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Linking Chemistry at the TiO/CHNHPbI Interface to Current-Voltage Hysteresis Ross A. Kerner, and Barry P. Rand J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 05 May 2017 Downloaded from http://pubs.acs.org on May 5, 2017

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The Journal of Physical Chemistry Letters

Linking Chemistry at the TiO2/CH3NH3PbI3 Interface to Current-Voltage Hysteresis Ross A. Kernera and Barry P. Rand*a,b a

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

b

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

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

ACS Paragon Plus Environment

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ABSTRACT:

We demonstrate that reversible chemical reactions occur at TiO2/gas and CH3NH3PbI3/gas interfaces on a time scale of seconds to minutes. The chemisorption strongly affects their electronic properties, mainly acting to deplete TiO2 of free electrons and passivate surface traps on the perovskite. Although the chemistry is not directly probed, we infer that reversible chemistry occurs at the solid-state TiO2/CH3NH3PbI3 interface. Equilibrium or steady-state concentrations established for adsorbed species associated with each material would be voltage and illumination dependent due to free or photocarriers being a main reactant. Interfacial chemistry provides an additional physical mechanism to explain the origins of normal and anomalous hysteretic current-voltage characteristics of perovskite devices. Furthermore, chemical reactions help to understand why measured perovskite ion transport properties and the nature of hysteresis are highly dependent on interfaces.

TOC GRAPHICS

Reversible Surface Chemistry eTiO2

e⇋ I2 ⇋

CH3NH3PbI3 Oxygen vacancy, VO Iodine vacancy, VI Chemisorbed Iodine Free electrons

Solid-State Interfacial Chemistry


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Metal halide perovskite materials of the chemical formula ABX3, where A is often a small organic cation, such as methylammonium, and X is a halide anion, possess highly desirable optical and electrical properties.1 Therefore, they have found many applications as the active layer in optoelectronic devices such as photovoltaics, photodetectors, light emitting diodes (LEDs), and lasers.1-5 The most commonly studied metal halide perovskite, methylammonium lead triiodide (MAPbI3), has reached a photovoltaic power conversion efficiency (PCE) exceeding 18%, and, in LEDs, an external quantum efficiency of approximately 10%.2,4 However, many critical issues remain unresolved, including reproducible and large area compatible processing routes, long term operational and shelf life device stability, and a firm understanding and elimination of hysteretic current-voltage behavior. Hysteresis in current-voltage (I-V) measurements of solar cells made from lead halide perovskite absorbers considerably complicates measurement interpretation and efficiency characterization, yet is poorly understood. It is becoming increasingly clear that there are both capacitive and non-capacitive contributions to hysteresis.6 It has been attributed to ferroelectricity, ion motion, and deep electronic traps at interfaces, although ferroelectricity has been ruled out.7 Most researchers have been converging on stoichiometric polarization as the cause of hysteresis.8-11 Stoichiometric polarization is defined as ion accumulation and the resulting space charge created at the respective carrier selective electrodes, leaving the overall stoichiometry and defect density of the material unchanged. Low ion mobilities presumably impede ions from redistributing on the time scale of an I-V sweep. That leads to hysteretic measurements and PCE values differing by as much as 50% for opposite sweep polarities.10,11 In general, discussions focus on the effects of mobile ions on device hysteresis and models qualitatively fit the experimental current-voltage measurements of perovskite solar cells.8,9


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Recent studies point out that room temperature hysteresis is usually much more dramatic for solar cells that employ an n-type metal oxide as an electron extracting electrode compared to devices that employ organic collection layers.11 Explanations that point to interface effects have been offered since: 1) even devices that show no hysteresis at room temperature become hysteretic at low temperature and 2) ion diffusion coefficients and ion migration activation energies vary widely for different deposition processes and electrode types (organic vs inorganic materials).11 In fact, hysteresis has also been attributed to oxygen migration within the metal oxide transport layer without invoking ions in the perovskite.12,13 These assertions are in line with the generally accepted idea that hysteresis is linked to ion motion. However, although surface interactions between TiO2 and perovskites have been alluded to, a direct chemical reaction between TiO2 and MAPbI3 has not yet been identified.6,14 It was recently demonstrated that lead iodide (PbI2), MAPbI3, and CsPbBr3 are strong oxidizing agents owing to the Pb2+ cation.15 When in contact with a metal, M, with a lower reduction potential than Pb0, electrons are transferred from the neutral metal M to form metallic Pb0 and oxidized Mn+, where n is typically the most common oxidation state of M. The organic cation, which is the most unique species in the highest performing perovskite materials, played little to no role in the chemistry as illustrated by identical reaction characteristics observed in the entirely inorganic CsPbBr3. Thus, reduction-oxidation (redox) chemistry between the perovskite and reactive metal contacts ultimately dominated the catastrophic device degradation. The redox properties of metal oxides are well established. Their redox chemistry is exploited to store energy, such as in lead acid batteries. In contrast to complete chemical conversion, on a microscopic scale incomplete reactions proceeding on the parts per millions or thousands scale can affect electronic properties, such as charge carrier concentration, by orders of magnitude.


