Precipitation of CH3NH3PbCl3 in CH3NH3PbI3 and Its Impact on

Apr 16, 2015 - A precipitation of CH3NH3PbCl3 (MAPbCl3) in CH3NH3PbI3 (MAPbI3) has been observed for deposition from solutions containing CH3NH3I (MAI...
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Precipitation of CH3NH3PbCl3 in CH3NH3PbI3 and Its Impact on Modulated Charge Separation Pongthep Prajongtat*,†,‡ and Thomas Dittrich† †

Institute of Heterogeneous Materials, Helmholtz-Centre Berlin for Materials and Energy, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany ‡ Department of Materials Science, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand S Supporting Information *

ABSTRACT: A precipitation of CH3NH3PbCl3 (MAPbCl3) in CH3NH3PbI3 (MAPbI3) has been observed for deposition from solutions containing CH3NH3I (MAI), PbCl2 and small amounts of ammoniumvaleric acid iodide (HOOC(CH2)4NH3I or AVAI) in N,N-dimethylformamide. The influence of the precipitation of MAPbCl3 on the electronic properties of the free perovskite layer and of the nanoporous TiO2/perovskite nanocomposite has been investigated by modulated surface photovoltage spectroscopy. The lowest characteristic energy of exponentially distributed defect states below the band gap was found for the highest amount of AVAI in the solution (2.6% for the given experiments). Stabilization of MACl by AVA and the role of coadsorption of ions on TiO2 surfaces for charge separation and layer formation are discussed on the basis of interaction and adsorption energies obtained by molecular modeling.

1. INTRODUCTION Solution-based processing of materials at low temperatures allows one to reduce the amount of energy required, for example, for the preparation of solar cells such as organic,1−3 hybrid organic−inorganic,4−6 and dye-sensitized7,8 solar cells. Recently, lead halide perovskites became very interesting for solar cells due to their excellent light-harvesting ability and simple preparation based on solution processing.9−11 The solution-based deposition of films of a high electronic quality requires appropriate solvents for dissolving the material being deposited and for wetting with the substrate surface as well as controlled evaporation of solvents allowing the formation of homogeneous layers. Methylammonium lead iodide perovskite (CH3NH3PbI3, MAPbI3) has a band gap of about 1.5−1.6 eV12 and is the most prominent lead halide perovskite absorber for related solar cells with high solar energy conversion efficiency (up to 20%).13 MAPbI3 layers can be deposited onto organic (for example, PEDOT:PSS14) or inorganic (for example, TiO215) materials in order to form charge-selective contacts. The electronic properties of the deposited layers and the performance of solar cells based on MAPbI3 can be improved if small amounts of CH3NH3PbCl3 (MAPbCl3) and/or other side phases exist in the MAPbI3 layer.16 MAPbI3‑xClx and/or coexisting phases of MAPbI3 and MAPbCl3 can be obtained by mixing MAI with PbCl2. Depending on the ratio between MAI and PbCl2 and on the annealing conditions, different phases appear in the deposited layers.16−19 However, very pure MAPbI3 has been prepared from solutions containing N,N-dimethylformamide (DMF) or mixed DMF and dimethyl sulfoxide as solvents and MAI and PbCl2 with a molar ratio of 3:1.16−18 © 2015 American Chemical Society

Very recently, hot casting of solution-processed lead halide perovskite from PbI2 and MACl dissolved in a solvent with a high boiling point resulted in highly efficient lead halide perovskite solar cells with large grains.14 Here, it is important to remark that the grains of lead halide perovskite showed a radial geometry giving evidence for precipitation starting at a center point. This suggests that the formation of solution-processed lead halide perovskite layers with high electronic quality requires the precipitation of seeds for crystal growth. However, the question about the underlying mechanism of layer formation has not been raised up. Long-term stability has also been a current issue for perovskite solar cells. Normally, the performance of perovskite solar cells decreased dramatically due to moisture- and oxygeninduced degradation of the perovskite absorbers.20 Interestingly, inclusion of an additive with a carboxylic group binding to TiO2 and an ammonium group binding to MAPbI3 such as ammoniumvaleric acid iodide (HOOC(CH2)4NH3I or AVAI) into the solution containing MAI and PbI2 can lead to a very stable solar cell.21,22 However, the role of AVAI has not been investigated yet for the deposition of MAPbI3 from solutions containing MAI and PbCl2. Obviously, interactions among chemicals and interactions between chemicals and the surface of the substrate are changed when changing the chemical compositions of the solution. Here, it will be shown that addition of small amounts of AVAI into a solution with MAI Received: February 18, 2015 Revised: April 16, 2015 Published: April 16, 2015 9926

