Complexities of Contact Potential Difference Measurements on Metal

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Complexities of Contact Potential Difference Measurements on Metal Halide Perovskite Surfaces Fengyu Zhang, Florian Ullrich, Scott H. Silver, Ross A. Kerner, Barry P. Rand, and Antoine Kahn J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03878 • Publication Date (Web): 10 Feb 2019 Downloaded from http://pubs.acs.org on February 10, 2019

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Complexities of Contact Potential Difference Measurements on Metal Halide Perovskite Surfaces

Fengyu Zhang, † Florian Ullrich, ‡ Scott Silver, † Ross A. Kerner, † Barry P. Rand,†, § Antoine Kahn †,*

†Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544, United States

‡ Materials Science Department, Technische Universität Darmstadt, 64287 Darmstadt, Germany

§Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey 08544, United States


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Corresponding Author *E-mail: [email protected]

ABSTRACT: Understanding the stability of metal halide perovskite (MHP) surfaces is of considerable interest for the development of devices based on these materials. We present here a vacuum-based study of the surface potential and response to illumination of two different types of perovskite films, methylammonium lead bromide (MAPbBr3) and the 2D Ruddlesden-Popper phase butylammonium lead iodide (BA2PbI4, n=1), using Kelvin probe-based contact potential difference and surface photovoltage measurements. We show that supra-band gap light irradiation can induce


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the loss of halide species, which adsorb on the Kelvin probe tip, inducing quasiirreversible changes of the MHP surface and tip work functions. If undetected, this can lead to misinterpretations of the MHP surface potential. Our results illustrate the effectiveness of the Kelvin probe-based technique in providing complementary information on the energetics of perovskite surfaces, and the necessity to monitor the work function of the probe to avoid erroneous conclusions when working on these materials.

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The field of metal halide perovskites (MHP) has grown rapidly over the past few years. Efficiencies of single-junction MHP solar cells have increased from 3.8%1 to 23.3%2 in less than a decade, making this the fastest-developing solar technology to date. Further advances demand careful consideration of key issues, such as band bending in the active layer, energy level alignment with adjacent layers, and electronic trap states at interfaces. A number of techniques, including ultraviolet and X-ray photoemission spectroscopy (UPS, XPS), contact potential difference (CPD) measurements using a Kelvin probe (KP), and Kelvin probe force microscopy (KPFM) have been applied to gather information at MHP surfaces and interfaces.3-6 CPD measurements give the difference between the work function of the surface under investigation and that of the KP tip.

Such measurements are often combined with surface photovoltage (SPV)

measurements, which provide the photon-induced change in surface potential of the material under investigation. The presence, magnitude and direction of band bending at MHP interfaces has been difficult to assess, given that the bulk Fermi level position is generally unknown. Standard photoemission spectroscopy, the technique of choice to determine the Fermi


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level position in the gap at a semiconductor surface, only probes the top few nm of the film and cannot distinguish between flat bands and band bending occurring over several tens of nanometers. Furthermore, photoemission can induce non-equilibrium conditions of SPV and unwanted band-flattening, providing distorted information on the equilibrium position of the Fermi level. Therefore, Kelvin probe based CPD and SPV measurements serve as important complementary techniques to sort out some of these issues. Contactless, non-invasive and non-destructive, CPD can be performed in the dark or under illumination, in vacuum, ambient or controlled atmosphere. Both CPD and SPV measurements provide reliable information on work function changes caused by the generation of electron-hole pairs near the surface/interface, and on charging and discharging of surface/interface defect states upon illumination. Therefore, SPV and surface photovoltage spectroscopy (SPS) have been applied to a number of MHP film surfaces and interfaces and have already provided important information.7-9 Yet, CPD measurements can also be skewed and lead to erroneous results owing to changes in chemical composition and electronic structure of the surface,10-12 photo-doping of lowdoped semiconductors in the presence of a selective contact,8,13 or trap states or band


