The Surface of Hybrid Perovskite Crystals: A Boon or Bane Banavoth Murali,‡ Emre Yengel, Chen Yang, Wei Peng, Erkki Alarousu, Osman M. Bakr, and Omar F. Mohammed* KAUST Solar Center, Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia ABSTRACT: Hybrid perovskite single crystals have garnered tremendous research attention and are expected to be nextgeneration materials for high-efficiency photoactive devices. Therefore, it is fundamentally important to understand the relationship between the optoelectronic properties of these materials and the marginally exploited surface chemistry in ambient air. For instance, a strong surface disorder, including hydration and ion migration, can possibly lead to extremely different optical and electronic properties at the surface compared to the bulk of the single crystal (SC). From this perspective, we evaluate the key variables that underlie the perovskite SC surface restructuring in ambient air and discuss their merits and limitations. In addition, a comprehensive picture of surface disordering, the remarkable change in the charge carrier dynamics and carrier mobility, surface hydration, and the effect of ion migration on the surface behavior will be discussed. Finally, surface passivation methods are highlighted to resolve or overcome the challenges for device integration.
A
humidity, such as during postprocessing exposure (for some minutes), can improve the performance;9,30−32 however, exposure to humidity for a number of days results in degradation in the performance.30 However, for SCs, it has been found that their larger absorption coefficients and properties of the transformed surfaces of SCs differ significantly from their bulk properties, including morphology, carrier mobility, photocurrent, carrier dynamics, optical bandgap, and device performance. Such effects have been marginally exploited in perovskite SCs; therefore, the surface recombination losses, which play a decisive role in determining the intrinsic optoelectronic properties and subsequently optimizing the device performance, need to be fully understood. Thus, a comprehensive study of the SC surface sensitivity to the environment is urgently required.33 Hence, with this Perspective, we will focus on the active interfaces of SCs, which have a great technological impact, by studying the variations in the structural, optical, and transport properties of the surface when it is exposed to ambient air. Optical Properties. Conventional spectroscopic and microscopic techniques have been extensively used to study the surfaces of SCs, and it has been proposed that dangling bonds, dislocations, and chemical contaminants on the surface possibly mask the intrinsic properties of the SC.34−36 Here, we discuss
n unprecedented revolution in photovoltaic research due to the advent of hybrid perovskites has marked a new era in which a tremendous improvement in the photoconversion efficiency from 3.8% to almost 22% in the time window of a few years has been accomplished.1−3 Holding the promise of solution processability, these semiconductor absorber materials exhibit galvanizing optoelectronic properties4−7 with potential applications that span from solar cells8−11 to electrically and optically pumped lasing,12−14 photodetectors,15,16 bright light-emitting diodes,17,18 color imaging devices,19 and phototransistors.20,21 Notably, most optoelectronic devices are primarily based on polycrystalline films, which unfortunately comprise a high density of trap states and grain boundaries that can severely affect the device efficiency and stability.22 Their single crystal (SC) counterparts with a low trap density and an absence of grains represent a strong candidate for overcoming these limitations.23−26 Apart from the issues related to the device integration arising from the thicker crystals, there are a number of significant challenges associated with the surface sensitivity of these SCs to various environmental factors, particularly, high levels of relative humidity (RH ≥ 55%).27 Specifically, for polycrystalline perovskite films, a low degree of humidity or moisture is a boon for high PCE; however, for SCs, the edge sites are a bane at harsher environmental conditions, as the surface transforms and becomes disordered due to hydration, thereby masking its true properties.28,29 Recent reports have revealed that previously unrecognized variations in films when they are exposed to © 2017 American Chemical Society
Received: December 12, 2016 Accepted: March 3, 2017 Published: March 3, 2017 846
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We focus on the active interfaces of SCs, which have a great technological impact, by studying the variations in the structural, optical, and transport properties of the surface when it is exposed to ambient air.
