Fast Dissociation and Reduced Auger Recombination of Multiple

Mar 24, 2015 - Exciton decay dynamics in chemically treated PbS quantum-dot (QD) films have been studied using femtosecond transient-absorption (TA) s...
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Fast Dissociation and Reduced Auger Recombination of Multiple Excitons in Closely Packed PbS Nanocrystal Thin Films Taishi Nishihara,† Hirokazu Tahara,† Makoto Okano,† Masashi Ono,‡ and Yoshihiko Kanemitsu*,†,§ †

Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Frontier Core-Technology Laboratories, Fujifilm Corporation, Ashigarakami-gun, Kanagawa 258-8577, Japan § Japan Science and Technology Agency, CREST, Kyoto University, Uji, Kyoto 611-0011, Japan ‡

ABSTRACT: Exciton decay dynamics in chemically treated PbS quantum-dot (QD) films have been studied using femtosecond transient-absorption (TA) spectroscopy. In photoconductive QD films, a decay component with a lifetime of a few nanoseconds appeared in the TA signals because of exciton dissociation under weak excitation. Increasing excitation fluence resulted in additional fast-decay components corresponding to the lifetimes of multiple excitons, which decreased with increasing photoconductivity of the closely packed QD films. Auger recombination in photoexcited QDs was suppressed in highly photoconductive films. Our findings clearly show that the carrier transfer between the QDs dominates the lifetimes of single and multiple excitons.

films: reduction of the nonradiative carrier recombination, increase of the mobility of carriers, and extraction of carriers confined in photoexcited QDs.19−34 In thin QD film structures, the sizes of surface ligands are of importance because they determine the interparticle spacing between the QDs. Shorter interparticle spacing results in the increase of overlap of the wave functions of adjacent QDs. Therefore, QD-based device structures treated with small and stable ligands are expected to be promising photovoltaic devices with high-efficiency energy conversion. Recently, it was reported that the QD films treated with small ligands are strongly coupled to each other, which increases the carrier mobility and carrier extraction from photoexcited QDs.20,29,30 A significant photocurrent enhancement is also observed in PbS QD films chemically treated by a small and inorganic ligand, e.g., potassium thiocyanate (KSCN).34 However, the dynamics of multiple excitons in chemically treated QD films remain unclear. Understanding the dynamics of multiple excitons is essential to realize highefficiency QD-based solar cells based on MEG processes. In this study, we investigated the exciton dynamics in chemically treated PbS QD films using pump−probe transientabsorption (TA) spectroscopy. For strongly coupled photoconductive QD films, TA signals show a fast decay in the subnanosecond time region even under weak photoexcitation. With increasing excitation fluence, additional fast-decay components, which are related to the decay of multiple excitons, appear in TA signals. The decay rate of multiple excitons depends on the surface treatment of QD films and increases with the film photoconductivity. We concluded that

O

ver the past two decades, semiconductor nanomaterials such as nanoparticles and carbon nanotubes have attracted much attention as promising solar-cell materials because they have unique optoelectronic properties and the potential to overcome the conversion efficiency limit of singlejunction solar cells (the Shockley−Queisser limit).1 Solar cells based on semiconductor nanomaterials take advantage of unique light-energy conversion processes such as multiple exciton generation (MEG)2−6 and upconversion.7−10 In the MEG process, a single high-energy photon creates two or more excitons (electron−hole pairs) using the excess energy, which results in higher conversion efficiency. The MEG process can be regarded as the inverse of the Auger recombination process,5 where, in Auger recombination, the electron−hole recombination energy is transferred to a third electron (or hole). Enhanced carrier−carrier interactions in nanomaterials play important roles in the MEG and Auger recombination processes.4,5 A precise understanding of the dynamics of multiple carriers in nanostructures is essential for realizing MEG-enhanced solar cells. Signatures of MEG and quantized Auger recombination processes have been identified in colloidal semiconductor quantum dots (QDs) using optical11−15 and photocurrent measurements.16 Strong carrier−carrier interactions and the breakdown of momentum conservation are caused by the spatial confinement of carriers within QDs. It is well-known that PbSe and PbS QDs show low MEG thresholds and high MEG efficiencies.2,3,11,16,17 MEG has been reported not only in solution but also in closely packed PbSe and PbS QD films, and the enhancement of photovoltaic efficiency and photocurrent have been experimentally demonstrated.3,16−18 In QD-based films, surface modifications of QDs are essential to improve the photoelectrical properties of closely-packed QD © 2015 American Chemical Society

