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Large-Scale Compositional and Electronic Inhomogeneities in CH3NH3PbI3 Perovskites and Their Effect on Device Performance Qing Sun, Paul Fassl, and Yana Vaynzof ACS Appl. Energy Mater., Just Accepted Manuscript • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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ACS Applied Energy Materials

Large-Scale Compositional and Electronic Inhomogeneities in CH3NH3PbI3 Perovskites and Their Effect on Device Performance Qing Sun†§, Paul Fassl†§ and Yana Vaynzof†§* †

Kirchhoff-Institut für Physik, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld

227, 69120 Heidelberg, Germany. §

Center for Advanced Materials, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld

225, 69120 Heidelberg, Germany. Corresponding Author * Email: [email protected].

ACS Paragon Plus Environment

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ABSTRACT

Photovoltaic devices based on lead halide perovskite materials are under extensive investigation due to their remarkably high efficiencies. However, it has become evident that the photovoltaic performance of these devices shows wide distributions, even across individual perovskite samples, resulting in the practice of presenting performance histograms, rather than single values. We demonstrate that these variations in performance are related to the large-scale compositional and electronic inhomogeneities of perovskite films, which we characterize using X-ray and ultra-violet photoemission spectroscopy mapping. These inhomogeneities are observed for three different fabrication methods of the perovskite layers, and while they are unaltered by storage in nitrogen and dry air, long-term exposure to vacuum increases the homogeneity of the surface structure. We demonstrate that perovskite films with a non-uniform surface structure result in broadly varying photovoltaic performance despite seemingly similar bulk properties such as absorption and microstructure.

Keywords: perovskite solar cells, photoemission spectroscopy, inhomogeneity, performance distribution, chemical composition, ionization potential


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Since the first application of hybrid organic-inorganic halide perovskites in solar cells by Kojima et al. in 20091, perovskites have gained significant attention from the photovoltaic research community, resulting in a rapid increase in their power conversion efficiencies (PCE) from 3.8%1 to over 22%2. This remarkable progress is mostly attributed to the advantageous optoelectronic properties of perovskite materials, such as high absorption coefficients3, long charge carrier lifetime and diffusion length4–7, high electron/hole mobility8 and low exciton binding energy9. Significant work has been dedicated to improving the PCE of perovskite solar cells with particular focus on active layer composition10–16, optimization of perovskite film morphology17,18 and adjacent transport layers3,19. While early reports often showed only the maximum PCE value of the fabricated solar cells1,20–24, in recent years researchers routinely provide statistics and/or histograms of photovoltaic device performance parameters17,25–28. To obtain sufficient statistics, most research groups fabricate their devices on small area substrates with multiple (68) solar cells (often referred to as ‘pixels’) on each substrate. A batch of devices typically consists of multiple substrates (8-12), allowing for the fabrication of several dozens of solar cells within a single batch. Most studies report a wide distribution of photovoltaic performance of up to 6-7 % in PCE19,29,30. To illustrate this problem, we fabricated a set of 8 substrates with 8 pixels on each (a total of 64 solar cells) under identical conditions and in the device architecture ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/BCP/Ag (Figure 1a). The variation in performance of pixels on each individual substrate as well as of the entire batch is shown in Figure 1b. Both the average and standard deviation of PCE vary widely from substrate to substrate. The combination of all PCE values of the 64 solar cell devices results in a broad distribution comparable to that reported in various literature studies (Figure 1c). While it is likely that differences in the reported


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performance of studies by different groups can be attributed to different fabrication conditions18,31–34, the origin of the wide performance distribution of identically prepared perovskite solar cells within single studies remains unclear.

(a)

(c)

(b)

(e) (d)

XPS

(f) I/Pb

(g)

(h) IP

UPS

Figure 1. (a) Illustration of the perovskite solar cell structure employed in the paper. (b) Variation in PCE of eight identical devices (8x8 = 64 pixels) from one batch (black line), as well as distributions among eight pixels from each device (colored lines). Lines displayed are the Gaussian fit to each distribution. The actual histograms of all eight devices are shown in Figure S1a. (c) PCE spread (i.e. PCEmax – PCEmin) reported in 50 representative studies (for a detailed list of references, see Supporting Table S1) from 2013 to 2017. (d) Illustration of the experimental approach of XPS and UPS mapping. (e) Pb4f and I3d spectra collected at each point. (f) I/Pb map – a two-dimensional plot of iodine to lead (I/Pb) ratio calculated by dividing the atomic percentage of iodine by the atomic percentage of lead for all measured points over the entire sample. (g) UPS spectra collected at each point. (h) IP map - a twodimensional plot of ionization potential (IP) for all measured points over the entire sample.


