How Lead Halide Complex Chemistry Dictates the Composition of

Letter pubs.acs.org/JPCL

How Lead Halide Complex Chemistry Dictates the Composition of Mixed Halide Perovskites Seog Joon Yoon,†,‡ Kevin G. Stamplecoskie,†,∥ and Prashant V. Kamat*,†,‡,§ †

Radiation Laboratory, ‡Department of Chemistry and Biochemistry, and §Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: Varying the halide ratio (e.g., Br−:I−) is a convenient approach to tune the bandgap of organic lead halide perovskites. The complexation between Pb2+ and halide ions is the primary step in dictating the overall composition, and optical properties of the annealed perovskite structure. The complexation between Pb2+ and Br− is nearly 7 times greater than the complexation between Pb2+ and I−, thus making Br− a dominant binding species in mixed halide systems. Emission and transient absorption measurements show a strong dependence of excited state behavior on the composition of halide ions employed in the precursor solution. When excess halide (X = Br− and I−) are present in the precursor solution (0.3 M PbX2 and 0.9 M CH3NH3X), the exclusive binding of Pb2+ with Br− results in the formation of CH3NH3PbBr3 perovskites as opposed to mixed halide perovskite.

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the band gap from 2.43 to 1.48 eV with a continuous red shift.22 Such mixed halide films also offer tunability of the emission following bandgap excitation.23 Such tunable and coherent light emission properties have been utilized in lasing applications.20,24 The halide composition in the mixed halide perovskite can be varied by varying the precursor halide (PbX2 or CH3NH3X, X = Br− or I−) concentration or exposing the CH3NH3PbI3 film to Br2 vapor.21,25,26 Several recent studies have focused on studying the absorption, emission, and photovoltaic performance of mixed lead halide perovskite systems.21,22,27,28 However, an important underlying criterion for achieving desired electronic and optical property for perovskite film is the nature of Pb2+ and halide ion complexation in the precursor solution and how it aids in the evolution of the overall perovskite structure. For example, trihalide and tetrahalide complexes of lead exhibit charge transfer properties with characteristic absorption and emission properties.12,29 Movement of halide ions to form an iodide-rich region during the illumination of mixed halide perovskite film has raised the curiosity to better understand the underlying plumbate chemistry of these materials.23 In order to elucidate the nature of binding between Pb2+ and Br−/I− ions in mixed halide systems, we have now probed the spectral features in the precursor solution as well as perovskite films. The role of Br− and I− in dictating the complexation with Pb2+ and its influence on the optical properties of the perovskite film are discussed.

he increased attention given toward organic metal halide perovskites has necessitated understanding of morphology, crystal structure, and excited state dynamics.1−4 The solvents with Lewis base properties (e.g, DMSO) have been shown to coordinate with Pb2+ ions and, thus, dictate the morphology of the annealed perovskite films.5,6 Similarly, the presence of Cl− in the precursor solution is also beneficial to influence the morphology of the perovskite films as well as the performance of solar cell.7−10 The initial interaction of Pb2+ ions with coordinating ligands becomes a major factor in dictating the structure, morphology, and photovoltaic properties of the perovskite film. The precursor film of plumbate complex upon annealing at elevated temperatures transforms into perovskite structure as the organic cation gets inserted into the cavity.11−13 Of the various organic metal halide perovskites tested so far, CH3NH3PbI3 continues to exhibit highest photovoltaic conversion efficiency.14 However, organic lead halide perovskite film is very sensitive to humidity and can undergo transformation under ambient operating conditions.15−17 It is proposed that H2O initially interacts with the perovskite surface, and then slowly permeates through to the center of larger crystallites to transform into hydrate form. Replacement of organic cation with Cs+ and iodide with bromide seem to increase the stability of the perovskite films.18,19 However, other pure-phase lead halide perovskite structures employed in photovoltaic cells have yet to exhibit performance comparable to CH3NH3PbI3. Another interesting aspect is the substitution of methylammonium cation with formamidium and halide substitution, which offers the possibility to tune the bandgap of the perovskite semiconductor.3,18−21 By increasing the I/Br ratio in the mixed halide perovskite film, it is possible to tune © 2016 American Chemical Society

Received: February 24, 2016 Accepted: March 23, 2016 Published: March 23, 2016 1368

DOI: 10.1021/acs.jpclett.6b00433 J. Phys. Chem. Lett. 2016, 7, 1368−1373

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Figure 1. Absorption spectra of 0.2 M PbBr2 in DMF with increasing concentration of CH3NH3Br of (A) 0−15 mM (spectra a−d) and (B) 15−120 mM (spectra d−g). Insets show the dependence of 1/ΔA versus 1/[CH3NH3Br] corresponding to equilibria 1 and 2.

