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Facet-dependent Property of Sequential Deposited Perovskite Thin films: Chemical Origin and Self-annihilation Tiankai Zhang, Mingzhu Long, Keyou Yan, Xiaoliang Zeng, Fengrui Zhou, Zefeng Chen, Xi Wan, Kun Chen, Pengyi Liu, Faming Li, Tao Yu, Weiguang Xie, and Jian-Bin Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11986 • Publication Date (Web): 08 Nov 2016 Downloaded from http://pubs.acs.org on November 9, 2016

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ACS Applied Materials & Interfaces

Facet-dependent Property of Sequential Deposited Perovskite Thin films: Chemical Origin and Self-annihilation Tiankai Zhang, 1, † Mingzhu Long, 1, † Keyou Yan, 1 Xiaoliang Zeng, 3 Fengrui Zhou,3 Zefeng Chen, 1 Xi Wan, 1 Kun Chen, 1 Pengyi Liu, 2 Faming Li, 4 Tao Yu, 4 Weiguang Xie, 2, *Jianbin Xu 1, * Affiliations: 1

Department of Electronic Engineering, The Chinese University of Hong Kong,

Shatin, New Territories, Hong Kong, SAR P. R. China. 2

Siyuan Laboratory, Guangzhou Key Laboratory of Vacuum Coating Technologies

and New Energy Materials, Department of Physics, Jinan University, Guangzhou, Guangdong, 510632, P. R. China 3

Shenzhen Institute of Advanced Technology, Chinese Academy of Science,

Shenzhen, 518055, P. R. China 4

National Laboratory of Solid State Microstructures & Department of Physics,

Nanjing University, Nanjing 210093, P. R. China

ABSTRACT Quantify on inter-grain length scale properties of CH3NH3PbI3 (MAPbI3) can provide further understanding of material physics leading to increased device performance. In this work, we noticed two typical kinds of facets, smooth and step-like in morphology, appears in sequential deposited perovskite (SDP) films. By mapping the surface potential as well as photoluminescence (PL) peak position, we revealed facets heterogeneity of SDP thin films that smooth facets are nearly intrinsic with PL peak at 775 nm while the step-like facets are p-type doped with 5 nm blue shifted PL peak. 1

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Considering the reaction process, we proposed the smooth facets have well-defined crystal lattices resulted from the interfacial reaction between MAI and PbI2 domains containing low trap states density yet the step-like facets are MAI rich originated from the grain boundaries of PbI2 film and own more trap states. Conversion of step-like facets to smooth facets can be controlled by increasing the reaction time through Ostwald ripening. The improved stability, photo-responsivity up to 0.3 A/W, on/off ratio up to 3900 and decreased photo response time to around 160 µs show that the trap states can be annihilated effectively to improve the photoelectrical conversion with prolonged reaction time and elimination of step-like facets. Our findings demonstrate the relation between facets heterogeneity of SDP films and crystal growth process for the first time and implies that the systematic control of crystal grain modification will enable amelioration of crystallinity for more efficient perovskite photoelectrical applications. KEYWORDS sequential deposited perovskite, facets heterogeneity, MAI rich, Ostwald ripening, trap states annihilation 1. INTRODUCTION Organic–inorganic perovskites such as CH3NH3PbX3 (X = Cl, Br, I) have attracted much attention as light absorption materials in solar cells application since 2009 and the photovoltaic conversion efficiency reached over 22% recently.

1-10

Besides, based

on the same opto-electrical conversion property, some research groups have developed perovskite photodetectors

11-16

, light emitting diodes

17-19

, lasers

20

and

water splitting photocatalyst 21 and achieved remarkable results. 2


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Despite the rapid advance in the performance of perovskite devices, the origin of high optoelectronic conversion efficiency as well as the mechanism of the energy loss during conversion have not been fully understood so far. Especially when it turns to polycrystalline perovskite thin films devices, the grain boundaries effect and facets heterogeneity will dominate the performance. As a result, the investigation on the fundamental property of perovskite facets or boundaries is vital to further performance engineering. The function of grain boundaries in polycrystalline films could be benign or detrimental. In polycrystalline Si thin-film solar cells, there are some dangling bonds at the grain boundaries forming deep trap states within the band gap, which could act as a non-radiative recombination centers upon generated carriers and decrease the power conversion efficiency.

