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Photoconductivity of CdTe Nanocrystal-based Thin Films: Te Ligands Lead to Charge Carrier Diffusion Lengths Over 2 µm Ryan Crisp, Rebecca A Callahan, Obadiah George Reid, Dmitriy S Dolzhnikov, Dmitri V. Talapin, Garry Rumbles, Joseph M. Luther, and Nikos Kopidakis

J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b02252 • Publication Date (Web): 16 Nov 2015 Downloaded from http://pubs.acs.org on November 17, 2015

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The Journal of Physical Chemistry Letters

Photoconductivity of CdTe Nanocrystal-based Thin Films: Te2- Ligands Lead to Charge Carrier Diffusion Lengths Over 2 µm Ryan W. Crisp,1,2,* Rebecca Callahan,1,3,* Obadiah G. Reid,4 Dmitriy S. Dolzhnikov,5 Dmitri V. Talapin,5 Garry Rumbles,1 Joseph M. Luther,1, ξ and Nikos Kopidakis1,ξ 1National

Renewable Energy Laboratory, Golden, CO, 80401 of Physics, Colorado School of Mines, Golden, CO, 80401 3Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309 4Renewable and Sustainable Energy Institute, University of Colorado at Boulder, Boulder, CO 80309 5Department of Chemistry, University of Chicago, Chicago, IL 60637 2Department

*These authors contributed equally to this work ξ Address correspondence to: [email protected], [email protected]

Abstract We report on photoconductivity of films of CdTe nanocrystals (NCs) using time-resolved microwave photoconductivity (TRMC). Spherical and tetrapodal CdTe NCs with tunable size-dependent properties are studied as a function of surface ligand (including inorganic molecular chalcogenide species) and annealing temperature. Relatively high carrier mobility is measured for films of sintered tetrapod NCs (4 cm2/Vs). Our TRMC findings show that Te2- capped CdTe NCs show a marked improvement carrier mobility (11 cm2/Vs) indicating that NC surface termination can be altered to play a crucial role in charge-carrier mobility even after the NC solids are sintered into bulk films. Introduction Semiconductor nanocrystals (NCs) offer many advantageous features for a variety of optoelectronic applications. Specifically, they can be solution-processed into a film where the benefits of quantum confinement can be exploited1-4 or they can be sintered into a bulk material where the NCs are used as an ink which can benefit from solution processing techniques to reduce manufacturing costs.5-7 The unique surface chemistry of NCs allows the possibility of new device structures and concepts such as p-n homojunction CdTe devices, CdTe absorber layers with tunable carrier concentrations from ligand-induced doping, and accurately tuned stoichiometry in quaternary compounds.8-11 However, colloidal NCs commonly display poor long-range (>500nm) transport properties as a result of complications due to energetic barriers between individual NCs from organic ligands and grain boundaries.11-14 Work on inorganic surface passivation of NCs opens new possibilities

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for absorber layers composed of nanocrystal arrays in conductive matrices.15, 16 Increasing the charge carrier mobility and lifetime of photogenerated charge carriers would enable optically dense absorber layers that could take advantage of the above-mentioned benefits of such absorber layers and efficiently extract all photogenerated charges.16-18 For quantum dot solar cells, which show beneficial multiple excitons produced from single photons, devices would greatly benefit from extracting charges from optically thicker layers.13, 19 In nanocrystal arrays, charge carrier mobility is largely constrained by an energetic barrier created by the encapsulating nanocrystal ligands. The native ligands (i.e. the ligands used to synthesize the NCs) often consist of a monolayer of long aliphatic moieties that bind to metal sites on surface of the NC. The ligand shell dictates the inter-nanoparticle spacing as well as the energetic barrier to charge transfer.20,

21

The effect of spacing on charge

transfer between nanocrystals has been previously well studied22,

23

and more recently

work has focused on quantifying the effects of the barrier height on charge transfer.24, 25 Our recent work has shown the performance of nanocrystal CdTe ink-based devices to be highly influenced by the nanocrystal shape.26 Similarly, Dayal et al. observed differences in charge transfer efficiency dependent on NC shape as well.27 New coupling strategies such as nanocrystals with inorganic ligands have demonstrated highly conductive films, some with reported “band-like” transport as indicated by temperature dependent mobility.8,

