Temperature Dependent Diffusion of DMSO in CH3NH3PbI3

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Cite This: ACS Appl. Energy Mater. 2019, 2, 5116−5123

Temperature Dependent Diffusion of DMSO in CH3NH3PbI3 Precursor Films During Layer Formation and Impact on Solar Cells Qin Tan,† Karsten Hinrichs,‡ Huang Mao-Dong,‡ Steffen Fengler,§ Joerg Rappich,† Pongthep Prajongtat,∥ Norbert H. Nickel,† and Thomas Dittrich*,†

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†

Institut für Si-Photovoltaik, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Kekuléstrasse 5, 12489 Berlin, Germany ‡ Leibniz-Institut für Analytische Wissenschaften−ISAS−e.V., Schwarzschildstrasse 8, D-12489 Berlin, Germany § Institut für Werkstoffforschung, Helmholtz-Zentrum Geesthacht, Zentrum für Material-und Küstenforschung, Max-Planck-Strasse 1, D-21502 Geesthacht, Germany ∥ Department of Materials Science, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand S Supporting Information *

ABSTRACT: The temperature dependent out-diffusion of dimethyl sulfoxide (DMSO) from CH3NH3PbI3 precursor layers was investigated by analyzing the S/Pb molar ratio in the layers by high-resolution continuum source absorption spectroscopy (HR-CSAS) and the evolution of the SO vibrational mode in the layers with infrared spectroscopic ellipsometry (IRSE). The diffusion coefficients were extracted by applying a diffusion model in a homogeneous layer. At 100 °C, for example, the diffusion coefficient of DMSO in CH3NH3PbI3 amounted to about 10−11 cm2/s. The diffusion constant was thermally activated by two processes with activation energies of 0.6 and 1.8 eV, respectively. The lower and higher activation energies can be explained by decomposition of DMSO complexes and by the activation of DMSO incorporated in the perovskite lattice structure. A strong influence of the S/Pb molar ratio on the fill factor and its standard deviation was observed for solar cells with CH3NH3PbI3 layers. With regard to the performance of solar cells with high efficiency, it seems that some residual DMSO is useful for the preparation of homogeneous CH3NH3PbI3 layers and for passivation of defect states in the material. KEYWORDS: lead halide perovskite, DMSO, infrared spectroscopy, molecular diffusion, solar cells

1. INTRODUCTION Hybrid organic−inorganic metal halide perovskites, such as methylammonium lead iodide (CH3NH3PbI3), are semiconductors which can be synthesized from precursor salt solutions, including methylammonium iodide (CH3NH3I) and lead iodide (PbI2)1 with excellent electronic properties as shown, for example, by the high efficiencies achieved for solar cells.2 As an example, Figure 1a shows schematically the formation of CH3NH3PbI3 from a solution containing dimethyl sulfoxide (DMSO) and dimethylformamide (DMF), a common mixture of solvents for precursor salts of CH3NH3PbI3. After spin-coating and vacuum flash,3 the constituents freeze due to the fast evaporation of excess DMSO and DMF and a layer of mainly CH3NH3I−PbI2− DMSO is formed due to the higher boiling temperature of DMSO.4 The formation of intermediate adducts avoids the rapid reaction between PbI2 and CH3NH3I during the following postannealing, leading to a better control of the crystallization process by a softer evaporation of the residual solvent. However, the evaporation of solvent molecules is limited by their transport through the forming layer, and some © 2019 American Chemical Society

residual solvent molecules will remain in the perovskite layer after the annealing step. DMSO molecules remaining in the CH3NH3PbI3 perovskite layer can stay at different sites. For illustration, Figure 1b,c shows DFT simulations of the idealized structure of CH3NH3PbI3, the structure with a CH3NH3+ vacancy and the structure with a DMSO molecule replacing CH3NH3+ (see the Supporting Information for details). For the given idealized structures, the binding energy of a DMSO molecule replacing a CH3NH3+ cation was found to be about −3.58 eV; i.e., DMSO molecules can be trapped and stabilized, for example, at the positions of CH3NH3+ vacancies during the formation of the perovskite structure. Only little is known about residual solvent molecules in metal halide perovskite layers and their impact on the behavior of solar cells. One of the reasons for this is the difficulty to measure precisely the concentration of solvent molecules in Received: April 23, 2019 Accepted: June 4, 2019 Published: June 4, 2019 5116

DOI: 10.1021/acsaem.9b00769 ACS Appl. Energy Mater. 2019, 2, 5116−5123

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ACS Applied Energy Materials

reflectivities. Working with ratios in short order measured polarization dependent intensities for IRSE has the advantage that the background effects can be minimized. This allows for relatively fast and highly sensitive measurements under rather robust conditions. Furthermore, bands in IRSE related to molecules diluted in thin films show a practically linear dependence of the amplitude with respect to the concentration. Therefore, IRSE was chosen as a method to measure the relative change of the amount of DMSO molecules remaining in perovskite layers after a certain annealing step. A simple model was developed for describing diffusion in a homogeneous layer with one reflecting boundary and one boundary acting as an ideal sink. The investigation of the diffusion of solvent molecules in forming layers demands excellent reproducibility of the initial boundary conditions. The so-called vacuum flash technique is based on the quick evaporation of physisorbed solvents from the surface of a perovskite precursor layer14,15 and therefore very suitable for the diffusion experiments of this work without introducing additional molecules for so-called antisolvent dripping. The vacuum flash technique is very reproducible and suitable for upscaling and in combination with the preparation of monolithic tandem solar cells.16 The precursor salts were dissolved in a DMSO/DMF solution, and the layers were prepared by applying the vacuum flash technique. Solar cells with high efficiency and different contents of DMSO in the perovskite layer were prepared in order to demonstrate a possible influence of DMSO.

