Precursor Concentration Affects Grain Size, Crystal Orientation, and

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Precursor Concentration Affects Grain Size, Crystal Orientation, and Local Performance in Mixed-Ion Lead Perovskite Solar Cells Sarah Wieghold, Juan-Pablo Correa-Baena, Lea Nienhaus, Shijing Sun, Katherine E Shulenberger, Zhe Liu, Jason Tresback, Seong Sik Shin, Moungi G. Bawendi, and Tonio Buonassisi ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00913 • Publication Date (Web): 06 Nov 2018 Downloaded from http://pubs.acs.org on November 6, 2018

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ACS Applied Energy Materials

Precursor Concentration Affects Grain Size, Crystal Orientation, and Local Performance in Mixed-Ion Lead Perovskite Solar Cells Sarah Wieghold†,*, Juan-Pablo Correa-Baena†, Lea Nienhaus†,¶, Shijing Sun†, Katherine E. Shulenberger†, Zhe Liu†, Jason S. Tresback§, Seong Sik Shin†,‡, Moungi G. Bawendi†, Tonio Buonassisi†,* †

Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139,

United States §

Center for Nanoscale Systems, Harvard University, 11 Oxford Street, Cambridge, MA 02139,

United States

Corresponding Author *SW: [email protected], TB: [email protected].

ACS Paragon Plus Environment

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ABSTRACT A key debate involving mixed-cation lead mixed-halide perovskite thin films relates to the effects of process conditions on film morphology and local performance of perovskite solar cells. In this contribution, we investigate the influence of precursor concentration on the film thickness, grain size, and orientation of these polycrystalline thin films. We vary the molar concentration of the perovskite precursor containing, Rb, Cs, MA, FA, Pb, I, and Br from 0.4 to 1.2 M. We use optical and electrical probes to measure local properties and correlate the effect of crystallographic orientation on the inter- and intra-grain charge carrier transport. We find that with increasing precursor concentration, the grain size of the polycrystalline thin films becomes larger and more facetted. Films with small grains show mostly random grain orientation angles, whereas films with large grains are oriented with {100} planes around an angle of 20° relative to the surface normal. These films with oriented large grains also show longer-lived charge carrier lifetimes and an improved charge carrier extraction at the surface. Our results provide new insights on the role of process conditions (precursor concentration) on film morphology (grain size and orientation), and consequently on the homogeneity of local performance, which could bring perovskite solar cells beyond the state-of-the-art.

KEYWORDS:. Mixed halide lead perovskites, photoconductive-AFM, photoluminescence mapping, pole figure, grain size, local performance


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1. INTRODUCTION Mixed-cation lead mixed-halide perovskites1

arise as a promising alternative to

methylammonium lead iodide (MAPbI3) due to a higher long-term stability2,3 and bandgap tunability for applications in e.g. perovskite/silicon tandems.4 However, deposition techniques such as spin-coating,5 thermal evaporation,6 vapor-assisted deposition7 or dip coating8 result in a polycrystalline thin-film perovskite absorber layer with a large number of grains and grain boundaries (GBs). Depending on the fabrication conditions of these films, the thickness, surface roughness, grain size distribution and GB characters are observed to change, as do the figures of merit of the resulting perovskite solar cells (PSCs).9–11 Hence, it is critical to gain deeper understanding of how process conditions, microstructure and performance are linked at a microscopic level. Process parameters of interest include i.e., temperature,12,13 precursor concentration14 and spinning speed.15 The constitution of the film can also be changed e.g. by incorporating additional cations16 or anions.17 Further, PSCs fabricated employing a pressureinduced crystallization process lead to a highly preferred-orientation of crystal grains.18 Recent effort has been invested in elucidating the link between PSC microstructure and local performance and stability. An increase in GB density (by decreasing the grain size) was suggested to cause hysteresis in current-voltage measurements by aiding ionic migration via vacancies at the GBs.19,20 GBs can be detrimental for charge carrier collection by enabling trap-assisted recombination, and can lead to formation of localized energy states in the band gap due to crystal defects and segregated impurities,21 locally and globally limiting solar cell efficiency.22–24 Thus, it was proposed that preferred oriented and enlarged grains with low-angle grain boundaries (LAGBs) could minimize interfacial defect sites near GBs.18


