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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Biexciton Generation and Dissociation Dynamics in Formamidiniumand Chloride-Doped Cesium Lead Iodide Perovskite Nanocrystals Navendu Mondal, Apurba De, and Anunay Samanta J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01666 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 19, 2018

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The Journal of Physical Chemistry Letters

Biexciton Generation and Dissociation Dynamics in Formamidinium- and Chloride-Doped Cesium Lead Iodide Perovskite Nanocrystals

Navendu Mondal, Apurba De and Anunay Samanta* School of Chemistry, University of Hyderabad, Hyderabad 500046, India

*Corresponding author’s Email: [email protected]

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Recent studies show that perovskite (ABX3 type) nanocrystals comprising mixed A or B cation and/or mixed halide (X) are more stable and efficient materials for photovoltaic applications than their respective pure forms. Herein we report how doping of small quantity of formamidinium and/or chloride ion influences the single and multi-exciton dynamics of CsPbI3 nanocrystals. With the help of ultrafast pump-probe spectroscopic measurements we show that chloride doping can enhance the biexciton lifetime of the system significantly by slowing down the Auger recombination (AR) process. The measured biexciton AR timescale (~ 195-205 ps) in some of these NCs is the longest among those reported till date for any similar size perovskites. We further demonstrate that suppression of the AR rate and consequent lengthening of biexciton lifetime allow harvest of these species for their utilization through rapid (18-45 ps) electron transfer to fullerene. The insights obtained from this study are expected to help design of more efficient doped perovskites for energy conversion purpose. Table of contents graphic:


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Lead halide perovskites are in limelight for their high solar energy conversion efficiency due to broad and intense absorption, low exciton binding energy and high charge carrier mobility of these materials.1-6 The iodide-based perovskites are most suitable in photovoltaic applications as their band-gap (~ 1.6-1.7 eV) is most appropriate for a single-junction solar cell.7-9 Pure iodide-based perovskites such as MAPbI3 and FAPbI3, where MA and FA represent methylammonium and formamidinium cation, respectively, show an efficiency of around 20%. However, low stability of these materials is indeed a matter of concern.10-14 Even though CsPbI3 is relatively more stable, it also undergoes phase transition like FAPbI3.10, 11 Though this phase transition can be prevented by mixing other halides, doping -

of Br increases the band-gap and also gives rise to halide segregation, which results in carrier trapping and reduction in efficiency.10,

12-14

-

This, however, is not the case with Cl

doped system. In addition, long electron-hole diffusion length in this system makes it a more suitable material for efficient photovoltaic device compared to the pure iodide one.3 Improved stability of mixed cation and halide containing perovskites, where halide segregation is prevented, is recently reported.8,

15

A connection between the crystallinity

(estimated by the tolerance factor) and stability of a perovskite with definite mixed composition with its optoelectronic properties is also established.8, 16, 17 These findings and the NREL report that five out of best six certified solar cells are based on mixed perovskites10 have generated great interest in mixed cation and/or halide based perovskites. Utilization of higher energy electron-hole pair (produced by high energy solar photons) of semiconductors prior to dissipation of their excess energy in the form of heat can enhance the efficiency of a photovoltaic device. For photon energy > 2hν, the excess energy released by a carrier can also generate another or more exciton.18-24 However, these bi/multiexcitons recombine nonradiatively through Auger process (in about 10-100s of ps) in



which the recombination energy of one exciton is transferred to another charge carrier of the same NCs.21,