Figure 1. Schematic of the formation of a CH3NH3PbI3 perovskite structure from a solution containing [PbI6]4−, DMSO, DMF, CH3NH3+, and I− after formation of a layer of adducts and after postannealing (a) and idealized structure of perovskite (b), perovskite with a CH3NH3+ vacancy (c), and perovskite with a DMSO molecule replacing a CH3NH3+ (d). Part a adapted by permission from ref 5. Copyright 2014 Springer Nature.

2. EXPERIMENTAL METHODS AND SIMULATION ANALYSIS 2.1. Sample Preparation and Annealing. Glass substrates coated with SnO2:In (ITO, sheet resistance of 15 Ω/sq, Automatic Research, size 2.5 × 2.5 cm2) were cleaned in an ultrasonic bath by washing subsequently in soap water, deionized water, acetone, and isopropanol for 15 min. For the preparation of solar cells, the ITO layers were structured in order to define the areas for the solar cells. After cleaning, the substrates were dried in a N2 flush, treated with O3 for 15 min, and coated with Au (thermal evaporation, layer thickness 100 nm) for IRSE measurements. The Au layer was omitted for the preparation of solar cells. A poly[bis(4-phenyl)(2,4,6trimethylphenyl)amine] (PTAA, Sigma-Aldrich) layer was spincoated (4000 rpm for 30 s) onto all substrates from a toluene solution (2 mg/mL) and annealed at 100 °C for 10 min in a N2-filled glovebox. The 1 M perovskite precursor solution was prepared with PbI2 (TCI) and CH3NH3I (dyenamo) in the molar ratio of 1:1 in a mixed solvent of DMF (Sigma-Aldrich) and DMSO (Sigma-Aldrich) with a volume ratio of 9:1, which was optimum for the preparation of solar cells with high efficiency. The precursor solution was spin-coated onto the PTAA layer at 3000 rpm for 7 s, and the substrate was transferred immediately into a small chamber within 2 s. The chamber was connected to a buffer tank with a volume of 80 L kept at low pressure (1.8 × 10−2 mbar). The pressure in the chamber reached 0.6 mbar immediately after connecting to the buffer tank. After evacuation for 60 s, the sample was taken out and annealed on a hot plate. The application of the identical vacuum flash for each sample provided excellent reproducibility of the samples before the final annealing step. The annealing temperature and the annealing time were varied between 60 and 110 °C and between 1 and 60 min, respectively. During annealing, CH3NH3PbI3 layers with thicknesses of about 400 nm were formed. Figure 2 shows the temperature calibration of the hot plate, i.e., the evolution of the temperature at the surface of a dummy sample for different annealing temperatures. The annealing temperature was reached in the experiments within 15−20 s.

perovskite films. As a model system in this work, the content of DMSO was investigated in CH3NH3PbI3 layers formed at moderate annealing temperatures. For this purpose, a methodology for the measurement of the DMSO content in CH3NH3PbI3 layers has been developed and applied to the analysis of the temperature dependent diffusion coefficients of DMSO in CH3NH3PbI3 layers. Furthermore, the influence of the final content of DMSO on the performance of solar cells was demonstrated. The absolute concentration of DMSO in CH3NH3PbI3 layers was determined by the high-resolution continuum source absorption spectrometry (HR-CSAS). The HR-CSAS provides a novel approach not only for the determination of metallic elements, but also for some nonmetal elements, e.g., sulfur, phosphorus, and halogens due to its ability of measuring corresponding diatomic molecular absorption.6−9 In the present case, the determination of sulfur was accomplished by using the CS molecular absorption at a wavelength of 258.056 nm.10,11 The quantification of DMSO is based on the fact that one DMSO molecule contains one sulfur atom, and that DMSO is the only sulfur-containing ingredient in the thin CH3NH3PbI3 films prepared here. Additionally, the change of the specific infrared vibrational modes of DMSO in the CH3NH3PbI3 layers was investigated by infrared spectroscopic ellipsometry (IRSE)12,13 as a function of annealing time and annealing temperature. IRSE can determine the complex reflectance ratio of thin layers which is defined as the ratio of the p-polarized and s-polarized 5117


