Effect of Halide Treatments on PbSe Quantum Dot ... - ACS Publications

Oct 5, 2015 - Australian Centre for Advanced Photovoltaics, School of Photovoltaic and Renewable Energy Engineering, and. ‡. School of. Chemistry, U...
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Effect of Halide Treatments on PbSe Quantum Dot Thin Films: Stability, Hot Carrier Lifetime, and Application to Photovoltaics Zhilong Zhang,† Jianfeng Yang,† Xiaoming Wen,† Lin Yuan,† Santosh Shrestha,† John A. Stride,‡ Gavin J. Conibeer,† Robert J. Patterson,† and Shujuan Huang*,† †

Australian Centre for Advanced Photovoltaics, School of Photovoltaic and Renewable Energy Engineering, and ‡School of Chemistry, UNSW Australia, Sydney, New South Wales 2052, Australia S Supporting Information *

ABSTRACT: Air stability and efficient multiple exciton generation (MEG) are crucial for the application of PbSe quantum dot (QD) thin film in next generation photovoltaics. Recently it was reported that PbS QD thin films with solid-state halide atomic ligands exhibited superior performance in terms of stability and efficiency. There is great interest in applying these halide ligands to PbSe QD thin films to improve the stability, while their additional effects on the hot carrier dynamics and hence MEG efficiency remain unknown. Here, we demonstrate that proper halide treatments can modify both the stability and hot carrier thermalization of a PbSe QD thin film. This confirms that using proper halide ligands in the solid-state ligand exchange step for film fabrication can significantly improve the stability. The film subjected to an iodide treatment exhibited the best air stability, and additionally its hot carrier thermalization time can be three times longer than that with a chloride treatment. We suggest that stronger bonding between the QD surface and the halide ligand can lead to suppressed intermediate-state assisted hot carrier thermalization, while the difference in ligand atomic mass may also play an important role. We conclude that ligands in QD thin films have significant impact on the stability and hot carrier thermalization.

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coupling in PbSe QD-based devices.8,10 For QDSCs it is crucial to have strongly coupled QDs because it directly affects carrier transport within the QD thin film.11 Also, it was reported that PbSe QDs have more efficient MEG compared to PbS QDs because of their slower hot carrier thermalization rate,12,13 and so far the only reported lead chalcogenide QDSC with MEG enhanced photocurrent was based on PbSe QDs.14 However, despite of the interesting properties of PbSe QDs, they are very sensitive to air and quickly oxidize.15,16 It is essential to fabricate air-stable PbSe QD thin films, and at the same time their advantage in MEG must be maintained for the application in photovoltaics. Many researchers have reported halide (Cl, Br, I) treatments to PbS/PbSe QDs and succeeded in producing air-stable QD dispersions, thin films, and high-efficiency QDSCs.8,16−26 The halide treatments can be implemented either to form a thin halide layer adsorbed on QD surface during synthesis (in situ) or as atomic ligands after solid-state ligand exchange during thin film fabrication. So far many reported halide treatments on PbSe QDs were based on in situ methods,8,16−18,22 while airstable PbS QDSCs with world recorded PCEs at the time were also reported by applying solid-state ligand exchange with

INTRODUCTION Colloidal quantum dots (QDs) are semiconductor nanocrystals that exhibit quantum confinement effects. Due to the tunable band gap and low-cost fabrication through wet chemical synthesis, they are promising materials for further cost reduction of thin film photovoltaic devices.1 In addition, it has been predicted and experimentally demonstrated that lead chalcogenide QDs (PbX, X = S, Se, Te) have efficient multiple exciton generation (MEG) or carrier multiplication (CM).2−4 This effect involves a high-energy photon from solar radiation generating two or more electron−hole pairs in a QD-based solar cell (QDSC). Hence it offers a theoretical power conversion efficiency (PCE) limit over 40%,5,6 compared to the Shockley−Queisser limit of 33% for a single junction device.7 Because of the potential for cost reduction and high theoretical device PCE, lead chalcogenide QDs are key candidate materials for next generation photovoltaic cells. In recent years the majority of research in QDSCs has been focused on PbS QDs because of the advantage of air stability compared to other lead chalcogenide QDs.8 The PCE of PbS QDSCs has been significantly improved after the first certified cell in 2010 and now is approaching 10%.9 Compared to PbS, PbSe has a larger Bohr radius and therefore stronger degree of quantum confinement effect for QDs of the same size. This should result in a more significant wave function overlap among QDs, and hence it is believed to have stronger electronic © 2015 American Chemical Society

Received: August 18, 2015 Revised: October 3, 2015 Published: October 5, 2015 24149

