Influence of the Grain Size on the Properties of CH3NH3PbI3 Thin

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The influence of the grain size on the properties of CHNHPbI thin films Oleksandra Shargaieva, Felix Lang, Joerg Rappich, Thomas Dittrich, Manuela Klaus, Matthias Meixner, Christoph Genzel, and Norbert H. Nickel ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10056 • Publication Date (Web): 17 Oct 2017 Downloaded from http://pubs.acs.org on October 17, 2017

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ACS Applied Materials & Interfaces

The influence of the grain size on the properties of CH3NH3PbI3 thin films Oleksandra Shargaieva,1* Felix Lang,1 Jörg Rappich,1 Thomas Dittrich,1 Manuela Klaus,² Matthias Meixner,² Christoph Genzel,² and Norbert H. Nickel1 1

Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institute Silicon Photovoltaics,

Kekuléstr. 5, 12489 Berlin, Germany 2

Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Department of Microstructure

and Residual Stress Analysis, Albert-Einstein-Str. 15, 12489 Berlin, Germany

ABSTRACT: Hybrid perovskites have already shown a huge success as an absorber in solar cells, resulting in the skyrocketing rise of power conversion efficiency to more than η = 22%. Recently, it has been established that the crystal quality is one of the most important parameters to obtain devices with high efficiencies. However, the influence of the crystal quality on the material properties is not fully understood. Here, the influence of the morphology on electronic properties of CH3NH3PbI3 thin-films is investigated. Post-annealing was used to vary the average grain-size continuously from ≈150 to ≈1000 nm. Secondary grain growth is thermally activated with an activation energy of Ea = 0.16 eV. The increase of the grain size leads to an enhancement of the photoluminescence and, hence, indicates an improvement of the material quality. According to surface photovoltage measurements, the charge-carrier transport length exhibits a linear increase with increasing grain size. The charge-carrier diffusion length is limited by grain boundaries. Moreover, an improved morphology leads to the drastic increase of power conversion efficiency of the devices. KEYWORDS: hybrid perovskite, secondary grain growth, morphology, post-annealing, diffusion length, large grains

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Introduction Over the course of a few years, hybrid perovskites have shown a remarkable development leading to solar cells with power conversion efficiencies (PCE) of more than 22 %.1 This is owed to its extraordinary properties comprising a high absorption coefficient,2 high charge-carrier mobility3, and large carrier diffusion-length.4 In addition, the broadly tunable band-gap renders perovskites one of the most promising hybrid materials for solar cell applications.3,5–7 Moreover, several reports have shown that the combination of perovskites with conventional inorganic semiconductors such as silicon or copper indium gallium diselenide in a tandem device has the potential to surmount the Shockley–Queisser limit for single junction solar cell.8–12 It has been shown that the efficiency of the perovskite solar cells often depends on the crystalline quality of the material.13 The solution based preparation methods often lack the control over the crystallization process leading to the formation of polycrystalline layers. Many reports have shown a beneficial influence of an enhanced grain size on the electronic properties of perovskite thin-films and thus, on the device performance.14–16 Moreover, it has been demonstrated that a homogeneous morphology within a perovskite thin-film is required to achieve solar cell efficiencies of more than 15 %.17 The improvement of the morphology is especially important for possible upscaling of the perovskite solar cells.18 Based on this, fabrication techniques have been developed, which allow annealing in the presence of a solvent or methylammonium iodide (CH3NH3I).19–22 In this work, we present a systematic study of the impact of the morphology on electronic properties of methylammonium lead iodide. We applied the very general approach of the secondary grain growth induced by a post-annealing. The post-annealing led to a continuous increase of the grain size from 150 nm up to 1 µm. The process was thermally activated with an activation energy of about 160 meV. An increase of the average grain-size was accompanied by an increase of the photoluminescence intensity and power conversion efficiency of the devices. Moreover, we showed the correlation between the diffusion length of the charge carriers and the


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grain size by direct measurement. The charge-carrier diffusion length increased linearly with grain size and reached values of up to 900 nm for large-grained samples.



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Results Figure 1 shows the surface temperature, TS, of CH3NH3PbI3 perovskite samples as a function of the annealing time, tann. The samples were placed on a hot plate at a given set-temperature, Tset, and the time dependence of the surface temperature was measured. Independent of Tset the surface temperature of the perovskite samples increased linearly for about 30 s and then approaches a saturation value asymptotically. When the surface temperature reached TS ≈ 190 °C, the specimens started to decompose. This is consistent with previous reports.23 The decomposition temperature, which amounts to Tdecomp = 190 °C is indicated by the arrow in Figure 1.