DOI: 10.1021/acs.jpcc.5b01667 J. Phys. Chem. C 2015, 119, 9926−9933

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Dyesol), AVAI and lead chloride (PbCl2, Carl Roth, Germany) in N,N-dimethylformamide (DMF, Sigma-Aldrich). Concentration ratios between ammoniumvaleric acid ion (AVA) and the sum of AVA and methylammonium ion (MA) ([AVA]/ ([AVA] + [MA]) of 0, 0.5, 1, 1.6, 2.1, and 2.6% were used whereas the concentration ratio ([AVAI] + [MAI]):[PbCl2] was kept constant at 3:1. In the following, ([AVA]/([AVA] + [MA]) will be called concentration of AVA. The precursor solutions were stirred at 60 °C overnight. The 80 μL samples of the precursor solutions were dispersed on the substrates by spin coating (2000 rpm for 10 s followed by 3000 rpm for 30 s) in a N2-filled glovebox. After that, the perovskite layers were formed during heating in the glovebox at 100 °C for 60 min. 2.2. Characterization. The phase composition near the surface of the lead halide perovskite layers was investigated by grazing incidence X-ray diffraction (GIXRD, Bruker AXS D8 Advance). Modulated surface photovoltage (SPV) spectra were measured in the fixed capacitor arrangement23 in vacuum (2 × 10−5 mbar) at a modulation frequency of 8 Hz. For the investigated samples, the lead halide perovskite existed in two configurations: a bottom nanocomposite layer of perovskite/ TiO2 and a top layer of free perovskite (see also Figure 1). For

and PbCl2 can induce precipitation of MAPbCl3 which has an impact on the electronic quality of MAPbI3. In this work, the influence of AVAI on the deposition of lead halide perovskite layers on np-TiO2 from a solution with MAI and PbCl2 and on the electronic properties of MAPbI3 is investigated. The concentration of iodide ions has been kept three times higher than the concentration of lead ions in the solution whereas the concentration of AVAI has been varied systematically. By this way and by fixing the annealing temperature and time to 100 °C and 1 h, respectively, a reference layer without formation of MAPbCl3 has been obtained for a concentration of AVAI equal to zero. The phase composition and electronic properties of the prepared lead halide perovskite layers were characterized by grazing incidence X-ray diffraction (GI-XRD) and modulated surface photovoltage (SPV) spectroscopy, respectively. Furthermore, in order to gain a deeper understanding of mechanisms of MAPbCl3 precipitation by AVAI, interaction energies between AVA, MA, Cl− and I− ions and adsorption energies of related ions at TiO2 surfaces were calculated by using density functional theory (DFT) calculations. It will be shown that the formation of stable cyclic complexes of AVA and MACl through hydrogen bonding and electrostatic interactions is a key feature of initiating the precipitation of MAPbCl3.

2. EXPERIMENTAL AND COMPUTATIONAL METHODS 2.1. Sample Preparation. Double layers consisting of a compact and a nanoporous TiO2 layers (cp-TiO2 and np-TiO2, respectively) were deposited onto glass substrates coated with SnO2:F (solaronix, 2.5 × 2.5 cm2). Before depositing the cpTiO2 layer, the substrates were precleaned successively in detergent (Mucasol, Sigma-Aldrich), distilled water, acetone, and 2-propanol for 10 min each step and dried with nitrogen. The precursor solution for the cp-TiO2 layer was prepared by dissolving 463 μL of titanium isopropoxide (≥97% purity, Sigma-Aldrich) in 5 mL of ethanol (≥99.8% purity, SigmaAldrich), first, and adding 35 μL of 2 M HCl, second. Sixty μL of the precursor solution were dispersed on the substrate by spin coating at 2000 rpm for 60 s. The films were dried at 150 °C in air for 10 min and afterward sintered in air at 500 °C for 30 min. After that, 100 μL of a commercial precursor solution (Ti-Nanoxide T/SP, Solaronix) were dispersed on the cp-TiO2 layers by spin coating at 3000 rpm for 60 s, dried in air at 150 °C for 10 min and cooled down for 5 min. After repeating this process for 3 times, the layers were sintered in air at 500 °C for 60 min. The resulting thickness of the np-TiO2 layers was about 250 nm. HOOC(CH2)4NH3I (AVAI) was prepared following the procedure described in the previous work.22 A stoichiometric aqueous solution of 5-aminovalerlic acid (HOOC(CH2)4NH2 Sigma-Aldrich) and HI was obtained by dropping gently a solution of 40 wt % of 5-aminovalerlic acid (Sigma-Aldrich) into a solution of 57 wt % of HI (Sigma-Aldrich) under stirring in an ice bath. After continued stirring in the ice bath for 2 h, the water was evaporated in a rotary evaporator at 50 °C at reduced pressure. For further cleaning, the precipitated AVAI was recrystallized twice. For this purpose, first, the AVAI was dissolved in ethanol, second, diethyl ether (Sigma-Aldrich) was added and, third, the solution was left at room temperature overnight. The white powder of AVAI was collected by filtering. Finally, AVAI was dried in vacuum at 40 °C for 24 h. Precursor solutions with 40 wt % of perovskite were prepared by dissolving methylammonium iodide (CH3NH3I, MAI