DOI: 10.1021/acs.jpcc.5b08021 J. Phys. Chem. C 2015, 119, 24149−24155

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The Journal of Physical Chemistry C halides.19−22,24 By such a method the atomic halide ligands within the QD thin film serve as protective layers, and hence the stability is improved. Also this may result in a shorter interdot distance by using atomic halide ligands compared to the commonly used organic ligands and hence better electronic coupling.27 Moreover, it was reported that surface ligands on cadmium selenide (CdSe) QDs can affect hot carrier thermalization (in dispersion).28 Since MEG efficiency is dependent on the competition between impact ionization and hot carrier thermalization,12,13 this implies the possibility of modifying MEG efficiency in QD thin films through careful selection of ligands. Here, we investigate the effect of halide treatments on PbSe QD thin films by comparing their stability as well as the hot carrier thermalization time. In this report the stability is defined as the resistance to oxidation. PbSe QDs with an average diameter of 5.2 nm were first synthesized and then used to fabricate thin films with different ligands. The halide ligands were applied by using tetrabutylammonium halide (fluoride, bromide, chloride, and iodide) solutions for the solid-state ligand exchange step of film fabrication. Improvements in stability, as well as longer-lived hot carrier features, were observed in bromide-, chloride-, and iodide-treated samples. The mechanism of the effect of halide treatments is discussed in this report.

followed by spinning at 2500 rpm (repeated twice). The rinsing process is supposed to remove the OA being replaced, as well as the tetrabutylammonium/OA compound formed during the ligand exchange process.19 For comparison one sample was fabricated with the same steps; however, EDT ligand solution (0.02 vol % in acetonitrile) and acetonitrile were used for ligand exchange and rinsing. Another sample was fabricated without ligand exchange and rinsing steps, so the OA ligands were remained. All samples were stored in an inert gas environment before the first measurement. The samples are denoted as PbSe−OA, PbSe−EDT, PbSe−F, PbSe−Cl, PbSe−Br, and PbSe−I representing the QD thin film with OA, EDT, fluorine, chlorine, bromine, and iodine ligands, respectively, for the remainder of this report. Absorption Spectra Measurement. The stability of the thin films can be tested by monitoring the position and the width of the first exciton peak in the absorption spectra over time.15,16,30 The optical absorption spectra of the QD thin films were measured by PerkinElmer Lambda1050 UV/vis/NIR spectrophotometer in ambient condition. The samples were stored in air after the first measurement. The first measurement was conducted approximately 4 h after film fabrication and repeated after 3, 4, 5, and 90 days. Fourier Transform Infrared Spectroscopy (FTIR) Measurement. FTIR spectra of the samples were measured to confirm that the as-synthesized ligands (OA) on the QDs were successfully exchanged. The measurements were conducted by PerkinElmer Spotlight 400 FTIR spectrometer with attenuated total reflectance (ATR) mode. X-ray Photoelectron Spectroscopy (XPS) Measurement. XPS measurement was conducted to identify the presence of halide ligands within the samples after ligand exchange. It also provided qualitative indications showing that the formation of lead oxide compounds in some samples. The measurement was performed on a Thermo Scientific ESCALAB250Xi, with a monochromatic Al X-ray source. The power was 150 W, and spot size was 500 μm. The QD thin films for XPS measurement were fabricated on silicon wafer substrates. Ultrafast Transient Absorption (TA) Measurement. The hot carrier dynamics of the samples, including the occupying energy levels and thermalization time at higher energy states above the first excited state, were characterized by ultrafast TA spectroscopy. The experiments were performed with a TA spectrometer (Helios, Ultrafast Systems). The laser system consists of a Ti:sapphire oscillator (Spectra-Physics, Tsunami), which seeded a regenerative amplifier (SpectraPhysics, Spitfire Pro XP). The output of the amplifier (800 nm with a repetition rate of 1 kHz and pulse duration of 100 fs) was then split into pump and probe beamlines. The 400 nm pump pulses were generated using a type-I BBO crystal, and the pump fluence was 30 μJ/cm2. The probe beam passed through a delay stage and was used to generate a white light continuum in a 13 mm sapphire crystal. The experiment was performed at room temperature.