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However, other experimental results suggested that GBs are not limiting the performance of state-of-the-art PSCs, as other recombination pathways are dominant.25 Compared to established PV technologies, a fundamental understanding is still lacking about the local effects of grain size and grain boundaries on electronic properties in PSC’s, and the causal correlation between film morphology and synthesis conditions. In this paper, we investigate the effects of precursor concentration on film morphology, grain size distribution, as well as grain orientations, and correlate these changes to charge carrier transport at the perovskite surface by combining micron- and macroscale characterization techniques. We fabricate thin films with different precursor concentrations from 0.4 M to 1.2 M (concentration relative to the DMF/DMSO solvent mixture) based on mixed-cation lead mixedhalide perovskites composed of (MAPbBr3)0.17(FAPbI3)0.83 with an addition of 5% CsI and 5% RbI, which we label here as “mixed ion” perovskites. We study the evolution of film morphology, including film thickness, grain size and GBs for different molar precursor concentrations, i.e. 0.4 M, 0.8 M and 1.2 M by atomic force microscopy (AFM). These mixed-ion perovskites make excellent solar cells with efficiencies of up to 21.6%, fill factor of 81% and a VOC of 1240 mV.1 We use grazing incidence X-ray diffraction (GIXRD) and in-plane pole figure measurements to determine the morphology and crystallographic orientation of each film. To correlate the change in precursor concentration to charge carrier transport, we perform photoluminescence (PL) and photocurrent mapping under ISC conditions. Our results show the inter-grain heterogeneity evolution with increasing precursor concentration. We also find that films fabricated with a 1.2 M concentration exhibit large grains which are oriented around an angle of 20° relative to the (100) plane (surface normal).


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2. RESULTS AND DISCUSSION Figure 1 shows the morphology change by the different precursor concentrations, namely 0.4 M, 0.8 M and 1.2 M (Figure 1a-c, respectively). This concentration range was used because it yielded the most homogenous film morphology for the studied molar concentrations, whereas lower or higher concentrations, i.e. 80° were not used in the analysis due to the influence of beam defocus at high angles.

We measured the transient PL emissions of these films on a non-quenching substrate (i.e., glass slide) to understand the optical response of these films fabricated from different molar precursor concentrations. Figure 3 shows the time-resolved dynamics of the 0.4, 0.8 and 1.2 M films. All different concentrations exhibit an initial fast decay which can be attributed to rapid non-radiative recombination via trap states, followed by a long-lived tail, which we attribute to be the free-carrier lifetime.33–35 Mono-exponential decays are fitted to the long-lived tail of the PL decays, yielding extracted carrier lifetimes of 0.29, 0.46 and 0.83 µs for the 0.4 M, 0.8 M and 1.2 M concentrations, respectively. Thus, by increasing the precursor concentration, the averaged grain size changes from 100 nm to 500 nm and a 3-fold increase in lifetime was achieved, suggesting longer-lived charge carriers for the large grain sample. It is important to note that these extracted values are averaged over large areas, which may mask the effects of localized defects such as GBs. We also note that the 1.2 M sample exhibits a larger amount of the early lifetime component compared to the 0.8 M sample, which we attribute to the local environment since it will play a major role in the fraction


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of early time quenching. In literature, several authors have observed a similar behavior for different grain sizes based on MAPbI3 where an enhancement of the non-radiative channels or trap density were attributed to an increase in the polycrystallinity.14,36,37

Figure 3. Time-resolved PL lifetimes of the different concentrations 0.4 M (orange), 0.8 M (green) and 1.2 M (blue).

To study the spatially-resolved inter-grain charge carrier transport on the micron scale, we perform micro-PL mapping (Figure 4). This technique allows for obtaining structures at subdiffraction limit due to differential sampling during a raster scan, which can give further insights of the dynamics of large vs. small grains. As the average grain size for the lowest 0.4 M sample is still significantly smaller than our diffraction limit, the signal is blurred across bright and dark grains, and fairly homogeneous PL intensities and lifetimes are obtained due to the contributions of multiple grains per diffraction limited spot (Figure 4a, d). On the other hand, the larger grains resulting from the 0.8 M (Figure 4b) and 1.2 M (Figure 4c) concentrations allow for a more precise determination of the grains and GBs. As a result, we can unambiguously identify bright and dark grains in the absolute counts and as a result, can obtain varying lifetimes for the different grain sizes (see Figure 4e and f). We attribute a high quantum yield (QY) to grains with low trap density, which will yield longer lifetimes. Since the absorption is expected to be uniform across the films due to a homogeneous thickness for each concentration, the QY can be directly correlated to the


