Laser Emission from Self-assembled Colloidal Crystals of Conjugated

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Laser Emission from Self-assembled Colloidal Crystals of Conjugated Polymer Particles in a Metal-halide Perovskite Matrix Annabel Mikosch, Sibel Ciftci, Gregory Tainter, Ravichandran Shivanna, Bastian Hähnle, Felix Deschler, and Alexander J.C. Kuehne Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00307 • Publication Date (Web): 08 Mar 2019 Downloaded from http://pubs.acs.org on March 8, 2019

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Chemistry of Materials

Laser Emission from Self-assembled Colloidal Crystals of Conjugated Polymer Particles in a Metal-halide Perovskite Matrix Annabel Mikosch1,2, Sibel Ciftci1, Gregory Tainter2, Ravichandran Shivanna2, Bastian Haehnle3, Felix Deschler2, Alexander J. C. Kuehne1,3,* 1

DWI – Leibniz Institute for Interactive Materials e.V., Forckenbeckstr. 50, 52074 Aachen, Germany.

2

Cavendish Laboratory, JJ Thomson Avenue, Cambridge, United Kingdom.

3

Institute of Organic and Macromolecular Chemistry, Ulm University, Albert-Einstein-Allee 11, 89081 Ulm, Germany.

ABSTRACT: Here we present a hybrid organic/inorganic photonic composite, which generates laser emission from the organic material after pumping the inorganic component. The composite consists of a methylammonium lead-halide perovskite matrix CH3NH3Pb(BrxCl(1-x))3 and monodisperse poly(fluorene divinylbenzene) particles, which have excellent optical feedback and gain. Micrometer-sized conjugated polymer particles are deposited together with the perovskite precursor from solution using a single-step vertical deposition method. The particles self-assemble into a photonic crystal and the perovskite forms an inorganic matrix in the interstitial space. Energy transfer to the polymer particles after optically pumping the metal-halide perovskite is studied in two systems with different halide ratios in the perovskite (Br to Cl: 1/9 and 4/6), to control the overlap of the perovskite emission energy with the absorption of the particles. From time-resolved photoluminescence experiments, we observe non-radiative energy transfer from the perovskite to the particle in both coassemblies; however, increased spectral overlap of perovskite emission and particle absorption enhances energy transfer efficiency by 37%. Because of the ordered assembly of the conjugated polymer particles we observe laser emission after energy transfer from the Cl-rich perovskite matrix at fluences of 13 mJ/cm². Our report of a hybrid material system that combines the excellent opto-electronic properties of metal-halide perovskites with the outstanding optical properties of conjugated polymers represents a new approach and progress in the pursuit of electrically pumped polymer lasers.

INTRODUCTION Methylammonium lead halide perovskites CH3NH3PbX3 (MAPbX3) combine high conductivity and low defect density typical for inorganic semiconductors with solutionprocessablity, a property usually reserved for organic semiconductors.1 Solution processing enables integration of perovskite films into multi-layer stacks and allows combination with organic layers for optoelectronic and photonic applications. This integration and combination are facilitated by the fact that MAPbX3 and organic semiconductors can be processed from orthogonal solvents (in which either the perovskite or the organic semiconductor is soluble). The resulting layered hybrid organic-perovskite semiconductor devices, where the organic layers often serve as charge transport layers regularly excel all-organic devices.2–4 Changing the type and ratio of the halides in the perovskite structure allows tuning of its electronic bandgap. Bromine-pure MAPbBr3 exhibits electro- and photoluminescence in the green spectrum, exchange of the bromines for chlorides – yielding mixed Br/Cl perovskites – shifts the emission hypsochromically into the blue spectrum, while chlorine-pure MAPbCl3 emits in the near-UV region.5–7 By contrast, the addition of iodide to produce Br/I mixed perovskites shifts the emission bathochromically into the red spectrum and into the near-IR spectrum

