Mesoscopic Perovskite Light-Emitting Diodes - ACS Applied Materials

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Mesoscopic perovskite light emitting diodes Alessandro Lorenzo Palma, Lucio Cinà, Yan Busby, Andrea Marsella, Antonio Agresti, Sara Pescetelli, Jean-Jacques Pireaux, and Aldo Di Carlo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07750 • Publication Date (Web): 26 Sep 2016 Downloaded from http://pubs.acs.org on September 26, 2016

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ACS Applied Materials & Interfaces

Mesoscopic perovskite light emitting diodes Alessandro Lorenzo Palma,1 Lucio Cinà,1 Yan Busby,2 Andrea Marsella,1 Antonio Agresti,1 Sara Pescetelli,1 Jean-Jacques Pireaux,2 and Aldo Di Carlo1* 1

C.H.O.S.E. (Centre for Hybrid and Organic Solar Energy), Department of Electronic

Engineering, University of Rome “Tor Vergata”, Via del Politecnico 1, 00133 Rome (Italy). 2

Department of Physics, Research Center in Physics of Matter and Radiation (PMR), LISE

Laboratory, Université de Namur ASBL, Rue de Bruxelles 61, 5000 Namur (Belgium).

ABSTRACT: Solution-processed hybrid bromide Perovskite Light-Emitting-Diodes (PLEDs) represent an attractive alternative technology which would allow overcoming the well-known severe efficiency drop in the green spectrum related to conventional LEDs technologies. In this work, we report on the development and characterization of PLEDs fabricated using, for the first time, a mesostructured layout. Stability of PLEDs is a critical issue; remarkably, mesostructured PLEDs devices tested in ambient conditions and without encapsulation showed a lifetime wellabove what previously reported with a planar heterojunctions layout. Moreover, mesostructured PLEDs measured under full operative conditions showed a remarkably narrow emission spectrum, even lower than what typically obtained by nitride or phosphide based green LEDs. A dynamic analysis has shown fast rise and fall times, demonstrating the suitability of PLEDs for display applications. Combined electrical and advanced structural analyses (Raman, XPS depth profiling and ToF-SIMS 3D analysis) have been performed to elucidate the degradation

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mechanism, which result to be mainly related to the degradation of the Hole Transporting Material (HTM) and to the perovskite/ HTM interface.

KEYWORDS: Perovskite, Mesoscopic, Light Emitting Diode, Aging effects, Characterization, Raman, XPS, ToF-SIMS.

INTRODUCTION

Green Light Emitting Diodes (LEDs) currently in commerce are typically made of AlGaInP or InGaN heterostructures, thus including, rare and expensive materials.1-3 Moreover, both nitride and phosphide technologies suffer of the so-called green-gap, namely a severe drop in efficiency of green emitters compared to blue and red ones,4, 5 which has a profound impact on the future development of ultra-efficient white LEDs. Over the last few years, hybrid organo-metal halide perovskite materials have shown outstanding properties for optoelectronic applications6 especially for photovoltaic technologies. The first perovskite based solar cell (PSC) was reported on 2009, achieving a Power Conversion Efficiency (PCE) of 4%7 on liquid solar cells, triggering the work of Kim et al.8 that obtained the milestone PCE of 9% on a solid-state solar cell, until the certified record PCE of 22.1% obtained on small-area devices.9 Similarly, the development of large active area mesostructured PSC-based prototypes led to a PCE of 13% on a 10 cm2 active area mini module10 and a PCE of 4.3% on larger active area (100 cm2).11 Advantages of lead halide perovskites include high diffusion lengths of charge carriers (up to 1 µm),12 low temperature solution-process deposition, compatibility with a wide range of electron/hole transporting materials,13, 14 bandgap tuning by modification of the halide content,15 and strong


