Imaging the Anomalous Charge Distribution Inside CsPbBr3

Sep 28, 2017 - Highly luminescent CsPbBr3 perovskite quantum dots (QDs) have gained huge attention in research due to their various applications in op...
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Imaging the Anomalous Charge Distribution Inside CsPbBr3 Perovskite Quantum Dots Sensitized Solar Cells Shrabani Panigrahi,*,† Santanu Jana,‡ Tomás Calmeiro,† Daniela Nunes,† Rodrigo Martins,† and Elvira Fortunato*,† †

CENIMAT/i3N, Departamento de Ciência dos Materiais, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa and CEMOP/Uninova, Campus de Caparica, 2829-516 Caparica, Portugal ‡ Laboratoire de Physique des Solides, CNRS, Université Paris-Sud, Université Paris-Saclay, 91405 Cedex Orsay, France S Supporting Information *

ABSTRACT: Highly luminescent CsPbBr3 perovskite quantum dots (QDs) have gained huge attention in research due to their various applications in optoelectronics, including as a light absorber in photovoltaic solar cells. To improve the performances of such devices, it requires a deeper knowledge on the charge transport dynamics inside the solar cell, which are related to its power-conversion efficiency. Here, we report the successful fabrication of an all-inorganic CsPbBr3 perovskite QD sensitized solar cell and the imaging of anomalous electrical potential distribution across the layers of the cell under different illuminations using Kelvin probe force microscopy. Carrier generation, separation, and transport capacity inside the cells are dependent on the light illumination. Large differences in surface potential between electron and hole transport layers with unbalanced carrier separation at the junction have been observed under white light (full solar spectrum) illumination. However, under monochromatic light (single wavelength of solar spectrum) illumination, poor charge transport occurred across the junction as a consequence of less difference in surface potential between the active layers. The outcome of this study provides a clear idea on the carrier dynamic processes inside the cells and corresponding surface potential across the layers under the illumination of different wavelengths of light to understand the functioning of the solar cells and ultimately for the improvement of their photovoltaic performances. KEYWORDS: CsPbBr3 QDs, perovskite solar cell, Kelvin probe force microscopy, surface potential, charge carrier dynamics, light illumination

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excitonic radius), their carrier dynamic processes become improved.15 There are very few reports on the CsPbBr3 perovskite solar cells.20−22 Liang et al.20 observed the longterm stability of the bulk CsPbBr3 perovskite solar cell compared to that of organic−inorganic hybrid halide perovskite-based cells. Recently, Akkerman et al.22 developed CsPbBr3 perovskite nanocrystal-based solar cells and observed their power-conversion efficiency (PCE) ranging from 0.67 to 5.4% with the change of the active layer thickness.22 However, to improve these cells, a detailed explanation on the charge transfer dynamics across the layers of the perovskite QD sensitized solar cell is still lacking. In a high-performance perovskite solar cell, the charge transport dynamics depend on the perfect band alignment across the layers of the perovskite

ybrid organic−inorganic halide perovskite materials have been considered as wonderful semiconductors with high performance for solar cells and other optoelectronic applications since the last few years.1−10 Their low exciton binding energy and ambipolar characteristics help charge carrier formation and transport in devices easily.11−13 Compositional variations such as mixed halide and mixed cation etc. were also studied to enhance the photovoltaic performances of the cells and for their long-term stability. However, thermal instability has been observed after certain temperatures for mostly used organic−inorganic perovskites (e.g., methylammonium lead halide).14 Recently, as an alternative, all inorganic cesium lead halide perovskites have become an exciting material in research due to their higher thermal stability, excellent optical properties, such as large optical absorption, high quantum yields, and long carrier diffusion length and mobility.15−19 When these perovskite materials are confined in a nanoscale space (below Bohr © 2017 American Chemical Society

Received: July 7, 2017 Accepted: September 25, 2017 Published: September 28, 2017 10214

