Optical Properties of Heterojunction between Hybrid Halide Perovskite

Sep 26, 2016 - The photoluminescence from exciplex states at lower energy than the bandgap of CH3NH3PbI3 (MAPbI3) was observed in both MAPbI3/Spiro-OM...
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Optical Properties of Heterojunction between Hybrid Halide Perovskite and Charge Transport Materials: Exciplex Emission and Large Polaron Xiao Yang,† Yuchen Wang,† Heng Li,†,‡ and ChuanXiang Sheng*,† †

School of Electronic and Optical Engineering, and ‡MIIT Key Laboratory of Advanced Solid Laser, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, China ABSTRACT: The photoluminescence from exciplex states at lower energy than the bandgap of CH3NH3PbI3 (MAPbI3) was observed in both MAPbI3/Spiro-OMeTAD and MAPbI3/PCBM bilayer films. At the same time, using continuous wave photoinduced absorption (PIA) spectroscopy, we observed the PIA band at 0.14 eV, which was due to the positive or negative polarons in perovskite for MAPbI3/Spiro-OMeTAD and MAPbI3/PCBM film, respectively. The binding energies of both kinds of polarons are about ∼40 meV, with the radius of phonon cloud of about 5.9 nm. Our finding may supply direct spectroscopic evidence for the existence of exciplex states in heterojunction of hybrid halide perovskite and charge transport materials as well as consequent large polarons in perovskite.

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perovskite and CTLs and of large polaron in perovskite after charge separation, illuminating the photophysics properties of the interface between perovskite and CTLs. The hybrid halide perovskite films were prepared using twostep spin-coating procedures to ensure the high-quality of the film (see Experimental Methods). The scanning electron microscopy (SEM) image of a MAPbI3 film exhibits highly compact morphology with good coverage and flatness on the glass substrate, which is shown in Figure 1a. We measured steady-state photoluminescence (PL) spectra of three samples at room temperature shown in Figure 1b. Besides the slight blue-shift in PL of the MAPbI3/PCBM and MAPbI3/SpiroOMeTAD as compared to the PL in the MAPbI3 film, the spectra are almost identical after normalization. However, in the PL spectra of low temperature (LT, 77 K), the most noticeable result from Figure 1c is the spectral features of new fluorescence peaked at ∼850 nm for both MAPbI3/PCBM and MAPbI3/Spiro-OMeTAD films. Along with the PL1 band, which is due to donor−acceptor-pair (DAP) transitions in perovskite,17 a new PL band, PL2, is observed in bilayer films only. For comparison, the same PCBM and Spiro-OMeTAD solutions are spin-coated on glass substrate directly to fabricate counterparts, in which there is no detectable PL emission using the same setup at LT. Therefore, the perovskite films are concluded to play an active role in the emission process in bilayer samples. On the other hand, considering the dramatic difference in PL spectra between single perovskite film and bilayer samples, as well as that the PL2 band for bilayer films peaks differently for MAPbI3/PCBM and MAPbI3/Spiro-

fter six years of progress, the efficiency of solar cell based on solution-processed hybrid halide perovskite is now 22.1% certified.1−7 Moreover, the high photoluminescence quantum yield has also been taken advantage of for the development of light emitting diodes (LEDs)8,9 and lasers.10,11 Now the easily prepared perovskite is often taken as an analogy to GaAs, which is for high-performance commercial optoelectronic devices of solar cells, LEDs, and lasers.12 To further explore the full potential of the perovskite, the studies of electronic properties of the perovskite interfacing with other related materials will be crucial both for fully understanding working mechanism of optoelectronic devices and for real world applications.13 Charge separation was observed at the junction of perovskite and carrier transport materials using transient spectroscopy and microwave photoconductivity measurements.14−16 There is no doubt that the interface between perovskite and charge transport layer (CTL) will influence the optoelectronic properties. However, the complete nature of photophysics related to interface is still unrevealed. In this work, bilayer films with an electron extraction layer of [6,6]-phenyl-C61-butyricacidmethylester (PCBM) and hole extraction layer of (2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene (Spiro-OMeTAD) spinning coated on perovskite were fabricated. Light emission from exciplex states at energy lower than the bandgap of perovskite was detected at low temperature in both bilayer films, but not in the corresponding single layer perovskite film. We then used continuous wave (CW) photoinduced absorption (PIA) spectroscopy with the spectral range of 0.1−2 eV for studying the long-lived photoexcitations. We found the charge separation happens at the junction of perovskite and SpiroOMeTAD (PCBM), resulting in the large polarons with a binding energy of ∼40 meV. Our finding may supply direct spectroscopy evidence for the existence of exciplex between © XXXX American Chemical Society

Received: September 20, 2016 Revised: September 24, 2016

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DOI: 10.1021/acs.jpcc.6b09515 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) The SEM image of MAPbI3. (b,c) The steady-state PL spectra at (b) 300 K and (c) 77 K for a single layer of MAPbI3, MAPbI3/SpiroOMeTAD, and MAPbI3/PCBM films. (d−f) The steady-state PL spectra at various temperatures for (d) MAPbI3, (e) MAPbI3/PCBM, and (f) MAPbI3/Spiro-OMeTAD films. The pump wavelength and intensity are 447 nm and 100 mW/cm2, respectively.