Figure 1. Schematic of the electrode configuration for surface photovoltage measurements in the fixed capacitor arrangement with front and back illumination and schematic sample structure containing a free perovskite surface layer and a nanoporous TiO2/perovskite nanocomposite layer on top of a compact TiO2 layer.

preferential illumination of the highly absorbing free perovskite or perovskite/TiO2 nanocomposite layers, front and back illumination were performed with a halogen lamp and a quartz prism monochromator. Figure 1 shows the schematic of the electrode configuration for SPV measurements in the fixed capacitor arrangement with front and back illumination. Inphase and phase-shifted by 90° SPV signals were detected with a high-impedance buffer and a double phase lock-in amplifier (EG&G 5210). The photovoltage amplitude, defined as the square root of the sum of the squared in-phase and phaseshifted by 90° signals, was analyzed. 2.3. Computational Analysis of Interaction and Adsorption Energies. Interaction energies between AVA 9927

DOI: 10.1021/acs.jpcc.5b01667 J. Phys. Chem. C 2015, 119, 9926−9933

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The Journal of Physical Chemistry C and MA and between AVA or MA and Cl− or I− and adsorption energies between AVA, Cl−, or AVA, I−, and the TiO2 surface were calculated by spin-unrestricted density functional theory (DFT) calculations implemented in the DMol3 module24−26 in the Materials Studio (MS) program (Accelrys Inc., version 5.5). Full geometry optimizations were performed by employing Perdew−Burke−Ernzerhof (PBE) exchange-correlation functional27 and the double numerical plus polarization (DNP) basis sets.28 The DNP basis sets generated the second set of basis functions for valence orbitals, and consequently doubling in basis set size or double-numerical (DN) sets. The DNP basis sets added the polarization d-function for all heavy atoms and the polarization p-function for all hydrogen atoms were similar to 6-31G(d,p) basis sets, whereas the DNP basis sets gave a higher accuracy than the 6-31G(d,p) basis sets.29 The convergence parameters for the energy change, maximum force, and maximum displacement were set at 2 × 10−5 Ha, 4 × 10−3 Ha Å−1, and 5 × 10−3 Å, respectively (note: 1 Ha = 27.2 eV). The global orbital cutoff was set to 4.5 Å. The interaction and adsorption energies (Eint/ads) were obtained by using the definition:

some cases, one of 3 × 3 × 1 were employed for the geometry optimization. For BB configuration, the values of Eads obtained from the k-point of 3 × 3 × 1 were equal to that achieved from the k-point of 2 × 2 × 1 within an error of 0.01%. Thus, the kpoint of 2 × 2 × 1 was chosen for further calculations. A test with water molecules adsorbed on the TiO2 surface resulted in a value of Eads of −18.8 kcal/mol which was very similar to previous simulations (Eads = −17.9 to −19.8 kcal/mol)33−35 but below experimental values (Eads = −11.5 to −16.1 kcal/mol).36

3. RESULTS AND DISCUSSION 3.1. Phase Analysis. Figure 2 shows the X-ray diffraction patterns of perovskites prepared with concentrations of AVAI