DOI: 10.1021/acsnano.7b04762 ACS Nano 2017, 11, 10214−10221

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ACS Nano absorber, electron transport layer (ETL), and hole transport layer (HTL), which makes it easy to separate the photoinduced charge carriers and transport them to the respective electrodes. Efficient charge transfer process also depends on the generation and extraction of the charge carriers at the interfaces.23−25 There are many spectroscopic techniques to investigate the photoinduced charge transfer dynamics inside the devices, which include electron-beam-induced current (EBIC) measurements, photoelectron spectroscopy (PES), surface photovoltage (SPV) technique, etc. Recently, EBIC measurements have been employed to analyze the local electrical properties of different devices.26−28 On the other hand, Jiang et al.29 and Gorji et al.30 used SPV technique to investigate the photoinduced charge transfer process inside the devices. However, Kelvin probe force microscopy (KPFM), a modified version of atomic force microscopy (AFM), is a noncontact surface technique used to measure the local contact potential difference (CPD) between a conducting AFM tip and the sample.31−33 The spatial resolution of this microscopy is higher than the other techniques, such as EBIC and PES, and it allows simultaneous imaging of the topography and surface potential with nanoscale resolution.31 In recent times, KPFM has been employed to observe the surface potential distribution across the layers of solar cell devices like inorganic, organic heterojunction, and bulk perovskite sensitized solar cells.34−40 To clarify the various charge transfer dynamics at the interfaces of the solar cells under different wavelengths of light in the range of the solar spectrum, we have observed the CPD distribution across the cross section of the solar cells using a planar ITO/TiO2/CsPbBr3 QDs/spiro-MeOTAD/Au structure as the framework. The influence of the different spectra of light on the generation and transport processes of the charge carriers inside the solar cell have been investigated here. Under steadystate solar illumination, a sharp difference in electrical potential is observed across the active layers of the solar cell. Opposite polarities of accumulated charges (electrons and holes) in two regions (ETL and HTL) of the device make the potential difference. The accumulation of holes in the perovskite layer indicates an unbalanced carrier separation at the junction. However, under the illumination of monochromatic light (ultraviolet (UV), green, and red light in solar spectrum), the difference in surface potential between ETL and HTL is less due to low carrier transport at the interfaces.

Figure 1. (a) Optical absorption and steady-state PL spectra of CsPbBr3 QDs dispersed in toluene. (b) Photograph of the CsPbBr3 QDs dispersed in toluene. (c) Time-resolved PL spectra recorded at emission energy (at the PL peak) for the CsPbBr3 QDs in toluene solution. (d) TEM image and X-ray diffractogram (inset) of CsPbBr3 QDs. (e) HRTEM image of the CsPbBr3 QDs.

CsPbBr3 QDs.41 We have further investigated the time-resolved PL decay of the CsPbBr3 QDs (Figure 1c) with the excitation of 400 nm, and the average radiative lifetime was 12.6 ns. According to transmission electron microscopy (TEM) images shown in Figure 1d,e, the as-obtained QDs have an average diameter of 7.2 ± 1.5 nm, with a cubic shape. High-resolution TEM image (Figure 1e) shows the high crystalline quality of the QDs, and they are very much monodispersed in solution. The size of the QDs is comparable to the exciton Bohr radius (∼7.4 nm).42 The crystallinity of the QDs was further confirmed by X-ray diffraction (XRD) analysis. The X-ray diffractogram of the QDs is presented in the inset of Figure 1d, showing cubic crystalline structure. We have fabricated the solar cell using these CsPbBr3 perovskite QDs on ITO-coated glass substrate serving as the transparent electrode by following the usual stacking of ITO/ TiO2/perovskite QDs/spiro-MeOTAD/Au. A full detail of the device fabrication is available in the Experimental Section. Figure 2a shows the cross-sectional scanning electron microscopy (SEM) image of the solar cell where all the layers have been clearly identified. The electrical measurement of the solar cell showed a high open-circuit potential of VOC = 0.859 V, a short-circuit current density of JSC = 8.55 mA/cm2, and a fill factor of 0.57 (Figure 2b), leading to a PCE of 4.21%. To observe the surface potential depth profile across the cross section of the solar cell using KPFM, at first we cleaved the cell and polished it mechanically. After that, it was mounted in a metallographic mounting resin in such a way that it can expose the cross section of the cell. KPFM measurements were performed on the flat cross-sectional areas of the solar cell. Figure 2c shows the schematic diagram of the cross-sectional surface of the solar cell under operating condition of KPFM with an applied voltage between tip and ITO. The KPFM tip moved easily across the cross section of the cell and measured CPD between the tip and the sample. When the tip is brought close to the sample surface, an electrical force is generated between them due to the difference of their work function. Figure 2d shows the energy level diagram of the different layers of the solar cell. The numerical values of CB and VB for the

