Quantum Confinement Effects in Organic Lead Tribromide Perovskite

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Quantum Confinement Effects in Organic Lead Tribromide Perovskite Nanoparticles Prashant Kumar, Chinnadurai Muthu, Vijayakumar C Nair, and Kavassery Sureswaran Narayan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06545 • Publication Date (Web): 26 Jul 2016 Downloaded from http://pubs.acs.org on July 29, 2016

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The Journal of Physical Chemistry

Quantum Confinement Effects in Organic Lead Tribromide Perovskite Nanoparticles.

Prashant Kumar,1 Chinnadurai Muthu,2 Vijayakumar C. Nair,2 K. S. Narayan1,*

1. Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560064, India 2. Photosciences and Photonics Section, CSIR-National Institute for Interdisciplinary Science and Technology (CSIR-NIIST), Trivandrum 695 019, India; Academy of Scientific and Innovative Research (AcSIR), New Delhi 110 001, India.

Receipt date: 29/June/2016

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Abstract The optical and electronic properties of nano particles/nano crystals (NC) of methylammonium lead tribromide perovskite (MAPbBr3) have been studied in detail. We observe the effect of quantum confinement in particles of average diameter of ≈ 6 nm and smaller, in form of an increase in excitonic nature with decrease in particle size. The differences in the photo-physical properties in bulk and NC forms of MAPbBr3 are clearly observed in the temperature dependent measurements, and provide insight into the length scales prevalent in this system. We demonstrate devices consisting of active layers of NC in conjunction with low band gap polymer semiconductors which exhibit the dual functionality of light emitting diode in the forward bias and a photodetector in the reverse bias.


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I. Introduction Organic-lead halide perovskites have attracted considerable attention in recent years due to their great potential as efficient light harvesters for applications in low cost, solution processed solar cells and light emitting diodes (LED).1-6 Remarkable power conversion efficiencies of ≈ 19 – 21 % in solar cells7-9 and quantum efficiency ≈ 9 % in LEDs10-12 have been established. Hybrid perovskite demonstrate additional properties like band gap tunability,13-14 low temperature processability,15 high absorption coefficient (α(E)) throughout the visible region along with sharp band edge,16 and large carrier diffusion lengths.17 Interesting observations have been reported with decrease in average crystallite size, which include shift in absorption edge,18-19 increase in photoluminescence (PL),20 and formation of nano sheets and platelets.21 NCs demonstrate enhanced stability against moisture compared to the bulk polycrystalline MAPbBr3 films (bulk), primarily due to the presence of hydrophobic capping groups.22 As a result, the NCs will be beneficial for realizing stable perovskite emitters.23 On this note, it is worthwhile to investigate the nature of NCs and compare it to the bulk perovskites. MAPbBr3 was selected as the material of interest because of its high stability, and larger band gap which exhibits optoelectronic response in the spectral regime of the human vision system. Additionally, the tunable electronic levels introduced by quantum confinement as the particle size decreases can also change the characteristics of the interface. We demonstrate this aspect in a bilayer structure where an appropriate acceptor-polymer of suitable energy levels is paired with the NC. The fundamental common aspect of similar crystalline features and electronic structure of the bulk and NC systems is evident from X-ray and α(E) studies. Excitonic features in α(E) get well resolved at low temperature; the binding energy values estimated via conventional models suggests a higher binding energy for NC than that in the corresponding bulk. PL at low 3 ACS Paragon Plus Environment