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PL intensity. For the 1.2 M sample, we can more readily isolate emission from within a grain (860 ns) (Figure 4f, label 1) compared to at a GB (3.6 ns, early time component) (Figure 4f, label 3), and conclude the fast early-time component to the lifetime is likely due to rapid non-radiative recombination at the GB (Figure S2 Supporting Information). For the lower concentrations, grains are smaller than our confocal spot, and therefore it is not possible to unambiguously separate GB and grain emission. These results can be correlated to the ensemble PL lifetimes obtained in Figure 3, as the laser beam spot of those measurements is ca.100 µm in diameter, much larger than the diffraction limit, sampling hundreds of grains and GBs in each measurement. Furthermore, for small grains the PL lifetimes are more similar from grain to grain compared to larger grains. These homogeneous vs. inhomogeneous PL dynamics give an indication that precursor concentration and thus, the grain size plays a critical role on the PL emission properties of these film.

Figure 4. (a-c) Normalized photoluminescence emission mapping of (a) 0.4 M, (b) 0.8 M and (c) 1.2 M concentration under 405 nm excitation. Scale bar in all images: 2 µm. (d-f) Lifetime plots of the three locations indicated in the PL maps (a)-(c) for the different concentrations, (d) 0.4 M, (e) 0.8 M and (f) 1.2 M.


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We perform c-AFM measurements which are used to obtain the short-circuit photocurrent (ISC) of the half devices (FTO/SnO2/mixed ion perovskite) to study the impact on the precursor concentration on the electrical properties of the films.20,38,39 Figure 5a-c shows the ISC response under white light illumination at a measured power intensity of approx. 2 mW/cm2 in AFM contact mode revealing the heterogeneity of the ISC distribution within each sample for the 0.4 M, 0.8 M and 1.2 M concentrations, respectively. For a better visualization, the photocurrent images are overlaid on 3D topography maps, which are obtained simultaneously during the scan. To avoid fast degradation, the samples were kept in a dry box before the measurements. The samples were then scanned in a controlled humidity below 20% in air (film hydration has been reported to require over 30% relative humidity)40,41. For the 0.4 M sample (Figure 5a), the obtained photocurrent maps show a broad distribution of ISC values ranging from 10 to 170 pA. In this map, grains with high current (blue/purple) are observed beside grains with low current (red). For the 0.8 M film with an average grain size of 250 nm (Figure 5b), we observe that neighboring grains show similar ISC values next to grains where only a small current could be measured. When the precursor concentration is further increased to 1.2 M (Figure 5c), the narrowest current distribution is obtained for all samples. Interestingly, a mostly uniform current collection that spans over multiple grains is obtained. For a better comparison of the current distribution within each sample, the normalized photocurrent is plotted in Figure 5d. To rule out artifacts which influence our current measurements and thus lead to misinterpretation of the local photocurrent maps,42,43 we collected consecutive scans under illumination and in the dark of the photoresponse of the film (Figure S3, Supporting Information). Indeed, an order of magnitude higher current was measured when the light source was turned on. Additional control experiments were conducted, which show that the recorded current comes from the photogenerated charge carriers (Figure S4, Supporting


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Information). Figure S5 in the Supporting Information shows that a good mechanical contact is mandatory to ensure reliable results because the AFM tip-sample contact area can change during the scan or the AFM tip can become coated with material due to frictional forces.42 Thus, the tip becomes non-conductive and areas with no current can be seen in the collected current images. These observations may suggest that for the smaller grains as given in the 0.4 M film, the charges (measured in c-AFM mode as current) are confined within the small grains, and the GBs can hinder inter-grain charge transport due to high energy barriers at the GB despite the longer diffusion length of the carriers compared to the grain size. Thus, we expect a broad current distribution which can be seen in yellow in Figure 5d. By increasing the precursor concentration and thus the grain size, as seen for the intermediate concentration 0.8 M, the photocurrent becomes more homogeneously distributed across several grains. However, some GBs still act as barriers for current transport. This implies that certain grains and GBs hinder the transport, which effectively then funnel current through local high-conductivity areas (including across certain apparently conductive grains).44 By further increasing the concentration to 1.2 M, the distribution of the measured charge carriers is homogeneously distributed across several grains, which may indicate that the orientational alignment of the GBs (as supported by our in-plane pole measurements) reduces the energy barrier the charges must overcome to pass over the GB. If the energy barrier at the GBs is reduced, the charge carriers can travel and we expect a narrower current distribution within the sample (Figure 5d, green). We also investigate the dependency of the collected current on the grain size. Here, no correlation is found (Figure S6, Supporting Information).


