Article Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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Influence of a Hole-Transport Layer on Light-Induced Degradation of Mixed Organic−Inorganic Halide Perovskite Solar Cells Takeyuki Sekimoto,* Taisuke Matsui, Takashi Nishihara, Ryusuke Uchida, Takashi Sekiguchi, and Takayuki Negami Institute for Energy and Material/Food Resources, Technology Innovation Division, Panasonic Corporation, Osaka 570-8501, Japan
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ABSTRACT: Organic−inorganic halide perovskites (OIHPs) have been intensively studied in recent years for use in solar cells. Many studies have reported the light-induced phenomena of OIHP materials and their solar cells. In this study, we investigated the influence of a hole-transport layer (HTL) on light-induced degradation (LID) of mixed OIHP solar cells, especially at the OIHP/HTL interface, by hard X-ray photoelectron spectroscopy (HAXPES) and impedance spectroscopy. HAXPES shows accumulation of iodine and metallic lead in the vicinity of the OIHP/HTL and electron-transport layer/OIHP interfaces, respectively, after LID. Under light illumination and in the dark after LID, characteristic impedance responses are observed for a sample with a highly damaged OIHP/HTL interface as an intermediate frequency arc and negative capacitance, respectively, because of the electrochemical reaction in the vicinity of the OIHP/HTL interface. Overall, the results indicate that light-induced iodide diffusion to the OIHP/HTL interface and the electrochemical reaction to form iodine molecules are important factors for LID of OIHP solar cells. KEYWORDS: perovskite solar cell, hole-transport layer, light-induced degradation, electrochemical reaction, hard X-ray photoelectron spectroscopy
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factors have been reported,14−22 and PCEs of more than 20% have now been achieved for MAPbI3- and FAPbI3-based OIHP solar cells.21,23−37 In the large volume of research on OIHP solar cells, in addition to the cell performance, the light and thermal stabilities have been improved by A+ and/or X− substitution, including Cs24,38 and Br substitution,39,40 use of MA/FA mixtures,41−43 and addition of Rb.25,27,30 Long-term light and thermal stabilities are essential for outdoor applications of OIHP solar cells. These stabilities of OIHP materials and cells have recently been reviewed in detail,44,45 and various causes of light-induced degradation (LID) have been identified, including light-induced phase separation,46−48 defect formation,49−51 dissociation of MA+,50,51 HTL/gold (Au) interfaces,52 the electron-transport layer (ETL)/OIHP interface,52 4-tert-butylpyridine (tBP) in 2,2′,7,7′-tetrakis(N,N-
INTRODUCTION Conventional three-dimensional perovskites are represented by the general formula ABX3, where the A site contains organic or inorganic cations such as cesium (Cs+), methylammonium (MA+, CH3NH3+), and formamidinium (FA+, HC(NH2)2+), the B site contains metals such as lead (Pb2+) and tin (Sn2+), and the X site contains halides such as chloride (Cl−), bromide (Br−), and iodide (I−). Organic−inorganic halide perovskites (OIHPs) have received much attention because of their interesting electronic and ionic properties1−5 and their promise in solar cells and other applications owing to their large absorption coefficients,6,7 high carrier mobilities,8−10 large polaron formation,11 and direct and slightly indirect bandgaps originating from the Rashba effect.12 Regarding the use of OIHPs in solar cells, Miyasaka’s group first reported that MAPbI3 acts as a photosensitizer for titanium dioxide (TiO2) in a dye-sensitized solar cell with a power conversion efficiency (PCE) of 3.8%.13 Various improvements of the hole-transport layer (HTL) and other © XXXX American Chemical Society
Received: April 7, 2019 Accepted: June 12, 2019
A
DOI: 10.1021/acsaem.9b00709 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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ACS Applied Energy Materials
Figure 1. Current density−voltage curves before and after light illumination for 185 min for OIHP solar cells with (a) PTAA and (b) spiro HTLs.