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bending at buried interfaces well below the investigated surface.14 Obtaining accurate SPV signals can therefore be challenging when working on perovskite surfaces, which are sensitive to, and degrade under, irradiation.10-12 Care must be taken to distinguish real SPV signal and reversible band flattening as a result of photo-excitation, which occurs over short time scales (s), from the long-term changes in the surface work function due to surface reorganization or changes in stoichiometry (irreversible, or slowly reversible over hours), e.g. when the perovskite film surface undergoes degradation and decomposition during the CPD or SPV measurement. Note that the typical response time of a standard CPD measurement setup is of the order of the second. We show here that this subtle process can also lead to a change in the surface potential of the probe used for KP measurements, caused by adsorption of decomposition products. This potential perturbation must be taken into consideration, especially when working under vacuum. Assuming a constant work function of the probe tip (WFTip) can lead to erroneous evaluations of the perovskite work function (WFPerovskite) and the SPV response. No systematic investigation of the impact of MHP degradation on CPD and SPV measurements has been performed to date.


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Here, we conduct CPD and SPV measurements on 3D methylammonium lead bromide (MAPbBr3) and 2D Ruddlesden-Popper phase (RPP) butylammonium lead iodide (BA2PbI4, n=1) films. 3D metal halide perovskites, in particular MAPbBr3, are known to be susceptible to degradation upon interaction with oxygen, moisture, and/or light.16 The material serves therefore as a good platform to expose the complexity of characterizing volatile perovskite surfaces. On the other hand, the 2D Ruddlesden– Popper phases have demonstrated enhanced resistance to moisture and light, presumably due to the protection provided by the dense array of organic ligands that separate the inorganic (Pb, X) layers.20 A comparison between the two types of materials is therefore extremely informative. Our results provide direct experimental evidence of a change of WFTip during SPV measurements on both 3D and 2D films. This change is attributed, in part to changes occurring at the film surface, and in part to an unexpected change in the KP tip work function caused by the adsorption of halide species released by the MHP surface. We emphasize that a careful calibration of the

WFTip under illumination is crucial for obtaining accurate SPV results. Finally, this study


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is complemented by direct measurements of changes in the work function of a gold surface upon adsorption of iodine or bromine.

To investigate the impact of light on the MAPbBr3 work function, the CPD between the KP tip and highly ordered pyrolytic graphite (HOPG) or MAPbBr3, i.e. CPDHOPG-Tip and

CPDMAPbBr3-Tip, respectively, were measured before and after deep red (660 nm) and green (530 nm) illumination of the perovskite film, according to the procedure described in the experimental section. The measured values are displayed in Figure 1(a). Figure 1(b) shows the absolute values of WFMAPbBr3 and WFTip, based on the calibration of the tip against WFHOPG (4.70 eV) measured via UPS. These measurements were done as a function of time, before, during and after illumination with photons of two different wavelengths. The pre-illumination work function WFMAPbBr3, measured over several minutes after the sample had spent 6 hours in the dark in vacuum, shows excellent stability at about 4.43 eV, indicating that the MAPbBr3 sample has reached an equilibrium state. This value is used here as the starting point.


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Illumination with deep red light (~ 1 min) does not induce any measurable change of

WFMAPbBr3 nor of WFTip. This observation is not surprising considering that MAPbBr3 has a band gap of about 2.30 eV and does not absorb the 1.88 eV photons.15 At first, illumination with green light (2.34 eV) does not appear to influence CPDMAPbBr3-Tip significantly either. Instead, Figure 1(a) shows that CPDHOPG-Tip decreases by 0.14 eV. This shift can have two causes: a change in the graphite surface or in the Au tip surface. As UPS shows that WFHOPG remains constant, we conclude that WFTip has changed during the experiment. To rule out the influence of green irradiation on the KP tip itself, CPDHOPG-Tip was evaluated separately without MAPbBr3 present in the vicinity and found to remain unchanged. Consequently, the experiment above demonstrates that WFTip increases from 4.10 eV to 4.24 eV during CPD measurement of the MAPbBr3 film under green light irradiation (see Figure 1(b)). Similarly, WFMAPbBr3 increases from 4.45 eV to 4.55 eV, a change that is not immediately reversible when the light is switched off (Figure 1b). This observation stands in direct contradiction to expectations for a simple SPV effect.


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Figure 1. CPD and WF responses to irradiation on MAPbBr3. The full colored dots indicate measurements with and without illumination. (a) CPD between perovskite sample and KP tip (grey) and HOPG and KP tip (black). (b) Calibrated WF of the KP tip (black) and the MAPbBr3 film (grey). The resolution of the CPD measurement is estimated to be  10 meV.