Received: February 10, 2015 Accepted: March 24, 2015 Published: March 24, 2015 1327

DOI: 10.1021/acs.jpclett.5b00293 J. Phys. Chem. Lett. 2015, 6, 1327−1332

Letter

The Journal of Physical Chemistry Letters

electronic interactions between the QDs exist in the untreated film and that exciton recombination occurs individually within each photoexcited QD. In the chemically treated films, the TA signal decays faster than that in the untreated film. A new decay channel of carriers appears in the chemically treated films. We extracted the decay lifetime τ1 by fitting the decay curves using a single exponential function. The fitting results are shown as solid curves in Figure 1b, and they reproduce the experimental results well. Figure 1c shows the correlation between the film photoconductivity and the decay rate (τ1)−1. The values of photoconductivity in Figure 1c are taken from ref 34, where we reported that photoconductivity and dark conductivity increase with decrease in interparticle spacing between the QDs. Among the three films, the KSCN- and EDT-treated films show the highest and lowest photoconductivities, respectively. The carrier lifetime shows a clear correlation with the photoconductivity; the carrier decay rate increases monotonically with photoconductivity. As shown in Figure 1a, the lowest-energy exciton absorption of the chemically treated films shows a broad spectrum, indicating inhomogeneous energy distribution due to variations of the QD size. We found that a carrier transfer process occurs in the chemically treated films.34 The carrier transfer from monitored QDs to QDs with different sizes contributes to the decay of TA signal intensities. In a previous study, we reported that larger photocurrents are generated in KSCN- and AE-treated films than in an EDT-treated film.34 This large photocurrent generation is ascribed to the increase in carrier transfer rate caused by the reduction of the interparticle spacing between the QDs. We conclude that the electronic interaction between the QDs affects the exciton dissociation due to carrier transfer between the QDs. To gain a precise and detailed understanding of exciton decay dynamics in closely packed QD films, we examined TA decay dynamics for various excitation fluences. Figure 2a,b show TA decay curves for the EDT- and KSCN-treated films at different excitation fluences for 2Eg excitation. An increase in the excitation fluence causes the appearance of fast-decay TA components in both the EDT- and KSCN-treated films. These results demonstrate that the lifetimes of excitons decrease on increasing the number of excitons in a single QD. To evaluate the dynamics of carriers in a single QD including multiple excitons, we performed the global fitting of the TA decay curves with different excitation fluences. The TA decay curves are well reproduced by the triple exponential functions: A1exp(−t/τ1)+A2exp(−t/τ2)+A3exp(−t/τ3). The fitting results are shown as solid curves in Figure 2a,b. Under weak excitation fluences, the TA dynamics approximately show a single exponential decay with a lifetime of τ1. As the excitation fluence increases, first, a fast decay component with a lifetime of τ2 appears, followed by τ3 in the chemically treated films. The dynamics of multiple-exciton recombination is also derived using a simple subtraction procedure.35 According to the simple subtraction procedure proposed in ref 35, we subtracted the TA decay curves at lower fluences from those at higher fluences to obtain the lifetimes of multiple excitons in a single QD. Figure 2c,d show the decay curves derived from the simple subtraction procedure. For each film, two single-exponential decay curves were obtained. This procedure for the evaluation of two decay times confirms the above global fittings. We assigned the longer lifetime τ2 and the shorter lifetime τ3 to the exciton lifetimes in a single QD including biexcitons and triexcitons, respectively. The decay times depend on the number of excitons initially

