Energetics of Non-Radiative Surface Trap States in Nanoparticles

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Surfaces, Interfaces, and Applications

Energetics of Non-Radiative Surface Trap States in Nanoparticles Monitored by Time of Flight Photoconduction Measurements on Nanoparticle-Polymer Blends Xiaoqing Guo, Qianxun Gong, Joanna Borowiec, Sijie Zhang, Shuo Han, Meng Zhang, Maureen Willis, Theo Kreouzis, and Kui Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07852 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 19, 2019

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ACS Applied Materials & Interfaces

Energetics of Non-Radiative Surface Trap States in Nanoparticles Monitored by Time of Flight Photoconduction Measurements on Nanoparticle-Polymer Blends Xiaoqing Guo,† Qianxun Gong,‡, Joanna Borowiec,‡ Sijie Zhang,‡ Shuo Han,† Meng Zhang,† Maureen Willis,*,‡ Theo Kreouzis,*,‡, Kui Yu*,†, †Institute

of Atomic and Molecular Physics, Sichuan University, Chengdu, Sichuan, 610065, People’s Republic of China

‡College

of Physical Science and Technology & Sino-British Materials Research Institute,

Sichuan University, Chengdu, Sichuan, 610065, People’s Republic of China §Engineering

Research Center in Biomaterials, Sichuan University, Chengdu, Sichuan, 610065, People’s Republic of China

School

of Physics and Astronomy, Queen Mary University of London, London, E1 4NS, United Kingdom

State

Key Laboratory of Polymer Materials Engineering, Chengdu, Sichuan, 610065, People’s Republic of China M. W. (email: [email protected]) or to T. K. (email: [email protected]) or to K. Y. (email: [email protected])

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ABSTRACT Nanoparticles (NPs) have had increasingly successful applications including in emissive or photovoltaic devices; however, trap states associated with the surface of NPs often drastically reduce the efficiency of devices and are difficult to detect spectroscopically. We show the applicability of photoconduction as means of detecting and quantifying trap states in NPs. We performed Time of Flight (ToF) photoconduction measurements, using semiconducting poly[bis(4-phenyl)(4-butyphenyl)amine] (P-TPD) doped with either core/shell CdSeS/CdS quantum dots (QDs) or perovskite CsPbBr3 NPs, both of which are carefully designed to be energetically matched. In the case of the QDs, a drop in hole mobility from ~10-3 to ~10-4 cm2V-1s-1 was observed when compared to a control sample, suggesting the presence of hole trapping. These trap states were found to be around -5.0 to -4.9 eV from the vacuum level. The presence of the trap states was further supported by a coincident reduction in photoluminescence (PL) quantum yield (QY) and lifetime of the core/shell QDs after purification. Using the measured reductions in the PL QY and lifetime, the surface trap state density was estimated to increase by between 20 and 40%, most likely due to ligand detachment. In the case of the perovskite NP doped samples, a mobility of ~10-3 cm2V-1s-1 was measured. Thus, doping with perovskite NPs did not generate any obvious hole trapping from the P-TPD matrix. The absence of trapping may be related to the reduced surface-to-volume ratio and NP number density of the perovskite NPs compared to the core/shell QDs, since the perovskite NPs are approximately 10 times larger in radius than that of the core/shell QDs. Our results suggest that to minimize the presence of trap states with a view to improving device performance, large-size perovskite NPs appear to be better than small-size QDs. TOC Graphic:

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KEYWORDS: perovskite nanoparticle (NPs), quantum dots (QDs), mobility, shallow traps, surface defects, semiconducting polymer, time of flight (ToF) INTRODUCTION Nanoparticles (NPs), which display narrow luminescence peaks with high colour purity, have great potential in emissive devices,1,2 owing in part to the tunability of their optical properties. In the case of semiconductor quantum dots (QDs) the optical properties are typically tuned by varying their size and composition, whereas in perovskite NPs they are adjusted primarily by their composition.1,3-5 In addition, QDs and perovskite NPs are compatible with solution processing, making it possible to engineer them into thin layers fabricated between the hole and electron transport layers in functional devices. In light emitting diodes (LEDs),2,6-10 charges are transferred from the hole and electron transport layers to the NPs, whereas in photovoltaics,11-15 electron-hole pairs generated in the NPs are separated and travel via the hole and electron transport layers to the electrodes. Specifically, emissive devices containing QDs require electrons and holes to be efficiently transferred from the respective electron and hole transporting elements of the device into the QDs to obtain emission. Likewise, photovoltaic devices require efficient charge transfer between the donor and acceptor components of the device to maximise efficiency. Additionally, the presence of ligands used to aid solubility and to passivate the surface of QDs could potentially hinder charge transfer in to and out of the QDs due to the insulating nature of the alkane chains forming the ligands. Not surprisingly, an in-depth understanding of the charge transfer between an organic semiconducting polymer (OSC) and the NPs would be of great use for the development of certain NP-based LED and photovoltaic applications. One method that has been widely employed to investigate the charge transport in various materials is the Time of Flight (ToF) technique,16-20 in particular, for the study of hole transport in QD-doped polymers. However, the effects of the QDs on the charge transport remain unclear,21-27 since in some cases the presence of the QDs improves the charge carrier mobility,21,22,26,27 whereas in other studies the QDs act as hole traps reducing the mobility.23,24 Furthermore, another case shows an initial reduction in carrier mobility at low QD concentrations followed by a recovery at high concentrations.25 In fact, when the valence band of the QDs is located below the highest occupied molecular orbital (HOMO) of 3



the OSC, it is ambiguous as to why the QDs even act as hole traps. In fact, in work related to the present, where PbS QDs are incorporated in a semiconducting polymer matrix, no hole mobility reduction is observed up to 60 wt% QD loading, as would be expected from the relative energetics of the polymer HOMO-LUMO levels and the QD valence and conduction band levels.28,29 In this study, we report an investigation on the effect of NPs on hole transport in an OSC using ToF, including the charge transfer between the OSC and NPs and the energetics of “trap” states in the NPs. The device structure used is illustrated in Scheme 1, in which the OSC layer consists of poly[bis(4-phenyl)(4-butylphenyl)amine] (P-TPD). The P-TPD layer is doped with core/shell CdSeS/CdS QDs or perovskite CsPbBr3 NPs which are closely matched energetically in terms of bandgap, valence band (VB) and conduction band (CB) edges to each other as well as to the host polymer as shown in the TOC graphic. In both cases the VB of the two types of NPs lies below the Highest Occupied Molecular Orbital (HOMO) level of the P-TPD. It was found that hole trapping can be detected in QD-doped P-TPD but not in perovskite NP-doped P-TPD. Accordingly, trap states (within the QD bandgap) are VB trap states and are located above the P-TPD HOMO level. We also evaluated the effect of QD purification on the formation of VB trap states. Our study suggests that large-size perovskite NPs may be better than small-size QDs, in terms of minimising the presence of trap states, to give improved device performance. Non-Radiative (NR) trap states cannot be detected optically, yet they reduce the PL QY of emissive devices and can provide additional electron-hole recombination pathways in photovoltaic devices. In this study, we detect the presence of such states using their effect on the hole mobility within a semiconducting polymer matrix by carefully selecting the relative energetics between the host HOMO and LUMO and the NP valence and conduction band levels. Additionally this approach can, in principle, distinguish between valence (occupied) band trap states and conduction (unoccupied) band trap states. We explain why QDs doped within semiconducting polymer matrices, at low concentrations, can lead to charge carrier trapping, even when this is at first glance energetically unfavourable. By studying the effect of QD purification on the trap states we also obtain insight into the nature and formation mechanisms of the trap states themselves by linking them to ligand detachment during purification. Finally, using the shallow trap mechanism behind the charge carrier mobility reduction, we are able to energetically “image” the trap states 4


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themselves, locating their presence relative to the vacuum. In terms of fundamental science, carefully designed ToF experiments incorporating QDs and other NPs have the potential of delivering a large amount of information, considering the simplicity of the experimental procedure itself.