The transformed surfaces of perovskite single crystals differ significantly from their bulk properties, including the morphology, carrier mobility, photocurrent, carrier dynamics, optical bandgap, and device performance. carrier density increases with time after 1p-excitation for various durations, which is presumably governed by diffusion and recombination processes (Figure 1f). In this case, the PL recombination dynamics are mainly classified into bulk and surface recombination channels, as illustrated in Figure 1g. Notably, the PL spectra are modulated when the MAPbBr3 SC is exposed to air or vacuum, which confirms that the surface trap sites play a crucial role in determining the optoelectronic properties. Such a PL modulation effect is attributed to the reversible physisorption of O2 and H2O, which further aids the neutralization of positive charges on the surface because of the donation of electron densities to the Pb2+ cation, in turn affecting the surface recombination rates. The high density of surface traps in the SC is also evident from the higher surface recombination velocities.42 It is worth mentioning that for the PL spectra of MAPbBr3 observed at 540 and 580 nm for the 1p- and 2p-excitations, respectively, possible light reabsorption phenomenon in the bulk cannot be excluded. Therefore, when the correction factor using the absorption coefficient43 and diffusion length are taken into account,38 the emission peak at 580 nm significantly reduced, and the peak at 555 nm became the new PL position for 2p-excitation (Figure 2a). This clearly suggests that because of the large penetration depths of the millimeter-sized SC, light reabsorption can dominate for the 2p-excitation and subsequently alter the position of the PL spectrum of the MAPbBr3 SC. This is very important information that needs to be taken into account when one deals with thick perovskite SCs.
the sensitivity of the SCs to ambient conditions and, in particular, the impact of moisture on the surface behavior. Interestingly, recent studies have shown that low humidity results in desired crystallization growth and dynamics;37,38 high humidity, on the other hand, accelerates degradation and diminishes the device efficiency.31,39,40 Controlled experiments were carried out under vacuum and for various humidity levels to gain more insight into the surface disorder. As observed in Figure 1a−c, a blue spectral shift in the photoluminescence (PL) from ∼796 to ∼770 nm along with a decrease in the full width at half-maximum (fwhm) by ∼22 nm at RH ∼ 50% is observed for the MAPbI3 SC, which can be attributed to the lattice distortion-induced surface reconstruction of the SC.41 It is known that the single-photon absorption (1p) can excite only the surface of the SC, and the two-photon absorption (2p) can penetrate much deeper and be used to acquire the true bulk properties. Being in this regime, the MAPbI3 SC under 1p (λex = 600 nm) and 2p excitation (λex = 1200 nm) surprisingly exhibited a clear contrast in their emission spectra (Figure 1d). The PL recombination lifetime increases from 4.9 to 109 ns upon moving from the surface to the bulk, respectively.42 A similar result is also observed for the case of MAPbBr3 SC (see Figure 1e). Recently a clear surface sensitivity to moisture was also observed for MAPbBr3 SC.28 Apparently, a change in the lifetime from the surface to the bulk is expected because of the density of accumulated traps at the surface, which results from hydration, ion migration, and surface disorder. Moreover, in the MAPbBr3 SC, the surface carrier density decreases and the bulk