N

E int/ads = Ecomplex −

∑ Ei i=1

(1)

where Ecomplex and Ei denote the total energy of a complex of N interacting components and of the isolated contributing components which can be AVA, MA, Cl−, I−, and TiO2 slab. A complex is stable for a negative value of Eint/ads. The starting geometry of bulk anatase TiO2 was obtained from the library of the MS program. The calculated lattice parameters of bulk anatase TiO2 are consistent to the experimental data,30 for example, calculated lattice constant a and c are 3.813 and 9.811 Å, respectively, while experimental lattice constants a and c are 3.782 and 9.502 Å, respectively. Microscopic studies of nanocrystalline powders and thin films of TiO2 have revealed that their surfaces are terminated most favorably at the (101) plane.31 Therefore, a slab of about 7 Å in thickness from 24 TiO2 units was constructed. This slab had a (101) surface with 12 atomic layers aligned in the z-direction. The shape of the surface is a parallelogram with the side lengths of 11.20 and 11.44 Å along the x and y axes, respectively. The anatase (101) surface consists of four possible adsorption sites: a 2-fold coordinated oxygen, O(2c); a 3-fold coordinated oxygen, O(3c); a 5-fold coordinated titanium, Ti(5c); a 6-fold coordinated titanium, Ti(6c). The unsaturated atoms, O(2c) and Ti(5c), provide more reactive binding sites than the saturated atoms, O(3c) and Ti(6c). Each TiO2 slab had a vacuum thickness of 15 Å in the zdirection to avoid mirror image effect. In contrast, the isolated ions were optimized in a supercell periodic box of 30 × 30 × 30 Å3 which was large enough to neglect any intermolecular interaction. Simulations were performed for bidentate bridging (BB), bidentate chelating (BC), and monodentate ester-type (MET) binding modes of AVA on the anatase TiO2 (101) surface. For the BB and BC binding modes, two oxygen atoms of the carboxylic group of AVA bound to two and one Ti(5c), respectively. In the case of the MET mode, only one oxygen atom of the carboxylic group of AVA bound to one Ti(5c) forming an ester bond. All binding modes were assumed to be dissociative adsorption in which the hydrogen atom of the carboxylic group of AVA was dissociated and adsorbed onto O(2C). A Monkhorst−Pack k-point32 of 2 × 2 × 1 and, in

Figure 2. Grazing incidence X-ray diffraction patterns of perovskites prepared with concentrations of AVAI of 0, 0.5, 1, 1.6, 2.1, and 2.6% (black, blue, green, magenta, red, and purple lines, respectively). The pattern for concentrations of 0.5, 1, 1.6, 2.1, and 2.6% were multiplied by factors of 3, 9, 27, 81, and 243, respectively. The arrows mark the (110) and (100) peaks of MAPbI3 and MAPbCl3, respectively. The inset shows the dependence of the ratio of the intensities of the (100) MAPbCl3 peak to the (110) MAPbI3 peak on the concentration of AVA.

of 0, 0.5, 1, 1.6, 2.1 and 2.6%. The most prominent peak observed at 14.13° is related to the (110) direction of MAPbI3.22 The position of the peak at 14.13° was constant for all samples. For the different samples, the intensity the peak at 14.13° varied within a deviation of only 20%. Therefore, the major phase consisted of pure MAPbI3 for all samples. A peak at 15.64° appeared for the samples which were prepared with added AVAI. The peak at 15.64° corresponds to the (100) direction of MAPbCl3.18 The position of the peak at 15.64° was constant for all samples which were prepared with added AVAI. Therefore, only a small amount of pure MAPbCl3, but not mixed halide perovskite phases of MAPbI3‑xClx, was formed in the lead halide perovskite layers which were prepared with added AVAI. The inset of Figure 2 presents the dependence of the ratio of the peaks at 15.64° and at 14.13° on the concentration of AVA. The intensity of the peak at 15.64° increased practically linearly with increasing concentration of AVA and the intensity ratios of the peaks are of the same order as the concentrations of AVA. This means that practically each AVA ion contributed to the incorporation of one MAPbCl3 into the lead halide perovskite layers. However, there was a tendency to saturation of the intensity ratios of the peaks when higher concentration of AVA, for example 5.2%, was added into the perovskite precursor 9928