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This aspect is exploited in the use of metal oxides for gas sensors. One would therefore expect that, due to the redox activity of both lead halide perovskites and metal oxides, these two material classes interact electrochemically. Indeed, there are reports of chemical reactions of highly reactive metal oxides with perovskites that lead to rapid degradation.16,17 Here, we expand upon the redox properties of lead halide perovskites by studying the chemical interactions between MAPbI3 and TiO2, one of the most common electron transport layers employed in perovskite solar cells. Typically, in contact with n-type oxides, perovskites are measured to be slightly n-type.18 We show that MAPbI3 and TiO2 interact chemically via a Faradaic reaction in which both charges and ions are heterogeneously exchanged across the interface. This implies that, in addition to electronic charge transfer due to work function differences, TiO2 can reduce MAPbI3, making it n-type, by an electrochemical mechanism. Although both result in n-type MAPbI3, an electrochemical, Faradaic mechanism has critical implications for device performance beyond that of modifying electrostatic double layers. Most importantly, an electrochemical mechanism allows for pseudo or hybrid capacitance, which can store up to two orders of magnitude larger charge per unit area than an electrostatic double layer capacitor.19 Such large capacitances, in combination with even moderate resistances, can easily lead to time constants larger than I-V sweep time scales, and be a significant contributor to hysteresis in perovskite devices.20


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Current (µA)

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N2 flow 0.5

O2 flow

illumination on

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Time (s) Figure 1. Lateral current versus time measured across thin films of TiO2 in N2 and O2 atmospheres (a) showing the effect of illumination (approximately 4 mW/cm2 at 405 nm) and dark recovery transients and (b) a zoom in of the first current transient in (a).

In order to characterize the nature of the surface chemistry, gas sensitive devices were fabricated and tested following the operating principles of Xie et al.21 The effect of illumination (approximately 4 mW/cm2 at a wavelength of 405 nm) on the lateral current measured across a TiO2 thin film device is shown in Fig. 1a. The films are illuminated for 1 min in N2 or O2 atmospheres followed by 9 min of recovery in the dark. The current response is very reversible. Figure 1b shows that the increase in current once the illumination is turned on takes several minutes.


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2.0

Current (µA)

a)

TiO2: O2 exposure O2

N2

O2

N2

O2

N2

1.5 1.0 0.5 0.0

b) Current (µA)

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0 1000 TiO2: I2 exposure

2000

3000

4000

6000

1 0.1 I2 off

0.01 0

I2 on 2000

Time (s)

Figure 2. TiO2 current response to (a) high and low O2 partial pressures, or (b) low and high I2 pressures. High O2 or I2 partial pressure gas was flowed during the shaded sections and N2 otherwise. The samples were illuminated at approximately 4 mW/cm2 intensity at 405 nm. The TiO2 films displayed excellent gas sensing behavior which improved following a 6-10 hour stabilization period under illumination. Figure 2a shows the current response of the TiO2 film under approximately 4 mW/cm2 illumination versus time while the O2 partial pressure was cycled from high (0.2 atm) to low (99.998% N2). The conductivity of TiO2 decreased when the O2 partial pressure was increased. Changes in current were highly reversible and reproducible, approaching the same saturation at steady state for each cycle. Figure 2b shows the current response on a semi-logarithmic scale, of the TiO2 film to pulses of I2 gas. The steady state photoconductivity of TiO2 under I2 flow (≈8 nA) was approximately 2 orders of magnitude lower than when exposed to O2 (≈500 nA). In contrast to O2, the kinetics of the TiO2 current response


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to I2 was extremely rapid and the recovery transients were very slow. Thus, for cycling response curves, the I2 was only pulsed for 5-10 s except the long pulse near 7000 s that shows the current dipping to approximately 10 nA. The variation in current response for the cycles stems from inconsistencies in the I2 dose which were controlled by hand with a multi-way valve. We also acknowledge that recovery could be limited by the experimental difficulty to rapidly purge I2 with N2.

