Probing Photocurrent Generation, Charge Transport, and

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Probing Photocurrent Generation, Charge Transport and Recombination Mechanisms in Meso-Structured Hybrid Perovskite through Photoconductivity Measurements Kári Sveinbjörnsson, Kerttu Aitola, Xiaoliang Zhang, Meysam Pazoki, Anders Hagfeldt, Gerrit Boschloo, and Erik M. J. Johansson J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b02044 • Publication Date (Web): 12 Oct 2015 Downloaded from http://pubs.acs.org on October 12, 2015

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The Journal of Physical Chemistry Letters

Probing Photocurrent Generation, Charge Transport and Recombination Mechanisms in Meso-Structured Hybrid Perovskite through Photoconductivity Measurements Kári Sveinbjörnsson†, Kerttu Aitola†, Xiaoliang Zhang†, Meysam Pazoki†, Anders Hagfeldt†,‡, Gerrit Boschloo† and Erik M. J. Johansson*,† † Uppsala University, Department of Chemistry – Ångström Laboratory, Physical Chemistry, Box 523, 751 20 Uppsala, Sweden ‡ École Polytechnique Fédérale de Lausanne, Laboratory of Photomolecular Science, EPFL SB ISIC LSPM , CH G1 523, Chemin des Alambics, Station 6, CH-1015 Lausanne, Switzerland Corresponding Author *E-mail: [email protected]

ABSTRACT

Conductivity of methylammonium lead triiodide (MAPbI3) perovskite was measured on different mesoporous metal oxide scaffolds: TiO2, Al2O3 and ZrO2, as a function of incident light

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irradiation and temperature. It was found that the MAPbI3 exhibits intrinsic charge separation and its conductivity stems from a majority of free charge carriers. The crystal morphology of the MAPbI3 was found to significantly affect the photoconductivity, whereas in the dark the conductivity is governed by the perovskite in the pores of the mesoporous scaffold. The temperature dependent conductivity measurements also indicate the presence of states within the band gap of the perovskite. Despite a relatively large amount of crystal defects in the measured material the main recombination mechanism of the photogenerated charges is bimolecular (bandto-band), which suggests that the defect states are rather inactive in the recombination. This may explain the remarkable efficiencies obtained for perovskite solar cells prepared with wetchemical methods.

TOC GRAPHICS

TEXT Recent breakthroughs in the field of perovskite solar cells (PSCs) have resulted in an ever growing attention owing much to their rapidly increasing power conversion efficiencies (PCE), that have risen from below 4% in 2009 and have now surpassed 20%.1–5 These hybrid organicinorganic perovskite materials have proven to be ideal as a light absorber for photovoltaic


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devices in view of their optical and electronic properties, which materialize in broad and strong absorption over a large part the solar spectrum as well as long photoexcited charge-carrier diffusion lengths.6,7 Despite the advances in PSC performance, the material studies of these new type of semiconductors have trailed behind the applied research. Photogenerated free charge carriers are important for charge transport and recombination in organic-inorganic perovskites, while small contributions from trapped charges and excitons have also been reported.8 Further insight of the material photoconductivity dependence on the free carrier concentrations, and possible contributions of trap states, can be of crucial importance for understanding of the PSC performance mechanism. To this day, the most efficient PSCs incorporate a thin layer of nanoparticle metal oxide (npMOx) electrodes which are infiltrated with a perovskite absorber but efficient devices with planar heterojunction structure have also been reported.5,9 The charge separation dynamics in organometallic perovskites are also highly dependent on the choice of the metal oxide scaffolding layer used in photovoltaic devices.2,8 However, the characterization of PSCs with different scaffolds lacks information on how the scaffold can affect the formation of the perovskite layer and consequently how that affects the material properties of the perovskite film. A large increase in the conductivity has been observed in the perovskite materials due to photodoping.10 Despite being large, the conductivity exhibits a slow increase which has been postulated to be a result of either slow filling of sub-band-gap trap states or structural changes in the material, which on the other hand may also be explained by the large dielectric constant exhibited by perovskite materials.11–14 Interestingly, Leijtens et al showed that the conductivity of perovskite on mesoporous alumina scaffold displays larger dark conductivity compared to a heterojunction thin film perovskite, but in light the opposite was observed.15 Although the