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Recently, Emara et al. have shown that the surface composition of perovskite films has a strong influence on their electronic structure and consequently on the device efficiencies35. In order to elucidate the origin of the different published ionization potential (IP) values for the same material CH3NH3PbI3 ranging from 5.1 to 6.6 eV in the literature36–41, they performed ultraviolet and X-ray photoemission spectroscopy (UPS and XPS) measurements on CH3NH3PbI3 films prepared by varying the precursor ratio composition as well as by using different deposition methods. Their results show that there exists a correlation between the surface composition (i.e. N/Pb ratio) and the IP of the perovskite layer. Varying the IP of the perovskite layer will affect the photovoltaic device performance, due to a higher accessible opencircuit voltage (VOC) and more efficient charge transport throughout the devices, when optimized energy level alignment between the perovskite and the adjacent transport layers are obtained38,42– 45

. In this work, we perform XPS and UPS mapping on perovskite films and demonstrate that

identically prepared samples may result in large compositional and electronic inhomogeneities, between both different substrates and across single samples. In order to probe the composition and electronic structure of the perovskite layers, we performed XPS and UPS mapping on CH3NH3PbI3 films deposited on PEDOT:PSS coated ITO glass substrates across a 7.2 x 7.2 mm2 area in the center of the substrate, which contains the active areas of the eight pixels of the final device (Figure S1b). The XPS map was generated by performing a serial acquisition of 324 XPS measurements with an X-ray spot size of 400 µm, generating a two-dimensional map across the measured CH3NH3PbI3 film area (Figure 1d). At each measurement point, the iodine to lead atomic ratio (I/Pb) is calculated by dividing the atomic percentage of iodine by that of lead as obtained from the collected Pb4f and I3d spectra (Figure 1e). A representative two-dimensional


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plot of I/Pb ratios (I/Pb map) across the entire sampled area is shown in Figure 1f, with the xand y- axis representing the probed position and the color change representing the variation in surface composition. UPS mapping was similarly performed by measuring the secondary photoemission onset and high-energy valence band features (Figure 1g) using a 400 µm UPS area. It was from these measurements that the IP across the sample was calculated (Figure 1h). We utilized several fabrication methods for the deposition of perovskite films, and show that the variation in surface composition and electronic structure are not limited to a single recipe. The first recipe, abbreviated as Pb(OAc)2 in the following discussion, utilizes lead acetate trihydrate (Pb(OAc)2 · 3H2O) as a precursor and has been widely applied in the fabrication of perovskite solar cells since its introduction by Zhang et al. in 201546. The second and third recipes, abbreviated as sol-eng and 1-1-1, are based on the anti-solvent treatment introduced by Jeon et al.25 and Ahn et al.27, respectively. The sol-eng method is based on the use of an antisolvent during deposition, which removes the solvent of the perovskite precursor solution and forms an intermediate phase of MAI-PbI2-DMSO. Similarly, based on an anti-solvent treatment, perovskite films were fabricated using the 1-1-1 recipe via the Lewis base adduct of PbI2. Finally, we compare the degree of electronic inhomogeneity to the distribution of the photovoltaic performance on the same substrate and find a direct correlation between the two. Figure 2 shows representative I/Pb maps of three CH3NH3PbI3 films fabricated using the Pb(OAc)2 recipe under identical conditions. The first two samples (Figure 2a and 2b) exhibit a rather uniform composition, albeit with a different average I/Pb ratio. The third sample (Figure 2c), however, reveals large compositional inhomogeneities across the sample surface. The differences between the three samples can also be seen in the histograms of their I/Pb distributions (Figure 2d).


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Figure 2. I/Pb maps of perovskite samples processed by Pb(OAc)2 recipe, where (a) is a uniform film, (b) is quite uniform but with a different averaged I/Pb ratio and c) is a sample with very large inhomogeneity. (d) Histograms of the I/Pb ratio distribution of these three samples (black – sample in panel (a), red – sample in panel (b), blue – sample in panel (c)).

It is important to note that the compositional inhomogeneities at the surface of the sample are not related to changes in microstructure of the perovskite film. Both the root-mean-square (RMS) surface roughness and the average grain size (obtained from atomic force and scanning electron microscopy measurements) remain largely unchanged at different spots across the sample (Figure S2, S3). The optical properties are also unchanged across the sample as can be seen from


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UV-visible absorption measurements (Figure S2c). Phase impurities as a cause for the observed inhomogeneities have also been ruled out, as no residual PbI2 or lead acetate were detected by Xray diffraction measurements (Figure S11).