Figure 2. Emission spectra of PbBr2 solution in DMF containing (A) 2.7 mM and (B) 90 mM CH3NH3Br. The spectra were recorded following excitation with (a) 310 nm and (b) 360 nm, corresponding to the absorption maximum of PbBr3− and PbBr42− complexes.

Post-transition metal ions such as Pb2+ readily complex with solvents and halide ions to produce trihalide or tetrahalide plumbate complexes.12,29 Complexation of Pb2+ ions with solvent molecules such as N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) through their six coordination sites is beneficially utilized in obtaining homogeneous organic metal halide perovskite films.7 The solvent molecules or ligands that can donate a pair of electrons are known to form Lewis adducts with Pb2+. Of particular interest is the complexation of Pb2+ ions with halide ions. PbI2, which is colorless, becomes darker yellow upon addition of excess iodide ions. As halide ions (X−) replace weaker coordinating solvent molecules, we observe formation of PbX3− and PbX4− complexes with characteristic charge-transfer absorption bands.29 In our previous study, we elucidated the complexation of Pb2+ with iodide ions through spectral changes associated with PbI3− and PbI42−.2,29Apparent association constants for these two complexation equilibria were determined to be 54 M−1 and 6 M−1 respectively. When working with mixed halide systems, however, it is important to know the association constants for different halide complexes of lead so that we can identify the dominance of individual halide ions in the complexation process. Figure 1A,B shows the absorption changes of PbBr2 solution with increasing Br− concentration. These spectral changes, which are plotted for two different concentration ranges of Br− ion, represent two lead−bromide complexes. At lower concentration range (0−15 mM) we see a decrease in 285 nm band of PbBr2 with concurrent increase in the absorption at 310 nm. The appearance of this new band corresponds to the formation of PbBr3− complex.30 The appearance of an isosbestic point in the absorption spectra

(Figure 1A) confirms the existence of two species in equilibrium, viz., complexed and uncomplexed species (equilibrium 1). PbBr2 + Br − ⇔ PbBr3−

(1)

With further increase in concentration of Br−, we see a decrease in 310 nm band along with simultaneous increase in absorption at 360 nm. Although these absorption changes are small, it is indicative of the formation of the higher degree halide complex PbBr42− (equilibrium 2).30 PbBr3− + Br − ⇔ PbBr4 2 −

(2)

We employed absorbance changes at 310 and 360 nm to obtain complexation constants for the two equilibria using Benesi− Hildebrand analysis.31 The insets in Figure 1 show the linear dependence of 1/ΔA versus 1/[Br−]. The apparent complexation constants for the formation of PbBr3− and PbBr42− determined from these plots were 220 M−1 and 22 M−1, respectively. The apparent complexation constant of a trihalide complex for bromide ions is 7 times greater than that of a triiodide complex and 3.5 times greater for a tetrabromide than a tetraiodide complex of Pb. These results clearly show that, as compared to I− ions, Br− has stronger affinity for complexation with Pb2+. The difference in complexation constant implies that when PbI2 or PbBr2 are dissolved in equimolar concentrations, the complexation with Br− dominates over I−. Thus, the complexation processes occurring in the precursor solution is likely to play an important role in determining the final composition and structure of mixed halide perovskite systems. 1369

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Figure 3. (A) Absorption spectra and (B) emission spectra (excitation: 400 nm) of mixed halide lead perovskite films prepared with different Br:I ratio with the same 0.3 M of PbX2 and CH3NH3X (X = Br− or I−). The halide compositions were (a) Br only (0.9 M Br−, 0 M I−) (b) Br−:I− = 1:1 (0.45 M Br−, 0.45 M I−) (c) Br−:I− = 1:3.5 (0.2 M Br−, 0.7 M I−), and (d) I only (0 M Br−, 0.9 M I−).