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dissolved by 100 μL of nitric acid (65%, Merck) and subsequently diluted in distilled water to a total volume of 5 mL. For the measurement of the peak volume selected absorbance (PVSA) of carbon sulfide or Pb, 20 μL of the S-sample solution with 5 μL of a palladium matrix modifier solution (Merck) or 5 μL of the Pbsample solution were filled into separate graphite tubes, respectively, and heated following the individually optimized temperature programs given in Table 1. The sensitivity for the measurement of

Table 1. Parameters of the HR-CSAS Analysis step

temperature (°C)

heating rate (°C s−1)

hold time (s)

drying step 1 drying step 2 pyrolysis

90 110 160 (for CS) 400 (for Pb) 2500 (for CS) 1900 (for Pb) 2600

4 4 200

20 30 10

3000

4

1000

3

vaporization atomization cleaning

Figure 2. Temperature calibration of a sample on the hot plate for annealing set-temperatures of 60, 80, and 100 °C (squares, circles, and triangles, respectively).

the amount of sulfur was increased by adding two S-sample solutions. The concentrations of sulfur and lead were calibrated with aqueous S (based on DMSO) and Pb standard solutions, respectively. 2.3. Infrared Spectroscopic Ellipsometry (IRSE). The IRSE setup is a custom-built infrared spectroscopic ellipsometer attached to an FTIR (Vertex 70 from BRUKER, Germany) equipped with a liquid nitrogen cooled photovoltaic mercury cadmium telluride (MCT) detector and permanently purged with dry air. The ellipsometric parameter tan Ψ, defined as the amplitude ratio of the reflected p- and s-polarized components, was measured at a spectral resolution of 4 cm−1, an incidence angle of 80° for 4 cycles of 64 scans. Further details on the ellipsometric method can be found in ref 12. Compared to a dense DMSO layer, the vibrational absorption of DMSO molecules diluted in the perovskite layer is much smaller because of the relatively low density of vibrational dipole moments. For an isotropic thin layer on a metallic substrate, then, the bands in a tan Ψ spectrum would correlate almost linearly with the vibrational absorption. The relative accuracy of the linear correlation between the integrated density of DMSO molecules homogeneously diluted in CH3NH3PbI3 and the IRSE signals at the SO vibrational mode better than 5% was estimated by using the Brüggemann effective medium approximation, a layer thickness of 400 nm, and a refractive index of the CH3NH3PbI3 equal to 2. Incidentally, the detailed IRSE analysis of CH3NH3PbI3 layers is beyond the scope of this work. 2.4. Diffusion Analysis. The concentration of DMSO molecules (C) is a function of time (t) and space (x). For the diffusion model, a homogeneous layer, a homogeneous distribution of DMSO in the layer before starting diffusion, a reflecting boundary at the layer interface with the substrate, and a sink at the surface where DMSO molecules disappear are assumed. In the simulation, t increases stepwise by Δt and the layer is separated into slices with the thickness Δx. The index i denotes the number of a slice (interface with substrate, i = 1; surface, i = imax). The diffusion constant is given by D. The values of Ci(t) were calculated by using the well-known recursion equation and the boundary conditions:

Therefore, it was reasonable to start the experiments with the annealing time of 1 min. Finally, solar cells were prepared by evaporating the electronselective contact (C60/BCP (bathocuproine)) and the copper contact onto the annealed perovskite layer via masks aligned with the prestructured ITO layer (ITO/PTAA/CH3NH3PbI3/C60/BCP/Cu, see also Figure S1). The morphology of perovskite layers is important for the diffusion analysis and for the performance of solar cells. Annealing at 60 °C led to the formation of holes inside perovskite films whereas compact perovskite films were formed at higher annealing temperatures (see also Figure S2). The average crystal size increased with increasing annealing time (see also Figure S3 for annealing at 100 °C). Furthermore, the volume of larger crystallites (size in vertical direction limited by the layer thickness) dominated the volume of the perovskite films annealed at higher temperatures and longer times so that transport phenomena were dominated by transport through grains. Three sets of samples have been prepared under identical conditions. The sets were used for the evaluation of the amounts of Pb and S in the layer by means of HR-CSAS, for the IRSE measurements, and for the basic characterization of solar cells under a sun-simulator at air mass 1.5. Two batches of solar cells (area 0.16 cm2) were prepared for each condition resulting in a statistic of 12 cells for analysis of standard deviations. The sun-simulator was calibrated with a calibrated c-Si reference solar cell (ISE Freiburg) under filtered light (Schott KG3). 2.2. Measurement of the S/Pb Ratio by Applying HighResolution Continuum Source Absorption Spectrometry. The absorption of the molecular absorption line of carbon sulfide at 258.056 nm and of the atomic absorption line of Pb at 261.418 nm was measured in a high-resolution spectrometer system (contrAA 700, Analytik Jena AG) with a transversely heated graphite furnace and pyrolytically coated standard graphite tubes with PIN-platform (Analytik Jena. part 407-A81.025). The spectrometer is based on a xenon arc lamp and a double monochromator (prism premonochromator and an echelle grating monochromator) with a linear CCDarray detector. The measurements were performed in an argon atmosphere (99.998% vol, Air Liquid). To separate the S- and Pb-containing species present in the prepared CH3NH3PbI3 layer, the film was completely dissolved using 300 μL of acetonitrile (Biosolve) and 200 μL of water. After addition of 3−5 mg of zinc powder (maximum particle size 45 μm, Merck), the solution was heated until its volume decreased to about 200 μL in order to reduce Pb2+ ions to neutral Pb. Consequently, Pb was precipitated from the solution. The S-containing solution was separated from the precipitated Pb by filtration, whereas the Pb was