25

Utilization of these multiexcitons for boosting the performance of a

photovoltaic device requires their extraction using an electron/hole acceptor prior to the AR process.26, 27 While harvesting of multiexcitons from metal chalcogenide quantum dots has been explored, 27-30 this has not been attempted so far with the perovskite NCs. Considering the potentials of the mixed perovskites and importance of the harvest of multiexcitons in enhancing the efficiency of perovskite-based solar cells, we have studied how the carrier dynamics of CsPbI3 NCs is influenced by FA+ and/or Cl- dopants under both low and high excitation fluence with the help of femtosecond pump-probe spectroscopic technique. For this purpose, we have chosen three doped systems, FAxCs1-xPbI3, CsPbI3-yCly and FAxCs1-xPbI3-yCly, (x and y < 0.1) apart from pure CsPbI3. Another system, FAPbI3, with a reduced band gap is also studied. The results show high biexciton lifetime (~195-205 ps) for CsPbI3-yCly and FAxCs1-xPbI3-yCly and indicate that it is indeed possible to extract the biexcitons from these systems using electron acceptor like fullerene. CsPbI3 and doped NCs were prepared by hot injection two-precursor method developed by Kovalenko and co-workers with some modifications (details are in experimental section, ESI)16, 31 and characterized by transmission electron microscopy (TEM) imaging and powder X-ray diffraction (PXRD) measurements. Energy dispersive spectroscopy studies indicated dopant content in these NCs to be less than 0.1. The FA content was estimated from the reduction of Cs-content upon doping.16 Pure CsPbI3 NCs were found to be cubic shape with mean edge length of 12-13 nm (Fig. 1A). Doping of small amount of FA and/or Cl did not introduce any noticeable change in the shape and size of the NCs. The spacing (~ 0.594 nm) between the (001) lattice planes, as measured from the highresolution TEM images, was found to be same for all these NCs. PXRD patterns also did not show any change of the cubic lattice upon doping (Fig. S2, ESI). The toluene-dispersed 4 ACS Paragon Plus Environment

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colloidal samples showed very similar band-gap (~1.82 eV) and photoluminescence (PL) peak position (680-685 nm) with similar FWHM (Fig. 1B). Unlike pure CsPbI3, which transforms into a non-perovskite phase within 4-5 days in our laboratory condition (consistent with earlier report),20, 35 the doped NCs exhibited much improved phase stability. The mixed halides and FA-doped NCs are found to be stable for more than 15 and 45 days, respectively, in their colloidal form. The PL quantum yields (QY) of CsPbI3 and FAxCs1-xPbI3 are measured to be 55 and 62 %, respectively. CsPbI3-yCly and FAxCs1-xPbI3-yCly NCs are less fluorescent and their PLQYs are estimated as 36% and 45 %, respectively. (A)

(B)

(C)

PL intensity

685 nm CsPbI3-yCly

500

680 nm

CsPbI3

682 nm

600 700 Wavelength (nm)

800

PL Intensity (a.u.)

FAxCs1-xPbI3-yCly

FAxCs1-xPbI3

Prompt CsPbI3 FAxCs1-xPbI3

10k

685 nm

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CsPbI3-yCly FAxCs1-xPbI3-yCly

1k

100

0

100

200

300

400

Time (ns)

Figure 1. High resolution TEM images (A) of the NCs. The scale bar shown in the images is 10 nm. Normalized absorption and PL spectra (λex = 530 nm) (B) and PL decay profiles (λex = 480 nm) monitored at their respective PL maxima (C) of CsPbI3, FAxCs1-xPbI3, CsPbI3-yCly and FAxCs1-xPbI3-yCly NCs. 5 ACS Paragon Plus Environment

500


-

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-

Even though doping of Cl or both FA and Cl did not improve PLQY of the system, PL lifetimes of the mixed halide perovskites increase significantly (Fig. 1C). The average PL -

lifetime of the Cl doped systems, which is estimated from the tri-exponential decay profile (with two major components (93-96%) (Table S1)), is found to be ~3 times longer than that of CsPbI3. A similar increase in lifetime is reported for MAPbI3-yCly.3 As replacement of Iwith smaller Cl- pushes the tolerance factor towards more stable regime, better crystallinity of -

