Ultrasensitive Solution-Processed Broadband PbSe Photodetectors

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Functional Inorganic Materials and Devices

Ultrasensitive Solution-Processed Broadband PbSe Photodetectors through Photomultiplication Effect Tao Zhu, Luyao Zheng, Xiang Yao, Lei Liu, Fei Huang, Yong Cao, and Xiong Gong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21966 • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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ACS Applied Materials & Interfaces

Ultrasensitive Solution-Processed Broadband PbSe Photodetectors through Photomultiplication Effect Tao Zhu,1 Luyao Zheng,1 Xiang Yao,2 Lei Liu,1 Fei Huang,3 Yong Cao,3 and Xiong Gong1* 1) Department of Polymer Engineering, College of Polymer Science and Polymer Engineering, The University of Akron, Akron, OH 44325, USA 2) Institute of Molecular Plus, Tianjin University, Tianjin 300072, P. R. China 3) State Kay Laboratory of Luminescent Materials and Devices, South China University and Technology, Guangzhou, 510640, P. R. China

ABSTRACT: Broadband photodetectors have important applications in both scientific and industrial sectors. In this study, we report room-temperature operated solution-processed photodetectors by PbSe quantum dots (QDs) with spectral response from 350 nm to 2500 nm. In order to boost both external quantum efficiency (EQE) and projected detectivity (D*), the holetrap assisted photomultiplication effect through EDT-PbSe QDs/TABI PbSe QDs double-thinlayer thin film, where EDT-PbSe QDs is 1, 2-ethanedithiol (EDT) capped PbSe QDs and TABIPbSe QDs is tetrabutylammonium (TABI) capped PbSe QDs, is applied. To further enhance D*, a thin layer of conjugated polyelectrolyte, which offers significant hole injection resistance for suppressing dark current but enhancing photocurrent under illumination due to the photo-induced self-doping process, is applied for reengineering the electron extraction layer in PbSe QDs based photodetectors. As a result, at room temperature, PbSe QDs based photodetectors exhibit over 450% EQE and over ~1012 Jones D* in the visible region, over 120% EQE and D* ~4×1011 Jones in the infrared region. These results demonstrate that our studies provide a simple approach to realize room-temperature operated solution-processed broadband photodetectors.

Keyword: PbSe quantum dots; broadband photodetectors; photomultiplication effects; conjugated polyelectrolyte; ligand exchange *Corresponding author, Email: [email protected], Fax: (330) 9723406 1 ACS Paragon Plus Environment

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INTRODUCTION Sensing from ultraviolet (UV) - visible to infrared (IR) is critical to environmental monitoring and remote sensing, fiber-optic communication, day-/night-time surveillance, and emerging medical imaging modalities.1 Today, separate photodetectors (PDs) or materials are required for different sub-bands within the UV to IR range.1 For example, bulk InGaAs based PDs was used for the near IR (NIR), 2 and bulk PbS based PDs was used for mid-IR (MIR) regions.3 However, these IR PDs must be operated at low temperatures (for example at 4.2 K) to obtain reasonable detectivity.2,3 Additionally, the fabrication of these IR PDs are very complicated and expensive. These difficulties have motivated the development of ultrasensitive novel broadband PDs, which is operated at room-temperature and under low applied voltages for scientific and industrial applications.4-7 Inorganic quantum dots (QDs), for example, PbS and PbSe QDs, with size-tunable optical properties and low-temperature solution processability, have been emerged as one of alternatives for realizing broadband PDs.8-13 Previously, we demonstrated broadband PDs by utilizing hybrid organolead halide perovskite materials and PbS QDs as the light harvesters. These PDs exhibited a broad spectral response from 375 nm to 1100 nm, but further improvement in spectral response was restricted by relatively weak quantum confinement.11 As compared with PbS QDs, PbSe QDs has a large Bohr radius of 46 nm,7 which creates strong quantum confinement effect and enables its optical band edge being tuned up to MIR region. Recently, Dang et al., reported PbSe QDs based PDs with a planar structure exhibited a spectra response ranging from 1.5 μm to 3.4 μm, and a projected detectivity (D*) and the external quantum efficiency (EQE) at 1400 nm were only ~109 Jones (1 Jones=1 cmHz1/2W-1) and 25% at a larger bias of 5V,14 respectively. Such poor device performance was ascribed to low absorption coefficient and poor electrical

