Photophysics of Hybrid Lead Halide Perovskites: The Role of

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Photophysics of Hybrid Lead Halide Perovskites: The Role of Microstructure Published as part of the Accounts of Chemical Research special issue “Lead Halide Perovskites for Solar Energy Conversion”. Ajay Ram Srimath Kandada and Annamaria Petrozza* Center for Nano Science and Technology @Polimi, Istituto Italiano di Tecnologia, via Giovanni Pascoli 70/3, 20133 Milan, Italy CONSPECTUS: Since the first reports on high efficiency, solution processed solar cells based on hybrid lead halide perovskites, there has been an explosion of activities on these materials. Researchers with interests spanning the full range from conventional inorganic to emerging organic and hybrid optoelectronic technologies have been contributing to the prolific research output. This has led to solar cell power conversion efficiencies now exceeding 20% and the demonstration of proofs of concept for electroluminescent and lasing devices. Hybrid perovskites can be self-assembled by a simple chemical deposition of the constituent units, with the possibility of integrating the useful properties of organic and inorganic compounds at the molecular scale within a single crystalline material, thus enabling a fine-tuning of the electronic properties. Tellingly, the fundamental properties of these materials may make us think of a new, solution processable, GaAs-like semiconductor. While this can be true to a first approximation, hybrid perovskites are intrinsically complex materials, where the presence of various types of interactions and structural disorder may strongly affect their properties. In particular, a clear understanding and control of the relative interactions between the organic and inorganic moieties is of paramount importance to properly disentangle their innate physics. In this Account we review our recent studies which aim to clarify the relationship between structural and electronic properties from a molecular to mesoscopic level. First we identify the markers for local disorder at the molecular level by using Raman spectroscopy as a probe. Then, we exploit such a tool to explore the role of microstructure on the absorption and luminescence properties of the semiconductor. Finally we address the controversy surrounding electron−hole interactions and excitonic effects. We show that in hybrid lead-halide perovskites dielectric screening also depends on the local microstructure of the hybrid crystals and not only on its chemical composition. This leads to the possibility of band gap engineering and the consequent control of the elementary photoexcitation dynamics that determine the perovskites’ performances in different optoelectronic devices.

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INTRODUCTION

experimental efforts which unveil the role of local disorder on the photophysics of this class of semiconductors. By combining vibrational and optical spectroscopies to investigate MAPbX3 polycrystalline thin films, we provide a key for correlating their optoelectronic properties with their processing conditions for an educated design of the devices.

To a first approximation, the electronic properties of hybrid perovskites in their 3D structural form, (CH3NH3)MX3−xYx (M = Pb; X,Y = I, Br, Cl) are defined by the (Pb)−(X3−xYx) network: the conduction band is composed of the p orbitals of lead, while the valence band is a hybridization of the s orbitals of lead and the p orbitals of the halogen element.1 In these terms, the organic cation might appear to have a mere structural role. Though it does not directly contribute to the frontier orbitals the organic moiety can nevertheless strongly manipulate them. Hydrogen bonding between the amine group and the halide network, and strong orientational disorder of the organic cation within the inorganic cage, given by its relatively high rotational mobility, induce strong lattice deformations that perturb the electronic landscape within the Pb−X cage.2 Thus, the structure of MAPbX3 is a fluctuating structure where titling and distortion of the octahedral networks and rotations and polarizability of the molecular dipole strongly affect the optoelectronic properties of the semiconductor. In this Account, we review our recent © XXXX American Chemical Society

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ORGANIC−INORGANIC INTERPLAY: RAMAN SPECTROSCOPY AS A PROBE Despite the rapid increase in the efficiency of hybrid perovskite based solar cells, there exists very little knowledge and control of the thin film properties. Various processing techniques have been documented so far that seem to strongly affect the optoelectronic functionalities of the compounds; however, there is no clear correlation that could provide a rational guideline to process perovskite films and to design the champion device architecture. Raman spectroscopy is a Received: October 15, 2015