Following this first illumination phase, both MAPbBr3 and HOPG samples were kept in vacuum in the dark and remeasured after 6 hours. As shown in Figure 1(b), WFTip


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almost recovered its initial value, but WFMAPbBr3 remained essentially unchanged, indicating a long-lasting impact of the green light, probably due to decomposition. It is known that metal halide perovskite films tend to decompose into PbX2, MAX and other components.11,16-20 Zhu et al. showed that the degradation of MAPbI3−xClx upon white light illumination occurs in ultra-high vacuum (UHV) over a short period of time.10 This decomposition process can be initiated by irradiation alone without the presence of H2O or O2.10 Juarez-Perez et al. studied the photodecomposition reactions of MAPbI3 and MAPbBr3 under simulated sunlight in vacuum.17 They found MAPbI3 decomposes into various volatile gas species, such as I2, CH3NH2, CH3I, HI and NH3, which sublimate in vacuum.18 The MAPbBr3 is found to decompose into PbBr2, CH3N2 and HBr, in which PbBr2 might further decompose into Pb and Br2 under irradiation.16,17 Significant losses in surface bromine and nitrogen concentrations in MAPbBr3 linked to light-induced degradation have been previously reported in another study.20 Following these previous studies,16-20 we propose here that species, such as Br2 and HBr, are emitted from the MAPbBr3 surface in vacuum and form an adlayer on the gold KP tip, thus changing

WFTip. It is worth noting that WFTip changes rapidly when the KP tip is close to the


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MAPbBr3 surface while being exposed to green light (increase of 140 meV in only 30 s). Tracking WFTip by measuring CPDHOPG-Tip before and after illumination is therefore essential in order to prevent erroneous CPD measurements on the perovskite surface. The recognized instability of some 3D metal halide perovskites, in particular MAcontaining MHPs, has led to considerable interest in the seemingly more stable Ruddlesden–Popper phases, also known as 2D MHPs. We examine here one of these 2D materials in the context of the experiments presented above for MAPbBr3. For SPV measurements, we choose to investigate BA2PbI4 (n=1), in which the butylammonium (BA) ligands sandwich a single 2D inorganic layer.20,22 This compound, previously investigated by Silver et al.,22 exhibits an optical bandgap of 2.38 eV and a single particle gap of 2.7 eV.


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Figure 2. Light response of BA2PbI4. The full colored dots show the measurements under illumination. (a) The CPD of HOPG and BA2PbI4, (b) the calibrated WF of the gold tip and the BA2PbI4. The resolution of the CPD measurement is estimated to be  10 meV.

The evolution of the CPDBA2PbI4-Tip and WFBA2PbI4 under deep red, green and blue light illumination is summarized in Figure 2(a) and (b), respectively. The HOPG reference


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surface WFHOPG was again checked with UPS and confirmed to have the same value of ~4.70 eV before and after the illumination, independent of the wavelength of the light. The 2D MHP sample was tested under red light 15 min after the first HOPG measurements. No significant change was detected on WFTip and WFBA2PbI4, i.e. ~0.02 eV on the former (within experimental error) and negligible on the latter, consistent with the results obtained for MAPbBr3. Considering the 2.38 eV band gap of BA2PbI4, the 660 nm light is not absorbed. The apparent discontinuity that appears at ~33 min on the figure is due to the fact that measurements were shifted to a different part of the surface exhibiting a slightly higher WFBA2PbI4. In contrast to the case of MAPbBr3 above, green (2.34 eV) or blue (2.63 eV) light illumination produces a rapid drop in both CPDBA2PbBr3-Tip and WFBA2PbI4 (Figures 2a,b, minutes 43, 61 and 130). The drop occurs within the response time of the KP setup, consistent with the occurrence of SPV. Measurements of CPDHOPG-Tip repeatedly under the same conditions allow us to track changes in the work function of the tip. A more systematic measurement of the contact potential difference between the KP tip and the BA2PbI4 or HOPG surface is given in Figure 3. Under green light irradiation, it shows


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that the measurement on the 2D material undergoes a rapid change in CPD attributed to SPV, followed by a slower shift that parallels the change in CPDHOPG-Tip. The latter is again attributed to the release of iodine species from the BA2PbI4 surface, adsorption on the tip and subsequent shift in WFTip . Zhao et al. have reported degradation of a 2D perovskite single-crystal under green laser illumination (520 nm), in which iodine loss was pointed out as an important degradation pathway.23

Figure 3. Change in CPDBA2PbI4-Tip under green irradiation. The initial rapid decrease (SPV) is followed by a slower decrease parallel to that of CPDHOPG-tip.