Scheme 1. Schematic of the Time of Flight devices used (a) Control device consisting of undoped P-TPD (b) sample device with NP doped P-TPD. The nanoparticles are dispersed throughout the P-TPD polymer matrix and the laser pulse generates electron-hole pairs near the illuminated electrode. The electric field direction is chosen so that holes drift towards the counter-electrode. (c) Chemical structure of P-TPD.

RESULTS AND DISCUSSION Having synthesized the NPs, we first characterised their absorption, PL and physical properties. Figure 1 shows the absorption and photoluminescence (PL) spectra, along with transmission electron microscopy (TEM) images of the two NPs, CdSeS/CdS core/shell QDs and perovskite NPs. The CdSeS/CdS QDs display a sharp absorption peak at 509 nm with a corresponding symmetrically shaped emission peak at 526 nm (Figure 1a). The core/shell nature is supported by Figure S5 showing the comparison of the optical properties of the core and core/shell QDs. The CdSeS/CdS QDs are spherical in shape with an average diameter of 3.7 ± 0.6 nm as indicated by the TEM image (Figure 1b). Comparatively, the perovskite NP sample exhibits absorption at 517 nm and a sharp emission peak at 519 nm, again, symmetrical in shape as shown in Figure 1c. We note that the long wavelength absorption displayed in Figure 1c is an instrumental artefact due to scattering from the large

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NP particles. The shape of the NPs is cubic with a relatively broad size distribution and an average size of 39 ± 10 nm (Figure 1d).

Figure 1. Optical properties (a and c) and TEM (b and d) of the core/shell CdSeS/CdS QDs (a and b) and perovskite CsPbBr3 NPs (c and d). (a) Normalized absorption (dashed trace, left axis) and PL (350 nm excitation, solid trace, right axis) spectra of the QDs (15 µL in 3 mL of toluene). (b) A typical TEM image of twice-purified QDs. The QDs have a spherical shape with a diameter of ~4 nm. (c) Normalized absorption (dashed trace, left axis) and PL (350 nm excitation, solid trace, right axis) spectra for the perovskite (10 µL in 9 mL of toluene). (d) A typical TEM image of twice purified perovskite NPs. The perovskite NPs are in the shape of cuboids with a size distribution of 30~100 nm.

Size distribution histograms for both types of NPs are shown in Figure S1. The PL of both types of NPs displays no trap emission. This is not a unusual as it has been acknowledged 6


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that often the presence of trap states in NPs does not result in characteristic “trap emission” at long wavelengths.30-33 On the other hand, surface effects have been shown to strongly affect the electronic and optical properties of NPs.34,35 It is important to note that both types of NPs exhibit very similar luminescence peak values upon excitation at 350 nm, highlighting the close matching in terms of optical bandgap, although the nanoparticles differ in size. It is then necessary to determine the positions of the valence band (VB) and conduction band (CB) edges for both types of NPs in order to compare them to the HOMO of the OSC. These were obtained using cyclic voltammetry (CV) (see Figure S3), and the VB and CB edge values were found to be -5.56 eV and -3.25 eV and -5.71 eV and -3.43 eV from the vacuum level for the QDs and NPs, respectively. A summary of the CV results for both NPs can be found in Table S1. The CV measurements support the optical behaviour and confirm the CdSeS/CdS and perovskite NPs used in this study are very closely matched energetically to the HOMO of the OSC, for which the complete schematic is shown in the TOC graphic. While the bandgaps of the NPs are well matched energetically, the surface-to-volume ratios are significantly different. This difference is key to studying the surface effects. In order to further characterise the different NP sizes, X-ray diffraction (XRD) measurements were performed and suggest particle sizes of 2.6 and 50 nm for the CdSeS/CdS QDs and perovskite NPs, respectively (see Figure S4 and associated text for details of the Scherrer equation analysis used). We note that these are in reasonable agreement with the sizes obtained from microscopy. In this case, the smaller CdSeS/CdS QDs should display significant surface effects while their larger perovskite NP counterparts should not.36 Having characterised the physical and optical properties of both types of NPs, the next step was to determine the mobility of holes in the OSC doped with the NPs. This was done using ToF. Figure 2 shows the resultant Poole-Frenkel plot of the room temperature ToF hole mobilities obtained from a number of un-doped and doped P-TPD samples (with examples of individual photocurrent transients shown in figure S8). For both the perovskite NP and CdSeS/CdS QD doped samples shown, the QDs and NPs were purified twice before being incorporated in the ToF samples. The inset in Figure 2 shows how the transit time, 𝑡𝑡, is obtained at each bias using the intersection of the tangents to the data in a double logarithmic plot. This was required as all samples displayed dispersive photocurrent transients. The mobility, 𝜇, is calculated at each 7