the lifetimes of multiple excitons are affected by the carrier transfer between adjacent QDs in the films. The enhanced dissociation of multiple excitons causes the reduction of nonradiative Auger recombination in photoexcited QDs and the enhancement of carrier extraction from photoexcited QDs, leading to high photocurrent due to MEG in QD films. We prepared 13 types of PbS QD films treated with different ligands and measured their conductivities. Since ethane-1,2dithiol (EDT)-, 2-aminoethanol (AE)-, and KSCN-treated films showed high conductivities,34 we used these three films in this study. Typical photoluminescence (PL) and optical absorption spectra of the EDT-, AE-, and KSCN-treated films are shown in Figure 1a. The optical absorption and PL peaks around 1.0−1.3

Figure 1. (a) PL (broken curves) and optical absorption (solid curves) spectra of the EDT-, AE-, and KSCN-treated films. The spectra are offset for clarity. (b) Carrier relaxation dynamics for the chemically treated and untreated films measured using TA spectroscopy. The decay curves are offset for clarity. The solid curves are the fitting curves. (c) Correlation between the film photoconductivity and the decay rate of carriers for various QD films. The dotted line is a guide for the eye.

eV are attributed to the lowest-energy excitons in all PbS films. Changes in absorption and PL energies in these films are correlated with the interparticle spacing between the QDs determined from X-ray and transmission electron microscopy.34 We studied these three chemically treated films by TA measurements. The decay dynamics of single excitons are sensitive to the chemical treatment of the QD film surface. Figure 1b shows TA decay curves of the lowest-energy excitons in the EDT-, AE-, and KSCN-treated and untreated films under weak excitation. The excitation energies were 2Eg, where Eg is the peak energy of the lowest-energy exciton in each film. The excitation fluences were 2−7 μJ/cm2 for all films. The TA signal intensities were normalized at zero time delay. For all the PbS QD films, the photoinduced transmission changes ΔT/T are positive because of the photobleaching of the lowest exciton state. As shown in Figure 1b, the TA signal for the untreated film was almost constant in the picosecond time region, up to 1 ns. Thus, the rate constant of the untreated film cannot be estimated in this experiment. We also observed that the TA signal for the untreated film is identical to that for isolated PbS QDs in octane solutions, which implies that no significant 1328

DOI: 10.1021/acs.jpclett.5b00293 J. Phys. Chem. Lett. 2015, 6, 1327−1332

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photoconductivity. The reproducibility of these trends was confirmed using different samples. To understand the decay mechanism of multiple excitons, we discuss the excitationfluence dependence of TA signal intensities at a longer delay time. Figure 3b shows the excitation-fluence dependence of TA signal intensities at a delay time of 1000 ps for EDT- and KSCN-treated films. In the EDT-treated film, the TA signal intensity increased and then saturated at approximately 200 μJ/ cm2. It is known that in isolated QDs, the saturation behavior of the TA signal intensity at longer delay times is caused by fast Auger recombination.36 Figure 3c shows a schematic view of carrier relaxation processes in isolated QDs films. When the fast recombination of multiple excitons occurs in highly photoexcited QDs, almost all photoexcited QDs include only a single exciton at a long delay time. Therefore, TA signal intensities at a longer delay time are determined by the initial number of photoexcited QDs immediately after photoexcitation. The probability of the initial number of photogenerated excitons is described by the Poisson distribution, Pn (⟨n0⟩), where n and ⟨n0⟩ are the number of excitons and the average number of excitons per QD immediately after photoexcitation, respectively. ⟨n0⟩ is given by the product of the photon fluence J and the absorption cross section σ.36 Thus, TA signal intensities at 1000 ps are given by

Figure 2. Excitation-fluence dependence of temporal changes in the TA signals of (a) EDT- and (b) KSCN-treated films. The solid curves are the fitting results. Differential TA curves obtained through a simple subtraction procedure for (c) EDT- and (d) KSCN-treated samples: decay curves due to exciton recombination in a single QD including biexcitons (2Ex) and triexcitons (3Ex).