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conductivity of PbSe QDs.15 In addition, it was reported that large-sized PbSe QDs were extremely sensitive to oxidative doping upon air exposure,16-18 which creates the sub-band trap states, inducing severe charge carrier recombination and reducing photocurrent, and consequently lowering down D* and EQE of PbSe QDs based PDs.18 Trap-assisted photomultiplication (PM) effect, where the trap-states were deliberately introduced into the surface of photoactive layer, forming a barrier-less Ohmic contact at the interface between photoactive layer and the electrode for efficient electron (or hole) injection from the electrode, has been demonstrated to be an effective way to boost D* and EQE of PDs.1921

Previously, by introducing PbS QDs into the surface of perovskite thin film for inducing

Ohmic contact at the perovskite/aluminum (Al) interface, we observed extremely high EQE (or R) and D* from perovskite based PDs.19 However, the photoresponse of this PDs was still limited to 800 nm, which substantially restricts its applications. In this work, we report solution-processed PbSe QDs based PDs with spectral response from 350 nm to 2500 nm. By forming a thin film with double-thin-layer structure of EDT-PbSe QDs/TABI PbSe QDs, where EDT-PbSe QDs is 1, 2-ethanedithiol (EDT) capped PbSe QDs and TABI-PbSe QDs is tetrabutylammonium (TABI) capped PbSe QDs, the pronounced trapassisted PM effect is applied in PbSe QDs based PDs. As a result, boosted EQE and D* are observed. To further enhance D*, a thin layer of conjugated polyelectrolyte, which offers significant hole transport resistance in dark and high electrical conductivity under illumination by photo-induced self-doping process, is applied in PDs for suppressing dark current density and boosting photocurrent density. Thus, at room temperature, solution-processed broadband PbSe QDs based PDs with EQE over 450 % and 120%, and D* over 1012 Jones (1 Jones=1 cmHz1/2W-1) and 4×1011 Jones in the visible and MIR regions, respectively, are observed.



EXPERIMENTAL SECTION Materials: PbO (99.999%) was purchased from Alfa Aesar. Oleic acid (OA) (90%), 1octadecene (ODE) (90%), trioctylphosphine (TOP) (97%), diphenylphosphine (DPP) (98%) and selenium (99.99%), 1, 2-Ethanedithiol (EDT) (98%), tetrabutylammonium iodide (TBAI) (98%), methanol (99.8) and acetonitrile (ACN) (99.8%) were purchased from Sigma Aldrich. All materials are used as received without further purification. Synthesis of PbSe QDs: PbO (0.54 g), OA (2 mL), and ODE (7 mL) were added in a 50 mL flask. The mixed solution was degassed by stirring under vacuum at 120 ℃ for 2 hours. Subsequently, the mixed solution was kept at 120 ℃ under nitrogen atmosphere. In another flask, the selenium precursor was prepared by mixing selenium powder (0.59 g) in 7.5 mL of TOP and 130 uL of DPP under N2 atmosphere. Then, the solution of Pb-oleate was heated to 190 ℃ under nitrogen atmosphere. At this temperature, the selenium precursor was rapidly injected and retained for 6 minutes. After the reaction, the flask was cooled down to room temperature rapidly. Then, the formed OA capped PbSe quantum dots (OA-PbSe QDs) were isolated by precipitation with acetone and subsequently re-dispersed into octane.22 Ligand exchange in PbSe QDs thin films: A solution of 1.7 mM EDT in acetonitrile and 30 mM TBAI in methanol were prepared separately. OA-PbSe QDs thin film was spin-casted from 50 mg/mL OA-PbSe QDs octane solution at a spin speed of 2000 rpm for 20 seconds (s). Then rinse the OA-PbSe QD thin film by either EDT acetonitrile solution or TBAI methanol solution for 30s. Afterward, either acetonitrile or methanol solvent is used to wash out the excess organic ligand. Repeat above steps for several times to obtain either EDT-PbSe QDs thin film or TBAIPbSe QDs thin film.