the doped system and more stronger ionic interaction between Pb2+ and Cl perhaps suppress the transition to non-perovskite phase.32 As doping enhances the PL lifetime, but not the PLQY, it is evident that increase in lifetime is not due to passivation of the trap states.33, 34 A recent report attributes the long time-component of PL decay to the trap states, which lie close to the band-edge state.35 The photogenerated charge carriers trapped in these shallow states can return to the band-edge state by thermal excitation and recombine. The excursion of the charge carrier to the trap state followed by its return to the band edge and recombination from there increases the PL lifetime of these systems.35 This phenomenon is similar to E-type delayed fluorescence in molecular systems.36 We attribute the increase in lifetime in the present case to the formation of shallow trap centre by doped Cl-, as observed in the case of Br--doped iodide perovskites,37 The carrier dynamics under single and multi-exciton regime is studied by TA measurements on colloidal samples dispersed in toluene. The photo-excitation was made at 530 nm (unless stated otherwise) maintaining low pump fluence (restricting the average number of excitons () to much less than unity) for the single exciton regime to avoid nonlinear processes like exciton-exciton annihilation. Fig. 2 shows time-evolution of the TA spectra of FA-doped CsPbI3 NCs for lowest excitation laser fluence corresponding to = 0.25. These spectra are characterized by excitonic bleach signal 6 ACS Paragon Plus Environment

38

corresponding to ground

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state absorption and a weak photoinduced absorption in the higher energy side of the bleach. These features are similar for other NCs (Fig. S3-S5, ESI) and consistent with literature.39

2

- 0.1 ps 25 ps 500 ps

(A)

(B)

2 ps 100 ps

0

0 -2

-6 -8

∆Α (mOD)

-4

0.0

∆Α/ normalized

∆Α (mOD)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CsPbI3 FAxCs1-xPbI3 FAxCs1-xPbClyI3-y CsPbClyI3-y

-0.4

-20

-40

-0.8

0

250

500

750

- 0.1 ps 25 ps 500 ps

2 ps 100 ps

500 ps

2 ps

1000

Delay time (ps)

-60 600

650

600

700

650

700

Wavelength (nm)

Wavelength (nm)

Figure 2. Representative TA spectra of FAxCs1-xPbI3 NCs at the indicated delay times under lowest pump (λex = 530 nm) fluence; 7 µJ/cm2 (A). Inset shows bleach recovery dynamics for the NCs monitored at their respective bleach maxima. Panel (B) depicts TA spectra at higher pump fluence; 162 µJ/cm2. Fig. 2(A) shows very little recovery of the bleach signal of FAxCs1-xPbI3 NCs in 500 ps. A similar observation is made for the other NCs (Inset of Fig. 2A). It is thus evident that the carrier dynamics of the systems in this time range is not affected by doping. The bleach recovery is found to be single exponential with a long lifetime (too long to be measured accurately in this setup) indicating negligible nonradiative carrier trapping centre,39-41 and electron-hole recombination process to be responsible for this bleach recovery process. The exact time constant of this process, which is very similar for all the NCs (8-9 ns, Table S1, ESI), is estimated from the PL decay dynamics. We attempted generation of multiexcitons both by increasing the fluence of the 530 nm excitation pulse and also by increasing the frequency of the pump laser keeping its fluence low. However, the latter could not be achieved even for the lowest band-gap material, 7 ACS Paragon Plus Environment


FAPbI3 when excited at 280 nm (corresponding to 2.7Eg), the bleach recovery dynamics was found single-exponential with no additional (fast) component due to the biexcitons (Fig. S6, ESI) indicating the biexciton generation threshold to be higher than 2.7 Eg (an observation consistent with literature)42 for this system. As the dynamics of multiexcitons does not depend on the method by which they are produced,18-24,

