MAPbI3 Electronic Coupling by Interface Modification

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Enhanced TiO2/MAPbI3 Electronic Coupling by Interface Modification with PbI2 Edoardo Mosconi, Giulia Grancini, Cristina Roldán-Carmona, Paul Gratia, Iwan Zimmermann, Mohammad Khaja Nazeeruddin, and Filippo De Angelis Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00779 • Publication Date (Web): 27 May 2016 Downloaded from http://pubs.acs.org on May 30, 2016

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Chemistry of Materials

Enhanced TiO2/MAPbI3 Electronic Coupling by Interface Modification with PbI2 Edoardo Mosconi,a,b Giulia Grancini,c Cristina Roldán-Carmona,c Paul Gratia,c Iwan Zimmermann,c Mohammad Khaja Nazeeruddin,c,* Filippo De Angelis,a,b * a

Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), CNR-ISTM, Via Elce di Sotto 8, I-06123, Perugia, Italy. b

CompuNet, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy.

c

Group for Molecular Engineering of Functional Materials, Institute of Chemical Sciences and Engineering, Ecole polytechnique fédérale de Lausanne, CH-1951 Sion, Switzerland.

ABSTRACT: We report a combined experimental and computational investigation to elucidate the effect of PbI2 on the electronic properties of the TiO2/methylammonium lead-iodide (MAPbI3) perovskite interface. We propose that excess PbI2 from the precursor solution or from a TiO2 pre-treatment may be predominantly located at the interface with the semiconducting oxide, enhancing the interfacial electronic coupling and favorably modifying the electronic energy levels landscape at the interface. This is in turn found to facilitate electron transfer from the MAPbI3 perovskite to the TiO2 electron transport layer as revealed by a fast quenching in the photoluminescence dynamics, confirming the beneficial role of PbI2 at the interface.

Organohalide lead perovskites are revolutionizing the landscape of emerging photovoltaic technologies. From their first application in 2009 by Kojima et al.1a as solar cells sensitizers and the first report of solid state solar cells by Kim et al.,1b photovoltaic devices based on these materials showed a fast and continuous increase in their efficiency,2-3 with very recent certified efficiency exceeding 22%.4 These materials can be solution-processed at low-temperature5 and vapor-deposited,2 realistically holding the promise to reach comparable efficiency as conventional thin-film photovoltaic technologies. Furthermore, they can be combined with organic electron acceptors/donors, to deliver flexible photovoltaic devices.6-8 Methylammonium lead-iodide, hereafter MAPbI3, and the related mixed halide MAPbI3-xClx analogue have dominated the field. These perovskites can support both electron and hole transport,5, 9-10 giving rise to a variety of device architectures, being employed as solar cell sensitizers when deposited on n-transporting mesoporous TiO2, or serving both as light absorber and electron transporter in meso-super-structured and planar heterojunction solar cells. In spite of the great body of research on different perovskite solar cells architectures, devices based on mesoporous TiO2 are still the most common choice in highest efficient devices.11 Understanding the properties of the MAPbI3/TiO2 interface is therefore crucial for device optimization although still mostly at a premature stage. This is partly due to the difficulty of conventional experimental techniques in probing such mesoporous interface, though a general understanding of the electronic energy

level alignment of the TiO2/MAPbI3 interface is now available through combined XPS/UPS measurements.12-14 Also, Edri et al. probed such interface by EBIC, disclosing the features of charge generation in TiO2-based perovskite solar cells.15 Various reports16-17 have also highlighted that despite the relatively favorable interfacial energetics, charge separation at the TiO2/perovskite interface is not as efficient as one could desire, giving rise to charge accumulation effects which could be related to the adverse hysteresis observed in the solar cell J-V curve.17-20 The use of a self-assembled fullerene monolayer helped in mitigating this effect.20 Computational modeling has also been employed by some of us21 and by Feng et al.22 to probe the structural and electronic properties of the TiO2/MAPbI3 interface. Very recently, two independent studies have disclosed a significant efficiency and stability enhancement in TiO2based MAPbI3 perovskite solar cells employing a nonstoichiometric MAI:PbI2 precursor ratio. In particular, Kim et al.,23 and Roldán-Carmona et al.,24 have reported on the beneficial effect of a 10-20% excess of PbI2 in the precursor solution compared to the usual 1:1 stoichiometric ratio in terms of solar cells efficiency, stability and suppressed hysteresis. Despite some effect on the morphology of the ensuing perovskite film was observed, in terms of slightly larger grains of the capping layer obtained when excess PbI2 was employed, similarly to what reported by Liu et al. 25 the mechanism behind the enhanced solar cell behavior is not completely understood. Motivated by these very recent results, here we report on a combined experimental and computational study to in-