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density of states within the band gap (traps/defect states) is rather similar for the perovskite in a scaffold compared to in a thin perovskite film, the states are to larger extent filled (electrons are observed in the band gap closer to the conduction band) in the meso-structured perovskite compared to in a thin film of perovskite, as has been confirmed by hard X-ray photoelectron spectroscopy (HAXPES) measurements.16 Those traps become predominantly filled under strong illumination and once filled the dominating recombination mechanism is a bimolecular (band-toband) recombination of free charge carriers.11 Excitonic charge carriers may also be present in the perovskite, but only in small amounts due to the low binding energies observed for excitons in perovskite at room temperature.17 In this work, we investigate the effects of photoconductivity of methylammonium lead triiodide (MAPbI3) perovskite on meso-structured titania (TiO2), zirconia (ZrO2) and alumina (Al2O3). Our results show that the magnitude of the photoconductivity is affected by the type of metal oxide through changes in crystallinity and morphology of the perovskite layer. The results also show that bimolecular recombination is the dominating recombination mechanism in these systems, and the generation of free charges after light absorption occurs on all the different metal oxide scaffolds. The temperature dependence of the conductivity suggests that trap states exist in the band gap, but interestingly they are not very active in the recombination processes. In order to study the sample morphologies, we performed scanning electron microscopy (SEM) on the samples used for photoconductivity measurements (Figure 1). The samples were prepared on glass substrates and consisted of MAPbI3 perovskite spin-coated at 1500 rpm, 3500 rpm and 6500 rpm on TiO2, and at 3500 rpm on ZrO2 and Al2O3, along with thermally evaporated gold electrodes. The thicknesses of the perovskite layer on all TiO2 samples were


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rather similar (~300 nm) but the film morphology changed so we refer to each sample by their MOx scaffold and respective spin-coating speed. The primary difference between the MAPbI3 on TiO2 samples (Figure 1a-c) is the morphology of the perovskite capping layer due to the different spin-coating speeds of the precursor solution, while the TiO2 scaffolds (~250 nm thick) were all prepared by the same procedure. The sample with the perovskite precursor solution spin-coated at 1500 rpm on TiO2 exhibited the most even MAPbI3 capping layer, which became less interconnected as the spin-coating speed was increased from 1500 rpm to 6500 rpm. For the samples of MAPbI3 on Al2O3 (Figure 1d) and ZrO2 (Figure 1e), both the thickness (~500 nm and 400 nm, respectively) and porosity of the MOx scaffolds were different than that of TiO2, and a morphology difference was also observed for the perovskite capping layers (~250 and 500 nm thick, respectively).


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Figure 1. Cross-sectional SEM images of MAPbI3 spin-coated at; a) 1500 rpm on TiO2, b) 3500 rpm on TiO2, c) 6500 rpm on TiO2, d) 3500 rpm on Al2O3 and e) 3500 rpm on ZrO2 scaffolds, along with thermally evaporated gold electrodes (not always visible due to the SEM sample preparation method).


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A schematic representation of the photoconductivity measurement sample is shown in Figure 2a. The total resistance between each neighboring electrode was measured and from that the conductivity was calculated (see Supporting Information for details). A slow increase of the photoconductivity was observed in the beginning of the measurement, which gradually stabilized over time (see Figure S2 in the Supporting Information), as has been reported previously.11,12 Most likely, this slow photodoping effect resulted from the gradual filling and equilibration of trap states. Due to the photodoping effect, the measured resistances of the perovskite spanned from GΩ in dark to MΩ under 1 sun illumination intensity (100 mW cm-2). The stabilized photoconductivity of the samples with MAPbI3 on TiO2 scaffold layer is shown in Figure 2b. The photoconductivities at 100 mW cm-2 illumination intensity are 4·10-4 S cm-1, 2·10-4 S cm-1 and 5·10-5 S cm-1, with MAPbI3 spin-coated at 1500 rpm, 3500 rpm and 6500 rpm, respectively. Interestingly, the measured dark conductivities (Table 1) were rather similar for all the different spin-coating speeds, in the range of 2·10-8 S cm-1, which is comparable to the conductivity of single crystal MAPbI3 with low trap state density measured by Shi et al.7 However, in a composite system consisting of MAPbI3 in a mesoporous structure and a crystalline capping layer on top, the dark conductivity appears to be governed by the perovskite in the mesoporous scaffold.15 This is also verified by the fact that we obtained the same conductivity values for MAPbI3 on TiO2 regardless of the spin-coating speed.