di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro),53 light-induced oxidation of spiro,54 and the operating conditions.55,56 Moreover, the light and thermal stabilities of cells are strongly affected by the experimental environment, such as the moisture content and presence of gases (air, nitrogen, and oxygen), but they can be improved by encapsulation.44 Although the light stability of cells has been extensively investigated, the influence of the OIHP/HTL structure on the light stability is still unclear, and the effect of coating a HTL on an OIHP layer has not been considered. Here, we investigated the influence of a HTL on the LID mechanism of OIHP solar cells using the conventional HTL materials poly[bis(4phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) and spiro. We use a combination of hard X-ray photoemission spectroscopy (HAXPES) and impedance spectroscopy to investigate the OIHP/HTL structures with different HTLs. We found that light-induced I− diffusion to the OIHP/HTL interface and the electrochemical reactions, most likely corrosion reactions, to form I2 in the vicinity of this interface are crucial processes in operating OIHP solar cells.
device-grade quality and thus suitable to investigate the effects of the HTL on the OIHP layer. Different effects of PTAA and spiro HTLs on the OIHP behavior are clearly observed after the LID tests. For the cell with spiro, LID of the cell parameters is small compared with the case of the cell using PTAA. Although some studies have reported low LID in spite of using a PTAA HTL,25,31 it is difficult to compare our data with the literature data because of the different cell conditions, such as holding temperature (room temperature or 85 °C), applied voltage (open circuit or maximum power point tracking), and other factors. Chemical Analysis near the OIHP/HTL Interface. To investigate the change in the chemical bonding states caused by the HTLs before and after LID, HAXPES measurements at the surface of the OIHP layer and OIHP/HTL interface were performed for sample A with the glass/ITO/ATO/c-TiO2/ mp-TiO2/OIHP structure and samples B and C with the glass/ ITO/ATO/c-TiO2/mp-TiO2/OIHP/HTL (50 nm) structure. The thickness of the OIHP layer and type of HTL of the HAXPES samples are summarized in Table 2. Note that the
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Table 2. Thickness of the OIHP Layer and Type of HTL in Samples A−D Used for the HAXPES Measurements
RESULTS AND DISCUSSION Light-Induced Degradation of Solar Cells. We fabricated encapsulated solar cells with glass/indium tin oxide (ITO)/antimony-doped tin oxide (ATO)/compact TiO2 (c-TiO2)/mesoporous TiO2 (mp-TiO2)/OIHP/HTL (PTAA or spiro)/Au structures. The cells were encapsulated in glass to prevent the environmental conditions, such as moisture and oxygen, from affecting LID. The current density−voltage (J−V) curves and cell parameters [opencircuit voltage (VOC), short-circuit current density (JSC), fill factor (FF), and PCE] of the cells before and after 1 sun illumination for 185 min are shown in Figure 1 and Table 1, respectively. The LID tests of the solar cells were performed under open-circuit conditions to accelerate LID.54 The cells were exposed to incident light from the glass side. The initial PCE values are ∼20%, meaning that the OIHP layers are of
PTAA spiro
light illumination
VOC (V)
JSC (mA/cm2)
FF
efficiency (%)
before after before after
1.046 0.937 1.074 1.033
24.48 7.84 23.76 23.16
0.804 0.397 0.812 0.737
20.58 2.92 20.72 17.63
OIHP thickness (nm)
HTL
A B C D
550 550 550 ∼50
PTAA spiro
HAXPES measurements were started more than 18 h after finishing the LID tests, and thus the irreversible changes of the samples caused by long-term light illumination are mainly detected. The standard deviations in the intensities of the Pb 4f7/2 peaks for sample B with the OIHP/PTAA structure and sample C with the OIHP/spiro structure are 0.2% and 1.3% before and after LID, respectively, and thus the X-ray penetration depth into the OIHP layers is almost the same for these samples. The C 1s X-ray photoelectron spectroscopy (XPS) spectra of the samples are shown in Figure 2a. The C 1s peaks are very similar to those reported in the literature.57,58 The C 1s spectra contain components at ∼286.6 eV originating from C−N bonds3,57 and ∼288.4 eV originating from CO bonds,57 in addition to a signal from C−C bonds at 284.8 eV. The decrease in the fraction of the C−N component in the C 1s spectrum has been suggested to represent the amount of MAI loss originating from decomposition of the OIHP layer.3 Interestingly, before and after LID, there is almost no difference in the C−N fraction of the surface of the OIHP layer in sample A, the OIHP/PTAA