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vestigate the effect of PbI2 on the electronic properties of the TiO2/MAPbI3 interface. We propose that excess PbI2 may predominantly be located at the interface with TiO2, enhancing the interfacial electronic coupling and favorably modifying the electronic energy levels landscape at the interface. This is in turn shown to facilitate electron transfer from MAPbI3 to TiO2 as revealed by photoluminescence quenching dynamics. Notably, a fast quenching also occurs using a 1:1 precursor composition but pretreating the mesoporous TiO2 with PbI2, confirming the beneficial role of PbI2 at the interface. Figure 1 a, b show the structural characterization in terms of XRD and micro-Raman measurements of the PbI2 pre-treated sample (see the scheme in Figure S3 explaining how the film has been prepared), deposited from a 1.2 M DMSO solution, and of the films fabricated with 5% and 20% PbI2 excess on mesoporous TiO2 following the protocol in Ref. [8] (see additional details in Supporting Information).

Figure 1. a, X-ray diffraction (XRD) pattern of the MAPbI3 samples with various PbI2 conditions. The peak marked with the asterisk is assigned to a PbI2. b, Micro-Raman spectra of the MAPbI3 fabricated with 5% (red dots), 20% (blue squares) of PbI2 excess and MAPbI3 pre-treated sample (black triangles). Sample were illuminated from the capping layer side at 532 nm wavelength. All the sample were encapsulated to prevent any moisture/oxygen effect during the measurements.

From the XRD data in Figure 1a we can observe the presence of a peak at 12.8° that is indicative of the presence of a small amount of PbI2 retained even in the pretreated film. Note that in such pre-treated MAPbI3 sample the initially formed PbI2 layer might partially re-dissolve upon the subsequent perovskite deposition, still a small amount of residual PbI2 is observed in the final films. Probing the entire volume of the sample, XRD does not allow us to identify where the PbI2 excess is distributed. Figure 1b shows the micro-Raman spectra for the same set of samples. Considering the device structure (see crosssectional SEM in Figure S2) the signal mainly comes from the thick perovskite capping layer formed on top of the mesoporous structure. Similarly to previous reports26,27 the Raman spectra show a broad feature with two dominant peaks below 130 cm-1 assigned to a combination of Pb-I stretching and bending and organic libration modes, along with a broader feature peaking at 250 cm-1 assigned to torsional mode of the organic cation. The 20% and pretreated samples show similar features, while the 5% sample displays a well defined peak at ∼160 cm−1. This peak is theoretically assigned to libration of the MA cations, 25

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which is expected to be sharper upon increased crystalline order at the molecular scale. This data thus suggests that the presence of the excess of PbI2 can impact on the crystallization dynamics and the local order of the capping layer, in agreement with Refs. 23-25. However, within the resolution limit of the instrument, we do not observe a clear indication of the presence of large PbI2 rich-phases as also supported from the SEM images (in Figure S4). This suggests a homogeneous distribution of excess PbI2 throughout the film thickness, leading to the hypothesis that it might be mostly retained at the TiO2 interface. Notably, in the pre-treated film the perovskite capping layer has a similar morphology to the film grown using the stoichiometric solution (0% PbI2 excess) (see Figure S2 and S4) further supporting the idea that the excess of PbI2 mainly influences the mesostructured TiO2 interface. As previously reported, devices fabricated from excess PbI2 solution lead to improved solar cells performances. Here we replicate the fabrication of such devices obtaining for a 10% PbI2 excess 18.5% average power conversion efficiency against the 16.5% for the 1:1 precursors with a very limited hysteresis and good stability (see Figure S5 and S6 for device characteristic). Aiming to target the specific interface-related processes, i.e. the electron injection to TiO2, we monitor the light induced phenomena probing the PL decay upon selectively exciting from the mesoporous TiO2 side or from the perovskite capping layer-side. Comparison of the PL decay for the 0, 10 and 20% PbI2 excess and the PbI2 pre-treated samples is shown in Figure 2 a, b. Note that since the light penetration depth at the employed 460 nm exciting wavelength is estimated in the 100 nm range, excitation of the perovskite film from the TiO2 side (scaffold thickness ∼ 150 nm) ensures to probe the specific interaction occurring at the mesoporous TiO2/perovskite interface.

