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. 2017 Apr 25;2(4):1617–1624. doi: 10.1021/acsomega.7b00097

Mechanism of Light-Soaking Effect in Inverted Polymer Solar Cells with Open-Circuit Voltage Increase

Takuji Kusumi †, Takayuki Kuwabara †,‡,*, Kyosuke Fujimori †, Takumi Minami †, Takahiro Yamaguchi †, Tetsuya Taima †,‡, Kohshin Takahashi †,‡,*, Tatsuya Murakami §, Vanadian Astari Suci Atina Rachmat ∥, Kazuhiro Marumoto ∥
PMCID: PMC6641021  PMID: 31457528

Abstract

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In this study, we present novel insights into the light-soaking effect of inverted polymer solar cells (PSCs), where the open-circuit voltage (Voc) of the cells improves over time under light irradiation. The effect was investigated by electron spin resonance (ESR) studies of bare indium tin oxide (ITO) and piperazine derivative-modified ITO/regioregular poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61 butyric acid methyl ester (PCBM) substrates. These results were combined with alternating current impedance spectroscopy (IS) measurements of inverted PSCs based on the above substrates. In ESR experiments with the substrates under white light irradiation, with a UV light component, many P3HT•+ radical cations were observed in the bare-ITO/P3HT:PCBM substrate. The number of radical cations was considerably suppressed in the ITO/P3HT:PCBM substrates with ITO modified by piperazine derivatives. This is because adsorbed oxygen molecules on the ITO acted as acceptor dopants for photoexcited P3HT, and the amount of adsorbed oxygen was decreased by modifying the ITO with piperazine derivatives. In IS measurements of the inverted PSCs under white light irradiation, a decrease in the electric capacitance (CPE2) of an electric double layer formed at the ITO/P3HT:PCBM interface was observed. A strong correlation was observed between the decrease of CPE2 and the increase of Voc. From these results, the light-soaking behavior was attributed to the removal of an electron injection barrier formed between ITO and PCBM, under white light irradiation.

Introduction

Polymer solar cells (PSCs) have attracted much attention as next-generation solar cells because of their lightweight, flexibility, and low cost of manufacturing.1−4 Inverted PSCs with indium tin oxide (ITO) as the bottom electrode and a high work function metal (Au or Ag) as the top electrode show excellent stability even in ambient air without encapsulation.5−7 ITO is the most widely used transparent electrode in electro-optical devices because it has high transparency and conductivity. The ionization potential (Ip) of bare ITO has been reported to be approximately 4.8 eV.8−11 Therefore, it is necessary to reduce the electron injection barrier between the ionization potential of ITO and the LUMO levels of [6,6]-phenyl C61 butyric acid methyl ester (PCBM) (ca. 3.8 eV) to achieve a high power conversion efficiency.8,12−14 We have previously improved the photovoltaic properties of inverted PSCs using ITO surfaces modified by aliphatic amine compounds having simple chemical structures.15 The Ip of the modified ITO electrodes decreased compared to that of untreated electrodes; however, we observed that the open-circuit voltage (Voc) in all of the PSCs containing bare ITO electrodes gradually improved under continuous light irradiation, the so-called light-soaking effect. Although this phenomenon is now well-known in the field of photovoltaics, including inorganic photovoltaics, such as CIGS solar cells,16−18 there are few satisfactory mechanistic explanations for the effect operating in organic photovoltaics.19−21

Alternating current impedance spectroscopy (IS) measurements can be used to quantify the electrical properties of bulk and interfacial materials that cannot be determined by direct current methods. We have previously used IS to investigate inverted PSCs containing metal oxides as electron collection electrodes.22−26 Electron spin resonance (ESR) spectroscopy is also a promising method for the microscopic characterization of charge-accumulation sites. Charge carriers in most organic semiconductors have a spin, which allows the use of ESR to directly observe charge carriers that accumulate during device operation.27−29 The ESR method has been successfully used to determine the microscopic properties of charge-carrier states in organic devices, including spin states and the spatial extent of wave functions in organic materials at device interfaces. We have previously investigated the mechanism of the light-soaking effect that accompanied a short-circuit photocurrent increase in inverted PSCs containing amorphous titanium oxide, using both ESR and IS methods.30

In this study, we investigated the mechanism of the light-soaking effect and the accompanying Voc increase by studying the features of the interface between amine-modified ITO substrates and an organic photoactive layer.

