Unraveling the Charge Extraction Mechanism of Perovskite Solar

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Unraveling the Charge Extraction Mechanism of Perovskite Solar Cells Fabricated with Two-Step Spin Coating: Interfacial Energetics between Methylammonium Lead Iodide and C60 Dongguen Shin, Donghee Kang, Junkyeong Jeong, Soohyung Park, Minju Kim, Hyunbok Lee, and Yeonjin Yi J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02562 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 23, 2017

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The Journal of Physical Chemistry Letters

Unraveling the Charge Extraction Mechanism of Perovskite Solar Cells Fabricated with Two-Step Spin Coating: Interfacial Energetics between Methylammonium Lead Iodide and C60 Dongguen Shin,† Donghee Kang,† Junkyeong Jeong,† Soohyung Park,† Minju Kim,† Hyunbok Lee*,‡ and Yeonjin Yi*,† †

Institute of Physics and Applied Physics, Yonsei University, 50 Yonsei-ro, Seodaemun-Gu,

Seoul 03722, Republic of Korea ‡

Department of Physics, Kangwon National University, 1 Gangwondaehak-gil, Chuncheon-

si, Gangwon-do 24341, Republic of Korea AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (H. L.) *E-mail: [email protected] (Y. Y.)

ACS Paragon Plus Environment

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ABSTRACT In organolead halide perovskite solar cells (PSCs), interfacial properties between the perovskite and charge transport layers are the critical factors governing charge extraction efficiency. In this study, the effect of interfacial energetics between two-step spin-coated methylammonium lead iodide (MAPbI3) with different methylammonium iodide (MAI) concentrations and C60 on charge extraction efficiency is investigated. The electronic structures of perovskite films are significantly varied by the MAI concentrations due to the changes in the residual precursor and MA+ defect content. As compared to the optimum PSCs with 25 mg ml-1 MAI, PSCs with other MAI concentrations show significantly lower power conversion efficiencies and severe hysteresis. The energy level alignment at the C60/MAPbI3 interface determined by ultraviolet and inverse photoelectron spectroscopy measurements reveals the origin of distinct differences in device performances. The conduction band offset at the C60/MAPbI3 interface plays a crucial role in efficient charge extraction in PSCs.

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Organolead halide [e.g. methylammonium lead iodide (MAPbI3)] perovskite solar cells (PSCs) have gathered tremendous attention in recent years due to their excellent light harvesting efficiency,1,2 low exciton binding energy,3,4 long electron-hole diffusion length,5,6 and high carrier mobility.7 Recent intensive studies on PSCs bring a rapid increase in their power conversion efficiency (PCE) and the reported highest PCE exceeds 22%.8 The PCE of PSCs greatly depends on crystallinity and surface properties of MAPbI3 perovskite films.9,10 One of the most promising methods to fabricate high quality MAPbI3 perovskite films is the so-called two-step spin coating, sequential deposition of PbI2 and methylammonium iodide (MAI) solutions.11,12 However, it is well known that the complete growth of MAPbI3 is highly limited by the concentration of MAI solution13 and this determines the interface with a charge transport layer which plays an important role in device performance. To achieve highperformance PSCs, therefore, the optimum MAI concentration should be established via a fundamental understanding of the physical and chemical properties (e.g. electronic structure and chemical composition) of fabricated MAPbI3 films.14,15 Despite the unconventionally high PCEs of PSCs, there are several unresolved issues that should be addressed for real-world application. For example, interfacial properties such as band offset and trap states affecting charge extraction efficiency are presented as origins of the PCE deterioration and hysteresis phenomenon.16-19 Accordingly, investigation on the electronic structure of PSC is of great importance to understanding its charge transport and collection abilities.20 However, the relationship between device performance and interfacial energetics has not been well understood. Therefore, a combined study of electronic structure analysis and device characterization in line with the same fabrication conditions is necessary. The charge transport levels, valence, and conduction bands can be accurately measured by ultraviolet photoelectron spectroscopy (UPS) and inverse photoelectron spectroscopy (IPES).21 However, the reported energy level alignments in PSCs show a large deviation even