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The rapid decrease in WFBA2PbI4 upon irradiation corresponds to surface photovoltage caused by the surface accumulation of photogenerated carriers. We cannot rule out some contribution of ion migration caused by the change in band bending.24 As the light is switched off, photogeneration stops and WFBA2PbI4 increases immediately, recovering partially by 0.06 eV in the first 20 s, followed by a slow positive shift towards its initial value after 3 min. The first phase of recovery is due to the recombination of photocarriers while the slower phase may be due to ion migration due to the change in band bending. As evidenced by the change in WFTip under illumination, halide species are ejected from BA2PbI4, leading to a halide deficient surface when the light is turned off. The ion distribution near the surface is not in equilibrium anymore, which leads to the rearrangement of ions. This latter process appears to be significantly faster than the reversal of the light-induced degradation on MAPbBr3. In contrast, ion migration, which is known to occur in perovskites, causes changes in CPD on compatible timescales ranging from seconds to minutes.25,26 Almadori et al. also found a fast shift in CPD after the light was switched off, followed by a slow stabilization to the initial level. They attributed this behavior to the difference between the rate of ion migration under


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illumination and the one in dark conditions.24 There is an additional slow and small increase in WFBA2PbI4 for the final part of the recovery (minutes 65 -130 in Figure 2(b)), which we tentatively attribute to changes in surface composition and possible surface reconstruction.17 This sequence of illumination and recovery times was repeated several times to ensure reproducibility. Key differences between CPD or WF responses to irradiation of the 3D and 2D MHP surfaces investigated above are the rate of degradation and the SPV response. The MAPbBr3 film does not exhibit any measurable SPV, suggesting the absence of band bending at or near the surface, a strong pinning of the Fermi level or fast degradation of MAPbBr3. The BA2PbI4 film, on the other hand, repeatedly exhibits a ~130 meV SPV with corresponding reduction of the WF. We consider this light-induced change in the 2D MHP WF to be an electronic response at the free BA2PbI4 surface, rather than the result of surface degradation as in the case of the 3D MHP for the following reasons. First, the response to the green and blue irradiation is fast, and reaches saturation within the time of the KP setup response (< 2 s). Second, the change in WFtip is small and the change in WFBA2PbI4 is reversible, recovering back to the initial value within 3


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min. Finally, any band bending at the buried ITO/BA2PbI4 interface is likely to have negligible contribution to the SPV signal, due to the large film thickness (200 nm) and the weak illumination intensity (power density about two orders of magnitude smaller than solar irradiation), ensuring that all the light is absorbed near the free surface and in the film. It is important to note that the magnitude of the SPV, and thus the efficiency of a perovskite solar cell, is strongly dependent on the details of film processing. Yet, the SPV measured on BA2PbI4 under green irradiation varied little between the three samples examined under these conditions, ranging only from 130 to 150 mV (Figures S1 and S2). Both green and blue irradiation consistently produced an SPV of about 130 mV corresponding to a change in band bending with little variation between experimental cycles. The energy levels deduced from a combination of UPS and inverse photoemission spectroscopy (IPES) measurements,15and from the CPD under dark and green light conditions are illustrated in Figure 4. The Fermi level is estimated to be below mid-gap, at about 800 meV above the valence band maximum (VBM) near the surface. The work function measurements and the deduced sign of the SPV (Figure 2b)


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indicate an upward band bending at the BA2PbI4 surface in the dark. This surface band bending is reduced by 130 meV under the present illumination conditions, as photogenerated carriers (holes in the present case) accumulate at the surface and induce a counter field that tends to flatten the bands.

Figure 4. Schematic energy diagram near the 2D n=1 BA2PbI4 surface in the dark (bold lines) and under green irradiation (dash lines), showing the vacuum level (Evac), conduction band minimum (CBM) and valence band maximum (VBM). The Fermi level is shown in red.