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Figure 2. a) Schematic illustration of the typical sample for the conductivity measurement. b) Conductivity (σ) response of photodoped MAPbI3 spun onto a mp-TiO2 substrate at three different speeds; 1500, 3500 & 6500 rpm and c) of photodoped MAPbI3 spun onto TiO2, Al2O3 and ZrO2 at 3500 rpm. The solid lines are a power function fit off the respective data. Table 1. Dark conductivities (σdark) of MAPbI3 spin coated on TiO2, Al2O3 and ZrO2 scaffolds. Scaffold

σdark [S cm-1]

*

TiO2

3·10-8 ± 1·10-8

Al2O3

1·10-8 ± 5·10-9

ZrO2

2·10-8 ± 1·10-8

*Average of all three spin-coating speeds.


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The conductivity behavior of the perovskite films shows almost an ideal square root dependence on light intensity (Figure 2b-c), which is characteristic of an intrinsic semiconductor that has approximately equal concentrations and mobilities of electrons and holes.18 It indicates that the charge-carriers in the material are unbound and that the majority of recombinations are due to band-to-band recombinations. We propose the following model to describe the photoconductivity (σ) (derivation in Supporting Information): = +

where e is the elementary charge, µn and µp are the mobilities of electrons and holes, respectively, I0 is the incident light irradiance, A is absorptance, is a spectral match factor of the light source with the absorber and krec is the recombination rate constant. The square root behavior results from the assumption of a pure bimolecular recombination mechanism, while in reality a fraction of the total recombinations may be monomolecular (excitonic). With an increased exciton fraction of the total charge carriers in the material, the conductivity light dependence is expected to approach linearity. As mentioned above, the MAPbI3 perovskite was formed on TiO2, Al2O3 and ZrO2 scaffolds, which have band-gaps of 3.2 eV, ~8 eV and ~6 eV, respectively.1,16,19 The conduction band of only TiO2 matches that of the MAPbI3 perovskite, making the TiO2 capable of accepting excited electrons from the perovskite absorber. Ponseca et al have shown that the mobility of an excited electron in the MAPbI3 perovskite conduction band is decreased once the electron is injected into the TiO2 conduction band, therefore a lower conductivity would be expected from samples where injection of electrons from MAPbI3 into TiO2 occurs.20 Our results (Figure 2c) show that the photoconductivity is not significantly affected by electron injection, since we observed that the photoconductivity of the samples on TiO2 was of similar magnitude to those on Al2O3 and ZrO2.


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The photoconductivities of MAPbI3 at 100 mW cm-2 illumination intensity are 4·10-5 S cm-1 on Al2O3 and 3·10-4 S cm-1 on ZrO2. The square root dependence of the conductivity versus light intensity observed for all the different samples also indicates that the recombination mechanism for the photogenerated charges is the same. Therefore, our observations confirm, that an electron acceptor material is not necessary for charge carrier generation or separation in the MAPbI3 perovskite material. We performed UV-Vis spectroscopy and X-ray diffraction (XRD) to characterize the material properties of the MAPbI3 perovskite, and the crystalline morphology of each sample, and found that the variations in photoconductivity between samples are related to differences in absorptance and perovskite crystal formation. Lead(II)iodide (PbI2) is a precursor of the MAPbI3 perovskite material and it remains in the sample if the perovskite crystal formation is incomplete. Traces of PbI2 in the films of all samples can be observed in the absorptance spectra (~530 nm) and the XRD spectra (12.6°) in Figure 3, which indicates that the perovskite formation was not fully complete.21,22 However, due to its low photoconductivity the contribution from PbI2 is negligible to the values presented in Figure 2.23 The MAPbI3 perovskite has a band gap of 1.51 eV and its dominant characteristic XRD peaks are found at 2θ diffraction angles of 14.0°, 28.4° and 31.8°.22,24 The absorptance spectra in Figure 3a clearly indicate that the amount of perovskite absorber on the TiO2 substrate became lower the faster the spin-coating speed of the precursor solution. The absorptance of the 6500 rpm sample was quite low in general, and the XRD spectrum (Figure 3b) confirms poor perovskite crystal formation. Both factors contribute to the low photoconductivity observed for this sample. The differences in light absorptance observed between samples prepared with 1500 rpm and 3500 rpm spin-coating speeds are mainly due to differences in perovskite crystal