for iodine-pure MAPbI3.8,9 This remarkable tunability of the electronic band-gap combined with their strong luminescence,10 long change carrier lifetimes and low defect density have enabled the development of highly efficient, solution-processed thin film perovskite solar cells,11,12 lightemitting diodes (LEDs),5,13 and optically pumped lasers.11,14 However, much like in organic semiconductors, electrical pumping of MAPbX3 perovskite laser geometries remains impossible to date. In organic semiconductors low charge carrier mobility is one of the major obstacles preventing electrically pumped lasing.15 To achieve optical feedback in lasers, the semiconductor waveguide needs to have dimensions of at least ~ 𝜆⁄2 of the supported mode. Driving such thick organic semiconductor layers electrically towards population inversion requires high voltages and high charge carrier densities, leading to detrimental quenching effects, impeding laser emission.16 By contrast, MAPbX3 perovskites exhibit extremely high charge carrier mobilities,17–19 enabling layer thicknesses in light emitting devices of up to microns.20 Despite their high charge carrier mobilities, perovskites suffer from thermal instability under injection conditions, preventing electrically pumped lasing.13,19 Device architectures, which overcome these challenges are absent to date.

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Due to the orthogonal solubility of organic semiconductors versus perovskites, conjugated polymer particles could be co-deposited and assembled within a perovskite matrix in a single fabrication step. This technique would yield new types of hybrid organic/inorganic device architectures, which could potentially overcome the drawbacks of the individual components. Dispersion polymerization gives access to such monodisperse conjugated polymer particles (CPPs) with controllable diameters in the range of the wavelength of visible light (ca. d = 100 nm and up to 1.5 µm).21,22 Handling of CPP dispersions is straightforward and they can be deposited by classical solution processing techniques like spin-coating, doctor-blading and ink-jet printing. Naturally, CPPs have been applied as active media in light-emitting diodes (LEDs)23 and organic solar-cells.24 Monodisperse CPPs can be self-assembled into periodic photonic crystal structures, where the particles act as the gain medium as well as an optical resonator simultaneously.25 To improve the ordering during the assembly process, reduce crack formation, and protect the CPPs from photo-oxidation and mechanical abrasion, the self-assembly process can be performed in presence of an organo-silicate sol-gel material, which condensates into a glass occupying the interstitial space.25–27 This concept yields laser structures that can be pumped optically.25 This way, the fabrication procedure for a stable, optically pumped conjugated polymer laser is reduced to a single fabrication step, avoiding nanofabrication techniques to impose a resonator structure, and no back-filling, infiltration or etching steps are required.28 Replacing the organo-silica matrix by perovskite could overcome the above-mentioned charge carrier mobility problems, while exploiting the best properties of both, the inorganic and the organic worlds. However, such device architectures remain inaccessible to date and the energy transfer from perovskite to conjugated polymer emitters is not understood. In this work, we fabricate ordered assemblies of monodisperse CPPs inside of a MAPbX3 perovskite matrix via

colloidal co-assembly. We investigate the non-radiative energy transfer from the MAPbX3 perovskite to poly(fluorene-co-divinylbenzene) CPPs under optical excitation. The composition of MAPb(BrxCl(1-x))3 is adjusted so, that (i) the respective absorption and emission bands of the CPPs and the perovskite overlap fully or (ii) so that the emission of the perovskite coincides partially with the emission of the CPPs, leading to reduced overlap cross-section of perovskite emission with the absorption of the CPPs. For CPP/perovskite co-assemblies with good energy transfer characteristics we demonstrate laser emission from the organic photonic crystal feedback structure, after pumping into the absorption of the perovskite. We elaborate that the energy transfer process from the perovskite to the CPPs follows a Förster resonant energy transfer (FRET) mechanism, enabling new CPP/perovskite hybrid architectures as optically pumped lasers and as potential geometries for electrically pumped hybrid organic/inorganic lasers of the future.