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broad band at 200−340 cm−1 are associated with librational and torsional motions, respectively, of the MA cations within the cage (see Figure 1). Then, on the basis of electronic structure calculations, we elucidated how the interactions between the organic cation and the inorganic counterpart may affect the vibrational frequency in MAPbI3. An increased level of order induces a sharpening, blue-shifting, and loss in strength (by symmetry selection rules) of the band at ∼250 cm−1, which is predicted to fully disappear in the orthorhombic phase. Additionally, the mode around 154 cm−1 is expected to become sharper and more intense in the more ordered arrangement. With such a model in mind, we have eventually compared the Raman spectra of MAPbI3 crystals grown on a mesoporous scaffold and on a flat glass surface4 (last panel in Figure 1). Notably, in the first sample the crystal growth is constrained, which on average reduces the grain size. A close evaluation of the spectra from the“meso” to the “flat” MAPbI3 sample immediately reveals an increasing degree of conformational order as the band at around 250 cm−1 blue-shifts and its intensity strongly decreases, while the band at around 160 cm−1 becomes sharper and more resolved. This trend strikingly follows the theoretical prediction. Moreover the peak at 119 cm−1 decreases in intensity and red shifts to 115 cm−1, while the peak at around 94 cm−1 gains strength. This behavior points to reduced stress in the Pb−I bond, which may be related to the interaction between the organic and inorganic moieties.5 We demonstrated that the crystallization of CH3NH3PbI3 in a mesoporous scaffold not only produces smaller crystallites but also induces an intrinsic form of disorder in the relative arrangement of the organic cation within the inorganic cage.4

spectroscopic technique that can probe vibrational, rotational, and other low-frequency modes in a compound. It involves shining monochromatic light on a sample and detecting the scattered light. While the majority of the scattered light is of the same frequency as the excitation source, that is, Rayleigh scattering, a very small amount of it is shifted in energy from excitation due to interaction between the incident electromagnetic field and the vibrational energy levels. Thus, the Raman spectrum represents the intensity of this “shifted” light versus frequency. The band positions will lie at frequencies that correspond to the energy levels of different functional group vibrations, representing specific modes. Note that the Raman spectra are represented with respect to the laser frequency such that the Rayleigh band lies at 0 cm−1. Based on such a simple introduction, it is clear that the technique is extremely powerful to probe molecular interactions through specific vibrational modes. De Angelis and co-workers3 first calculated the nonresonant Raman spectra for three different model structures composed of four MAPbI3 units (see insets in Figure 1). Two

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LIGHT ABSORPTION AND PHOTOLUMINESCENCE MAPbX3 perovskites exhibit optical absorption of conventional direct bandgap semiconductors such as GaAs.6 The optical transitions involve excitation of carriers only within the frontier orbitals of the metal and the halide ions, while the cation MA+ does not participate in the absorption.2,6−8 However, it has a deep impact on the electronic structure and thus on the bandgap of perovskite.2,4 By replacing the MA+ cation with smaller (e.g., Cs) or larger9 (e.g FA+) ions, the bandgap can be tuned because of a change in the I−Pb−I bond angle.2,3,7,10 However, changing the chemical composition (and size) of the cation is not the only way to engineer the bandgap in this class of semiconductors. A modulation of the electronic structure can be also induced by a variation of the orientation of the organic cation within the lattice. This can be achieved by varying the degree of polycrystallinity of the thin film,4 which in fact strongly affects the optical properties of the material. In Figure 2a we show the absorption and steady state photoluminescence (PL) spectra of MAPbI3 polycrystalline thin films deposited in a mesoporous scaffold of Al2O3 (2 μm thick), which limits the perovskite crystal size to a few tens of nanometers, and on a glass substrate, which generally allows the formation of much larger grains.11 The crystals grown in the scaffold clearly show a larger band gap evident from the blueshifted optical band edge. Such a widening is also accompanied by a shortening of the PL decays. In Figure 2b, we show timeresolved photoluminescence (tr-PL) maps collected from the perovskite polycrystalline film grown on a 100 nm thin Al2O3 scaffold. The sample also presents a perovskite capping layer thicker than 400 nm, consisting of larger crystallites.11 By selecting the appropriate excitation wavelength (700 nm), a penetration depth of the light of about 200 nm within the

Figure 1. Theoretical (top three panels) and experimental Raman spectra of MAPbI3. Data reproduced with permission from refs 3 and 4. Copyright 2014 American Chemical Society.