Figure 4. (A) Time-resolved difference absorption spectra recorded after 5 ps after 387 nm laser pulse excitation of mixed halide perovskite films with different halide composition: (a) Br only (0.9 M Br−, 0 M I−) (b) Br−:I− = 1:1 (0.45 M Br−, 0.45 M I−) (c) Br−:I− = 1:3.5 (0.2 M Br−, 0.7 M I−), and (d) I only (0 M Br−, 0.9 M I−). (B) Transient recovery as monitored for the same films at bleach maximum and (C) corresponding secondorder kinetic fit representing bimolecular recombination.

Emission Properties of PbBr3− and PbBr42− Complexes. Both PbBr3− and PbBr42− complexes are photoactive and can be characterized from their emission spectra with maxima at 600 and 560 nm, respectively.30 The emission spectra recorded at low and higher Br− concentrations are shown in Figure 2A,B, respectively. At low concentrations (2.7 mM), we have mainly PbBr3−as the major contributing species to the emission. Indeed, the emission spectrum recorded with 310 nm excitation shows the dominance of PbBr3−as it responds only to lower wavelength excitation. At higher concentrations, both PbBr3− and PbBr42− coexist in the solution, and we see a broad emission when excited at 310 nm. This broad emission arises as both these complexes are excited at 310 nm. However, when excited at 360 nm, we see a single emission band with maximum at 560 nm in response to the excitation of PbBr42−. These results parallel the previous observation of emission from plumbate complexes and thus indicate their optical activity.29 It is interesting to note that the absorption and emission maxima of these complexes are well separated. The large Stokes shift with low energy emission bands point out to the charge transfer nature of the excited state.29,30 According to Horvath and Miko, the ligand-to-metal charge transfer interactions are capable of forming Pb(I) intermediates either photochemically or thermally as a result of intramolecular ligand-to-metal charge transfer process.32 By employing laser flash photolysis and steady state photolysis. they were able to characterize the metallic lead and I3− as the final products resulting from charge transfer interactions. Similarly, EPR studies have also been

employed to probe the charge trapping in halogenoplumbate compounds.33 Since both PbBr3− and PbBr42− are photoactive, it is important to take into account the emission arising from these complexes while assessing the excited state dynamics of CH3NH3PbBr3 and mixed halide perovskite samples. Optical Properties of Mixed Halide Lead Perovskite Films with Stoichiometric Composition (0.3 M PbX2 and 0.3 M MAX). We prepared precursor solutions containing a stoichiometric composition of PbX2 and CH3NH3X (MAX), where X = I− + Br−. We kept the Pb2+-to-halide ratio constant at equimolar (0.3 M for both PbX2 and MAX), but varied the ratio between Br− and I−. The solutions were spin coated onto a mesoporous Al2O3/glass slide, and pressed onto another glass slide with sealed edges. The mesoporous Al2O3 layer was selected as a support in the present experiments to exclude any possible interaction of oxide support with the lead halide perovskite film. (Charge transfer interactions have been reported between lead halide perovskite and TiO2 support layer under visible excitation.34) The films were annealed at 373 K for 5 min. The XRD experiments confirmed the transformation of plumbate complex into perovskite films (Figure S1 in the Supporting Information). The evolution of perovskite from the plumbate complex has been discussed in our earlier studies.12 The absorption and emission spectra of four different films with different halide composition are shown in Figure 3. The film of CH3NH3PbBr3 alone shows an absorption onset around 520 nm and an emission peak at 525 nm. With increasing iodide concentration, the absorption edge and the emission peak shift 1370

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Figure 5. Time-resolved difference absorption spectra recorded after 5 ps after 387 nm laser pulse excitation of lead halide perovskite films using 0.3 M for both PbX2 and MAX (Br−:I− = 1:1 for A and B) and 0.3 M PbX2 and 0.9 M MAX (PbX2:MAX = 1:3 for C and D). The samples in A and C had Br only, and the samples in B and D had an equimolar ratio of bromide and iodide (B: 0.45 M Br− or I−, D: 0.75 M Br− or I−).