RESULTS AND DISCUSSION We apply poly(fluorene-co-divinylbenzene) (F8DVB) as the conjugated polymer laser medium due to its high gain and low optical threshold.29 We produce F8DVB CPPs in a dedicated dispersion polymerization to obtain monodisperse colloids with diameters of d = 1 µm (± 10 nm).22 This diameter is optimized to achieve feedback in a hexagonal resonator lattice, taking into account the previously determined refractive indices of perovskite with n = 2.330 and F8DVB with n = 1.9 at the emission wavelength of F8DVB.25 For the co-assembly process, the particles are combined with the respective precursors CH3NH3X (MAX, X = Br,Cl) and PbX2 (X = Br,Cl) to form MAPb(BrxCl(1-x))3 perovskites. We choose a 4:1 (by volume) solvent mixture of dimethylsulfoxide (DMSO) and acetonitrile. DMSO is a coordinating solvent for [PbX6]4-, retarding the formation of perov-

Figure 1. Fabrication and characterization of co-assembled F8DVB/perovskite films. a) Schematic display of the vertical deposition method. Dissolved perovskite precursors (PbX2, CH3NH3X) form CH3NH3PbX3 (MAPbX3) around the nanoparticles, which are driven to self-assemble at the meniscus of the drying dispersion. (b-e) Scanning electron micrographs of co-assembled films. b) Close-up of an x = 0.1 (CPP/MAPb(Br0.1Cl0.9)3) perovskite co-assembly. c) Overview of a x = 0.4 (CPP/MAPb(Br0.4Cl0.6)3) perovskite co-assembly in top view. d) Cross-section of a co-assembly with x = 0.1, showing the CPP monolayer and perovskite crystallites and debris from breaking the sample in the background). e) Cross-section of the co-assembly with x = 0.4. Some deformation of the particles is visible towards lower layers due to limited swelling. f) WAXS of x = 0.1 and x = 0.4 co-assemblies with a zoom-in from q = 10 - 25 nm-1 to better identify the first four scattering peaks.


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Chemistry of Materials skite crystals in solution.31 This is critical for the co-assembly process, as we want to prevent crystal formation until the F8DVB particles are assembled. Acetonitrile is added as a drying control agent, which reduces swelling of the CPPs in contrast to when using pure DMSO. The molar ratios of the chloro- and bromo-precursors are adjusted in accordance with a previously reported method to obtain MAPb(Br0.1Cl0.9)3 and MAPb(Br0.4Cl0.6)3 with maximum photoluminescence at λem = 430 nm and λem = 460 nm, respectively.5 The absorption maximum of the F8DVB CPPs is at λabs = 415 nm for large overlap integral of the x = 0.1 perovskite emission and absorption of the particles, while in the x = 0.4 perovskite the emission spectra partially overlap (x denotes the stoichiometry of the bromine as indicated in the sum formula MAPb(BrxCl(1-x))3). We adapt a previously developed vertical deposition method, where polymer colloids are co-assembled with an organo-silicate sol-gel system to achieve large area opal structures. Adaptation of this protocol to our perovskite precursor system enables us to co-assemble CPPs inside of the respective perovskite matrix (see Figure 1a).27,32 To assemble the CPPs inside of the perovskite matrix via vertical deposition, we place a cleaned glass substrate vertically in a colloidal dispersion containing the CPPs (d = 1 µm) and the perovskite precursors in DMSO:acetonitrile. While the solvents evaporate, the colloids are dragged to the liquid meniscus at the glass substrate via capillary forces forming a hexagonally ordered assembly of CPPs. At the same time, formation and crystallization of the perovskite takes place

in the interstitial space between the colloids (see Figure 1a). The vertical assembly process is performed at 55°C to increase the evaporation rate of the low vapour pressure solvents and under nitrogen atmosphere to prevent oxidative degradation. Inspection of the obtained assemblies using scanning electron microscopy (SEM) shows that we obtain well ordered, crystalline arrangements with colloidal crystal domain sizes in the range of 10 µm (see Figure 1b-d). As observed previously in the organo-silica sol-gel co-assembly approach, the formation of perovskite also has a positive effect on the colloidal self-assembly as it mitigates stresses otherwise induced by capillary interactions between the particles.27,32 Crack-formation is often observed in colloidal crystals without matrix - we do not observe cracks over cm distances, as also reported by others when co-assembling the particles with matrix (see Figure 1be).27,32 Furthermore, the mechanical stability of the colloidal assemblies is improved in comparison to colloidal crystals without a matrix. To prove that the co-assembly process indeed leads to the formation of the desired perovskite material in the interstitial space we perform wide angle Xray diffractometry (WAXS) on the hybrid inorganic-organic assemblies. WAXS gives insight to the crystallinity as well as the composition of the perovskite matrix. The scattering peaks of our co-assemblies can be indexed according to the cubic space group 𝑃𝑚3̅𝑚, as previously reported for pure Br,33 Cl,34 and also the mixed Br/Cl perovskites.7 The absence of PbX2 reflections indicates full conversion of the precursors to the desired perovskite. For x = 0.1 the (001)