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EXPERIMENTAL SECTION Chemicals. Lead oxide (PbO, 99.99%), selenium powder (Se, 99.99%), trioctylphosphine (TOP, technical grade, 90%), oleic acid (OA, technical grade, 90%), 1-octandecene (ODE, technical grade, 90%), tetrabutylammoium fluoride hydrate (TBAF, 98%), tetrabutylammoium chloride (TBAC, 97%), tetrabutylammoium bromide (TBAB, 99%), tetrabutylammoium iodide (TBAI, 98%), 1,2-ethanedithiol (EDT, technical grade, 90%), octane (98%), and acetonitrile (anhydrous, 99.8%) were purchased from Sigma-Aldrich. Methanol (anhydrous), ethanol (absolute), and hexane (95%) were purchased from Chem-Supply Pty Ltd., Australia. All chemicals were used as received. Synthesis of PbSe QDs. The PbSe QDs were synthesized according to the method reported by Yu et al. with some modifications.29 Typically, 0.892 g of PbO, 2.825 g of OA, and 12.5 g of ODE were loaded in a 50 mL three-neck flask and degassed for 1 h under stirring. The mixture was then heated to 150 °C in nitrogen environment to form a colorless lead oleate solution and was further heated to 170 °C for hot injection. TOPSe was prepared by dissolving 0.64 g of Se powder in 6 mL of TOP within an inert gas environment, and it was quickly injected into the lead oleate solution. The temperature of the mixture dropped to approximately 150 °C upon hot injection and was kept for 30 s. The mixture was then cooled down to room temperature by a water bath. The PbSe QDs were purified twice by centrifugation using hexane and ethanol and finally redispersed in octane at a concentration of approximate 50 mg/mL. PbSe QD Thin Film Fabrication. The PbSe QD thin films were fabricated by spin-coating method. For each layer the PbSe QD dispersion in octane was applied to cover the surface of a precleaned glass or silicon substrate and spun dried at 2500 rpm for 20 s. For halide treatments, the halide ligand solution (TBAF, TBAC, TBAB, and TBAI, 30 mM in methanol) was applied on the thin film for 30 s and then spun dried at 2500 rpm for 20 s. The film was further rinsed with methanol

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RESULTS AND DISCUSSION The as-synthesized PbSe QDs were capped by OA ligands and stabilized in hydrophobic solvents. The average size of the monodisperse QDs is approximately 5.2 nm in diameter, as shown in Figure 1a. The first exciton peak of the QDs in dispersion is at about 1520 nm, which corresponds to band gap energy of 0.82 eV, and it is optimal for MEG.5,6 In this report 24150

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Figure 1. (a) Transmission electron microscope (TEM) image of the as-synthesized PbSe QDs. (b) Absorbance of the as-fabricated PbSe QD thin film (PbSe−Cl) as the reference in the stability study.

solid-state QD thin films were studied; therefore, the reference spectrum for stability comparison was chosen from the asfabricated thin film samples instead of the dispersion, since the spectrum of the QDs may vary slightly after ligand exchange.31 PbSe−Cl initially had its first exciton peak at about 1512 nm (Figure 1b), which is the closest to the QD dispersion. Because of this it was chosen as a reference for other measurements in the stability study. During thin film fabrication, the ligand exchange process is supposed to replace the OA ligands on the as-synthesized PbSe QDs by the EDT or halide ligands used in this study. As shown in Figure 2a, the FTIR measurements demonstrated significant reduction in the C−H stretching peaks at 2854, and 2929 and 2961 cm−1 for the thin films with halide and EDT treatments, which suggests that the OA ligands were successfully removed.32 This is consistent with the XPS results for the C 1s peak of the samples, which indicated significant increase in Pb/COA ratio after halide treatments (see Figure S1 in the Supporting Information). In XPS measurements clear peaks corresponding to the halides (F 1s, Cl 2p, Br 3d, and I 3d) could also be observed, which indicates the existence of the halide ligands in the thin films after ligand exchange, as shown in Figure 2b−e. From both the FTIR and XPS results, it is concluded that the PbSe QDs within the thin films were capped by halide ligands after the solid-state ligand exchange process. Since oxidation of QDs shrinks effective cores and hence leads to higher degree of quantum confinement and band gap energy,30 we studied the stability of the PbSe QD thin films by monitoring their first exciton peak positions in absorption spectra, as well as comparing the peak broadening. The thin film samples were stored in an inert gas environment after fabrication; however, they were exposed to air during and after the first measurement, which was conducted 4 h after film fabrication. Figure 3a summarizes the shifts of the first exciton peak positions for the five samples. It is obvious that the blue

Figure 2. (a) FTIR spectra of the PbSe QD thin films with different ligands. The reduction in C−H stretching peak indicates that the OA ligands on the QD surface were successfully removed. (b)−(e) XPS results confirmed the existence of halogens (fluorine, chlorine, bromine, and iodine, respectively) within the thin films after ligand exchange.