Figure 1. Normalized PL emission spectra obtained under (a) vacuum, (b) low-humidity atmosphere, and (c) high-humidity atmosphere. The laser fluence used was ∼10 μJ cm−2. (d) MAPbI3 α/s (absorption/scattering), bulk emission (λex = 1200 nm), and surface emission (λex = 600 nm) profiles. Inset: MAPbI3 SC image. (e) PL emission spectra of the MAPbBr3 SC with 400 nm (1p at ≈20 μJ cm−2) and 800 nm (2p at ≈200 μJ cm−2) excitations. Inset: Image of a typical MAPbBr3 SC. (f) Photocarrier density profiles obtained at various time scales, as marked in the figure at laser excitation λex = 400 nm. Inset: Image of the MAPbBr3 SC under single-photon excitation (SPE) with λex= 400 nm. (g) Schematic showing the classification of PL recombination dynamics into bulk and surface recombination channels. [Adapted with permission from refs 33 (Copyright 2016 AAAS), 41 (Copyright 2015 Royal Society of Chemsitry), and 42 (Copyright 2016 Wiley).] 847
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carrier lifetimes (1.68 μs) than the surface (34.88 ns), in accordance with previously reported results.42 It should be noted that the shorter lifetime measured on the surface upon 1p excitation can be attributed to the carrier trap and diffusion on the surface.24 Structural Properties. The structural characterizations are carried out on both the surface and bulk of the SC to account for any phase transformations within the same SC. The X-ray diffraction pattern obtained from the surface resembled that of the thin film, with a highly oriented (100) Bragg reflection; however, at the bulk, it showed the (200) reflection as the dominant orientation (Figure 2d−f). Figure 3a shows the aged surface and bulk of the SC. In general, the hk0 reciprocal lattice plane reconstructed from raw SC X-ray diffraction (XRD) data shows sharp spots, which indicates the high quality of the crystals (Figure 3b). Nevertheless, in most cases, the reported XRD for the thin films or SCs (grounded into powder) showed only the highly oriented (100) peak; therefore, we consider if there is an SC surface disordering into polycrystalline features [oriented along the (100) plane], which yields similar XRD patterns. The difference in the texture coefficient of the highly oriented peaks along the (100) and (200) planes at the surface and bulk is attributed to the gradual homogeneous nucleation that initially drives crystal growth and to the rapid surface transformation upon exposure to ambient air.28 To account for the hydration-induced surface transformations, a comprehensive study using state-of-the-art scanning tunneling microscopy (STM) was conducted in our recent work to show how the surface is prone to hydration and demonstrate the formation of grain boundaries at the surface of the SC.28 Briefly, the bulk of the SC showed well-ordered bright protrusions of Pb octahedra with a Pb atom at the center. Interestingly, the octahedra are found to be distorted from the ideal case, which was presumably driven by the lattice strain, forcing the octahedra to tilt because of an alteration of the metal−halide−metal angle.45 On the other hand, the volume expansion of the perovskite unit cell due to hydration resulted in local stress, leading to a relaxation of the surface structure, which accounted for the polycrystalline appearance of the aged surface. This study indicates that regardless of the high-quality SC synthesis technique employed, the surface restructures in ambient air because of perovskite hydrate formation and turns into a polycrystalline film. Such restructuring can impact the carrier transport when SCs are integrated into devices.
Figure 2. Comparison of MAPbBr3 SC optical properties: (a) PL spectra obtained at λex ∼ 400 nm and ∼800 nm and after the correction for light reabsorption. (b) Absorption spectra. Inset: Tauc plot showing the bandgaps. (c) Carrier lifetimes of the SC measured using the streak camera. For 1p, the PL peak is monitored at 540 nm after 410 nm excitation, and for 2p, the PL is probed at λem = 580 nm after 820 nm excitation. XRD pattern for the (d) aged (surface), (e) pristine (bulk) surfaces, and (f) thin film, respectively. (Adapted from ref 28. Copyright 2016 American Chemical Society.)