20 Current (nA)

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10

initial current saturated current

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1.7 mW/cm 2 3.4 mW/cm 2 5.0 mW/cm

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Time (s) Figure 3. Lateral current versus time measured for a thin film of MAPbI3 in N2 atmosphere showing the effect of 405 nm illumination. The inset shows a square root dependence on illumination intensity of both initial and saturated current. The effect of illumination on the lateral current measured across a MAPbI3 thin film device in a N2 atmosphere is shown in Fig. 3. In contrast to TiO2, the current response to illumination was


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instantaneous. The current decreased with time under illumination. Both the instantaneous and saturated currents were proportional to the square of the illumination intensity as shown in the inset.

Current (nA)

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MAPbI3: I2 exposure

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3000 I2 on

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Figure 4. (a) MAPbI3 and (b) PbI2 gas sensor photocurrent response to low (pure N2) and high I2 partial pressures at 1 V and 5 V biases, respectively. High I2 partial pressure gas was flowed during the shaded sections and N2 otherwise. The sensors were illuminated at approximately 4 mW/cm2 intensity at 405 nm. Figure 4a shows the current response versus time for an MAPbI3 gas sensor periodically exposed to doses of I2 gas. The current response of MAPbI3 is observed to increase for exposure to I2. Like the TiO2, MAPbI3 responds quickly and reversibly to I2 exposure with a 5-fold increase in current (10 to 50 nA). Again, the long recovery time includes the time needed to purge I2 down to ppm levels, and may not be intrinsic. Similar response is observed for a PbI2


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film (Fig. 4b), indicating that the organic cation does not play a significant role. The rise and decay of the PbI2 conductivity was much slower than the MAPbI3. The TiO2 films displayed a slow, initial response to illumination saturating after many hours that was irreversible. Following the stabilization, the films were stored in the dark and O2 for several hours, illuminated again, and the maximum stabilized current returned in less than one hour. We interpret the irreversible changes as reduction of the bulk, which has been attributed to a chemistry or diffusion limited increase in the number of oxygen vacancies which act as donors.22 The faster, reversible transients are due to gas molecule chemisorption which depletes the film of free electrons by trapping them at the surface.22 Even the fast transients were much slower than electronic processes (including carrier trapping/detrapping) and the measured current cannot be assigned to photoconductivity. In the case of TiO2, the interaction with O2 is a surface reaction. As an example, the simplified reaction between TiO2 and O2 at the surface forming a superoxide as the product can be expressed as: e + O () ⇋ O

(1)

where the product is a surface chemisorbed superoxide ion. The thin film is easily depleted of electrons and the conductivity is reduced. Illumination creates a steady state population of electronic carriers. As discussed, the photocarrier contribution to conductivity for the TiO2 was negligible. Instead, ultraviolet light significantly increases the film’s conductivity by driving the reaction of the chemisorbed ions with the photogenerated holes. hν ⇀ e + h

(2)

h + O ⇋ O ()

(3)


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Note that the e- in equation 1 and h+ in equation 3 both increase the same amount under illumination, inversely affecting the concentration of adsorbed species via Le Chatelier’s principle. However, while the electron concentration in a natively n-type oxide will only increase by a fraction, the hole concentration increases by much more, possibly multiple orders of magnitude. Thus, when the sample is initially illuminated, reaction 3 will dominate until the new steady state concentrations are reached. Finally, the reaction of TiO2 with I2 can be written as:

e + I () ⇋ I

(4)