18, 28, 29

The use of nanocrystal-ligand systems offers the ability to further tune macroscopic properties by the adjusting NC-ligand matrix. CdTe represents a near-ideal model system to study because the Cd-based surface chemistry is well understood and controllable and it is applicable to highly efficient commercially-relevant devices. With the ever-growing library of ligand species available it is difficult to assess the feasibility of each ligand/NC combination (e.g. spheres, rods, tetrapods, etc.) in completed photovoltaic devices; thus, we employ a higher throughput unique optical technique to rapidly quantify and study the photogenerated carrier dynamics in nanocrystal arrays. Carrier mobility and conductivity measurements in electronic devices rely on contacts and interfaces that can have certain associated complications; e.g. Fermi level pinning, Schottky barriers, and shunting through pinholes or cracks. Furthermore, casting films from both very high vapor pressure (hexane) and low vapor pressure (N-


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methylformamide) solutions can lead to uneven covering of the substrate surface producing varying degrees of surface heterogeneity across the sample. As mentioned, a wide breadth of ligand systems have been developed to create high conductivity nanocrystal films but the direct measurement and assessment of each ligand system has so far been reliant upon optimization of the device architecture and film processing. In this work, time-resolved microwave conductivity (TRMC) is employed to study the effects of nanocrystal shape, size and ligand-type on photoconductivity. TRMC is a transient electrodeless technique that detects photoexcited carriers by their absorption of a 9 GHz CW microwave probe beam. As such, it can accommodate a variety of sample morphologies and electronic properties that would be difficult to measure via device techniques.30 TRMC has previously been applied to study photoinduced charge generation and charge carrier dynamics in conductive polymers, organic small molecules, dyesensitized TiO2, hybrid organic-inorganic heterojunctions, and, similar to this work, neat nanocrystalline films.30-33 For the purpose of our work here, TRMC provides a method for screening potential inorganic ligand/NC systems for high carrier mobility and directly comparing prospective ligands without the complications caused by electrical contacts. We find that annealing CdTe NCs with ligands of molecular chalcogens (specifically Te2-) leads to higher photoconductance than other CdTe NC-based films commonly used in photovoltaic devices.5 The observed increase in the mobility and lifetime of photogenerated carriers inspire the use of inorganic ligand chemistry for higher efficiency photovoltaic devices to increase the charge extraction.

Results and Discussion For purposes of comparing different ligand/NC systems, we are interested both in the peak photoconductance after laser excitation, and the lifetime of the carriers that result. The photoconductance due to a laser pulse (4 ns) is measured for sample inside a microwave cavity with a response-time of ca. 10 ns, leading to an under-reported value of the actual yield of photogenerated charges from the measured transient peak. In order to extract accurate values for both lifetime and initial photoconductance, we fit each transient to the sum of three exponentials, convoluted with the instrument response function (Eqn 1). We take the weighted average of the time constants as representative of the carrier


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lifetime, and the sum of the pre-exponential factors as a measure of initial photoconductance. We assume that the carrier mobilities (Σμ = µelectron+µhole) are constant in the measurement timescale (>1 ns). ∆G = −

3 − t 1 ∆P = IRF ⊗ ∑ ai e τ i K P i =1

Eq. 1

Here, the relative change of the microwave power in the cavity (P/P), as microwaves get absorbed by photoinduced free charges, is related to the photoconductance of the sample (G), using the measured sensitivity factor of the cavity, K. G can be expressed in terms of the product of the yield for free carrier generation per photon absorbed (ϕ) and the sum of the carrier mobilities (Σμ) by Eqn. 2, where β is the ratio between the short and long dimensions of the microwave waveguide, qe is the elementary charge, Io is the photon fluence incident on the sample and FA is the fraction of light absorbed by the sample at the excitation wavelength.