Table 1. Cell Parameters of OIHP Solar Cells with PTAA and Spiro as HTLs before and after Light Illumination for 185 min HTL
sample label
B
DOI: 10.1021/acsaem.9b00709 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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ACS Applied Energy Materials
The Pb 4f7/2 XPS spectra are shown in Figure 2b. For sample A, small signals are detected at ∼136.5 eV, which are assigned to metallic Pb (Pb0).3,28,58,60 Snaith and co-workers61 found that metallic lead is present in OIHP films. Regarding OIHP films exposed to light and oxygen, metallic Pb and PbOx have been detected in degradation experiments at low oxygen levels under 1 sun illumination.62 Pb0 is considered to be a nonradiative trap site63,64 that needs to be prevented in OIHP layers, because it is related to recombination of carriers in OIHP solar cells. The Pb0 peaks are not derived from the damage induced by X-ray irradiation because the peak intensity does not change during repeated measurements at the same position. It has recently been reported that OIHP films partially degraded to form Pb0 during toluene treatment.3 This indicates that the use of toluene as an antisolvent might partially induce decomposition of OIHPs. The Pb0 peak decreases or almost disappears when HTL is present. For sample B, the Pb 4f7/2 peak clearly shifts to lower binding energy after LID. According to the literature, an increase of the Pb−I bond length or incorporation of excess PbI2 shifts the binding energy of the Pb 4f7/2 peak to lower binding energy.58 In this study, the possibility of excess PbI2 is ruled out because there is almost no MAI loss after LID, resulting in lattice expansion of the OIHP layer near the OIHP/PTAA interface after LID. The I 3d5/2 XPS spectra are shown in Figure 2c. Upon coating a HTL on the OIHP layer, I0 (∼620.5 eV)65 and small I5+ (∼623.8 eV)66 components appear. These additional peaks are attributed to the changes in the bonding states near the OIHP/HTL interface. Immediately after the OIHP/HTL junction is made, electron−hole exchange between OIHP and HTL occurs in the vicinity of the interface, resulting in formation of a space charge region. The carrier exchange may lead to the formation of I2 and Pb2+ (2I− → I2 + 2e− and Pb0 → Pb2+ + 2e−) in the OIHP layer near the OIHP/HTL interface; for example, the redox potentials of I2/I− (∼4.98 eV) and PbI2(+2)/Pb0 (∼4.075 eV)67 are shallower than the highest occupied molecular orbital (HOMO) level of the HTL (∼5.2 eV for PTAA and spiro). Interestingly, the initial additional components do not affect the cell parameters, as shown by the initial PCE values exceeding 20% (Table 1), and thus the initial I0 peak can be attributed to neutral iodine atoms in the lattice rather than I2 molecules. For sample A, there is no I0 peak both before and after LID. This indicates that the OIHP surface under illumination does not decompose to PbI2 and then Pb0 + I2 with MAI loss without an HTL. In other words, irreversible formation of I2 does not occur near the OIHP surface for the ETL/OIHP (550 nm) structure without an HTL. We determined the degree of LID using the change in the peak area ratio AI0/(AI−Pb + AI0) before and after LID (δA), where AI0 and AI−Pb are the areas of the I0 and I−Pb (∼619.0 eV)65 components, respectively. For samples B and C, the peak area ratios clearly increase after LID, showing enhanced accumulation of iodine at the OIHP/HTL interface induced by light illumination. For sample B, the δA value of 0.016 is approximately three times larger than that for sample C (0.006). Note that the larger surface recombination velocity (S) with a more defective OIHP/HTL interface (Table S1), the larger the amount of iodine accumulation near the interface after LID (Figure 2c). We also measured the changes in the chemical bonding states in the vicinity of the ETL/OIHP interface before and after LID using samples A and D with different OIHP layer
Figure 2. XPS spectra of samples A−D before and after LID: (a) C 1s, (b) Pb 4f7/2, and (c) I 3d5/2.
