving a reduction of surface or interface recombination, Fig. 4 a, b Temperature-dependent Hall mobilities of the MAPbI 3 films before and after annealing at 120°C and 145°C Fig. 5 The TRPL decay spectra of the MAPbI 3 films before and after annealing at 120°C and 145°C which finally results in an increase of reduced emission lifetime τ after increasing thermal annealing temperature. In the previous works, Wang et al. also investigated the charge recombination in MAPbI 3 films by analyzing the emission lifetime. They found that longer emission lifetime would indicate the enhanced suppression of the charge recombination, which could be attributed to the decomposed PbI 2 effectively passivating the charge traps at GBs in MAPbI 3 films [40]. Therefore, in this work, the enhanced τ could be ascribed to the increasing passivation effect of the decomposed PbI 2 locating at GBs which fills the GBs and suppresses the interface charge recombination in MAPbI 3 films. This is another powerful evidence for the passivation effect of the decomposed PbI 2 at the MAPbI 3 GBs.

Conclusions
In summary, MAPbI 3 films were fabricated by a vapor solution reaction method. The microstructures as well as the optical and electronic properties were investigated before and after thermally annealing. All the films show a pure perovskite phase and present typical optical properties of MAPbI 3 . However, after thermal annealing at 145°C, the decomposed PbI 2 species occurring at GBs can be revealed in MAPbI 3 films, leading to a successful passivation at GBs. Therefore, the scattering from GBs, which dominates the charge transport process in the unannealed and 120°C-annealed MAPbI 3 films, is obviously suppressed after thermal annealing at 145°C due to the effective passivation of PbI 2 that successfully reduces the potential barrier height of GBs. Meanwhile, the scattering from acoustic phonons turns into the prime scattering mechanism in the 145°C-annealed MAPbI 3 film. Consequently, Hall mobility is reached to 5 cm 2 /V s, which is significantly higher than that of unannealed one (0.6 cm 2 /V s).