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of in-phase and phase-shifted by 90° signals means that, for example, the formation of a polarization charge contributed significantly to the SPV signals. The polarization charge is of the opposite sign of the photogenerated free charge carriers separated in space and does not follow the fast recombination of the free charge carriers after switching off the light. The action of a polarization charge or trapping on modulated charge separation is illustrated in Figures S2 and S3 in Supporting Information. In a solar cell, a polarization charge will lead to a reduction of the open circuit voltage. 3.2.2. Analysis of the Onset Energies. Figures 4 (linear scale) and 5 (logarithmic scale) depict spectra of the

solution, but a new phase of PbI2 occurred (see Figure S1 in Supporting Information). In this case, the model proposed cannot be applied. The peak at 23.49° is related to the (111) direction of MAPbI322 and remained practically constant for all samples. It is interesting to remark that addition of AVA to a precursor solution containing MAI and PbI2 led to a strong increase of the peak at 23.49° and to a decrease of the peak at 14.13° which has been related to a preferential growth of the perovskite in the c-direction.22 This was not observed for the given experiments which included PbCl2 as a precursor. Therefore, the presence of AVA in the precursor solution containing MAI and PbCl2 did not lead to a change of the growth direction of MAPbI3. 3.2. Modulated Charge Separation. 3.2.1. Overview Spectra. Figure 3 shows in-phase and phase-shifted by 90° SPV

Figure 3. In-phase (black lines and filled circles) and phase-shifted by 90° (thin red lines and open circles) overview surface photovoltage spectra for front and back illumination (symbols and lines, respectively) for concentrations of AVA of 0, 0.5 and 2.6% (a−c, respectively).

Figure 4. Spectra near the band gap of MAPbI3 for the surface photovoltage amplitudes normalized to the maximum for front and back illumination (a and b, respectively) at concentrations of AVA of 0, 0.5, 1 and 2.6% (squares, circles, triangles and stars, respectively). The inset shows the dependence of the onset energies on the concentration of AVA for front and back illumination (squares and circles, respectively). The solid lines mark some values of onset energies.

spectra for front and back illumination for concentrations of AVA of 0, 0.5, and 2.6%. For front illumination, the in-phase signals were positive for all samples. This means that photogenerated electrons were separated preferentially toward the substrate. Furthermore, the maximum of the in-phase signals was much higher for the samples prepared with AVA (see Figure 3, parts b and c) than for the sample prepared without AVA (see Figure 3a). For back illumination, the inphase signals were negative for the sample prepared without AVA but positive for the samples prepared with AVA. This means that photogenerated electrons were separated preferentially toward the external surface for the sample prepared without AVA. Therefore, the direction of modulated charge separation changed between the free perovskite and the perovskite/TiO2 nanocomposite layers for the sample prepared without AVA. In the case that charge separation is caused by only one mechanism and that the relaxation of SPV signals is caused by recombination, the in-phase and phase-shifted by 90° signals have the opposite sign which is the case for the sample prepared at concentrations of AVA of 0 and 0.5% (as well as for 1%, not shown in the figure). However, the signs of the inphase and phase-shifted by 90° signals were positive at photon energies between 1.58 and 2.89 eV for the sample prepared at a concentration of AVA of 2.6% (between 1.8 and 2.56 eV and between 1.67 and 2.13 eV for the concentrations of AVA of 2.1 and 1.6%, respectively, not shown in the figure). The same sign

Figure 5. Spectra near the band gap of MAPbI3 for the surface photovoltage amplitudes normalized to the maximum in a logarithmic scale for front and back illumination (a and b, respectively) at concentrations of AVA of 0, 0.5, 1, and 2.6% (squares, circles, triangles and stars, respectively). The dashed lines mark the shape of the photon flux spectrum for front illumination. The inset shows the dependence of the characteristic energy of exponentially distributed defect states below the band gap on the concentration of AVA for front and back illumination (squares and circles, respectively). The solid lines mark some tails related to exponentially distributed defect states below the band gap. 9929

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Figure 6. Optimized geometries and important geometrical parameters of AVA-MA-Cl, AVA-MA-I, AVA-MA-AVA-Cl and AVA-MA-AVA-I (a−d, respectively) calculated by a PBE/DNP method. The shown interaction energies are defined as the difference between the total energy of the complex and the energy of the corresponding isolated ions.