shifts in PbSe−EDT and PbSe−F were much more significant compared to that of PbSe−Cl, PbSe−Br, and PbSe−I. PbSe− EDT had a significant shift (−68 nm) in its first exciton peak for the first measurement, which indicates the sample suffered serious oxidation within only a few hours. The absorption feature of PbSe−EDT over time showed continuous blue shifts and broadening in its first exciton peak, and eventually the peak feature became unrecognizable after 90 days, as shown in Figure 3b. Similar to PbSe−EDT, the absorption spectra of PbSe−F initially had an obvious peak shift (−80 nm), indicating that it was also being oxidized quickly. XPS result also shows that the Pb 4f peaks in PbSe−F suffered significant broadening and shift (Figure 4), which can be attributed to the formation of lead oxide.16 In addition to shifts to higher energies, the exciton peak broadening was more significant in PbSe−F than PbSe−EDT for the first 5 days, as shown in Figure 3c. We attribute the peak broadening to the overlaps of absorption features from QDs with different degrees of oxidation. Therefore, it is suggested that PbSe−F oxidized in a way such that was less uniform compared to PbSe−EDT for 24151

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Figure 3. Time evolution of the absorbance of the PbSe QD thin films fabricated with five different ligands. (a) Summary of shifts in the first exciton peak position of the samples over time; the inset compares the peak width of PbSe−Cl, PbSe−Br, and PbSe−I after 90 days. (b)−(f) Absorbance evolution of PbSe−EDT, PbSe−F, PbSe−Cl, PbSe−Br, and PbSe−I, respectively.

The air stabilities of the thin films were significantly improved after chloride, bromide, and iodide treatments. As shown in Figure 3a and d, although with minor peak broadening, PbSe−Cl did not have blue shift in its first exciton peak even after 4 days, and on the fifth day the shift was still minor (−4 nm). After 90 days the peak shifted about −58 nm from its original position, and peak broadening became obvious, which was likely caused by nonuniform oxidation of the QDs. As shown in Figure 3e−f, PbSe−Br and PbSe−I first exhibited minor peak shifts (−4 and −14 nm); however, the oxidizing process was also very slow in the first 5 days, which indicates the thin films were relatively stable. As shown in Figure 4, from XPS measurement the Pb 4f peaks in PbSe−I were very narrow and no splitting can be observed, indicating that the oxidation of the film was almost negligible. To further elucidate the stability of PbSe−Cl, PbSe−Br, and PbSe−I over a longer term, their absorption features after 90 days were compared. Interestingly the final positions of the first exciton peaks of all three samples were almost the same, as shown in Figure 3a. Moreover, the exciton peak broadening was less significant in PbSe−Br and PbSe−I compared to PbSe−Cl, and PbSe−I maintained the narrowest peak, as shown in the inset of Figure 3a. From this observation, we conclude that the chlorine, bromine, and iodine atomic ligands in the QD thin films significantly improved the stability, and within a relatively short period chlorine seems to provide the most stable film according to the position of first exciton peak. However, over a period of 90 days because of the same final position of first exciton peaks in all three samples, as well as the peak in PbSe−I being the narrowest, we clarify that the stability was indeed the best in PbSe−I. This is consistent with reported high performance and

Figure 4. Pb 4f peak information on PbSe−F and PbSe−I obtained from the XPS measurements (7 days after fabrication). The peaks in PbSe−I maintained narrow and in position, while for PbSe−F there is clear peak broadening (splitting) and shifts. Such changes in the Pb 4f peaks suggest the formation of lead oxide.

the first 5 days. Interestingly, as shown in Figure 3a the blue shift in the first exciton peak of PbSe−F slowed down after 1 day and remained less significant compared to PbSe−EDT, which suggests the fluorine atomic ligands may partially improve the stability to some degree after initial oxidation. However, since its blue shift and peak broadening were still significant over the study period, we conclude that neither EDT nor fluoride treatment can produce stable PbSe QD thin films. 24152

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Figure 5. Ultrafast transient absorption (TA) spectroscopy measurement results for (a) PbSe−Cl, (b) PbSe−Br, and (c) PbSe−I in their higher energy states. ΔOD stands for change in optical density; negative value of ΔOD in the figure denotes bleaching, i.e., increase in carrier occupation near the second (1200 nm) and third (950 nm) excited states of the QD thin films.