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electric field by substituting the transit time as well as the device thickness, 𝑑 and applied bias, 𝑉 into equation (1). 𝑑2

(1)

𝜇 = 𝑉𝑡𝑡

Figure 2. Poole-Frenkel plot of the room temperature hole mobilities obtained from un-doped (triangular symbols) and doped (square symbols for perovskite NPs and circular symbols for the QDs) P-TPD samples. The sample thickness ranges from 1 to 1.5 µm. The inset shows a double logarithmic plot (orange line) of a hole photocurrent transient at 20 V bias obtained from a 1.5 µm pure P-TPD sample at room temperature; the intersection point (as indicated by the arrow) of the two tangents (dashed lines) is used to determine the precise transit time. The shaded area in the main plot represents the average hole mobility measured at room temperature over 12 “control” samples ± 1 standard deviation. Both the QDs and perovskite NPs were purified twice, before being included in the polymer matrix.

The grey triangle data plotted from six pure P-TPD samples indicate that the hole mobility in the “control” samples is close to 10-3 cm2V-1s-1 with significant sample to sample variation, but is relatively field independent. Over the 12 “control” samples measured we obtained an average hole mobility of 9.1 × 10-4 cm2V-1s-1 ± 17%. This is higher by almost an

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order of magnitude than the literature value reported by Thesen and coworkers using FET measurements37, but in close agreement to the hole mobility measured by Barard and coworkers on a similar polytriarylamine of comparable molecular weight (including the electric field independence of the mobility).38 The hole mobilities measured in the 2 wt% perovskite NP doped samples (open squares) are virtually indistinguishable from the those in un-doped P-TPD. They display negligible electric field dependence and lie within the range of mobilities dictated from sample to sample variation. This is in contrast to the hole mobilities obtained from the 2 wt% CdSeS/CdS QD doped samples (open circles) which are clearly distinguishable and have an average value of 1.5 × 10-4 cm2V-1s-1. Again, they display no significant electric field dependence. The results of the sample with perovskite NPs shown in Figure 2 are exactly as expected given the perovskite NPs cannot act as hole traps since this is energetically unfavourable (as shown in the TOC graphic). Since the perovskite VB lies below the P-TPD HOMO, holes present within the HOMO cannot be trapped by the perovskite and no effect on the hole mobility is expected. In other words, the transiting holes can easily diffuse around the energetic barriers formed by the perovskite. What is surprising is that the QD doped sample mobility results show a significant drop in hole mobility. This is completely unexpected since hole trapping is disallowed energetically as in the case of the perovskite NPs (see TOC graphic). In fact, the average mobility from the QD doped sample in Figure 2 differs by five standard deviations from the average “control” mobility and thus is statistically significant. This is clearly indicative of shallow trapping occurring when the P-TPD is doped with 2 wt% of QDs. Polytriarylamines, such as the P-TPD matrix used in the present study are amorphous, thus any hole mobility reduction cannot be due to morphological changes in the polymer structure due to the presence of the QDs.38 Since the film formation occurs at room temperature, the QDs may aggregate but they do not fuse and hence retain their individual optical and electronic properties. Other mechanisms, such as energy transfer from the polymer to the QDs or hole injection from the QDs to the polymer matrix or recombination effects between excited electrons in the QDs and holes in the polymer can also be eliminated as explanations for the hole mobility reduction observed since the bulk of the semiconducting polymer, and any QDs or NPs included, are not illuminated by the incident laser light. Specifically, the laser penetration depth has been measured to be approximately 9