interface in sample B, and the OIHP/spiro interface in sample C. Therefore, irreversible decomposition of the OIHP layer related to MAI loss is not a type of LID in this study. To confirm if decomposition of the OIHP layer occurred, we performed X-ray diffraction (XRD) measurements. The XRD patterns of sample A before and after LID are shown in Figure 3. There are no differences in the XRD patterns before and
Figure 3. XRD patterns of sample A before and after LID.
after LID. Considering the results of HAXPES, it is concluded that remarkable decomposition reactions with the increase in PbI2 did not occur in the OIHP layer under light illumination in this study. Wang and co-workers proposed a degradation mechanism related to I2, in which, under illumination, I2 breaks into two atomic I (I•), a mobile I− ion reacts with I• to form I2•−, and then I2•− reacts with MA+ to form CH3NH2, I2, and H2 (degradation mechanism involving chain reactions of I2).59 However, almost no MAI loss is observed after LID, so this chain reaction does not occur near the OIHP/HTL interface. C
DOI: 10.1021/acsaem.9b00709 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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Figure 4. Apparent capacitance (Im(Z−1)ω−1) against frequency for solar cells containing a PTAA HTL under dark conditions: (a) initial state (0 min) and (b) light-induced degraded state after light illumination for 185 min, and under light illumination for (c) 5 and (d) 185 min. The dashed lines in panel b represent the absolute values of the negative capacitance in the region −Z″ < 0. Apparent capacitance (Im(Z−1)ω−1) against frequency for solar cells containing a spiro HTL under dark conditions: (e) initial state (0 min) and (f) light-induced degraded state after light illumination for 185 min, and under light illumination for (g) 5 and (h) 185 min. The applied bias voltages (Vdc) were 0.4, 0.6, 0.8, and 1.0 V.
Figure 5. Nyquist plots of the solar cells in the dark before and after LID caused by light illumination for 185 min. The solar cells contained PTAA and spiro HTLs. The applied bias voltages (Vdc) are (a) 0.4, (b) 0.6, (c) 0.8, and (d) 1.0 V. The impedance signals of the PTAA-based devices after LID in panels c and d were divided by 100. The right-angle triangles represent the guides of 45°.