stable QD dispersion and QDSCs with iodide treatment.20−22,24,26 In previous reports it was suggested that for PbS QDs, Pb2+ has stronger bonding with I− compared to Cl− and Br− according to the hard soft acid base (HSAB) theory and density functional theory (DFT) calculations.16,20,33 Hence the PbI2 layer formed on the QDs surface is more stable and resistant to oxidation. Also larger iodine atoms can form a complete protective layer on the QD surface.20,33 We suggest these mechanisms also apply to the halide-treated PbSe QD thin films investigated in this report. We studied hot carrier relaxation of the samples by ultrafast TA spectroscopy and discovered that PbSe−Cl, PbSe−Br, and PbSe−I showed very obvious hot carrier features above the band gap (corresponding to 1520 nm) of the QDs. Two absorption bleaching peaks (negative ΔOD) were detected clearly in all of the three stable samples (PbSe−Cl, PbSe−Br, PbSe−I) around 1200 and 950 nm as shown in Figure 5a−c, which correspond to the second and third excited states of the QDs (as denoted in Figure 1b). Such clear bleaching at the higher energy states corresponds to hot carrier occupation at these energy levels within the first few picoseconds after the excitation and therefore reduces the optical transition. The most popular hot carrier theory in QD system is the so-called “phonon-bottleneck” effect. In this theory, the enlarged electronic intraband spacing of carriers by the spatial quantum confinement in QD prohibits the carrier relaxation by single longitudinal optical (LO) phonon emission.28,34 Therefore, the excited carriers can have a longer dwell time at those highenergy quasi-quantized states in QD thin films, e.g., second and third excited states as shown in Figure 5. However, since we did not observe such hot carrier dynamics in PbSe−EDT and PbSe−F, and because the dynamics of bleaching in PbSe−Cl, PbSe−Br, and PbSe−I is different (given the same excitation condition), we conclude that the nature of the different halide ligands also has significant effect on the hot carrier dynamics in the PbSe QD thin films. In order to reveal the effect of different halide ligands on hot carrier dynamics in the thin films, we compared the hot carrier thermalization time at the second and third excited states in PbSe−Cl, PbSe−Br, and PbSe−I. The thermalization times are obtained from the decay of the bleaching signals directly. The observed decay time τ as shown in Figure 6 denotes the average dwell time of the carriers at these higher excited states. Apart from the carrier thermalization, this dwell time is also influenced by carrier relaxation mechanisms at lower energy

Figure 6. Hot carrier thermalization dynamics in PbSe−Cl, PbSe−Br, and PbSe−I at the (a) second excited state (1200 nm) and (b) third exciton state (950 nm). The indicated thermalization times are obtained by fitting exponential functions to the TA data.

levels, such as recombination at the ground state. However, typically the relaxation of carriers at the ground state is much slower than the decay observed here. Therefore, it is reasonable to approximate this average dwell time to the carrier thermalization time. The hot carrier thermalization times near the second excited state are 1.9, 3.9, and 5.9 ps for PbSe−Cl, PbSe−Br, and PbSe−I respectively, as shown in Figure 6a. This indicates the halide ligands in PbSe QD thin film definitely have significant effects on the hot carrier thermalization. From the XPS results the atomic ratios of Pb/Cl, Pb/Br, and Pb/I were approximately 2.5, 2.8, and 2.4. Using bulk constant of PbSe, the surface Pb atom to halide ratios (chloride, bromide, and iodide correspondingly) were estimated to be 0.83, 0.93, and 0.80 for a 5.2 nm PbSe QD. The ratios suggest that the degree 24153

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advantage in shorter interdot distance and hence stronger electronic coupling by atomic ligands,27 we conclude that the iodine ligand has great potential to produce highly efficient PbSe QDSCs. Finally, from our observations within this study, we suggest that it should be variable to study other ligands that have similar properties to iodine when coordinating with PbSe QDs. For example, ligands that can bind more strongly than others with the PbSe QD surface may introduce improvements to stability and hot carrier thermalization time of the thin film. Heavier ligands may also have similar effects. However, if somehow the surface ligands can provide vibration modes such that couple well with the energy states of the QD, it may create an additional pathway for hot carrier thermalization.34 Therefore, more study is required to fully understand the selection criteria of ligands for PbSe QDSCs. Nevertheless, we have demonstrated the significance of ligand selections by comparing the important physical properties of different halide treated PbSe QD thin films.