Figure 2. Photoluminescence properties of CPP/perovskite co-assemblies and the pure components. a) Fluorescence spectra of films of the pure perovskite with x = 0.1 (blue line, per/0.1), the co-assembly of perovskite and CPPs (cyan line co/0.1), and pure CPPs (black line), all excited at λex = 355 nm. The CPP absorption is depicted as a grey line (abs. CPP). b and c) Photoluminescence maps of sub-monolayers of co-assemblies with x = 0.1 perovskite: (b) shows the emission of the perovskite (donor) at λ em = 430 nm when excited with a 407 nm, and (c) shows a PL map of the same film at λem = 510 nm to show the location and emission of the CPPs (acceptor). d) Photoluminescence lifetime decay curves of the donor emission (left) detected at λ em = 430 nm (excited at λex = 375 nm) in the pure x = 0.1 perovskite (blue, per/0.1) and the co-assembly with CPPs (cyan, col/0.1). The decays are fitted with a bimolecular model. The emission of the acceptor (right) is detected at λem = 510 nm (excited at λex = 375 nm) in the pure CPPs (black) and the co-assembly (cyan, co/0.1). The decay of the pure CPPs is fitted with a monoexponential function. The decay in the co-assembly is fitted with a biexponential function. (e-h) present the same experiments as in (a-d); however, with x = 0.4 perovskite composition. The donor emission is detected at λem = 460 nm, the maximum emission of the pure perovskite, and also the same wavelength as for the PL map in (f). The acceptor emission is detected at the same wavelength as for x = 0.1 case.



reflection is a doublet peak and two lattice parameters can be derived using 𝜆⁄2𝑎 = sin(𝜃 ⁄2) which are a1 = 5.75 Å and a2 = 5.72 Å (see Figure 1f). In case of x = 0.4 the (001) reflection is a single peak and only one lattice parameter is determined with a = 5.75 Å. For the x = 0.1 sample (with more Cl) we consistently observe shifting of the reflections to higher angles due to the smaller size of the chloride-ions compared to bromide. The observed doublet reflections for the (001) and (002) crystal planes in x = 0.1 is due to compositional heterogeneities that have previously been observed by others for mixed halide perovskites by WAXS experiments.5–7 This indicates that in x = 0.1 there is formation of more Br-rich domains (similar to x = 0.4) and Brpoor domains. The x = 0.4 composition with its sharp singlet peaks produces a homogeneous distribution of Cl and Br in the perovskite film (see Figure 1f). To determine the optical properties of the produced perovskites we spectrally investigate films with and without added CPPs. As indicated above, the respective perovskite compositions are chosen to overlap more or less well with the absorption of the CPPs. The pure colloids show absorption with a maximum at λabs = 415 nm and a broad fluorescence spectrum of λem = 460 – 630 nm with three distinct vibronic transitions (at 470 nm, 510 nm and 550 nm), typical for conjugated polymers (see Figure 2a and e). The emission of the x = 0.1 perovskite overlaps well with the CPP absorption, fulfilling the prerequisite for non-radiative energy transfer,35 while the x = 0.4 perovskite emission overlaps also with the fluorescence of the CPPs, which we use as a reference material to check our hypothesis of energy transfer from the perovskite to the CPPs (see Figure 2a and e). To study the energy transfer processes, we fabricate co-assemblies of sub-monolayer (incomplete, defective monolayer) CPPs in the perovskite matrix. We apply sub-monolayers to avoid any diffractive effects from hexagonally ordered particle assemblies, which could influence the optical performance of the composite material. When we excite the CPP/perovskite sub-monolayer co-assembly with x = 0.1 at λex = 355 nm, we observe emission resembling that of the CPPs. However, the spectrum appears bathochromically shifted and the 0-0 transition is strongly weakened (see Figure 2a). There is no contribution by the perovskite to the emission spectrum, which we interpret as efficient excitation transfer from the excited perovskite donor to the CPP acceptor (see Figure 2a). By contrast, when we excite the perovskite with x = 0.4 we observe a fluorescence spectrum that reproduces all transitions of the CPP spectrum, while the co-assembly