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photoluminescence.16 These electrical and photophysical properties17 allow using perovskites not only in PSCs but even in a wide range of new generation optoelectronic devices, like photodiodes,18 lasers,19 and light emitting diodes (LEDs).20 After the first attempt, in 1992, to induce electroluminescence (EL) in perovskite materials,21 the first perovskite 2D light emitting diode (PLED) was demonstrated in 1994, mimic of the Organic LEDs structure,22 with a luminescence registered only at liquid nitrogen temperature. In 1999, Mitzi et al.23 reported the first room-temperature PLED realized with (H2AEQT)PbCl4, where AEQT was a dye molecule designed and synthesized by the same group. Lately, Tan et al.24 reported two structures, an infrared PLED with structure ITO/TiO2/Al2O3/CH3NH3PbI3–xClx/F8/MoO3/Ag, and a green PLED with structure ITO/PEDOT:PSS/CH3NH3PbBr3/F8/Ca/Ag. For the IR PLED, a radiance of 13.2 W sr−1 m−2 at a current density of 363 mA cm−2 and an External Quantum Efficiency (EQE) of 0.76% were obtained, while the green PLED showed a luminance of 364 cd m-2 at a current density of 123 mA cm−2 and an EQE of 0.1%. Wang et al.25 achieved a luminance of 20000

cd

m-2

and

EQE

of

0.8%

on

a

0.0324

cm2

PLED

with

a

FTO/ZnO/PEI/CH3NH3PbBr3/TFB/MoOx/Au structure, while Bade et al.26 reported a mixed PEO-CH3NH3PbBr3 active layer without Hole Transporting Material (HTM) and Electron Transporting Material (ETM) with a luminance of 21014 cd m-2 and EQE of 1.1%. Finally, a much higher EQE of 8.53% was obtained by Cho et al.20 by preventing the exciton quenching induced by the metallic lead formation and confining excitons in MAPbBr3 nanograins. Unfortunately, stability analysis was not mentioned in the PLED studies cited above. The first report on PLED stability by Jaramillo-Quintero et al.27 refers to a PSC-like planar FTO/TiO2/CH3NH3PbI3–xClx/2,2',7,7'-Tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'spirobifluorene (Spiro-OMeTAD)/Au PLED heterojunction structure. A visible-infrared radiance


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of 7.1 W Sr-1 m-2 at a current density of 232 mA cm-2 and a maximum EQE of 0.48% were achieved, although a 50% emission drop was observed after a few seconds and the complete EL disappearance occurred after only 2 min probably due to an upward in non-radiative losses. A more recent result on stability of PLEDs has been reported by Shi et al.28 where a PEDOT:PSS/CH3NH3PbBr3/ZnO sandwiched structure has shown an endurance of less than 5 minutes in air at 15 °C, 30% humidity. Therefore, concerning LEDs application of perovskite, several structures were proposed to lower injection barriers29 and increase quality and surface coverage of spin coated perovskite layers24 obtaining good results; nevertheless, stability is clearly one of the major issue for PLEDs development and it deserves additional studies namely on the physical/chemical origin of the degradation.30 So far, PLEDs where realized exploiting planar structures; surprisingly, although most efficient small31 and large area10, 11 PSCs use a mesoporous TiO2 (mp-TiO2) as a scaffold layer, perovskite growth on mp-TiO2 was not yet considered. Moreover, advantages induced by the TiO2 scaffold have been clearly highlighted in many studies,32-34 in particular its beneficial effects on long term stability.35, 36 In this work, we report the first mesostructured PLED based on a mp-TiO2 scaffold. This turned out to be a winning choice in the 2D devices field, since a lifetime exceeding 50 min in ambient conditions and without encapsulation was achieved, overwhelming the previous record of less than 5 min. Moreover, mesostructured PLEDs showed a remarkably narrow emission bandwidth in operative conditions, even lower than what is typically obtained in commercial green LEDs.37 Furthermore, structure-to-properties were investigated by a complete set of device characterizations including photoluminescence, impedance spectroscopy, Raman spectroscopy, High Resolution X-ray Photoelectron Spectroscopy (H-R XPS) depth profile chemical analysis,


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and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) 3D molecular imaging,38 and dynamic analysis.