∞

ΔT /T (t = 1000 ps) ∝

∑ Pn(⟨n0⟩) = 1 − exp(−Jσ ) n=1

(1)

generated in a QD, and the discretization (quantization) of the decay times occurs similar to the case of isolated QDs. The decay rates of biexcitons and triexcitons are summarized as a function of the film photoconductivity in Figure 3a. The decay rates of multiple excitons show clear correlations with photoconductivity, similar to the case of single exciton decays; the decay rates of biexcitons and triexcitons increase with

The solid curve in the upper panel of Figure 3b follows eq 1, suggesting that the multiple exciton dynamics can be well described by Auger recombination in the EDT-treated film, for which the obtained absorption cross section is σ = 6.2 (± 0.3) × 10−15 cm2. The decay behaviors of multiple excitons in the EDT-treated film were similar to those of isolated QDs, indicating weak electronic interactions between adjacent QDs.

Figure 3. (a) Correlation between the film photoconductivity and the decay rates of biexcitons (blue circles) and triexcitons (red circles). The dotted lines are guides for the eye. (b) TA signals of EDT- (upper panel) and KSCN-treated (lower panel) samples as functions of the excitation fluence (J) at a delay time of 1000 ps. The solid curves are the fitting results. Schematic views of carrier relaxation processes in (c) isolated QDs and (d) coupled QD films. 1329

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The Journal of Physical Chemistry Letters In the KSCN-treated film, the excitation-fluence dependence of the TA signal at 1000 ps is entirely different from that of the EDT-treated film; the TA signal intensity increased with excitation fluence and showed no clear saturation behavior even under high excitation fluences. This result indicates that Auger recombination is not the only determinant of the decay processes of multiple excitons in the KSCN-treated film. This indication is supported by the correlation between the film photoconductivity and the decay rate of multiple excitons in photoexcited QDs shown in Figure 3a. Trap-assisted Auger recombination and surface trapping rates can be reduced in the highly photoconductive films, because of the reduction of midgap trap states by the passivation of the dangling bonds at the surface.34,37 The increase of multiple exciton decay rates in the KSCN-treated film implies the existence of additional decay processes. A possible decay process of multiple excitons is the exciton dissociation, because exciton dissociation is the dominant decay process of single excitons in the KSCN-treated film. To explain no-saturation behavior of the excitation-fluence dependence of the TA signal at 1000 ps in the highly photoconductive KSCN-treated film, we propose here a simple model accounting for the carrier transfer. Figure 3d shows a schematic view of this simple model. In KSCN-treated films, fast carrier transfer to adjacent QDs occurs as a result of the electronic interparticle interactions because the length between adjacent QDs is shorter than that in EDT-treated films. Multiple excitons in a single photoexcited QD are dissociated into the nearest-neighbor QDs. The influence of this carrier transfer can be expressed by modifying the TA signals, as discussed with only Auger recombination. It is assumed that the carrier transfer occurs with the probability k. When the carrier transfer in a single QD including n excitons generates n QDs including a single exciton, the carrier transfer component kPn(⟨n0⟩) contributes to the TA signals as knPn(⟨n0⟩) at 1000 ps. Therefore, under the above simple assumption, the TA signal intensities in the highly photoconductive QD films at 1000 ps are given by