22, 23

While in inorganic CZTS, CIGS, and CdTe thin

film solar cells, first principle computation shows that the grain boundaries are electrically benign and could even enhance the minority carriers’ collection though impurity charge and band bending. 24-26 The condition is similar in polycrystalline perovskite thin films, theoretical calculation indicates that point defects in organic inorganic halide perovskite have high formation energies due to the strong sp coupling at valence band maximum (VBM), which suggests the dominating defects are shallow defects acting more like doping instead of recombination centers.

27

This

superior defects property has been observed by Yun et al. in experiments. For CH3NH3PbI3/TiO2 heterojunction,

28

Kelvin probe force microscope (KPFM)

measurement shows that a potential barrier is formed along the grain boundaries, 3



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manifesting the band bending at grain boundaries. The Conductive-AFM measurement demonstrates the higher short circuit current collection near grain boundaries compared with that within grain interior, confirming that photo generated carriers are effectively separated and collected at grain boundaries. Even though these point defects are benign, some detrimental defects will be introduced into the perovskite film during fabrication process, typically like the defects originates from DMF coordination and release or the poor morphology of the film.

29

To avoid these

unwelcomed grain boundaries defects, two main strategies have been come up with. One is to enlarge crystal domains by crystallization engineering, like hot-casting intra-molecule exchange

31

and vacuum assisted spin-coating

passivation on grain boundaries using fullerene pyridine) 34 , self-induced PbI2

35

, MAI

36

32

30

,

. Another is

33

, Lewis bases (thiophene and

or slight doped halogens 37. Moreover, ion

migration has been observed in perovskite with broad attention and is speculated to play the key role in many unusual phenomenon in perovskite materials and devices such as current–voltage hysteresis dielectric constant

38

, switchable photovoltaic effect

40

39

, giant

, diminished transistor behavior at room temperature

photo-induced phase separation

42

41

,

and so on. Here, grain boundaries are found to be

the critical route for ion migration and contribute largely to those aforementioned behaviors. 43, 44 Facets or grain heterogeneity is another common issue in organic inorganic perovskite thin films. Different facets tend to grow with slightly variant chemical composition and naturally earn different trap states density as well as mobility. At the inter-grain 4


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length

scale,

studies

cathodoluminescence

46

found

surprisingly

strong

inter-grain

PL

45

and

heterogeneity, which suggests that optoelectronic properties,

such as open-circuit voltage (Voc), short-circuit current (Jsc) may also exhibit strong local variations 47. For solution processed perovskite, it is obvious that film quality, namely the facets heterogeneity, grain boundaries effect, trap distribution and density, largely depends on the fabrication method and the crystal growth process. Typically, two important and widely used solution processed methods: sequential deposition method and one step spin-coating method, could lead to perovskite with huge imparity. Through the interfacial reaction of PbI2 and MAI then followed by a fine dissolution and re-deposition process, SDP is composed of different oriented meso-crystals.

48, 49

So

far, there are some researches revealing grain boundaries effects and facets heterogeneity of one step spin coating perovskite films, but few researchers have noticed the difference between facets and the microscale trap states distribution has been rarely discussed in SDP films. In this work, we pay attention to the facets properties and trap states location in inter-grain length scale of SDP films. Two typical crystal facets, one with many edge states and the other with smooth surface, were observed. Based on the KPFM surface potential and PL mapping characterizations, we compared the trap states density of these two facets. Furthermore, considering the crystal growth process, an interfacial reaction followed by the dissolution and redeposition (Ostwald ripening), in sequential deposition method, we proposed that compact PbI2 crystal domain reacts 5