Results and Discussion

Characterization of Bare ITO or Amine-Modified ITO/Poly(3-hexylthiophene) (P3HT):PCBM Substrates

Zhou et al. have previously reported a decrease in the ionization potential (Ip) of bare ITO under UV light irradiation, as measured by UV photoelectron spectroscopy and Kelvin probe force microscopy. From these measurements, it was concluded that the decrease of Ip was caused by the desorption of oxygen on the ITO.31 When the ITO substrate absorbs UV light, electrons in the valence band are excited to the conduction band. This causes oxygen to desorb from the ITO surface, removing the electrostatic interaction acting between the ITO surface and the adsorbed oxygen molecules.

Figure 1 shows the chemical structures of piperazine, N-(2-aminoethyl)piperazine (NAP), and 1,4-bis(3-aminopropyl)piperazine (BAP), which were the three amines used in this study. Amine-modified ITO substrates were prepared from the piperazine, NAP, and BAP solutions of various concentrations from 20 μM to 4 M. Figure 2 shows the relationship between the Ip of the modified ITO substrates and the amine concentration in the cast solutions. The Ip decreased with increasing amine concentration because the amine molecules are adsorbed to ITO by displacing the adsorbed oxygen molecules. The minimum Ip values of the piperazine-, NAP-, and BAP-modified ITO substrates were 4.43, 4.42, and 4.40 eV, respectively. The Ip was reduced more effectively for a larger number of amino groups in the amine molecule, in the order of piperazine < NAP < BAP.15 However, the Ip of the modified ITOs suddenly increased when BAP and NAP solutions with concentrations greater than 1 and 2 M, respectively, were spin-cast. This result suggests that when a large excess of piperazine derivatives was deposited on ITO, the amines acted as an insulator, which interfered with photoelectron emission from the ITO.

Figure 1.

Figure 1

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Chemical structures of piperazine derivatives used for preparing the modified ITO electrodes.

Figure 2.

Figure 2

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Plots of Ip of various amine-modified ITO electrodes against the concentration of the amines in ethanol solutions used to prepare the modified electrodes.

Piperazine-, NAP-, and BAP-modified ITO substrates that were adjusted to Ip = 4.6 eV were analyzed by X-ray photoelectron spectroscopy (XPS). From the high-resolution N 1s XPS spectra shown in Figure 3a, we observed one broad N 1s peak at a binding energy of 401 eV, which had a full width at half-maximum of 3.5 eV. Figure 3b shows an example of peak-fitting analysis of the XPS spectrum of the BAP-modified ITO substrate. The spectrum consisted of three components with peak-binding energies of 400.5, 401.5, and 402.8 eV. The component with the lowest peak-binding energy of 400.5 eV is assigned to neutral BAP, whereas other components are assigned to protonated BAP adsorbed to the ITO substrate.14,15 From these results, we confirmed that the adsorption of amine molecules was caused by a neutralization reaction between the basic amino groups and the acidic hydroxyl groups of ITO.15 Thus, an electric double layer was formed between the protonated amino groups and the deprotonated hydroxyl groups on the ITO surface, which promoted the release of photoelectrons from ITO. This may be one reason for the lower Ip of the amine-modified ITOs.

Figure 3.

Figure 3

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High-resolution N 1s XPS spectra of piperazine derivative-modified ITO substrates with Ip = 4.6 eV (a), and peak fitting of the XPS spectrum of the BAP-modified ITO substrate (b).