φΣµ =

∆G β qe I o FA

Eq. 2

This coupling of yield and mobility in the photoconductance transient, presents both a challenge and an opportunity. If one is known, the other can be calculated, but in the absence of constraining experimental or theoretical evidence, the two cannot in general be separated based on TRMC results alone. Also, G in principle includes contributions from both electrons and holes in the film. In the following we compare the photoconductance across a series of samples as a function of processing and choice of ligand, without the need to specify which sign of carrier dominates G. Photoconductance transients were measured across a range of light intensities, typically from 1012-1014 photons/cm2. In the majority of samples measured with the TRMC technique

a

nonlinear

relationship

exists

between

the

absorption-normalized

photoconductance and the light intensity. This has been attributed to nonlinear loss processes such as exciton-hole quenching,34, 35 or exciton-exciton quenching.36 An empirical equation modified from the work of Dicker et al.33, is fit to the double-logarithmic plot of yield-mobility (t=0) as a function of absorbed photon fluence (I0FA), Eq. 3. At low absorbed


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fluence, the fit asymptotically approaches the A-value which acts as an indication of the linear regime of the ϕΣμ(I0FA) function. The A value thus acts as an indication of the yieldmobility product at low fluences at which higher order loss processes are negligible, such as those at the solar fluence, and allows for the direct comparison of the maximum yieldmobility product between samples.

φt=0 Σµ =

A 1+ BI 0 FA + CI 0 FA

Eq. 3

Imaginary contributions to the photoconductance were detected by measuring microwave transients on-resonance, fo and off-resonance at frequencies both larger and smaller than fo; Ref.31 and Ref.30 give detailed descriptions of this process. For samples using short ligands that give rise to longer-range (>100 nm) conductivity the imaginary component is negligible and we focus only on the real component of the photoconductance.

Effect of Tetrapod Size on Photoconductance Prior to studying the ligand-based coupling strategies we sought to determine the effect of CdTe tetrapod size on photoconductivity. CdTe nanocrystals were synthesized to produce both spheres and various sizes of tetrapods as discussed in our previous work.26 TRMC measurements were made on films of the oleic acid capped tetrapods to compare size effects. Normalized absorption spectra (offset for clarity) of the tetrapods in hexane is also shown in Figure 1A indicating the variation in the first exciton peak, and thus degree of size-induced quantum confinement, across three samples of nanocrystals. The arms, as determined from transmission electron microscopy (TEM) images, vary in both length (8 nm to 14 nm) and diameter (2.5 nm to 5.5 nm). Long-lived signal seen in the normalized ΔG transients (Figure 1B) indicate the lifetime of photoexcited carriers in oleic acid-capped NCs increases with increasing tetrapod size. However, it is interesting to note that the yieldmobility product is inversely related to the size of tetrapods as determined from the fit Avalues shown in the inset of Figure 1B. It is important to note that despite the NCs having their native long-chained oleate ligand, charge separation (i.e. exciton dissociation) is still occurring and that they are not corrected for the imaginary change in photoconductance as


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we seek to simply screen and compare these as-deposited samples before applying more conductive ligands as discussed below. The three size samples are all capped with the same partially isolating ligand, for which the sum of mobilities for carriers hopping between NCs will be similarly very small. Intuitively, the larger tetrapods have higher mobility because of the increased crystal volume available to the carriers before interacting with an interface and scattering. The increased lifetime of carriers can be explained by this reasoning, i.e. the ability of carriers to move farther away from the interface in the larger particles lengthens their lifetime.27 However, the inverse relationship between tetrapod size and the yieldmobility product implies that natively-capped uncoupled films the carrier generation yield increases with decreasing particle size faster than the expected decrease in mobility. Hence we conclude that, qualitatively, more excitons separate in smaller tetrapod nanocrystals than the larger crystals but these charges recombine on faster time scales perhaps through increased bimolecular recombination.

Figure 1. A) Absorbance spectra of three sizes of CdTe tetrapods capped with OA. B) Normalized photoconductivity transients after excitation with 4 ns pulses of 520 nm at an absorbed photon flux near 4x1012 cm-2 (intensity-dependent data is shown in the supporting information Fig S1) show that the lifetime of excited carriers increases as the size of the NC increases.

Comparison of ϕΣμ of Spherical and Tetrapodal CdTe Nanocrystals


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Figure 2. TEM images of spheres A) and tetrapods B) with insets showing higher magnification images of the NCs. C) Yield-mobility product (obtained from the sum of pre-exponential factors from fits to the individual photoconductance transients) plotted as a function of absorbed photon flux for oleic acid-capped and fully sintered tetrapods and spheres, as well as pyridine-capped spheres. D) Solution and film absorbance spectra of OA-capped tetrapod and spheres indicating that the characteristic size of the NCs is similar so the intra-dot mobility is also expected to be similar.

