Lead Sulfide Quantum Dot Photodetector with Enhanced Responsivity

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Lead Sulfide Quantum Dot Photodetector with Enhanced Responsivity through a Two-step Ligand Exchange Method Haodong Tang, Jialin Zhong, Wei Chen, Kanming Shi, Guanding Mei, Yuniu Zhang, Zuoliang Wen, Peter Muller-Buschbaum, Dan Wu, Kai Wang, and Xiao Wei Sun ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00889 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 19, 2019

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ACS Applied Nano Materials

Lead Sulfide Quantum Dot Photodetector with Enhanced Responsivity through a Two-step Ligand Exchange Method Haodong Tang,†,▽ Jialin Zhong,†,▽ Wei Chen,‡,▽ Kanming Shi,†,§ Guanding Mei,† Yuniu Zhang,† Zuoliang Wen,† Peter Müller-Buschbaum,‡ Dan Wu,∥, * Kai Wang,†* Xiao Wei Sun,†,§* †Guangdong

University Key Lab for Advanced Quantum Dot Display and Lighting, Shenzhen

Key Lab for Advanced Quantum Dot Display and Lighting, and Department of Electrical and Electronic Engineering, Southern University of Science and Technology, 518055 Shenzhen, China ‡Physik-Department,

Lehrstuhl für Funktionelle Materialien, Technische Universität München,

James-Franck-Straße 1, 85748 Garching, Germany ∥Academy

for Advanced Interdisciplinary Studies, Department of Electrical and Electronics

Engineering, Southern University of Science and Technology, 518055 Shenzhen, China §Shenzhen

Planck Innovation Technologies Co. Ltd, Shenzhen, China

KEYWORDS: Quantum dots; Photodetectors; Solution Process; Ligand exchange; GISAXS.

ACS Paragon Plus Environment

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ABSTRACT: Recently, lead sulfide (PbS) quantum dots (QDs) have demonstrated great potential as becoming one of the most promising next-generation photoelectrical materials for photodetectors. PbS QDs provide fascinating properties including size-controllable spectral sensitivity, wide and tunable absorption range, cost-efficient solution processability and flexible substrate compatibility. One of the key problems that limit the performance of PbS QDs based photodetectors is the inefficient carrier transfer. Long ligands decorating the outside surface of PbS QDs to protect them against degeneration inhibit the transfer of electrical charge carriers and thereby limit the device performance. To overcome this problem, the long ligands need to be effectively exchanged. Here, a two-step ligand exchange method is demonstrated. The QDs are pretreated using methylammonium iodide (MAI) in solution as the first step ligand exchange before the layer-by-layer (LBL) deposition process and solid-state ligand exchange. The grazingincidence small-angle X-ray scattering (GISAXS) and X-ray photoelectron spectroscopy (XPS) analyses prove a smaller spacing among the QDs and an increased ligand exchange ratio by adopting the two-step method. This strongly indicates a better capability of charge transfer than the traditional one-step solid-state ligand exchange technology. Devices fabricated using the twostep method present an enhancement of the charge transfer capability with a lager current. The efficient charge transfer is further demonstrated by a significant 94 % increase of the responsivity and a 57 % enhancement of the detectivity of the PbS QDs-based photodetector, reaching 3302 mA/W and 5.06 ×1012 J, respectively.