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ZnO thin film preparation: ~ 40 nm ZnO thin film was spin-casted on the top of pre-cleaned FTO glass from ZnO precursor solution, which was synthesized by sol–gel method, followed with thermal annealing at 200 °C for 60 mins.23 Afterward, ZnO thin film was ultrasonicated in isopropyl alcohol and subsequently dried in an oven for further application. Thin film characterizations: Absorption spectra of PbSe QDs thin films were measured by Lambda 750 UV/Vis/NIR spectrometer from PerkinElmer Company. The transmission electron microscopy (TEM) images were measured by Model JEOL JSM-1230. The t-DOS of the diodes by either a 5-thin-layer EDT-PbSe QDs thin film or a 4-thin-layer EDT-PbSe QDs/1-thin-layer TBAI-PbSe QDs thin film were characterized by Keithley model 82-WIN simultaneous capacitors versus voltage system. The grazing-incidence wide-angle X-ray scattering (GIWAXS) measurement was carried out in beamline 8-ID-E, Advanced Photon Source, Argonne National Lab. Fabrication of photodetectors: 30 nm PNDIT-F3N layer was spin-casted on the top of FTO/ZnO substrate from 10 mg/mL PNDIT-F3N methanol solution at a spin-speed of 1000 rpm for 10s and. Afterward, PbSe QDs thin film was deposited on the top of PNDIT-F3N thin film using layer-by-layer method (5-thin-layer EDT-PbSe QDs thin film, a 4-thin-layer EDT-PbSe QDs/1-thin-layer TBAI-PbSe QDs thin film or 3-thin-layer EDT-PbSe QDs/2-thin-layer TBAIPbSe QDs thin film). Finally, 10 nm MoO3 and 100 nm Ag were sequentially deposited by thermal deposition in a vacuum disposition system. The device area is measured to be 0.045 cm2. Characterization of photodetectors: The current density versus voltage (J-V) characteristics of PDs were conducted in dark and under white light with the light intensity of 100 mW/cm2, under monochromatic light at wavelength (λ) of 1300 nm illumination with the light intensity of 0.40 mWcm-2, and λ of 2400 nm with the light intensity of 0.10 mWcm-2 by Keithley model



2400, respectively. The EQE spectra were measured by quantum efficiency measurement system (QEX10) with a 300 W steady-state xenon lamp as the source light. The transient photocurrent was measured by optical chopper controlled at λ of 640 nm laser pulse at a frequency of 3 KHz.

RESULTS AND DISCUSSION In order to enhance the electrical conductivity of PbSe QDs, 1, 2-ethanedithiol (EDT), which is with short thiol (S-H) chain, is used to substitute OA, which is with long carbon chain (C-C), through ligand exchange process,24-28 for generating EDT capped PbSe QDs (termed as EDTPbSe QDs). Moreover, in order to enhance both electrical conductivity and stability of PbSe QDs, tetrabutylammonium (TABI), which is with iodine (I-) and is insensitive to air16-18 is used to substitute OA through ligand exchange process as well,24-29 for generating TABI capped PbSe QDs (termed as TABI-PbSe QDs). Molecular structures of OA, EDT and TABI are shown in Figure 1a. The crystal orientations of PbSe QDs thin films are confirmed by GIWAXS profiles. Figure 1b displays the azimuthally integrated intensities of the GIWAXS patterns based on twodimension GIWAX profiles of OA-PbSe QDs thin film, TBAI-PbSe QDs thin film, EDT-PbSe QDs thin film and 4-thin-layer EDT-PbSe QDs/1-thin-layer TBAI-PbSe QDs thin film (Figure S1). All thin films exhibit the identical scattering rings, indicating that crystallographic orientations of PbSe QDs are the same. The azimuthally integrating the scattering intensities over the scattering vector q (where q = 4π sinθ/λ, θ is the half of the scattering angle, λ = 0.116 nm is the X-ray wavelength) affords the GIWAXS patterns. As shown in Figure 1b, the peaks at q=1.30Å-1, q=-1.70Å-1, q=-1.96Å-1, and q=-2.80Å-1 are for all PbSe QDs thin films, no matter these PbSe QDs were capped by different molecules (OA, EDT or TABI) or with different layer