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interfacially with MAI and formed perovskite free of structure defects. In the opposite, MAI inserted from the grain boundaries of PbI2 films and leave these facets MAI rich and showed obvious p-type characteristics. With increased reaction time, imperfect lattice on the MAI rich facets tend to dissolve, re-coordinated in the solution and redeposited on the defined facets, which explained the origin and annihilation of trap states considering the sequential reaction process. At last, the good crystalline perovskite photodetector synthetized by sequential method earned a gain of 3.9×103 under 1 V bias, 20 mW/cm2 input light and a lifetime over 150 hours even under 60% humidity air, which is comparable to other reported perovskite photodetectors 50. This proves that sequential deposition method is a self-modified crystal growth method which can lead to high quality meso-crystals. 2. MATERIALS AND METHODS 2.1 Materials. . Methylammonium iodide (MAI, Dyesol), PbI2 (Sigma-Aldrich, 99%), N, N-dimethylformamide (Sigma-Aldrich, anhydrous, 99.8%) 2.2 Perovskite films synthesis. First, the 15mm×15mm glass substrates were prepared and cleaned in an ultrasonic bath containing acetone, isopropanol and ethanol for 20 min serially. Then, 25µl 1M PbI2/DMF solution was spin-coated on the substrate at 3000rpm for 20s. The solution and substrate needed to be both preheated at 343K for 30min before spin-coating. After annealing at 343K for 20min, the PbI2 film was dipped into a 6mg/ml CH3NH3I/2-propanal solution and converted into perovskite. Here we control the dipping time for 10s, 1min and 10min, respectively. By this way we synthesis the perovskite active layers with different crystal size. The 6


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perovskite film was annealed at 383K for 30min in glovebox. 2.3 Material and device characterizations. UV-Vis absorption spectra were taken on a Hitachi U-3501 ultraviolet/visible/near-infrared spectrophotometer. Morphological images of the perovskite films were observed by HR-FESEM (FEI, Quanta 400). The crystalline structures for the perovskite films were characterized by XRD (Rigaku, Smart Lab). The photo induced current–voltage (I-V) curves were measured using Keithley 4200 semiconductor analyzer. The response time was measured using RHK IVC 100 with bandwidth of 250 kHz. The rise/decay time of the laser is smaller than 1ms and it is modulated by a square wave with frequency of 30 Hz. KPFM is a technique combining topography and surface potential information and naturally be suitable for microanalysis on surface potential of the perovskite meso-crystal film. The KPFM and AFM images were taken with a Bruker Dimension Icon. PL measurements were taken on a 1 µm2 laser spot size with a power of 1 mW at room temperature using commercially available Raman/PL spectrometer (Renishaw, Inc.) with 514 nm laser source. 2.4 Perovskite conductivity and photo response measurement. For photo-electrical response measurement, an 80nm-thick gold film was thermal deposited as electrodes on perovskite film forming the smooth ohm contact between them. The channel width is around 20 µm and length is around 2 mm. 3. RESULTS AND DISCUSSION 3.1 Perovskite crystal growth and morphological heterogeneity

7



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Figure 1. (a-c) SEM of SDP film with reaction time of 10s, 1min and 10min and (d-f) the crystal size distribution statistics, respectively; (g) XRD patterns; (h) UV-Vis absorption spectra of PbI2, perovskite with reaction time of 10s, 1min and 10min; (i) PL of perovskite with reaction time of 10s, 1min and 10min. Three typical reaction times: 10 s, 1 min and 10 min, were chosen to represent the whole reaction process. To compare the morphology of the controlled perovskite films, Scanning electron microscopy (SEM) measurements were conducted. Figure 1(a-c) show that the three controlled perovskite films are composed of omnidirectional cubic grains with similar shape yet different sizes. Initially, PbI2 film is flat and composed of small rod-like crystals (Figure S1). With the insertion of MAI for 10 s, some small MAPbI3 perovskite crystals with size range mainly from 150 nm to 200 nm form on the contact surface. As the reaction time increases to 1 min, perovskite crystals grow 8


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larger to the scale of around 250-350 nm and detach with each other. For the film after 10 min reaction, the crystals size extends to 700-800 nm. In addition, crystals grow departing with each other and processes different-oriented facets in one crystal domain. This kind of morphology is different with that perovskite prepared by conventional one-step methods. 51 We also measured the X-ray diffraction (XRD) patterns of different controlled perovskite films to check the phase conversion from PbI2 to CH3NH3PbI3 in figure 1(g). The starting lead iodide is hexagonal 2H layered polytype and the structure is confined by the corresponding (001) peak at 12.7° in XRD pattern.