42

in the present case, we have

generated the multiexcitons by using high pump fluence (in which a single NC absorbs multiple photons and generates the multiexcitons) and studied their dynamics. At higher pump fluence, the bleach amplitude as well as the bandwidth is increased (Fig. 2B and S3-S5, ESI) indicating filling of the states near the band-edge. Under this condition, a significant recovery of the bleach signal is observed within 500 ps. For example, the recovery is now 50% (in contrast to ~ 5% for lower fluence) in the case of FAxCs1-xPbI3. The effect of pump fluence on the bleach recovery kinetics of the NCs is depicted in Fig. 3. The acceleration of dynamics at higher pump fluence is clearly due to Auger-assisted recombination of the multiexcitons. The lifetime of the multiexcitons is estimated by normalizing the bleach recovery signals for different pump fluences at a longer time (1 ns), then subtracting the bleach recovery decay profile at lowest fluence from that at highest fluence and finally, by fitting the resulting decay profile (Fig. S7, ESI). A single exponential fit of the latter indicated absence of higher order excitons other than the biexcitons. This observation is consistent with two-fold degeneracy of the band-edge states of these lead halide perovskites.42, 43


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0

0

FAxCs1-xPbI3

CsPbI3 -2

-2

Normalized ∆ A

Normalized ∆ A

-1

~ 0.09 ~ 0.40 ~ 0.67 ~ 0.85 ~ 2.50 ~ 4.29

-3

-4

-4

~ ~ ~ ~ ~

-6

0.25 0.45 2.1 3.6 6.5

-8

0

100

200

300

400

500

0

100

Delay time (ps) 0

200 300 Delay time (ps)

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500

0

CsPbI3-yCly

FAxCs1-xPbI3-yCly

Normalized ∆A

-1

Normalized ∆A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-2 ~ 0.25 ~ 0.50 ~ 1 ~ 2.1 ~ 3.2 ~ 4.8

-3

-4

-1

-2

~ 0.10 ~ 0.40 ~ 0.70 ~ 1.5 ~ 2.7 ~ 4.5

-3

-5 0

150

300

450

600

750

Delay time (ps)

0

150

300 450 Delay time (ps)

600

Figure 3. Bleach recovery kinetics of individual NCs monitored at their respective bleach maximum as a function of excitation laser fluence. values for the individual NCs are estimated using the measured values of absorption cross-section (see later section). The biexciton lifetimes estimated from these fits are 115±6.1, 101±2.0, 197±8.5 and 204.6±8.1 ps for CsPbI3, FAxCs1-xPbI3, CsPbI3-yCly and FAxCs1-xPbI3-yCly, respectively. A similar study yielded a biexciton lifetime of ~145±4.4 ps for FAPbI3 NCs. The measured biexciton lifetime of 115 ps for CsPbI3 is slightly higher than its literature value (94 ps) presumably due to size difference of the NCs.42, 43 The biexciton lifetimes of 197 and 190 ps for the two Cl- doped systems are the highest values observed till date for any perovskite of similar size and shape indicating that the biexcitonic AR process is the slowest in these systems. As AR rate is governed by the overlap of electron-hole wave function and Coulomb


750


interaction (between electron and hole) arising from spatial confinement,44-48 smaller particles with greater confinement of electron and hole are expected to exhibit a faster AR than the larger ones. In core-shell (say, of type-II) quantum dots, the separation of electron and hole in shell and core reduces the overlap of their wave functions and hence, slows down the AR rate.45 In our case, we think the shallow trap centre created by Cl- doping helps delocalization of one of the charge carriers, and consequent decrease in overlap of the electron-hole wave function, leading to a decrease of the rate of AR process. Recently, in the case of Cl- doping of MAPbI3, a reduction of the electron-hole wavefunction overlap is demonstrated theoretically by time-domain ab initio analysis.49 As the valence band (VB) and conduction band are mainly due to the contributions of halogen and lead atoms, respectively, according to this work, replacement of I- by Cl- does not affect the band-edge states of the system, but it reduces the contribution of I- in the VB states and consequently, results in a reduction of the overlap. We have ascertained that the (100-200 ps) decay component at higher fluence is solely due to biexciton recombination by examining the TA amplitude at a longer time when the NCs have fully relaxed to a single-exciton state. This late-time TA amplitude is directly related to the number of excited NCs. Assuming Poisson statistics for photon absorption, the probability (Pi) of an NC having i-no of excitons is given by21, 42, 50

Pi =

< N >i exp ( − < N > ) i!