mixed halide films show bleaching in the intermediate range following the trend seen in the absorption spectra. The induced absorption is seen at higher energy region relative to the corresponding bleached absorption band. It is interesting to note that the difference absorption spectra of all mixed halide perovskite films (Figure 4A) exhibit similar spectral fingerprints of induced absorption and bleaching of absorption band. The only difference is the position of the maxima, which shifts to red as the iodide content increases. The position of the bleaching maximum agrees well with the absorption edge for the corresponding perovskite film. This spectral feature in turn indicates a common semiconducting behavior of mixed halide perovskites despite the difference in the halide composition. As confirmed in the case of CH3NH3PbI3, the bleaching represents the charge separation following the laser pulse excitation.38 The bleaching recovery is marked by the bimolecular electron−hole recombination without significant interference from trap sites. Figure 4B shows normalized bleaching recovery recorded at the bleaching maximum, and Figure 4C shows the kinetic analysis corresponding to bimolecular charge recombination (see Table S1 for the detailed kinetic analysis). CH3NH3PbBr3 films exhibited the shortest lifetime of 0.126 ns. With increasing iodide content, the lifetimes increased to 0.386 and 1.518 ns for 1:1 (0.45 M Br−, 0.45 M I−) and 1:3.5 (0.2 M Br−, 0.7 M I−) of halide composition films, respectively. The CH3NH3PbI3 has slightly shorter lifetime of 0.787 ns. These results suggest that the charge recombination in mixed halide films is slower than the one observed in CH3NH3PbBr3 film. Earlier, impedance spectroscopy measurements have also pointed out similar reduces rates of charge recombination following the insertion of mixed halides in the perovskite lattice.38

toward the red region. CH3NH3PbI3 film exhibits its characteristic absorption edge at 760 nm and emission peak at 770 nm. Any shifts in the emission peak arising from the grain size is considered to be rather negligible since such effects cause only a small shift (10−20 nm).35,36 As shown in earlier studies,21,25,27,28 the increase in Br−:I− ratio causes a hypsochromic shift in the absorption and emission bands, and thus enables the tuning of the bandgap in the range of 1.48 to 2.43 eV. It is important to note that the absorption and emission properties of perovskite films (Figure 3) differ significantly from the corresponding precursor plumbate complexes (Figure 2) and thus confirm the transformation into perovskite structure during annealing. The presence of a single emission peak close to the absorption edge is an indication of a single phase of perovskite structure. The emission peaks are narrow with a small Stokes shift. These results confirm that the observed emission arises from charge carrier recombination within the band structure of mixed halide perovskite films. The mechanistic and kinetic aspects of charge recombination processes in pristine CH3NH3PbI3 have been discussed in earlier studies.37 It should be of interest to know how the presence of mixed halides in the lattice might further influence the excited state dynamics. We recorded time-resolved transient absorption spectra following 387 nm laser pulse excitation of samples (Figure 4). The samples were placed in an evacuated cell and subjected to 387 nm laser pulse excitation. The difference absorption spectra recorded 5 ps after the laser pulse excitation, show a sharp bleaching band and an induced absorption band (Figure 4A).The bleach bands at 525 nm for CH3NH3PbBr3 and 760 nm for CH3NH3PbI3 films corresponds to the wavelengths of the absorption shoulder seen in Figure 3A. The films with 1371