RESULTS AND DISCUSSION A colloidal hot injection method was used to synthesize CsPbBr3 perovskite QDs (details in Experimental Section). The absorption and emission spectra in Figure 1a were recorded on the purified colloidal CsPbBr3 QDs dispersed in toluene. The absorption spectrum shows two peaks at 510 nm near the longwavelength edge and another small peak located at 430 nm, which is due to optically allowed transition between discrete defect levels. From the peak position of absorption spectra and bulk band edge position, we have calculated the potential of conduction band (CB) and valence band (VB) with respect to the vacuum level that are at ∼3.6 and ∼6 V, respectively. The QDs dispersed in toluene show the emission peak centered at 518 nm with a narrow fwhm value of only ∼25 nm (Figure 1a). It shows an intense green color emission in daylight (Figure 1b), and the dispersion was found to exhibit the best photoluminescence (PL) quantum yields (QY) of ∼83% at 400 nm excitation. It is reported that high PL QYs can be attributed to minor electron or hole trapping pathways in 10215

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Figure 2. (a) Cross-sectional SEM micrograph of the solar cell. (b) J−V characteristics and the value of photovoltaic parameters (JSC, VOC, FF, η) of the solar cell. (c) Schematic diagram of the cross-sectional KPFM measurement system. (d) Energy level diagram of the device layer structure.

Figure 3. (a,b) AFM topography and the corresponding line profile across the cross section of the solar cell. (c,d) Phase contrast image and the corresponding line profile of the solar cell. (e,f) Surface potential image and potential depth profile across the layers of the solar cell under dark condition. (g,h) Surface potential image and potential depth profile across the layers of the solar cell under solar illumination.

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Figure 4. (a) (i) Vacuum level alignment along n-type/perovskite/p-type material in solar cell before contact. (ii) Fermi level alignment after contact in the dark (equilibrium condition), (iii) separated quasi-Fermi levels for electrons and holes under illumination. (b) Schematic diagram for charge separation in the solar cell under the illumination of the full solar spectrum (white light).

electrodes and the charge-extracting layers have been used according to the previous literature.22,39 The alignment of these levels makes it easy to separate charge carriers across the interfaces. KPFM measurement was carried out by Olympus AC240TM probes (details in Experimental Section) with an AC tip voltage tuning at 3 V in a double pass mode in air ambient. The topography image in Figure 3a shows the different layers across the cross section of the solar cell. The layers in the topography image are related with the positions of the material interfaces of the device shown in above cross-sectional SEM image. The height difference among the layers is shown in the corresponding line profile in Figure 3b. The phase image in Figure 3c shows a sharp contrast among the layers due to different levels of energy dissipation between the tip and the organic−inorganic layers.43−45 The corresponding line profile of the phase channel is shown in Figure 3d. KPFM measures the CPD between the tip and sample surface, which is equal to the difference of their work function ϕTip and ϕSample by probing and nullifying the Coulomb force between them (Figure S2). During the measurement, the work function of the tip remains constant. The surface potential is determined by the amount of trapped charge carriers at the surface of each layer. Panels e,f and g,h of Figure 3 show the CPD images and the corresponding line profiles across the cross section of the solar cell under dark and white light illumination conditions, respectively. In an equilibrium condition, the Fermi levels are aligned across any interfaces of the layers in the solar cell, and consequently, there is a drop in vacuum level due to their different work function (Figure 4a). Under dark condition, the potential variation is just linear, which confirms a homogeneous electric field present throughout the device. The line profile of the surface potential across the cross section of the solar cell