Figure 1. Surface temperature, TS, as a function of annealing time, tann. CH3NH3PbI3 perovskite samples were heated on a hot plate with set temperatures, Tset, ranging from 200 to 300 °C. The decomposition temperature, Tdecomp is indicated by the red arrow. The schematic setup is depicted in the inset.

The influence of TS on the microscopic structure of the samples is shown in Figure 2. At settemperatures of Tset = 200, 250, and 300 °C, the annealing time was kept constant at tann = 15 s, and the surface temperature reached values of TS = 125, 158, and 193 °C, respectively. Note


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that for each time a freshly prepared sample was used. Top view scanning electron microscopy micrographs of the annealed samples are shown in Figure 2. With increasing surface temperature from 125 to 158 °C [Figure 2 (a) and (b)], an increase of the average grain-size from = 271 to = 469 nm was observed, respectively. However, annealing at TS = 193 °C resulted in the appearance of distinguishable features such as holes due to the decomposition of the perovskite (see Figure 2 (c)). Figure 3 shows the influence of the annealing time on the morphology of the perovskite samples for Tset = 200 °C. At annealing times of tann = 5, 35, and 60 s the surface temperature of the perovskite samples amounted to 61, 170, and 176 °C, respectively. The sample annealed at the lowest surface temperature of TS = 61 °C did not exhibit visible changes in morphology [(Figure 3 (a)]. However, a further increase of TS to 170 and 176 °C induced recrystallization that resulted in an increase of the average grain-size to 644 and 1048 nm, respectively [(Figure 3 (b) and (c)]. Furthermore, a comparison of the micrographs for TS = 170 and 176 °C showed a pronounced increase of . It is likely that this increase is due to the prolonged annealing time required for the temperature saturation.

Figure 2. Top view scanning electron microscopy (SEM) micrographs of CH3NH3PbI3 perovskite layers after annealing for tann = 15 s at Tset = 200 (a), 250 (b), and 300°C (c). An annealing time of tann = 15 s, leads to surface temperatures of TS = 125, 158, and 193 °C, respectively.



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Figure 3. Top view scanning electron microscopy (SEM) micrographs of CH3NH3PbI3 perovskite layers after annealing at Tset = 200 °C. For annealing times of (a) tann = 5 s, (b) tann = 35 s, and (c) tann = 60 s surface temperatures of TS = 61, 170, and 176 °C were reached, respectively.

In Figure 4, cross-sectional micrographs of an as-prepared (a) and an annealed perovskite sample (b) are depicted. Annealing was performed at Tset = 200 °C for 60 s to reach a surface temperature of TS = 176 °C. The specimens had a thickness of about 345 nm. Annealing changed the microstructure of the samples significantly. Crystalline grains extended from the substrate to the surface and grain boundaries parallel to the substrate vanished completely due to secondary grain growth. This led to grains with diameters much larger than the thickness of the sample. Consequently, grain boundaries occurred only normal to the surface and the number of grain boundaries decreased significantly after annealing.


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Figure 4. Cross-section SEM micrographs of an as-prepared (a) and an annealed CH3NH3PbI3 perovskite sample (b) on top of c-TiO2 and FTO. Annealing was performed at Tset = 200°C for 60 s to reach a surface temperature of TS = 176 °C.

For all post-annealed samples, the average grain-size was determined from SEM micrographs. In Figure 5, is plotted as a function of TS (data points). The error bars indicate the standard deviation of the grain size. For TS < 120 °C the average grain-size was constant. At higher TS, a modest increase of is observed that is independent of the set temperature. Moreover, post-annealing resulted in a fairly homogeneous distribution of grain sizes. However, for TS >150 °C the increase of the average grain-size depended on Tset. increased and reached values of about 1 µm (black squares and blue triangles in Figure 5). For these specimens, the grain-size distribution increased significantly. However, while similar average grain-sizes were obtained for Tset = 200 and 300 °C, the higher set temperature resulted in the decomposition of the sample since the surface temperature exceeded the decomposition threshold. Interestingly, the largest grains were observed in these samples.



Figure 5. Average grain-size, , of annealed samples as a function of TS, obtained from top view SEM micrographs. The CH3NH3PbI3 perovskite layers were annealing at Tset = 200, 250, and 300 °C. The error bars represent standard deviation.