On the other hand, the current through the MAPbI3 film increased instantaneously when illuminated (Fig. 3), allowing us to conclude that the photoconductivity contribution dominated. In addition, both the instantaneous and saturated currents are proportional to the square root of the illumination intensity (Fig. 3 inset), expected for steady state carrier concentrations in the high injection regime where the recombination rate is proportional to the square of the excess carriers. We can estimate the steady state carrier concentrations at 1.7 mW/cm2 to be of the order 1013 cm-3 assuming an equal electron and hole mobility of 10 cm2/Vs, placing an upper bound on the native carrier concentration. The slow transients in the photoconductivity and the dark current recovery suggest that some chemistry affecting photoconductivity was also occurring. It is known that the surfaces of perovskites contain a much higher density of deep traps than in the bulk (possibly undercoordinated or metallic Pb due to abundant halide vacancies) and are a main contributor to non-radiative recombination.23 Note that undercoordinated Pb at surfaces has been shown to cause trap mediated recombination in PbS.24 The slow transient for the MAPbI3 photoconductivity is ascribed to surface chemistry as volatile species are lost. The defects created at the surface decrease the steady state concentrations of photocarriers in the film.


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From the MAPbI3 photoconductivity data, we conclude that I2 passivates traps at the perovskite surface: e + trap ⇋ trap

trap + I ⇋ I

(5) (6)

By occupying and passivating the recombinative surface traps, the steady state carrier concentrations under illumination increase upon exposure to I2 as seen in Fig 4a.23 These reactions reveal that both TiO2 and MAPbI3 surfaces react strongly and reversibly with gases such as O2 and I2. Thus, at an interface between the two materials, there would be strong competition for specifically adsorbed ions at surface sites. Assuming that the TiO2 is at least moderately n-type (and, thus, O2 thermodynamic activity is very low) we can gain insight into the nature of the interfacial chemistry by combining reactions 4, 5, and 6 to eliminate the I2 (the I2 partial pressure/thermodynamic activity within the solids is still a meaningful quantity) and use subscripts to indicate on which material the species reside: e( ) + I (!"#$ ) ⇋ I ( ) + e(!"#$% ) + trap(!"#$% ) %

(7)

In reaction 7, an iodine in the perovskite moves across the interface to adsorb onto the TiO2, depleting the TiO2 of electrons.18 A free electron and a surface trap are left on the MAPbI3. The energy level of the defect would dictate whether the electrons remain trapped or are free to conduct. Hence, the MAPbI3 can be chemically reduced by the TiO2 in addition to charge transfer due to work function differences. This is a Faradaic reaction, defined as heterogeneous charge transfer at an interface, since mass is transferred and chemical species are created. We recognize that creation or depletion of species at the interface would likely be followed by their diffusion to/from the bulk and the rate would be mass transport limited. However, we note that


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the time scales for diffusion at room temperature may be too long to be observed in our measurements. The chemical species in reaction 7 are similar in oxidation state before and after reacting, and they are not in very large amounts (we estimate 10 to 100 ppm surface density from Fig. 1) making them difficult to distinguish by methods such as X-ray photoelectron spectroscopy. The given equations are not exhaustive. Electrochemistry of the oxygen, metal ions, impurities, or transport layer dopants (such as dopants in hole transport polymers) may also contribute or even dominate depending on the original stoichiometries and defect populations but are beyond the scope of this study.25 Effect on Capacitive I-V Hysteresis. Faradaic reactions that occur throughout the bulk will continue until the reactants are consumed, storing charge like a battery. If the reaction is limited to the surface, such as in the RuO2/H2SO4 system, pseudocapacitance will prevail.26 Such chemical or pseudocapacitances, Cchem, can be one to two orders of magnitude larger than capacitances from electrostatics alone.18 A giant dielectric effect measured by impedance spectroscopy at low frequencies has been reported for MAPbI3 in contact with TiO2 as well as other contacts.27 This is strongly suggestive evidence that a chemical pseudocapacitance (or possibly a hybrid capacitance) is present at interfaces between MAPbI3 and other materials.26 Combining these capacitances with an ionic resistance, Rion (either ion drift or diffusion resistance) would result in very large RionCchem time constants. Furthermore, the transients may instead be limited by the adsorption/desorption kinetics which we can assume to be on the order of many 10s of seconds from Figures 1 and 3. Measurement hysteresis has also been observed in conventional dye sensitized solar cells (DSSCs) employing