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In this final part, the assumption that the changes of the KP gold tip’s work function are caused by the adsorption of halide species released by the MHP surface shall be crosschecked through measurements of the WF of an Au surface intentionally exposed to iodine (and bromine) species. Hereby, we place a thermally evaporated gold film (50 nm on glass) and iodine flakes in a covered petri dish for 60 s, and measure the film WF with an ambient KP (also calibrated against HOPG). The WFs of this exposed film as well as the one of a non-exposed control sample are compared in Figure 5. The exposed film has a WF of about 5.50 eV, which is almost 700 meV larger than that of the control film. It is found to be stable, decreasing by only 30 meV within 2 h. The control film was then also exposed to iodine under similar conditions for 15 s, which leads to a value of 5.39 eV. Both gold films then were left in air and remeasured after 12 h. WFs only decreased slightly and stabilized at 5.34 ± 0.01 eV (marked by an oval in Figure 5).


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Figure 5. The WF of gold films with (circle) and without (square) iodine exposure, measured by ambient Kelvin probe (a-KP). The arrow shows the change of the WF of the gold film after a 15 s short exposure to iodine.

In further tests, the iodine-exposed Au films were subjected to various treatments to investigate the stability of this effect: (a) annealing at 100 ºC for more than 10 h; (b) exposure to UV (Thorlabs M470L3 LED, 375 nm, maximum irradiance) for 10 min; (c) illumination with a UV laser (405 nm, 5 mW) for 5 min; and (d) storage in high vacuum (10-7 Torr) for 12 hours. Interestingly, the work function all of these films remained above 5.30 eV, which is at least 0.50 eV higher than the WF of air-exposed gold without iodine exposure. A chemical reaction between Au and I2, leading primarily to gold


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monoiodide, is known to take place at high temperature (> 120 ℃) and relatively slowly.27 However, the change observed here in the WF of the film exposed to iodine occurs rapidly at room temperature and does not appear to be reversible. Furthermore, the films were characterized with XPS. As expected from the KP experiments a pronounced I signal was measured (Figure S4). The high work function above 5.20 eV was confirmed by ambient Kelvin probe and was retained after more than 1 hour of X-ray exposure, despite a slight decrease in the I:Au ratio (see Figure S4). Besides, no direct evidence of a chemical reaction between Au and I2 was obtained from the Au 4f and I 3d core levels (Figures S5 and S6), confirming our previous interpretation of the increased WF of the Au film and Au KP tip as solely due to an iodine-induced surface dipole. In summary, the Kelvin probe-based CPD/SPV study presented here shows that supra-band gap illumination of both (3D) MAPbBr3 and (2D) BA2PbI4 leads to halide loss from the surface of the material, accompanied by a contamination-induced modification of the KP tip. The effect is in line with previously demonstrated irradiationinduced halide loss from MHP surfaces. This, in turn, is consistent with the reported


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higher environmental stability of the 2D perovskite structure, although one cannot discard in the present case the known volatility of the MAX components. In contrast to MAPbBr3, BA2PbI4 exhibits a significant SPV corresponding to a partial flattening of an upwards surface band bending. From an experimental point of view, the results show that Kelvin probe based CPD and SPV measurements on MHP surfaces should be approached with caution, given the potential risk for erroneous conclusions due to halide-induced changes of tip surface potential. A repetitive calibration of the work function of the tip with HOPG is necessary to ensure a reliable KP result. Along the same line, one should keep in mind that halide loss with the formation of a density of gap states likely results from supra-band gap irradiation in any photoemission experiments on MHP surfaces.