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Figure 5. (a–c) ISC maps overlaid on a 3D AFM topography image under white light illumination in contact mode for the (a) 0.4 M, (b) 0.8 M and (c) 1.2 M concentration. Scale bar in all images: 500 nm. (d) Calculated normalized photocurrent counts for the 0.4 M (yellow), 0.8 M (blue) and 1.2 M (green) concentration.

To further correlate the measured photocurrent to morphological features, we calculate the surface angle, i.e. inclination angle, of each point (pixel) in the topography images to test for if the observed inter-grain heterogeneity in ISC is related to the grain morphology.42,44 We calculate surface inclination by connecting three adjacent pixels in the AFM data by forming a triangular plane, and calculating the surface inclination angle 𝛼 as the angular difference between the local surface normal and the global substrate z-axis (schematic is shown in Figure S7 in the Supporting Information, and detailed in Ref. 45). For the analysis, the obtained photocurrent is normalized because we are interested in the current distribution for small vs. large grains rather than absolute current values. Also, because the film thickness is changing by different precursor concentrations, we can’t unambiguously draw a conclusion about the amount (maximum and minimum values of the current) of the overall response. The 2D histogram of the surface inclination angle 𝛼 and the normalized photocurrent obtained from the c-AFM data is plotted in Figure S8 in the Supporting Information. It can be seen that for all precursor concentrations, the distribution peaks around a surface inclination angle of 10–20°. This implies that there is an enhanced current collection for


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low-angular orientations, measured by the difference between the local surface normal and the global substrate z-axis. In other words, this low-angular orientation suggests that the majority of grains are flat in all three films prepared by the different precursor concentrations from which we conclude that flatness of the films is prerequisite for charge extraction at the interface. Note, this surface inclination is not to be compared with a crystallographic orientation obtained earlier by XRD. Correa-Baena et al.19,25 have previously shown, that the PSC performance is mainly influenced by the grain size. Device VOC values were shown to decrease slightly however, JSC values ranging from 11.2 mA/cm2 for the lowest concentration (grain size of ~40 nm) up to 23.0 mA/cm2 were measured for their champion device (concentration of 1.2 M, grain size of ~400 nm). In addition, colloidal chemistry studies revealed that a reduced photocurrent can be associated with a smaller grain size induced by a smaller colloidal size.46,47 Our study revealed additional insights into the performance of different film thicknesses by changing the precursor concentrations of mixed-ion perovskite thin-films. Here, we showed that the process conditions, i.e. the precursor concentration, changes the film morphology and more importantly the film orientation. These changes affect the microstructure of the film and thus, highly impact the local performance of PSCs. Figure 6 shows a schematic of the perovskite thin film process using the anti-solvent method along with the crystal growth, wherein grains grow bigger for a higher precursor concentration. Chlorobenzene was used as anti-solvent to promote fast nucleation and growth.48 It was previously shown, that the grain size is a function of supersaturation where the mean grain size increases if the supersaturation increases.49 In a one-step process, all nucleation takes place during the antisolvent stage and grains grow during the annealing phase in contrast to a two-step method where the grain size can be mainly influenced in the nucleation-dominated region.49 Figure 6b depicts the


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evolution of grain size in the growth-dominated regime for the different precursor concentrations. To shed more light on the growth mechanisms for the different grain sizes, the Gibbs free energy diagram as a function of nuclei radius is used to explain the difference in grain size growth (Figure 6c). During the nucleation phase, nuclei are formed and result in almost the same nuclei size for the different precursor concentrations. Slightly larger nuclei are expected for the higher precursor concentrations due to an increase in the system entropy ∆S. The nuclei start to grow into indistinct small grains in the range of 10s of nanometer.50 During longer annealing times, the intermediates disappear and the grains begin to grow. Here, the thermodynamic driving force is related to the number of GBs which represent defects in the crystal structure. Hence, to reduce the energy of the system, the total area of GBs has to be reduced. Figure 6d shows a schematic of how the grain orientation might be influenced by the film growth process. First, various number of nuclei form during the nucleation stage. Here, the underlying substrate morphology has a big influence on the nucleation process and grain growth dynamics due to surface roughness and different density of nucleation sites.51 After nucleation, the grains start to grow during the annealing stage due to an increase in the critical free energy at elevated temperatures.48 In general, for a cubic perovskite phase, the [100], [010], and [001] directions are equivalent.51 When the grains start to grow, we observe a preferred orientation of the grains with [100] planes around an angle of 20° relative to the surface normal. When comparing the different precursor concentrations, we see an increase in the intensity peak at this angle around α = 20° of 2.5 and 3.8 in comparison to the 0.4 M for the 0.8 M and 1.2 M concentrations, respectively. This suggests that when the grains grow they become more oriented exhibiting a preferred orientation.