of ligand exchange (or coverage) was similar after these three halide treatments, and it is unlikely to cause major difference in hot carrier thermalization time. Interestingly the improvement in thermalization time exhibits the same trend as the improvement of stability by chloride, bromide, and iodide treatments. A possible reason for the difference in hot carrier thermalization time is the modified surface state assisted carrier thermalization.35 The surface states of the PbSe QDs are shifted under the influence of the nearby ligands according to hybridization theory. In this case, it may lead to a reduced density of intermediate states due to an antibonding hybridization (between the first and second excited states) that can contribute to a suppression in intraband carrier relaxation and hence a longer hot carrier thermalization time. As mentioned previously, it is suggested that the bonding energy between Pb2+ and halides is stronger with I− than Cl− according to the HSAB theory and DFT calculation,20 and it is possible that there is a stronger/deeper hybridization (due to Pb−I bonding) in the PbSe QDs which effectively reduces the intermediate electronic states below the second excited state. As a result, the intermediate-state assisted hot carrier thermalization is suppressed more effectively in PbSe−I, compared to PbSe−Br and PbSe−Cl. Similar observation was reported on cadmium selenide (CdSe) QDs and other organic ligands.35 The hot carrier thermalization times at the third excited state also have the same trend, although the difference is less significant compared to the second excited state, and the lifetime is generally shorter, as shown in Figure 6b. Another possible reason for the difference in hot carrier thermalization time is the effect of increasing atomic mass from chlorine to bromine and then to iodine. Padilha et al. reported that hot carrier thermalization time ratio (from second to first excited state) for PbS, PbSe, and PbTe QDs is approximately τPbS:τPbSe:τPbTe = 1:2:4.12 Interestingly this is similar to the ratio of the anion atomic mass, i.e. mS:mSe:mTe = 1:2.5:4. Moreover, it was suggested that larger overall mass of the compound may result in longer LO phonon lifetime, and hence potentially longer hot carrier thermalization time.36 Therefore, it seems that higher atomic mass of the anions in lead chalcogenide QDs can result in longer hot carrier thermalization times, and the relationship seems to be directly proportional to the masses. From our study, the hot carrier thermalization time ratio from the second excited state is about τCl:τBr:τI = 1:2:3, while the atomic mass ratio for the halide ligands is mCl:mBr:mI = 1:2.3:3.6. The two ratios are also similar. Due to the large amount of QD interface area within the QD thin film compared to bulk material, we believe the nature of the surface ligands has significant impact on the overall hot carrier thermalization of the film, and the relationship may also be proportional to the atomic mass of the ligands. MEG efficiency is dependent on the competition between impact ionization and hot carrier thermalization, while the impact ionization rate is mainly dependent on the size of QD.12,13 Therefore, for thin films fabricated from the same batch of QDs, the MEG efficiency may be modified by engineering the carrier thermalization rate. From our results PbSe−I showed longer hot carrier thermalization time (i.e., slower rate) compared to PbSe−Cl and PbSe−Br; therefore, we suggest PbSe−I may have higher MEG efficiency, which should further increase the PCE of PbSe QDSCs. Also, we confirmed that iodide treatment produced the most air-stable QD thin film in a longer period. Because of the improvement in stability and potential for higher MEG efficiency, as well as the reported

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CONCLUSION In summary, we have demonstrated that proper halide treatments of PbSe QD thin films can improve the air stability and modify the hot carrier thermalization time. By monitoring the absorption spectra of the samples over time, we confirmed that the iodide treatment can produce the most air-stable QD thin film, while chloride and bromide treatments also significantly improve the stability. On the other hand, fluorine and the commonly used EDT ligands in PbSe QDSCs showed poor performance for thin film stability. We also discovered that there were obvious longer-lived hot carrier features in all of the three stable samples, while PbSe−I had the longest hot carrier thermalization time. It is suggested that the longer thermalization time is due to the stronger Pb2+ to halide bonding, which reduces the number of intermediate states for hot carrier thermalization from higher excited states. We also suggest that heavier atomic mass of the ligands on the QD surface may result in slower hot carrier thermalization. It is clear that the selection of ligands in PbSe QD thin film has significant effect on hot carrier dynamics and hence potentially on MEG. From the ligands studied in this work, because of the significant improvement in stability, as well as the advantage in slower hot carrier thermaliztion, we suggest that the use of iodine atomic ligands in PbSe QD thin films has great potential to produce air-stable, MEG-assisted high-PCE photovoltaic devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b08021.

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Additional XPS results showing the degree of ligand exchange (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +61-2-9385-5057. Fax: +61-2-9385-5104. Notes

