Cation-Dependent Hot Carrier Cooling in Halide ... - ACS Publications

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Cation Dependent Hot Carrier Cooling in Halide Perovskite Nanocrystals Junsheng Chen, Maria E Messing, Kaibo Zheng, and Tõnu Pullerits J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11867 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on January 28, 2019

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Journal of the American Chemical Society

Cation Dependent Hot Carrier Cooling in Halide Perovskite Nanocrystals Junsheng Chen1, 2, Maria E. Messing,3 Kaibo Zheng1,4, Tonu Pullerits1* 1

Chemical Physics and NanoLund, Lund University, Box 124, 22100 Lund, Sweden 2

Nano-Science Center & Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark

3

4

Solid State Physics and NanoLund, Lund University, Box 118, 22100 Lund, Sweden

Department of Chemistry, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark

1 ACS Paragon Plus Environment

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Abstract Lead halide perovskites (LHPs) nanocrystals (NCs), owning to their outstanding photophysical properties, have recently emerged as a promising material not only for solar cells but also for lighting and display applications. The photophysical properties of these materials can be further improved by chemical engineering such as cation exchange. Hot carrier (HC) cooling, as one of the key photophysical processes in LHPs, can strongly influence performance of LHPs NCs based devices. Here we study HC relaxation dynamics in LHP NCs with cesium (Cs), methylammonium (MA, CH3NH3+) and formamidinium (FA, CH(NH2)2+) cations by using femtosecond transient absorption spectroscopy. The LHP NCs show excitation intensity and excitation energy dependent HC cooling. We investigate the details of HC cooling in CsPbBr3, MAPbBr3 and FAPbBr3 at three different excitation energies with low excitation intensity. It takes longer time for the HCs at high energy to relax (cool) to the band edge, compared to the HCs generated by low excitation energy. At the same excitation energy (350 nm, 3.54 eV) all the three LHP NCs show fast HC relaxation ( MAPbBr3 (0.27 ps, 4.6 meV/fs) > FAPbBr3 (0.21 ps, 5.8 meV/fs). The cation dependence can be explained by stronger interaction between the organic cations with the Pb-Br frameworks compared to the Cs. The revealed cation dependent HC relaxation process is important for providing cation engineering strategies for developing high performance LHP devices.


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Introduction In recent years, lead halide perovskites (LHPs) with the general formula APbX3 (X = Cl, Br and I), where A is a cation such as cesium (Cs), methylammonium (MA, CH3NH3+), or formamidinium (FA, CH(NH2)2+), have emerged as promising active material for optoelectronic applications. For example, the LHPs-based solar cells have reached more than 23% light-to-electricity conversion efficiency.

1

Their nanomaterial counterparts show

potential for fabricating various other devices for our daily life such as lighting (e.g. light emitting diodes, LEDs), displays (e.g. TVs), photo-detectors and lasers. The high performance of LHPs is related to their broadband absorption, easily tunable optical band gap, efficient charge generation and transportation, as well as their luminescence properties. These properties can be further refined by chemical engineering, such as halogen atoms X mixing and cations A exchange.2 All above applications are based on light absorption or emission and involve a complex sequence of different photophysical processes. A comprehensive understanding of these processes in LHPs is a key for further improving the design and utilization of these materials.3 Consequently, photophysics of LHPs have been extensively studied through various theoretical and experimental approaches.4 Hot carrier (HC) cooling is one of such photophysical processes. HCs are created by absorption of photons with excess energy above the bandgap. The mechanisms and dynamics of HC cooling in LHPs and other semiconductor materials are of fundamental importance for the performance of many types of devices.5 For example, the potentially highly efficient hot carrier solar cells depend on the details of carrier cooling dynamics and the lattice energy recycling capability.6-8 In light emitting devices, fast HC relaxation is preferred because of the competition between intraband HC cooling and charge trapping from higher energy hot levels.7, 9 Recent experimental results have revealed that bulk LHPs have slower HC dynamics at the excess energy of 1.4 eV (~0.4 ps) 3 ACS Paragon Plus Environment

10-11

compared to


most organic semiconductors (~0.1 ps).12-13 Moreover, it is possible to slow down the HC relaxation by more than one order of magnitude via increasing the excitation intensity.3, 11, 14-15 Slow HC relaxation has been also observed in Sn-based perovskites.16 The slow HC relaxation in bulk LHPs has been related to various mechanisms such as hot-phonon bottleneck,3 large polaron screening,15 or Auger heating effect.5 Also in LHP nanocrystals (NC), Li et al. demonstrated NC size dependent slow-down of the HC relaxation and efficient HC extraction in a device.17 This exemplifies the potential of LHP NCs for HC solar cells.6, 17 HC relaxation in LHP NCs can be further tuned by chemical engineering. Chung and Mondal et al. demonstrated that the HC relaxation time is sensitive to the type of halogen atoms in CsPbX3 NCs.7,

18

Also cation mixing can play an important role for HC relaxation in LHPs as

demonstrated in bulk LHPs both experimentally and theoretically.4, 15, 19-20 Cation mixing, as one of the most practical and successful strategies for chemical engineering, can enhance the performance of LHP bulk and NC materials.2,

21-22

The influence of the cations on HC

relaxation dynamics in LHP NCs has not been explored so far. A comprehensive understanding of the cation-dependent HC relaxation dynamics in LHP NCs is not only essential for understanding the fundamental photophysics but also for improving the performance of the LHP NC-based optoelectronic devices.