Figure 3. I/Pb maps of perovskite films processed by sol-eng recipe: (a) uniform and (b) an inhomogeneous film and using the 1-1-1 recipe: (c) uniform and (d) an inhomogeneous film. Corresponding histograms of I/Pb ratio distribution of (e) sol- eng samples and (f) 1-1-1 samples.

Changing the fabrication method of the perovskite films yields similar results. Figure 3 depicts I/Pb maps obtained for the sol-eng and 1-1-1 recipes. Both recipes can lead to compositionally uniform samples (Figure 3a and 3c) or result in films with noticeable inhomogeneities (Figure 3b


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and 3d). Similar to perovskite films prepared using the Pb(OAc)2 recipe, compositional inhomogeneities at the film surface are not related to variations in the microstructure across the sample. AFM and SEM images (Figure S4a and S4b, S5 and S6) display no obvious changes in microstructure at different areas of samples. The optical properties are also unchanged as can be seen from UV-visible absorption measurements (Figure S4c and S4d). The variation in the chemical composition can be attributed to the fact that films cannot be produced in identical environmental conditions, even within the same batch. The first sample is normally prepared in a low solvent vapor concentration atmosphere. Subsequent samples will always have a slightly higher concentration of solvent vapor in the atmosphere than the previous ones. Different solvent atmospheres during spin-coating of perovskite films has been shown to have an impact on the crystal formation and film morphology21,31,47 and it is likely to also influence the chemical composition at the sample surface. Another possible reason for the variations could be small differences in the temperature across the heating plate used for the annealing of the samples. The conditions under which the samples are stored after fabrication might also have an effect on the surface composition. We have stored initially inhomogeneous samples in three sets of environments typically used by researchers: a N2-filled glovebox, a dry air glovebox and vacuum for 50 hours. There was no obvious change in I/Pb ratio, and its distribution, for the samples stored in N2 and dry air, while the sample kept in vacuum showed a significantly reduced I/Pb ratio over the entire sample’s surface (Figure S7). We attribute this change to loss of methylammonium iodide from the perovskite surface due to prolonged exposure to vacuum48. Similar loss of iodine species was also observed for sol-eng and 1-1-1 samples stored in vacuum. We note that during the photoemission spectroscopy experiments all the samples are exposed to


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vacuum, but we kept the measurement time below 1.5 hours in order to minimize the effect of exposure to vacuum on the surface composition. Compositional inhomogeneities on the surface of perovskite films could result in electronic inhomogeneities. To probe the electronic properties of the sample, UPS mapping measurements were carried out in a similar fashion to the XPS mapping: a series of UPS spectra over the same area of the perovskite film is collected with a measurement spot size of approximately 400 µm. To generate the IP map, the work function (WF) at each spot was calculated from the secondary photoemission onset, while the energetic difference between the Fermi level and the valence band (EV) was determined from the x-axis intercept of the linear fit of the valance band onset. The ionization potential was then obtained by the sum of the two, i.e. IP = WF + EV. IP maps obtained from UPS mapping experiments on Pb(OAc)2 and sol-eng perovskite films are shown in Figure 4. It can be seen that similar to the XPS mapping results, the same recipe and fabrication parameters may still lead to some samples showing large-scale electronic inhomogeneities, while others being rather uniform. Overall, we observe that films fabricated using the Pb(OAc)2 recipe (Figure 4a and 4b) are more homogeneous than those made using the sol-eng recipe (Figure 4c and 4d) or the 1-1-1 recipe (Figure S8). During the fabrication process using the sol-eng and 1-1-1 recipes, a droplet of anti-solvent is cast on the wet film. The spot where this anti-solvent is dropped can be clearly seen in the IP maps of samples prepared by both recipes, indicating that a different electronic structure is formed at this spot as compared to the rest of the film. This suggests that it is more difficult to form energetically homogenous samples using solvent quenching methods. Overall, our measurements indicate that roughly 15% of Pb(OAc)2 prepared samples show significant inhomogeneities, while for the sol-eng and 1-1-1 recipes, this fraction increases to 25-


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30%. Additionally, UPS and XPS mapping experiments performed on the same samples reveal a correlation between the standard deviations of IP and I/Pb atomic ratio (Figure S10), suggesting that compositionally inhomogeneous samples will also exhibit a large variation in their electronic properties.