……. (1)

Here, is the average number of excitons per NC and is equal to Jσ, where, J is the pump fluence (in photons/cm2) and σ is the absorption cross section.20, 22 As the late-time (say, at 1 ns) bleach signal (∆AL) is solely due to the contribution from the single exciton, one can


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write, ∆AL = (1-P0), where P0 is the fraction of un-excited NCs. Using equation (1) and replacing as Jσ, one can write ∆AL = (1-P0) = (1-exp (-Jσ)) ………. (2) Fig. 4 depicts the variation of the bleach amplitude at 1 ns versus the pump fluence and fits to the Poisson statistics, indicating the fast component to arise from biexciton recombination. The absorption cross-section (σ) for the NCs at 530 nm, obtained from these fits lie between ~1×10-14 - 3×10-14 cm2. These values, which are consistent with literature,42, 51 have been used for the estimation of used in Fig 3.

FAxCs1-xPbI3

CsPbI3

∆ΑL

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

1

2

3

0

2

1.5

3.0

6

FAxCs1-xPbI3-yCly

CsPbI3-yCly

0.0

4

4.5

0.0

1.5

3.0

4.5

Pump fluence (1014photons/cm2)

Figure 4. Plot of bleach amplitude at long time (t ~ 1 ns) as a function of laser fluence and fit to equation 2.

Though multiexciton generation using a higher photon energy (corresponding to λ = 280 nm, hν = 2.7 Eg) of low fluence in the case of FAPbI3 was unsuccessful, the biexciton



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formation is observed with an increase in fluence of the 280 nm laser pulse corresponding to an increase of from 0.05 to 0.25 (Fig. S8, ESI). In contrast, for 530 nm excitation, biexciton formation was observed only for much higher threshold of laser fluence ( > 1). This is due to the availability of higher kinetic energy to the charge carriers for 280 nm excitation compared to the 530 nm excitation. The former enhances the probability of threebody collisions and AR process. The estimated single- and bi-exciton lifetimes and absorption cross-sections (at 530 nm) for different NCs are collected in Table 1. Table 1. Single exciton and biexciton lifetime and absorption cross-section (at 530 nm) of the studied NCs.

Composition

Size (nm)

CsPbI3

12±1.5

FAxCs1-xPbI3

Single exciton lifetime (ns)

Biexciton lifetime (ps)

Absorption crosssection (×10-14 cm2)

50.9±1.0

115.0±6.1

1.59

12±1.5

44.4±0.5

101.4±2.0

1.22

CsPbI3-yCly

12±1.5

150.7±1.0

197.3±8.5

1.06

FAxCs1-xPbI3-yCl y

12±2.0

144.4±1.5

204.6±8.1

2.72

FAPbI3

10±1.5

74.2±1.0

145.0±4.4

1.75

We have also explored whether the biexcitons can be extracted using commonly used electron acceptor like C60, which has been used recently for electron transfer from FAPbBr3 NCs.52 For this purpose, we excited the system containing C60 at 530 nm (where C60 has negligible absorption53) to ensure that the number of photons absorbed by the NCs is not changed. We observe that under single-exciton condition, the spectral feature is not affected, but the bleach recovery dynamics becomes faster (Fig. 5) in the presence of C60. Analysis of the time-dependence of the bleach signal revealed a biexponential dynamics of the composite (unlike single exponential one for the bare NC) with a fast component of 18-45 ps, which can


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be assigned to the ET process from the perovskites to C60. Similar assignment of additional fast component as ET has been reported earlier for other nanocomposites.54, 55 2

3

(B)