under dark condition also demonstrates the same nature (Figure 3f). However, a huge difference in surface potential is observed across the layers of the solar cell under white light illumination. When photons hit the solar cell, the perovskite QD layer absorbs light and electron−hole pairs are generated in the material. After that, charge separation occurs by following two steps: injection of photogenerated electrons into TiO2 (ETL) and injection of holes into spiro-MeOTAD (HTL).46,47 That means excitons have dissociated into two separate charge carriers with opposite polarity, which move to the appropriate contacts through the layers. Hence, the contacts must be selective for a specific carrier. An increase in CPD in the perovskite layer and HTL is related to the presence of extra positive charge carriers (holes), and they can be extracted to the Au electrode. Accumulation of holes in the perovskite layer signifies that the efficiency to extract the holes from the perovskite layer via spiro-MeOTAD is less than that for electron extraction from the perovskite layer through TiO2; consequently, an irregular charge transport occurred in the device. Accumulation of the charge carriers with two opposite polarity (electron and hole) in ETL and HTL, respectively build the potential difference between them. The shift of the work function is mostly related to the carrier density (Figure 4a,iii).48 The corresponding line profile (Figure 3h) also demonstrates the difference in the surface potential between ETL and HTL. It shows an efficient charge separation across the junction of the active layers. Marchioro et al.47 also investigated that the most efficient charge separation occurred in this device, where perovskite is in contact with both TiO2, as an electron acceptor and transporter and organic HTL. The schematic diagram for the charge separation and transport in a perovskite sensitized solar cell is shown in Figure 4b. In contrast, to check the excitation wavelength-dependent charge carrier distribution across the cross section of the solar 10217

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Figure 5. (a−c) Surface potential images across the cross section of the solar cell under shorter (UV), medium (green), and longer (red) wavelength of light, respectively. (d−f) Potential depth profiles across the layers of the solar cell under UV, green, and red light illuminations, respectively.

profile, it is clear that the difference in surface potential between the active layers under green light illumination is better than that of the UV light illumination but poorer than that of the white light illumination. Therefore, the charge transfer from the perovskite to ETL and HTL is strongly dependent on the electronic charge distribution at the VB and CB maxima of perovskite layer. However, when we have used a longer wavelength of light (red LED, λ = 660 nm) to excite the perovskite material, charge carrier generation was not possible due to its insufficient energy to excite the perovskite absorber material (Figure S3c). From the absorption spectra of CsPbBr3 perovskite, it is clear that there is no absorption range after 530 nm. Therefore, there is no such change in surface potential across the layers of the solar cell due to no generation of carriers in the perovskite layer. It is clear from this observation that perovskite generates more charge carriers in steady solar illumination than any monochromatic light illumination and shows an efficient charge transport dynamics inside the solar cells. To enhance the carrier generation as well as the performances of the cells, low band gap perovskite nanocrystals with superior properties are essential, which will result in highly efficient solar cells in future.