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structure. All these results indicate that neither EDT nor TBAI nor OA has any effect on the crystal structure of PbSe QDs. These observations are consistent with others.30 Figure 1c presents the J-V characteristics of two-terminal diodes fabricated by EDT-PbSe QDs, TBAI-PbSe QDs and OA-PbSe QDs thin films. Although the thickness of these thin films is approximatively the same, the current densities at the same biased voltages are dramatically different. The current density of the diode by the EDT-PbSe QDs thin film is higher than that by the TBAI-PbSe QDs thin film, which is higher than that by the OA-PbSe QDs thin film. All these results indicate that the electrical conductivity of the EDT-PbSe QDs thin film is higher than that of the TBAI-PbSe QDs thin film, which is higher than that of the OA-PbSe QDs thin film. The TEM image of PbSe QDs thin film is presented in Figure 1d. Based on TEM image, average particle size of PbSe QDs is estimated to be ~7 nm. Figure 1e shows the absorption spectra of single-layer of OA-PbSe QDs thin film and a double-layer of EDT-PbSe QDs/TBAIPbSe QDs thin film. The optical bandgap (Eg) is estimated to be 0.5 eV. According to quantum confinement effect,31 the particle size of PbSe QDs is estimated to be ~ 7 nm, which is in good agreement with the TEM result. Interestingly, the normalized absorbance, particularly in IR region, of double-layer of EDT-PbSe QDs/TBAI-PbSe QDs thin film is higher than that of single-layer of OA-PbSe QDs thin film, indicating that IR absorption coefficient from PbSe QDs is boosted by double thin layer structure, i.e. n-type TBAI-PbSe QDs thin layer on the top of ptype EDT-PbSe QDs thin layer.27 This observation is in good agreement with others.30 By utilization of thin film with double thin-layer, i.e. n-type TBAI-PbSe QDs thin layer on the top of p-type EDT-PbSe QDs thin layer, weak absorption coefficient from PbSe QDs is circumvented (Figure 1c).27 Thus, spectral response, in particular in IR region, from PbSe QDs



thin film with double thin-layer structure can be realized. However, typically, IR PDs possess poor EQE in IR region.14 In order to boost both EQE and detectivity,19-21 trap-assisted PM effect is applied in the PD with a device structure of FTO/ZnO/EDT-PbSe QDs/TBAI-PbSe QDs/MoO3/Ag, as shown in Figure 2a, where FTO is fluorine tin oxide and acts as the cathode, ZnO is zinc oxide and acts the electron extraction layer (EEL). MoO3 is molybdenum oxide and acts as the hole extraction layer (HEL), Ag is silver and acts as the anode, respectively. The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels of ZnO, EDT-PbSe QDs, TBAI-PbSe QDs and MoO3, and the work functions of FTO and Ag are shown in Figure 2b. The double-layer of EDT-PbSe QDs/TBAI-PbSe QDs thin film as a photoactive layer not only boosts the IR absorbance (Figure 1e), but also has other twofold functions. Firstly, the photo-generated holes captured by the trap-state in the EDT-PbSe QDs thin film shifts both the LUMO and HOMO energy levels of the EDT-PbSe QDs thin layer. After the work function of Ag is pinned with the Fermi level of the EDT-PbSe QDs thin layer, the electron-injection barrier from the Ag anode is diminished. Secondly, deeper LUMO of TBAI-PbSe QDs thin layer compared with that of EDT-PbSe QDs thin layer would produce more pronounced PM effect in the double-layer of EDT-PbSe QDs/TBAI-PbSe QDs thin film. Additionally, the air-stable TBAI-PbSe QDs thin layer on the top of the EDT-PbSe QDs thin layer circumvents overall poor stability of PbSe QDs. Thus, once Fermi level of TBAI-PbSe QDs thin film is pinned with the work function of the Ag anode, the injected electrons from the Ag anode is easily tunneled through the MoO3 HEL at low external biases,19,20 predicting high EQEs (or R). In order to verify above hypothesis, a 5-thin-layer EDT-PbSe QDs thin film, a 4-thin-layer EDT-PbSe QDs/1-thin-layer TBAI-PbSe QDs thin film, a 3-thin-layer EDT-PbSe QDs/2-thin-