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for the same interface (e.g. MAPbI3/C60 interface).22-24 The reason would be mainly due to the energy levels of perovskite materials being significantly varied by its subtle stoichiometry change. For example, the ionization energy (IE) of MAPbI3 can vary by more than 0.7 eV depending on fabrication methods.25 In addition, it has been recently reported that a logarithmic spectral analysis should be used when determining the valence band maximum (VBM) and conduction band minimum (CBM) using UPS and IPES due to the dispersive band structure of perovskite.26 Another possible reason for discrepancies is that the perovskite fabrication site and the analysis site are far away. A long sample travel can cause considerable degradation and reduce the reliability of the measurement. Therefore, a scrupulous analysis of the energy level alignment at the same site is required. In this regard, we investigated the impact of interfacial energetics between the MAPbI3 perovskite and C60 electron transport layers on charge extraction efficiency in PSCs. The MAPbI3 was prepared with two-step spin coating with different MAI solution concentrations (10, 15, 20, 25, 30 and 35 mg ml-1) and its electronic structure was explored using UPS, IPES and X-ray photoelectron spectroscopy (XPS). To understand the effect of MAPbI3 energy levels on device performance, we fabricated the PSCs with the architecture of Ag/bathocuproine (BCP)/C60/MAPbI3/ITO prepared with each concentration of MAI solution. All sample fabrications and electronic structure measurements were performed at the same site. Finally, we revealed that the efficient charge transport through the band offset reduction between MAPbI3 and C60 plays a critical role in PSC optimization. To investigate the electronic structure of perovskite films with various MAI concentrations, the UPS and IPES spectra were obtained as presented in Figure 1a. The secondary electron cutoff (SEC) region is displayed with the kinetic energy in abscissa to directly show the work function (Ψ) of the sample. To clearly see the energy shifts, the SEC spectra were normalized. The Ψs of MAPbI3 are 4.45 eV for the MAI concentration of 10‒20


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mg ml-1 and increases to 4.80 eV as the MAI concentration increases to 35 mg ml-1. The positions of the VBM and CBM are shown with vertical bars determined from the logarithmic onset of the dispersive MAPbI3 band (see SI, Figures S1 and S2).26 The measured band features including energy positions and photoemission intensities are in good agreement with the density of states from density functional theory (DFT) calculation.26-28 According to the DFT results, the valence band features indicated as a, b and c in Figure 1a are mainly derived from the I 5p orbital with minor contributions from the Pb 6s and 6p orbitals while those indicated as d and e are composed of the MA molecular orbital. The conduction band feature indicated as f is derived from the Pb 6p orbital with minor contributions from the I 5p orbital. Interestingly, the VBM and CBM positions are markedly changed by varying the MAI concentration. As the MAI concentration increases from 10 mg ml-1 to 35 mg ml-1, the VBM moves towards a lower binding energy from 1.40 eV to 1.00 eV. On the other hand, for the 10 mg ml-1 MAPbI3, the CBM is observed at 0.60 eV above the Fermi level (EF) and it shifts toward the EF by 0.25 eV as the MAI concentration increases until 20 mg ml-1. However, further increases in the MAI concentration moves the CBM away from the EF. The energy level diagrams were illustrated in Figure 1b. The IEs of all MAPbI3 films are the same within the experimental error margin (5.85±0.05 eV). On the other hand, it is noteworthy that the electron affinity (EA) is significantly varied for the entire range of the MAI concentration. The EA gradually increases from 3.85 eV to 4.15 eV as the MAI concentration changes from 10 mg ml-1 to 30 mg ml-1 and then decreases to 3.95 eV for the 35 mg ml-1 concentration. Thus, the band gap (Eg) is changed with the MAI concentration and the 20 mg ml-1 and 25 mg ml-1 MAPbI3 perovskite films have the narrowest Eg of 1.70 eV whereas the 10 mg ml-1 MAPbI3 film has the widest Eg of 2.00 eV. The narrow Eg with a middle range of MAI concentrations implies the formation of the optimum MAPbI3 film for solar cell application.23,29 We remark that the optical gap from UV-vis absorption was almost