Because of the large penetration depths of the millimeter-sized perovskite single crystals, light reabsorption can dominate for the 2p-excitation and subsequently alter the position of the PL spectrum of the perovskite single crystals. Interestingly, the resemblance between the surface and thinfilm PL spectra with a slight change in the fwhm at 540 nm is a clear signature of the surface transformation of the SC to a polycrystalline nature (Figure 2a). Such broadening of the emission peaks may originate from different degrees of hydration and/or various hydrogen bonding interactions between the H2O and the organic cation (CH3NH3+) and halide ions.41 It is therefore reasonable to consider strain incorporation into the crystal due to such hydration effects. Strain-induced symmetry changes are known to alter the optical bandgaps,44 and in the current scenario, we have observed a similar effect, where the absorption features and band edges of the aged surface and the film were at 2.27 and 2.28 eV, respectively; however, these features were at 2.22 eV for the pristine surface (Figure 2b, inset showing the Tauc plot), which suggests that the electronic properties of the surface are different from those of the bulk. Notably, the carrier lifetimes of the SC measured using a streak camera at low pump fluences of 22.8 μJ/cm2 for 2p and 160 nJ/cm2 for 1p showed that the SC bulk has longer average
The volume expansion of the perovskite unit cell due to hydration resulted in local stress, leading to a relaxation of the surface structure by forming the nanostructure features and grains, which accounted for the polycrystalline appearance of the aged surface. Charge Carrier Mobilities of SCs. Recent studies have shown that RH ≥ 55% levels can severely affect both the crystallization and device performance because of degraded perovskite/metal interfaces.39,46 Notably, it has been reported that the SC surfaces behave similarly to polycrystalline films in humid ambient; thereby, the full potential of such promising materials can be achieved by eliminating the grain boundary and hydration effects. Therefore, a fundamental study of the intrinsic 848
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Figure 3. (a) Schematic showing the surface (aged surface) and bulk (pristine surface) of the MAPbBr3 SC. (b) hk0 reciprocal lattice plane reconstructed from raw SC-XRD data. Inset showing the MAPbBr3 SC. Current density−voltage characteristics (J−V) showing the SCLC under dark conditions on the (c) aged surface and (d) pristine surfaces. Transient photocurrent measurements carried out on the (e) pristine and (f) aged surfaces of an MAPbBr3 SC. (g) Calculated mobilities from the transient decay as a function of the illumination light intensity. (h, i) Hall measurement parameters obtained from various batches of SCs. Ranges of minimum and maximum values are selected to show variations in the mobility, Hall coefficient, carrier density, and resistivity. (Panel b is adapted with permission from ref 33. Copyright 2016 AAAS.)
characterization method are detailed in Materials and Methods. The SCL current in a typical configuration was measured by applying a potential difference between the two metal contacts connected in parallel on both sides of the SC. The SCLC method is widely accepted especially for the thick samples because of its versatility in directly obtaining the mobility from the sample resistivity and the conductivity, without the need for further correction factors.59,60 The SCLC of the pristine surface was measured after cleaving both ends of the same aged crystal (used for Figure 3c) in vacuum, and new Au contacts were deposited on both the pristine surfaces. Importantly, the hydration of a near-surface perovskite crystal can directly affect its charge carrier mobility and optoelectronic properties. MAPbBr3 is an intrinsic semiconductor in which the carrier mobility, μ (μ = μp ≈ μn, where μp and μn are the hole and electron mobilities, respectively),26,42 is estimated from the SCLC under dark current−voltage conditions following the Mott−Gurney power-law (I ∝ Vn) dependence.61 As shown in Figures 3c,d, the carrier mobilities are derived from the Child’s regime, where a quadratic transition was observed at higher bias conditions using eq 1:
properties, especially the carrier mobility of the SCs, in harsh environments is of immediate concern.47−49 The reality in the complex competition is always argued on the modest or higher carrier mobility values50,51 reported by several experimental and theoretical groups.52−56 However, we follow the widely accepted methods for carrier mobility extraction reported to date for perovskite SCs.25,26,57 In this Perspective, we choose only MAPbBr3 SC as a model system to provide a comprehensive study of the extraction of carrier mobilities using three methods: space-charge-limited current (SCLC), transient photocurrent (TPC), and Hall effect. As previously mentioned, because the surface of SCs is prone to hydration or degradation, we refer to this surface as the aged surface, and the carrier mobility is obtained from it prior to cleaving of the crystal under ultrahigh vacuum (UHV) to form the pristine surface, where the device is reconstructed to remeasure the carrier mobility of the pristine surface. A freestanding SC and a monocrystalline film of MAPbBr3 perovskite were synthesized using the antisolvent crystallization (AVC) and cavitation-triggered asymmetric crystallization (CTAC) techniques, respectively.26,58 The as-synthesized SCs were aged under RH ≥ 55% levels. The aged surface showed clear difference from the bulk, as the outer surface of the SC gradually turned to polycrystalline nature, and is depicted schematically in Figure 3a, where the resulting surface is hereafter referred to as the pristine or bulk surface. Being in this regime, we have focused on the carrier mobilities of the aged and pristine surfaces by considering three different methods as stated before. The device fabrication procedures for each
J=
9 V2 με0εr 3 8 L
(1)
where μ is the charge carrier mobility, V the applied voltage, ε0 the free space permittivity, εr the dielectric constant of the semiconductor (25.5 for MAPbBr3 SC62), and L the separation between the contacts or the thickness of the semiconductor 849
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layer. As expected, a lower carrier mobility of ∼25 cm2 V−1 s−1 and a higher carrier mobility of ∼65 cm2 V−1 s−1 were obtained for the aged and pristine surfaces of the SC, respectively. After SCLC, we then obtained the carrier mobility using the transient photocurrent (TPC) measurement. TPC is a widely used technique to extract and analyze the time-dependent charges in a semiconductor, which is sandwiched between two electrodes, using a short square light pulse, where the resulting current is detected by an oscilloscope. The charge carrier recombination kinetics, such as trapping and detrapping, the density of states, and the mobilities, are determined using TPC.63−67 Here, we used TPC to extract the total charges from the phototransient decay currents when the excitation pulse is turned off. The light intensity-dependent decay currents followed an exponential behavior (Figure 3e,f), which upon integrating between the maximum current time (t1) and first zero-current time (t2) yields the total charge (Qtotal), as described by eqs 2−4: Q total =
∫t
t2
I dt
(2)
1
Q half =
μ=
Vd , 2
1 Q = 2 total
Vd =
The carrier mobilities obtained are in agreement with the values previously reported in the literature (Figure 3i, right panel).26 Clearly, the average carrier mobilities obtained from several batches of crystals are lower on the aged surface (∼24 cm2 V−1 s−1) than the pristine surfaces (∼52 cm2 V−1 s−1). It is also worth noting that the average carrier mobilities obtained from the Hall measurements are in good agreement with those obtained from the SCLC measurements. Therefore, the Hall measurements provided another strong piece of evidence that the carrier mobility of the pristine surface is higher than that of the aged surface, which is also consistent with SCLC and TPC. Such a relatively higher mobility was recently observed upon integrating the SC into a thin film, which could be attributed to the formation of pristine surfaces upon integration.68 On the other hand, the relatively low mobilities obtained from the TPC experiments are expected, as the top Au electrode shielded most of the SC surface, and only a smaller portion of the surface was available for exposure. Although we draw the same conclusions on the carrier mobilities of the pristine and aged surfaces of these SCs, as extracted from the transient decays in the TPC measurements, the values are relatively lower, and they are dependent on the illumination intensity, which can be attributed to second-order recombination, charging of trap states, and/or any chargemediated phonon scattering mechanisms69,70 present in the SCs. Comparisons of the estimated mobilities from various techniques in this study are presented in Table 1. Implications for Device Performances. To investigate the surface-modulated electronic properties of the MAPbBr3 SC, Fang et al. fabricated a simple photoconductor based on the SC, and the photocurrent dependence was monitored under vacuum and in air.33 In a typical device architecture with Au top electrodes that create a channel length of 120 μm (Figure 4a−c), the dark