Figure 2. Time resolved photoluminescence decay at 770 nm obtained for the stoichiometric (0%) and non-stoichiometric films (10, 20% of PbI2 excess) and for the pre-treated film (called “PbI2 pre-spin”) upon excitation at λexc = 460 nm, ex2 citation density of ~10 nJ/cm . All the samples have been en-


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capsulated to prevent degradation or any oxygen/moisture induced effects. a. Excitation from the TiO2 mesoporous side and b. Excitation from the perovskite-capping layer side. Solid lines represent the fitting curve resulting from exponential fitting. Notice that a short time window in the first 30 ns is monitored.

When exciting from the TiO2 side we observe that increasing the PbI2 excess a faster PL quenching takes place. For a better visualization we use a simple mathematical fitting that indicates that an additional exponential function is need to properly fit the PL decay in presence of PbI2 excess (see Table S1 for fitting parameters). This analysis does not pretend to describe in an exhaustive way the photophysical model behind, but simply indicates that a faster PL dynamics happens at the TiO2 interface with PbI2 excess. Table 1. Time constants retrieved from a mono or biexponential fit of the PL decays curves in Figure 2. A1 and A2 are the amplitudes of the mono- and bi-exponential fit functions and τ1 and τ2 the corresponding time constants.

% PbI2

A1 (%)

A2 (%)

τ1 (ns)

τ2 (ns)

0

100

--

18

--

10

77

23

13

4.2

20

26

74

7

2

Pre-treated

78

22

18

1.5

On the same timescale, the PL decay upon 460 nm excitation from the capping perovskite side does not show any appreciable change, exhibiting a mono-exponential decay with a time constant beyond the temporal window monitored here, similarly to the 0% sample. No changes are observed within the 40 ns time window, indicating that the PbI2 excess is not affecting the electron-hole dynamics in the capping layer within this temporal window. This indicates that the PbI2 excess manifests predominantly when exciting from the TiO2 side. Overall, this is a clear indication that the PbI2 excess has an important effect on the interfacial processes, possibly leading to a more efficient electron injection to TiO2, although we cannot exclude that excess PbI2 might also influence the perovskite grain boundaries. To gain insight into the possible role of interfacial PbI2 we investigate the MAPbI3/TiO2 interface by first principles computational modeling. We consider the tetragonal (110) perovskite surface as this was previously shown to be the favored facet.21 While the structural properties of the investigated systems are nicely reproduced by scalarrelativistic (SR) DFT,28 inclusion of spin-orbit coupling (SOC) is crucial for a correct interfacial electronic structure of lead perovskites,21 so we include SOC on the SRoptimized geometries, see Computational Details in Supporting Information. We consider two MAPbI3 terminations at the interface with TiO2, i.e. a MAI-terminated 20,21

and a PbI2-terminated perovskite, see Figure 3. This latter system is representative of a MAPbI3 film grown under PbI2-rich conditions, i.e. the PbI2 pre-treated sample or under excess PbI2. The perovskite models were “deposited” onto a 5x3x2 slab of anatase TiO2 made by 120 TiO2 units, exposing the majority (101) surface, see Figure 3. We use throughout the experimental TiO2 cell parameters to build our supercells,20 which introduce a lattice mismatch below 2%. The MAI-terminated perovskite/TiO2 interaction occurs mainly through the binding of perovskite halide atoms to under-coordinated Ti(IV) atoms of the TiO2 surface.20 The interaction between the PbI2-terminated perovskite with TiO2 mainly occurs via Pb-O and I-Ti interactions, see Figure 3. The MAI-terminated system has a slightly higher binding energy to TiO2 than the PbI2terminated system, by less than 0.1 eV per surface site, thus both terminations are likely to be observed depending on the growth conditions and the stoichiometry of the precursor solution. 29