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the same (~1.6 eV) for all MAI concentrations (see Figure S3 in SI). Thus, exciton binding energy, which is the energetic difference between the transport gap and the optical gap, is lowest (~0.1 eV) for ~25 mg ml-1 (see Table S1 in SI) and efficient exciton dissociation is expected.30 The wide Eg of MAPbI3 with low MAI concentrations is due to the residual precursors identified by the strong X-ray diffraction (XRD) peak of PbI2 (see SI, Figure S4). Since the precursors for MAPbI3 synthesis have wide Egs (2.35 eV for PbI2 and 4.65 eV for MAI, Figure S5), MAPbI3 with low MAI concentrations containing residual precursors has the wider Eg than that of the optimum concentration. On the other hand, at high MAI concentrations, the high Ψ and slightly wider Eg of MAPbI3 are attributed to the MA+ defect which induces a self-doping transition.27 In the case of 35 mg ml-1, the Ψ shows the highest value of 4.80 eV and the EF is situated at 1.00 eV above the VBM, which is close to the center of Eg and corresponds to the band shift for the p-type doping as compared other films. In literature, it has been reported that the EF of MAPbI3 moves toward the valence band when MA+ defects are formed,31 which accords well with current measurements. The MA+ defect formation would induce somewhat reconversion to PbI2 having high Ψ (5.50 eV, see Figure S5), thus the MAPbI3 with high MAI concentration shows the higher Ψ and slightly wider Eg than those of the optimum concentration.


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Figure 1. (a) UPS and IPES spectra and (b) electronic structure of MAPbI3 perovskite films with different MAI concentrations.

The EA, IE and EF changes are replotted in Figure 2 with the N/Pb ratio. The defects in MAPbI3 perovskite can be crosschecked by elemental stoichiometry with XPS measurements. We obtained the XPS spectra of Pb 4f7/2 and N 1s of MAPbI3 perovskite films with different MAI concentrations (see SI, Figure S6) and the calculated N/Pb ratio of each film is shown in Figure 2. Although the MAI concentration increases, the N/Pb ratio is nearly stoichiometric


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(~unity) up to 25 mg ml-1. Then the ratio rapidly drops down to 0.65 with the emergence of metallic Pb peak at ~137.0 eV for 35 mg ml-1.32,33 These evidences indicate that the MA+deficient MAPbI3 containing PbI2 is formed with high MAI concentration. We remark that this PbI2 mainly exists on the surface since it is observed with surface-sensitive UPS and XPS measurements while the PbI2 peak in XRD patterns is not increased (see Figure S4). These MA+ deficiencies induce the p-doping (gradual p-doping for the films >25 mg ml-1, blue shaded region in Figure 2), which accords well with reported DFT and experimental results.27

Figure 2. Atomic ratio of N/Pb, EF, EA and IE of MAPbI3 perovskite films with different MAI concentrations from XPS measurements.

These different MAPbI3 formations can be explained by a previously reported MAPbI3 crystallization process.12,13 At low MAI concentration, the deposited MAI reacts with the PbI2 surface to produce MAPbI3, and the formed MAPbI3 layer prevents further MAI diffusion into the PbI2 bulk, resulting in incomplete MAPbI3 conversion. Thus, MAPbI3 grown with low MAI concentration contains significant precursor residues. On the other hand, the MAI of high concentrations dissolves and reacts with most PbI2 to produce MAPbI3, resulting in larger grains and smoother surfaces as shown in Figure S7 (see SI). However, it would need a


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much longer reaction time for full-bulk crystallization. Due to the short reaction time (spin coating) in device fabrication, full crystallization has not yet been reached. As a result, unreacted volatile MA+ can evaporate from the surface first, and thus MA+-deficient MAPbI3 is formed with high MAI concentration. With the optimum MAI concentration, MAPbI3 conversion without defects for high performance PSCs is expected. To investigate the effect of MAPbI3 electronic structure changes on device performance, we implemented current density-voltage (J-V) measurements on the PSCs with both forward and reverse directions under 1 sun illumination. The device structure and measured J-V curves for each best device are shown in Figure 3 and the main photovoltaic parameters are summarized in Table 1. Significant changes in J-V characteristics are observed with the MAI concentration changes. The 25 mg ml-1 PSCs show the highest PCE among the devices, in which values of short-circuit current density (JSC), open-circuit voltage (VOC) and fill factor (FF) are 18.57 mA cm-2, 1.00 V and 58.40%, yielding a PCE of 10.84% and negligible hysteresis. As the MAI concentration is decreased from 25 mg ml-1 to 20 mg ml-1, hysteresis becomes observable and the PCE is slightly decreased to 10.18%. Moreover, PCEs are significantly deteriorated to 2.40% and 0.90% when the MAI concentration is decreased to 15 mg ml-1 and 10 mg ml-1, and extreme hysteresis is observed at 10 mg ml-1 PSCs. For 30 mg ml-1 PSCs, the PCE is the same with 25 mg ml-1 PSCs but hysteresis starts to occur. Eventually, 35 mg ml-1 PSCs shows a very low PCE of 3.17% and strong hysteresis. Although somewhat difference between the average value from 8 devices and the best value is seen, the overall tendency with different MAI concentrations is the same. We note that these results cannot be simply correlated with their morphological properties from scanning electron microscopy (SEM) studies shown in Figure S7. It has been speculated that the larger grains improve the device performance and reduce hysteresis.34,35 The grain size effect seemingly matches our device results from 10 mg ml-1 to 25 mg ml-1;


