Terahertz Spectroscopic Probe of Hot Electron and Hole Transfer from

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Terahertz Spectroscopic Probe of Hot Electron and Hole Transfer from Colloidal CsPbBr Perovskite Nanocrystals 3

Sohini Sarkar, Vikash Kumar Ravi, Sneha Banerjee, Gurivi Reddy Yettapu, Ganesh B Markad, Angshuman Nag, and Pankaj Mandal Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b02003 • Publication Date (Web): 23 Aug 2017 Downloaded from http://pubs.acs.org on August 23, 2017

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Terahertz Spectroscopic Probe of Hot Electron and Hole Transfer from Colloidal CsPbBr3 Perovskite Nanocrystals Sohini Sarkar, Vikash Kumar Ravi, Sneha Banerjee, Gurivi Reddy Yettapu, Ganesh B. Markad, Angshuman Nag, and Pankaj Mandal* Department of Chemistry, Indian Institute of Science Education and Research, Pune 411008, Maharashtra, India. *Email: [email protected]; Phone: +91-20-25908030

ABSTRACT: Colloidal all inorganic CsPbX3 (X = Cl, Br, I) nanocrystals (NCs) have emerged to be an excellent material for applications in light emission, photovoltaics, and photocatalysis. Efficient interfacial transfer of photogenerated electrons and holes are essential for a good photovoltaic and photocatalytic material. Using time-resolved terahertz (THz) spectroscopy (TRTS) we have measured the kinetics of photogenerated electron and hole transfer processes in CsPbBr3 NCs in presence of benzoquinone (BQ) and phenothiazine (PTZ) molecules as electron and hole acceptors, respectively. Efficient hot electron/hole transfer with a sub-300 fs timescale is the major channel of carrier transfer, thus overcomes the problem related to Auger recombination. A secondary transfer of thermalized carriers also takes place with time scales of 20-50 ps for electrons and 137-166 ps for holes. This work suggests that suitable interfaces of

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CsPbX3 NCs with electron and hole transport layers would harvest hot carriers, increasing the photovoltaic and photocatalytic efficiencies.

KEYWORDS. CsPbBr3 nanocrystals, Terahertz spectroscopy, electron and hole transfer, hot carrier transfer.

Colloidal all inorganic CsPbX3 (X = Cl, Br, I) nanocrystals (NCs) have emerged as a potential material for applications in electro-optic devices.1-3 These NCs show incredibly high photoluminescence (PL) quantum yield (QY)1, narrow emission bandwidth,1 low threshold lasing

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, reduced fluorescence blinking7, high carrier mobility,8 and long diffusion length.8

Defect tolerant nature of perovskite NCs is regarded to be responsible for such properties.9,10 These NCs have already been used in light emitting devices11-13, photovoltaics14,15 and photocatalysis.16 Efficient ultrafast electron/hole transfer at the device interface is one of the major requirements of an efficient photovoltaic and photocatalytic system. Solar energy conversion efficiency can be improved beyond Shockley-Queisser limit if the photo-induced hot carriers can be transferred across such interfaces prior to their relaxation to the band edge.17,18 Here, we establish such hot carrier transfer from CsPbBr3 NCs to a molecular carrier acceptor using time resolved terahertz (THz) spectroscopy (TRTS). Femtosecond transient absorption (TA) is frequently used to study the carrier transfer process.19 TA primarily monitors the exciton bleaching (XB) recovery to probe the kinetics of the electron/hole transfer from the NCs. XB is the result of band filling.19,20 Hence, TA is unable to capture if the carriers get transferred to the acceptor molecules from a hot state since the hot carriers would not contribute to the band filling or XB. On the other hand, time-resolved