current measured under a 405 nm laser illumination at a 1.5 V applied bias showed a slight increase under air exposure; however, the photocurrent under laser illumination was significantly increased from 10 μA in vacuum to 24 μA in air; moreover, it is reversible in vacuum (Figure 4d,e). This experiment provided a clear indication that the surface trap sites play an important role in the optoelectronic SC devices because the 405 nm laser with a short penetration depth could excite only the surface. Such a process was possibly due to the reversible physisorption process, in which O2 and H2O can donate electron density to the positively charged surface, thereby modulating the surface recombination velocity. However, in our controlled experiments on a simple visible (AM 1.5 g illumination) photodetector (PD) in the Au/MAPbBr3/Au sandwiched architecture, a different trend was observed for the freshly cleaved surface (pristine) and the aged surface in humid ambient. The aged surface exhibited a photocurrent lower than that of the pristine surface, which we attributed to the increase in the interfacial contact resistances due to the perovskite hydrate formation at the aged surface. From both experiments, it is clear that although the surface neutralization in air resulted in a slight change in the photocurrent, the true potential of the SC will be the bulk contribution to the overall photocurrent of the device. In general, MAPbBr3 SC devices are known for their low photostability under ambient air.49 Therefore, we explored the effect of such surface neutralizations and degradations at RH ≥ 55% by constructing a thinner 6 μm, hole-transporterfree SC-based solar cell device with the ITO/TiO2/MAPbBr3/Au configuration.49 A PCE of 4.24% (Figure 4f) with an impressively
∫t
t
I dt
(3)
1
L /2 , t
μ=
L2 2tVbi
(4)
where t is the time at which half of the total charges are extracted. The mobility can be extracted using these relations. A built-in voltage (Vbi) of 1.2 V is used for the calculation, which was extracted from the Au and MAPbBr3 SC interface, as reported elsewhere.58 The extracted charges for the pristine surface are comparatively higher than those of the aged surfaces, which further supports the higher mobilities obtained from the SCLC technique (Figure 3g). We believe that this reduction is due to the presence of the perovskite hydrate, which masks the true carrier transport of the pristine material. Additionally, the carrier mobility measurements were further confirmed from the TPC measurements63−65 (Figure 3g) and the Hall-effect measurements (Figure 3h,i and Table 1). Special Table 1. Comparison of Carrier Mobilities from Three Techniques surface
avg. hall mobility (cm2/V s)
SCLC mobility (cm2/V s)
TPC Mobility (cm2/V s) at 10% light illumination
aged pristine
24.53 52.66
24.98 64.74
∼7.5 ∼10
care was taken during sample preparation, and several batches of SCs were selected for the Hall measurement to estimate the mobilities. A positive Hall coefficient (RH) was obtained for the aged and pristine surfaces, which indicates that the SC is p-type, and there are no n-type electronic impurities present (Figure 3e, left panel). A lower carrier concentration (n) and a higher hall mobility (μ) for the pristine surfaces compared to the aged surfaces is expected from the following relations: n = 1/RH·q and μ = RH·σ, where the conductivity σ = 1/ρ and q is the electron charge. A high Hall resistance of the pristine surface is likely due to the increased surface roughness upon cleaving and thus a higher contact resistance (Figure 3i, left panel). 850
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Figure 4. (A) Schematic SC device used for current−voltage (I−V) measurements. (B, C) Cross-sectional view and top view of an SC device. (D) Laser illumination I−V characteristics of the MAPbBr3 SC device. (E) Dark I−V characteristics of the MAPbBr3 SC device in air and under vacuum. (F) Lamp ON and OFF response cycles under AM1.5G illumination. (G) J−V curves for the device under AM1.5G illumination, showing the effect of hydration with time, as indicated. (Adapted with permission from refs 28 (Copyright 2016 American Chemical Society) and 33 (Copyright 2016 AAAS).]
Figure 5. (a) Possible ion migration paths of I− ions in the MAPbI3 crystal. (b) Illustration showing the passivation of the Pb−I antistite grain boundary defect by fullerene.88 (c) PL spectra obtained from the passivation of the MAPbBr3 SC at various volumes of pyridine, as indicated from top to bottom, with corresponding (d) confocal laser scanning micrographs exhibiting the enhanced color contrast difference due to passivation of the SC surface with pyridine. (Panel b adapted with permission from ref 88.)