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temperature also illustrates an increased excitonic character with larger quantum yield. The bilayer devices of NC/acceptor type structure exhibit characteristic wavelength dependent photocurrent (Iph(E)) and electroluminescence (EL). This combination demonstrates a device which can double up as a single pixel emitter and a photodetector with an extended range of detection. II. Experimental methods Materials. MAPbBr3 NCs synthesized using a solvent extraction route, as reported by Schmidt et al.24 were used in this study, the details of NC synthesis and characterization was covered elsewhere.22 The NC obtained via this route contains a blend of various size particles ranging from ~ 3 - 10 nm with a larger fraction of NCs ≈ 6 nm average diameter. High conductivity PEDOT:PSS (Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)) (clevios PH – 1000, specific conductivity ~ 800 S/cm) was procured from Heraeus. Activeink-N2200 (P(NDI2OD-T2)) (Mw ~ 84,000 g/mol) was procured from Polyera, and PCBM-C70 (PhenylC71-butyric acid methyl ester) was obtained from lumtec, Taiwan. Device Fabrication. Films for optical measurements were prepared by spin coating NC and bulk perovskite films on quartz substrates. The devices were fabricated on ITO coated glass substrates, the ITO substrates were cleaned by the standard procedure followed for optoelectronic devices. PEDOT:PSS was spin coated on ITO substrates, at 3000 rpm, for 60 s in air, followed by annealing at 150 0C for 30 min in a nitrogen filled glove box. Further processing was performed at room temperature inside a nitrogen filled glove box for maintaining a moisture and dust free environment. NC layer was spin coated at 1000 rpm, on PEDOT:PSS coated substrates, from a 20 mg/ml suspension in toluene; film thickness of the order of 60 – 80 nm was


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obtained. Bulk perovskite layer was coated from 20 wt% solution in Dimethylaformamide. Following the perovskite layer, an n-type layer of thickness ≈ 40 – 50 nm of N2200 (PCBMC70) was spin coated from 10 mg/ml (20 mg/ml) solution in chlorobenzene. N2200 being a high mobility, low band gap, semiconductor polymer allows a suitable pair with NC for low energy optical detection. Thick aluminum back contact ≈ 150 – 200 nm was evaporated at the rate of ≈ 0.5 – 1.0 Å/s in a thermal evaporator at ≈ 10-6 mbar pressure, the active area was controlled using a physical mask. Optical and Electrical characterization Absorption. All measurements were performed in cryostats, flushed with dry nitrogen and evacuated to ~ 10-2 mbar pressure. Results were verified by testing adequate number of samples and devices. The α(E) measurements at different temperature were carried out in liquid nitrogen cooled cryostat with quartz optical window, the substrate temperature was recorded using a calibrated Pt-100 RTD and multi meter. Zolix monochromator in conjunction with a tungsten lamp was used for monochromatic light source, a calibrated silicon (Si) detector was used as a reference, and transmission signals were recorded using SR830 lock-in amplifier. The α(E) at 300 K was verified using a calibrated PerkinElmer UV-Vis spectrometer in the wavelength range of (400-700 nm). Photoluminescence. The PL measurements were carried out in laser PL setup; the samples were placed in a cryostat. A dot laser, emission at 405 nm (output power ≈ 100 mW), was used as an excitation source, and the PL was collected at 90 degree, in reflection geometry, using a plano – convex lens and fiber coupled Si CCD based spectrometer (Hamamatsu high sensitivity mini spectrometer C10083CA). The excitation wavelength was filtered using a 450



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nm high pass interference filter. PL at low temperature’s were carried out in temperature controlled cryostat, (CTI cryogenics) cooled using a closed cycle helium pump. Substrate temperature was recorded using calibrated Pt-100 RTD and Keithley 6514 electrometer. Electroluminescence. The EL was measured in bilayer sandwich architecture, a Keithley 2400 source meter was used to apply bias. Electroluminescence was collected using a fibercoupled Si CCD based spectrometer. Low temperature EL was measured in a liquid nitrogen cooled cryostat; substrate temperature was measured using Pt-100 RTD and Keithley 6514 electrometer. Photocurrent. The Iph(E) spectra was measured in bilayer architecture identical to the one used for EL measurements. Zolix monochromator (Omni-λ500, resolution 0.05 nm at 435 nm) coupled with tungsten lamp was used as a monochromatic light source. Iph(E) was recorded using a lock-in amplifier (SR830) and the incident power was calibrated using a UV enhanced Si photodetector (DSI-200, Zolix). III. Results and Discussion Confinement effects on band gap. The recorded α(E) of the NC and bulk films were corrected for scattering effects using the refining technique introduced by S.J. Leach and H. A. Schera.25-26 The band gap estimate is extracted from the modeled α(E) close to the band edge using the modified Elliot’s theory of Wannier excitons as reported by Saba et al.27 This method provides a more accurate estimate for the band gap as compared to the value arrived by extrapolating the absorption edge response (α ≈ (E – Eg)1/2).1 The α(E) response of bulk and NC MAPbBr3 thin films, measured at 300 K, is shown in Figure 1(a) and 1(b) respectively. The model fit to the modified Elliot’s formula indicates a band gap (Eg) of ≈ 2.36 eV and ≈ 2.39 eV 6 ACS Paragon Plus Environment