RESULTS AND DISCUSSION

The PLED device structure has the following layer sequence: Glass-FTO/50 nm compact TiO2 (c-TIO2)/50 nm mesoporous TiO2 (mp-TiO2)/100 nm CH3NH3PbBr3 perovskite/200 nm Poly(3hexylthiophene) (P3HT)/100 nm Au (Figure 1). The choice of the Br-based perovskite was motivated by its energy gap of about 2.3eV,15 corresponding to an emission wavelength peaked at 539 nm, i.e. an emission in the green visible light. The energy level diagram of the complete device structure is depicted in Figure 1 (b). In this layout, the c-TiO2 acts as an ETM, facilitating the electron injection from the FTO to the perovskite conduction band. Similarly, P3HT acts as HTM, favoring the hole injection from the gold electrode to the perovskite valence band. P3HT was chosen instead of Spiro-OMeTAD, the most used HTM in the perovskite solar cells field, to move toward low-cost devices, while maintaining good transport properties.39 Moreover, preliminary experiments with Spiro-OMeTAD did not show appreciable light emission in our PLED configuration. The reason for this phenomenon is still unclear and it deserves additional investigations. For PSCs we found40 that by increasing the P3HT Molecular Weight, the trapassisted interface recombination at the Perovskite/P3HT is reduced also with respect to the Perovskite/Spiro-OMeTAD. This could be very crucial for PLEDs emission and could explain this phenomenon. The perovskite layer was deposited by solvent engineering method41 ensuring


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a good film quality and high surface coverage (fabrication details are reported in the Methods section).

a

b

c

Figure 1. Working principle and structure of PLEDs. (a) Photo of a working mesoscopic PLED. (b) Energy levels diagram of the PLED showing electrons and holes injection mechanism. (c) PLED structure and cross-section SEM image of a realized device.

The absorbance spectrum of CH3NH3PbBr3 (Figure 2 (a)) shows an onset at 546 nm corresponding to a band gap of ∼2.27 eV. Photo-luminescence (PL) and electro-luminescence (EL) emission peaks are centered at 543 and 536 nm respectively. The blue-shift in the EL


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spectrum could be ascribed to free carrier emission as already demonstrated in CH3NH3PbI3 in CH3NH3PbI3-xClx42 and other perovskite materials.24, 43 This is one of the lowest FWHM ever reported for a PLED in operating conditions;25 moreover, by mapping the EL spectrum into the color space CIE (Commission Internationale de l’Eclairage), the extracted point results at x=0.25 and y=0.71 (Figure 2 (b)) which makes the emission spectrum very appealing for display technology application. In fact, found the “dominant wavelength”, i.e. the interception point of the line from the E point (x=1/3, y=1/3) to the extracted point with the edge of the CIE space, at 542 nm, we found an outstanding color purity44 of 92% that outperforms the one of commercial hi-performance LEDs (e.g. Luminus SBT-70 G, color purity 84%). Despite of a relatively high current density (980 mA/cm2), the PLEDs brightness is quite low (< 27 cd m-2). The turn-on of light emission is clearly observed at about 1.8 V, i.e. a value even lower than the photon emission energy. This phenomenon, already observed for colloidal quantum dots LEDs45, 46 and polymer LEDs,47 has been also reported for a planar PLED by Wang et al.25 and related to an efficient and barrier-free charge injections into the perovskite emitter. In our staggered structure, the ETM and the HTM present a conduction band lower and a valence band higher with respect to the perovskite ones, respectively. Therefore, a turn-on voltage lower to the perovskite energy gap suggests a pinning of the Fermi level at the perovskite/heavily-doped P3HT interface. A very low emission is indeed observed in the below-gap regime, while, as the applied voltage approaches to the band gap level (∼2.27 eV), the curve slope presents a strong increase, testifying a higher photons production (Figure 2 (c)). The PLED EQE and current efficiency as a function of the driving voltage (Figure 2 (d)) show maximum values of 1.7×10-5 % and 2.4×10-3 cd/A, respectively, demonstrating that a high charge density is required to promote the radiative recombination.


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b

Figure 2. Optical emission properties of PLEDs. (a) Absorbance (orange), EL spectrum at 2.5 V bias (green) and steady-state fluorescence emission (blue) of CH3NH3PBBr3 PLED deposited on Glass/FTO/m-TiO2 substrate, as obtained with a 350 nm excitation wavelength. (b) CIE coordinates for the EL spectrum under an applied voltage of 2.5 V. A picture of a working device is shown in the inset. (c) Current density and luminance vs applied voltage. (d) External quantum efficiency and current efficiency from 2 to 5.5V.