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formation. This can be seen from the XRD spectra, where the ratio of 14.0° MAPbI3 main peak to the 12.6° PbI2 peak is relatively higher, for the 1500 rpm sample. Combining this with the results in Figure 2 confirms that the photoconductivity is increased with a larger amount of crystalline perovskite absorber material on the sample. In the SEM images (Figure 1a-c) it can be seen that the MAPbI3 capping layer for the sample spin-coated at 1500 rpm is also more interconnected, but as the spin-coating speed increases the capping layer becomes more disconnected, which also results in a lower photoconductivity. As evident in the absorptance and the XRD spectra of the sample on Al2O3 (Figure 3c-d), a large amount of unreacted PbI2 was present, which is responsible for the low photoconductivity of this sample. Despite the absorptance of the perovskite sample on the ZrO2 scaffold being lower than the absorptance of the perovskite on the TiO2 scaffold, we observed a three times larger photoconductivity for the former. This might be due to a more complete perovskite crystal formation on ZrO2 (see Figure 3d) and a more interconnected MAPbI3 capping layer as compared to those on TiO2 and Al2O3 (see Figure 1b, d, e). In which case, these two properties have a relatively greater beneficial effect on the photoconductivity compared to the slight differences in film absorptance.


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Figure 3. a) UV-Vis absorptance and b) XRD spectra of the MAPbI3 perovskite on TiO2 with the precursor spin-coated at three different speeds: 1500, 3500 and 6500 rpm. c) UV-Vis absorptance and d) XRD spectra of MAPbI3 with the precursor solution spin-coated at 3500 rpm on TiO2, Al2O3 and ZrO2 scaffolds. We also studied the temperature dependence of the conductivity of MAPbI3 spin-coated at 3500 rpm onto a mesoporous TiO2 scaffold (Figure 4) and coated with a layer of poly(methyl methacrylate) (PMMA) to protect the perovskite layer from effects of ambient humidity. In the


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dark, we observed an exponential increase in conductivity with respect to temperature, which is in general characteristic of intrinsic semiconductors. In the temperature region where a semiconductor acts intrinsic, its conductivity depends mainly on its band gap and the temperature available for thermal excitations. The band gap can then be calculated from a well-known expression (see Supporting Information). In the inset of Figure 4 we display the natural logarithm of the dark conductivity versus the inverse of the temperature and the linear-fit of which, when using the expression for the band gap of intrinsic semiconductors, gives a thermal excitation energy of 1.0 eV for the MAPbI3. This energy is well below the optical band gap of 1.51 eV and the results therefore suggest that there are defect states within the band gap of the perovskite, which can be thermally populated, and the excitation of which follows a Boltzmann distribution.15,16,25,26

Figure 4. Temperature dependence of the conductivity of MAPbI3 on TiO2, with the precursor solution spin-coated at 3500 rpm, under 100 mW cm-2 (blue), 46 mW cm-2 illumination intensity (light blue) and in dark (red). Inset: Natural logarithm of the dark conductivity vs. the inverse of