For the samples with the concentrations of AVA of 0.5 and 1.0%, the values of Eon were 1.539 and 1.532 eV for front illumination (Figure 4a) and 1.535 and 1.559 eV for back illumination (Figure 4b), respectively. Therefore, the starting precipitation of MAPbCl3 or interaction of np-TiO2 with AVA led to a decrease or increase of the band gap of MAPbI3 for front and back illumination, respectively. The onset energies did not change strongly, as compared with the samples prepared at a concentration of AVA of 1.0%, for the samples prepared at concentrations of AVA of 1.6 and 2.1% for front illumination (1.538 and 1.532 eV). For back illumination of the samples prepared at concentrations of AVA of 1.6 and 2.1%, the onset energies decreased to 1.542 and 1.540 eV, respectively. For the sample with the concentration of AVA of 2.6%, the onset energies increased to 1.544 and 1.556 eV for front (Figure 4a) and back (Figure 4b) illumination, respectively. As a conclusion, the strongest changes of the onset energies appeared for concentrations of AVA between 0 and 1%. 3.2.3. Analysis of the Characteristic Energies of Exponentially Distributed Defect States below the Band Gap. On a logarithmic scale (Figure 5), the normalized PV amplitudes showed an exponential behavior follow the onset energies. The exponential increase is described by the characteristic energy of exponentially distributed defect states below the band gap (Et).37 The values of Et, called also energy of the exponential tails, could be obtained for front and back illumination for all samples excluding for back illumination of the sample prepared at the concentration of AVA of 2.6% for which the sign of the phase-shifted by 90° signal changed near the band gap which led to a dip in the PV amplitude spectrum. As noted, the values of the energy of the exponential tails increase with increasing disorder caused by electronic states being involved in modulated charge separation in semiconductors or at their interfaces.

normalized surface photovoltage amplitudes (PV amplitudes) for front and back illumination of samples with the concentrations of AVA of 0, 0.5, 1, and 2.6% in the spectral range close to the band gap of MAPbI3. In comparison to Figure 3, the spectra were measured with a reduced energy step and the energies of the maxima of the PV amplitudes were, excluding the sample with the AVA concentration of 0% for back illumination, between 1.68 and 1.80 eV (this range is not shown in the Figures 4 and 5). The precise direct measurement of the band gap by spectral dependent modulated SPV is usually impossible since the relation between the in-phase and phase-shifted by 90° spectra often changes near the band gap and since the PV amplitude cannot be treated as a linear function of the photon flux through a spectrum for this and other reasons. Therefore, the onset energy of the PV amplitude (Eon) was obtained as the intersection point at zero amplitude of the tangent at the maximum slope of the spectrum (see Figure 4). Most of the onset energies were not identical for the samples with different concentrations of AVA and for front and back illumination despite the fact that the XRD measurements showed the same dominating MAPbI3 phase. For the sample with the concentration of AVA of 0%, the values of Eon were 1.549 and 1.524 eV for front (Figure 4a) and back (Figure 4b) illumination, respectively (see also the inset of Figure 4). Therefore, the band gap of MAPbI3 was decreased by 25 meV due to the interaction with np-TiO2. A pronounced peak with a maximum at 1.58 eV and a minimum at 1.60 eV appeared in the spectrum of this sample for back illumination but not for front illumination. As a speculation, the density of states slightly above the band gap of MAPbI3 was influenced by interactions at the interface between TiO2 nanoparticles and MAPbI3. 9930

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Figure 7. Optimized adsorption geometries and important geometrical parameters at an anatase TiO2 (101) surface for chloride ions without (a) and with (b) coadsorbed AVA (monodentate ester-type configuration). The shown adsorption energies are defined as the difference between the total energy of the whole complex and the energy of the AVA adsorbed TiO2 and the energy of the isolated Cl ion.