The reduced EQE can be attributed to a non-optimal uniformity of the mesoporous layer achieved by using a spin coating deposition technique. In fact, despite this technique is useful for a rapid prototyping, when spin-coating the mp-TiO2 on the substrate, mp-TiO2 “fingers” were noticed to form from the sample centre to the baseplate, creating a non-uniform light emission


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(Figure 4 (a), inset). In the future, screen printing or blade coating methods could be adopted to obtain a more uniform, thin and smooth mp-TiO2 layer, providing for an enhanced photoemission. In order to underline the applicability of mesostructured PLEDs in the display technology, we investigated the response speed by using a time-varying square wave bias voltage with a 50% duty cycle at 38 Hz. The rise and fall profiles of EL signal are shown Figure 3. Rise (fall) time is extracted from the 10-90% (90-10%) pulse amplitude increase (decrease). Interestingly, both rise and fall times are lower than 1 ms, which is faster respect to the typical reference value for large display applications of few milliseconds, and in the same category as the fastest OLED smartphone displays.

Figure 3. EL signals of the PLED modulated at 38 Hz with a square wave bias voltage (0 to 2.5 V). The PLED rise time (EL variation from 10% to 90%) is 670 μs, while the fall time (from 90% to 10%) is 750 μs. The inset shows the PLED EL signal on a larger time scale.


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The PLEDs stability was investigated under ambient conditions by testing the EL emission from a non-encapsulated device at a constant bias of 2.5 V, i.e sufficiently above the turn-on voltage (1.8 V). During the test, EL and current density signals were both monitored over 50 minutes. Figure 4 (a) shows that the EL signal at the peak value, decreases by 70% after 34 min and by 90% after 50 min. In comparison, the two previously reported planar heterojunction PLEDs in Ref 26 and 27 reported a life time of 2 and 5 minutes, respectively. The significant improve of the mesoscopic PLED stability with respect to planar PLEDs27 is possibly ascribed to (i) the good perovskite layer quality, as achieved by solvent engineering method41 and (ii) to the use of a scaffold-based structure, which was already shown to be beneficial in increasing the stability of PSCs.34 The inset of Figure 4 (a) shows the active area at the starting and ending of the stability test. Interestingly, after 50 min, the EL emission results to be confined to the device perimeter. The faster luminescence drop in the central device area was also recently observed in an OLED device;48 as it will be discussed later, the origin of this phenomenon is possibly related to a faster material degradation in the central device area. Current injection through the PSCs is mainly limited by the material that offers the lowest conductivity13, 49 and the highest resistance, which is typically the HTM layer. It is than reasonable to expect that in PLEDs, the higher current density (i.e. the higher Joule heating) would tend to cause more damages in the central area of the P3HT layer. This will be further discussed from the results of the ToF-SIMS 3D analysis. The current density and the corresponding luminescence characteristics versus applied voltage are depicted in Figure 4 (b) before and after the 50 min stress test at 2.5 V. We notice an increase of the reverse and subthreshold current in the stressed device with respect to the fresh one, while the opposite applies at large forward currents. The higher current density at low voltages, as observed in stressed devices, indicates an increase of non-radiative current losses.


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This can be associated to the degradation of the active layer, namely to the insurgence of defects (pin holes) in the perovskite film25 that can bring to localized short circuits. On the other side, the lowering in the onset current in stressed PLEDs can be related to the degradation of the transporting layers with a consequent increase of the layers’ resistivity. These currents losses can generate local Joule heating that brings to the device failure.28 To identify degradation mechanism, several analyses were performed on both fresh and stressed PLEDs. Impedance spectroscopy (IS) measurements have been extensively carried on photovoltaic devices for highlighting physical processes such as charge accumulation and transport/recombination resistances.50 To avoid an additional stress, IS data were recorded at 0 V bias. The IS analysis was focused on the aging effect over the recombination resistance (Rrec) which can be roughly evaluated from the semicircle diameter in the Nyquist plot.27 An evident reduction of Rrec occurs during the stress test from a Rrec(fresh)=1.6 kΩ to a Rrec(aged)=0.8 kΩ (Figure 4 (c)). Consequently, an increase of the recombination rate and, hence, of the recombination current, is expected to occur under low bias condition. The Bode plot of the real capacitance, extracted from the IS data, is shown in Figure 4 (d). This capacitance is mainly associated to the perovskite layer, where recombination processes take place. In particular, Kim H.S et al.51 demonstrated, for the first time, charge accumulation capability of the nanostructured perovskite layer by analyzing the overall capacitance of the device. This phenomenon is attributed to a density of states (DOS) that is larger in the perovskite than in either ETM or HTM.51,