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terahertz spectroscopy (TRTS), a non-contact method of measuring local AC conductivity, can probe the hot electron/hole transfer kinetics.21-23 This technique measures the photoconductivity which is proportional to the carrier density irrespective of them being hot or at the band edge. However, TRTS is limited by its sub-ps temporal resolution. There are only a few reports probing the interfacial charge carrier dynamics of these NCs.24-28 Wu et al. have, for the first time, studied the carrier dynamics within CsPbBr3 NCs, and the NCs in presence of electron and hole acceptors using ultrafast TA spectroscopy.24 At very low excitation fluence (average electron-hole pairs per NC, = 0.025) they observed an ultrafast exciton dissociation in CsPbBr3 using benzoquinone (BQ) and phenothiazine (PTZ) as molecular electron and hole acceptors respectively. The efficient electron and hole transfer occurred with half lives of 65±5 and 49±6 ps, respectively, indicating CsPbBr3 NCs as a good choice for solar cell material. However, at higher excitation fluence these NCs exhibit efficient nonradiative Auger recombination with a time scale of 20-40 ps which can limit the efficiency of carrier extraction.8,29 Auger recombination may not be the prominent decay pathway for solar cells operating at 1 sun-intensity. However, for concentrator photovoltaics, where light intensity is enhanced several folds using optics, Auger recombination will certainly become a very important factor.30,31 This will also be relevant in the case of multiple exciton generations from absorption of very high energy photons.32,33 In such a scenario interfacial carrier transfer will have to compete with Auger recombination. Hence it is very important to study the interfacial carrier transport rates and mechanism in presence of Auger annihilation to reveal the real potential of these NCs as future photovoltaics and photocatalyst. In this work, we utilized TRTS to understand the electron and hole transfer processes and recombination dynamics of photo-excited charge carriers in CsPbBr3 NC-molecular acceptor


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complexes at excitation intensities resulting in the range from 0.54 to 1.91. Understanding of carrier transfer process in the NC-molecule interface will help in understanding the carrier transfer process in the bulk interface. For a rational comparison of our results with that of Wu et al., we chose BQ and PTZ as electron and hole acceptor molecules, respectively.24 We observe a very efficient hot electron and hole transfer occurring in sub-300 femtosecond time scale and secondary slower transfer processes of thermalized carriers with a time scales of 20-250 ps in the presence of BQ and PTZ. CsPbBr3 NCs were synthesized following a previously reported hot injection method.1 Details of synthesis, characterization, and preparation of NC-BQ and NC-PTZ complexes are documented in Supporting Information (SI). The average edge length of the nanocubes with an orthorhombic crystal structure is 11±1 nm (See figure S1 in SI). These NCs have their lowest energy exciton absorption at 504 nm (2.46 eV) and excitonic PL at 514 nm (see Figure S2 in SI). The BQ and PTZ molecules form adsorption complex with NC surfaces governed by Langmuir adsorption isotherm.24,26,28 The NC-acceptor molecule adsorption complex formation is a stochastic process that follows a Poissonian distribution.24,26,28 In our experiments the average numbers of BQ and PTZ molecules attached to a NC have been determined to be 5.85±0.12 and 3.02±.02, respectively (see SI for details). On complex formation, with BQ and PTZ, the UV-visible absorption spectrum (Figure S2) of the NCs remains mostly unchanged, but the PL intensity quenches significantly (Figure S3) due to electron and hole transfer. This PL quenching is not due to energy transfer from the NCs to the molecular acceptors as there is no overlap between the emission spectrum of the NCs and the absorption spectra of the molecular acceptors. Recent cyclic voltammetry (CV) study reported the valence band (VB) and conduction band (CB) edges of these NCs to be -5.85 eV and -3.35 eV, respectively.34 Using CV, we determined the LUMO


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for BQ to be -4.16 eV and HOMO for PTZ to be -5.0 eV (see figure S6 in SI). Thus, the driving forces (-∆G) for electron and hole transfer are 0.81 eV and 0.85 eV, respectively. Description of our TRTS setup, experimental details, and the related data analysis procedure are reported elsewhere8 and also provided in SI. In a typical TRTS experiment of semiconductor NCs, a THz probe pulse, arriving at a certain pump-probe delay, measures the photoconductivity induced by the presence of electrons and holes created by an ultrafast optical pump pulse.22 We measured the temporal evolution of pump induced change in photoconductivity in neat CsPbBr3 NCs, NC-BQ and NC-PTZ complexes, all dispersed in 2,2,4,4,6,8,8-heptamethylnonane (HMN), at very similar experimental conditions. We have used 400 and 480 nm pump wavelengths with different intensities to excite the samples to produce hot carriers with ~640 and ~120 meV of excess energy, respectively, over the band edge. One should note that the acceptor molecules have near negligible absorption cross section at the pump wavelengths (see Figure S7) of this study. The transient THz photoconductivity in NC-BQ(PTZ) complex differs from that in neat NCs due to additional processes related to electron (hole) transfer from NCs to the molecular acceptors. One should note that the THz response observed is solely due to the presence of mobile charge carriers in NCs.22,35 Localized carriers in acceptor molecules do not respond to the low energy THz probe. The primary excitations at the experimental conditions of this study are hot excitons. In the absence of any acceptor molecule (in neat NCs), all excitons relax to the band edge (sub-ps) prior to different recombination processes. However, in the presence of carrier acceptor molecules, the photo-generated hot excitons have two or more options. They may choose to 1) relax to the band edge, 2) dissociate quickly and get transferred to the acceptor molecules prior to relaxation to the band edge, or both (1) and (2).