cell, we have illuminated the device with three different wavelengths of light in the range of the solar spectrum (λ = 365, 505, and 660 nm), separately. Figure 5a−c shows the CPD images across the cross section of the solar cell under shorter (UV), medium (green), and longer (red) wavelength of light in the solar spectrum, respectively, and the corresponding line profiles are shown in Figure 5d−f. To selectively excite the device, at first we have used shorter wavelength of light in the solar spectrum (UV LED, λ = 365 nm) as an excitation wavelength. When UV light passed through the window layer, TiO2 absorbed UV light because of its suitable band gap and perovskite was also excited due to the absorption of UV light. In this case, the probability of charge transfer from the perovskite to TiO2 is prohibited as the conduction band of TiO2 is already occupied by the excited state electrons (Figure S3a). The lower CPD in TiO2 is due to the presence of excess electrons, which changed its surface potential. Thus, the charge separation is poor across the junction of perovskite/ETL and perovskite/HTL. The corresponding CPD image and the line profile show no such change in surface potential across the active layers of the solar cell. When we have selected a middle range wavelength of light in solar spectrum (green LED, λ = 505 nm) to excite the device, CsPbBr3 absorbed the light due to its appropriate band gap and generated electron−hole pairs. However, under the illumination of monochromatic light, charge carrier generation in CsPbBr3 perovskite QDs is lower than that in the full spectrum of light due to its large optical absorption range from UV to visible up to λ = 530 nm. Thus, after the separation of the charge carriers, a small amount of gathered electrons in TiO2 and holes in HTL shift their work function and make the potential difference between them (Figure S3b). From the CPD image and the corresponding line

CONCLUSION In summary, we have successfully developed fully inorganic CsPbBr3 perovskite QD sensitized solar cells and showed the electrical potential distribution across the layers of the solar cell under different wavelengths of light in the solar spectrum. Using KPFM, we were able to observe the anomalous surface potential depth profile and correlated with the local structure of the layers of the solar cell. The selective transfer of photoinduced charge carriers to the two different electrodes 10218

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compact layer was deposited by spin-coating technique using the solution of titanium isopropoxide [Ti(OC3H7)4] in 2-methoxy ethanol at 3500 rpm for 30 s. Then, the samples were dried at 125 °C for 15 min, followed by an annealing treatment at 500 °C for 30 min. The TiO2 mesoporous layer was deposited by spin-coating technique with a solution of the TiO2 powder (200 mg) in water (1.2 mL), acetylacetone (90 μL), and Triton X-100 (3 drops) at 3500 rpm for 30 s. After that, the heating (125 °C for 15 min) and annealing (500 °C for 30 min) treatment were followed. Next, the perovskite solution (CsPbBr3 suspension solution) was deposited by spin-coating technique at 3500 rpm for 30 s. To increase the layer thickness, we followed several deposition cycles. The samples were dried for 5 min between each spinning. For hole transporting materials, 50 μL of the spiro-MeOTAD solution was spin-coated at 3500 rpm for 30 s. The spiro-MeOTAD solution was made by dissolving 72.3 mg of spiro-MeOTAD in 1 mL of chlorobenzene; 17.5 μL of LiTFSI stock solution (520 mg of LiTFSI in 1 mL of acetonitrile) and 28.8 μL of 4-tBP were also added to the solution as additives. The cells were finally completed by depositing 100 nm of gold as a cathode using thermal evaporation (Vinci Technologies) technique under a vacuum. Characterization Details. Optical Characterization. The absorption spectra were taken using a UV−vis−NIR spectrophotometer (Agilent Cary series). Samples were prepared by diluting the QD solutions in toluene. Photoluminescence spectra were collected using a Horiba Jobin Yvon Fluoromax-4 spectrofluorometer. The PL QY of the perovskite QDs was measured using a quantum yield measurement accessory (Fluoromax-4) with a 150 W xenon lamp coupled to a monochromator for wavelength discrimination and a 10 cm integrating sphere as sample chamber. Lifetime Measurement. For the time-correlated single photon counting measurements, the samples were excited by 370 nm based picosecond diode laser (IBH NanoLED-07) in an IBH Fluorocube apparatus (JY-IBH-5000M). The fluorescence decays were collected on a Hamamatsu MCP photomultiplier. The fluorescence decays were analyzed using DAS6 software. The average lifetime was calculated using equation below. The data are fitted with second-order exponential decay.