Measurements of the ESR of a bare ITO (Ip = 4.8 eV)/P3HT:PCBM substrate, a BAP-modified ITO (Ip = 4.4 eV)/P3HT:PCBM substrate, and a BAP-modified ITO (Ip = 5.1 eV)/P3HT:PCBM substrate were performed to characterize the charge accumulation at the ITO/P3HT:PCBM interfaces. Figure 4a shows the ESR spectra measured in the dark and after white light irradiation, with a UV light component, for the bare ITO (Ip = 4.8 eV) and the BAP-modified ITO (Ip = 5.1 eV) substrates. Almost no signal was observed under dark conditions, but clear ESR signals with similar intensities were observed for the three substrates directly after white light irradiation, as shown by the blue lines in Figure 4a. The g value and the peak-to-peak linewidth (ΔHpp) for the substrates were approximately 2.002 and 0.22 mT, respectively. The signals originated from the P3HT•+ radical cations (positive polarons) in the charge-transfer complexes of P3HT+:PCBM–, which are produced transiently by light absorption.27 ESR signals due to adsorbed oxygen molecules on the ITO substrates were not observed because the range of the resonance magnetic field was outside the measured magnetic field.32 In addition, ESR signals due to oxygen radical anions O2– were not observed because of the broadening of the ESR linewidth resulting from fast relaxation due to orbital rotation.33,34 The number of spins (Nspin) of P3HT•+ radical cations was estimated from the double integration of the ESR signals.27Figure 4b shows the time dependence of the Nspin under light irradiation and in the dark. The Nspin increased gradually with increasing light irradiation time. However, after continuous white light irradiation for 18 h, the Nspin accumulated in the bare ITO substrate was much larger than that in the BAP-modified ITO substrates. Even when the bare ITO substrate was left in the dark for 10 h, its Nspin value was about 70% of that observed just before turning off the light, as shown by the black plots in Figure 4b. A similar decay behavior was observed for the BAP-modified ITO (Ip = 4.4 eV) substrate, as shown by the red plots in Figure 4b, although the Nspin declined to zero after the substrate was left in the dark for 10 h. Adsorbed oxygen molecules on the ITO were released by UV light exposure,31 and these molecules behaved as acceptor dopants in the P3HT:PCBM active layer. Thus, the P3HT was gradually oxidized by the released oxygen molecules. As a result, more P3HT•+ radical cations were produced from the bare ITO substrate, which had initially adsorbed more oxygen molecules. In this way, we indirectly showed the release of oxygen molecules adsorbing to ITO using ESR measurements under white light irradiation with a UV light component.

Figure 4.

Figure 4

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(a) ESR spectra in the dark (black line) and after white light irradiation for 1 h (blue line) and 15 h (red line) for bare ITO (Ip = 4.8 eV)/P3HT:PCBM (left-hand side) and BAP-modified ITO (Ip = 5.1 eV)/P3HT:PCBM (right-hand side) substrates, and (b) the time dependence of P3HT•+ spin number (Nspin) for bare ITO (Ip = 4.8 eV)/P3HT:PCBM (black symbols), BAP-modified ITO (Ip = 4.4 eV)/P3HT:PCBM (red symbols), and BAP-modified ITO (Ip = 5.1 eV)/P3HT:PCBM (blue symbols) substrates.

In contrast to the increase in the Nspin in the BAP-modified ITO (Ip = 4.4 eV) substrate under continuous light irradiation, the Nspin in the excess BAP-modified ITO (Ip = 5.1 eV) substrate hardly increased, after a small initial increase, as shown by the blue symbols in Figure 4b. Furthermore, after the light was turned off, the Nspin in the latter substrate decreased rapidly compared with the slow decline in the former. This means that no P3HT•+ radical cation was formed by oxidization from oxygen molecules released from the ITO surface because fewer oxygen molecules were adsorbed to the BAP-modified ITO substrate with Ip = 5.1 eV than those to the BAP-modified ITO substrate with Ip = 4.4 eV.

In Figure 5, we summarize a plausible schematic process by which the P3HT•+ radical cations gradually accumulate in the ITO/P3HT:PCBM substrates under continuous white light irradiation: (a) generation of oxygen anions, such as O2–, by the adsorption of oxygen molecules onto ITO, (b) spin coating of the P3HT:PCBM active layer onto the ITO, (c) electron transfer from oxygen anions to ITO by UV irradiation, (d) oxygen desorption, and (e) oxidization of P3HT by the released oxygen molecules.

Figure 5.

Figure 5

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Schematic of proposed processes for the accumulation of P3HT•+ radical cations in the ITO/P3HT:PCBM substrate by the generation of oxygen anions, such as O2–, from adsorbed oxygen molecules on the ITO (a), spin coating of the P3HT:PCBM active layer (b), electron transfer from oxygen anions to ITO by UV irradiation (c), subsequent oxygen desorption (d), and oxidization of photoexcited P3HT by released oxygen molecules (e).