TiO2.20 Computational methods

using experimentally measured


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capacitances have been successful at very accurately modeling DSSC hysteresis. These models do not invoke ion motion in the TiO2 but use measured values of the chemical capacitance at the TiO2/dye/electrolyte interface and the diffusional resistance of the liquid electrolyte.20 Thus, in addition to the fact that many high performing DSSCs do not display hysteresis, mobile oxygen vacancies in the TiO2 as the sole source of I-V hysteresis is highly unlikely.12,20 However, reaction 7 clearly illustrates how changing the oxygen vacancy concentration in the TiO2 layer will affect the chemistry, capacitance, and the differences in hysteresis characteristics observed by Zhang et al.13 We can now offer an a more detailed explanation for capacitive hysteresis in perovskite solar cells sourcing from Faradaic reactions that enable large chemical capacitances. Large capacitances lead to large RC time constants when combined with even moderate resistances. If the electrochemistry is extensive, then the pseudocapacitances can be large enough that they approach or exceed the time scale of an I-V measurement (e.g. typical measurement delay times of 0.01 to 0.1 s, or voltage step size/sweep rate) and hysteresis will be observed. For example, the chemical capacitance measured in DSSCs (note that DSSCs employ high surface area, mesoporous TiO2) of 100 mF/cm2 would only need an ionic resistance of 10 Ω to have an RionCchem = 1 s.20 These capacitances are similar in magnitude to the low frequency capacitances measured in both planar and mesoporous TiO2 perovskite solar cells (10 mF/cm2 requiring only Rion = 100 Ω for τ = 1 s).27 It is also important to note that the maximum theoretical capacitance without a chemical reaction at a planar interface is limited to around 0.05 mF/cm2.19 Finally, we cannot exclude the large capacitance could source from the TiO2 interface appearing as a Schottky diode for which the capacitance diverges when the depletion width approaches zero. Such a mechanism is still enabled by chemistry.


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Influence on Ionic Transport Measurements. As pointed out by Levine et al., there are wide variations in the ionic diffusion coefficients (10-8 to 10-12 cm2/s) and activation energies (0.1 to 0.6 eV) measured for MAPbI3 for different film processing conditions and in contact with various electrode materials.11 If stoichiometric polarization were responsible for hysteresis, then hysteretic characteristics should be independent of the contacts. Instead, a hypothesis was offered suggesting that the different time constants for MAPbI3/oxide versus MAPbI3/organic interfaces was due to ions leaching and diffusing through the oxide or organic layers in contact with the perovskite.11 Very different diffusivities and activation energies would then be expected. Disregarding possible differences in film morphology, the higher activation energies measured on perovskite devices with oxide (0.5-0.6 eV) versus purely organic contacts (0.1-0.3 eV) could be due to the activation energy for the proposed interfacial chemistry at the TiO2/MAPbI3 interface.11 Effect on Anomalous I-V Hysteresis. In contrast to TiO2 scaffold/organic dye DSSCs, attempts to model perovskite hysteresis involving only mobile ion accumulation effects contributions do not precisely fit the data for perovskite solar cells.8,28-30 The models show that built in fields and extraction barriers are modified by the accumulated ions, but extraneous effects such as large non-radiative recombination rates must be invoked to better fit the experimental data.28-30 The models also fall short at properly explaining the Voc or Jsc transient dynamics when the illumination is turned on.30 Although these voltage bias and illumination history dependent trap densities have been used in models, their origin has been unclear. Electrochemistry at interfaces provides a mechanism of the voltage dependent electronic trap density invoked to explain these observations.28 The traps that form affect the carrier recombination rate and, thus, significantly influence device performance. Additionally, a recent study suggested that interfacial defects on