The grain size distribution of as prepared and after annealing at TS = 170 and 176 °C samples is shown in Figure 6. As prepared samples exhibited a monomodal size distribution with a mean value of = 154 nm in a range from 50 to 250 nm. After annealing at TS = 170 °C, the sample showed the appearance of a second fraction leading to a value of = 644 nm. Furthermore, an increase of the temperature to 176 °C stimulated an additional growth of the grains, which resulted in a broad monomodal distribution with = 1048 nm.

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Ts = 22°C

10 Ts = 170°C Ts = 176°C

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(nm) Figure 6. Grain size distribution of as prepared (22 °C) and annealed samples at TS = 170 and 176°C obtained from top view SEM micrographs of the corresponding layers.

To ensure that annealing did not change the lattice of the CH3NH3PbI3 perovskites, XRD measurements were performed. Figure 7 shows x-ray diffractograms of a perovskite reference sample (a) and two specimens annealed at TS = 176 °C (b) and TS = 193 °C (c). According to the position of the reflections, the reference sample was crystallized in the tetragonal perovskite phase.24 The x-ray reflection peaks (211) and (213), that are indicative of the tetragonal phase, ACS Paragon Plus Environment

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are depicted in red [Figure 7 (a)]. The diffractogram of the sample annealed at TS = 176 °C is shown in [Figure 7 (b)]. For a better comparison, the peak positions of the reference sample are indicated by dashed lines. The diffractogram of the annealed sample shows a close similarity of the peak positions and also contains the reflexes that are assigned to the tetragonal perovskite phase [Figure 7 (b)]. Since the perovskite layer was prepared on top of a FTO/TiO2 coated substrate, additional reflexes originating at the substrate were observed. These reflexes are marked with red asterisks. It is important to point out that the sample did not show a diffraction peak at about 12.6 ° that is indicative of the presence of hexagonal lead iodide (PbI2).

Figure 7. X-ray diffractograms of CH3NH3PbI3 perovskite layers. The diffractogram of a reference sample is depicted in (a). Perovskite thin films annealed at TS = 176 °C and TS = 193 °C are shown in (b) and (c), respectively. The samples were fabricated on FTO/TiO2 coated substrates and reflexes attributed to the substrates are marked by red asterisks. The green hash tag (#) in (c) indicates the (001) reflex of hexagonal PbI2.



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The specimen annealed at TS = 193 °C exhibited the same diffraction peaks as the reference sample. Hence, it was also crystallized in the tetragonal phase [see Figure 7 (c)]. However, an additional peak occurred at 12.65 ° indicating the presence of hexagonal PbI2 [see # in Figure 7 (c)]. Hence, annealing at TS > Tdecomp led to a loss of CH3NH3+ ions that was accompanied by a structural rearrangement from tetragonal perovskite to hexagonal lead iodide. Since both crystal structures are visible in Figure 7 (c), it can be concluded that the decomposition commenced at the surface of the sample. In addition, this is consistent with the degradation observed in SEM micrographs [see Figure 2 (c)]. The influence of the post-annealing step on the electronic quality of the perovskites was investigated using photoluminescence measurements, PL. In Figure 8 (a), PL spectra of a perovskite samples annealed at Tset = 200 °C are plotted. The measurements were performed after the indicated surface temperature, TS, was reached. The spectrum obtained for TS = 22 °C represents the as-prepared specimen. This reference sample exhibited a PL peak at λ = 777 nm. This wave length corresponds to an optical band-gap of 1.6 eV and is in good agreement with values from the literature.25 With increasing surface temperature from TS = 22 to 176 °C, the PL peak shifted to longer wave lengths and reaches a value of λ = 782 nm for an annealing at TS = 176 °C. This corresponds to a decrease of the optical band-gap by about 20 meV. In addition, post-annealing resulted in an increase of the PL intensity by about 80 %, which is due to the increase of the average grain size (see Figure 5). Moreover, the specimens after post-annealing at Tset = 200 °C exhibited a strong increase of the life-time as can be seen in Figure 8 (b) where the PL intensity is plotted as a function of time. The life-time increases with increasing Ts visualized by a slower decay of the PL intensity.


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a)

b)

Figure 8. a) Photoluminescence spectra and b) time-resolved photoluminescence intensity of CH3NH3PbI3 perovskite layers on top of glass before (black curves) and after annealing at Tset = 200 °C. The data were taken after the indicated surface temperatures, TS, were reached; excitation wavelength was a) λex = 365 nm and b) λex = 500 nm.