59 nm, which is much smaller than the inter-electrode distance of 1 µm or more, which is a prerequisite for the ToF technique (see supporting information section S2 for details). The photocurrent measured by the ToF technique is purely due to holes, as dictated by the applied electric field direction in our setup. Thus, any high mobility electron transport occurring via the NPs themselves can also be excluded.39 The low, sub percolation threshold, concentrations of the dopants also rule out any bulk charge transport occurring not in the PTPD matrix but in via the NPs themselves. Thus, the interpretation that the hole mobility reduction is due to shallow trapping resulting from the inclusion of the QDs in the polymer matrix is in agreement with similar examples of ToF studies found in the scientific literature.23-25 The next logical step is to change the doping concentration and study the variation in hole mobility. Figure 3 shows a plot of the variation in the average hole mobility, obtained across the measured electric field range, as a function of the NP doping. The average mobility was determined using the gradient of a reciprocal transit time versus electric field plot (for typical plots and analysis, see Figures S9 and S10). As in the case of the 2 wt% samples used in Figure 2, the NPs in the samples used to obtain the data shown in Figure 3, were doped into the P-TPD following two stages of purification. It is worth noting that the electric field independence of the hole mobility presented in Figure 2, for both doped and un-doped samples, leads to a close agreement between the average mobility determined using the graphical method and the mobility calculated at a specific field given by equation (1). In the case of perovskite NP doping between 1 wt% and 4 wt% the hole mobility is essentially unchanged, within sample to sample variation. This is in agreement with the results for the 2 wt% perovskite NP sample shown in Figure 2. In stark contrast, the results for the samples doped with CdSeS/CdS QDs show a strong dependence of the hole mobility on the amount of NP doping, with a minimum 1 wt% QD doping content required for a distinguishable mobility drop.

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Figure 3. Average hole mobility at room temperature across the electric field range measured versus NP loading of perovskite NP and CdSeS/CdS QD dopants. The un-doped P-TPD average hole mobilities of six “control” samples appear as 0 wt% loading. The shaded area represents the average room temperature hole mobility measured over 12 “control” samples ± 1 standard deviation.

For QD doping of 0.5 wt% or less, the mobility measured in the doped sample is indistinguishable from that of the un-doped P-TPD within the range dictated by sample to sample variation. On increasing the amount of QDs from 1 wt% to 2.5 wt%, the mobility of the doped samples shows a rapid decrease, confirming that the QDs are acting as hole traps. However, when the QD content in the sample is higher than 2.5 wt%, it is not possible to measure the hole mobility by ToF as the trapping becomes too severe, thus the photocurrent decays become featureless with no discernible transit time (see Figure 11). This is not first time hole trapping by core/shell QDs has been reported. A study with a similar (CdSe/ZnS) structure and at similar concentrations as that of CdSeS/CdS QDs used in this study showed the presence of hole trapping by the QDs.25 Despite purifying the NPs twice before doping, attention is now turned to the role of impurities on the hole mobility. Figure 4 shows a Poole-Frenkel plot of the room temperature ToF hole mobilities obtained from un-doped P-TPD samples, samples doped

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with CdSeS/CdS QDs obtained from various conditions and samples doped with the impurities in the supernatant from the purification process of QDs.

Figure 4. Poole-Frenkel plot of the room temperature hole mobilities obtained from un-doped P-TPD and doped P-TPD with CdSeS/CdS QDs as indicated. The sample thickness ranges from 1.1 to 1.3 µm. The un-doped P-TPD data obtained from various experimental sample batches (open triangular symbols) and from the same experimental batch as the doped sample results presented in this figure (solid triangular symbols), are indicated. The diamond symbols represent hole mobilities measured using a P-TPD sample doped with 1 wt% of the supernatant obtained from a third purification step of the QDs. The shaded area represents the average room temperature hole mobility measured over 12 “control” samples ± 1 standard deviation.