52

After 10 s, the

diffraction peak at 14.1° emerges and corresponds to tetragonal phase of perovskite. There still remains substantially unreacted PbI2 for the obvious (001) peak at this time. With reaction time increases, we can see that (110) peaks of CH3NH3PbI3 in samples get sharper and higher thus have a decreased full width at half maximum (FWHM), and the peaks of PbI2 get smaller, which means the remaining PbI2 further reacts with MAI and converts to perovskite. The decreased FWHM indicates that the perovskite crystals grow larger and own higher crystallinity after long time reaction. 53 The absorption curves of these controlled layers are shown in Figure 1(h). The absorption edge shifted from 480 nm to 780 nm shortly after we loaded MAI solution due to the formation of perovskite. Besides, we can see that the absorption increases above the wavelength of 560 nm as the grain size increases. From the absorption curves we can also find that there is absorption even in the near infrared region, which may be contributed by the scattering effect of surface cavity.

54

The PL spectra of 9



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perovskite films with different crystal size are presented in Figure 1 (i). As the reaction time increases, the PL intensity increased dramatically, which shows the heavily reduced trap induced non-radiative recombination process. The decreased FWHM implies that larger perovskite crystals with longer reaction time have more concentrated band edge states. For conclusion, it is clear to see the increase of crystal size as well as crystallinity after long reaction time.

Figure 2. (a) SEM of SDP, smooth facet (green) and step-like facet (red). (b) AFM profile of 3×3 µm SDP film. Line profiles representing smooth facet (with capping layer) (c) and step-like facet (d). Furthermore, SEM image in Figure 2 (a) shows that there are two main types of facets with different morphology in the SDP film. One kind of facets have the feature 10


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of flat and smooth surface, like the facets in green blocks in Figure 2 (a). As shown in Figure 2 (b) and Figure 2 (c), AFM profile shows that the smooth facet has ultra-flat surface with roughness less in 2 nm. On some smooth facets, we could also observe a capping layer with thickness about 10 nm and grows in epitaxial on the surface. The other one, however, have several step-like edges on it, corresponding to the facets in red blocks in Figure 2 (a). Figure 2 (d) the surface profile of one step-like facet showing roughness more than 100 nm. 3.2 Surface potential heterogeneity of facets

Figure 3. (a) AFM topography of perovskite thin film. (10×10 µm) (b) and (c) corresponded surface potential imaging of the sample in dark and under illumination respectively. For better observation and understanding of the surface potential evolution with/without light illumination, we draw a line across grain surface containing both the smooth facets, illustrated by red arrow, and the step-like facet, illustrated by black arrow, respectively. (d) The topography and surface potential line profiles corresponding with (a), (b) and (c) cases separately. Schematic of band 11



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structure and Fermi level diagram of smooth (e) and step-like (f) facets.

To reveal the difference in optical and electrical property of these different facets and investigate the origin of the heterogeneity, we first observed the surface potential of the deposited perovskite film using KPFM. The topography of a 10×10 µm area on SDP film with reaction time for 10 min is shown in Figure 3 (a). Gold film with 80 nm thickness was mounted closely on the perovskite surface as the reference to compare the surface potential of different facets quantitively. Figure 3 (b) shows the surface potential of SDP film is not uniform across the film in the dark. In general, we found that the smooth facets in morphology usually have high potential, while the step-like facets possess much lower potential with difference ~0.16 eV. When the perovskite film was illuminated with a 633 nm 3 W/cm2 laser in Figure 3 (c), the surface potential heterogeneity did not vanish but even enlarged. The originally high surface potential facet A remains the unchanged high potential while the low surface potential facet B shows lower potential after the illumination forming deeper potential valley across the film. The exact surface potential line profiles across the smooth and step-like facets surface before and under illumination are demonstrated in Figure 3 (d). Once illuminated, the surface potential of smooth facet is almost unchanged. However, the surface potential of step-like facet drops by 0.06 eV under light with reduced potential difference from 0.16 eV to 0.10 eV between facet A and B. Q. Wang et al. reported that perovskite could be either n- or p-doped by changing the ratio of MAI and PbI2. MAI-rich and PbI2-rich perovskite films are p and n self-doped, 12