of a solar cell has been credited the potential difference between active layers. Due to gathering of excessive number of electrons and holes generated by the illumination, the Fermi energy splits up into two quasi-Fermi energies. Under steadystate solar illumination, the difference in CPD between ETL and HTL is higher than any single wavelength illumination. When we have selected the longer wavelength of light for excitation, more than the absorption range of CsPbBr3, there is no change in CPD across the layers of the solar cell because of no carrier generation in perovskite layer. However, we have observed an unbalanced charge carrier separation across the perovskite layer under steady-state solar illumination. The accumulation of extra holes in this layer developed a potential barrier which is one of the causes for poor photovoltaic performances of the devices. New materials for HTL in the future might be helpful to overcome this problem by extorting more positive charge carriers effectively from the perovskite material. It will be simultaneously effective to increase the lightharvesting abilities of perovskite and enhance the performances of the corresponding solar cell. Our report on the anomalous charge distribution inside the solar cells under different illuminations helps to understand the basic charge transport mechanism across the interfaces and shows the possibility to design high-performance fully inorganic perovskite QD sensitized solar cells in the future.

EXPERIMENTAL SECTION Materials. Indium tin oxide (ITO)-coated glass substrate, titanium isopropoxide [Ti(OC3H7)4], 2-methoxyethanol (C3H8O2), acetic acid (CH3COOH), TiO2 powder, acetylacetone (C5H8O2), and Triton X100 were employed for the preparation of compact and mesoporous TiO2 layer. Cesium carbonate (Cs2CO3, 99.9%), octadecene (C18H36, 90%), oleic acid (C18H34O2), lead(II) bromide (PbBr2), and oleylamine (C18H37N) were employed for the preparation of CsPbBr3 perovskite. For HTL, spiro-MeOTAD, 4-tert-butylpyridine (4-tBP, 96%), bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI), and chlorobenzene (C6H5Cl) were employed. Deionized water from Millipore equipment was used. All the chemicals and solvents were purchased from Sigma-Aldrich and were employed without any further treatment or purification. Synthesis of CsPbBr3 Perovskite QDs. CsPbBr3 perovskite QDs were synthesized according to the previously reported hot injection colloidal synthesis procedure.49 The QDs were prepared by using twostep methods: at first the preparation of Cs-oleate and then synthesis of CsPbBr3 QDs. Cs2CO3 (0.814 g,) was loaded into 100 mL threeneck flask along with octadecene (40 mL, ODE) and oleic acid (2.5 mL, OA), dried for 1 h at 120 °C, and then heated under N2 up to 150 °C until all Cs2CO3 reacted with OA. Since Cs-oleate precipitates out of ODE at room temperature, it has to be preheated to 100 °C before injection. ODE (5 mL) and PbBr2 (0.069 g, 0.188 m mol) were loaded into 25 mL three-neck flask and dried under vacuum for 1 h at 120 °C. Dried oleylamine (0.5 mL) and dried oleic acid (0.5 mL) were injected at 120 °C under N2. After complete solubilization of a PbBr2 salt, the temperature was increased to 160 °C, and Cs-oleate solution (0.4 mL, 0.125 M in ODE, prepared as described above) was quickly injected; 5 s later, the reaction mixture was cooled by the ice−water bath. The crude solution was cooled with a water bath, and NCs were separated by centrifugation by adding a small amount of acetone. After centrifugation, the supernatant was discarded and the particles were redispersed in toluene or hexane, forming long-term colloidal stable solutions. Device Fabrication. After the ITO-coated glass substrates were etched, they were thoroughly washed with soap solution and then rinsed by deionized water, acetone, and isopropyl alcohol and finally dried with compressed air. Before the compact layer deposition, the substrates were cleaned with a UV-ozone treatment for 15 min. TiO2

y = a1 × exp(− x /τ1) + a 2 × exp(− x /τ2) τaverage =

Σaiτi 2 Σaiτi

(1)