The authors declare no competing financial interest. 24154

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(18) Bae, W. K.; Joo, J.; Padilha, L. A.; Won, J.; Lee, D. C.; Lin, Q.; Koh, W. K.; Luo, H.; Klimov, V. I.; Pietryga, J. M. Highly Effective Surface Passivation of Pbse Quantum Dots through Reaction with Molecular Chlorine. J. Am. Chem. Soc. 2012, 134, 20160−8. (19) Tang, J.; Kemp, K. W.; Hoogland, S.; Jeong, K. S.; Liu, H.; Levina, L.; Furukawa, M.; Wang, X.; Debnath, R.; Cha, D.; et al. Colloidal-Quantum-Dot Photovoltaics Using Atomic-Ligand Passivation. Nat. Mater. 2011, 10, 765−71. (20) Ning, Z.; Voznyy, O.; Pan, J.; Hoogland, S.; Adinolfi, V.; Xu, J.; Li, M.; Kirmani, A. R.; Sun, J. P.; Minor, J.; et al. Air-Stable N-Type Colloidal Quantum Dot Solids. Nat. Mater. 2014, 13, 822−8. (21) Chuang, C. H.; Brown, P. R.; Bulovic, V.; Bawendi, M. G. Improved Performance and Stability in Quantum Dot Solar Cells through Band Alignment Engineering. Nat. Mater. 2014, 13, 796−801. (22) Kim, S.; Marshall, A. R.; Kroupa, D. M.; Miller, E. M.; Luther, J. M.; Jeong, S.; Beard, M. C. Air-Stable and Efficient Pbse Quantum-Dot Solar Cells Based upon Znse to Pbse Cation-Exchanged Quantum Dots. ACS Nano 2015, 9, 8157−8164. (23) Oh, S. J.; Berry, N. E.; Choi, J. H.; Gaulding, E. A.; Lin, H. F.; Paik, T.; Diroll, B. T.; Muramoto, S.; Murray, C. B.; Kagan, C. R. Designing High-Performance PbS and PbSe Nanocrystal Electronic Devices through Stepwise, Post-Synthesis, Colloidal Atomic Layer Deposition. Nano Lett. 2014, 14, 1559−1566. (24) Crisp, R. W.; Kroupa, D. M.; Marshall, A. R.; Miller, E. M.; Zhang, J.; Beard, M. C.; Luther, J. M. Metal Halide Solid-State Surface Treatment for High Efficiency PbS and PbSe Qd Solar Cells. Sci. Rep. 2015, 5, 9945. (25) Marshall, A. R.; Young, M. R.; Nozik, A. J.; Beard, M. C.; Luther, J. M. Exploration of Metal Chloride Uptake for Improved Performance Characteristics of PbSe Quantum Dot Solar Cells. J. Phys. Chem. Lett. 2015, 6, 2892−9. (26) Sayevich, V.; Gaponik, N.; Plotner, M.; Kruszynska, M.; Gemming, T.; Dzhagan, V. M.; Akhavan, S.; Zahn, D. R. T.; Demir, H. V.; Eychmuller, A. Stable Dispersion of Iodide-Capped Pbse Quantum Dots for High-Performance Low-Temperature Processed Electronics and Optoelectronics. Chem. Mater. 2015, 27, 4328−4337. (27) Weidman, M. C.; Yager, K. G.; Tisdale, W. A. Interparticle Spacing and Structural Ordering in Superlattice Pbs Nanocrystal Solids Undergoing Ligand Exchange. Chem. Mater. 2015, 27, 474−482. (28) Peterson, M. D.; Cass, L. C.; Harris, R. D.; Edme, K.; Sung, K.; Weiss, E. A. The Role of Ligands in Determining the Exciton Relaxation Dynamics in Semiconductor Quantum Dots. Annu. Rev. Phys. Chem. 2014, 65, 317−39. (29) Yu, W. W.; Falkner, J. C.; Shih, B. S.; Colvin, V. L. Preparation and Characterization of Monodisperse Pbse Semiconductor Nanocrystals in a Noncoordinating Solvent. Chem. Mater. 2004, 16, 3318− 3322. (30) Weidman, M. C.; Beck, M. E.; Hoffman, R. S.; Prins, F.; Tisdale, W. A. Monodisperse, Air-Stable Pbs Nanocrystals Via Precursor Stoichiometry Control. ACS Nano 2014, 8, 6363−71. (31) Sandeep, C. S.; ten Cate, S.; Schins, J. M.; Savenije, T. J.; Liu, Y.; Law, M.; Kinge, S.; Houtepen, A. J.; Siebbeles, L. D. High ChargeCarrier Mobility Enables Exploitation of Carrier Multiplication in Quantum-Dot Films. Nat. Commun. 2013, 4, 2360. (32) Nag, A.; Kovalenko, M. V.; Lee, J. S.; Liu, W.; Spokoyny, B.; Talapin, D. V. Metal-Free Inorganic Ligands for Colloidal Nanocrystals: S2−, HS−, Se2−, Hse−, Te2−, HTe−, Tes3(2−), OH−, and NH2− as Surface Ligands. J. Am. Chem. Soc. 2011, 133, 10612−20. (33) Buonsanti, R.; Milliron, D. J. Chemistry of Doped Colloidal Nanocrystals. Chem. Mater. 2013, 25, 1305−1317. (34) Pandey, A.; Guyot-Sionnest, P. Slow Electron Cooling in Colloidal Quantum Dots. Science 2008, 322, 929−32. (35) Guyot-Sionnest, P.; Wehrenberg, B.; Yu, D. Intraband Relaxation in Cdse Nanocrystals and the Strong Influence of the Surface Ligands. J. Chem. Phys. 2005, 123, 074709. (36) Bhatt, A. R.; Kim, K. W.; Stroscio, M. A. Theoretical Calculation of Longitudinal-Optical-Phonon Lifetime in Gaas. J. Appl. Phys. 1994, 76, 3905−3907.