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respectively. The SDP showed typical p-type behavior in Hall measurement. 55 Based on this report and our KPFM results shown above, here we draw the band diagram of these two different facets in Figure 3 (e) & (f). For CH3NH3PbI3, the conduction band minimum and valence band maximum are -3.9 eV and -5.5 eV, respectively.

56

The

work function of reference gold usually can be regarded as -4.8 eV in the air. 57 In the dark, for the smooth facets, we find its Fermi level is -4.74 eV since the KPFM results shows it is 0.06 eV higher than that of gold. This work function is very close to the intrinsic Fermi level of CH3NH3PbI3 which is -4.7 eV judging from its VBM and CBM. As a result, we regard the smooth facets are nearly intrinsic and have negligible amount of structure defects as well as trap states. However, the Fermi level of step-like facets are around -4.92 eV from the KPFM profile, which is -0.22 eV lower than CH3NH3PbI3 intrinsic Fermi level showing that the step-like facets has many structure defects and trap states, which would lead to the p-type doping of perovskite. As revealed by Q. Wang et al., SDP with PbI2 full conversion is MAI rich showing p-type doping property. We can propose the step-like facets contents more MAI than the smooth facets. First-principles calculations by P. Delugas et al. show methylammonium fragmentation in amines as source of localized trap levels in perovskite. Typically, molecular level of the methyl-type fragment is 0.2 eV below the CBM and be able to trap electrons.

58

Then after the illumination, large amount of

electrons and holes can be generated simultaneously, and the population of generated carriers should be much higher than the intrinsic carriers. The Fermi level of smooth facets would not change and sustain at around -4.7 eV due to its low trap state 13



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property, while Fermi level of the step-like facets would drop further to -4.98 eV as partial generated electrons are trapped by half-filled electronic orbitals derived by CH2NH3+ ions. 58 3.3 PL heterogeneity of facets

Figure 4. (a-b) SEM and the corresponding PL peak position mapping of SDP film and (c-d) perovskite meso-crystal with longer reaction time. In the following, we conducted PL mapping of the perovskite film as another evidence for facets heterogeneity of trap state density. We observe the two kinds of typical facets on the large crystal circled in Figure 4 (a) with the corresponding PL peak position mapping of this crystal in Figure 4 (b). Three facets align to different directions are marked by A, B and C, corresponding to smooth (A) and step-like (B & C) facets respectively. Referring to the PL peak position mapping, we found the PL peak position of step-like facet has an obvious blue shift than that of smooth facet for 14


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around 5 nm. G. Grancini et al. found defects and lattice reconstructions of MAPbI3 could cause several nanometers’ blue shift of PL peak.

59

As a result, we can ascribe

this blue shift to the incomplete inorganic cage with I defect or the organic cation rich in the form of MA+ interstitial. To more clearly elucidate the crystal facets heterogeneity during reaction process, later we chose one meso-crystal SDP around 5 µm in size after 6 hours reaction with morphology view and PL peak mapping, showing in Figure 4 (c) and (d), to observe the evolution more clearly of the crystal facets heterogeneity during reaction process. The enlarged crystal has two smooth facets (marked by D and F) and one step-like facet (marked by E). We can find the smooth facet of this kind of large crystal is maintained completely and defect less after long time soaking in MAI/IPA solution. The phenomenon of PL peak position blue-shift of step-like facet also remains (Figure 4 (d)). Besides, we could observe crystals gradually grew into shape of cubic/cuboid along with all their facets turned into smooth facets and demonstrate PL peak position at 775 nm, showing the intrinsic perovskite properties. Based on these evidences, we conclude that the crystal facets heterogeneity can be annihilated during the Ostwald ripening process with longer reaction time. 3.4 Chemical explanation on facets heterogeneity origin and annihilation