The data plotted with open triangles is obtained from six pure P-TPD samples forming the “control” samples, with the individual control fabricated as part of the same batch as the QD doped samples in this figure highlighted in grey. For comparison, the hole mobilities in P-TPD samples doped with 1 wt% of impurity extracted from the supernatant are shown by the diamond data points. In this case, during the purification process the QDs precipitate out while the impurities remain in solution such that the supernatant will be comprised of

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the impurities themselves. Consequently, any impurities extracted during a third purification stage would have been present with the QDs used to dope the 2 wt% sample, since these would have only undergone two purification stages. Thus it is precisely these impurities that are deliberately doped in the P-TPD sample reported in Figure 4. The hole mobility obtained from the (supernatant) impurity doped sample is, again, indistinguishable from the pure PTPD “control” sample mobility, within the sample to sample variation range. Thus the QDs themselves must be responsible for the hole trapping, i.e. the trapping is not caused by impurities present in the QD samples. We note that although the hole mobilities measured in the 2 wt% ToF sample where the CdSeS/CdS QDs were purified once (grey circles) are virtually indistinguishable from the un-doped P-TPD with negligible electric field dependence, they are consistently lower compared with the mobilities obtained from those produced as part of the same batch of ToF samples (grey triangles). We additionally note that the purification procedure carried out on the QDs between the purified once and purified twice cases does not remove a significant fraction of the QDs themselves. Using PL measurements it is estimated that approximately 0.5% of the original QDs are present in the supernatant (see supporting information Figure S14). Thus the composition of the purified once and purified twice QD stock used to dope the P-TPD matrix is not significantly different and meaningful comparisons can be made between the two samples of equal nominal 2 wt% doping. The mobility drop from doping with 2 wt% of QDs (following two purification steps) observed in Figure 2 is once again observed in Figure 4, indicating very good reproducibility of the mobility effect across different QD synthetic batches and different ToF samples. The purification process does not affect the optical properties of the QDs, as evident from the absorption and PL spectra shown in Figure S12. Thus the purification method used does not affect the bandgap of the QDs. The trapping effect of the QDs, however, is enhanced by purification as evident by the larger hole mobility drop in the twice purified case compared to the once purified and control cases. This is understandable if one considers that the surface of the QDs is surrounded by ligand molecules, which passivate and stabilize the QDs.40 During purification, the ligand molecules may be partially removed, which may also lead to the removal of surface atoms.41,42 The removal of surface atoms can give rise to surface trap states and, given the hole mobility reduction observed from Figures 2 to 4, we can identify these states as VB trap states (see TOC graphic). We note that these states are 13



not distinguishable using absorption or PL spectroscopy, as shown in Figure S12, but their presence can be detected by the ToF method used here. In order to confirm the increase in trap states following purification, time resolved PL was performed on unpurified and twice purified CdSeS/CdS QDs samples. Figure 5 displays the time resolved PL results obtained from the unpurified and twice purified CdSeS/CdS QD samples. In both cases the data can be fitted with a single exponential decay function, from which, we can obtain PL decay lifetimes for both cases. These were 𝜏0 = 18.6 ± 0.2 ns and 𝜏2 = 17.3 ± 0.3 ns for unpurified and twice purified QDs, respectively. The errors in the lifetimes have been defined by the interval corresponding to a 10% increase in 𝜒2, that is, where the fitted function visibly deviates from the experimental data. We note a significant decrease in the PL lifetime following two purification steps of ∆𝜏 = 1.3 ± 0.36 ns, corresponding to 3.6 standard deviations. In the case of emissive and non-radiative processes the resultant PL lifetime, 𝜏, can be defined by equation (2).43 1 𝜏

(2)

= Γ + 𝑘𝑁𝑅

Where Γ is the emissive rate and 𝑘𝑁𝑅 is the non-radiative decay rate. The decrease in lifetime following purification must thus be due to an increase in the non-radiative recombination rate, since the emissive rate remains constant (as it is defined by the “natural” lifetime). This non-radiative recombination rate increase coincides with a decrease in the photoluminescence quantum yield (PLQY) after purification,44 from 81% in the unpurified case to 60% in the twice purified case. The decrease in both PL lifetime and PLQY following purification of the CdSeS/CdS NPs is consistent with the presence of an increased amount of surface trap states (defects) and supports the decrease in the hole mobility shown in Figure 4. The PLQY is related to the emissive and non-radiative rates as described in equation (3).43 Γ

𝑃𝐿𝑄𝑌 = Γ + 𝑘𝑁𝑅

(3)

Substituting the unpurified and twice purified PL lifetimes and PLQY values into equations (2) and (3) we can obtain an emissive (natural) lifetime of 26 ± 3 ns for the CdSeS/CdS QD photoluminescence at 526 nm. The lifetime value obtained for this system compares favourably with literature lifetimes of approximately 20 ns measured in similar quantum dots.45 We can, in turn, estimate the increase in the non-radiative decay rate, and 14


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therefore in the amount of trap states. This was determined to be between 20% and 40% as a result of the two step purification of the CdSeS/CdS QDs pre-doping.