Strong differences in the values of Et were observed for front illumination of the samples, as shown in Figure 5a. The corresponding values of Et were low and amounted to 19.5, 24, 18, 21, 21, and 14 meV for the concentrations of AVA of 0, 0.5, 1, 1.6, 2.1, and 2.6%, respectively (see also the inset of Figure 5). For comparison, the exponential tails were of the order of 80−140 meV for In2S3 layers deposited from a solution-based process.38 Therefore, disorder in free MAPbI3 increased at low level of precipitation of MAPbCl3 (concentration of AVA of 0.5%) but reduced at the highest level of precipitation of MAPbCl3 (concentration of AVA of 2.6%). For back illumination (Figure 5b), the values of Et were 18, 19, 20, 19.5, and 20 meV for the concentrations of AVA of 0, 0.5, 1, 1.6, and 2.1%, respectively. This means that the disorder of MAPbI3 being in contact with TiO2 nanoparticles was practically not influenced by AVA and/or by precipitation of MAPbCl3. 3.3. Analysis of Interaction and Adsorption Energies. 3.3.1. Stabilization of Chloride Ions in the Solution in the Presence of HOOC(CH2)4NH3+. The XRD measurements indicated that chloride ions were not contributing to a crystalline phase without AVA despite the high concentration of chloride ions in the solution. The reason for this is that the MAI is more stable than MACl as indicated by thermogravimetric analysis, i.e., MACl was evaporated during evaporation of the solvent by thermal annealing.19 In contrast, chloride ions were incorporated into MAPbCl3 precipitates if AVA was present almost at small amounts in the solution. This means that chloride ions got stabilized in the solution by AVA. The interaction energies of chloride ions with MA or AVA are −4.16 or −4.23 eV, respectively. For comparison, the interaction energies of iodide ions with MA or AVA are −3.37 or −3.36 eV, respectively. Therefore, the interaction energies between chloride and iodide ions and AVA or MA are practically independent of the length of the organic molecule. This is not surprising since the ammonium group dominates

the interaction. Therefore, the interaction of one chloride ion with only one AVA cannot explain the stabilization of chloride ions in the solution in the presence of AVA. The formation of hydrogen bonds between the carboxylic and ammonium groups shall be the reason for the stabilization of chloride ions in the solution. As remark, a hydrogen bonding interaction between AVA and MA has been proposed for the formation of (AVA)xMA1−xPbI3.22 Figure 6 depicts the stabilized structures of chloride and iodide ions with one MA and one or two units of AVA. Ammonium groups interact with the chloride or iodide ions as well as with the carboxylic groups. This leads to the formation of cyclic structures with interaction energies of −6.29, −7.16, −5.60, and −6.54 eV for AVA-MA-Cl (Figure 6a), AVA-MAAVA-Cl (Figure 6c), AVA-MA-I (Figure 6b), and AVA-MAAVA-I (Figure 6d), respectively. The interaction energy is larger for complexes with chloride ions than for complexes with iodide ions, i.e., complexes with chloride ions are more stable than complexes with iodide ions. The interaction energy is larger for the AVA-MA-AVA-Cl than for the AVA-MA-Cl complex. This shows that more than one AVA can contribute to the stabilization of one chloride ion. This fact can explain the relatively large scatter in the dependence of the intensity ratio of the peaks at 15.64° and at 14.13° as a function of the concentration of AVA. The formation of complexes containing chloride ions, MA and AVA in the solution avoids the evaporation of MACl so that MAPbCl3 can precipitate during thermal annealing. 3.3.2. Role of Coadsorption of Cl− and AVA at TiO2 Surfaces. The sign of the SPV spectra for front and back illumination changed depending whether AVA was added or not added to the solution. Therefore, the adsorption of AVA together with the adsorption of chloride or iodide ions at the TiO2 surface has to be considered. Here, plausible mechanisms are proposed for effects observed in modulated charge separation. 9931

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The Journal of Physical Chemistry C

serve as seeds for crystallization of MAPbI3 and can therefore influence the electronic properties of MAPbI3. To our opinion, the formation of stable precipitates of MAPbCl3 is important for further crystallization of MAPbI3 with high electronic quality. In this sense, a reduction of the density of MAPbCl3 precipitates would increase the diameter of crystallized grains of MAPbI3. We speculate that hot casting from solvents with a high boiling point14 leads to a reduction of the density of seeds for the crystallization of MAPbI3. It shall be noted that the proof of this hypothesis needs dedicated experiments correlating the observed precipitation of CH3NH3PbCl3 in CH3NH3PbI3 with the performance of corresponding solar cells. Adsorption of AVA at the TiO2 surface is preferred due to the carboxylic groups so that a monolayer of chemisorbed AVA could be formed on TiO2 even for the lowest concentration of AVA. This caused the similar qualitative behavior of the SPV signals for all samples prepared with AVAI. It has been shown that the characteristic energy of exponentially distributed defect states below the band gap of MAPbI3 can be reduced by introducing AVAI in comparison to the preparation of MAPbI3 from a solution containing only MAI and PbCl2. The related reduced disorder may have an impact to the fabrication of solar cells based on MAPbI3 absorbers.