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layer TBAI-PbSe QDs thin film as the photoactive layer are under investigation with a device structure as shown in Figure 2a. The total thickness of photoactive layer is adjusted to be ~150 nm for comparison studies. Figure 2c presents the EQE spectra of PDs. It was found that the PDs fabricated by a 5-thin-layer EDT-PbSe QDs exhibits an EQE of 120% in the visible region and an EQE of 40% in the NIR region. Over 100% EQE indicates that the trap-assisted PM effect, which is attributed to EDT-PbSe QDs thin film that captures electron injected from the Ag anode,19-21 is occurred in this PDs. But the intrinsic property of p-type EDT-PbSe QDs thin film (with the Fermi level lies in the middle of the HOMO and LUMO energy levels) and the shallow trap states would allow holes to be transferred.32 As a result, the PDs by a 5-thin-layer EDT-PbSe QDs thin film possess limited EQEs. Through intentionally fabrication of a n-type TBAI-PbSe QDs thin layer on the top of p-type EDT-PbSe QDs thin layer, the surface energy levels between n-type TBAI-PbSe QDs thin layer and the Ag anode is ultimately reengineered, which significantly diminishes the electron-injection barrier from the Ag anode (Figure 2b) and provides deep holes trap-state and blocks holes to be transported as well. As a result, the PDs by the EDT-PbSe QDs/TBAI-PbSe QDs thin films (a 4-thin-layer EDT-PbSe QDs/1-thin-layer TBAI-PbSe QDs thin film, and a 3-thin-layer EDT-PbSe QDs/2-thin-layer TBAI-PbSe QDs thin film) possess tremendously improved EQEs in both visible and NIR regions. Particularly, the PDs by a 4-thin-layer EDT-PbSe QDs/1-thin-layer TBAI-PbSe QDs thin film exhibit the highest EQEs in both visible (~ 450%) and NIR (~ 120%) regions among all PDs. These results demonstrate the PDs by a 4-thin-layer EDT-PbSe QDs/1-thin-layer TBAI-PbSe QDs thin film possesses much more pronounced PM effect than those by a 3-thin-layer EDT-PbSe QDs/2-thinlayer TBAI-PbSe QDs thin film.



Instead of as the function of an interfacial layer, TBAI-PbSe QDs thin layer plays an important role in the PDs by a 4-thin-layer EDT-PbSe QDs/1-thin-layer TBAI-PbSe QDs thin film, since it converts to an inverse p-n junction (with n-type semiconductor on the anode electrode), which is adversely to photo-generation because of the charge carrier recombination at the p-n depletion region.33 Figure 2d shows the J-V characteristics measured in dark for the PDs fabricated by either a 5thin-layer EDT-PbSe QDs thin film, or a 4-thin-layer EDT-PbSe QDs/1-thin-layer TBAI-PbSe QDs thin film, or a 3-thin-layer EDT-PbSe QDs/2-layer TABI-PbSe QDs thin film, respectively. All PDs exhibit high dark current densities, which would result in poor D*.34 In order to suppress the dark current density, a thin layer of conjugated polyelectrolyte, 9,9-bis(3 ′ -(N,Ndimethylamino)propyl)- 2,7-fluorene)-alt-5,5 ′ -bis(2,2 ′ -thiophene)-2,6-naphthalene- 1,4,5,8tetracaboxylic-N,N′-di(2-ethylhexyl)imide (PNDIT-F3N)25 is deposited on the top of the ZnO EEL, as a hole blocking layer (HBL). The molecular structure of PNDIT-F3N is shown in Figure 3a. PNDIT-F3N used in above PDs has twofold functions. In dark, the PNDIT-F3N layer possesses poor electrical conductivity and low hole mobility that greatly suppresses dark current density (Jd). While under illumination, photo-induced n-type doping is occurred in the PNDITF3N thin film, which tremendously improves its electrical conductivity, consequently, boosts the electron transporting efficiency from PbSe QDs thin film to the ZnO EEL, resulting in enhanced photocurrent (Jph).35 The J-V characteristics of the PDs with a device structure of FTO/ZnO/4-thin-layer EDT-PbSe QDs/1-thin-layer TBAI-PbSe QDs/MoO3/Ag measured in dark and at white light are shown in Figure S2. The J-V characteristics of the PDs with a device structure of FTO/ZnO/PNDITF3N/4-thin-layer EDT-PbSe QDs/1-thin-layer TBAI-PbSe QDs/MoO3/Ag measured both in dark 10 ACS Paragon Plus Environment