Suppression of the Light-Soaking Behavior of Inverted PSCs with Amine-Modified ITOs

Figure 6 shows the time dependence of the current–voltage (I–V) curves for inverted PSCs containing bare ITO and BAP-modified ITO electrodes under continuous white light irradiation. The performance of the cells with bare ITO (Ip = 4.8 eV) was improved because the open-circuit voltage (Voc) gradually increased through the light-soaking effect. The Voc increased from its initial value of 0.25 to 0.54 V after light irradiation for 1 h, whereas the short-circuit current (Jsc) remained almost unchanged. Finally, the maximum power-conversion efficiency (PCE) of 2.22% was obtained under light irradiation for 2 h. When the BAP-modified ITOs were used as the electron-collection electrodes, the light-soaking behavior gradually disappeared as the Ip of the modified ITO decreased. Thus, the Voc of the device based on BAP-modified ITO, with the lowest Ip of 4.4 eV, increased from its initial value of 0.48 V to a maximum of 0.56 V after a few minutes of light irradiation. This device also exhibited a better Jsc and fill factor (FF) compared to those of the devices based on ITO substrates with a higher Ip and achieved the best PCE of 3.40%. All P3HT:PCBM based devices using bare ITO and amine-modified ITOs showed a quite high stability even after light soaking, and they have sufficient reproducibility for the performance and the light-soaking behavior.

Figure 6.

Figure 6

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Time-dependent change of the photo I–V curves for inverted PSCs containing bare ITO and BAP-modified ITO electrodes under continuous irradiation by AM 1.5G-100 mW cm–2 simulated sunlight. The Ip values of the electrodes were 4.8 (a), 4.6 (b), and 4.4 eV (c), respectively.

IS measurements were performed to gain a better understanding of the light-soaking effect. Figure 7a,b shows typical Nyquist plots that were obtained for inverted PSCs containing bare ITO under an applied DC voltage of 0.5 V, near the Voc, in the dark and under white light irradiation, respectively. The plots consist of an arc at a high frequency of more than ca. 150 kHz and a second arc at a low frequency of less than ca. 150 kHz. The former and the latter are denoted as arc 1 and arc 2, respectively. The plots were analyzed with the equivalent circuit shown in Figure 7c, and a reasonable fit to the simulated curve was obtained. Here, Rs represents the series resistance, comprising ohmic components, and R1 and R2 are resistance components that form a parallel circuit with the constant phase elements CPE1 and CPE2, respectively. The CPEs are roughly equal to the differential capacitances because there was almost no depression of the arcs. On the basis of our previous work,22−24 arcs 1 and 2 are related to the components in the electron-collection electrode and the P3HT:PCBM layer, respectively. In the dark, the arc size was almost unchanged over 30 min, as shown in Figure 7a, whereas arc 2 rapidly decreased from 1300 Ω cm2 in the dark to about 50 Ω cm2 directly after irradiation, as shown in Figure 7b. This large decrease in the resistance component is attributed to the increase in photoconductive carriers in the P3HT:PCBM film during light irradiation. However, there was a poor correlation between the R components and the Voc. Although the R2 increased from 20 Ω cm2 just after irradiation to 50 Ω cm2 after irradiation for 30 min, the Jsc did not change as shown in Figure 6a. This R2 value is directly unrelated to the Jsc, that is, the photocurrent under short-circuit condition, because it is a resistance component in active layer under open-circuit condition. On the other hand, we found a strong correlation between the time dependence of the Voc and CPE2 under continuous light irradiation. As shown in Figures 8 and S1, for inverted PSCs containing bare ITO and BAP-modified ITOs, which had Ip values of 4.80, 4.60, 4.50, and 4.40 eV, the increase in Voc is strongly correlated with the decrease in CPE2. Because the differences (ΔCPE2) between the initial and saturated CPE2 values became smaller with decreasing Ip, as summarized in Table S1, the CPE2 was related to the capacitance of the electric double layer, as depicted in Figure 5b. In addition, the difference (ΔVoc) between the initial and saturated Voc values also became smaller as Ip decreased. Thus, the electric double layer at the ITO/P3HT:PCBM interface acted as an electron injection barrier, which disrupted the electron injection from PCBM to ITO. Figure S2 shows the time dependence of Voc and CPE2 for inverted cells containing piperazine-, NAP-, and BAP-modified ITO electrodes, both of which had a similar Ip of 4.6 eV. The profiles for these three PSCs were almost the same. These results show that the light-soaking effect is not influenced by the type of amine compound but by the Ip value, that is, the charge density of the electric double layer is determined by the adsorbed oxygen coverage.