ACKNOWLEDGMENTS This Program has been supported by the Australian Government through the Australian Renewable Energy Agency. Responsibility for the views, information, or advice expressed herein is not accepted by the Australian Government. Authors also thank Prof. Tak W. Kee and Prof. Takaaki Harada of University of Adelaide for the access to ultrafast spectroscopy, Dr. Bin Gong of UNSW for XPS measurements, and the Electron Microscopy Unit of UNSW for TEM imaging support.

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REFERENCES

(1) Talapin, D. V.; Lee, J. S.; Kovalenko, M. V.; Shevchenko, E. V. Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications. Chem. Rev. 2010, 110, 389−458. (2) Nozik, A. J. Quantum Dot Solar Cells. Phys. E 2002, 14, 115− 120. (3) Beard, M. C.; Luther, J. M.; Semonin, O. E.; Nozik, A. J. Third Generation Photovoltaics Based on Multiple Exciton Generation in Quantum Confined Semiconductors. Acc. Chem. Res. 2013, 46, 1252− 60. (4) Semonin, O. E.; Luther, J. M.; Beard, M. C. Quantum Dots for Next-Generation Photovoltaics. Mater. Today 2012, 15, 508−515. (5) Hanna, M. C.; Nozik, A. J. Solar Conversion Efficiency of Photovoltaic and Photoelectrolysis Cells with Carrier Multiplication Absorbers. J. Appl. Phys. 2006, 100, 074510. (6) Klimov, V. I. Detailed-Balance Power Conversion Limits of Nanocrystal-Quantum-Dot Solar Cells in the Presence of Carrier Multiplication. Appl. Phys. Lett. 2006, 89, 123118. (7) Shockley, W.; Queisser, H. J. Detailed Balance Limit of Efficiency of P-N Junction Solar Cells. J. Appl. Phys. 1961, 32, 510−519. (8) Zhang, J.; Gao, J.; Church, C. P.; Miller, E. M.; Luther, J. M.; Klimov, V. I.; Beard, M. C. Pbse Quantum Dot Solar Cells with More Than 6% Efficiency Fabricated in Ambient Atmosphere. Nano Lett. 2014, 14, 6010−5. (9) Best Research-Cell Efficiencies, http://www.Nrel.Gov/Ncpv/ Images/Efficiency_Chart.Jpg (Accessed July 07 2015). (10) Shabaev, A.; Efros, A. L.; Efros, A. L. Dark and PhotoConductivity in Ordered Array of Nanocrystals. Nano Lett. 2013, 13, 5454−61. (11) Tang, J.; Sargent, E. H. Infrared Colloidal Quantum Dots for Photovoltaics: Fundamentals and Recent Progress. Adv. Mater. 2011, 23, 12−29. (12) Padilha, L. A.; Stewart, J. T.; Sandberg, R. L.; Bae, W. K.; Koh, W. K.; Pietryga, J. M.; Klimov, V. I. Carrier Multiplication in Semiconductor Nanocrystals: Influence of Size, Shape, and Composition. Acc. Chem. Res. 2013, 46, 1261−9. (13) Klimov, V. I. Multicarrier Interactions in Semiconductor Nanocrystals in Relation to the Phenomena of Auger Recombination and Carrier Multiplication. Annu. Rev. Condens. Matter Phys. 2014, 5, 285−316. (14) Semonin, O. E.; Luther, J. M.; Choi, S.; Chen, H. Y.; Gao, J.; Nozik, A. J.; Beard, M. C. Peak External Photocurrent Quantum Efficiency Exceeding 100% via Meg in a Quantum Dot Solar Cell. Science 2011, 334, 1530−3. (15) Sykora, M.; Koposov, A. Y.; McGuire, J. A.; Schulze, R. K.; Tretiak, O.; Pietryga, J. M.; Klimov, V. I. Effect of Air Exposure on Surface Properties, Electronic Structure, and Carrier Relaxation in PbSe Nanocrystals. ACS Nano 2010, 4, 2021−34. (16) Woo, J. Y.; Ko, J. H.; Song, J. H.; Kim, K.; Choi, H.; Kim, Y. H.; Lee, D. C.; Jeong, S. Ultrastable Pbse Nanocrystal Quantum Dots Via in Situ Formation of Atomically Thin Halide Adlayers on PbSe(100). J. Am. Chem. Soc. 2014, 136, 8883−6. (17) Zhang, J.; Gao, J.; Miller, E. M.; Luther, J. M.; Beard, M. C. Diffusion-Controlled Synthesis of Pbs and Pbse Quantum Dots with in Situ Halide Passivation for Quantum Dot Solar Cells. ACS Nano 2014, 8, 614−22. 24155

DOI: 10.1021/acs.jpcc.5b08021 J. Phys. Chem. C 2015, 119, 24149−24155