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however, device performance is deteriorated again even when the grains monotonically grow larger as MAI concentration increases further. Therefore, the changes in device performance do not mainly originate from the morphology of perovskite but there are other mechanisms which govern charge extraction efficiency in PSCs.

Figure 3. The schematic device structure and J-V curves of MAPbI3 PSCs with different MAI concentrations with forward scan (solid symbol-line) and reverse scan (hollow symbolline) under 1 sun illumination.


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Table 1. Photovoltaic parameters of PSCs fabricated with different MAI concentrations. Among 8 devices for each concentration, the values for the devices with the best PCE over all forward and reverse scans are shown (average value and standard deviation from 8 devices are shown in parentheses). MAI concentrations (mg ml-1)

sweep direction forward

10 reverse forward 15 reverse forward 20 reverse forward 25 reverse forward 30 reverse forward 35 reverse a)

VOC (V)

JSC (mA cm-2)

FF (%)

PCE (%)

0.48 (0.63±0.24) 0.87 (0.60±0.27) 0.96 (0.90±0.05) 0.90 (0.70±0.19) 1.00 (0.99±0.06) 1.04 (1.00±0.02) 1.00 (0.99±0.03) 0.99 (1.00±0.01) 0.99 (0.96±0.05) 0.95 (0.94±0.08) 0.95 (0.75±0.17) 0.80 (0.65±0.23)

1.81 (2.41±0.79) 2.18 (3.05±1.65) 8.28 (5.30±2.78) 9.37 (5.60±2.49) 17.20 (15.86±0.88) 16.62 (16.11±0.75) 18.57 (18.21±1.03) 18.75 (18.26±1.10) 17.00 (17.11±1.05) 17.46 (17.25±0.72) 5.18 (7.89±2.53) 5.29 (8.21±3.28)

23.79 (29.99±9.92) 47.55 (42.63±3.01) 25.14 (29.25±3.96) 28.50 (42.68±18.90) 59.27 (62.38±4.25) 55.75 (54.93±3.47) 58.40 (58.18±2.34) 54.36 (56.28±4.57) 64.65 (62.55±2.08) 54.12 (53.13±5.02) 64.39 (41.66±15.47) 45.67 (36.98±7.23)

0.21 (0.48±0.31) 0.90 (0.65±0.18) 1.99 (1.35±0.62) 2.40 (1.49±0.60) 10.18 (9.71±0.37) 9.60 (8.82±0.48) 10.84 (10.46±0.27) 10.09 (10.23±0.28) 10.84 (10.20±0.64) 9.02 (8.61±1.38) 3.17 (2.25±0.60) 1.92 (1.74±0.29)

HI a)

0.52 (0.56±0.15)

0.21 (0.45±0.2)

0.06 (0.04±0.02)

0.03 (0.02±0.01)

0.04 (0.05±0.03)

0.19 (0.26±0.15)

(Hysteresis Index, see SI)

As shown above, the electronic structure of MAPbI3 perovskite is remarkably changed with varying MAI concentration, and thus it also influences the energy level alignment in contact with the C60 charge transport layer. The band offset between MAPbI3 and C60, which critically affects the charge transport and accumulation, should be understood since this would be the critical reason of distinctively different PSC performances. Therefore, we investigated the energy level alignment at the C60/MAPbI3 interface with in situ UPS measurements. Three representative MAPbI3 films with 10, 25 and 35 mg ml-1 MAI concentrations were investigated, which exhibit the greatest difference in device performances and two different MAPbI3 conversion regimes explained. The UPS spectra were measured during the stepwise deposition of C60 on MAPbI3 without breaking the