52

As previously reported by Jaramillo-Quintero et al.27 for CH3NH3PbI3-xClx based

planar PLEDs, the capacitance does not change after the stress test. On this basis, we could claim that the perovskite layer of our PLEDs did not show major degradation effects, as detectable by IS analysis.


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Figure 4. PLEDs stability EL and IS analyses. (a) Evolution of PLEDs current density (dark) and EL (blue) signals at a constant voltage (2.5V) under ambient condition and without encapsulation. After 34 minutes the EL amplitude reduces to 30% of the initial value and to 10% after 50 min. Left and right insets show PLEDs before and after the ageing, respectively. (b) IS analysis showing the current density and EL versus the applied voltage on a fresh and an aged device at 0 V bias. The device turn-on occurs at 1.8 V. Dashed arrows are guidelines for eyes. (c) Nyquist impedance plot. (d) Bode plot of the real capacitance.


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Complementary investigations of the degradation process were conducted by Raman spectroscopy, a widely-used technique allowing for a non-destructive, in-situ monitoring of the structural changes in active layers. In particular, micro-Raman characterization was carried out on a complete device by focusing the laser beam onto the perovskite layer. Since the PL emission from CH3NH3PbBr3 is centered at 543 nm (Figure 2 (a)), a 785 nm laser excitation wavelength was chosen in order to avoid the fluorescence background. Raman bands from MAPbBr3 have been accurately simulated by non-periodic DFT calculations by B.W. Park and co-workers.53 The theoretical approach resulted in an assignment of fundamental vibrations for an isolated perovskite cluster defined as an inorganic octahedron unit, PbBr6, combined with two MA dipole cancelling cations. The Raman bands identification in halide-substituted perovskites is still under debate, however, a very good agreement was found between the theoretical data on MAPbBr3 and the spectrum obtained on a fresh perovskite active layer (Figure 5 (a)). Characteristics vibrational modes of perovskite crystals are found at low frequencies (< 250 cm1

). Vibrational frequencies below 140 cm-1 have been ascribed to the inorganic cage vibrations;

in particular, the band at 78 cm-1 is assigned to a triple degenerated asymmetric X-Y, X-Z, and Y-Z vibrations, the band at 100 cm-1 is the asymmetric “breathing” mode, while the symmetric one is at 130 cm-1. On the other hand, Stokes peaks from organic cations are related to the rotational mode at 144 cm-1 and to the vibrational wagging mode at 168 cm-1. Finally, the prominent band at about 180 cm-1 corresponds to the symmetric stretching mode between adjacent methylammonium cations (MA-MA). The complete perovskite crystal formation is testified by the absence, see Figure 5 (a), of the characteristic sharp Raman bands of PbBr2 at 84, 107, 120 cm-1 reported in Figure 5 (c). The Raman spectrum from the degraded device area (after 50 min at 2.5 V) is shown in Figure 5 (b). Raman analysis on the degraded device allows


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evidencing only a slight decrease in the MA-MA vibrational bands intensity, testifying a modified interaction between adjacent octahedron units in perovskite film. This is likely to be due to the prolonged heating of the active layer under working conditions. However, the absence of typical Raman bands from PbBr2 excludes a degradation of the perovskite layer involving a back-conversion process from CH3NH3PbBr3 to PbBr2 species.

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a) Fresh LED b) Aged LED c) PbBr2

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Intensity (a.u.)