1. Introduction


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High responsivity, wide spectral sensitivity and low-cost photodetectors are core components for applications in sensing, optical fiber communication and imaging systems.1-4 Currently commercialized photodetectors are mostly based on gallium phosphide (GaP), silicon (Si), indium gallium arsenide/germanium (InGaAs/Ge) or lead sulfide/ selenide (PbS/PbSe) prepared through an epitaxial growth method to cover the most commonly used wavelength range (ultraviolet (UV), visible and infrared region (IR)), respectively. Colloidal quantum dots (QDs) provide a new opportunity for solution-processed thin-film photodetectors, not only with low costs, but also in a wide detection range covering the entire wavelength regime from UV to IR wavelengths.5-10 Moreover, highly sensitive colloidal QDs based photodetectors have been demonstrated to cover a broad spectrum from the UV-visible to the short-wavelength and mid IR range. Specifically, PbS QDs outperform other colloidal QDs for photodetector applications.9-11 PbS QDs have a particular broad and tunable absorption spectrum from the UV to the near IR region, showing its potential as a photodetector with an ultra-wide detection range.12 In addition, they have a weak exciton binding energy of 100 meV as a result of their long radiation combination lifetime and high dielectric constant, resulting in a better charge carrier transport and charge extraction.12-15 Moreover, the multiple exciton generation observed in PbS QDs enables the generation of multiple electron-hole pairs when absorbing one photon, which has provided exciting possibilities for improving the energy conversion efficiency for these photodetectors.16-19 In the literature, there is a number of methods to synthesize monodisperse PbS QDs, and most of them involve different kinds of long ligands (typically, oleic acid (OA-)) on the surface of QDs’ to maintain their stability. However, the presence of the long capping ligands directly influences the conductivity of the PbS QDs solid serving as the active layer in the photodetectors. The charge tunneling process is strongly influenced by the separation between neighboring QDs, which is


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mainly determined by the length of the ligands. The long-length OA- ligands (~2.5 nm) act as a tunneling barrier for charge carriers, resulting in a weak coupling between the individual QDs, because the wave function of the charge carriers in each QD is still effectively confined within it.20-21 Limited efficiency of charge tunneling confines the transport of photogenerated charge carriers between QDs and finally results in a limited device responsivity. One widely used solution to the problem is exchanging the long ligands with short ligands such as hydroxide ions (OH-), sulfide (S2-) or halide ions (I-, Br-, Cl-).21-24 To achieve this, a solid-state ligand treatment method (exchange process after solid formation) was used to replace the OA- ligands with shorter ligands by a chemical treatment to improve their performance. In combination with the solid-state ligand treatment, normally a layer-by-layer (LBL) deposition was used to reach a certain film thickness as well as to offset cracks and voids generated during the exchange in the PbS QDs solids. The use of such short ligands decreased the gap between QDs and enhanced the charge transportation. Consequently, photodetectors with ligand exchange on solid-state base proved to be effective and related photodetectors demonstrated responsivity values over 500 mA/W.25,26 Although this method is simple and effective, further improvements were achieved by exchanging the residual long ligands via a two-step method.28,29 Based on such two-step method, recently we have demonstrated an enhanced responsivity of PbS QD based photodetectors.30 In this early-stage work, however, the PbS QD synthesis was far from being optimized and high dark current limited the entire device performance. Moreover, the decreased separation between QDs after two-step ligand exchange missed experimental prove. In the present study, we optimize our initial two-step method for the fabrication of QDs based photodetectors demonstrating a significantly improved device performance. Briefly, the QDs’ twostep treatment is performed by using methylammonium iodide (MAI) to pretreat the PbS QDs in


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solution as a first step, which is carried out before the deposition and the solid-state ligand exchange process (the second step). The improved synthesis and device fabrication process enables a decrease in the dark current by a factor of 10 as compared with our early stage results.30 Based on a more comprehensive material and device characterization, we provide an in-depth discussion to explain the enhanced responsivity. Thus, taking advantages of this pretreatment, we demonstrate PbS QDs based photodetectors with an overall high responsivity. Moreover, in the first step, the PbS QDs are treated by an optimized concentration of MAI in solution to replace the long chain ligands (OA-) with iodide ions (I-). The second step is a solid-state ligand treatment integrated with an LBL deposition to reach the desired film thickness. The binding energy of the ligands is studied with X-ray photoelectron spectroscopy (XPS). Time-resolved photoluminescence (TRPL) is used to analyze surface defects and charge carrier lifetimes. Moreover, grazing-incidence small-angle X-ray scattering (GISAXS) is applied to observe and compare the inner structure of QDs’ solids fabricated via a classical “one-step” and our optimized “two-step” method. Based on the developed two-step treatment of the PbS QDs forming the active layer of a photodetector, we achieve an excellent device performance with the responsivity of 2121 mA/W at 860 nm and the responsivity of 3302 mA/W at 500 nm which significantly outperforms our previous work.30 This translates into an enhancement of 94 % in the responsivity as compared to the device with one-step treatment.