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the perovskite act as recombinative trap centers and that they can be reduced by altering the TiO2 chemistry.30 Since the electronic carrier concentrations are involved in the reversible chemical reactions, the concentration of the reactants and products will be affected by applied voltages and illumination. Either of these stimuli will shift the steady state, local electrochemical potentials. Even more non-capacitive, hysteretic contributions can be expected given that the surface doping state of the TiO2 and MAPbI3 are directly affected by the extent of reaction 7 (as well as competing reactions like reactions 1 and 3), possibly modifying the built-in field or carrier extraction barriers. Thus, the steady state electronic carrier, trap concentrations, and extraction barriers will be a function of voltage and their formation will be rate-limited by chemical kinetics or mass transport across the interface. In conclusion, we have studied and revealed electrochemical processes occurring at TiO2/air, MAPbI3/air and TiO2/MAPbI3 interfaces. Such Faradaic processes can give rise to chemical capacitances that can be orders of magnitude larger than the estimated capacitances originating from stoichiometric polarization. When combined with ionic resistances, these chemical capacitances can result in extremely large time constants on the order of seconds and produce capacitive hysteretic effects in device I-V measurements. Carrier densities, trap densities, and recombination rates are influenced by the formation of new species and/or defects at this interface. The equilibrium concentrations of these species are dependent on the illumination and voltage bias conditions. Coupled with the long transient associated with the chemistry, these effects further contribute to hysteresis in an anomalous, non-capacitive fashion. Our results strongly indicate that hysteresis is not solely due to ion motion and stoichiometric polarization. A better understanding of the chemical nature of the interfaces of perovskites with


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oxides, as well as other organic and inorganic materials, should lead to a concerted effort to elucidate key mechanisms associated with them and to their subsequent mitigation or exploitation. EXPERIMENTAL METHODS Patterned indium tin oxide (ITO) electrodes on Eagle XG glass substrates were sonicated consecutively in detergent, deionized water, acetone, and isopropanol, followed by 5 min exposure to O2 plasma. Thin films (5-15 nm) of TiO2 were deposited onto the patterned ITO substrates with an Arradiance atomic layer deposition (ALD) instrument using titanium isopropoxide (held at 70 oC) and water as precursors with a 200 oC chamber/substrate temperature. The TiO2 samples were annealed at 350 oC for 10 min in air to increase crystallinity and conductivity. Approximately 160 nm thick films of MAPbI3 were spin coated at 5200 RPM onto the ITO substrates from 0.75 M MAI:PbI2 1:1 dissolved in dimethylformamide (DMF). A solvent exchange with toluene was performed after 4 s of rotation. The MAPbI3 films were not annealed and left in a solvent free glovebox for at least a week before characterization. The substrates were diced into individual devices. The channel width and length were 1 mm and 1 cm, respectively. Lateral, in-plane current of the films between the ITO electrodes was measured under illumination with two 405-nm LEDs (~20 mW each with 15o viewing angle). The TiO2 films were stabilized under illumination for several hours before taking measurements. The measurements were made under flowing gas, either N2, or compressed dry air (O2), or N2:I2 generated by directing N2 through a flask containing I2 crystals. The I2 flask was kept at 0 oC to minimize the I2 partial pressure. The upper limit of the high I2 partial pressure was 10-4 atm. A


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Keithley Instruments Model 2602B was used to measure current versus time at a constant voltage bias.

AUTHOR INFORMATION The authors declare no competing financial interests. ACKNOWLEDGMENT We would like to thank Jake T. Herb and Professor Sigurd Wagner for many helpful discussions. We acknowledge the ONR Young Investigator Program (Award #N00014-17-12005) and the U.S.-Israel Binational Science Foundation (Award #2014199) for research funding. REFERENCES (1) Sutherland, B. R.; Sargent, E. H. Perovskite Photonic Sources. Nat. Photon. 2016, 10, 295-302 (2) Heo, J. H.; Han, H. J.; Kim, D.; Ahn, T. K.; Im, S. H. Hysteresis-Less Inverted CH3NH3PbI3 Planar Perovskite Hybrid Solar Cells with 18.1% Power Conversion Efficiency. Energy Environ. Sci. 2015, 8, 1602-1608 (3) Dou, L.; Yang, Y. (M.); You, J.; Hong, Z.; Chang, W.-H.; Li, G.; Yang, Y. SolutionProcessed Hybrid Perovskite Photodetectors with High Detectivity. Nat. Comm. 2014, 5, 5404 (4) Xiao, Z.; Kerner, R. A.; Zhao, L.; Tran, N. L.; Lee, K. M.; Koh, T.-W.; Scholes, G. D.; Rand, B. P. Efficient Perovskite Light-Emitting Diodes Featuring Nanometre-Sized Crystallites. Nat. Photon. 2017, 11, 108–115


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Linking Chemistry at the TiO - American Chemical Society

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