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Figure 1. a) Typical THz waveforms in TRTS measurements at λexc = 400 nm. E0 is the pumpoff signal plotted in black solid line. Pump induced change of THz field is plotted as ∆E. THz conductivity of neat NCs, NC-BQ and NC-PTZ complexes normalized with respect to density of absorbed photons when b) λexc = 400 nm and = 1.31, c) λexc = 480 nm and = 1.27. Real (red symbols) and imaginary (blue symbols) conductivity spectra at λexc = 480 nm, ~ 72 µJ/cm 2 fluence ( ~ 1.3) for d) neat NCs, e) NC-BQ and f) NC-PTZ complexes. Solid lines are the fits to the Drude-Smith (DS) plus two Lorentz (d), DS plus one Lorentz (e), and only DS (f). The Lorentz oscillator(s) is used to model the contribution of phonon vibrations to the conductivity spectra. Details are given in SI.


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Figure 1a shows typical THz transmission waveforms recorded in this work. Figure 1b,c show the pump-induced conductivity transients of neat NC solution, NC-BQ and NC-PTZ complexes normalized with respect to the density of absorbed photons (initial carrier density N0) at 400 and 480 nm pump wavelengths, respectively. The average number of electron-hole pairs () at the experimental pump fluence is ~1.3. It is evident from the positive real conductivity (Figures 1df) that the THz conductivity observed here is mainly due to mobile electrons and holes, rather than excitons.22,35 The most striking feature of the observed THz kinetics shown in Figures 1b,c is a large quenching (with respect to that in neat NC) of the initial conductivity (at pump-probe delay, tp=0) in presence of molecular carrier acceptors. The quenching is as high as ~90% in NCBQ system and ~80% in case of NC-PTZ complex (Table S1 in SI). This clearly indicates to a huge reduction in charge carrier density within the NCs at an ultrafast timescale, even faster than our instrument response time (IRF) of ~300 fs, in the presence of the carrier acceptors. This reduction in carrier density is only possible by a sub-300 fs electron (hole) transfer from the photoexcited NCs to BQ (PTZ) molecules. The carrier transfer process is so fast that it may have occurred even before thermalization of the hot carriers. Recently, Maity et al. reported hole transfer from CsPbBr3 NCs to a hole acceptor molecule, 4,5-dibromofluorescein, to occur at 11.25 ps timescale.25 Ponseca Jr. et al. also reported a sub-200 fs hole transfer from CH3NH3PbI3 polycrystalline film to Spiro-OMeTAD, a frequently used organic hole acceptor.36 At the excitation fluences of this study CsPbBr3 NCs undergo efficient Auger recombination with a time scale of 20-40 ps.8,29 Above results clearly prove that in presence of BQ and PTZ as electron and hole acceptor molecules, respectively, the interfacial electron and hole transfer takes place at a time scale much faster than the Auger recombination. Thus the photovoltaic and photocatalytic efficiencies should not be affected by the detrimental Auger process.