2. Result and discussion


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Figure. 1 (a) Representative HRTEM image of synthesized PbS QDs shows that the QDs have a diameter around 3.5 nm. (b) Absorption and PL spectra of PbS QDs films using one-step (blue) and two-step (red) ligand exchange methods.

The PbS QDs synthesis follows previous works with modifications as seen in SI.12,30 Notably, the purification process has been modified by reducing the number of washing times. Multi-times centrifugation could result in rapid falling off of OA- ligands on QDs and cause the aggregation of QDs. High-resolution transmission electron microscopy (HRTEM) (Figure 1a) provides detailed information of the synthesized OA-capped QDs with high quality. The PbS QDs are uniform and have an average diameter of 3.5 nm. The QDs films are fabricated using the classical one-step and our optimized two-step ligand exchange processes to acquire the absorption and emission spectra as shown in Figure 1b. The MAI pretreatment used in the two-step process followed previous literature with slight modifications.6,30 Two miscible solvents, N, N-dimethylformamid (DMF) and toluene, are mixed to make sure that the ligand exchange pretreatment is successful by providing a combination and reaction of QDs and I-.20 In addition, the shell of MA+ and OA- pairs offer a steric hindrance to avoid aggregation of QDs (The HRTEM of QDs after the first ligand exchange


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process in solution is given in Figure S1).29-31 One-step fabricated QD solid films exhibit an absorbance spectrum with a first exciton absorption peak at a wavelength of 860 nm and a photoluminescence (PL) spectrum with a peak at 1005 nm as shown in Figure 1b. In contrast, the two-step treated QD solid films reveal a slight red-shift of the absorption spectrum as seen in Figure 1b. This phenomenon has been found related to the shift of electron coupling energy caused by decreased separation between QDs.32,33

Figure. 2 (a) Schematic of the fabrication process of the photodetector, including two steps of ligand exchange in steps (i) and (iv). (b) Schematic of ligand exchange process in which the long ligands are replaced by short ligands during the solution phase (i) and solid phase (iv).


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The LBL depositions for all samples are cycled for 10 times to reach the desired film thickness. All these fabrication processes are shown in Figure 2a. The detailed device fabrication process is given in the SI. The long chain OA- are supposed to be replaced by I- ions in an optimized situation during the two-step treatment (Figure 2b) due to the kinetic binding energy.32 After films’ fabrication, XPS testing is applied on the QD solids to quantitatively confirm the ligand exchange process as seen in Figure 3a. As shown in Figure 3b, a higher absorption of iodine on the surface of the two-step ligand exchanged PbS QDs solids indicates that more I- was attached on the QDs, while a decreased adsorption of oxygen and carbon (Figure 3c and 3d) show more stripping of the long OA- ligands. Table 1 illustrates the variations of the atomic content in the ligand exchanged PbS-QDs solids. We normalize these values to the amount of the Pb element in order to have a more quantitative comparison. The two-step treated solids show less carbon and oxygen due to the decrease of the carbon and carboxylate oxygen amount after the MAI exchange, which further confirms that the OA- ligands have been exchanged successfully. Whereas the increase of iodine illustrates that more iodine ion ligands have been successfully introduced through the applied twostep method. The enhancement of the iodine ion ligands will decrease the separation and increase the coupling of the charge carriers between the QDs and thereby facilitate the tunneling of charge carriers, resulting in an enhancement of the performance of the devices. In developing the MAI pretreatment process, we find that the amount of applied MAI plays a critical role. The amount of MAI applied for the solution-phase process of PbS QDs was systematically investigated with a set of different concentrations in order to avoid the overtreatment, which will be discussed as follows.