Complementary to the PL measurements, it is necessary to analyze the influence of the microscopic structure on the transport length of the material. Therefore, the modulated surface photovoltage (SPV) method after Goodman26 was used for the evaluation of the diffusion or transport length (L) of the light-induced charge carriers as a function of grain size. For SPV measurements, perovskite samples were prepared on top of FTO coated glass without a TiO2 layer. The roughness of the FTO layer led to thicker perovskite layers. Figure 9 shows the lightintensity dependence of L while the SPV signal was kept constant (Goodman plots) of the sample annealed at TS = 176 °C as a representative for all other samples. The absorption length of the CH3NH3PbI3 perovskite was taken from literature.27 The SPV measurements after Goodman were performed near the band gap of CH3NH3PbI3. The light intensity required for keeping the SPV signal constant is in direct correlation with wavelength and intensity of the SPV signal. For the used photovoltages from 20 to 50 µV, the dependence of the light intensity on the



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absorption length was fitted with straight lines. The negative value of the diffusion length, L, of the light-induced charge carriers was obtained from the intersection point of the linear fits with the x-axes. The diffusion length amounted to L ≈ 0.9 µm, which is in good agreement with the average grain-size = 1048 nm of the sample.

Figure 9. Dependence of the light intensity required for keeping different surface photovoltage signals of 20, 30, 40, and 50 µV constant on the absorption length of the annealed CH3NH3PbI3 specimen.

In the same manner, the transport-length measurements were performed on as-prepared samples and on samples annealed at surface temperatures of TS = 125, 148, and 170 °C. In Figure 10, the charge carrier diffusion length is shown as a function of the average grain size, .


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Figure 10. Charge carrier diffusion length, L, as a function of the average grain size, , of as-prepared sample (blue triangle) and after annealing at Tset = 200°C with a surface temperature of TS = 125, 148, 170, and 176 °C (red circles). The black dash line represents a slope of unity. Error bars represent standard deviation values of grain diameter and the accuracy of the SPV method.

The increase of for a constant layer thickness shows a direct correlation between average grain-size and L with a slope of 0.9.



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Figure 11. (a) Current – voltage characteristics of the solar cell with as-prepared perovskite layers (red dashed curve) and post-annealed (black solid curve) perovskite layers (Ts = 176°C). The arrows indicate a direction of the J-V scan and the circles mark maximum power points obtained after power-point tracking of 60 s. The inset depicts the power conversion efficiency during maximum power-point tracking of the corresponding devices. (b) Power conversion efficiency of the devices in the forward and reverse scan directions depicted as a function of the average grain size. The data were obtained after post-annealing at Tset = 200°C. Error bars represent the standard deviation for the power conversion efficiency values and the grain diameter.

Increased values of charge carrier diffusion length indicated possible benefits of the enhanced grain-size on the photovoltaic performance of the devices. Therefore, perovskite layers subjected to the post-annealing at different surface temperatures were used as an absorber in solar cells. Figure 11 (a) shows current–voltage characteristics of the devices (FTO/TiO2/perovskite/spiro-OMeTAD/Au) under AM1.5G illumination. The red and black curves were measured on devices, where the average grain size amounted to 150 and 1050 nm, respectively. The device with ≈ 150 nm exhibited Voc values of 1.02 V and Jsc amounted to 19.8 mA/cm2. An increase of the grain size of the perovskite layer to ≈ 1050 nm resulted in a pronounced enhancement of the device parameters to Voc ≈ 1.1 V and Jsc ≈ 21.5 mA/cm2. Both devices exhibited a hysteresis. To compare the hysteresis of the devices, the hysteresis index (H) was calculated according to the literature.28 The H values of the reference and prepared by post-annealing at Ts = 176°C solar cells amounted to 0.38 and 0.25 respectively. In addition, the H values were of the same order as reported by other groups.29 Therefore, for the correct determination of the power conversion efficiency, maximum power-point tracking was performed over a period of 60 s. The stabilized power conversion efficiencies of as prepared and postannealed specimens amounted to η ≈ 9.5 and ≈14 %, respectively. Figure 11 (b) depicts power


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conversion efficiency of the devices annealed at Tset = 200°C in comparison to as-prepared devices. The increase of the efficiency with increasing average grain-size was observed for both scanning directions. For the distribution analysis, data from 12 different devices were used for each grain size.