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180

210

240

-1

Raman shift (cm )

Figure 5. Raman spectroscopy analysis. Comparison between Raman spectra from a fresh (black), aged (red) CH3NH3PbBr3 PLED and from a PbBr2 layer (dotted) deposited on a glass/FTO substrate. PLEDs spectra were acquired by focusing a 785 nm laser beam onto the active layer; the intensities were normalized to the maximum value. The PbBr2 spectrum intensity was adapted to optimize the graph readability.

The PLED degradation mechanism was further investigated by H-R XPS depth profile chemical analysis and ToF-SIMS 3D molecular imaging on both fresh and aged devices (Figure 6). The obtained compositional profiles were very similar in fresh and degraded devices (Figure S1), apart from a slightly higher Au diffusion possibly related to field assisted metal


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percolation;54 a more accurate chemical analysis revealed ageing effects mainly occurring at the P3HT/perovskite interface. Namely, aged devices resulted in (i) a higher diffusion of Br inside the HTM (as testified by the Br/Pb at% ratio increase from 1 to over 3), (ii) a more oxidized lead (testified by the PbO/Pb at% ratio increase) and (iii) the appearance of a non-negligible component in the Br3d spectrum ascribed to Br bounded to an organic compound (see the Br(2) component in Figure 6 and the H-R Br3d spectrum in Figure S2). These results were confirmed by the ToF-SIMS molecular profile analysis, as testified by the broader profiles of PbO- and Br2and CBr- fragments observed in aged devices (Figure 6 (d)). Interestingly, a broader intensity profile was also observed for C5H4S- fragment (from P3HT). Since a broader interface may possibly be ascribed to two effects, namely diffusion from the underlying layer or the presence of defects (holes) in the overlying layer, the 3D distribution of intense fragment ions was accurately monitored in fresh and aged devices. The depth-integrated 2D (XY) color maps of C5H4S(P3HT) and PbBr2- (perovskite) in an aged device is shown Figure 6 (e) and (f). Dense localized defects can be clearly identified (blue-regions) in the P3HT layer, while fewer defects are observed in the perovskite layer. Defects correspond to holes which propagate through the entire layer, as shown by the YZ cross-section reconstructions (Figure S3). These defects were absent in fresh devices and more rarely observed on border regions in aged devices. According to the previous results, PLEDs degradation results to be mainly driven by local Joule effects induced by non-radiative currents. Local heating degrades the HTM layer and occasionally create holes that propagates through the perovskite.


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Figure 6. HR-XPS ((a), (b)) and ToF-SIMS(c-f) analyses on fresh and aged PLEDs. In degraded device are characterized by higher Pb2+/Pb0 at% and Br/Pb ratios and a more intense Br component (Br-2) ascribed to Br bound to an organic compound. Pb oxidation (PbO-) and Br (Br2-) diffusion are confirmed by the ToF-SIMS 3D analysis. A more diffused C5H4S- profile (from P3HT) is observed in the ToF-SIMS profile (d). ToF-SIMS XY maps show the z-


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integrated intensity of PbBr3- fragment from the perovskite (e) and C4H5S- fragment from P3HT (f) from an aged PLED. Light-blue areas correspond to degradations extending through the entire layer depth (see also Figure S3).

It has been recently reported55 that humidity acts with perovskite in a hydrolysis reaction, back converting it to its precursors. Moreover, Matteocci et al.38 demonstrated that halogen diffusion from the perovskite to the HTM occurs, under certain conditions, even in fresh devices. These facts suggest that the Br diffusion in the HTM could be correlated to a back-conversion of the perovskite. Nevertheless, our Raman analysis tends to exclude this process in aged PLEDs. Therefore, we can conclude that the presence of the Br in the HTM has to be related to Joule heating induced damages observed in the aged device. This is also supported by the recent observation that temperature stress increases halogens diffusion into the HTM.36