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for bulk and NC respectively, which is in agreement with the reported values.28-29 Additional peaks are visible in the high energy regime of α(E) of NC, these features are not observed in bulk perovskite films, suggesting the possibility of a small fraction of lower size particles. The shift observed in Eg (~ 0.03 eV) signifies the onset of confinement effects in NC (particle diameter ≈ 6 nm). The effective mass approximation is used for estimating the exciton Bohr radius ( ) from =

, where µ is the effective mass and ε is the dielectric constant. The value for ≈ 3

nm is close to the average particle radius, the µ ≈ 0.117me, and ε ≈ 7.5 values are used from literature as previously reported,30 thus the excitons will be weakly confined in the NCs (average diameter ≈ 6 nm). NC band gap as a function of the particle size can be written as, , = ℏ

, + − 1.786 , where the second term represents the confinement induced shift in Eg and the third term arises from the Coulomb interaction between the electron – hole pair in exciton.31 Using the above equation with the reported values of µ and ε gives a shift of ≈ 0.3 eV which is an order of magnitude larger than the experimentally observed shift in Eg. This simple picture does not include other complex interactions such as spin orbit coupling,32 and hence over estimates the Eg,NC. Additionally, peaks observed in the higher energy regime indicate stronger confinement effects, arising due to the finite presence of smaller particles. The α(E) spectra reveal sizable changes upon lowering the temperature, with the shoulder feature at the band-edge getting resolved into a well-defined peak. This local maximum at the band-edge spectra can be associated with the excitonic transition (at temperature ≈ 105 K) which is prominent both for bulk and NC films as shown in Figure 1(c) and 1(d) respectively. However, the excitonic feature is more sizable for the NC compared to bulk films. The methodology involved in calculating the exciton binding energy using the α(E) measurements 7 ACS Paragon Plus Environment


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has been well described in the literature of inorganic band semiconductors like GaAs.33-34 The model fit to modified Elliot’s theory for α(E), as shown Figure 1(a) for bulk and 1(b) for NC, has been used to estimate the exciton binding energy (Eb). Value obtained for Eb at 300 K is ≈ 50 meV for bulk, which is in the range of previously reported values. Increased confinement effect is visible in NC in terms of higher Eb value ≈ 75 meV as compared to that of bulk. The additional high energy peaks observed in NC α(E) spectra are red shifted (≈ 50 – 60 meV) at low temperature, analogous to the band edge maxima, this trend is consistent with the typically observed behavior for band edge excitonic transitions.


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Figure 1. Normalized α(E) plot measured at different temperature for perovskite bulk and NC films. a) Bulk α(E) measured at 300 K (red, square) and model fit (red solid line). b) NC α(E) measured at 300 K (blue, circle) and model fit (blue solid line). c) α(E) for bulk at 300 K (red, filled square) and 105 K (red, empty square). d) α(E) for NC at 300 K (blue, filled circle) and 105 K (blue, empty circle).