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spectrum appears hypsochromically shifted, indicating less efficient energy transfer (cf. Figure 2e). To further test the energy transfer efficiency from perovskite to CPP in case of the two bromide to chloride ratios (x = 0.1, x = 0.4), we record photoluminescence maps (with diffraction-limited resolution) of the CPP sub-monolayers inside the perovskite matrix. We excite the sub-monolayer co-assemblies at 407 nm and detect at wavelengths specific for the individual components. In case of the x = 0.1 perovskite we observe weak residual perovskite emission (the donor of the energy transfer process) when we detect at 430 nm and fluorescence of only the CPPs (acceptor) when we detect at 510 nm. By contrast, in case of the bromide rich x = 0.4 perovskite, we see perovskite (donor) emission at 460 nm and simultaneous emission from both the CPPs as well as the perovskite matrix when detecting at 510 nm. The simultaneous emission from the perovskite as well as the CPPs, indicates a less effective energy transfer from the perovskite to the CPPs (see Figure 2g). Both samples show similar morphology, which rules out emission artifacts from excitation diffusion. To confirm our hypothesis and reveal the energy transfer mechanism from the perovskite to the CPPs, we compare the photoluminescence lifetimes of the pure perovskites, the pure CNPs, and the co-assembled sub-monolayers. The samples are excited using a pulsed ps-laser at λex = 375 nm with a repetition rate of 80 MHz. The photoluminescence lifetimes of the samples are determined by time-correlated single-photon counting (TCSPC). The lifetime of the pure perovskite with x = 0.1 and its co-assembly with CPPs is detected at λem = 430 nm matching the maximum emission of the pure perovskite (donor emission). For the perovskite with x = 0.4 and the respective co-assembly the donor emission is recorded at 460 nm. These are the same wavelengths, at which we also recorded the photoluminescence maps. The resulting decay signal of the perovskite (donor) emission is fitted assuming a bimolecular decay mechanism using the following solution of the rate equation −𝐴 𝐼(𝑡) = (1). The parameters A, B in equation (1) con𝐵−𝑒 𝐶𝑡 tain the radiative and non-radiative rate constants (kr and knr,) for the decay of the excited state, while C = knr. Using τnr = 1/knr we can directly derive the lifetime of the nonradiative decay component. (See supplementary information for a detailed description of the model and fitting parameters.) In the co-assembly the non-radiative energy transfer forms an additional decay channel, which is expressed as Cco = knr + kET, where kET is the rate constant of the energy

Table 1. Summary of non-radiative rate constants and lifetimes calculated for the perovskites and co-assemblies using a bimolecular model knr [ns-1] MAPb(Br0.1Cl0.9)3 (per/0.1)

1.11

MAPb(Br0.1Cl0.9)3/CPP co-assembly (co/0.1)

1.70

MAPb(Br0.4Cl0.6)3 (per/0.4)

0.81

MAPb(Br0.4Cl0.6)3/CPP co-assembly (co/0.4)

2.07

kET [ns-1]

τ nr[ns]

τET [ns]

0.90 0.59

0.59

1.69

1.24 1.26


0.48

0.79

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Chemistry of Materials an average lifetime of τco/0.1 = 0.30 ns. We account this increase in lifetime by 0.11 ns to the energy transfer τETco/0.1 = τco/0.1 – τCPP = 0.11 ns. In case of x = 0.4 the co-assembly has an average lifetime of τco/0.4 = 0.23 ns, which is only a marginal increase of τETco/0.4 = 0.04 ns compared to τCPP.