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The Journal of Physical Chemistry Letters Optical Properties of Mixed Halide Films with Excess Halide Concentration (0.3 M PbX2 and 0.9 M MAX). It is a common practice to employ excess MAX (usually 3 times the concentration of PbI2) while preparing perovskite films for solar cells. The excess CH3NH3I allows slow evolution of perovskite film during the annealing process, thus producing a better quality films with larger crystallites. As confirmed from the thermogravimetric analysis, the excess MAX sublimes when annealed under open atmosphere.11 The obvious question is how the mixed halide composition can influence the spectral evolution if excess halide ions are present during the annealing process. We prepared perovskite films with excess MAX (0.3 M PbX2 and 0.9 M MAX) similar to those prepared with the equimolar ratio in earlier experiments. We recorded transient absorption spectra following 387 nm laser pulse excitation of four methylammonium lead halide perovskite samples (Figure 5), two with stoichiometric composition (A and B in Figure 5, 0.3 M for both PbX2 and MAX, X = Br− + I−) and two with excess halide ions (C and D in Figure 5, 0.3 M PbX2 and 0.9 M MAX). The reference samples (A and C in Figure 5) employed only bromide while the mixed halide samples (B and D in Figure 5) contained equimolar ratio of bromide and iodide. All samples were sealed before annealing and were excited with 387 nm laser pulse after transferring it into an evacuated cell. The absorption spectra of these films are shown in the Supporting Information (Figure S2). Films prepared with stoichiometric composition of Pb2+ and halide (0.3 M for both PbX2 and MAX) show spectral characteristics similar to those discussed in Figure 3A. However, for films prepared with excess MAX (0.3 M PbX2 and 0.9 M MAX), we observe a blue-shifted transient absorption/bleaching with no observable dependence on the iodide concentration. This is indicative of the possibility that the perovskite films obtained with excess MAX concentration constitute Br-rich structure. In the case of CH 3 NH 3 PbBr 3 films prepared with stoichiometric composition or excess bromide ions did not produce any noticeable difference in the transient absorption features (panels A and C in Figure 5). Both stoichiometric composition and excess halide samples showed characteristic bleaching around 520 nm and an induced absorption below 500 nm. This shows that in both these cases, the samples exhibit characteristics similar to that of CH3NH3PbBr3 excited state. However, the excited state behavior of films containing mixed halides with stoichiometric composition or excess halide ions showed marked difference in the transient absorption spectra. Both these samples contained identical bromide or iodide concentration (0.45 or 0.75 M for Figure 5B or 5D, respectively), but the total halide concentrations were stoichiometric and 5 times that of Pb2+ concentration to form CH3NH3PbX3. The transient absorption spectrum of the film with identical 0.3 M of PbX2 and MAX in Figure 5B shows characteristic excited state behavior of mixed halide perovskite film in accordance with the observation in Figure 4A. It is interesting to note that the films prepared using excess halide ions (0.3 M PbX2 and 0.9 M MAX) exhibit a transient absorption feature, closely resembling that of CH3NH3PbBr3. The bleaching around 500 nm in Figure 5D distinguishes itself from the mixed halide perovskite film seen in Figure 5B. These results point out that when excess bromide ions are present in the precursor solution, it promotes exclusive binding of Pb2+ with Br− and not I−. Since the association constant for the complexation of Pb2+ with Br− is greater than that with I−, we

expect to see such preferential complexation with bromide ions. Upon annealing, the same coordination geometry with octahedral structure is retained, thus resembling the formation of CH3NH3PbBr3. XRD analysis (Figure S1) also indicates the formation Br-rich perovskites when there are excess halide ions in the precursor solution. We can thus conclude that when excess bromide ions are present in the precursor solution, the iodide remains as a spectator without influencing the coordination with bromide ions. To summarize, the chemical interaction between lead and halide ions is a fundamentally important factor in determining the final composition and structure of organic metal halide perovskites. Whereas most of the studies reported to date have focused on varying synthetic steps to achieve better morphology and superior photovoltaic performance of organic metal halide perovskite films, the basic chemistry underlying the complexation between Pb2+ and halide ions is still an important factor, and it cannot be overlooked. In particular, the difference in the complexation constants between Pb2+ and Br−, and Pb2+ and I− ions and their concentration in precursor solution dictates the final structure of mixed halide perovskites. In addition to the morphology of the perovskite films, it is important to pay attention to underlying principles of precursor chemistry.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b00433. Experimental methods including lead halide film preparation, various characterizations, and X-ray diffraction patterns (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; twitter: @kamatlabND. Present Address ∥

Queens University, Canada (kevin.stamplecoskie@queensu. ca) Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the support from the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DE-FC02-04ER15533. This is document No. 5701 from Notre Dame Radiation Laboratory. The authors would like to thank Ms. Rose T. Bernier for her assistance during preliminary screening of mixed halide complexes.

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REFERENCES

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DOI: 10.1021/acs.jpclett.6b00433 J. Phys. Chem. Lett. 2016, 7, 1368−1373