of those are tetragonal: one, indicated as tet-1, has the organic dipole randomly oriented within the cage; the other, tet-2, has the dipoles arranged in an ideal head-to-tail configuration. The third one is the orthorhombic structure (indicated as ortho), which is the ultimate ordered structure, stable at low temperature, where organic dipoles tend to be locked. In such configurations, the conformational disorder/order of the organic cation will be transferred onto the inorganic network through hydrogen bonding interactions inducing different structural distortions. Based on these systems, first, key Raman markers were identified. The modes between 60 and 94 cm−1 are mainly associated with the I−Pb−I bending and Pb−I stretching. The spectral region between 100 and 200 cm−1 (in particular the modes at 119 and 154 cm−1) and the B

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Figure 2. (a) UV−vis absorption and cw-PL spectra of MAPbI3 flat and mesostructured films. Data reproduced with permission from ref 4. Copyright 2014 American Chemical Society. (b) PL maps of mesostructured MAPbI3 from the (top) substrate side and the (bottom) capping side excitation. Data reproduced with permission from ref 12. Copyright 2014 AIP Publishers. (c) (top) Spectral positions of UV−vis absorption band and cw-PL peak position and (bottom) PL lifetime as functions of average crystallite size within thin films of MAPbI3. (d) PL lifetime as a function of the excitation density for MAPbI3 samples with small (black circles) and large (red squares) crystallites. Data reproduced with permission from ref 13. Copyright 2014 American Chemical Society.

MAPbI3 film is achieved.12 Therefore, by changing the side of excitation, we can either predominantly excite the hybrid perovskite grown in the mesoporous oxide (substrate excitation) or the capping layer, which leads to the observation of two distinct behaviors. Smaller crystallites excited through the substrate side show shorter lived and blue-shifted PL (Figure 2b, top panel) with respect to the larger crystallites of the capping layer (Figure 2b, bottom panel). The correlation between the bandgap, PL lifetime, and polycrystallinity of the sample is further confirmed by the light absorption and emission spectra collected from samples with controlled crystal dimensions, that is, with different degrees of polycrystallinity, grown by a two step deposition method on a quartz substrate,13 therefore in the absence of any oxide scaffold (Figure 2c). In the previous section we have shown that orientational disorder of the cation could be induced in smaller perovskite crystals, together with a strain in the Pb−I lattice4(Figure 1). Accordingly, we correlate the modulation of the bond angles to the blue shift in both the absorption edge and the PL peak. In order to discuss the trend observed in the PL dynamics, we first recall the photophysical model that is currently used to describe it.13−17 Photoluminescence results from the bimolec-

ular recombination of electrons and holes, while any carrier trapping phenomenon will result in nonradiative loss channels. Thus, the temporal evolution of the excited carrier population, n(t), can be represented by dn/dt = −An − Bradn2, where A is the trapping rate and Brad is the bimolecular intrinsic radiative R recombination coefficient, (Brad = nrad2 , where Rrad is the i

radiative rate and ni is the intrinsic carrier concentration).18 At low pump fluences, the PL dynamics are dictated by the trapping dynamics (A) and hence exhibit a monomolecular and intensity independent behavior. As the pump fluence is increased, the carrier density exceeds the available trap density, enabling an efficient radiative recombination process. This is seen as an increase in the PL quantum yield with the pump fluence.13,17 Please note that this model does not take into account Auger-like non-radiative channels (∼n3), which may kick-in at higher excitation densities.13,16 Brad depends on intrinsic radiative rate (number of electron− hole recombination events per unit time and per unit volume). Both Rrad and ni decrease exponentially with the bandgap energy (Eg), but ni2 overcompensates the difference in Rrad.18,19 This typically results in smaller Brad (and longer PL lifetimes) for materials with smaller Eg. C

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Accounts of Chemical Research Coming back to the observation of elongation of PL lifetimes with an increase in the average crystal size, this can be the result of a change in either the trap density or the intrinsic radiative coefficient. These parameters can be extracted by fitting the intensity dependence of the PL lifetime with the simple relation, τPL = 1/(A + Bradn0) where n0 is the initial excitation density. In Figure 2d, we show fits for samples with small (1 μm as shown in Figure 6c; however it has a lifetime of only a few picoseconds (please refer to ref 40 for details). Therefore, different morphologies of MAPbI3 thin films show (i) only free carrier population, (ii) an excitonic fraction upon temperature reduction (in agreement with a recent investigation by optical pump−terahertz probe spectroscopy (OPTP) on polycrystalline thin films), and (iii) a fraction of F