transfer from the perovskite to the CPP. We calculate kET through 𝑘ET = 𝐶co − 𝐶per . Since the photoluminescence of the perovskite as a donor will be quenched during the non-radiative energy transfer in the co-assembly, we compare the non-radiative decay mechanisms of the excited states in the pure perovskites and the co-assembly. The pure perovskite with x = 0.1 has a non-radiative decay constant of knrper/0.1 = 1.11 ns-1 with a lifetime of τ nrper/0.1 = 0.90 ns. When we add CPP as acceptors in the co-assembled structure, we enable an additional non-radiative decay pathway with energy transfer rate of kETco/0.1 = 0.59 ns-1. We do not expect additional nonradiative losses in the perovskite matrix in the co-assembly compared to the pristine film. We therefore obtain an overall increased non-radiative decay rate knrco/0.1 = knrper/0.1 + kETco/0.1 = 1.70 ns-1 and a reduced non-radiative lifetime of the perovskite (donor) excited state in the co-assembly of τnrco/0.1 = 0.59 ns. The perovskite excited state of the co-assembly decays faster than in the pure perovskite. The increased decay rate indicates a more efficient transfer of the excited states from the perovskite into the CPPs (see Figure 2d).

The timescale of these energy transfers is indicative of a Förster resonant energy transfer (FRET).36 The efficiency of the FRET transfer in the x = 0.1 system is interesting if we want to produce a laser from the co-assembled CPPs, which are pumped by the surrounding perovskite. Our results suggest that the complete spectral overlap in case x = 0.1 leads to more efficient energy transfer to the excited states of the CPPs than for x = 0.4 with only partial overlap. However, we have shown above that in the co-assembly with x = 0.4 the donor lifetime quenching is more pronounced than in the chlorine-rich (x = 0.1) co-assembly, which points to more excitation transfer into the CPPs for the co-assembly with x = 0.4. One reason for this discrepancy could lie in the variation of crystal domain sizes in the perovskites, which entail perovskite emission regions lying closer to the CPPs in the x = 0.4 case than for the x = 0.1 case.5 Another reason could lie in the broad CPP absorption spectrum with different absorption depths at the applied wavelengths. The conjugated polymer particles possess a broad variety of possible acceptor states, of which those overlapping with the x = 0.4 perovskite emission might be populated and decaying faster than those overlapping with the x = 0.1 perovskite emission.

In the x = 0.4 case, we observe similarly increased decay rates when adding the CPP acceptor to the perovskite. The pure perovskite excited state has a non-radiative decay constant of knrper/0.4 = 0.81 ns-1, whereas in the co-assembly the non-radiative energy transfer rate constant is kETco/0.4 = 1.26 ns-1 accelerating the decay and leading to an overall reduced lifetime of the donor excited state in the co-assembly of τnrco/0.4 = 0.48 ns (see Figure 2h). It is surprising that the energy transfer in the x = 0.4 perovskite/CPP co-assembly (τETco/0.4= 0.79 ns) with the improper spectral overlap is faster than in the co-assembly with x = 0.1 (τETco/0.1 = 1.69 ns), which is characterized by an ideal spectral overlap of the perovskite emission and the CPP absorption spectra.

To test the quality of the co-assembled CPP colloidal crystal inside the perovskite matrix, we first excite the CPPs at 470 nm with increasing power using a pulsed laser (tpulse = 10 ns, frep = 20 Hz). Beyond a threshold, the intensity increases super-linearly and the broad photoluminescence spectrum gives way to a single sharp line (FWHM < 0.5 nm) indicative for single mode lasing. The threshold of the photonic crystal CPPs lasers are similar for both perovskite matrices at 16 mJ/cm2 for x = 0.1 and 23 mJ/cm2 for x = 0.4 (see Figure 3a-d). This similarity is not surprising as the quality of the CPPs is the same and the refractive index of the perovskite is similar for x = 0.1 and x= 0.4, resulting in comparable conditions for lasing when directly exciting the CPPs.