Figure 4. IP maps of a uniform (a) and an inhomogeneous (b) perovskite sample prepared by the Pb(OAc)2 recipe. IP maps of a uniform (c) and an inhomogeneous (d) perovskite film prepared by the sol-eng recipe. Note the spot in (d) resulting from the deposition of the antisolvent during film fabrication. To investigate the relationship between the electronic inhomogeneities and the variations in photovoltaic device performance, a batch of 12 samples was fabricated using the Pb(OAc)2 method. The samples were characterized using UPS mapping and the standard deviation of the


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ionization potential values (σIP) was calculated to quantify the degree of inhomogeneity of each sample. Directly afterwards, the samples were completed into functional photovoltaic devices with 8 pixels on each device by depositing the electron extraction layers and electrodes (PCBM/BCP/Ag). The devices were characterized under the solar simulator (AM 1.5 G, 100 mWcm-2) and the standard deviations of the photovoltaic parameters (σVoc, σJsc, σFF and σPCE) were calculated for each substrate. The list of the average device parameters, and their corresponding standard deviations, of the twelve solar cells are shown in Table S2. J-V curves of the champion pixels from each device are also shown in Figure S9. Figure 5 summarizes the standard deviations of IP and the standard deviation of device performance for all devices. There is a clear correlation between the presence of electronic inhomogeneities and the variation in photovoltaic performance across the device. Electronically homogeneous samples result in devices with a narrow performance distribution, while those with a large distribution in IP result in a wide spread of PV parameters. The relationship between the ionization potential and the photovoltaic performance has several origins. The value of the ionization potential at the surface of the perovskite layer will determine the built-in potential in the device, which in turn will have an effect on the device’s open circuit voltage35. Additionally, the ionization potential at the surface will influence the efficiency of hole blocking and recombination at the perovskite/PCBM interface49,50. Since the bandgap of the active layer remains the same across the entire sample, the observed differences in the ionization potential show that the electron affinities also vary at the surface of the perovskite film. These variations in the energetic position of the perovskite conduction band result in different energetic alignment at the perovskite/PCBM interface, which in turn influences the electron extraction efficiency and the accessible VOC38,42–45. Since no changes in absorption or microstructure were


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present across the active layer, changes in the short-circuit current and fill factor are also likely to be associated with the extraction efficiency at the perovskite/PCBM interface. The combination of these effects results in a broad distribution of PCE values for inhomogeneous samples, which explains the wide variation in performance reported in the literature shown in Figure 1b.

Figure 5. (a) Standard deviation of IP (σIP) derived from UPS mapping experiments of 12 perovskite films, which afterwards were completed to solar cells and measured under AM1.5 G simulated sunlight (100 mWcm-2). Standard deviation of VOC (b), JSC (c), FF (d) and PCE (e) extracted from IV-measurements of the corresponding solar cell devices. In conclusion, we demonstrate that large-scale chemical composition and electronic inhomogeneities are present at the surface of CH3NH3PbI3 films. The extent of these variations determines the spread of photovoltaic performance parameters across the sample area. This


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phenomenon was observed for three different fabrication methods, suggesting a widespread issue affecting many research groups in the field. Our results demonstrate that special attention should be devoted to the study of the surface properties of the perovskite layers and not only to bulk properties like microstructure and absorption. To obtain constant and reliable photovoltaic performance across large areas, there is a need to develop perovskite fabrication procedures that would result in uniform surface structure. Alternatively, post processing surface treatments should be applied prior to the deposition of subsequent device layers.

TOC GRAPHICS


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ASSOCIATED CONTENT Supporting Information Supporting Information is available from the ACS Publications website. Experimental methods, lists of references for obtaining Figure 1c, table of average device parameters and standard deviations, the real histogram of Figure 1b, illustration of mapping area and substrate area, AFM and UV-vis plots, SEM images, I/Pb maps of samples stored under various conditions and IP maps of perovskite samples by 1-1-1 recipe, J-V curves of champion pixels from devices in Figure 5, correlation of standard deviation of IP and I/Pb, XRD of perovskite samples by Pb(OAc)2 recipe. AUTHOR INFORMATION Corresponding Author * Email: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors would like to kindly thank Prof. Uwe Bunz for providing access to the device fabrication facilities. The authors are also grateful to Prof. Annemarie Pucci and Prof. Jana Zaumseil for access to AFM and SEM, respectively. P.F. thanks the HGSFP for scholarship.


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