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Figure. 3 (a) XPS scan for one-step and two-step films, respectively. Normalized XPS scan about (b) iodide, (c) oxide and (d) carbon give the percentage change of elements between one-step and two-step solids. Table 1. Atomic percentage of PbS QDs with and without MAI treatment Atomic %

Pb

C

O

I

One-step

5.5

79.8

13.3

1.4

Normalized one-step

8.9

130.1

21.7

2.4

Two-step

8.9

67.9

19.9

3.3

For photodetectors, a wide detection range or absorption spectrum of the active material is highly desirable. Another key factor is the responsivity ( R ). It is determined by the amount of photo-


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generated current I ph per incident light power Pin and describes the light-current conversion efficiency of the device. It can be calculated using equation (1):

R ( ) 

I ph ( ) Pin ( )



I p ( )  I d ( ) Pin ( )

(1)

where I p and I d refer to the currents through the device under illumination and in dark, respectively. Moreover, the detectivity ( D* ) is used to evaluate the efficiency of the device considering the active area and noise current, which is determined by equation (2),

D * ( ) 

R ( ) I

2 n

 A

(2)

where A stands for the effective area, and I n stands for the noise current spectral density. The noise current spectral density contains flicker noise, thermal noise and shot noise.34-36 Here, we simplified the calculation following previous works (for accurate calculation, the 1/f noise should be considered):25, 26, 29

D * ( )  R ( ) 

A 2qI d

(3)


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Figure. 4 (a) Dark and (b) illuminated current of devices prepared via the one-step and the twostep method. (c) Cross-section SEM image of device prepared via one-step method. (d) Responsivity (R, blue) and detectivity (D, red) for the devices shown as function of the MAI concentration. The one step-solid method is in the gray shaded area. All devices are illuminated under 500 nm, 12.2 uW/cm2. The R and D are calculated under a bias of 40 V.

During the pretreatment with MAI solution varying the concentration from 0.01 to 0.06 M, we achieve well mono-disperse QDs without QD aggregation. The devices based on these 6 groups of MAI pretreated QDs are compared to a group of un-pretreated QDs (resembling the one-step method). As shown in Figure 4a, the dark current increases with increasing the MAI concentration, which is due to the enhanced ability for a charge carrier transport due to a closer separation between the QDs in combination with less surface defects on the QDs. Benefitting from


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a higher binding energy32, at higher MAI concentration, a larger amount of OA- could be replaced by I- resulting in a decreased separation between neighboring QDs. On the other hand, an increased concentration of I- could decrease the surface defects of QDs caused by exposed dangling bonds. As a result, the QD solids treated with a higher MAI concentration should have better electrical properties, which however will also result in larger dark currents as a negative side effect. In contrast, the illuminated currents (Figure 4b) show a different tendency as compared with the dark currents. The obvious improvement in the illuminated current is observed for a MAI concentration of only 0.01 M. Using more than 0.01 M MAI causes an excess of MAI which lowers the current. The enhanced illuminated current for all two-step samples can originate from an increased mobility of the charge carriers, as a result of having short ions attached on the QDs’ surfaces. To better understand this phenomenon, the surface morphology of the QDs films is probed with scanning electron microscopy (SEM) for various MAI pretreatments as shown in Figure 5 (For larger scale, see Figure S2). The QD solids, build from pretreated QDs using 0.01 M MAI, reveal smooth surfaces, whereas with increasing MAI concentration, the QD solids start to show pinholes and an increase number of defects. When we increase the MAI concentration over 0.02 M for the pretreatment in the first step, the surface morphology for the final QD solids turns worse due to the increased numbers of defects. QDs dissolved in toluene exist in a colloid state, but such colloidal state could be affected by introducing halide ions with a contain concentration. In our case, the increased MAI concentration affects this balance for the colloidal state and results in a worse film quality. These defects are less crucial for the dark current as compared with the illuminated current. Without illumination, the dark current is mainly dominated by the intrinsic conductivity of the QD solids (The conductivity is given in Table S1). Though the defects also resist the transport of carriers, the enhancement of carrier transport efficiency outplays the