thicknesses of 550 and ∼50 nm, respectively (the sample structure was glass/ITO/ATO/c-TiO2/mp-TiO2/OIHP in both cases). For sample D, the intensity of the Pb0 peak clearly increases after LID (Figure 2b). We confirmed that the Pb0 peak is not caused by X-ray damage; that is, the peak intensity does not change during repeated measurements at the same position. Figure 2a suggests that there is almost no MAI loss because there is almost no change in the fraction of the C−N component after LID, indicating that the Pb0 peak is not derived from decomposition of the OIHP layer. Ionic Diffusion and Interfacial Electrochemical Reactions. Impedance spectroscopy was performed to understand the difference in LID for samples with PTAA and spiro HTLs, in addition to the changes in the chemical bonding states. The apparent capacitance (=Im(Z−1)ω−1) against the frequency for solar cells containing PTAA and spiro HTLs in
the dark and under light illumination before and after LID is shown in Figure 4. The complex impedance is composed of real and imaginary parts: Z = Z′ + jZ″, j = (−1)0.5, and ω is the angular frequency. The plateau at intermediate frequencies (∼102−105 Hz) can be mainly attributed to the geometrical capacitance.68−70 The apparent capacitances in the lowfrequency region (∼10−1 to 100 Hz) are related to electrode polarization caused by interfacial ion accumulation.68−70 Under light illumination, the plateau region became narrower with greater LID, as shown in Figure 4c,d,g,h. Interestingly, comparing the initial states for solar cells containing PTAA and spiro HTLs in the dark, the plateau region for the PTAA HTL became narrower than that for the spiro HTL with increasing bias voltage (Vdc), indicating that applying too high Vdc degraded the cell even in the dark. The hollow of the apparent D
DOI: 10.1021/acsaem.9b00709 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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Figure 6. Nyquist plots under light illumination for 5 (blue), 15 (brown), 40 (green), 60 (magenta), 120 (light blue), and 185 min (orange) for solar cells containing a PTAA HTL at Vdc of (a) 0.4, (b) 0.6, (c) 0.8, and (d) 1.0 V and a spiro HTL at Vdc of (e) 0.4, (f) 0.6, (g) 0.8, and (h) 1.0 V. The insert in panel h shows the low-frequency component.
capacitances at ∼102 Hz shown in Figure 4f indicates resistance against the occurrence of negative capacitance. Nyquist plots obtained in the dark for the cells before and after LID are shown in Figure 5. Warburg ion diffusion is characterized by a straight line with a slope of 45°, as shown in Figure 5a,b. In Figure 5a, for the solar cells before LID, ion diffusion in the OIHP bulk (arctan(−Z″/Z′) = 45°) at intermediate frequencies and ion accumulation at low frequencies are identified, similar to the behavior of the TiO2/monocrystalline MAPbBr3/Au structure.71 When ion diffusion into the HTL occurs, the contact boundary impedance decreases, resulting in a decrease in arctan(−Z″/ Z′) and a small arc in the low-frequency region.71 This behavior has been confirmed for the TiO2/polycrystalline MAPbBr3/spiro/Au structure.71 The arctan(−Z″/Z′) and arc in the low-frequency region for the solar cell with the PTAA HTL were smaller than those for the solar cells using spiro HTL, indicating that the charge-transfer rate, that is, the amount of ion diffusion into the HTL, for the PTAA HTL was larger than that for the spiro HTL. In Figure 5d, for the solar cell with the PTAA HTL before LID, a new arc appeared at intermediate frequencies, indicating that the intermediatefrequency arc resulted from charge accumulation near the newly formed degraded interface and that the low-frequency arc can be attributed to ion diffusion into the HTL. In the lowfrequency region (∼100 Hz) at high Vdc (1.0 V) under light illumination, the apparent capacitances for the PTAA HTL were smaller than those for the spiro HTL (Figure 4c,g), indicating that ion accumulation at the OIHP/HTL interface for the PTAA HTL was smaller than that for the spiro HTL. This is consistent with other results from HAXPES and impedance analysis, where neutral I0 formation and ion diffusion into the HTL were larger for PTAA than spiro, leading to an increase in defects in the OIHP bulk near the OIHP/PTAA interface owing to corrosion. We concluded that because of these factors and the influence of the solvent on the OIHP layer,3 the bulk lifetime for the PTAA coating was lower than that for the spiro coating (Table S1). When PTAA is used as the HTL, negative capacitance is observed in the lowfrequency range at Vdc ≥ 0.6 V in the dark after light illumination for 185 min (Figure 5). Applying higher Vdc leads to a narrower space charge region, meaning that the