Finally, we briefly comment on solar cells based on highly conductive films (e.g., KSCN-treated film). Immediately after the excitation of high-energy photons, the MEG process occurs and multiple excitons are formed. Before biexcitons relax to a single exciton through Auger recombination, exciton dissociation occurs rapidly via the tunneling of carriers to the adjacent QDs. There is no change in the number of carriers in the process of exciton dissociation. Therefore, the enhancement of the photocurrent is experimentally observed in the KSCNtreated film.34 The exciton dissociation process in a single QD including multiple excitons plays an important role in the enhancement of photocurrent due to MEG. In summary, we studied the exciton decay dynamics in chemically treated PbS QD films using femtosecond TA spectroscopy. For closely packed and photoconductive QD films, fast decay components with lifetimes of a few nanoseconds appeared in the TA signals under weak excitation. The difference in TA decay curves of various QD films indicates that the exciton decay depends on the conductivity of the films and corresponds to the carrier transfer between the adjacent QDs. Increasing excitation intensity results in the appearance of the fast decay components of the TA decay profiles on the order of subnanosecods, indicating that the decay rate of multiple excitons is also affected by the conductivity of the films.

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EXPERIMENTAL METHODS PbS QD film samples were fabricated using the layer-by-layer (LBL) method.19,34 The PbS QD films were deposited onto a glass substrate by spin-coating about 10 mg/mL QD-octane solution (the original ligand being oleic acid) at 2500 rpm. After the 100 mM ligand solution was poured onto the films, the films were left undisturbed for 1 min to displace originally bound oleic acids and subsequently spun at 2500 rpm to dry them. Then, the films were rinsed using a solvent and octane. Subsequently, the films were dried using a dryer for 10 s. The entire process was iterated 15 times to obtain smooth 90−100 nm-thick films with QDs bound to the ligands. Finally, the LBL films were annealed at 90 °C for 5 min to remove organic residues. Femtosecond TA spectroscopy was performed using a pump−probe technique. The light source was a wavelengthtunable optical parametric amplifier (OPA) based on a regenerative amplified mode-locked Ti:sapphire laser with a pulse duration of 150 fs and a repetition rate of 1 kHz. The probe−pulse energies were tuned to be resonant with the lowest exciton-absorption transition of each QD film. For PL measurements, the excitation light source was an OPA system based on a Yb:KGW (potassium gadolinium tungstate) regenerative amplified laser with a pulse duration of 300 fs and repetition rate of 200 kHz. PL emission was detected with a liquid-nitrogen-cooled InGaAs diode array. The laser spot size on the sample surface was measured carefully using the knifeedge method. All measurements were performed at room temperature.

ΔT /T (t = 1000 ps) ∞

∞

∝ (1 − k) ∑ Pn(⟨n0⟩) + k ∑ nPn(⟨n0⟩) n=1

n=1

= (1 − k)(1 − exp(−Jσ )) + kJσ

(2)

where the first and second terms denote the Auger recombination and dissociation of multiple excitons due to carrier transfer, respectively. The exciton dissociation due to carrier transfer does not change the total number of excitons. Therefore, the TA signal intensity shows no saturation behavior with increasing excitation fluence. In the lower panel of Figure 3b, the solid curve is the fitting result with eq 2, which reproduces the experimental result very well. The absorption cross-section σ and the coefficient k were estimated to be 3.1 (± 1.5) × 10−14 cm2 and 0.067, respectively. Here, note that the value of k should be affected by the structural inhomogeneity (size and surface effects) and other complex processes (reversible and multiple carrier transfer). The enhancement of exciton dissociation in the KSCN-treated film results in the relative suppression of Auger recombination because rapid carrier transfer into neighboring QDs occurs as a result of the spatial extension of the wave functions in the QD films.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 1330

DOI: 10.1021/acs.jpclett.5b00293 J. Phys. Chem. Lett. 2015, 6, 1327−1332

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ACKNOWLEDGMENTS The authors would like to thank Dr. M. Suzuki and Dr. A. Tanaka of Fujifilm Corporation and Dr. T. Ihara of Kyoto University for experimental assistance. Part of this work was supported by JSPS KAKENHI (25247052), JST-CREST, and the MEXT Project of Integrated Research on Chemical Synthesis.

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