To fully understand the energy transfer and determine whether the effect is efficient enough to enable laser emission from the CPP after energy transfer from the perovskite, we study the photoluminescence of the CPPs after exciting the respective perovskite. The photoluminescence decay of the pure CPPs is fitted using a monoexponential function and the photoluminescence decay of the co-assemblies is fitted using a biexponential function: I(t) = ACPP exp(t/τCPP)+ Aper exp(t/τper). The biexponential model is chosen to account for the additional energy transfer pathway. The parameters ACPP and Aper denote the excitation densities in the CPP and the perovskite, respectively. From these fits, the photoluminescence lifetimes τ are calculated (see table 2). The pure CPPs have a photoluminescence lifetime of τCPP = 0.19 ns. The co-assembly with x = 0.1 has

When we change the excitation wavelength to 410 nm to excite the x = 0.4 perovskite, we see neither laser emission nor the desired threshold behavior (see Figure 3g and h). However, when we move to the x = 0.1 perovskite co-assembly with better FRET behavior, we observe a threshold at 13 mJ/cm2, at which we see narrow lines emerging in the photoluminescence spectrum (see Figure 3e and f). Above the threshold the intensity and the number of lines increases, which is indicative for random lasing. We also ob-

Table 2. Photoluminescence lifetimes of pure CPPs (monoexponential fit and their co-assemblies with donor perovskite (biexponential fits) detected at the CPP (acceptor) emission maximum of 510 nm

MAPb(Br0.1Cl0.9)3/CPP co-assembly (co/0.1) MAPb(Br0.4Cl0.6)3/CPP co-assembly (co/0.4) CPPs

ACPP

τ CPP [ns]

Aper

τ per [ns]

τET [ns]

6.43 x 1013

0.30

2.77 x 104

1.80

0.11

0.23

104

1.46

0.04

1.91 x

1016

5.06 x

1019

0.19


4.72 x


Figure 3. Laser emission from co-assemblies under optical excitation using a ns-pulsed laser at varying wavelengths and intensities. (a-d) Laser spectroscopy experiments for direct excitation of the CPP within the perovskite matrix. a) PL spectra of co-assembly with x = 0.1 (co/0.1) excited at λex = 470 nm with increasing pump intensity, b) emission versus pulse power. c) PL spectra of co-assembly with x = 0.4 (co/0.4) excited at λex = 470 nm with its corresponding emission intensity versus pump power plot (d). The solid lines extrapolate the threshold in their crossing point, while the dotted line is a guide to the eye for the data. The PL spectra in (a) and (c) exhibit a sharp cut-off at 500 nm as a low-pass filter with a 500 nm edge was applied to filter out the excitation laser signal. (e-h) Laser spectroscopy experiments for excitation of the perovskites followed by energy transfer to the CPP. e) PL spectra of a x = 0.1 (co/0.1) co-assembly excited at λex = 410 nm and corresponding emission versus pump power plot. (f). g) PL spectra of co-assembly with x = 0.4 (col/0.4) excited at λex = 410 nm and corresponding threshold plot in (h). The insets depict the type of excitation (into particles or perovskite) and emission from the CPPs.

serve that laser emission saturates quickly and does not increase to such high intensities as we see for the directly pumped CPPs. When we consider energy transfer to take place between the perovskite and the CPPs, then FRET will only transfer energy effectively over a distance known as the Förster radius, which for our system we determine to be R0 = 11.9 nm (with an average distance between donor and acceptor of R = 6.87 nm) using a model for a 2 point source (see Supplementary Information).36

and multiple photonic crystal domains as well as their disordered grain boundaries contribute to laser emission. Therefore, we observe random laser emission for the coassemblies pumped by energy transfer. However, the lower lasing threshold shows that the energy transfer allows very efficient injection from the perovskite into the CPPs.