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Figure 2. Schematic representations of the possible mechanism of carrier transfer and charge recombination processes in CsPbBr3 NC-BQ and NC-PTZ system upon photoexcitation. a) Ultrafast (< 300 fs) hot carrier transfer. 1: photoexcitation, 2: hot e/h transfer, 2-3-4: Auger assisted e/h transfer, 5: back transfer b) Fast (20-250 ps) thermalized carrier transfer. 1: photoexcitation, 6: thermalized e/h transfer, 7: back transfer. Both electrons and holes are expected to have similar contributions to the observed photoconductivity as their effective masses are predicted to be similar (me=0.22, mh=0.24).8 Hence, in NC-BQ (NC-PTZ) complex, one should not expect the initial conductivity to reduce more than half (with respect to that in neat NCs) even if all photo-generated hot electrons (holes) are transferred to LUMO (HOMO) of BQ (PTZ). Such unexpected quenching of carrier density can occur due to multiple reasons. An Auger assisted mechanism for electron and hole transfer can partially explain the above observation.37 As shown schematically in Figure 2a (right panel, arrow 2-3-4), in Auger assisted electron/hole transfer process the excess energy (∆G) released due to electron (hole) transfer reaction is consumed by the hole (electron). The energized hole (electron) may attain enough energy to be at par with the HOMO (LUMO) of BQ (PTZ), and may as well get transferred to the HOMO (LUMO) of the acceptor molecule. According to the


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band alignment, this mechanism seems to be feasible in case of NC-BQ complex when photoexcited with 400 nm pump. However, in the case of NC-PTZ complex, the energy analysis does not support such a mechanism. The second plausible mechanism is an ultrafast interfacial back transfer (process 5 in Figure 2a) soon after the carrier transfer from NCs to molecular acceptors, thus reducing the carrier density. Both of these processes essentially lead to interfacial charge recombination (CR) involving slightly different timescales. One should note that such CR should be avoided if these NCs are to be used in photovoltaics. We anticipate that in the simultaneous presence of suitable electron and hole transport layers, CR can be minimized as the initial transfer of both electrons and holes are equally fast. In frequency resolved TRTS experiments, one measures the photoinduced change in complex conductivity (∆σ) from the observed photoinduced change in THz transmission (-∆E/E0) (see SI for details). Most often a Drude (or Drude-Smith) type response, arising from the free carriers generated by photoexcitation, is observed. In addition, if, upon photoexcitation, there is any change in the amplitude of IR-active phonon vibrations having frequencies in the range of THz probe light, there will be additional resonant absorption at those particular phonon frequencies. The photo-induced change in phonon amplitude (spectrum) will appear in the complex conductivity spectra (complex dielectric function) obtained from the TRTS measurements. Figures 1d, e and f show the complex conductivity spectra induced by 480 nm excitation ( = 1.3) at 0 ps pump-probe delay (at the peak of the THz transients) for neat NCs, NC-BQ and NCPTZ systems, respectively. In neat NCs hot carrier relaxation occurs in sub-ps time scale as evidenced by the appearance of strong phonon modes at ~1.8 THz and ~3 THz in the real conductivity spectrum. The appearance of phonon modes on pump induced conductivity spectra is an indication of strong carrier-phonon coupling, and a multiphonon process to be responsible


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for hot carrier relaxation. Similar observations have also been reported earlier in organo-metalhalide perovskite systems.35,38-40 Recent TA studies also report the hot carrier relaxation in CsPbBr3 NCs to occur in time scale in the range of 310-700 fs.24,41-43 On the other hand, only a weak phonon band at ~3 THz is observed in the photoconductivity spectrum of NC-BQ complex whereas the spectrum of NC-PTZ complex is completely devoid of any phonon absorption. This observation strongly indicates that hot carrier relaxation and carrier transfer take place at similar time scale in NC-BQ complex whereas carrier transfer is probably even faster than hot carrier relaxation to the band edge in case of NC-PTZ system. It is expected that the carrier cooling rate may not alter much on complex formation. Hence, it may be inferred that the hot hole transfer process is relatively more efficient compared to hot electron transfer process. This observation also justifies why Wu et al. did not observe any sub-ps electron/hole transfer in their TA experiment of the same systems. As mentioned earlier, if hot carriers are transferred to the molecular acceptors prior to thermalization, they will not contribute to the band-filling and related exciton bleaching. Previously, hot electron transfer has been observed from PbSe NCs to TiO2 surface in sub-50 fs time scale44, from graphene quantum dots to TiO2 surface at

Terahertz Spectroscopic Probe of Hot Electron and Hole Transfer from

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