Ci(t + Δt )|1 < i < imax = Ci(t ) +

DΔt (Ci + 1(t ) − 2Ci(t ) + Ci − 1(t )) ΔxΔx

C1(t + Δt ) = C1(t ) +

DΔt (C 2(t ) − C1(t )) ΔxΔx

(1) (2)

DΔt (0 − 2Cimax(t ) + Cimax − 1(t )) ΔxΔx (3) At t = 0, all Ci values were set to a constant value. For keeping convergence, Δt was set as 0.01 times Δx2/D. The diffusion constant Cimax(t + Δt ) = Cimax(t ) +

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ACS Applied Energy Materials of impurities in solids close to the melting point, for example, for crystalline silicon, is on the order of 10−13−10−8 cm2/s.17 A similar range can be expected for diffusion constants of DMSO in perovskite. As an example, the inset of Figure 3 shows the depth dependencies of the concentration for a layer thickness of 200 nm after different

D ≈ 1.64

d2 τ

(4)

3. RESULTS AND DISCUSSION 3.1. Contents of DMSO in Differently Annealed CH3NH3PbI3 Layers. From one sample, the amount of lead was 1.54 ± 0.04 μmol which corresponds to a mass 0.32 ± 0.01 mg or, if taking the layer area and thickness into account, to a density of about 7.6 g/cm3. The reproducibility of the mass of lead was within 3−5%, giving evidence for excellent reproducibility of layer deposition and HR-CSAS measurements. The amounts of S and Pb and the S/Pb molar ratio are summarized in Table 2 for samples annealed at 100 °C for 1, 5, Table 2. Amount of S and Pb and S/Pb Molar Ratio in Samples Annealed at 100 °C for 1, 5, and 10 min

Figure 3. Example for depth dependencies of the concentration for a layer thickness of 200 nm after different times of diffusion assuming D = 2 × 10−11 cm2/s (inset) and time dependencies of the concentrations integrated over the depth for diffusion coefficients of 10−11, 10−12, and 10−13 cm2/s (stars, triangles, and spheres, respectively).

annealing time (min)

S (μmol)

Pb (μmol)

molar ratio (S/Pb)

1 5 10

0.021 0.0065 0.0037

1.52 1.48 1.58

1.38% 0.44% 0.23%

and 10 min. The amount of S decreased from 0.021 to 0.0065 and 0.0037 μmol for the annealing times of 1, 5, and 10 min, respectively. At the same time, the amount of Pb remained constant. The S/Pb molar ratios decreased from 1.38% to 0.44% and 0.23% for the annealing times of 1, 5, and 10 min, respectively. Figure 5 shows IRSE spectra of CH3NH3PbI3 layers annealed at 100 °C for 1 min (solid line) and for 30 min

times of diffusion for D = 2 × 10−11 cm2/s. The experiments were sensitive to the integrated concentrations. Therefore, the time dependencies of the integrated concentrations are depicted in Figure 3 for different values of D. At longer times, the decay of the integrated concentrations can be well-fitted with an exponential (time constant τ). This opens the opportunity for fast and robust analysis independent of the layer thickness. The ratio of the squared layer thickness (d2) and τ contains the information about the diffusion constant. The values of d2/τ are correlated with the corresponding values of D in Figure 4a. The values of d2/τ are proportional to D with a proportionality factor of about 0.61 for analysis at longer times and D > 2 × 10−13 cm2/s. For the given work, the accuracy of the proportionality factor of 0.6 is sufficient with respect to the accuracy of measurements, i.e.

Figure 5. Referenced IRSE spectra of CH3NH3PbI3 layers annealed at 100 °C for 1 min (black solid line) and for 30 min (blue dashed line). The peaks marked with asterisks disappeared during annealing and are due to DMSO. Some of the peaks marked by dashed vertical lines are assigned to their origin vibration modes.

(dashed line). For this annealing temperature and these annealing times, some of the lines of the IRSE spectra disappeared within the resolution of IRSE after the annealing whereas some of the lines stayed permanent. The positions of the peaks disappearing or staying permanent after annealing were identical for all samples, i.e., independent of annealing temperature and annealing time.