Experimental Methods Films of the Ruddlesden–Popper phase (2D perovskite) BA2PbI4 (n=1) were prepared by dissolving butylammonium iodide, BAI, (Greatcell Solar) and lead (II) iodide, PbI2 (TCI, 99.9%) in N, N- dimethylformamide (DMF, Sigma-Aldrich, 99.8% anhydrous) in a


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molar ratio of 2:1 to prepare a 0.5 M solution. The solution was annealed for 30 min at 60 °C and then spin-coated onto ITO-coated glass substrates (Thin Film Devices) at 5500 rpm for 35 s in a nitrogen-filled glovebox. No anti-solvent was necessary to produce smooth films. The films were then annealed for 20 min at 60 °C. Electronic, optical and structural characterizations of BA2PbI4 (n=1) films, prepared under identical conditions, were presented in a previous publication.22

These polycrystalline films

exhibited grains sizes of about 500 nm. Films of the 3D perovskite CH3NH3PbBr3 were prepared on ITO-coated glass substrates (Thin Film Devices). The precursor solution was prepared by dissolving methylammonium bromide (MABr) (Greatcell Solar) in N,N- dimethylformamide (DMF, Sigma-Aldrich, 99.8% anhydrous) and adding lead (II)-bromide (PbBr2) in a molar ratio of 1.02:1. The solution was stirred at 60 °C for about 15 min and deposited via spincoating in a nitrogen-filled glovebox for 35 s at 5500 rpm. After about 6 seconds, toluene was added as an antisolvent. Finally, the films were annealed for 12 min at 100 °C in the same glovebox. The average grain size for these films was about 100 nm.


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The CPD measurements were performed using a standard vibrating (~220 Hz) Kelvin probe (Au tip, 0.5 mm diameter) in UHV with a base pressure of 10-9 Torr. The resolution of KP is around 10 meV. All samples were transferred without ambient exposure from a nitrogen-filled glovebox into UHV and initially kept in the dark for several hours to minimize the impact of light, in particular the formation of Pb0-related surface states.10 The WF of each sample was then measured multiple times to ensure that the film had reached equilibrium in vacuum and in the dark. SPV was evaluated using high-power deep red, green and blue LEDs (Thorlabs: M660L4, M530L3 and M470L3). For both materials, the entire 1 cm x 1 cm surface of the films was irradiated in the vacuum system through a Kodial glass window, which is transparent in the visible and UV region down to 300 nm (transmission ~ 90%). With a distance of approximately 10 cm between the LEDs and the samples, the power densities at the sample surface were estimated to be 3.36 mW/cm2, 1.60 mW/cm2, and 3.58 mW/cm2 for the 660 nm, 530 nm, and 470 nm LEDs. Samples were exposed to each wavelength for less than 30 s to track changes in the CPD and minimize degradation of the perovskite layer. The

WF of the gold probe (WFTip) was calibrated promptly after each measurement using a


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vacuum-peeled highly ordered pyrolytic graphite sample (HOPG, Mosaic spread value of 0.8 ± 0.2°, NanoAndMore), which provided a consistent WF of 4.70 ± 0.05 eV in vacuum, as verified via ultraviolet photoemission spectroscopy (UPS). The calibration steps are expressed through the equations below: (1) determination of the initial WFTip by measuring the contact potential difference between the HOPG surface and the tip (CPDHOPG-Tip); (2) measurement of CPDPerovskite-Tip; (3) second measurement of

CPDHOPG-Tip to determine whether the tip has been impacted by the perovskite; (4) determination of WFPerovskite from equation (III); (5) repeat of steps (1) - (3) under different illumination conditions for SPV measurements.

𝐶𝑃𝐷HOPG ― Tip = 𝑊𝐹HOPG ― 𝑊𝐹Tip (I) 𝐶𝑃𝐷Perovskite ― Tip = 𝑊𝐹Perovskite ― 𝑊𝐹Tip (II) 𝑊𝐹Perovskite = 𝐶𝑃𝐷Perovskite ― Tip ― 𝐶𝑃𝐷HOPG ― Tip + 𝐶𝑃𝐷HOPG,

calibrated_with_UPS (III)

ACKNOWLEDGMENTS


26


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This work was supported in part by a grant from the US-Israel Binational Science Foundation (Grant #2014357) and from the ONR Young Investigator Program (award #N00014-17-1-2005). X-ray photoelectron spectroscopy (XPS) and Ambient Kelvin probe measurements on Au films exposed to iodine and bromine vapors

Supplementary Information Available

X-ray photoelectron spectroscopy (XPS) and Ambient Kelvin probe measurements on Au films exposed to iodine and bromine vapors

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Work Function (a.u.)

Vacuum Kelvin probe measurement


Illumination

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Complexities of Contact Potential Difference Measurements on Metal

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