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vacuum. Figure 4 shows the measured UPS spectra of the SEC region and highest occupied molecular orbital (HOMO) region of C60 (0.2, 0.5, 1, 2, 5 and 10 nm) on 25 mg ml-1 MAPbI3. In the SEC region, the Ψ of bare MAPbI3 is 4.55 eV. As the C60 layer thickens, the Ψ slightly decreases by 0.05 eV. In the HOMO region, the VBM of MAPbI3 is observed at 1.3 eV and the HOMO features of C60 are seen near 2.5 eV and its intensity is gradually increased as it thickens. The HOMO peak is slightly shifted toward a higher binding energy by 0.05 eV after the formation of the C60 monolayer (~1 nm). The HOMO peak shift is also saturated at 5 nm thickness as the SEC does. As a result, the HOMO shift of C60 was evaluated to be 0.05 eV, which indicates the band bending (Vb) to equilibrate the EF during the formation of C60/MAPbI3 interface. This C60 HOMO shift coincides with the SEC shift, meaning no interface dipole (eD). The HOMO onset is observed at 1.95 eV below the EF for the 10 nmthick C60 layer. To see the band offset changes with the variation of MAI concentration, the same measurements were carried out for the 10 mg ml-1 and 35 mg ml-1 MAPbI3 films as shown in Figure S8 and Figure S9 (see SI).


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Figure 4. UPS spectra of C60 (0.2, 0.5, 1, 2, 5 and 10 nm) on 25 mg ml-1 MAPbI3.

Combining all spectral changes in Figure 4, Figure S8 and Figure S9, the energy level diagrams were illustrated in Figure 5. All energy levels except the C60 LUMO were obtained from the current measurements whereas the C60 LUMO was estimated from the reported transport gap of C60 (2.30 eV).23,36 The eD was evaluated from the relationship of eD = −(∆Ψ + Vb), where ∆Ψ denotes the Ψ change measured from the SEC shift. Since electrons are transported from MAPbI3 to C60 during device operation, we focused on their conduction band-LUMO offset (Φe). For 10 mg ml-1 MAPbI3, the CBM and VBM are located at 0.60 eV above the EF and 1.40 eV below the EF, and the Ψ is 4.45 eV. At the C60/(10 mg ml-1 MAPbI3) interface (a), the C60 HOMO is located at 1.95 eV below the EF. The eD is measured to be 0.05 eV while no Vb is observed. As a result, the Φe between MAPbI3 and C60 is evaluated to be 0.25 eV. For 25 mg ml-1 MAPbI3, the CBM and VBM are located at 0.40 eV above the EF


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and 1.30 eV below the EF, and the Ψ is 4.55 eV. At the C60/(25 mg ml-1 MAPbI3) interface (b), the C60 HOMO is located at 1.90 eV (=1.95−Vb) below the EF, and no eD but small Vb of 0.05 eV is observed. Thus, although there is a slight difference of thickness-dependency in energy shift, the C60 contacts with 10 mg ml-1 MAPbI3 and 25 mg ml-1 MAPbI3 are essentially the same. This is attributed to their nearly identical Ψs of MAPbI3. The energy level alignment of deposited C60 is governed by the Ψ of underlying substrate,37 and the same is true for the perovskite underlayers. However, the Eg of 25 mg ml-1 MAPbI3 (device optimum) is smaller than that of 10 mg ml-1 MAPbI3 containing precursor residues as shown in Figure 1. Therefore, the CBM closer to EF results in the Φe-free interface at the C60/(25 mg ml-1 MAPbI3). On the other hand, for 35 mg ml-1 MAPbI3, the CBM and VBM are located at 0.85 eV above the EF and 1.00 eV below the EF, and the Ψ is high as 4.80 eV due to MA+ defects explained. The C60 HOMO is located at 1.65 eV at the C60/(35 mg ml-1 MAPbI3) interface (c), and both eD of 0.20 eV and Vb of 0.10 eV are observed. This different C60 HOMO position originates from the higher Ψ of 35 mg ml-1 MAPbI3 than those of other concentrations. As a result, the CBM of MAPbI3 and C60 LUMO forms the Φe of 0.20 eV. Overall, the energy level alignment and the resulting Φe are significantly affected by the Ψ and Eg of perovskite films due to the residual precursors and MA+ defect content described before.