information about the OIHP bulk near the ETL/OIHP and OIHP/HTL interfaces can be mainly extracted at high Vdc. Negative capacitance has been previously observed for electrodes covered by adsorbents, such as a passivation film on a metal and electrochemical reactants.72−75 This arc resistance is called the charge-transfer resistance (Rct), and Rct can be used to evaluate the progress of electrode corrosion. The decrease in the cell parameters caused by the increase of Rct for Vdc ≥ 0.8 V has the same tendency as the decrease in the cell parameters because of δA. Thus, we conclude that accumulation of I2 at the OIHP/HTL interface mainly affects the increase of R ct, in other words, the increase of electrochemical corrosion at the OIHP/HTL interface. In contrast, negative capacitance is not observed using a spiro HTL (the sign of −Z″ is always positive). As shown in Figure 5a,c,and d, there are inductive loops, which result from an additional contribution by carriers or ions to the impedance behavior. This type of inductive loop has been reported for OIHP in many studies, although the origin of the loop remains unclear.76−79 The sample with a spiro HTL after LID shows anomalous impedance behavior at low frequency, as indicated by the arrow in Figure 5d. This is the first demonstration of such behavior for OIHP solar cells. Such impedance behavior has already been reported in the field of electrochemistry,72,74,80 and it can be explained by suppression of the electrochemical reactions through formation of an adsorption layer on the electrode.72,80 Regarding this adsorption layer, it has been reported that a spiro−I complex forms as a result of iodide accumulation near the OIHP/spiro interface.81 Our data indicate that adsorption of the spiro−I complex suppresses the electrochemical reaction with iodide at the OIHP/spiro interface and prevents accumulation of I2 under light illumination. As a result, we conclude that the low LID of the sample with a spiro HTL compared with that with a PTAA HTL can be attributed to the following factors: small accumulation of I2 near the OIHP/spiro interface with low defect density and formation of a spiro−I complex as an adsorption layer that suppresses electrochemical corrosion. Nyquist plots of the solar cells under light illumination for 5, 15, 40, 60, 120, and 185 min are shown in Figure 6. In the cells with a PTAA HTL, an additional capacitive arc appears in the E
DOI: 10.1021/acsaem.9b00709 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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Figure 7. C−2 versus Vdc plots for solar cells containing (a) PTAA and (b) spiro HTLs, and R versus Vdc plots for solar cells containing (c) PTAA and (d) spiro HTLs under dark conditions for the initial (0 min) and light-induced degraded states after light illumination for 185 min, and under light illumination for 5 and 185 min.
lower Rrec value means that the recombination kinetics is faster and more unfavorable. The doping densities (NB) before light illumination were determined from the slopes of the Mott− Schottky plots. Before light illumination, the NB values for the cells with PTAA and spiro HTLs were 1.9 × 1016 and 2.2 × 1016 cm−3, respectively, where we used a dielectric constant of 25 for the OIHP layer. In the dark after LID, the NB values for the cells using PTAA and spiro HTL increased to 3.4 × 1016 and 3.1 × 1016 cm−3, respectively, indicating an increase in defects in the OIHP layers after LID. The flat-band potential (Vfb) was determined from the intercept with the x axis.84 For the PTAA HTL, Vfb under light illumination shifted to the negative side because of LID, indicating an increase in the light-induced dipole84 owing to charge accumulation of photogenerated carriers at the corroded interface. R under light illumination as a function of the illumination time is shown in Figure 8. For the PTAA HTL, R at 185 min illumination was lower than that at 5 min illumination when