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increased defects under higher concentration of MAI. With illumination using light, the photogenerated carriers are restrained severally by reduced extraction efficiency caused by increased number of aggregation of QDs under high concentration of MAI. Thought the carrier transport efficiency is enhanced, less extracted photogenerated carriers resulted in reduced illuminated current. The cross-section SEM image (Figure 4c) demonstrates a thickness of ~130 nm of the QDs solid, which was fabricated on a bare glass substrate using 10 times the LBL process. According to equations (1) and (3), the responsivity and detectivity of these devices are calculated (Figure 4d). As compared to the one-step sample, the two-step samples exhibit an enhanced performance only if the MAI concentration is lower than 0.03 M. The best optimized devices (pretreated by 0.01 M MAI) show 94 % enhancement in the responsivity (from 1701 mA/W to 3302 mA/W). Moreover, the detectivity is also enhanced by 57 % from 3.23 ×1012 J to 5.06 ×1012 J. Notably, both the responsivity and the detectivity of the two-step method-based devices are even higher than that of most devices reported in previous literature. A similar device architecture (photoconductor) as well the power intensity (in µW/cm2 level) of the shining light have been taken into account in this comparison.19, 25-27 A detailed comparison is given in Table S2.


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Figure. 5 SEM images for two-step ligand exchanged samples with increasing the MAI concentration from 0.01 M to 0.06 M in steps of 0.01 M (a-f).

The improvements for the two-step devices can be explained by two factors: 1) An enhanced charge transfer efficiency is achieved by the decreased gap between neighboring QDs, which causes higher dark and illuminated currents. 2) More defects in the two-step solids prepared with MAI concentration above 0.03 M, restrained the extraction of photogenerated carriers and result in lower illuminated currents. The first factor can be recognized from the continuously increasing dark current with increasing MAI concentration. Moreover, the illuminated currents are also enhanced with the two-step method, but when further increasing the MAI concentration, this enhancement cannot balance losses due to defects, which results in a constantly decrease of the current. Concerning the device performance, illuminated currents dominate the responsivity calculation because the dark currents are smaller in magnitude than the illuminated currents. Thus, we find the best optimized responsivity when using 0.01 M MAI for the first step, which gives rise to the largest illuminated current. A decrease in the illuminated current caused by the precipitation of QDs results in a decrease of the responsivity as well. Within the probed range of MAI concentrations, the responsivity decreases from 3302 mA/W to 1662 mA/W if the pretreatment is performed with 0.06 M MAI. The responsivity, device area and dark current affect the detectivity of the photodetector. In case of the best optimized two-step sample with 0.01 M MAI, also the highest detectivity is found. Since the active area remains the same for all devices, the detectivity


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follows the same trend as the responsivity when changing the MAI concentration and higher MAI concentrations give rise to a decreasing detectivity.

Figure. 6 Responsivity for one-step (blue) and two-step (0.01 M MAI, red) method-based devices at (a) different wavelengths with a light intensity of 12.2 uW/cm2 and (b) different light intensities, illuminated with 500 nm light. A bias voltage of 40 V is applied for all devices. Considering the effects of the light intensity and wavelength on the performance of the photodetectors, further measurements of the device performance are done under different conditions. The two-step devices are fabricated with 0.01 M MAI pretreatment. The probed wavelength range and light intensities are indicated in Figure 6. The calculated responsivities at different wavelengths are shown in Figure 6a. The two-step device shows a similar wavelength dependence of the responsivity as the one-step device, but the absolute values are significantly improved in the entire test range for the two-step device. A maximum responsivity of 3266 mA/W is reached at 500 nm. For shorter wavelength below 500 nm, although the absorption coefficient is higher in this region, photogenerated carriers more likely recombine at the surface of the device. These photogenerated carriers can no more contribute to the photocurrent and thereby cause a


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reduced responsivity.37 Moreover, at 860 nm, where the absorption peak of the PbS QDs is located, another small peak in the responsivity of 2121 mA/W is observed due to the QDs’ first exciton absorption peak. Different ligand exchange methods barely change the absorption spectra as indicated in Figure 1b, which ensures the similar spectral response for both devices. The obviously enhanced responsivity comes from not only a higher photogenerated current as a result of a decreased trap density, but also originates from the modified transport of the charge carriers between the QDs. Concerning light intensities, the responsivities for devices based on one-step and two-step methods show the same trend. The values decrease with increasing the light power density for the excitation as seen in Figure 6b. Thus, a nonlinear response as a function of light intensity is observed. The maximal responsivity of the device prepared through the two-step method reaches 3266 mA/W under 12.2 uW/cm2 with a wavelength of 500 nm, which is 101 % higher than the maximal responsivity (1623 mA/W) of the device based on the one-step method. This high responsivity indicates a long photogenerated charge carrier lifetime as compared with the carrier transit time between the electrodes. The increased illumination light power fills the long-live trap states inside the QD solid, reduces the carrier lifetime and finally results in a lower responsivity.5,10