Figure 4. Dependence of the squared layer thickness, d2, divided by the decay time constant, τ, on the diffusion constant, D. 5119


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ACS Applied Energy Materials The DMSO related IR-absorption peaks (see asterisks in Figure 5) disappear during annealing. The low IR absorption at about 1650 cm−1 is due to the CO stretching mode in the amide of some residual DMF.18 However, the very low intensity of this peak tells us that the amount of DMF in the layer is very low after 1 min of annealing at 100 °C. The N−H, C−H, and C−N related vibrations at about 3100−3200 cm−1 (incidentally, due to the overlapping with the thickness interference the band point upward), 1471 cm−1, and 1250 cm−1, respectively,19 remain nearly unchanged and show that the amount of methylammonium ions in the perovskite layer is not varied during the annealing process. Figure 6 shows IRSE spectra of CH3NH3PbI3 layers annealed at 100 °C for 1, 4, 8, and 30 min in a reduced

Figure 7. Correlation between the S/Pb ratio obtained from HRCSAS measurements and integrated peak of the vibrational mode at 1020 cm−1 (DMSO) obtained from IRSE for a series of samples annealed at 100 °C for 1, 5, and 10 min.

was integrated for the different annealing temperatures and annealing times. For all annealing temperatures, there was a time range in which the dependence of the integrated peak could be very well-fit with an exponential decay (see Figure 8). Therefore, the diffusion model in a homogeneous layer can be applied in this time range.

Figure 6. Referenced IRSE spectra of CH3NH3PbI3 layers annealed at 100 °C for 1, 4, 8, and 30 min (black, red, blue, and green lines, respectively) in the lower spectral range. The spectra were shifted vertically for clarity.

spectral range between 930 and 1050 cm−1. In this range, C−H deformation modes at 945 cm−1 (in DMSO), 962 cm−1 (in CH3NH3I),20 and 991 cm−1 (in DMSO) and the SO stretching mode at 1020 cm−1 (in DMSO with PbI2)20 are well-resolved. The amplitudes of the peak related to CH3 in CH3NH3I were independent of the annealing process whereas the peaks related to CH3 and SO in DMSO decreased with increasing annealing time and were below the resolution of the IRSE setup after annealing for 30 min. Since the SO vibration is the strongest of the DMSO vibrations, this IR-absorption peak was used for the calculation of the relative change in the number of SO groups during annealing by integration of the peak area. These values are plotted as a function of the S/Pb ratio as obtained from the HR-CSAS analysis for samples annealed at 100 °C for 1, 5, and 10 min, respectively (see Figure 7). An excellent correlation was found within the experimental error. Therefore, the area of the DMSO related peak at 1020 cm−1 in the IRSE spectra can be applied as a parameter proportional to the amount of sulfur in annealed perovskite layers. This allowed for the characterization of numerous samples within a relatively short time period as presented in the next paragraph. 3.2. Analysis of Diffusion Coefficients. The largest peak of specific vibrations in the solvent molecules was related to the νs[SO] stretching vibration mode in DMSO. This peak

Figure 8. Dependence of the integrated IRSE peak of the νs[SO] vibrational mode on the annealing time for different annealing temperatures.

The integrated IR peaks of the νs[S = O] vibrational mode measured for the as-prepared sample were much higher than expected from the diffusion model. Additionally, at longer times and higher annealing temperatures, the integrated peak area decreased with increasing time faster than expected from the diffusion model. Consequently, the model of diffusion in a homogeneous layer cannot be applied for very short annealing times when the complete CH3NH3PbI3 layer is not yet formed and solvents and complexes dominate out-diffusion of DMSO (uncontrolled range in Figure 8) and for longer times and higher annealing temperatures. Nevertheless, in an intermediate time period the time constants of the decay of the integrated νs[S = O] peaks amounted to 125, 65, 30, 13, 4.3, and 1.3 min for the annealing 5120


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ACS Applied Energy Materials temperatures of 60, 70, 80, 90, 100, and 110 °C, respectively. These time constants correspond, with respect to a layer thickness of 400 nm, to values of d2/τ of 2.1 × 10−13, 4.1 × 10−13, 8.9 × 10−13, 2.05 × 10−12, 6.2 × 10−12, and 2.05 × 10−11 cm2/s, respectively, as calculated by eq 4. The Arrhenius plot of the values of d2/τ is visualized in Figure 9.

Diffusion of DMSO in CH3NH3PbI3 causes a reduction of the concentration of DMSO molecules at lattice sites in time. Highly efficient solar cells can be prepared after annealing at 100 °C for 50−60 min. The concentration of DMSO will be decreased by about 6 orders of magnitude after annealing at 100 °C for 1 h. Depending on the initial concentration of DMSO, i.e., after annealing for 1 min, this would result in a concentration of DMSO molecules on the order of 1014−1015 cm−3. It is not clear which kind of defects and defect states can be caused by DMSO trapped at lattice sites. However, residual DMSO is present in CH3NH3PbI3 and will cause the formation of defects due to its chemical activity and varied complexation, for example, under illumination. 3.3. Impact of the DMSO Content on Solar Cells. The amount of DMSO in a perovskite layer may influence the performance of solar cells. In order to find correlations between the amount of DMSO in a perovskite layer and solar cell parameters, several batches of solar cells were prepared on perovskite layers annealed at 100 °C for 1, 5, 10, and 20 min. The S/Pb molar ratio was 0.02% after annealing at 100 °C for 20 min with regard to the presented diffusion model. Figure 10 shows typical forward and reverse current voltage (I−U) characteristics of solar cells prepared on CH3NH3PbI3

Figure 9. Arrhenius plot of d2/τ (circles), plots for activation energies of 0.64 and 1.48 eV (blue and red short dashed lines, respectively), and fit with two diffusion constants with activation energies of 0.6 and 1.8 eV (green line).