Figure 5. Comparison of the PL decay dynamics of CdSeS/CdS QDs in the un-purified (a) and purified twice (b) cases. For the un-purified sample (a), a PL lifetime τ = 18.6  0.2 ns is obtained using a single exponential fit. For the purified twice sample (b), a PL lifetime τ = 17.3  0.3 ns is obtained. Following purification the PL lifetime is reduced by ∆𝜏 = 1.3 ± 0.36 ns, corresponding to 3.6 standard deviations. The decreased PL lifetime is consistent with increased non-radiative recombination which suggests that, following purification, non-radiative recombination through surface defect trap states is increased.

Having established the behaviour of the mobility and the presence of trap states, we turn our attention to the nature of the trap states. The reduction in hole mobility in the presence of purified CdSeS/CdS QDs, compared to un-doped P-TPD, is indicative of shallow 15



trapping. From literature, we know that in disordered OSC systems shallow traps which give rise to charge carrier mobility reductions lie within 100 to 200 meV of the transport state levels (i.e. at an energetic separation comparable to the energetic disorder present in the OSC itself).46 Li and co-workers have interpreted the effect of shallow traps on mobility as stemming from an increase in the energetic disorder of the OSC and have shown that the presence of traps with far greater energetic separations (deep traps) does not lead to a reduction in charge carrier mobility but to a reduction in charge density. Thus, given the HOMO of P-TPD is at -5.1 eV, we can estimate the absolute position of the hole trap states to be in the region -5.0 to -4.9 eV relative to the vacuum level. While the trap states are identifiable in the CdSeS/CdS QDs it is still not clear why, for the same wt% doping in P-TPD, no trap states are detected due to the perovskite NP. Nevertheless a clearer understanding can be obtained if one considers the NP densities in both types of doped samples. In order to measure a significant hole mobility reduction in the CdSeS/CdS QDs a nanoparticle density of 1.5 × 1017 (cm)-3 (for the 2 wt% case) is required. This number density is comparable to that of CdSe/ZnS QDs in a PVK OSC matrix reported by Khetubol and coworkers.25 The difference lies, however, in the fact that the perovskite NPs are approximately ten times larger in size, which will lead to a perovskite NP number density one thousand times smaller, compared to the CdSeS/CdS QD case, for the same wt% loading. Likewise, if one considers the total surface area of the NPs one can calculate that the total surface area of the perovskite NPs is one fiftieth of the CdSeS/CdS QDs surface area for the same wt% loading (even after having taken into account the cuboid shape of the perovskite NPs). Both of these considerations mean that we cannot conclude, using the results in Figure 2, that there are no trap states present in the perovskite NPs, since the reduced number density and contact area with the polymer can account for the failure to detect trapping by mobility reduction. CONCLUSIONS We have embedded energetically matched perovskite NPs and CdSeS/CdS QDs within a PTPD matrix and measured the hole mobility in the polymer by ToF. Both types of NPs have lower lying valence bands compared to the polymer HOMO and should not give rise to hole trapping. For the perovskite NPs no mobility reduction was observed for a NP loading up to 4 wt%. In the case of the twice purified CdSeS/CdS QDs a significant hole mobility reduction 16