CONCLUSION

This work describes the fabrication and characterization of the first mesoscopic TiO2 based PLED by using solvent engineering method for the perovskite deposition. The emission from PLED under full operative conditions resulted in an excellent color purity, characterized by a remarkably narrow spectral width (FWHM of 20.8 nm). Moreover, the dynamic analysis showed fast rise (670 μs) and fall (750 μs) times, making this structure suitable for display applications. While the material stability is probably the main issue limiting this technology, the stability of the mesoscopic PLEDs under ambient conditions and without encapsulation was found to be more than 10 times higher with respect to PLEDs based on planar heterojunctions. Degradation mechanisms were accurately investigated by combining multiple advanced


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techniques before and after the ageing procedure. A clear increase of the recombination channels resulted in stressed devices, not directly related to a major degradation of the perovskite layer (IS analysis). Back-conversion of the perovskite into its precursors in aged PLEDs was excluded but some ageing effects occur in the perovskite layer (Raman analysis). A detailed investigation based on XPS depth profiling and ToF-SIMS 3D analysis finally revealed, for the aged PLED, degradation at the interface between perovskite and the P3HT HTM with a clear diffusion of Br into the HTM. Moreover, dense localized defects in the P3HT layer corresponding to holes propagating through the entire layer were identified in ToF-SIMS XY maps. Thus, failure mechanisms are mainly related to the formation of localized non-radiative current fluxes which degrade, by Joule heating, mostly the resistive HTM. A higher degradation occurs in the central device area, i.e. where the current density is maximum, while border regions are less affected by this phenomenon. We believe this work can guide the fabrication of stable solution-processed mesoscopic PLED; the implementation of the mesoporous scaffold deposition would allow for a more uniform morphology which could both enhance the EQE and the long term stability by the lowering of non-radiative current paths that were shown to bring to the material degradation, in fact extended device lifetime results to rely mostly on the improvement of the layers uniformity at the HTM/perovskite interface together with the optimization of transport properties of the P3HT. In parallel, device lifetime could benefit from efficient encapsulation, avoiding external agents, e.g. moisture or oxygen, to degrade the materials forming the PLED.


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Methods

PLEDs fabrication: The PLED was fabricated on a Glass/FTO substrate (TEC 7, Pilkington). FTO/glass substrates were etched by a raster scanning laser (Nd:YVO4, λ=1064 nm, average output power P=10W)56 to create the desired electrode pattern (four devices for each 2,5 cm x 2,5 cm substrate). Substrates were successively cleaned by sequential ultrasonic baths in acetone and ethanol. A silver mask was deposited by screen printing. c-TiO2 solution was prepared by adding acetylacetone (4 ml), titanium diisopropoxide (6 ml) and ethanol (90 ml). c-TiO2 was deposited by spray pyrolysis at 450°C. The silver mask was, then, removed by dipping the device into a 0.1M HCl solution and letting it under constant pressured airflow. mp-TiO2 was then deposited by spin coating at 4000 rpm for 60 s and successive sintering at 525°C. CH3NH3PbBr3 40%wt solution was prepared by mixing CH3NH3Br and PbBr2 in 1:1 molar ratio in DMSO and by subjecting the solution to a continuous stir overnight at 80°C. CH3NH3PbBr3 solution was, then, spin coated by means of a two steps deposition (1000 rpm for 10 sec followed by 5000 rpm for 30 sec). 1 ml of toluene was dipped over the device during the last 10 sec of the last step. P3HT solution in chlorobenzene (Merck 15 mg/mL, MW=94,100 g/mol) was doped with tert-butylpyridine (TBP, 26 mM) and Lithium Bis(Trifluoromethanesulfonyl)Imide (LiTFSI, 55 mM). The solution was stirred overnight at 60°C and deposited by means of spin coating in nitrogen atmosphere at 2000 rpm for 40 sec. Au was deposited by vacuum thermal evaporation.

IV, EL and stability characterizations: IV-EL characteristics and stability measurements were carried out with a programmable modular platform (ARKEO, Cicci Research) composed by a source meter unit with 10 pA of current sensitivity, a fiber coupled CCD spectrometer and a


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calibrated silicon photodiode. Emission peaks and the related FWHM values were extracted by employing a nonlinear least-squares fitting of the emission spectra with Gaussian line shapes. Luminance was calculated assuming Lambertian model for emission profile.