Figure 5. Energy level diagrams of C60 on 10 mg ml-1 MAPbI3 (a), 25 mg ml-1 MAPbI3 (b) and 35 mg ml-1 MAPbI3 (c) (unit: eV).


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The evaluated Φes explain current device results quite well, no Φe for high device performance and large Φe for poor device performance, indicating that the band offset is a key parameter for PSC performance and hysteresis. The photo-generated electrons would be accumulated at the interface of C60/(10 mg ml-1 MAPbI3) and C60/(35 mg ml-1 MAPbI3) due to the large Φe. This large Φe impedes the charge extraction and the accumulated charges contribute to the backward sweep hysteresis. On the other hand, electrons can efficiently transport through the interface of C60/(25 mg ml-1 MAPbI3) due to the absence of Φe and thus negligible charge accumulation. This shows that understanding and controlling the electronic structure in PSCs is vital to optimize device structure. In summary, we investigated the charge extraction mechanism in MAPbI3 PSCs fabricated with two-step spin coating through electronic structure measurements and solar cell characterization. UPS, XPS and IPES measurements reveal that the electronic structure of MAPbI3 perovskite is significantly changed with the variation of MAI concentration. At low MAI concentration, the Eg of MAPbI3 is increased due to residual precursors. At high MAI concentration, MAPbI3 shows increased work function and p-doped electronic structure owing to MA+ defects. With varying MAI concentrations, the photovoltaic performance is dramatically changed: The highest PCE without hysteresis in J-V characteristics is observed for the PSC with 25 mg ml-1 MAI, whereas low PCEs and severe hysteresis are observed for the unoptimized PSCs with other concentrations. These phenomena originate from the changes in energy level alignments at the C60/MAPbI3 interface. The C60/(25 mg ml-1 MAPbI3) interface shows the absence of Φe for efficient electron extraction and conduction, which corresponds well to the highest PCE and hysteresis-free J-V characteristics in PSCs. However, the large Φes of C60/MAPbI3 with other MAI concentrations induce low PCEs and severe hysteresis in PSCs. Therefore, the band offset minimization is extremely important for high-


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performance PSCs. This investigation on the correlation between device performance and the electronic structure of PSCs can contribute to future PSC studies, especially device optimization.

▪ ACKNOWLEDGMENTS This study was supported by National Research Foundation of Korea [NRF2015R1C1A1A01055026 and 2017R1A2B4002442, and 2017R1A5A1014862 (SRC program: vdWMRC center)] and Samsung Display Company and an Industry-Academy joint research program between Samsung Electronics and Yonsei University.

▪ ASSOCIATED CONTENT Supporting Information. Experimental details, log-scale UPS and IPES spectra, Tauc plots, exciton binding energy, XRD patterns, electronic structure of MAI and PbI2, XPS spectra of MAPbI3 films with different MAI concentrations, SEM images, HI evaluation, and UPS spectra of C60/MAPbI3 (10 mg ml-1 and 35 mg ml-1) (PDF).

▪ AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected] (H. L.)

*

E-mail: [email protected] (Y. Y.)

Notes The authors declare no competing financial interests.

▪ REFERENCES


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TOC graphics 49x49mm (300 x 300 DPI)


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Figure 1. (a) UPS and IPES spectra and (b) electronic structure of MAPbI3 perovskite films with different MAI concentrations. 277x421mm (300 x 300 DPI)



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Figure 2. Atomic ratio of N/Pb, EF, EA and IE of MAPbI3 perovskite films with different MAI concentrations from XPS measurements. 130x128mm (300 x 300 DPI)


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Figure 3. The schematic device structure and J-V curves of MAPbI3 PSCs with different MAI concentrations with forward scan (solid symbol-line) and reverse scan (hollow symbol-line) under 1 sun illumination. 180x102mm (300 x 300 DPI)



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Figure 4. UPS spectra of C60 (0.2, 0.5, 1, 2, 5 and 10 nm) on 25 mg ml-1 MAPbI3. 146x141mm (300 x 300 DPI)


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Figure 5. Energy level diagrams of C60 on 10 mg ml-1 MAPbI3 (a), 25 mg ml-1 MAPbI3 (b) and 35 mg ml-1 MAPbI3 (c) (unit: eV). 106x44mm (300 x 300 DPI)


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