Discussion The second annealing step showed a significant influence on the microscopic structure of CH3NH3PbI3 perovskites. Our study demonstrates a strong dependence of the grain-size growth on the parameters of the post-annealing step. Thereby, the grain size of CH3NH3PbI3 perovskite can be tuned from 150 nm to more than 1 µm by changing the surface temperature, TS, of the perovskite layer (see Figure 5). Moreover, the morphology of the layers annealed at three different set temperatures showed a strong dependence of the saturation rate at a chosen Tset. The largest grains were achieved at the highest value (Tset = 300 °C). However, already after 15 s, the surface temperature of the samples exceeded 190 °C and degradation of the specimens commenced. On the other hand, an enhanced control of the secondary grain growth is possible at lower Tset while simultaneously avoiding degradation of the layers. Secondary grain growth has been observed for many materials comprising ceramics, perovskites,30,31 and semiconductors such as silicon32,33 and germanium.34 The process of the secondary grain growth can be described as a post growth process of certain energetically favorable grains nucleating within the pre-existing crystalline matrix. During this type of growth, pre-existing grains are growing at the expense of smaller grains and without the formation of new nuclei.35 The small grains may also increase in size but large grains are growing with faster growth rates. Ideally, the process continues until all large growing grains have impinged and all small grains are annihilated. Often, secondary grain growth is driven by the minimization of the surface energy or grain boundary energy.35,36 In the bulk, secondary grain growth is driven by the energy difference of ACS Paragon Plus Environment


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atoms at different sites of a grain boundary. Therefore, the system tends to minimize the grain boundary area by secondary grain growth. In thin films, however, the surface energy is dominating with an inverse correlation to the film thickness.32,35 Secondary grain growth can be depicted as grain boundary migration that is induced by an external force and limited by a migration of species at an applied external force. For many materials, the observed activation energy is related to an activation energy of self-diffusion.37 For example, values for the grain-growth activation energy for silicon are comparable with selfdiffusion energies that amount to about 4.1 – 5.1 eV.38 Empirical results have shown that the presence of impurities and/or dopants can result in a significant decrease of the activation and self-diffusion energies.39–41 Figure 12 shows an Arrhenius plot for the average grain size. The activation energy was obtained from the slope of a least-squares fit of the data (dashed line). The activation energy for amounts to EA = 0.16 eV indicating that the diffusion barrier for self-diffusion and hence, secondary grain growth is rather small. However, it is consistent with the fact that CH3NH3PbI3 perovskites dissociate at low temperatures of only ≈190 °C.23


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Figure 12. Dependence of the average grain diameter on inverse temperature (1/T in K) for samples annealed at Tset = 200, 250, and 300 °C. The error bars represent standard deviation.

Besides of an improvement of the microscopic structure, secondary grain growth also resulted in enhanced electronic properties. An excellent measure of this improvement was the observed increase of the PL intensity with increasing grain size (see Figure 8). Radiative recombination increased, while non-radiative recombination due to the presence of localized defects decreased.42 In addition, the observed red shift of about 20 meV of the optical band-gap is indicative of an improved arrangement of the organic cation within the inorganic cage. Most likely, this is caused by the self-migration of lattice atoms during the secondary growth that results in the reduction of structural disorder at the surface and grain-boundary interfaces.15,43,44 The improvement of the electronic properties is corroborated by SPV data (see Figure 10). With increasing , a pronounced increase of the transport length of up to L = 900 nm is observed. Previously, such long transport lengths were observed only in mixed chloride/iodide perovskites4 and single crystal perovskites.45 The linear dependence of L on the average grainsize shows a slope of 0.9 (see Figure 10). Most likely, the small deviation from unity is due to different defect densities in the bulk of the material and at the grain boundaries. A higher defect concentration at the grain boundaries results in a potential barrier in perovskites that reduces the charge-carrier diffusion length to a value that is smaller than the actual grain size. It is conceivable that the origin of the grain-boundary defects is related to a compositional change with a deficiency of organic cations or the presence of additional lead iodide.

Summary In summary, we presented a detailed investigation on the structural and electronic properties of post-annealed CH3NH3PbI3 perovskites. Secondary grain growth was observed for TS > 70 ACS Paragon Plus Environment


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°C. At the post-annealing temperatures of 178 °C the average grain-size amounted to = 1048 nm. The secondary grain-growth was thermally activated and exhibits an activation energy of EA ≈ 0.16 eV. In conjunction with the enhancement of the structural properties of CH3NH3PbI3, an improvement of the electronic properties was observed. With increasing , an increase of the PL intensity by about 80 % was measured. In addition, the transport length of charge carriers, which is correlated to the average grain-size, increases with increasing . For an average grain-size of ≈1050 nm a diffusion length of L = 900 nm was obtained. This value exceeds the thickness of CH3NH3PbI3 absorption layers in state-of-the-art solar cells. The solar cells based on perovskite layers with improved morphology by secondary grain growth exhibited strong enhancement of the power conversion efficiency.