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Accounts of Chemical Research these structural changes to the variability of Eb through the modulation of the effective dielectric permittivity. The organic cations, which have permanent dipole moment, can generate a gradient of electrostatic potential when free to rotate.1,9 Walsh and co-workers simulated its standard deviation in a perfect single crystal of MAPbI3 as a function of temperature35 by multiscale modeling. The variance in the potential reduces when the temperature is lowered, as the rotational motions of the organic dipole are reduced, thus forming twinned dipole domains. Twinned dipoles within neighboring unit cells interact via dipole−dipole couplings, and this will increase the cumulative energy needed for their individual rotations and lead to the “locking” of the cations in an orderly fashion. Apart from thermal fluctuations, the dipole twinning can also be disrupted when moving from a perfect single crystal to a polycrystalline sample, i.e., by adding different densities of point defects in the simulated perfect slab of MAPbI3. As a result, the standard deviation of the electrostatic potential increases at a fixed temperature. Therefore, the variance of the electrostatic potential and thus the local screening can be controlled by the local order within the crystal. While larger and less defective crystals provide lower screening, the electron−hole separation due to electrostatic disorder can be significant in smaller crystals. The intrinsic correlation between the microstructure and excitonic effects is not limited to the MAPbI3 perovskite, but it can be generalized and further strengthened by our studies on MAPbBr3, where the excitonic transition is visible even at room temperature but only when the film is grown on flat glass, resulting in micrometer large crystals (absorption spectrum shown in Figure 7a, as large crystals). When it is deposited on the mesoporous alumina scaffold, the excitonic transition is quenched (referred to as small crystals in Figure 7a). The TA spectra of this sample also shows only PB (Figure 7b), concurrent with a free carrier picture, while the TA spectra of large crystals show the MA* feature. It is worth noticing that our observations find similarities in thin films of 2D perovskites,37 as well as in nanocrystal solutions,42 which, in principle, are not affected by spurious effects such as change in reflectivity.43 In Figure 7c, we reproduce the TA spectra as reported by Wu et al, gated at 1 ps, for 2D hybrid perovskites (C4H9NH3I)2(CH3NH3I)n−1(PbI2)n, with n = 3 and 2 defining the quantum well thickness. In such samples, Eb goes from 150 to about 250 meV, respectively. Larger confinement, i.e., stronger electron−hole correlation, amplifies the role of manybody interaction in the relative photoinduced spectra; therefore the weight of the MA* feature (induced by the blue shift of the excitonic transition) becomes stronger over the simple PB feature.37,39 There is of course still a need for comprehensive and quantitative analysis of the TA spectra of these materials to properly disentangle different overlapping spectral features.

Figure 7. (a) UV−vis absorption spectra of MAPbBr3 films composed of small crystals and large crystals. (b) Transient absorption spectra of the same samples at a pump−probe delay of 1 ps. Data reproduced with permission from ref 35. Copyright 2015 Macmillan Publishers. (c) Absorption and TA spectra at 1 ps of representative 2D perovskites. Adapted with permission from ref 37. Copyright 2015 American Chemical Society.

extent carriers can delocalize or localize within the lattice or defects, with relevant consequences on the transport properties.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Ajay Ram Srimath Kandada holds a Ph.D. in physics from Politecnico di Milano, and currently he is a postdoctoral researcher at the Istituto Italiano di Tecnologia (IIT).

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Annamaria Petrozza received her Ph.D. in Physics from the University of Cambridge (U.K.) in 2009. Then, she worked as Research Scientist at the Sharp Laboratories of Europe, Ltd. In 2010, she joined the Istituto Italiano di Tecnologia (IIT) where she currently holds a Tenure Track Researcher position.

CONCLUSIONS AND OUTLOOK We have established the inherent relationship between the photophysical properties and microstructure in hybrid perovskites, with more emphasis on the MAPbI3. The outcome of such studies on one side may suggest a guide for an educated processing of these materials for optoelectronic applications. On the other side, it calls for deeper investigations on a series of still unexplored topics such as the study of the photophysical dynamics in the high density excitation regime, the description of electron−phonon coupling in order to understand to what

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ACKNOWLEDGMENTS The authors acknowledge all their collaborators and the funding sources (EU 7th Frame work Program under the grant agreements No. 604032 (MESO), No. 316494 (DESTINY), EU Horizon 2020 Research and Innovation G

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program under the grant agreement No. 643238 (SYNCHRONICS), Fondazione Cariplo Project GREENS (No. 2013-0656), Fondazione Cariplo Project IPERLUCE (ref. 2015-0080).

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