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and under light illumination are shown in Figure 3b. As compared Jd observed from Figure S2, Jd observed from Figure 3 are depressed by approximatively three orders of magnitude (from 101

mA/cm2 for the PDs without the PNDIT-F3N HBL (Figure 2d) to ~10-4 mA/cm2 for the PDs

with the PNDIT-F3N HBL, biased at -0.5V), which could be attributed to poor electrical conductivity and low hole mobility of the PNDIT-F3N HBL. Under illumination, the electrical conductivity of the PNDIT-F3N HBL is quickly increased on account of the photo-induced ndoping mechanisms,35 which efficiently transports the photo-generated electrons from PbSe QDs layer to the ZnO EEL. As a result, a dramatically increased Jph is observed from PDs. For example, under an external biases of -0.5 V, Jph measured under the monochromatic light at λ=1300 nm with an incident light intensity (𝐿𝑙𝑖𝑔ℎ𝑡) of 0.40 mW/cm2 and λ=2400 nm with a 𝐿𝑙𝑖𝑔ℎ𝑡 of 0.10 mW/cm2 are 1.2 × 10-1 mA/cm2, and 5 × 10-3 mA/cm2, respectively. While under an external bias of -1.0 V, Jph at λ=1300 and λ=2400 nm is 4 × 10-1 mA/cm2 and 2 × 10-2 mA/cm2, 𝐽𝑝ℎ

respectively. According to the equations of R = 𝐿𝑙𝑖𝑔ℎ𝑡 and 𝐷 ∗ = 𝑅/ 2𝑞𝐽𝑑 (where 𝐽𝑑 is dark current density, q is the elementary electron charge and equals to 1.6×10-19 C),34 R are estimated to be 300 mA/W and 1000 mA/W, and D* are 1.4 × 1012 Jones and 1.9×1012 Jones, respectively, for PDs at λ=1300 nm and biased at -0.5 V and -1.0 V. R are estimated to be 50 mA/W and 200 mA/W, and D* are 2.3 × 1011 Jones and 3.7×1011 Jones, respectively, for PDs λ=2400 nm and biased at -0.5 V and -1.0 V. Noted that the doping process of PNDIT-F3N is a reversible process, which is indicated in Figure 3c. Such reversible process certainly ensures PDs to possess suppressed Jd and boosted Jph. In addition, the EQE values of the PDs incorporated with the PNDIT-F3N HBL is enhanced from 350 nm to 800 nm (Figure S3) as compared with that without the PNDIT-F3N HBL, which



is attributed to the PNDIT-F3N thin film possessing strong absorption ranging from 350 nm to 800 nm (Figure S4). Based on absorption spectrum (Figure 1c), the EQE spectrum (Figure S3), and R and 𝐷 ∗ at λ=1300 nm and λ=2400 nm, R and D* versus λ for the PDs biased at either -0.5 V or -1.0 V are estimated, and the results are shown in Figure 3d. Under a bias of -0.5 V, PDs exhibit R of over 300 mA/W in the visible region, over 100 mA/W in the NIR region (1100 nm-1500 nm), and over 40 mA/W in the MIR (1500 nm to 2500 nm); the PDs exhibit D* of over 1012 Jones ranging from 350 nm to 1500 nm, and over 1011 Jones ranging from 1500 nm to 2500 nm. Furthermore, under a bias of -1 V, the PDs exhibit R of over 1000 mA/W in the visible region, over 800 mA/W ranging from 1100 nm-1500 nm, and over 100 mA/W ranging from 1500 nm to 2500 nm, and D* of 1012 Jones ranging from 350 nm-1500 nm, and over 1011 Jones ranging from 1500 nm to 2500 nm. Such high R and D values are attributed to the PM effect, the double-layer structure of PbSe QDs, and durable function of the PNDIT-F3N HBL. These results are comparable to 2D metal dichalcogenide-based PDs.36 The linear dynamic range (LDR), another key parameter used to evaluate PDs, is estimated according to the equation of LDR = 20log