high VOC of 1.2 V was accomplished from a fresh device. A gradual ∼17% decrease in the PCE with time (on day 10) under extremely humid conditions indicated that the surfaces are transformed or disordered because of the perovskite hydrate formation. For instance, the CH3NH3PbI3−xClx-based solar cells exhibit a significant PCE reduction from 4% to less than 2% due to interface degradation by moisture.46 Some studies have shown that although monohydrate formation lowers the PCE of perovskites, it is however reversible upon dehydration.71
Notably, the polyhydrate formation can induce surface restructuring as a result of the permanent effect of grain boundary formations, and it may even cause a complete failure of the devices.72 Thus, we combine our fragmented understanding of the role of the humidity level with a thorough, comprehensive understanding of several variables associated with water intake. It is therefore very important to obtain high-quality SCs to improve the performance upon their integration into real-time devices. 851
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Role of Ion Migration. Another key variable component that can affect the device operation is the ion migration at the surface. After it was initially proposed to explain the hysteresis behavior in perovskite solar cells,73−75 it is now believed to be a driving force for reducing various phenomena, such as the fieldswitchable photovoltaic effect,76,77 photoinduced giant dielectric constant,78 photoinduced phase separation,79 and photoinduced self-poling.80 According to a number of theoretical reports on the MAPbI3 system, I¯ ions are more likely to migrate to the surface because of their lowest activation energy (0.58 eV) among available ions (Figure 5a).81−83 Although ionic migration in the bulk crystal is more controlled by the activation energy, other possible migration channels on the surface are the grain boundaries due to surface hydration,48 charge accumulation,84 a change in the chemical bonding, and surface strain85 can introduce additional properties in perovskite-based devices by increasing the ionic conductivity. Shao et al. have showed that the grain boundaries are the dominant migration channels for ions in the perovskite system.86 Conductive atomic force microscopy (c-AFM) measurements performed on the grains and grain boundaries showed increased ion migration on the grain boundaries, which resulted in the redistribution of ions. Such an effect led to hysteresis in the devices, thereby affecting the photostability of the devices.86 It was also observed that the dark current hysteresis decreases as the grains become larger, which results from less ion migration. Being in this regime, using cathodoluminescence in scanning transmission electron microscopy, Hentz et al. have demonstrated that the rearrangement of ions on the surface directly affects the fundamental optical and structural properties of perovskite films, which results in a change in the surface emission for iodide-enriched regions at energies higher than the bandgap.87 In an attempt to suppress the ion migration-dependent hysteresis, Shao et al. showed that passivating the grain boundaries with fullerene decreases the possible ion migration channels.89 In explaining the passivation of possible charge traps on the grain boundaries, it was shown that the diffusion of fullerenes into the grain boundaries can reduce the trap state density by 2 orders of magnitude (Figure 5b). In addition, Xu et al. have demonstrated both theoretically and experimentally that while passivating the grain boundaries, the fullerene−PbI3− antistite interactions suppress the deep trap states as well, which results in shifting of the trap state energy level toward the conduction band.88 Importantly, the hydration-induced grain boundaries due to the surface transformation to polycrystalline on perovskite SC surfaces needs to be either size-controlled or passivated to suppress surface recombination.90,91 Additionally, these boundaries on the SC surface can lead to pronounced ion migration, which is similar to that observed in the thin-film counterparts.92 Therefore, either actuated by grain boundaries or surface vacancies, ion migration and its effects on perovskite SC surface are still open to debate.72 Ion migration in the bulk and on the surface is assumed to be the reason for the strange properties in perovskite solar cells. Although changing the chemical structure seems to be the only way to suppress the migration of ions in the bulk, it is more important to control the ion migration in the surface to eliminate the interfacial problems that affect the device performance. Controlling the surface morphology or passivating the surface trap states are some of the possible strategies that are required when designing perovskite solar cell devices.
Controlling the surface morphology or passivating the surface trap states are some of the possible strategies for suppressing the ion migration at the surface of single crystals to eliminate the interfacial problems that can possibly affect the device performance. Here, we provide preliminary results on the surface passivation of MAPbBr3 SC using the pyridine molecule. Upon addition of pyridine, a gradual enhancement in the PL intensity was observed with respect to thickness, as shown in Figure 5c (bottom to top panel). We believe such a passivation can modulate the surface traps and hence the recombination velocities. Moreover, the surface passivation is further corroborated from the clear contrast of the brightness before and after pyridine addition according to the confocal laser scanning microscopy results (Figure 5d, left to right). Under different thickness, the MAPbBr3 SC showed PL enhancement. Various passivation approaches with different small molecules and correlations with ion migration effects are currently at a premature stage, and their development is ongoing. Summary and Future Outlook. We reveal that the intrinsic structural and optoelectronic properties of the surface and bulk SCs are very different. For instance, we have compared the carrier mobilities obtained from the MAPbBr3 SC using various techniques such as SCLC, TPC, and the Hall effect, and we observed that the average mobilities from different batches of SCs showed an almost three-fold increase for the pristine surface compared to the hydrated-aged surface. Thus, surface hydration is directly correlated to the decrease in the carrier mobilities under harsh ambient conditions. We have focused on new insights into the surface restructuring and disorder of the perovskite SC by providing a novel picture of how surfaces are obscured because of various factors including hydration, strain, and ion migration. Notably, polyhydrate formation can irreversibly affect the SC surfaces and transform them to polycrystalline surfaces by modifying the trap densities and diffusion lengths. Therefore, SC-based devices suffer from parasitic losses and underperform compared to polycrystalline thin films. We believe such a study can provide a foundation in the convergence to a fundamental understanding of the unusual optoelectronic properties in SCs. Surface passivating strategies at the expense of marginal PCE losses need to be implemented for future amelioration of photostable devices.