The activation energy of d2/τ was not constant. At the lower and higher temperatures, the activation energies could be approximated by 0.64 and 1.48 eV, respectively. The temperature dependence of d2/τ could be fitted with a diffusion constant having two activation energies (EA1 and EA2); i.e., the out-diffusion of DMSO was limited by two different processes. The prefactors of the diffusion coefficients of the two processes are denoted by D01 and D02.

ji E zy ji E zy DDMSO = D01 expjjj− A1 zzz + D02 expjjj− A 2 zzz j kBT z j kBT z (6) k { k { −11 2 The values of EA1, D01, EA2, and D02 are 0.6 eV, 10 cm /s, 1.8 eV, and 3.6 × 10−4 cm2/s, respectively. Diffusion processes are limited by transport barriers. Two different activation energies for the diffusion of DMSO in CH3NH3PbI3 give evidence for two different kinds of barriers. It seems that the lower barrier is related to relatively weak bonding of DMSO in complexes which may include, for example, DMF and components of the precursor salts such as Pb2+. The higher barrier is related to a more stable configuration of DMSO molecules in CH3NH3PbI3, for example, at a lattice position of the perovskite. Periodic DFT calculations were performed to get information about the stability of DMSO molecules trapped inside the CH3NH3PbI3 structure. Figure 1 shows the computational model of the perfect CH3NH3PbI3 structure, the CH3NH3PbI3 structure with a CH3NH3+ vacancy, and the CH3NH3PbI3 structure with a DMSO molecule trapped at the position of the CH3NH3 vacancy. The BE of DMSO molecules trapped at the position of the CH3NH3+ vacancy was found to be −3.58 eV. Therefore, DMSO molecules incorporated into a perovskite crystal during the crystal growth are stable. The high diffusion barrier of DMSO in CH3NH3PbI3 is therefore obviously related to the diffusion via lattice sites.

Figure 10. Typical forward (thick lines) and reverse (thin lines) I−U characteristics of solar cells prepared on CH3NH3PbI3 annealed at 100 °C for 1, 5, 10, and 20 min (black, red, blue, and green lines, respectively). Illumination was performed at AM1.5.

annealed at 100 °C for 1, 5, 10, and 20 min under illumination at AM1.5. For the given I−U characteristics and samples, the values of fill factor (FF) increased from about 0.72 to 0.78 with increasing annealing time. The values of the open circuit voltage (VOC) and of the short circuit current density (ISC) tended to increase with increasing annealing time. Figure 11 summarizes the values of VOC, ISC, and FF for two preparation batches of 6 solar cells for each annealing condition as a function of the corresponding S/Pb molar ratio. Incidentally, the values for forward and reverse I−U characteristics were not distinguished in Figure 11 since the hysteresis was low (within the standard deviations). Furthermore, the reproducibility between two batches was within the standard deviation of a separated batch. The values of VOC changed, with respect to the standard deviations, in a range between about 1.05 and 1.08 V whereas the highest values were obtained for the lowest S/Pb molar ratio. The highest standard deviations of VOC (ΔVOC about 5121


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ΔISC, and FF ± ΔFF of the two batches were (1.09 ± 0.01) V, (23.6 ± 0.4) mA/cm2, and (73 ± 4)%, and (1.07 ± 0.02) V, (21.6 ± 0.5) mA/cm2, and (74 ± 1)%, respectively (see Figure 11). This resulted in efficiencies of (18.9 ± 1.4)% for the first and (17.0 ± 0.7)% for the second batch. The values of ISC, VOC, FF, and efficiency amounted to 23.73 mA/cm2, 1.1055 V, 76.94%, and 20.18%, respectively, for the solar cell with the highest efficiency (see also Figure S4a). The value of ISC was confirmed by the measurement of the external quantum efficiency (23.3 mA/cm2, see also Figure S4b). In comparison to the solar cells with CH3NH3PbI3 layers containing a small amount of DMSO, the standard deviations within one batch of the solar cells with CH3NH3PbI3 layers without DMSO increased for ΔVOC and ΔFF but decreased for ΔISC, whereas there was no overlap of the ΔISC for the two batches.