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Figure 6. (a) Schematics of the thin film process using the anti-solvent method. (b) Evolution of grain growth during the annealing stage. (c) Gibbs free energy plot as a function of nuclei radius (d) Schematics of the film growth of the different molar concentrations.

3. CONCLUSION In summary, we showed that the precursor concentration affects the grain size and film thickness but more importantly the overall crystallographic texture of mixed-ion perovskite films. Our results indicate how charge-carrier transport is influenced by the morphology of the films: Small grains grow more randomly compared to the surface normal of the substrate whereas larger grains exhibit a preferred growth direction. For the small grains, charges seem to be spatially confined within a grain and GBs may act as barrier for charge transport. When the grains become larger and more oriented, grain-to-grain alignment is improved and charges can travel across aligned GBs resulting in a more homogeneous charge extraction at the interface. This preferred growth direction may be also the crucial factor to obtain ordered regions in the film where an improved charge


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carrier extraction can be achieved due to a facilitation of inter-grain charge transport. By correlating several different micro- and macroscopic characterization techniques, i.e. micro-PL, AFM, c-AFM, GIXRD and pole figures, our results open a pathway to engineer and design highlyoriented polycrystalline mixed-ion perovskite films. 4. EXPERIMENTAL METHODS 4.1 Perovskite film fabrication FTO glass of 10 Ω/sq was cleaned by sonication in 2% Hellmanex water solution for 30 min. After rinsing with deionized water and ethanol, the substrates were further cleaned with UV ozone treatment for 15 min. SnO2 layers were grown as electron selective layers by chemical bath deposition on FTO. 0.5 g urea was dissolved in 40 mL deionized water, followed by the addition of 10 µL mercaptoacetic acid (Sigma Aldrich) and 0.5 mL HCl (37 wt%, Sigma Aldrich). Finally, SnCl2×2H2O (Sigma Aldrich) was dissolved in the solution at 0.001 M followed by stirring for 2 min. The deposition was carried out by placing the substrates vertically in a container filled with the above solution, in a 70 °C lab oven for 3 h. The mixed-cation lead mixed-halide perovskite solution was prepared from a precursor solution made of FAI (1 M, Dyenamo), PbI2 (1.1 M, TCI), MABr (0.2 M, Dyenamo) and PbBr2 (0.22 M, TCI) in a 9:1 (v:v) mixture of anhydrous DMF:DMSO (Sigma Aldrich). A 1.5 M stock solution of CsI (Sigma Aldrich) in DMSO was then added to the above solution in 5:95 volume ratio. Similarly, a 1.5 M stock solution of RbI (Sigma Aldrich) in DMF was added to the above solution in 5:95 volume ratio. The perovskite solution was deposited through a two-step spin coating program (10 s at 1000 rpm and 20 s at 6000 rpm) with dripping of chlorobenzene as anti-solvent during the second step, 15 s before finishing the second step. The perovskite layers were then annealed at 100 °C for 45 min. To achieve different


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concentrations, the perovskite precursor solutions (with a final concentration of 1.2 M) are diluted down to 0.8 M, and 0.4 M, by way of adding the solvent mixture of 9:1 (v:v) DMF:DMSO. 4.2 Characterization Techniques AFM topographic measurements were performed with an MFP-3D AFM (Asylum Research) in tapping mode. Images were collected with a PtIr-coated silicon probe with a resonant frequency of 70 kHz and a spring constant of 2 N/m. Pc-AFM experiments were performed with the same AFM and tip in contact mode in air. Current detection was achieved by an ORCA cantilever holder. The white light was focused from above the sample with a measured power of 2.85 mW (approx. 2 mW/cm2, measured with a silicon reference cell). The dark current was set to 0 pA after approaching the AFM tip to the sample surface. Transient PL lifetimes were obtained by timecorrelated single photon counting (TCSPC). The samples were excited by a pulsed 405 nm wavelength laser (PicoQuant LDH-C 400) with an average incident power of 100 nW at a repetition rate of 100 kHz. Excess laser scatter was removed by 450 nm and 550 nm long-pass filters (ThorLabs). The emission was focused onto a silicon single-photon avalanche photodiode (Micro Photon Devices $PD-100-C0C) using parabolic mirrors. Photon arrival times were recorded using a PicoHarp 300 (PicoQuant). Charge carrier lifetimes are estimated by fitting the long-time component of the decay to a mono-exponential decay. Micro-PL maps were obtained by integrating the photoluminescence from the perovskite film for 10 ms in an evenly distributed 5 µm square grid with each pixel corresponding to a 10 nm step size. It is important to note that due to the diffraction limited spot there will be mixing of adjacent pixels giving an average at each spot, smoothing out any intensity transitions within the film. Samples are excited by a 405 nm wavelength pulsed laser (PicoQuant LDH-C 400) with an average incident power of 10 nW at a repetition rate of 100 kHz in a confocal geometry. Excitation is focused and emission is collected