15



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Figure 5. Illustration picture of chemical origin and self-annihilation of defects in SDP crystals (from both morphological and molecular view) Considering the perovskite growth process monitored by GISAXS

48

and SEM &

TEM 49, we may give a hypothesis on the formation and evolution of these two kinds of facets on SDP films. As shown in Figure 5, the pre-deposited PbI2 crystals have packed layered structure and obvious grain boundaries. After loading MAI solution, the whole reaction process could be divided into three main stages. In the Stage I, two kinds of facets would form instantly after the reaction as shown in Figure 1A. Smooth facets with the well-defined perovskite would be formed through the interfacial reaction of MAI with the PbI2 crystal grain interior. Besides, the step-like facet would form with the MAI inserting into layered PbI2 from the grain boundaries area and the 16


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crystal structure started to expand instantly. There were many defects on the starting PbI2 grain boundaries, so that after insertion of MAI, the layered defective PbI2 induced imperfect inorganic cage, leading to the steps in morphology (facet B). In addition, the insertion of the MAI to react with inner PbI2 is promoted by MAI concentration gradient, so the step-like facet could be MAI rich. From the morphological view, reacted perovskite crystals in Stage I show two typical facets, smooth one (A) and the step-like one (B); from the molecular view, the smooth facets own well-defined crystal lattice and the step-like facets are MAI rich. The inner part of crystals may also remain substantial unreacted PbI2. In the following Stage II, perovskite crystals will continue to grow up by Ostwald ripening. Namely, imperfect lattices dissolve into IPA solution and coordinate with each other then re-deposited on larger perovskite crystals. The ripening process at Stage II could be verified by checking the UV-Vis absorption of MAI solution in IPA after PbI2 long time soaking. The absorption edge of the solution shifted from 380 nm to 500 nm, indicating the dissolution of the defected perovskite into MAI IPA solution in the form of [PbI6]− framework. (Figure S2)

29

. By the notice of the flat 10 nm capping layer on the

smooth facets of the 10 min sample in Figure 2 (B), we can conclude perovskite based on these well-coordinated [PbI6]− framework would deposit and grow in epitaxial on the smooth surface and form the additional perfect lattice. Several hours later, the crystal growth of SDP films enters the Stage III. After long time Ostwald ripening 66, 67

, in the morphological view, crystals grow larger with more regular shape and

step-like facets (B) gradually turn to smooth facets (B’). In the molecular view, MAI 17



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and PbI2 reacted more completely and all facets own high crystallinity, as shown by the long time reacted perovskite meso-crystal in Figure 4 (c). In short, after referring to the three stages in the whole sequential reaction process, we firstly reveal that the facets heterogeneity in surface potential and PL peak position originates from the initial disparate reaction condition of PbI2 at crystal domains interior and grain boundaries with MAI. Then the large crystal with more perfect facets can be ascribed the annihilation of this heterogeneity to following long time Ostwald ripening process in sequential reaction. 3.5 Facets heterogeneity effect on photo response and stability

Figure 6. (a) I-V characteristics in dark, (b) photo current and (c) photo response time comparison of different perovskites with different reaction time. (d) Responsivity and on/off ratio of perovskites as increase of light intensity. (e-f) Photocurrent and on/off ratio decay of perovskite photodetectors. After the investigation on the chemical origin and evolution of facets heterogeneity of 18


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SDP, it is vital to reveal its impact on the basic photo-electric response. Larger crystal film contains more smooth facets while smaller crystal film has more step-like facets, so that we can check this facets heterogeneity through measuring the different sized SDP films’ conductivity and photo response. Figure 6 (a) shows the current-bias curves of three perovskite films in the dark. The dark current decreases as the crystal size increases after prolonged reaction time. Also, all three samples showed hysteresis between forward scan and reversed scan, which is consistent with the hysteresis phenomena in perovskite solar cell I-V curves