0

0

-3

-2 - 0.1 ps 10 ps 100 ps 1000 ps

-6 -9

∆Α/ mOD

∆Α/ mOD

(A)

2 ps 50 ps 500 ps

- 0.1 ps 10 ps 100 ps 1000 ps

-4 -6

2 ps 50 ps 500 ps

-8

-12 550

600

650

-10 550

700

600

Wavelength (nm) = 4.8

only CsPbI3-yCly

(C)

650

700

Wavelength (nm)

0.0

(D)

CsPbI3-yCly- C60

Normalized ∆ A

-0.2

normalized ∆ A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.4 -0.6 -0.8

only CsPbI3-yCly CsPbI3-yCly-C60

= 2.1

-1.0 0

250

500

750

Delay time (ps)

0

250

500

750

1000

Delay time (ps)

Figure 5. Representative TA spectra (λex = 530 nm) of CsPbI3-yCly in the absence (A) and presence (B) of C60, under single-exciton condition. Panel (C) compares the bleach recovery dynamics with and without C60 under single-exciton regime. Panel (D) compares bleach recovery dynamics of CsPbI3-yCly in the absence and presence of C60 at different pump laser fluence. The effect of C60 on the bleach recovery dynamics due to the biexcitons is also studied using higher laser fluence corresponding to values between 2 and 4.8. The bleach recovery dynamics is faster in the presence of C60 (Fig. 5D) for = 2. However, with further increase in fluence the difference in bleach recovery dynamics in the absence and



presence of C60 gradually narrows down and for ~ 4.8, no difference could be observed (Fig. 5D). This observation is understood considering that for moderate laser fluence, when the biexciton population is low, they are dissociated due to extraction by C60. However, for very high laser fluence, when the biexciton population is high, very few of them are extracted by C60. The number of dissociated biexcitons per NC can be controlled by varying the number of adsorbed species (in the present case, C60), which participate in one-electron reduction process. Under large population of biexcitons, all of them may not undergo dissociation through ET when the all adsorbed C60 molecules are reduced following the first ET. It is thus evident that one can extract majority of these long-lived biexcitons by proper selection of electron/hole acceptor and using chloride–doped perovskites with longer biexciton lifetime. Further search in this direction is currently underway. Lack of long-term stability of CsPbI3 is an issue, which has triggered research in quest for more stable alternative system with similar band-gap. Even though FA-doped CsPbI3 NCs offers a better stability, TA measurements on singly- and doubly-excited NCs reveal that moderately stable Cl-doped systems could be more appropriate for photovoltaic applications. The biexcitonic AR (~195-205 ps) rate is significantly suppressed in CsPbI3-yCly and FAxCs1xPbI3-yCl y

NCs compared to the other systems. This is attributed to the delocalization of one

of charge carriers over the band-edge and shallow trap states of the NC resulting in a reduced overlap between the electron and hole wavefunctions. The long biexciton lifetime also provides an opportunity for their extraction, which is realized here using fullerene. The demonstration that introduction of doping elements can suppress the AR process and allow extraction of the biexcitons can help improving the efficiency of photovoltaic device.


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Acknowledgements The work is supported by Grant No: EMR/2015/000582 and J. C. Bose Fellowship (to AS) of the Science and Engineering Research Board (SERB), Department of Science and Technology (DST), India. We acknowledge the TEM facility of the Centre for Nanotechnology of our University and thank Mr. M. Durga Prasad for the measurements. The ultrafast pump-probe measurements have been carried out using the femtosecond pump-probe facility of the School. NM and AD thank the Council of Scientific and Industrial Research (CSIR) and the University Grants Commission (UGC), respectively, for Fellowships. Supporting information available: Synthesis of perovskite NCs, details of femtosecond pump-probe technique, PXRD patterns, TA spectra of NCs at different pump fluence, Biexciton decay profiles, bleach recovery kinetics of FAPbI3 NCs as a function of excitation laser fluence, Time-resolved PL decay parameters of the NCs.

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