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Figure. 7 The rise (to 90 % max) and fall (to 10% max) time for (a) one-step and (b) two-step devices. (c) Normalized TRPL of PbS QD solids fabricated through two-step (red dots) and onestep (blue dots) methods. Black lines show best fits. (d) Frequency response of the normalized peak to peak current amplitude. The device is illuminated under 500 nm light with 12.2 uW/cm2 intensity.

Next, the response speed of the fabricated device is characterized through the analysis of light on-off processes and frequency response. Figure 7a and 7b show the rise (to 90%) and fall (to 10%) time of the devices fabricated through the one-step and two-step methods, respectively. As indicated from the Figures, the rise and fall times for one-step device are slightly longer than that


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of two-step devices. The reason is that the two-step ligand exchange process produced a QD solid with smaller number of traps, which is explained by the TRPL result of QD solids shown in Figure 7c. Both decay of PL intensity could be fitted with a biexponential model as shown with black lines in Figure 7c. The faster decay factor (τ1) could result from the charge carrier hopping while the slower decay factor (τ2) is related to radiative recombination.22,25 Benefitting from a higher binding energy38, the two-step ligand exchange process is able to address a higher amount of I- on the surface of QDs to decrease the surface traps. Less traps in the two-step fabricated QD solids results in less charge trapping during the charge transport process. Moreover, we assume that a closer separation between the QDs in two-step fabricated QD solids assists the carrier transport and results in a faster tunneling rate and a longer carrier lifetime with an enhanced responsivity. Importantly, all these parameters affect the bandwidth of the fabricated device. Figure 7d shows the frequency response of the normalized peak to peak current amplitude. The light is modulated by a light chopper run at specific frequencies. The -3 dB bandwidth is marked and the two-step fabricated device exhibit an optimized bandwidth of 50 Hz, as compared with the one-step fabricated device which has a bandwidth of 17 Hz.


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Figure. 8 2D GISAXS data for (a) one-step solid and (b) two-step solid, respectively. (c) Vertical line cuts with corresponding Yoneda peak positions, (d) Horizontal line cuts at the Yoneda region and best fits (black lines) based on a sphere model. (e) Domain analysis in solids according to the information from horizontal line cuts.

To further explain the benefits from our optimized two-step treatment, we use GISAXS to probe the inner structure of the QD solid films and explain the relations between the device performance and the inner film structure.39,40 Figure 8a and 8b show the 2D GISAXS data of QD solid films prepared with the classical one-step and with the two-step method, respectively. Vertical and the horizontal line cuts, as indicated in Figure 8a, are performed to study each sample’s solid density,


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inner structures and domain information. Figure 8c shows the vertical line cuts for both types of samples with the specular beam positioned at 0.4 °. According to the scattering length density, the Yoneda peaks for Si and SiOx are at 0.20 and 0.22 ° for the used X-ray photon energy of 8 keV. Therefore, the Yoneda peaks located at 0.26 ° for the one-step sample and at 0.30 ° for two-step sample can be assigned to the stacked QDs structures. The calculated critical angle of a densely packed QD solid with a density of 7.6 g/cm3 would be positioned at 0.36 °, which indicates that both solids are not that densely packed. In more detail, the two-step processing results in a higher solid density than the one-step method. The horizontal line cuts are done at the individual Yoneda peak positions and are shown in Figure 8d. We have used a three-sphere model (form factors are based on hard spheres with different sizes) to fit the horizontal line cuts and determine the structure information.41,