Absorbance measurements: Absorbance spectrum was measured by Shimadzu UV-2550 Spectrometer.

Impedance spectroscopy: Impedance spectroscopy (IS) measurements were performed in dark conditions at room temperature using an Autolab 302N Modular Potentiostat from Metrohm in the two-electrode configuration with a bias voltage of 0 V. The sinewave ac perturbation used was 10 mV of amplitude with frequencies from 1 MHz to 1 Hz.

Raman and PL characterizations: Raman measurements were performed using a JobinYvon-Horiba micro-Raman system (LabRAM ARAMIS) equipped with an air cooled intra cavity regulated laser diode (785 nm) as excitation source. The Horiba micro-spectrometer is coupled with a confocal microscope that allows the spatial resolution of the sample through detector pinhole aperture. The cut-off from the filter in the spectrometer is less than 80 cm-1. The laser light reached the sample surface at normal incidence by means of ultra-long working distance (50X) objective with 10.5 mm focal distance. The scattered radiation was collected in a backscattering geometry. Spectral de-convolution was carried out by nonlinear least-squares fitting of the Raman peaks to a mixture of Lorentzian and Gaussian line shapes, providing the peak position, width, height, and integrated intensity of each Raman band. The spectrometer is equipped with a diffraction grating of 1800 lines/mm coupled with a CCD camera. Steady-state


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fluorescence experiments were carried out on a Fluoromax spectrofluorimeter (Jobin-Yvon) operating in single-photon counting mode.

XPS and Tof-SIMS characterizations: XPS depth profiling (K-Alfa spectrometer, by Thermo Scientific) was performed with a monochromatic Al kα excitation by alternating sputtering with low-energy (500eV) Ar+ beam and the acquisition of HR spectral from all elemental species (C1s, S2p, Pb4f, N1s, O1s, Ti2p, Au4f and Br3d), in snapshot mode. Data analysis was performed with Avantage © software. ToF-SIMS depth profiles were acquired on a IonTof IV spectrometer, in non-interlaced mode with low energy (500 eV) Cs+ as a sputtering beam and Bi3+ for the analysis beam. Low-energy Cs+ was used for preserving molecular information while achieving convenient sputtering rates on organic and inorganic materials.57 Multiple analyses were acquired typically on 120×120µm area. Characteristic molecular ions were selected based on the XPS results, and by choosing well-isolated (to avoid mass conflicts) and high sputtering yield negative fragments (Au3- for gold, C5H4S- for P3HT, PbBr3- or Br2- for the perovskite, TiO3- for TiO2, and SnO2- for the FTO). For helping the comparison, the depth profiles in the fresh ad aged devices, the profiles intensities were normalized to get the same PbBr2- signal in the perovskite layer.

ASSOCIATED CONTENT

Supporting Information


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Supporting information available: XPS compositional depth profiles, XPS spectrum of Br 3d acquired in the aged PLED device at 550 sputtering time corresponding to the P3HT/perovskite interface, and ToF-SIMS cross-section reconstructions along YZ directions.

AUTHOR INFORMATION

Corresponding Author

* e-mail: [email protected]

Notes

The authors declare no competing financial interest.

ABBREVIATIONS

PLED, Perovskite Light-Emitting-Diode; HTM, Hole Transporting Material; psc, perovskite based solar cell; pce, Power Conversion Efficiency; LED, Light-Emitting-Diode; EL, electroluminescence; EQE, External Quantum Efficiency; ETM, Electron Transporting Material; Spiro-OMeTAD, 2,2',7,7'-Tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene; mpTiO2, mesoporous TiO2; H-R XPS, High Resolution X-ray Photoelectron Spectroscopy; ToFSIMS, Time-of-Flight Secondary Ion Mass Spectrometry; c-TIO2, compact TiO2; P3HT, Poly(3hexylthiophene; PL, Photoluminescence; FWHM, Full Width Half Maximum; CIE, Commission


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Internationale de l’Eclairage; IS, Impedance spectroscopy; Rrec, recombination resistance; MAMA, adjacent methylammonium cations; TBP, tert-butylpyridine; Li-TFSI, Lithium Bis(Trifluoromethanesulfonyl)Imide.

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