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PL measurement of bulk and NC films. The features present in the temperature dependent α(E) are also evident in the emission measurements. Figure 2(a) show the normalized PL emission spectra from NC and bulk films measured at 300 K. The NC films show a remarkably higher PL yield of over an order of magnitude compared to the bulk as shown in the inset of Figure 2(a). The NC PL shows an inhomogeneous blue shift compared to bulk PL; the shift observed in peak position (Epeak) (∆Epeak ≈ 0.037 eV) is smaller than the shift in PL edge (Eedge,NC), at FWHM, towards high energy (∆Eedge ≈ 0.076 eV). Incidentally, the ∆Epeak magnitude is similar to the ∆Eg (≈ 0.03 eV) calculated from α(E) measurement of the two systems. The PL emission of the NC exhibits a broader line width (FWHM) of ≈ 0.145 eV compared to that of bulk (≈ 0.102 eV). Increase in FWHM points towards the fact that, factors such as foster energy transfer process from smaller to the larger particles, re-absorption losses and direct emission of smaller NCs can play a significant role in determining the PL(E) of NC.


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Figure 2. a) PL(E) spectra for bulk (red, square) and NC (blue, circle) films. Inset shows the PL emission of NC at 300 K against bulk PL at 170 K (PL at higher temperature > 200 K is not well resolved for comparison). b) The bulk PL(E) at low temperature (≈ 100 K) (red, filled square) and at 175 K (orange, empty square). c) NC PL at low temperature (≈ 100 K) (blue, filled circle) and at 300 K (green, empty circles). Additionally, well resolved emission from smaller particles can be seen at low temperature (≈ 100 K) in NC emission spectra, they are not well resolved at 300 K. Dependence of PL on temperature is shown in Figure 2(b) and 2(c) for bulk and NC films respectively. The general trend of increase in peak intensity and decrease in FWHM is visible for both bulk and NC as the temperature is lowered. The emission intensity and its increase, with lowering of temperature, are sizably higher for NC. Observed large emission in the NC is consistent with the increase in the excitonic nature, which originates from the quantum confinement. The appearance of local maxima, in the low temperature PL(E) of NC, in the absorbing region, can be attributed to the emission contribution from particles of smaller average diameter (< 6 nm) and is consistent with the local maxima features in α(E). The other distinct trend, is the red shift of Epeak in PL(E) at low temperature for both NC and bulk, as shown in Figure 3(a). The Epeak,NC vs. temperature plot exhibits a linear response with a slope ≈ 0.3 meV/K, which is comparable to the slope reported for the red shift in Eg.28 In the case of bulk film, a discontinuity is observed in Epeak,bulk(T) at temperature ≈ 155 K, indicating the phase transition. Analogous shift in α(E,T) has not been observed. The slope of the linear response is ≈ 0.55 meV/K in orthorhombic phase and changes to a lower value of ≈ 0.1 meV/K, above the phase transition. Phase transition in the MAPbBr3 system from tetragonal (high temperature) to orthorhombic (low temperature) occurs at 155 K and is attributed to the decrease in the rotational 12 ACS Paragon Plus Environment

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degree of freedom of organic cation.35-36 It is interesting to note the presence of transition in the Epeak(T) characteristics in the bulk film, and its absence in quantum dot regime. Particle size dependence in many systems exhibiting structural transition has been observed, where the transition temperature decreases with size of nanoparticles. This structural stability of NC systems, in general, has been highlighted in perovskite 2D layers, CuS and CdSe quantum dots.37-39 Integrated intensity (Iint(T)) is fitted to the standard expression for thermal quenching as a &