Figure 7.

Figure 7

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Typical Nyquist plots of inverted PSCs containing bare ITO at an applied voltage of 0.5 V in the dark (a) and under AM 1.5G-50 mW cm–2 simulated sunlight irradiation (b). Solid lines indicate the curves calculated from the equivalent circuit (c).

Figure 8.

Figure 8

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Time dependence of Voc (black lines) and CPE2 (blue open circles) for inverted PSCs containing bare ITO (Ip = 4.8 eV) (a) and BAP-modified ITO (Ip = 4.4 eV) (b).

As shown in Figure 6, the light-soaking effect of the PSCs was not attributed to changes in the Jsc but instead to changes in the Voc. However, the Jsc values depended on the initial Ip of the BAP-modified ITO, as shown in Figure 9 and Table S2. As the Ip of the modified ITO decreased from 4.80 to 4.40 eV, the Jsc of the PSCs increased from 7.76 to 9.93 mA cm–2. A lower Ip can increase the built-in potential of the P3HT:PCBM layer, which is limited by the difference of both the Fermi levels of ITO and the poly(3,4-ethylenedioxylenethiophene):poly(4-styrene sulfonic acid) (PEDOT:PSS) layer, which sandwich the active layer. Hence, the increase in Jsc is caused by the drift migration of photoproduced charge carriers occurring more effectively owing to the higher built-in potential. Figure S3 shows the saturated I–V curves of the inverted cells fabricated with piperazine-, NAP-, and BAP-modified ITO set to about the same Ip value of 4.6 eV. Jsc and other parameters in the three PSCs were similar regardless of the type of amine compound used to modify the ITO layer, as summarized in Table S3.

Figure 9.

Figure 9

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Photo I–V curves for the inverted PSCs containing bare ITO and BAP-modified ITO with various Ip after white light irradiation for 120 min.

Although the built-in potential increased because of the increase in Voc caused by the light-soaking effect, the Jsc did not change, as shown in Figure 6a. As discussed in relation to the ESR results, P3HT can be oxidized by oxygen molecules released from the ITO surface under UV light exposure, as shown in Figure 5e. Thus, P3HT•+ radical cations are formed, and localized dopant anions, such as O2–, are produced near the ITO/P3HT interface. Ions close to the ITO may interfere with the injection of the photoproduced electrons into ITO because of electrostatic interactions. In fact, as shown in Figure 9, the Jsc value increased as the Ip of the BAP-modified ITO decreased. Therefore, the Jsc value is determined by the initial amount of oxygen molecules adsorbed at the ITO/P3HT:PCBM interface, which in turn depends on the coverage rate of amine compounds.

Conclusions

The Ip of ITO substrates treated with ethanol solutions containing piperazine derivatives decreased. The amine modification coupled with the removal of oxygen molecules adsorbed to the bare ITO resulted in a decrease of the Ip. Furthermore, the ESR measurements of the bare ITO/P3HT:PCBM substrates under continuous light irradiation, with a UV light component, revealed P3HT•+ radical cations oxidized by desorbed oxygen molecules. Thus, using ESR, we indirectly demonstrated the release of oxygen molecules adsorbed on bare ITO, which acted as acceptor dopants.

In inverted PSCs containing bare ITO/P3HT:PCBM, we observed a remarkable light-soaking effect, in which Voc gradually increased under continuous light irradiation. This phenomenon was suppressed when the ITO was modified with piperazine derivatives. For the light-soaking effect, the increase in Voc was strongly correlated with the decrease in CPE2, which represented the capacitance of an electric double layer formed at the ITO/P3HT:PCBM interface. From these results, we concluded that the light-soaking effect in these PSCs occurs by the removal of an electron injection barrier from PCBM to ITO.