4. CONCLUSIONS Methodologies for the measurement of the content of DMSO and of diffusion coefficients of DMSO in CH3NH3PbI3 layers have been developed by applying HR-CSAS and IRSE in combination with a diffusion model in a homogeneous layer. The amount of DMSO was calibrated with HR-CSAS in terms of the S/Pb molar ratio for measurements by IRSE. It was shown that diffusion coefficients of DMSO in CH3NH3PbI3 can be obtained from the dependencies of integrated peaks of specific vibration modes such as the SO stretching mode on annealing time and annealing temperature. It was found that the diffusion coefficient of DMSO in CH3NH3PbI3 is a superposition of two thermally activated processes with EA1, D01, EA2, and D02 being 0.6 eV, 10−11 cm2/ s, 1.8 eV, and 3.6 × 10−4 cm2/s, respectively. Different coordination and/or binding states of DMSO molecules are the origin for the rather different diffusion processes. It is obvious that the lower activation energy corresponds to activation of loosely bond molecules whereas the higher activation energy belongs to activation of DMSO trapped in the lattice of CH3NH3PbI3. Furthermore, it was found that out-diffusion of DMSO can have a strong influence on the fill factor of solar cells and its standard deviation. It seems that CH3NH3PbI3 layers get very homogeneous at concentrations between 0.44% and 0.23% of the S/Pb molar ratio as indicated by a strong increase of FF and a strong decrease of ΔFF. However, the standard deviations of ΔVOC and ΔFF strongly increased for solar cells based on CH3NH3PbI3 layers without DMSO. Therefore, it seems that some residual DMSO in CH3NH3PbI3 layers is useful for reaching a high homogeneity of solar cells based on metal halide perovskites and to passivate recombination active defects within the lattice. This is important for the preparation of solar cells on large areas. Incidentally, an additional influence by DMF molecules cannot be ruled out. Unfortunately, a comparably detailed investigation of the diffusion of DMF is not possible by the given approach. In the future, it will be very interesting to apply this method of investigation to more complex systems with larger numbers of cations, variations of boundary conditions, for example, by antisolvent dripping, changes in solar cell architectures, etc., and to correlate this method with more detailed investigations of, for example, a suitable influence of residual DMSO on lifetime of devices.

Figure 11. Correlation of the values of VOC (a), ISC (b), and FF (c) with the S/Pb molar ratio for forward and reverse I−U characteristics of two batches of solar cells for each annealing condition (Tann = 100 °C, tann = 1, 5, 10, and 20 min, and Tann = 120 °C, tann = 10 min, filled and open symbols, respectively). Two batches are distinguished for Tann = 120 °C.

0.011 V) were obtained for S/Pb molar ratios of 0.44 and 0.23 whereas the lowest ΔVOC (0.007 V) was obtained for the lowest S/Pb molar ratio. In general, VOC was practically independent of the content of DMSO. Therefore, residual DMSO in CH3NH3PbI3 did not lead to the formation of recombination active defects. The lowest and highest values of ISC were measured for the lowest and highest S/Pb molar ratios ((18.8 ± 0.2) and (19.4 ± 0.3) mA/cm2, respectively). There was no clear trend in the behavior of the standard deviation of ISC with regard to the S/ Pb molar ratio. The values of FF were nearly identical for the S/Pb molar ratios of 1.38 and 0.44 ((72 ± 2)% and (72 ± 3)%, respectively). Surprisingly, the values of FF increased strongly and the values of ΔFF decreased by about 1 order of magnitude between the S/Pb molar ratios of 0.44 and 0.23 (FF ± ΔFF was (76.2 ± 0.4)% for S/Pb molar ratio of 0.23). The highest value of FF and the lowest value of ΔFF were obtained for the lowest value of the S/Pb molar ratio ((78.2 ± 0.3)%). Therefore, FF and ΔFF showed by far the strongest dependence on the S/Pb molar ratio. With a decreasing S/ Pb molar ratio, the efficiency increased monotonously from (14.4 ± 0.3)% to (16.3 ± 0.2)%. After annealing at 120 °C for 10 min and with regard to the presented diffusion model, the amount of DMSO was below 10−9% of the S/Pb ratio; i.e., DMSO disappeared practically completely from the CH3NH3PbI3 layer. Two batches of solar cells were prepared for layers annealed at 120 °C for 10 min. Despite the fact that both batches were prepared under identical conditions, the variations of the parameters of the solar cells of both batches were larger than the standard deviations. The corresponding values of VOC ± ΔVOC, ISC ± 5122