The different configurations of AVA adsorbed at the anatase TiO2 (101) surface result in different adsorption energies. The adsorption energies for AVA at the anatase TiO2 (101) surface are −3.63, −3.00, and −3.86 eV for the BB, BC and MET configurations, respectively. In the following, the MET configuration (see Figure 7) will be considered since this configuration is the most stable one. Without AVA, chloride or iodide ions can adsorb at the anatase TiO2 (101) surface. The adsorption energies for chloride and iodide at the anatase TiO2 (101) surface are −1.47 and −0.49 eV, respectively. Therefore, the surface of the TiO2 nanoparticles can be assumed to be covered mainly with adsorbed chloride ions if AVA was not added to the solvent (see Figure 7a). Co-adsorption of AVA and chloride ions has been analyzed for the anatase TiO2 (101) surface. The adsorption energies of chloride ions at the anatase TiO2 (101) surface is only −0.50 eV (and even +0.38 eV for iodide ions) in the case that AVA is coadsorbed (see Figure 7b). Therefore, the concentration of chloride ions is drastically reduced at the surface of TiO2 nanoparticles in the presence of AVA. The qualitative change of the presence and absence of chloride ions at the surfaces of TiO2 nanoparticles in the absence and presence of coadsorbed AVA, respectively, shall be a key for understanding the qualitative changes of the SPV spectra for back illumination. In the case that AVA is absent, a preferentially negatively charged layer of chloride ions is formed at the surface of TiO2 nanoparticles. A negative ionic charge at TiO2 nanoparticles avoids the transfer of photogenerated electrons from MAPbI3 into TiO2 nanoparticles. In the case that AVA was present in the solution, the positive charge at the ammonium site attracts electrons photogenerated in MAPbI3 what caused the change in the sign of the in-phase SPV signal. Chloride ions coadsorbed with AVA have low adsorption energy. Therefore, in the case of coadsorption of AVA and chloride ions, the addition of negative charge into TiO2 nanoparticles due to injection of photogenerated electrons from MAPbI3 can initiate the desorption of chloride ions from the negatively charged TiO2 nanoparticles which diffuse into MAPbI3 toward the external surface of the sample. As consequence, desorption of chloride ions from the surface of TiO2 nanoparticles causes a change of the direction of charge separation. After switching off the light, electrons recombine with holes, TiO2 nanoparticles get neutral and therefore chloride ions can be readsorbed at surfaces of TiO 2 nanoparticles. The diffusion of chloride ions is slow in comparison to the transport of photogenerated electrons or holes. Therefore, modulated desorption and readsorption of chloride ions appeared as a change in the sign of the phaseshifted by 90° SPV signal.

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ASSOCIATED CONTENT

S Supporting Information *

Grazing incidence X-ray diffraction patterns for perovskite prepared with a concentration of AVAI of 5.2% and discussion of the formation of modulated SPV signals for the cases where charge separation is caused by only one or by two mechanisms with electrons or holes separated toward the internal or external interfaces, respectively. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(P.P.) Telephone: +49 30 8062-43213. Fax +49 30 806243199. E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Development and Promotion of Science and Technology Talent Project (DPST, Thailand) for financial support and F. Lang for providing AVAI.

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ABBREVIATIONS AVAI, ammoniumvaleric acid iodide (HOOC(CH2)4NH3I or AVAI); MAI, methylammonium iodide; MAPbI3, methylammonium lead iodide; MAPbCl3, methylammonium lead chloride; DMF, N,N-dimethylformamide; np-TiO2, nanoporous titaniumdioxide; SPV, surface photovoltage; PBE, Perdew− Burke−Ernzerhof exchange-correlation functional; DNP, double numerical plus polarization

4. CONCLUSIONS It has been shown that addition of small amounts of AVAI into a solution containing PbCl2 and MAI caused the formation of stabilized MAPbCl3 in the deposited MAPbI3 layers even after annealing at 100 °C for 1 h. The amount of MAPbCl3 increased with increasing concentration of AVAI. In the solution, AVAI forms preferentially cyclic complexes with MACl. The cyclic complexes are stabilized by electrostatic interaction and by the formation of hydrogen bonds and strongly reduce the evaporation of MACl. It can be assumed that precipitation of MAPbCl3 starts at agglomerated cyclic complexes with ongoing evaporation of solvents. Stabilized precipitates of MAPbCl3 can

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DOI: 10.1021/acs.jpcc.5b01667 J. Phys. Chem. C 2015, 119, 9926−9933