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the temperature. Through a well-known expression (see Supporting Information) the band gap of MAPbI3 can be calculated from the linear-fit of the data. Under illumination the photoconductivities of the perovskite materials were not greatly affected by variations in temperature, as can be observed in Figure 4. As previously stated, under illumination the trap states (or defect states) of the material are expected to be filled. As the material absorbs a photon, the excited charges may recombine with the trapped charges, and when the temperature is increased, the probability of this type of recombination also increases. The decrease of the photoconductivity with rising temperature (Figure 4) therefore suggests an increase of trap-assisted recombinations. However, the ideal square root dependence of the photoconductivity of perovskite on light intensity (Figure 2) shows that band-to-band recombination is the main recombination mechanism at room temperature, since trap-assisted (monomolecular) recombinations would result in a linear dependence of the photoconductivity on light intensity (see Supporting Information). Due to the relatively large number of crystal defects in the perovskite films, we would expect that increasing the temperature would result in strongly increased trap-assisted recombinations. Surprisingly, the filled traps do not appear to be prone to recombination due to only a small decrease in photoconductivity with increased temperature. This may explain the remarkable efficiencies obtained for the perovskite solar cells prepared by wet-chemical techniques, which results in a multicrystalline perovskite material. In conclusion, we show in this paper that the MAPbI3 perovskite film displays a large photoresponse in conductivity that increases by four orders of magnitude when going from dark to 100 mW cm-2 illumination intensity. A square root relationship between the photoconductivity and the light intensity was observed, which strongly suggests the presence of free charge carriers and a bimolecular (band-to-band) recombination mechanism. The free charges are the main


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contributors to the measured conductivity and the square root behavior was observed both in presence and absence of an electron acceptor material (TiO2), which means that an intrinsic charge separation occurs in the perovskite material itself. It was also observed that the conductivity photoresponse is directly related to the amount of the perovskite absorber in each sample. However, the quality of the perovskite film has a greater effect on the photoconductivity than its absorptance. In the dark, the perovskite conductivity on TiO2 scaffold increased exponentially with temperature, and the thermal excitation energy was estimated to be 1.0 eV in contrast to the optical band gap of 1.51 eV. This suggests that there are defect states within the band gap of the perovskite from which the thermal excitations occur. The photoconductivity of the MAPbI3 perovskite decreased slightly with rising temperature due to an increase in trap-assisted recombinations, however the results point to relatively inactive trap states and to band-to-band recombination mechanism being the dominating recombination mechanism. Experimental Methods The MOx scaffold layers were prepared by spin-coating of nanoparticle pastes/dispersions in weight ratio of 1:5 TiO2 (DSL 18NR-T, Dyesol):terpineol, in volume ratio of 1:2 Al2O3 dispersion (20 wt. % in isopropanol):isopropanol and in weight ratio of 1:3 ZrO2 (homemade screen-print paste):terpineol on plain glass substrates followed by sintering at 500°C in air for 30 minutes. A solvent-processed MAPbI3 perovskite was deposited on top of the MOx scaffold layers using a one-step method from a 40 wt. % 1:1 lead(II) iodide:methylammonium iodide solution in 3:1 dimethylformamide:dimethylsulfoxide (more experimental details can be found in the Supporting Information). The perovskite precursor solution was spin-coated for 30 seconds on top of TiO2 scaffold at three different speeds: 1500 rpm, 3500 rpm and 6500 rpm, and on top


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of Al2O3 and ZrO2 scaffolds at 3500 rpm, followed by heating the substrates at 110°C for 1 hour. Four lateral gold electrodes (thickness ~80 nm) of identical shape (8 x 4 mm2) were thermally evaporated onto the samples with a different channel lengths (1, 2 and 4 mm) between each neighboring electrode pair to yield the sample structure shown in Figure 2a. The conductivity measurements were carried out with a two-point probe method similar to the method of Horváth et al.10 In the photoconductivity measurements, a Newport Solar Simulator (model 91160) with an AM 1.5 G spectrum and fitted with a 400 nm longpass filter to avoid exciting the MOx semiconductors was used as the light source and the lamp intensity was controlled by neutral density filters. ASSOCIATED CONTENT Supporting Information. Sample preparation and experimental details; description of conductivity evaluation; slow photoresponse; photoconductivity model derivation; band gap evaluation; top-down SEM images. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT


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We thank Dr. Michael Wang and Dr. Leif Häggman for their technical support. We acknowledge the financial support obtained from the Göran Gustafsson Foundation, Swedish Energy Agency, and Swedish Research Council (VR). REFERENCES (1)

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Page The Journal 23 of 26 of Physical Chemistry Lettersa) Gold MAPbI3 1 1500 rpm 2

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Probing Photocurrent Generation, Charge Transport, and

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