42

In the model, the smallest form factor is given by the QD’s radius and the

corresponding structure factor represents the inter-dot distance. Besides the differences in the QDs’ inter-dot distance, also the presence of larger inner structures contributes to the increased density of the two-step treated solid. These larger structures are formed by close packed QDs with excellent QD crystal facet-to-facet alignment and can be considered as quasi solid regions providing higher density. In order to roughly estimate and compare sizes and distances in our fit model, the used form factors and structure factors are summarized in Table 2. We observe that the two-step solid exhibits a smaller inter-dot spacing and a larger size domain distribution than the one-step solid method as sketched in Figure 8e. The larger size of the QDs domains and the smaller inter-dot spacing in the two-step solid can be assumed to outperform the one-step solid with respect to the photoelectric performance when being used as photodetectors. The two-step solid has a higher density than the one-step solid due to a closer packing of the individual QDs. The closer


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distances between individual QDs and QD domains are beneficial for the electronic coupling behavior and conductivity of the solids. Table 2. Parameters derived from a three-sphere model-based line-cut fitting Domain size / Inter distance (nm)

One-step

Two-step

Large

74 / 120

274 / 510

Media

4 / 15

6 / 35

Small (single QD)

3.0 / 3.2

3.0 / 3.0

3. Conclusion In conclusion, we have designed, fabricated and optimized a process flow for two-step ligandexchanged photodetector based on PbS QDs. This method combines a MAI solution-phase pretreatment with a LBL solid-state ligand exchange process. The XPS results confirm the more successful exchange of I- ligands, which causes the significant enhancement of the electrical performance and of the responsivity of the device. The concentration of MAI plays an important role in the pretreatment process, since the fabricated QD solid could suffer from a large amount of defects at higher MAI concentrations. When using a concentration of 0.01 M MAI for the pretreatment of the QDs, the optimized responsivity of corresponding devices reaches 3302 mA/W, with an enhancement of 94 % as compared with the traditional one-step method. Moreover, the optimized detectivity reaches to 5.06 ×1012 J with an improvement of 57 %. TRPL results show a more efficient hopping rate and an enhanced charge carrier lifetime for the two-step fabricated solids, corresponding to fewer surface defects on QDs and shorter separation between QDs, respectively. In addition, the GISAXS measurements indicate that the two-step solid reveals a


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higher solid density and a decreased gap between neighboring QDs inside the QD films, which explain very well the improved device performances.

AUTHOR INFORMATION Supporting Information: Experimental details including PbS QDs Synthesis, ligand exchange treatment, device fabrication process and characterizations of QD solids and devices; Additional data and diagram including: HRTEM image for MAI pretreated PbS QDs; energy band diagram of the device; additional SEM images of QD solids; device comparison with the previous work; conductivities of QD solids; comparison of device performances. Corresponding Author * E-mail: [email protected], [email protected], [email protected] Author Contributions ▽These

authors contributed equally to this work.

Notes The authors declare no competing financial interest. Acknowledgement This work was supported by the National Key Research and Development Program of China administrated by the Ministry of Science and Technology of China (No. 2016YFB0401702), National Natural Science Foundation of China (Nos. 61875082, 61674074), Natural Science


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Foundation of Guangdong (No.2017B030306010), Shenzhen Innovation Project (No.JCYJ20160301113537474), Shenzhen Key Laboratory for Advanced Quantum Dot Displays and Lighting (No.ZDSYS201707281632549), Shenzhen Peacock Team Project (No.KQTD2016030111203005), Development and Reform Commission of Shenzhen Project (No. [2017]1395), and Tianjin New Materials Science and Technology Key Project (No.16ZXCLGX00040). W. C. is grateful for financial support from the China Scholarship Council (CSC). P.M.-B. acknowledges the Nanosystems Initiative Munich (NIM).

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TOC：

(For Table of Contents Only) PbS quantum dot with a two-step ligand exchange treatment has been employed achieving better performance photodetector. QDs, with an extra pre-ligand exchange in solution, have revealed an optimized carrier transfer property in solid and GISAXS analysis has proved the inner structure optimization of the two-step treated solid. The improved device shows the responsivity over 3302 mA/W with enhancement of 94.1% compared with one-step device.


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Lead Sulfide Quantum Dot Photodetector with Enhanced Responsivity

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