60

and may be related to the motion of

ions 61 and charge or discharge of defects. 62 10 min sample has the smallest hysteresis among all three samples, which implied the least defects and best crystalline quality, consistenting with the PL results discussed above. Meanwhile, this sample also has the outperformed photo response in Figure 6 (b). Namely, it has lowest dark current of 5.5×10-11 A and highest photo current of 2.2×10-7 A under a 633 nm laser with intensity of 20 mW/cm2. The on/off ratio reaches 3.9×103 and the responsivity increases to 0.14 A/W at light intensity of 2×10-5 mW/cm2 shown in Figure 6 (d). For the 1 min and the 10 s samples, the on currents are just 2.84×10-8 A and 6.98×10-9 A respectively. Figure 6 (c) shows the transient response curves of the three types of devices. The response time is defined as the time when current reach 90% of its maximum after the illumination. The 10 min sample has a shortest response time around 160 µs, while the response time of 1 min and 10 s sample are 230 µs and 300 µs, respectively. We find that the response time is not sensitive to the applied voltage, see Figure S3-S5. These results mean that the mobility of perovskite is rather high in 19



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the channel and generated carriers could reach the electrode in a shorter time scale compared to response time. The response time is an intrinsic property concerning with the film quality, which is actually limited by the trapping of the excited electrons and holes. The difference of response time could only be ascribed to different trap states density in these three samples. To give more quantitative information on the photo-electricity property of the smooth facets and step-like facets, we fabricated the electron-only device and hole-only device of 10 s and 10 min sample and calculated the difference in their mobilities. It is reasonable to assume that the 10 s sample contains much step-like facets and 10 min sample is mainly composed of smooth facets so that we can further give comparison of the mobility and trap states density between the step-like and the smooth facets. Using space-charge-limited current (SCLC) technique, upon applied the enough bias, the current will increase linearly with the voltage then transfer onto the trap-filled limit (TFL) regime in which all the available trap states were filled and finally reach the trap-free space charge limit current (SCLC) regime, the current of the perovskite can be fitted by the Mott-Gurney law

68, 69, 70

,μ is the mobility, L is the channel

length:

JD =

9εε 0 µVb2 8L3

(1)

The onset voltage VTFL is linearly proportional to the density of trap states nt. VTFL =

ent L2 2εε 0

(2)

the red line in Figure S7 and S8 represents the JD∝V2 regime. Using equation (1), we 20


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could get the electron mobilities of 10 s and 10 min samples are 0.0051 cm2 V–1 s –1 and 0.42 cm2 V–1 s –1, respectively. The hole mobilities of 10 s and 10 min samples are 0.016 cm2 V–1 s –1 and 1.3 cm2 V–1 s –1, respectively. this result shows that the mobility of smooth facets is nearly 80 times higher than step-like facets. The traps states densities are calculated from equation (2). The VTFL of 10 s and 10 min samples are around 13.0 V and 2.4V, respectively. Refer to the equation (2), the trap states density ratio of the two samples is around 5.4:1, showing that the step-like facets contains trap states density 6 times more than smooth facets. This result also corresponds the 0.06 eV variance in Fermi level of the two facets shown in KPFM under illumination. In conclusion, based on the dark current, on/off ratio and response time measurement and the SCLC technique, we find the trap states of perovskite crystals gradually annihilate as the crystals grow larger. It is known that organic inorganic perovskite is unstable and lose the extinguished photo-electric response when exposed in humidified air. More trap states could accelerate the degradation process. To compare the trap states density in SDP films with different sizes, we further investigate its stability in 60±5% relative humidity. The degradation process would accompany with the decrease of the photo current for less active perovskite and the decrease of the on/off ratio due to induced trap states. For the large sized 10 min sample, Figure 6 (e) shows its photo current gradually decayed with the storing time. For the first six days, the photo current of this device remained to be around 1×10-7 A. But it decreased to 3×10-8 A when measured at the 12th day. The on/off ratio shown in Figure 6 (f) correspondingly remains 1000 after 6 21



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days and 100 after 10 days respectively. However, the small sized samples show much worse behavior in lifetime tests. The photo current decreases dramatically from the second day on, along with the increased dark current. Also, the on/off ratio of these two devices drop sharply for the first two days, shown in Figure 6 (f). This decay just result from the degradation of perovskite as well as the divorced iodide ions during degradation procedure react with the gold electrode and destroy the ohm contact of perovskite and electrode.