Figure 2 shows the increase in yield-mobility product for tetrapod CdTe samples relative to their spherical analogs. The NCs were synthesized to have similar first exciton absorption peak position to minimize the effects of energetic differences on the sample. Tetrapods systematically have a larger yield-mobility product and lifetime as given in Tables 1 and 2. Similar results were observed in a comparison of CdSe nanocrystal shapes on photoconductivity.27 In that work, the arm length, which allows for larger separation of charges, was proposed as the mechanism that increased the charge yield. In our previous


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work, we fabricated CdTe solar cells using the same method for sintering the tetrapods and spheres shown here and reported improved device efficiency using sintered tetrapodal NCs vs sintered spheres with a CdCl2 treatment to facilitate large grain growth.26 Carrier transport is greatly improved due to one order of magnitude larger grain size using inks composed of the sintered tetrapods compared to the sintered spheres. The improved yieldmobility product found here corroborates the results found previously. Table 1. The A-values from fits using Eq. 3 and the average lifetime using Eq. 5. Sample

φΣμ (cm2/Vs) τave (ns)

OA-capped spheres

0.00047

24

OA-capped TPs

0.011

58

Yield-Mobility Product for CdTe Nanocrystals with Inorganic Ligands With this work, we aim to explore the photoconductivity of a film as a function of annealing temperature as well as type of ligand on the surface. To this end, inorganic ligands were exchanged for the original oleic acid ligands as previously described by Nag et al. and given in Supporting Information.10 The solution absorption of the ligand-exchanged nanocrystals as compared to their original absorption (Figure 3A) shows that the inorganiccapped nanocrystals retain quantum confinement, and that the ligand exchange only redshifts the absorption profile presumably due to increased coupling between the NCs.15 Although the tetrapodal CdTe nanocrystals show a higher yield-mobility product, we wanted to systematically study the effect of the ligand selection on the photoconductance without surface area dependence as we saw in the tetrapod size study. Thus isotropic (spherical) CdTe nanocrystals were prepared with S2-, Se2-, and Te2- ligands via a two-phase solution ligand exchange from hexane to N-methylformamide (NMF). Dissolved Na2X (X=S, Se, Te) in NMF displaces the native oleate ligands and attaches the chalcogenide to the surface of the NC during a phase transfer of the NCs from hexane to NMF.10 Measuring TRMC on spherical NCs minimizes the effects of packing and percolation of free charges through the NC array thus eliminating carrier anisotropy that may arise in tetrapodal or nanorod NCs. This class of sodium chalcogenide ligands was recently developed as molecular solders for NCs allowing for the highest carrier mobility measured in field-effect


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transistors.18 It has also been shown that thiol groups binding to surface of CdTe NCs reduce the electron trapping rate37 where a large density of trap states occurs near the valence band due to uncoordinated Te at the surface.38 For these reasons, we were motivated to investigate the effects of chalcogen ligands on photoconductivity. Figure 3B shows the yield-mobility product as a function of absorbed photon flux for films of spherical CdTe NCs ligand-exchanged with Na2X and annealed at 350°C. The Te-capped NCs show high mobility with the A-value from the fit using Eq. 3 equal to 11.6 cm2/Vs. Plotting the Avalues as a function of annealing temperature for each of the ligand exchanges explored shows drastic improvement by two orders of magnitude for the Te-capped NC (Figure 3C). By comparing the Te-capped NCs to the sintered spheres (originally capped with pyridine and CdCl2-treated in a procedure used in NC solar cells), we find that the Te-capped NCs reach nearly an order of magnitude higher yield-mobility product, indicating that the Tecapped NCs could out-perform the established procedure in devices where the carrier mobility plays a significant role in performance.39 To further assess the improvement in electronic properties of the films we calculate an effective diffusion length for photoexcited carriers.


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Figure 3. A) Solution absorbance spectra of spherical CdTe NCs before (OA-capped) and after ligand exchange with Na2X (X = S, Se, Te) normalized to the first exciton peak and off-set for clarity. B) Yield-mobility product as a function of absorbed photon flux for films of spherical CdTe NCs ligand-exchanged with Na2X and annealed at 350°C. Lines are fits to the data using Eq. 3 C) The A-values of the fits from Eq. 3 plotted as a function of annealing temperature for the ligand-exchanged samples. D) Yield-mobility product as a function of absorbed photon flux for films of spherical CdTe NCs capped with Na2Te ligands compared to CdTe NCs that have been sintered using a protocol known to yield high efficiency solar cells from our work in Ref.26 Note: only four fluences were measured because at >1012 photons/cm2 the photoconductance was high enough to saturate the detector.