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MATERIALS AND METHODS Chemicals and Reagents. The SC synthesis involved the following chemicals, which were used without further refinement: lead bromide (≥98%) and dimethylformamide (DMF) (anhydrous, 99.8%) procured from Sigma-Aldrich and methylammonium bromide (MABr) from Dyesol Limited (Australia). FTO-coated glass substrates (15 Ω sq−1) were purchased from Pilkington. SC Growth and Device Fabrication. The MAPbBr3 SC was grown using the AVC technique, for which the detailed experimental setup is reported elsewhere.26 Briefly, the crystal was grown using equimolar MABr and PbBr2 in N,N-dimethylformamide (DMF). MAPbBr3 monocrystalline films on FTO substrates were grown using the cavitation-triggered asymmetrical 852
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crystallization (CTAC) technique, for which the detailed experimental setup is reported elsewhere.26 The device in a glass/ FTO/MAPbBr3/Au configuration was fabricated by depositing a geometry-masked Au top electrode by thermal evaporation at a deposition rate of 0.25 Å/s. Measurement and Characterization. Time-resolved photoluminescence spectra were recorded using a high-resolution Hamamatsu C10910 streak camera where the pump beam is generated with Spectra-Physics MaiTai eHP and Inspire HF-100 OPO. The repetition rate of the pump beam is adjusted to 1 MHz using a APE Pulse Select pulse picker. SCLC measurements were carried out on a thicker SC for the aged surface, and then the SC was cleaved from both sides to obtain the pristine surface. All the SCLC measurements were carried out on the Au/MAPbBr3/Au geometry under dark conditions using a Keithley 2400 source measurement unit. TPC measurements are carried out using a Paios system equipped with a white-pulsed LED source under dark conditions. The LED source is specified with a pulse length range of 1 μs to 5 s, and the rise and fall time of the LED was 100 ns, with a variable 2 kW/m2 maximum intensity in the spectral width of 350− 700 nm. The decay transients were simulated exponentially to extract the charge, and the model was used to extract the mobilities. In the TPC measurements, surfaces of 100 and 400 μm thick freshly prepared crystals (in a glovebox; O2 and H2O levels 175 μm in SolutionGrown CH3NH3PbI3 Single Crystals. Science 2015, 347, 967−970. (58) Peng, W.; Wang, L.; Murali, B.; Ho, K.-T.; Bera, A.; Cho, N.; Kang, C.-F.; Burlakov, V. M.; Pan, J.; Sinatra, L.; et al. Solution-Grown Monocrystalline Hybrid Perovskite Films for Hole-Transporter-Free Solar Cells. Adv. Mater. 2016, 28, 3383−3390. (59) Chiang, C.-H.; Wu, C.-G. Bulk Heterojunction Perovskite− PCBM Solar Cells with High Fill Factor. Nat. Photonics 2016, 10, 196−200. (60) Han, Q.; Bae, S.-H.; Sun, P.; Hsieh, Y.-T.; Yang, Y.; Rim, Y. S.; Zhao, H.; Chen, Q.; Shi, W.; Li, G.; et al. Single Crystal
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