OMeTAD, it is straightforward to conclude that the new PL band observed in Figure 1c is due to the synergistic interaction of perovskite and CTLs; in other words, the exciplex states involving both perovskite and CTLs are formed, resulting in the emission of the PL2 band.13 For completeness, we also present the PL spectra at various temperatures for a single layer of perovskite, MAPbI3/PCBM, and MAPbI3/Spiro-OMeTAD in Figure 1d−f, respectively. The PL2 bands for bilayer films are undetectable after films temperatures were over 120 K. On the other hand, all three samples presented luminescence properties, indicating an orthorhombic−tetragonal phase transition with T ranging from 120 to 150 K. Therefore, the capping CTLs did not change the properties of perovskite, which gives additional evidence that the PL2 band comes from the interaction of perovskite and CTLs. Besides the radiative recombination directly, exciplex states between interfaces also could result in charge separation to generate long-lived charge carriers. To illustrate the properties of photoexcitations in bilayer films further, we applied broadband CW photoinduced absorption spectroscopy shown in Figure 2. For a single layer of MAPbI3 (Figure 2a), a single photobleaching (PB) band peaked at 1.58 eV (785 nm), which is the same as the PL position shown in Figure 1c. We also included absorption spectra in Figure 2a for comparison, in which the exciton feature is shown at 1.67 eV (743 nm). Nevertheless, the PB band is below the absorption edge obviously; therefore, the PB band is attributed to the sub-band states, which are responsible for PL1 emission in Figure 1.18,19 However, with same pump intensity (∼100 mW/cm2), both bilayer films present four different features in PIA spectra. First, the values of ΔT/T are enhanced roughly 10 times, which suggest the much more long-lived photoexcitation existing. Second, a new PB band (PB2) at the higher energy side of PB1 emerges. The PB2 share the same energy position of bandedge. Therefore, we attribute the PB2 to the band filling effect at low

Figure 2. Broadband CW photoinduced absorption spectroscopy at 77 K for (a) MAPbI3, (b) MAPbI3/Spiro-OMeTAD, and (c) MAPbI3/ PCBM films.

temperature. The third and fourth spectral features are shown in mid-infrared, where a PB0 ranges from 0.2 to 0.5 eV (∼1610 to ∼4030 cm−1) and a PA0 band peaks at 0.14 eV (∼1130 cm−1). To describe the optical processes in Figure 1 and Figure 2, a schematic energy diagram of MAPbI3 and MAPbI3/SpiroOMeTAD is proposed in Figure 3. For a single layer of MAPbI3, the origin of PB1 and PL1 is ascribed to a naturally existing acceptor (A) and donor (D) within the bandgap (Figure 3a).17 For bilayer films, here we took the MAPbI3/ Spiro-OMeTAD as the example; the charge transfer between the perovskite and Spiro-OMeTAD forms an exciplex state resulting in the PL2 band (Figure 3b). At the same time, in perovskite film, charge transfer processes generated excess B

DOI: 10.1021/acs.jpcc.6b09515 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

the function of temperature and cannot be detected by the CW quasi steady pump probe technique at room temperature. In Figure 2b and c, the clear onset of polaron absorption at 0.12 eV is presented, resulting in the binding energy of ΔE ≈ 40 meV for both positive and negative polarons.23 Therefore, at room temperature, a large amount of (83% in thermal equilibrium) carriers will be polaron instead of free charges. This could be used to explain the relatively small carrier mobility measured in perovskite. The energy of the interacting longitudinal optical phonon modes in MAPbI3 was determined to be 11.5 meV recently,24 and the effective mass of polaron can

Figure 3. Schematic energy diagram of (a) MAPbI3 and (b) MAPbI3/ Spiro-OMeTAD. CT: charge transfer.