intermediate-frequency range after 40 min illumination (Figure 6a−d). Similar intermediate-frequency arcs have been reported by Guerrero and co-workers.76 They suggested that the intermediate capacitive process may be related to an additional electronic surface state.76 In contrast, for the cells with a spiro HTL, additional intermediate arcs such as those observed for the cells with a PTAA HTL are not observed. In addition, inductive loops appear at all illumination times when Vdc = 0.4 (Figure 6e) and 1.0 V (Figure 6h), and the shapes of the loops in Figure 6e,h are different. Because these loops do not include negative capacitance caused by electrode corrosion, they are considered to be related to the inductive behavior as a result of good electrical contact, as previously reported.76 From these results, we conclude that the intermediate arcs are derived from accumulation of photogenerated carriers at the electrochemically corroded OIHP/PTAA interface. We observed three types of inductive loops with different shapes: loops that lean to the right (loop 1, Figure 6a,e), upright loops (loop 2, Figure 6h), and loops that lean to the left (loop 3, Figure 5a,c). Interestingly, for these loops, the sign of −Z″ is always positive. Because these loops appeared in the low- and/or intermediatefrequency regions, we speculate that these loops are closely related to the ion diffusion, accumulation, and reaction. Shapes similar to loops 1 and 2 have been reported at Vdc = 0 and 1.016 V in the dark,78 respectively, indicating the possibility of a common origin with the loops in this study. In contrast, loop 3 changed to negative capacitance for Vdc ≥ 0.6 V in Figure 5, indicating that this loop is related to interfacial corrosion. C−2 versus Vdc (Mott−Schottky) and R versus Vdc plots for solar cells containing PTAA and spiro HTLs under dark conditions and light illumination are shown in Figure 7, where the capacitance (C) and resistance (R) components were determined by fitting to high-frequency arcs. The R under light illumination is called the recombination resistance (Rrec) related to surface recombination mechanisms near the contacts,82,83 and it is distinguished from Rct in the dark. A
Figure 8. High-frequency resistance (R) as a function of the 1 sun illumination time for solar cells containing PTAA and spiro HTLs at Vdc = 0.4, 0.6, 0.8, and 1.0 V. F
DOI: 10.1021/acsaem.9b00709 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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ACS Applied Energy Materials Vdc < VOC, indicating an increase in activated recombination owing to the presence of Pb0 and I0. In contrast, for the spiro HTL, the decrease in R at illumination times from 5 to 185 min was lower than for the PTAA HTL, indicating suppression of LID for the spiro HTL. We presume that the following electrochemical reactions occur in the cells:
reaction at the ETL/OIHP interface, (ii) suppressing iodide migration along grain boundaries, (iii) decreasing the interfacial defects of the OIHP bulk near the OIHP/HTL interface (suppressing the anode reaction), (iv) using a highly iodine-resistant HTL, and (v) improving the hole extraction capability of the HTL (suppressing the anode reaction). There are many reports on light-stability improvement with regard to countermeasures i−v. Regarding i, it has been reported that a La-doped BaSnO3 ETL greatly improves the light stability of OIHP solar cells compared with a TiO2 ETL because of reduction of ultravioletinduced damage.31 In addition, functionalization of the TiO2 surface with a fullerene self-assembled monolayer86−89 and edge-enriched graphene nanoribbons,22 or the use of different ETLs such as SnO2 and NaYF4:Yb/Er,19 may be effective to suppress the cathode reaction. Regarding ii, it has been reported that OIHP cells with a large grain size OIHP layer are more resistant to LID,21 indicating suppression of ion migration through the OIHP surface and grain boundaries owing to the large grain size.90 Bi and co-workers37 reported that bifunctional molecular modulator SN moieties, which are contained in the OIHP layer, passivate the surface defects and induce formation of large grain crystals, leading to light-stability improvement. In addition, incorporation of exfoliated montmorillonite (exMMT) at the grain boundaries greatly improves the light stability of OIHP solar cells in spite of operating under ∼50% humidity.91 exMMT strongly bonds to the OIHP crystalline grains through cationic exchange with MAI,91 and thus the migration path of iodide can be effectively limited. Regarding iii, it has been reported that including 1adamantylamine (ADA) as an interfacial layer between the OIHP and HTL is effective in improving the operational stability of solar cells,26 indicating suppression of corrosion owing