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through a 100x oil immersion objective (Nikon, NA 1.25). Samples are mounted to a 3-axis piezo stage (Physik Instrumente, P-517.3CL) which is moved in the x and y directions through homebuilt python software to obtain raster scans. Excitation and emission are separated using a 425 nm long-pass filter (Thor Labs, DMLP425) and spatially filtered with a one-to-one 50 µm telescoping pinhole with 10 cm achromatic lenses (Thor Labs). Excess laser light is further filtered with a 425 nm long-pass filter (Edmund Optics). Emission is focused on to a single photon counting detector (SPCM-AQR-13, Perkin Elmer) with a 10 cm achromatic lens (Thor Labs). Photon arrival times are recorded for PL-decay measurement by a PicoHarp 300 (PicoQuant) or integrated for PL-mapping using a DAQ (National Instruments, USB-6221 BNC). Photoluminescent lifetimes correspond to 10-minute integration at specific sample positions chosen after performing a raster scan. Grazing incidence X-ray Diffraction (GIXRD) and in-plane pole figure measurements were performed with Rigaku SmarLab with Cu-Kα radiation (λ = 1.54050 Å). For the grazing incidence X-ray diffraction, out-plane incident angle was set at 1°. Lattice parameters were deduced from Pawley refinement using TOPAS Academic V6. Applying cubic symmetry, (100) pole figure measurements were taken around 2θ = 14.09° on the 0.4 M, 0.8 M and 1.2 M samples. In the pole figure measurement, α and β denote the tilt and rotation angle respectively. The center of the pole figure is defined as tilt angle, α = 0° and the outer end is defined as α = 90°. The lattice plane normal is perpendicular to the sample surface normal at α = 0°. Line profiles were extracted after absorption correction to quantify the diffracted intensity change across α at β = 45°. Intensities at α > 80° were discarded in the analysis due to the influence of beam defocus at high angles.


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ASSOCIATED CONTENT Supporting Information. Grazing incidence X-ray diffraction patterns, lattice parameters of the thin films, lifetime fits, artifacts by c-AFM, correlation between grain size and photocurrent, schematics of 3D surface analysis, correlation between inclination angle and photocurrent. AUTHOR INFORMATION Corresponding Authors *Email: [email protected], [email protected] Present Address ¶ Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 32306 ‡ Korea Research Institute of Chemical Technology (KRICT), Daejeon 34114, Republic of Korea ORCID Sarah Wieghold: 0000-0001-6169-3961 Juan-Pablo Correa-Baena: 0000-0002-3860-1149 Lea Nienhaus: 0000-0003-1412-412X Zhe Liu: 0000-0001-7268-6214 Moungi G. Bawendi: 0000-0003-2220-4365 Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the National Science Foundation and the Department of Energy (DOE) under Grant NSF CA EEC-1041895 and in part at the Center for Nanoscale Systems (CNS),


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a member of the National Nanotechnology Coordinated Infrastructure Network (NNCI), which is supported by the National Science Foundation under NSF award no. 1541959. CNS is part of Harvard University. Part of the work was supported by the NSF grant CBET-1605495 and Skoltech 1913/R. L.N. and K.E.S. were supported by the Center for Excitonics, an Energy Frontier Research Center (EFRC) funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001088 (MIT). J.P.C.B is supported by a postdoctoral fellowship awarded by the U.S. Department of Energy, Office of Science, Office of Energy Efficiency and Renewable Energy.

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Precursor Concentration Affects Grain Size, Crystal Orientation, and

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