(*)

function of 1/T (K-1) using, !" ($) = +,- '() ./0 /23 4 , where Iint(0) is intensity at 0 K and A is a constant.40 The Iint(T) trend, calculated by integrating the PL over entire emission range, along with the fit, is shown in Figure 3(b). Iint(T) for bulk has been fitted in the temperature range from ≈ 155 K – 250 K (no emission observed above 250 K ), while for NC it is fitted in the temperature range ≈ 193 K – 300 K. Anomalous behavior in Iint(T) of NC PL was observed below ≈ 193 K, where the Iint(T) decreases as the temperature is lowered up to ≈ 100 K, which is concurrent with previous report.41 Observed decrease in Iint(T) can be attributed to presence of non-radiative decay channels which gets activated below ≈ 195 K. The Eb, in NC is found to be ≈ 148 meV, which is slightly higher than that in the bulk (≈ 139 meV). The estimated value of Eb for both, NC and bulk, is much larger than that estimated from α(E) at ≈ 300 K. The observed difference in the magnitude of Eb, suggests that the thermal quenching model provides over estimated values, and may not be applicable for current system.



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Figure 3. a) The Epeak as a function of temperature for bulk perovskite (red, square) and NC films (blue, circle). b) Iint(T) for bulk (red, square) and NC (blue, circle), model fit are shown as continuous lines. c) FWHM as a function of temperature for bulk (red, square) and NC (blue, circle), model fit in the temperature range ≈ 155 K – 300 K is shown as continuous line. Thermal broadening of PL, and estimation of phonon interaction energy. The emission attributes as a function of temperature can be analyzed by fitting to the exciton model of Toyozawa.40 The FWHM shows a monotonic increase with temperature for both bulk and NC films as shown in Figure 3(c). Increase in FWHM of PL peak is associated with the phononassisted broadening. The FWHM as a function of 1/T (K-1) has been fitted for temperature > 155 K, using the independent boson model given as, Γ($) = Γ* + σT +

89: ℏ ?

;

, where Γ0 is the

@+

contribution from inhomogeneous broadening, σ and Γop are contributions from exciton-acoustic phonon interaction and exciton-optical phonon interaction respectively.42 It is observed that the quality of fit is not sensitive to variations in σT,41 which suggests that the acoustic phonons have negligible contribution towards the thermal broadening of line-width at high temperature (> 155 K). The Γ0 value for NC and bulk is around ≈ 10 meV, the energy of optical phonon associated with broadening (ħωop) is found to be around ≈ 6 meV for NC while for bulk it is ≈ 4 meV. The ħωop value for NC is closer to the reported value for longitudinal optical phonon energy;28 which can be related to the presence of pure perovskite phase in NC without the grain boundary defects. NC demonstrates a larger Γop (≈ 32 meV) compared to that of bulk (≈ 14 meV) suggesting a strong exciton – optical phonon interaction and hence an increased PL quenching with temperature.



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Spectral line shape analysis. Temperature dependence of spectral line shape indicates the extent of phonon-exciton coupling.43 The line shape of PL(E) for NC varies significantly with temperature; the line shape in the low temperature ( < 155 K) is a lorentzian (Figure S1).44 A lorentzian profile is associated with weak exciton-lattice interaction; the line shape broadening is related to the reciprocal of the excited state lifetime. The shape changes to pseudo-voigt (Figure S2)44 as the temperature increases above 155 K, pseudo-voigt profile is derived from the convolution of lorentzian and gaussian profiles. In the temperature range 230 K – 300 K the emission takes up a gaussian profile suggesting strong exciton – phonon interaction which leads to a characteristic reduction of PL upon heating (Figure S3).44 Above observation is consistent with the observed higher Γop value for NC. At low temperature (≈ 100 K), the PL(T) from the bulk film is asymmetric around the maxima with a long tail extending towards lower energy (figure S4(a)). Presence of asymmetric emission has been associated with below gap states. This nature persists throughout the orthorhombic phase, the peak shape changes to a lorentzian above the phase transition (temperature > 155 K) (figure S4(b)). This is concurrent with the lower Γop for bulk, inferred from independent boson model. The additional blue shifted peaks, visible in the low temperature PL(E), in the NC films originate from smaller size NCs of diameter < 6 nm. At high temperature the re-absorption and energy transfer processes dominate and mask these features of NCs, while at low temperature well resolved features appear due to the increased fluorescence. Multiple peak parameters in the temperature range of ≈ 90 K – 300 K have been derived using peak-fit software (sample fit shown in Figure S5).44 Variation in Epeak, Iint(T) and FWHM as a function of temperature is significantly different compared to that of ≈ 6 nm crystals. Epeak shows a consistent trend for all emission bands, with temperature independent regime (84 – 190 K) followed by a strong increase of ≈ 70 – 100 meV, for all the peaks observed