Experimental Section

Materials

Piperazine, NAP, BAP, regioregular P3HT (Mn 30 000–60 000), Triton-X 100, and 1,2-dichlorobenzene (DCB) were purchased from Sigma-Aldrich. 2-Propanol (IPA) and ethanol were purchased from Kanto Chemical Co. PEDOT:PSS dispersion in water (Clevios P) (1.3 wt %) was purchased from H.C. Starck. PCBM was purchased from Frontier Carbon. ITO substrates (sheet resistance = 10 Ω sq–1) and Au wires were purchased from Furuuchi Chemical Corporation. Quartz substrates were purchased from Iiyama Precision Glass Co., Ltd. ITO layers (10 Ω sq–1) on the quartz substrates were fabricated by Geomatec Co., Ltd. Masking tape E-MASK TP200 was provided from Nitto Denko. Lamination film Cellel F1550H was purchased from Kureha Extech.

Preparation of Amine-Modified ITO

The ITO electrode was ultrasonic-cleaned in IPA, washed in boiling IPA, and then dried in air. The substrate was partly covered with E-MASK TP200 to prevent any excess modification of the amines. Ethanol was used as the solvent for piperazine, NAP, and BAP. The amine solutions were dropped onto the ITO substrates and then spin-coated at 5000 rpm for 1 min. The substrates were then dried at 120 °C for 10 min on a hot plate. To remove any excess amine from the ITO, the same operation was performed using ethanol. These operations were performed in air. Glass–ITO substrates were used for most experiments; however, quartz–ITO substrates were used for the ESR measurements.

Fabrication of Inverted PSCs

The photoactive layer was prepared from a DCB solution containing P3HT (16.7 mg mL–1) and PCBM (13.3 mg mL–1). The solution was spin-coated onto the amine-modified ITO substrates, and the resulting films were solvent-annealed for 40 min in an airtight container. Subsequently, a PEDOT:PSS dispersion in water containing 0.5 wt % Triton-X 100 as a surfactant was spin-coated onto the hydrophobic P3HT:PCBM layer, and the resulting PEDOT:PSS film was dried at room temperature for 30 min. These operations were also performed in air with the relative humidity maintained at less than 30%. The thicknesses of the P3HT:PCBM and PEDOT:PSS layers were approximately 200 and 150 nm, respectively. The Au rear electrode was vacuum-deposited at 2 × 10–5 Torr on the PEDOT:PSS layer. Finally, the device was mechanically protected by thermal compression using Cellel F1550H.35 The effective area of the cells was 1 cm2.

Measurements

The ionization potential (Ip) of the modified ITO substrates was estimated in the dark by photoelectron spectroscopy in air with a Riken Keiki AC-2. XPS measurements were performed with a Shimadzu AXIS-ULTRA DLD XPS apparatus. The time-dependent change of the I–V curves of the PSCs was measured by a linear sweep voltammetry at a scan rate of 5 V min–1 in combination with a rest voltage measurement under AM 1.5G-100 mW cm–2 simulated sunlight irradiation. The time-dependent change of the Voc under AM 1.5G-50 mW cm–2 simulated sunlight irradiation was measured under the photocurrent at zero. The DC measurements were implemented with a Hokuto Denko HZ-5000 electrochemical analyzer. The IS measurements were performed with an Agilent Technologies E4980A precision LCR meter in the dark and under AM 1.5G-50 mW cm–2 simulated sunlight irradiation. The frequency range was from 20 Hz to 1 MHz, and the alternating signal magnitude was 5 mV. The data obtained were fitted with Scribner Associates Z-VIEW software v3.1 with the appropriate equivalent circuits. The light source was a SAN-EI Electric XES-301S solar simulator, which was calibrated with a standard silicon photovoltaic detector. These measurements were carried out in ambient atmosphere, that is, at 25 °C, at a relative humidity of 40–60%.

ESR measurements were performed with a JEOL RESONANCE JES-FA200 X-band spectrometer under nitrogen atmosphere at room temperature. The measurements under AM 1.5G-100 mW cm–2 simulated sunlight irradiation were obtained with a Bunkoukeiki OTENTOSUN-150BXM solar simulator as the light source. The number of spins and the g values of the ESR signals were calibrated using a standard Mn2+ marker sample.

Acknowledgments

XPS measurements were conducted in JAIST, supported by Nanotechnology Platform Program (Molecule and Material Synthesis) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grants-in-Aid for Scientific Research (B,C) and for Young Scientists (A) (Grant Nos. 24350092, 16K04929, and 25708029, respectively).

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b00097.

  • Time-dependence of Voc and CPE2; I–V curves; photovoltaic properties of inverted PSCs (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao7b00097_si_001.pdf (268.7KB, pdf)

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