Figure 3. Optimized geometrical structures of the (110) TiO2/perovskite interface for the MAI-terminated and PbI2terminated MAPbI3. Colors: Pb: light blue; I: purple; N: blue; C: green; H: white.

The calculated interface electronic structure is analyzed in Figure 4. In the MAI-terminated interface the perovskite CB edge is calculated ca. 0.8 eV above the TiO2 CB edge, see Figure 4 and Supporting Information.21 This value is overestimated compared to experimental band offsets of 0.4 eV reported for MAPbI3/TiO2 by Lindblad et al.12 but a qualitatively correct picture of interfacial energy levels is retrieved. A notable difference between the MAIand PbI2-terminated perovskite/TiO2 interfaces is the slight downshift of the perovskite VB and CB main features in the latter, see Figure 4 and Supporting Information, for which we also calculate a tail of perovskite states extending ca. 1 eV below the main CB feature. This tail is absent in the MAI-terminated case, for which a rather sharp CB edge is calculated. These results suggest an increased interfacial coupling in the PbI2-terminated case compared to the MAI-terminated case, which is mediated by the closer proximity (2.8 vs. 3.3 Å) between the Pb 6p orbitals, mainly constituting the perovskite conduction band, and the unoccupied Ti 3d states contributing the TiO2 CB. Considering the typical exponential decay of the electronic coupling with the distance, this geometrical



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difference readily explains the stronger coupling observed for the PbI2-terminated interface. A similar observation was reported by Geng et al. 30 on reduced MAPbI3/rutile TiO2 models.

ucation, Research and Innovation (SERI). We thank Prof. Hubert Girault for providing laser facilities.

The strong interfacial modification associated to the PbI2-terminated perovskite system is readily visualized by exploring the local interface DOS in Figure 4. As it can be noticed, while for the MAI-terminated system only a weak perovskite CB bending is observed when approaching the TiO2 surface, a pronounced CB bending is observed for the PbI2-terminated interface, reflecting the stronger interfacial electronic coupling discussed above. Such electronic landscape will definitely assist charge separation of photo-generated electrons in the perovskite, leading to a more efficient and overall faster charge injection into the TiO2, as it is experimentally observed.

1. a) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050-6051; b) Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; Grätzel, M.; Park, N.-G. Lead Iodide Perovskite Sensitized AllSolid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591.

Figure 4. Isodensity plot of the integrated DOS as a function of the distance from the TiO2 surface for the MAI- and PbI2terminated TiO2/MAPbI3 interface. A blue to red color variation indicates an increase of the DOS value. The arrow in the lower panel indicates the sizable band bending observed in the case of PbI2-terminated interface.

In summary, our joint experimental and theoretical study discloses the positive effect of interfacial PbI2 in enhancing the electronic coupling and the associated photoexcited electron injection from perovskite to the TiO2, thus possibly leading to overall improved solar cell performances by virtue of a simple PbI2 pre-treatment. Supporting Information. Computational and Experimental details; SEM images. This information is available free of charge via the Internet at http://pubs.acs.org/. The authors declare no competing financial interest. Corresponding Author: [email protected]; [email protected] ACKNOWLEDGMENT We thank FP7-MESO project, grant agreement n° 604032, the Swiss Federal Office for Energy (Energy program fund 563074), the H2020-ICT-2014-1, SOLEDLIGHT project, grant agreement N°: 643791 and the Swiss State Secretariat for Ed-

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MAPbI3 Electronic Coupling by Interface Modification

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