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(from approximately 10-3 cm2V-1s-1 to 10-4 cm2V-1s-1) was observed with increasing wt% for QD densities in the order of 1017 (cm)-3. The hole mobility reduction observed with CdSeS/CdS QDs doping is indicative of shallow trapping, the levels of which are estimated to be between -5.0 and -4.9 eV relative to the vacuum level. Also, the trapping effect was increased with successive purification steps. The larger perovskite NP size, compared to the CdSeS/CdS QDs, results in approximately one thousand fold smaller NP number densities for the same wt% loading and one fiftieth total surface area in contact with the polymer, which can explain why no trapping was detected using the perovskite NPs. In addition, there was no indication that the trapping was due to impurities present in the QDs. Successive purification steps led to a reduction of the PLQY from 0.81 to 0.60 and a concurrent decrease in the PL lifetime of the CdSeS/CdS QDs from 18.6 ± 0.2 to 17.3 ± 0.3 ns for the unpurified case and after two purification stages, respectively. We attribute the hole trapping to valence band trap states associated with the surface of the QDs forming as a result of ligand detachment during purification and estimate an increase between 20% and 40% in the number of trap states after two stages of purification. These surface defect trap states formed, however, are not emissive and can be difficult to detect spectroscopically indicating the viability of photoconduction measurements on NP doped OSC samples as a more general technique for probing the formation, the nature and energetics of trap states on the NPs themselves. More generally, this study has highlighted the importance of the NP surface and surface modification in determining the number of trap states and their effects. EXPERIMENTAL SECTION For detailed descriptions of the NP synthesis, cyclic voltammetry, absorption spectroscopy, photoluminescence spectroscopy and time resolved photoluminescence please see the experimental section of the Supplementary Information. For the Time of Flight samples, the P-TPD was purchased from Xi’an Polymer Light Technology Corporation and used as purchased. The devices were fabricated on pre-patterned Indium Tin Oxide coated glass slides by spin coating a 60 mg/mL solution of P-TPD (or P-TPD + NP dopant) in chlorobenzene at 1000 rpm for 30 s at room temperature. The devices were annealed at 120 C for 10 minutes before evaporating approximately 100 nm of Al as a common cathode at a base pressure of 10-4 to 10-3 Pa. The NP doping was achieved by dissolving a known mass of material in chlorobenzene, followed by serial dilution to achieve the desired 17



concentration, and using the NP containing solvent to dissolve the P-TPD. The OSC film thickness was measured using an Ambios Technology XP-200 stylus profilometer. The 355 nm wavelength, 6 ns pulsed output of a frequency tripled Nd:YAG laser provided the photoexcitation for the ToF measurements, while the bias was supplied by a Keysight B2901A source-measure unit. The photocurrents were detected as the voltage drop across a 50 Ω termination at the input of a Keysight MSO-X4024A digitising oscilloscope (for more details regarding the ToF measurements see supporting information section S2). All photoconduction measurements were carried out with the samples under vacuum at a base pressure 10-1 Pa.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx Experimental detailed including of the synthesis details, cyclic voltammetry, optical absorption and photoluminescence spectra, TEM, XRD, and time-resolved photoluminescence (PDF) AUTHOR INOFRMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] ORCID Joanna Borowiec: 0000-0002-4004-9285 Shuo Han: 0000-0003-0880-1833 Meng Zhang: 0000-0002-2852-2527 Maureen Willis: 0000-0002-7530-4258 Theo Kreouzis: 0000-0003-2326-5338 Kui Yu: 0000-0003-0349-2680 18


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS K.Y. thanks the National Natural Science Foundation of China (NSFC) 21773162 and 21573155, the State Key Laboratory of Polymer Materials Engineering of Sichuan University (Grant No. sklpme2018-2-08), the Fundamental Research Funds for the Central Universities, and the Open Project of Key State Laboratory for Supramolecular Structures and Materials of Jilin University for SKLSSM 201935. J. B. and S. H. and M. Z. acknowledge Post-doctoral Research and Development Fund of Sichuan University 2019SCU12070, 2017SCU12012, and 2019SCU12073, respectively. We thank Ms Jing Zhang for the discussion on the QD synthesis, Ms Tingting Zhu for the perovskite nanoparticles kindly supplied, and Mr Chengming Li for the cyclic voltammetry measurements, all at Sichuan University. We thank Dr. Shanling Wang (Analytical &Testing Center, Sichuan University) for TEM.

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Energetics of Non-Radiative Surface Trap States in Nanoparticles

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