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spectrometry. A critical review. Anal. Bioanal. Chem. 2014, 406, 2239−2259. (10) Huang, M. D.; Becker-Ross, H.; Florek, S.; Heitmann, U.; Okruss, M. Determination of sulfur by molecular absorption of carbon monosulfide using a high-resolution continuum source absorption spectrometer and an air-acetylene flame. Spectrochim. Acta, Part B 2006, 61, 181−188. (11) Heitmann, U.; Becker-Ross, H.; Florek, S.; Huang, M. D.; Okruss, M. Determination of non-metals via molecular absorption using high-resolution continuum source absorption spectrometry and graphite furnace atomization. J. Anal. At. Spectrom. 2006, 21, 1314− 1320. (12) Ellipsometry of Functional Organic Surfaces and Films; Hinrichs, K., Eichhorn, K.-J., Eds.; Springer International Publishing AG, 2018; Vol. 52. (13) Hinrichs, K.; Gensch, M.; Esser, N. Analysis of Organic Films and Interfacial Layers by Infrared Spectroscopic Ellipsometry. Appl. Spectrosc. 2005, 59, 272A−282A. (14) Li, X.; Bi, D.; Yi, C.; Décoppet, J.-D.; Luo, J.; Zakeeruddin, S. M.; Hagfeldt, A.; Grätzel, M. A vacuum flash-assisted solution process for high-efficiency large-area perovskite solar cells. Science 2016, 353, 58−62. (15) Ding, B.; Gao, L.; Liang, L.; Chu, Q.; Song, X.; Li, Y.; Yang, G.; Fan, B.; Wang, M.; Li, C.; Li, C. Facile and Scalable Fabrication of Highly Efficient Lead Iodide Perovskite Thin-Film Solar Cells in Air Using Gas Pump Method. ACS Appl. Mater. Interfaces 2016, 8, 20067−20073. (16) Wu, Y. L.; Yan, D.; Peng, J.; Duong, T.; Wan, Y.; Phang, S. P.; Shen, H.; Wu, N.; Barugkin, C.; Fu, X.; Surve, S.; Grant, D.; Walter, D.; White, T. P.; Catchpole, K. R.; Weber, K. J. Monolithicperovskite/ silicon-homojunction tandem solarcellwith over 22% efficiency. Energy Environ. Sci. 2017, 10, 2472−2479. (17) Tang, K.; Øvrelid, E. J.; Tranell, G.; Tangstad, M. Critical assessment of the diffusivities in solid and liquid silicon. JOM 2009, 61, 49−55. (18) See, for example: Chalapathi, V. V.; Ramiah, K. V. Normal vibrations of N,N-dimethylformamide and N,N-dimethylacetamide. Proc. Ind. Acad. Sci. A 1968, 68, 109−122. (19) The IR spectra of CH3NH3PbI3 were analyzed in detail: PérezOsorio, M. A.; Milot, R. L.; Filip, M. R.; Patel, J. B.; Herz, L. M.; Johnston, M. B.; Giustino, F. Vibrational properties of the organicinorganic halide perovskite CH3NH3PbI3 from theory and experiment: factor group analysis, first-principles calculations, and lowtemperature infrared spectra. J. Phys. Chem. C 2015, 119, 25703− 25718. (20) Wharf, I.; Gramstad, T.; Makhija, R.; Onyszchuk, M. Synthesis and vibrational spectra of some lead(II) halide adducts with O-, S-, and N-donor atom ligands. Can. J. Chem. 1976, 54, 3430−3438.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00769.

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DFT calculation details, information about device structure, and morphology and performance details (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Joerg Rappich: 0000-0003-4219-6964 Pongthep Prajongtat: 0000-0001-9618-2504 Thomas Dittrich: 0000-0002-2698-9481 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank I. Engler (ISAS e.V.) for laboratory assistance. Financial support by the Ministerium fü r Innovation, Wissenschaft und Forschung des Landes Nordrhein-Westfalen, the Regierende Bü rgermeister von BerlinSenatskanzlei Wissenschaft und Forschung, and the Bundesministerium für Bildung und Forschung is gratefully acknowledged, as well as the European Union through EFRE 1.8/13.

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REFERENCES

(1) See, for example: Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Solvent engineering for high-performance inorganic-organic hybrid perovskite solar cells. Nat. Mater. 2014, 13, 897−903. (2) See, for example: Ahn, N.; Son, D.-Y.; Jang, I.-H.; Kang, S. M.; Choi, M.; Park, N.-G. Highly Reproducible Perovskite Solar Cells with Average Efficiency of 18.3% and Best Efficiency of 19.7% Fabricated via Lewis Base Adduct of Lead(II) Iodide. J. Am. Chem. Soc. 2015, 137, 8696−8699. (3) Gao, L. L.; Liang, L.-S.; Song, X.-X.; Ding, B. V.; Yang, G.-J.; Fan, B.; Li, C.-X.; Li, C.-J. Preparation of flexible perovskitesolarcells by a gaspumpingdryingmethod on a plastic substrate. J. Mater. Chem. A 2016, 4, 3704−3710. (4) Boiling points: 189 °C (DMSO, https://en.wikipedia.org/wiki/ Dimethyl_sulfoxide), 153 °C (DMF, https://en.wikipedia.org/wiki/ Dimethylformamide). (5) Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Solvent engineering for high-performance inorganic-organic hybrid perovskite solar cells. Nat. Mater. 2014, 13, 897−903. (6) Welz, B.; Becker-Ross, H.; Florek, S.; Heitmann, U. HighResolution Continuum Source AAS; Wiley-VCH: Weinheim, 2005. (7) Welz, B.; Vale, M. G.; Florek, S.; Okruss, M.; Huang, M. D.; Becker-Ross, H. High-Resolution Continuum Source Atomic Absorption SpectrometryTheory and Applications. In Encyclopedia of Analytical Chemistry; Meyers, R. A., Ed.; John Wiley & Sons Ltd., 2010. (8) Butcher, D. J. Molecular absorption spectrometry in flames and furnaces atomization: A review. Anal. Chim. Acta 2013, 804, 1−15. (9) Resano, M.; Flórez, M. R.; García-Ruiz, E. Progress in the determination of metalloids and non-metals by means of highresolution continuum source atomic or molecular absorption 5123


Temperature Dependent Diffusion of DMSO in CH3NH3PbI3

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