to the anode reactions at the OIHP/HTL interface caused by termination of interfacial defects by ADA molecules. Regarding iv and v, various HTLs are also effective for improving the light stability of OIHP solar cells.32−36 Incorporation of iodide into the HTL should affect the electrical properties of the HTL itself. The decrease in the dopant concentration of the HTL in the vicinity of the OIHP/ HTL interface caused by LID would lead to a deeper HOMO level and poorer electrical conductivity in this region, hindering hole-transfer from the OIHP to the HTL and then to the Au electrode. It has been reported that an S-shape J−V curve is observed near the VOC because of chemical degradation of the metal electrode.84 In this study, the J−V curves did not show an obvious S-shape (Figure 1), indicating that the chemical reaction of the migrating iodide with the Au electrode, which results in insulating metal−iodide materials,84 did not greatly occur with 1 sun illumination for 185 min. It has been reported that this chemical reaction can be detected by capacitive methods.92 If accumulation of I2 proceeds, the influence of film peeling between the OIHP and HTL on the cell performance is also of concern. To simultaneously suppress both the anode and cathode reactions, incorporation of europium (Eu) ion pairs at surfaces and grain boundaries has recently been suggested.35 The redox potential of Eu3+/Eu2+ (∼4.080 eV) is located between those of PbI2(+2)/Pb0 and I2/I−, and thus the Eu3+−Eu2+ pair acts as a redox shuttle that selectively oxidizes Pb0 to Pb2+ and reduces I0 to I− in a cyclical transition, resulting in great improvement in the light stability of OIHP solar cells.35
anode: 2I− + 2h+ → I 2
cathode: Pb2 + + 2e− → Pb0
where h+ and e− are photogenerated holes and electrons, respectively. Almost no change in the C−N fraction (Figure 2a) and iodine accumulation (Figure 2c) near the OIHP/HTL interface after LID indicate that I− mainly diffuses to the anode side under illumination, which is consistent with the suggestion that the majority of mobile ions are I− (or iodide vacancies, VI+) in the literature.1,3,5 Although the origin of mobile I− is not revealed in this study, it has been suggested that mobile iodide and immobile iodide vacancy pairs are generated at the ETL/OIHP interface.85 From the change in the bonding state of Pb, the proposed cathode reaction is shown in Figure 2b. It has been reported that the ETL affects the resistance and capacitance of the HTL,86 supporting our claim that the pair reactions of Pb2+ to Pb0 (cathode side) and I− to I2 (anode side) occur in an OIHP solar cell under light illumination. The I 3d5/2 peak intensity for sample D decreases by 33% after LID, whereas that of sample A shows almost no change (a slight increase of 2%). This indicates occurrence of a pair reaction with the cathode reaction, that is, 2I− + 2h+ → I2↑. Note that these reactions require holes and electrons, and carrier accumulation and/or trapping near the interfaces should affect the reactions. Evaporation of I2 in sample D is deduced from the structure without a HTL, because a HTL should trap I2 at the OIHP/HTL interface to some extent. The small change of the I 3d5/2 peak intensity after LID for sample A indicates that interfacial defects contributed to occurrence of the pair reaction, because the OIHP surface itself has a low defect content, as deduced from the relatively low S (Table S1). Comprehensive Model of Light-Induced Degradation. On the basis of our data and previous reports, we summarize the model of LID in Figure 9. This figure indicates that the light-stability issue can be completely solved by the following countermeasures: (i) suppressing the cathode
Figure 9. Schematic representation of the pair reactions of Pb2+ to Pb0 (cathode side) and I− to I2 (anode side) in an OIHP solar cell under light illumination. G
DOI: 10.1021/acsaem.9b00709 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Energy Materials
BAS) using a triple-A xenon light source (BPS X300BA, Bunkoukeiki). The light intensity was calibrated to 1 sun with a silicon photodiode before the measurements. The voltage scan rate was 100 mV s−1, and no device preconditioning, such as light exposure or long-term forward-bias voltage application, was performed before starting the measurements. The cells were masked with a black mask (aperture, 0.1 cm2) to fix the active area and to decrease the influence of scattered light. The measurements were performed in dry air (