Therefore, following an energy transfer from the perovskite to the CPPs, only a 7 - 12 nm thick shell of each CPP at the perovskite interface will contribute to lasing. This is in stark contrast to the case, where we pump directly into the CPP absorption and use the entire CPP with 1 µm in diameter as gain medium. In the case of the energy transfer from perovskite to the CPPs a much smaller fraction of the conjugated polymer gain material is contributing to fluorescence and laser emission. The reduced laser intensities after energy transfer and early saturation, compared to the directly pumped CPP devices, are manifestations of the lower fraction of conjugated polymer material contributing to gain. Likewise, the volume required to generate sufficient gain for laser emission will be larger when we utilize the energy transfer compared to when we pump the CPPs directly. This could explain why we observe single mode laser emission in the directly pumped CPPs versus random lasing in the CPPs pumped by energy transfer from the perovskite. While the colloidal crystal domain size in our co-assemblies is large enough to support laser emission in the directly pumped co-assemblies; the energy transfer systems require larger volumes to gather sufficient gain

We demonstrate a new hybrid perovskite/conjugated polymer composite material, which enables laser emission after energy transfer from the perovskite to a self-assembled CPP photonic crystal. The perovskite serves not only as an encapsulating matrix assisting ordering of the particles but also as a donor, facilitating laser emission of the CPPs after an energy transfer from the pumped perovskite to the organic semiconductor. This allows us to combine the superior electrical properties of perovskites with the excellent optical properties of conjugated polymers. We hope that the developed energy transfer effect and the exceptionally low lasing thresholds of our hybrid material architecture will contribute to the long sought-after goal of a solution processed electrically pumped solid-state laser.

CONCLUSION

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. The Supplementary Information contains experimentals for the synthesis of the materials, fabrication of films, electron mi-


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Chemistry of Materials croscopy, WAXS, laser spectroscopy and fluorescence imaging, and a description of the theoretical fitting models applied to the photoluminescence decay data.

Perovskites for Optoelectronic Applications. Nano Lett. 2015, 15, 6095–6101. (6)

Kumawat, N. K.; Dey, A.; Kumar, A.; Gopinathan, S. P.; Narasimhan, K. L.; Kabra, D. Band Gap Tuning of CH3NH3Pb(Br1-xClx)3 Hybrid Perovskite for Blue Electroluminescence. ACS Appl. Mater. Interfaces 2015, 7, 13119–13124.

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Comin, R.; Walters, G.; Thibau, E. S.; Voznyy, O.; Lu, Z.-H.; Sargent, E. H. Structural, Optical, and Electronic Studies of Wide-Bandgap Lead Halide Perovskites. J. Mater. Chem. C 2015, 3, 8839–8843.

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Sadhanala, A.; Deschler, F.; Thomas, T. H.; Dutton, S. E.; Goedel, K. C.; Hanusch, F. C.; Lai, M. L.; Steiner, U.; Bein, T.; Docampo, P.; et al. Preparation of Single-Phase Films of CH3NH3Pb(I1–xBrx)3 with Sharp Optical Band Edges. J. Phys. Chem. Lett. 2014, 5, 2501–2505.

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Pellet, N.; Teuscher, J.; Maier, J.; Grätzel, M. Transforming Hybrid Organic Inorganic Perovskites by Rapid Halide Exchange. Chem. Mater. 2015, 27, 2181–2188.

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Deschler, F.; Price, M.; Pathak, S.; Klintberg, L. E.; Jarausch, D. D.; Higler, R.; Hüttner, S.; Leijtens, T.; Stranks, S. D.; Snaith, H. J.; et al. High Photoluminescence Efficiency and Optically Pumped Lasing in Solution-Processed Mixed Halide Perovskite Semiconductors. J. Phys. Chem. Lett. 2014, 5, 1421–1426.

AUTHOR INFORMATION Corresponding Author * Email: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources A. Mikosch thanks the German Academic Exchange Service (DAAD) for providing funds for a research internship in Cambridge. The authors thank the Deutsche Forschungsgemeinschaft (DFG) for funding this research (grant no. KU 2738/32). This work was performed in part at the Center for Polymer Technology (CPT), which is supported by the EU and the federal state of North Rhine-Westphalia (grant no. EFRE 30 00 883 02).

ACKNOWLEDGMENTS The authors thank Timo Linzenmeier for support with the preparation of CPPs and Dr. Khosrow Rahimi for performing WAXS experiments. The authors declare no conflict of interests.

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Chemistry of Materials

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Laser Emission from Self-assembled Colloidal Crystals of Conjugated

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