The calculated charge diffusion length, Ld, is a useful characterization parameter for predicting the ability for charges to be extracted from a device. The Ld is determined from the lifetime of charges, τave, and the diffusivity of charges a parameter based on sample temperature and the charge carrier mobility, μ following the work of Ref.32

Ld =

6τ ave (kBT )µ e


Eq. 4

10

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Similar TRMC studies of nanocrystal arrays have used a transient half-life time constant, τ1/2, for the calculation of carrier diffusion lengths.22 A half-life measurement is an appropriate gauge of lifetimes for signals that undergo single-exponential decay mechanisms. The photoconductivity decays shown herein are more complex, and as previously mentioned can be fit to a tri-exponential function, representing three separate lifetimes and thus recombination mechanisms leading to signal decay. To more accurately compare across the samples measured herein we have instead utilized a weighted average lifetime, as calculated in Eq. 5 and tabulated in Table 2.

τ ave =

a1τ 1 + a2τ 2 + a3τ 3 a1 + a2 + a3

Eq. 5

Table 2. The A-values from the fits using Eq.3, the average lifetime from Eq. 5, and the calculated diffusion length from Eq. 4 of the samples explored. Sample

Σμ (cm2/Vs)

τave (ns) Ld (nm)

Pyr. Spheres

0.011

17

53

Sintered Spheres

0.44

15

323

Sintered TPs

4.3

2

334

S2- 80°C

0.015

38

94

S2- 250°C

0.0038

22

36

S2- 350°C

0.61

21

449

Se2- 80°C

0.0057

14

35

Se2- 250°C

0.034

26

116

Se2- 350°C

0.15

20

215

Te2- 80°C

0.024

1190

661

Te2- 250°C

0.05

18

117

Te2- 350°C

11.6

39

2660

In some cases, annealing of inorganic capped nanocrystals leads to a sizeable increase in the calculated charge diffusion length compared with that of pyridineterminated NCs. As indicated in Table 2, sintering spherical NCs capped with pyridine


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induces a 40x increase in the sum of electron and hole mobilities, thus leading to a 6x increase in the Ld. Using the sodium chalcogenide ligands, we find similar increases with annealing, however Te2- shows unique properties. At low temperatures, Te2- treatment shows a long lifetime not seen in the other ligands and upon annealing the lifetime returns to a typical value of 39 ns, but the mobilites increase significantly. Therefore, the Te2capped CdTe spheres show the largest diffusion length (over 2.5 μm), and are therefore a promising candidate for use in solution processed optoelectronic devices.

Conclusions We have used time-resolved microwave photoconductivity to measure the photoconductance transients of NCs of CdTe in various forms. We find tetrapods with native ligand have a size-dependent weighted-average lifetime (using Eq. 5) that increases from 2.5 ns to 5.3 ns to 128 ns as the first exciton is increased from 600 nm to 622 nm to 670 nm. In conjunction with previously published PV devices, films of sintered tetrapods produce longer-lived carriers with roughly an order of magnitude higher yield-mobility product. Moreover, photoconductivity results show a distinct improvement in the yieldmobility product between sintered spherical CdTe NCs samples ligand exchanged with Na2Te compared to pyridine-capped and CdCl2-treated NCs following a procedure known to yield high-efficiency solar cells. Improving the diffusion length of sintered films of CdTe NCs through ligand exchanges shows unique promise for improving solution-processed CdTe solar cells. Microwave conductivity experiments presented here can probe films without the need to optimize contacts and film morphology so the challenge of these optimization steps remain.

Acknowledgements The work presented here is supported by the U.S. Department of Energy (DOE) SunShot program under Award No. DE-EE0005312.

The time-resolved microwave

conductivity experiment was developed and supported by the Solar Photochemistry Program of the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences, under Contract No. DE-AC3608GO28308 to NREL.


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Supporting Information The Supporting Information with intensity-dependent TRMC data and experimental methodology is available free of charge on the ACS Publications website at DOI: INSERT AFTER PUBLICATION

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Photoconductivity of CdTe Nanocrystal-Based Thin ... - ACS Publications

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