63, 64

From the decay results, we conclude perovskite with

smaller crystals from short time reaction have poorer stability than that of larger crystals, inferring that step-like facets could accelerate the degradation speed. AFM image of slightly degraded perovskite meso-crystal film also proves our estimation. From figure S6, the slightly degraded perovskite crystals generally keep the topography of original fresh perovskite crystals, especially those large smooth facets. However, some tiny dots appear showing that partial perovskite has reacted with H2O and O2 in the atmosphere and their lattice has been broken after one week storing. Those small dots of degraded perovskite mainly distribute among the step-like facets and can hardly be observed on large smooth facets. This facets heterogeneity in degradation of perovskite reveals that different facets of SDP has different moisture stability. For MAPbI3, MAI terminated and defects rich facets are proved to be more potential to react with H2O and O2, introducing impurities into perovskite lattice while PbI2 terminated and perfect lattice tends to be more chemical stable.

65

Considering

the chemical reactivity with the facet dependent stability, the hypothesis that smooth facets are stoichiometric defects free and step-like facets are MAI rich can be further 22


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confirmed. The results of the photoelectrical response as well as the stability check of SDP films with different reaction time correspond well with the facet heterogeneity analysis and proved our chemical illustration on that of step-like facets with high density of defects and the following annihilation process. 4. CONCLUSIONS In summary, this work reveals facets heterogeneity of SDP and further links it to the crystal growth process. Two typical facets: the smooth facets and the step-like facets with structure defects are observed in morphology. KPFM results show that smooth facets are electrically intrinsic yet the step-like facets are p-type doped with MAI rich composition. PL peak position mappings illustrate step-like facets has a 5 nm blue shift than that of smooth facets. This facets heterogeneity actually comes from the crystal growth process in sequential deposition method. The PbI2 crystal grain interior reacts with MAI to form smooth facets, while the original PbI2 grain boundaries after MAI insertion could deliver MAI rich and step-like facets. The smooth facets and step-like facets in SDP just correspond to the crystal domains and grain boundaries in one step spin coating perovskite. In the end, we find the facet heterogeneity has huge impact on photo-electric response and stability. By measuring the photo response and moisture stability of perovskite films after different reaction time, we found trap states will be effectively inhibited and crystallinity of perovskite will be improved as the going of long time Ostwald ripening, which means the sequential deposition method is a self-modified crystallization progress for perovskite crystals growth and high 23



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quality mesoscopic perovskite crystals with high photo response performance can be obtained after long time reaction. The analysis not only provides insights into the microscale property of SDP films but also guides the control of crystallinity which is meaningful for further improvement of perovskite photo electrical applications. ASSOCIATED CONTENT Supporting Information Supporting Information is available from the ACS Publications website. Supporting Information includes SEM image of pre-deposited PbI2 thin film, Absorption curves of MAI/IPA solution at the beginning and after long reaction time, Photo response of 10 s, 1 min and 10 min perovskite sample under different bias, respectively and AFM image of SDP thin film after one week exposure in air, J-V curves of hole-only device and electron-only device.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] * E-mail: [email protected] †

These authors contribute equally to this work.

Funding The work is in part supported by Research Grants Council of Hong Kong, particularly, via Grant Nos. AoE/P-03/08, N_CUHK405/12, T23-407/13-N, AoE/P-02/12, 14207515, 14204616, and CUHK Group Research Scheme,

24


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J. B. Xu would like to thank the National Science Foundation of China for the support, particularly, via Grant No 61229401. W. G. Xie would like to thank the National Natural Science Foundation of China (11574119, 61674070) Notes The authors declare no competing financial interest.

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TOC Figure:

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Facet-Dependent Property of Sequentially ... - ACS Publications

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