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in the high energy regime (Figure S6).44 The presence of internal conversion processes alters the trend in temperature dependence of intensity and FWHM of the emission in this spectral regime (Figure S7 and S8).44 Opto-electronic properties. The large emission intensity and size controlled energy gaps in NCs opens up the possibility of tailored devices. Both NC and bulk forms of these hybrid perovskite were studied as active layers for their emission and photo-detection properties. The ability to disperse NC in common organic solvents allows coating of very thin layer of perovskite over wide range of surfaces (Figure S9).44 While in the case of bulk, the surface energy plays a crucial role in film formation. In order to obtain efficient charge transport across the interface, the NC layers is further planarized. This is achieved to some extent in a bilayer structure, in conjunction with organic n-type acceptor layer of N2200 or PCBM-C70. These bilayer devices of NC/acceptor exhibit characteristic Iph(E) and EL. It should be noted that the emission from acceptor layer (N2200) is not present and was separately verified. Interestingly, the photocurrent contribution from the acceptor layers in the bilayer structure was significantly higher in NC devices compared to bulk devices. Electroluminescence from bulk and NC bilayer devices. A bimolecular recombination process is responsible for EL, where the formation of exciton depends on the carrier mobility and capture cross section, which makes it a useful tool to investigate the band-edge radiative recombination. Observation of EL in the same spectral range as PL suggests that the recombination centers are located in the NC layer. This suggests that the electron and hole transport are balanced in case of both NC and bulk films. EL spectra from bulk and NC film are shown in Figure 4(a). The Epeak in NC EL spectra shows a blue shift of ≈ 0.040 eV compared to that of bulk, which is comparable to the shift observed in NC PL (≈ 0.037 eV). The FWHM for 17 ACS Paragon Plus Environment


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NC EL is ≈ 0.114 eV which is much less than that of NC PL (≈ 0.145 eV) and close to that of bulk PL and EL (≈ 0.102 eV) at 300 K. The observed decrease in EL line width for NC can be related to the low energy, band edge transition from larger crystals (6 nm). EL profile for bulk and NC fits well to a voigt profile at 300 K, which is a gaussian in the case of NC PL; voigt profile suggests contribution from life time and thermal broadening in EL process. At lower temperature (≈100 K) the NC EL profile becomes asymmetric; the peak fits well to a lorentzian shape. The emission peak shifting towards lower energy by ≈ 0.047 eV, is less compared to the shift observed in the case of NC PL (≈ 0.058 eV) at temperature ≈ 100 K, which can be related to the Joules heating effect in EL process. Additional blue shifted peaks visible in low temperature PL are not visible in low temperature EL spectra, suggesting that the smaller crystals (< 6 nm) are not accessed and do not contribute to the EL process. The possibility of energy transfer processes or internal re-absorption losses can also mask the contribution from the smaller particles. A schematic of the bilayer device and a representative image of working NC LED are shown in Figure 4(b) and 4(c) respectively. The devices exhibit uniform EL throughout the active area (8 mm2) measured over multiple devices (~ 30 in number).