carriers, which results in a 10 times larger PIA signal in bilayer films; moreover, after fulfilling the sub-band states, the bandedge states will be filled, resulting in a PB2 state as well. However, the band structured PA0 cannot be simply ascribed to the free carriers, which follows the Drude model. In this model, the absorption scales inversely with the square of the light frequency, ∼N/[1 + (ωτs)2], where N is the density of free carriers, ω is the angular frequency of light, and the τs is the average time between subsequent collisions of an electron or hole. Another structured PA band could be due to excitons, but the exciton should have been recombined within the nanosecond scale;19 therefore, it cannot be detected by CW pump probe, which is only sensitive to the long-lived photoexcitations; also the peak of excitonic transition in ultrafast PIA spectra is at 0.8 eV20 instead of 0.14 eV shown here. The third possibility is the absorption from the donor (or acceptor) states to bandedge, between which the energy difference however cannot be larger than 0.1 eV because the energy position of PB1 and PB2 is 1.58 and 1.67 eV, respectively. This is on the contrary to PA0 peaked at 0.14 eV. Therefore, the other and maybe only possibility is so-called large polarons,21,22 which features a low frequency cutoff (3 times the binding energy of polaron) in the absorption spectrum.23 The methylammonium lead halide perovskite is known by a large difference between the static and the high frequency dielectric constants.12,21 Thus, the Coulomb interaction between excess electron (hole) and ionic lattice of MAPbI3 will lead to large polaron formation. The existence of polaronic effect in perovskite had been discussed extensively to explain the extraordinary optoelectronic properties in MAPbI3.21,22 Further, we check the pump intensity and sample temperature dependence of the PA0 band, and we found the peak position is not sensitive to either temperature or pump intensity (Figure 4a,b), as predicted by the theory of large polaron.23 At the same time, the magnitude of ΔT/T is sensitive to the temperature (Figure 4b). This suggests that the lifetime of polaron could be

(

polaron r.l. ≅ me,h 1+ be expressed as me,h

ΔE 6E LO

) = 1.58m

r.l. 25 e,h ,

where the index r.l. stands for “rigid lattice”. Also, the radius of the phonon cloud in the polaron is estimated by polaron re,h =

ℏ r.l. 2ωLOme,h

,25 where ωLO is the frequency of the LO

phonon. The effective mass of both electron and hole in MAPbI3 are estimated to be about ∼0.1m0 (m0 is the mass of an electron),21,22 and then the radius of the phonon cloud is estimated to be about 5.9 nm. Thus, the effective mass of large polaron is close to the free carriers in rigid lattice, and the radius of polaron is larger than lattice constant of MAPbI3,26 consistent with the definition of large polarons.25 Furthermore, we found that the PB0 band shares the same intensity dependence with polaron at LT (Figure 4a), but is less sensitive to the temperature than the polaron shown in Figure 4b. In FTIR, we observed background absorption in the range of 0.2−0.5 eV (1700−3700 cm−1) beneath the absorption peak around 3200 cm −1 (0.4 eV) associated with CH3 NH3 + molecular vibrations (Figure 4c). Therefore, the PB0 band is not due to the molecular vibration, but probably results from a partial filling of intragap states by carriers.27 Very recently, at the similar energy range of PB0 in TiO2/MAPbI3/SpiroOMeTAD films, Narra et al. observed submicrosecond photobleaching signals, which were ascribed to be the results of the hole transport to the Spiro-OMeTAD layer.28 Thus, the PB0 observed in this work could also supply evidence for charge transfer between perovskite and Spiro-OMeTAD (or PCBM). In summary, at energy lower than the gap of perovskite in both MAPbI3/Spiro-OMeTAD and MAPbI3/PCBM bilayer films, we have observed the photoluminescence from exciplex states, which is formed after the charge transfer between the perovskite and Spiro-OMeTAD (PCBM). At the same time, in same bilayer films, the CW PIA spectroscopy exhibits a PA band peaks at 0.14 eV, which was attributed to positive or

Figure 4. (a,b) The PA0 and PB0 bands of MAPbI3/Spiro-OMeTAD film with various (a) excitation intensities and (b) temperatures. (c) The FTIR spectra for Spiro-OMeTAD, PCBM, MAPbI3, MAPbI3/Spiro-OMeTAD, and MAPbI3/PCBM films. C

DOI: 10.1021/acs.jpcc.6b09515 J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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negative polarons in perovskite films for MAPbI3/SpiroOMeTAD and MAPbI3/PCBM film, respectively. The binding energies of both polarons are about ∼40 meV. Our finding may supply direct spectroscopic evidence for the existence of charge transfer states as well as consequent large polarons in perovskite, providing fundamental insights into its photophysics.

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EXPERIMENTAL METHODS Sample Preparation. All of the sample film preparation was completed under a N2 atmosphere in a glovebox with H2O and O2 levels of