Methods Sample preparation. 300 – 400 nm thick layers of perovskite were obtained by spin-coating of the precursor solution on top of FTO/TiO2 for SEM and XRD, on top of glass substrates for PL or pristine FTO-coated glass for SPV measurements. All perovskite layers were spin-coated in a nitrogen-filled glove box. First, 100 µL of an 1.2 M equimolar solution of methyl ammonium iodide (MAI)46 and PbI2 in a mixture of DMSO and GBL (30/70) was spin-coated on top of a substrate at 1000 rpm for 30 s and 5000 rpm for 10 s. At the final spin-coating step, 100 µl of toluene was dropped on the sample. Next, samples were annealed at Tset = 100 °C for 10 min according to literature.47 Then, obtained perovskite layers were subjected to post-annealing at different Tset. Post-annealing was conducted in a nitrogen atmosphere with an overpressure of p = 2.5 mbar. The surface temperature measurement was performed on a FTO-glass substrate with a Ktype thermocouple, and a temperature controlled heating plate inside of the nitrogen-filled glove box. The substrate connected to the thermocouple was placed on top of the heating stage when


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the temperature of the stage reached the desired set values, Tset. Temperature readings were taken every five seconds. The

samples

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diffraction

and

photoluminescence spectroscopies. The photoluminescence spectra were excited with 365 nm pulse laser (pulse width of 0.5 nm, pulse energy of 10-20 nJ, and 10 Hz repetition rate). The time-resolved photoluminescence spectra were excited with 500 nm laser and measured at 780 nm. The grain size analysis was performed by processing the top view SEM images with ImageJ software. The diffusion length of the charge carriers was obtained from surface photovoltage measurement after Goodman on FTO/perovskite/polymethylmethacrylate (PMMA) stack according to literature.26,48,49 Surface photovoltage (SPV) can be applied for the direct measurement of the diffusion or transport length of photo-generated charge carriers in semiconductors. In the approach after Goodman, the SPV signal is kept constant in order to keep constant surface recombination. The corresponding light intensity is measured as a function of the wavelength and plotted as a function of the related absorption length (also called Goodman plot). The diffusion length is found from the intersection point at zero intensity. In polycrystalline thin-films, one has to keep in mind that the measured transport length cannot be larger than the dimension of the largest grains, which occupy most of the surface area. In this sense, the measured transport length gives a lower limit of the diffusion length. Solar cells of FTO/c-TiO2/perovskite/spiro-OMeTAD/Au architecture were prepared according to the literature. First, SnO2:F – coated glass (FTO) was cleaned with detergent/de-ionized water, acetone and isopropanol. On top of SnO2:F glass a 60 nm thick compact TiO2 layer was deposited via spin-coating from a sol-gel solution and calcined at 500 °C for 1 hour.50 Next, a 300 – 400 nm thick layer of perovskite was deposited as described above. Then, samples were post-annealed at different Tset until desired was obtained. After post-annealing, samples cooled-down to room temperature and the hole conductor 2,2',7,7'-Tetrakis[N,N-di(4methoxyphenyl)amino]-9,9'-spirobifluorene (spiro-OMeTAD) was spin-coated at 2000 rpm from a



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precursor solution. The solution contained 80 mg of spiro-OMeTAD in 1 mL of chlorobenzene, 46.4 µL of a bis(trifluoromethane) sulfonimide lithium salt (LiTFSI) stock solution (170 mg/mL in acetonitrile), and 8.5 µL of 4-tert-butylpyridine. Finally, 80 nm thick Au contacts were deposited by thermal evaporation through a shadow mask. The active area of the devices amounted to 0.16 cm2. Current-voltage characterization was performed in air under AM1.5G illumination generated by “Steuernagel Lichtechnik” solar simulator. The solar simulator was adjusted to 100 mW/cm2 by calibrated silicon solar cell (Fraunhofer ISE). The delay-time was set to 60 s and the acquisition time to 40 s.

AUTHOR INFORMATION Corresponding Author * [email protected] Author Contributions The manuscript was written through contributions of all authors. ACKNOWLEDGMENT The authors are grateful to C. Klimm for taking SEM images. O.S. acknowledges the Graduate School Hybrid4Energy for financial support.

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