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Figure1. (a). Absorption spectrum (black solid line) and photoluminescence spectrum (red solid line, λex=400 nm) of CsPbBr3 NCs, the purple, blue and green arrow represents three different excitation wavelengths/energies (350 nm: 3.54 eV, 400 nm: 3.10 eV and 490 nm: 2.53 eV), respectively. (b). Crystal structures of lead bromide perovskite NCs with three different cations: cesium (CsPbBr3), methylammonium (MAPbBr3) and formamidinium (FAPbBr3). (c). Pseudocolour representation TA spectra of colloidal CsPbBr3 NCs for 350 nm excitation

at

high

excitation

intensity

with

≈34

(corresponding

to

I = 2.4×1014 photon/cm2/pulse, n~4.3×1019 cm-3), black dashed line arrow indicates the initial negative GSB signal increases and reaches the maximum. (d). Normalized GSB dynamics probed at the band-edge for CsPbBr3 NCs with 350 nm excitation at seven different excitation intensities

(I1=1.0×1012 ,

I2=4.9×1012 ,

I3=7.2×1012 ,

I4=2.4×1013 ,

I5=5.9×1013 ,

I6=1.4×1014 and I7=2.4×1014 photon/cm2/pulse, corresponding to ~ 0.1, 0.7, 1.0, 3.4, 8.4, 20, and 34; n~1.8×1017, 8.8×1017, 1.3×1018, 4.3×1018, 1.0×1019, 2.5×1019 and 4.3×1019 cm-3, respectively). Here we have investigated HC relaxation dynamics in lead bromide perovskite NCs with three different cations (Figs. 1 and S1): cesium (CsPbBr3), methylammonium (MAPbBr3) and



formamidinium (FAPbBr3) using transient absorption (TA) spectroscopy. The method has been widely used for studies of HC relaxation dynamics in LHPs.3, 5, 7, 11, 14, 17, 23 Consistent with earlier studies, we see strong excitation intensity dependence of HC relaxation dynamics. With increasing excitation intensity, HC dynamics slows down due to the hot-phonon bottleneck and Auger heating effects. At low excitation intensity limit we reveal clear cation dependence of HC dynamics. Perovskite NCs based on inorganic cation (Cs) have the slowest, while the FAPbBr3 NCs have the fastest HC relaxation. This can be explained by the increased carrierphonon coupling due to the interaction between organic cation with Pb-Br framework. Results and Discussion Figure 1c shows a pseudocolour TA spectrogram of CsPbBr3 NCs excited at 3.54 eV (350 nm) with an intensity of 2.4×1014 photons/cm2/pulse corresponding to the initial average generated electron-hole pairs per NC ~34 and average carrier density n~4.3×1019 cm-3 (=I·σ, evaluated from the excitation intensity I and absorption cross section σ of the NCs, see supplementary information S2). The average carrier density per NC volume is determine as n=/VNC, where VNC is the NC volume estimated by the average size of the cubic shape NCs (see supplementary information S1). Two obvious features appear: i) a negative ground state bleach (GSB) signal located around the bandgap (2.45 eV), which is induced by the band filling effect; ii) a positive photoinduced absorption (PIA) signal below the bandgap (less than 2.41 eV). The shift of the PIA signal in respect to the GSB is a typical signature of excitonexciton interaction and can be used to evaluate the so called bi-exciton binding energy.7, 24 While HCs relax to the lowest-energy states, the positive PIA signal decays and is replaced by a strong GSB signal. At the same time, the initial negative GSB signal increases and reaches the maximum (see the arrow in Figure 1c). All three types of NCs with different cations show analogous GSB and PIA features while the time-constants of kinetics vary. (see Figs. 1c, S3 and S4) The PIA decay and GSB buildup are the result of the carriers arriving at the band edge 6 ACS Paragon Plus Environment

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during the initial thermalization and relaxation.7, 18 Here we determine the time-constant of the process via global fitting of the TA kinetics by a sum of exponentials with global time constants (lifetimes) as variables (see supplementary information S4). We point out that polaron formation can be involved in the early time excited state dynamics, as one of the consequences of the carrier-phonon coupling. 25-26 Since here we focus on the hot carrier cooling process, no further discussion of the polaron formation will be carried out in the current work. Table1. Excitation energy dependence of the time constant of the rising signal (τc) at the bandedge for colloidal NCs with three different cations with low excitation intensity conditions.a,b

a

NCs

CsPbBr3

MAPbBr3

FAPbBr3

τc (fs, λex=350 nm)

390±20

270±20

210±20


310±20

235±20

180±20


Cation-Dependent Hot Carrier Cooling in Halide ... - ACS Publications

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