Structural and Microstructural Characterization. TEM images were taken on a JEOL JEM-2010 electron microscope using a 200 kV accelerating voltage. Specimens were prepared by dipping a Formvarcoated copper grid into a toluene solution of the QDs, and the grid was dried in air. Structural characterization of the perovskite material was performed with X-ray diffractograms using the computer-controlled Panalytical Xpert PRO system (Cu Kα radiation; λ = 1.5405 Å) and X’Celerator 1D detector. Cross-sectional analysis of the device was performed by SEM using a Carl Zeiss AURIGA cross beam workstation. For focused ion beam experiments, Ga+ ions were accelerated to 30 kV at 100 pA, and the etching depth was kept around 1200 nm. Electrical Characterization. For electrical characterization, a digital source meter (Keithley, model 2400) was used to apply the voltage to the cell while the current was recorded. Illumination was provided through AM 1.5 illumination conditions with a Sciencetech SS1.6 kWA-2-Q system consisting of a Xe lamp with a light intensity of 100 mW cm−2. KPFM. For KPFM study, after the cross-sectional surface of the solar cell was polished, the sample was transferred to the atomic force microscopy setup, where it was fixed in a sample holder with electrical connections to both of the electrodes. KPFM was done using Olympus AC240TM probes (spring constant = 2 N/m, resonant frequency = 70 kHz) in an Asylum Research MFP-3D stand-alone system with an AC tip voltage tuning at 3 V in a double pass mode. We have illuminated the device with a Fiber-Lite MI-150R light source for white light 10219

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ACS Nano illumination. For single wavelength illuminations, we have used UV (λ = 365 nm), green (λ = 505 nm), and red (λ = 660 nm) collimated LEDs (THORLABS). Before all illumination and imaging sessions, the device was kept in the dark for a few hours to stabilize the sample.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b04762. Supporting Figures S1−S4 (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Shrabani Panigrahi: 0000-0003-2294-8348 Santanu Jana: 0000-0002-4136-1852 Daniela Nunes: 0000-0003-3115-6588 Elvira Fortunato: 0000-0002-4202-7047 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was funded by National Funds through FCTPortuguese Foundation for Science and Technology, Reference UID/CTM/50025/2013 and FEDER funds through the COMPETE 2020 Programme under the project number POCI-01-0145-FEDER-007688 as well as by the BI_86_16 under the project BET-EU-Materials Synergy Integration for a better Europe, reference number GA 692373. S.J. acknowledges European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie, Grant Agreement No. 661199. REFERENCES (1) Liu, D.; Yang, J.; Kelly, T. L. Compact Layer Free Perovskite Solar Cells with 13.5% Efficiency. J. Am. Chem. Soc. 2014, 136, 17116− 22. (2) Zhang, W.; Eperon, G. E.; Snaith, H. J. Metal Halide Perovskites for Energy Applications. Nat. Energy 2016, 1, 16048. (3) Berhe, T. A.; Su, W.-N.; Chen, C.-H.; Pan, C.-J.; Cheng, J.-H.; Chen, H.-M.; Tsai, M.-C.; Chen, L.-Y.; Dubale, A. A.; Hwang, B.-J. Organometal Halide Perovskite Solar Cells: Degradation and Stability. Energy Environ. Sci. 2016, 9, 323−356. (4) Yang, L.; Barrows, A. T.; Lidzey, D. G.; Wang, T. Recent Progress and Challenges of Organometal Halide Perovskite Solar Cells. Rep. Prog. Phys. 2016, 79, 026501. (5) Xing, J.; Yan, F.; Zhao, Y.; Chen, S.; Yu, H.; Zhang, Q.; Zeng, R.; Demir, H. V.; Sun, X.; Huan, A.; Xiong, Q. High-Efficiency LightEmitting Diodes of Organometal Halide Perovskite Amorphous Nanoparticles. ACS Nano 2016, 10, 6623−30. (6) Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; Grätzel, M.; Park, N.-G. Lead Iodide Perovskite Sensitized AllSolid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (7) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050−6051. (8) Pellet, N.; Gao, P.; Gregori, G.; Yang, T.-Y.; Nazeeruddin, M. K.; Maier, J.; Grätzel, M. Mixed-Organic-Cation Perovskite Photovoltaics for Enhanced Solar-Light Harvesting. Angew. Chem., Int. Ed. 2014, 53, 3151−3157. 10220

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