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Figure 4. a) Measured EL from bulk (red, square) and NC (blue, circle) bilayer devices. b) Schematic of a bilayer device and c) working NC device of ≈ 8 mm2 area with uniform EL. Photocurrent (Iph(E)) response from bulk and NC bilayer devices. The Iph(E) along with the α(E) in the band edge region provides an additional insight in the free carrier generation process. The signature of exciton can appear as a shift between the onset of α(E) and Iph(E), provided extrinsic processes such as interfaces or defects do not alter the carrier generation processes. Figure 5(a) and 5(b) show responsivity (R) for bulk and NC devices measured with N2200 and PCBM-C70 as n-type layers, respectively. The Iph(E) edge for NC is blue shifted from the bulk Iph(E) by a magnitude (≈ 0.01 eV) as shown in the inset of Figure 5(a). However, the Iph(E) exhibits only a negligible shift from the respective α(E) edge in both sets of devices. Further, the Iph(E) contribution from the organic n-type layer can be clearly seen in NC based devices, which suggests that the charge transfer between perovskite NC layer and the n-type layer is favored to a larger degree (which is not the case in bulk films). Bulk and NC exhibit R ≈ 10 µA/W at the band edge and increases to ≈ 40 µA/W at 3 eV, compared to the perovskite layer, 19 ACS Paragon Plus Environment


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the n-type layers of N2200 and PCBM-C70 show low R (< 10 µA/W) for NC devices. The observed selectivity for charge transfer in bulk perovskite can be related to the well-defined energy levels with negligible gap states. This condition can be relaxed in NC, due to the quantum size effects and the presence of significant surface states. Resulting in the energy level pinning, giving rise to a greater flexibility for charge transfer in bilayer structure. Furthermore, the intensity dependence of Iph(E) in NC has been studied at various excitation wavelengths (405 nm, 543 nm and 632 nm). The Iph(E) varies linearly at low intensities (~ 1011 – 1015 photons/s) (Figure S10).44 The observed property is advantageous for enhancing the optical window of detection beyond the band gap of perovskite. The additional option of acceptor characteristics featuring in the photodiode spectral response of the NC perovskite device can extend the optical window to NIR region.


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Figure 5. Responsivity (R) for, a) MAPbBr3 bulk/N2200 (red, filled square) and bulk/PCBMC70 (red, empty square), the bulk α(E) (continuous red line) is shown for comparison. Inset shows R of NC (blue, circle) plotted against R of bulk (red, square). b) NC/N2200 (blue, filled circle) and NC/PCBM-C70 (blue, empty circle) bilayer devices, NC α(E) (blue continuous line)



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is shown for comparison. Iph Contribution from particles of average diameter < 6 nm is also visible in NC/N2200 device response. IV. Summary In summary, we observe interesting features of quantum confinement effects in perovskite NC. The size dependence, higher Eb and improved stability are the key attributes of the NC systems. Estimates for Eb are arrived independently both from examining the temperature dependence of PL and α(E). Temperature dependence of emission shows a clear signature of structral phase transitions in bulk while such changes are not observed for NC system. Additionally, an increase in exciton-lattice coupling is observed with decrease in particle size. The two fold increase in exciton-optical phonon coupling in NC also reflects in modification of emission spectral profile which evolves from a lorentzian at low temperature to a gaussian at high temperature. The sizable PL observed in the NCs is translated in a device form where EL emission is readily observed, along with Iph(E) showing well-separated spectral features. The bilayer device architecture shows potential to be employed as an emitter and detector in visibleNIR range.

Acknowledgement All authors acknowledge Department of Science and Technology, Government of India for the funding.

Supporting information: Figure S1, S2 and S3, Peak shape fitting of temperature dependent NC PL; Figure S4, peak shape fitting for bulk emission; Figure S5 representative fit for PL emission from smaller NCs; Figure S6, S7 and S8, Ipeak, Iint(T) and FWHM of PL peak from


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smaller NCs; Figure S9, AFM topography and phase image of NC films coated on ITO substrates; Figure S10; Intensity dependent photocurrent from NC/N2200 bilayer device. Author Information * Corresponding author, Phone: +91 080 2208 2813. Email: [email protected]



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Table of content image: Dual functionality of single bilayer device.


Quantum Confinement Effects in Organic Lead Tribromide Perovskite

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