∗ 𝐽𝑝ℎ

( ), 𝐽𝑑

34,37

∗ where 𝐽𝑝ℎ is the photocurrent measured at the

𝐿𝑙𝑖𝑔ℎ𝑡 of 1 mW/cm2. As indicated in Figure 3e, at room temperature, the LDR is 68 dB for PbSe QDs based PDs under 1300 nm, which is comparable to that (66 dB, at 4K) for InGaAs PDs.1,2 The photoresponse time is also an important parameter for PDs. The rise time is defined as the time required for output signals from 10% to 90% of saturated Jph. Similarly, the fall time is defined as the time required for output signals from 90% to 10% of saturated Jph.38 As indicated in Figure 3f, a rise time of 140 μs and a fall time of 410 μs are observed from PbSe QDs based PDs. The slow photoresponse time is probably attributed to numerous traps induced in PbSe QDs 12 ACS Paragon Plus Environment

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surface by either EDT or TBAI. Under the illumination, the photo-generated charge carrier is captured by these traps, resulting in prolonged time for the photocurrent to be reached the steady state. As a result, the current grows slowly. After the light is turned off, charge carrier would undergo a de-trapping process, which also prolongs the time for the photocurrent from the steady state to the off state. Thus, PbSe QDs based PDs possess relative slow photoresponse time. To further understand boosted EQE (or R) and D*, the trap density of states (t-DOS) distributions in a 5-thin layer EDT-PbSe QDs thin film and a 4-thin-layer EDT-PbSe QDs/1thin-layer TBAI-PbSe QDs thin film are investigated. The photodiodes with the device structures of FTO/ZnO/5-thin-layer EDT-PbSe QDs/MoO3/Ag and FTO/ZnO/4-thin-layer EDT-PbSe QDs/1-thin-layer TBAI-PbSe QDs/MoO3/Ag, are applied for determining the chemical capacitance (𝐶𝜇) values from the capacitance-voltage (C-V) characteristics.33 The measurement was performed under an external bias which equals to open-circuit voltage (VOC) and under different illumination intensities. Under these conditions, there’s no net internal field, and all the photo-generated electrons/holes contributes to the Cµ change. Thus, the capacitance change dC

obtained from C-V measurement is approximately equal to the Cµ change (dV ≈ spectra, the

dC dV

at each VOC is obtained. By integrating the

dC dV

dCµ dV

). From C-V

vs VOC curves, the t-DOS

2 distribution can be plotted according to the equation of 𝐶(𝑛) µ = 𝐿𝑞 𝑔𝑛(𝐸𝐹𝑛). Figure 4a, b shows

the dC/dV as a function of VOC and the chemical capacitance as a function of Voc. Based on the results shown in Figure 4a, b, the t-DOS as a function of VOC is shown in Figure 4c. As compared with the PDs by a 5-thin-layer EDT-PbSe QDs thin film, the t-DOS of PDs by a 4-thin-layer EDT-PbSe QDs/1-thin-layer TBAI-PbSe QDs thin film are dramatically increased, which verifies the trap-assisted PM effect occurred in PbSe QDs based PDs. Thus, as illustrated in Figure 4d, the PDs by a 4-thin-layer EDT-PbSe QDs/1-thin-layer TBAI-PbSe QDs 13 ACS Paragon Plus Environment


thin film possesses larger amount of deep hole trap-states than that by a 5-thin-layer EDT-PbSe QDs thin film. Such deep hole trap-states effectively captures the photo-generated holes and reduces the electron-injection barrier at the PbSe QDs/Ag interface. To further verify hole trap assisted PM effect occurred in PDs, the hole-only and the electrononly devices are fabricated and the Mott-Gurney law39,40 is applied to estimate electron and hole mobilities, respectively. The structures of the electron-only device and the hole-only device are ITO/ZnO/4-thin-layer EDT-PbSe QDs/1-thin-layer TBAI-PbSe QDs/Al and ITO/PEDOT:PSS/4thin-layer EDT-PbSe QDs/1-thin-layer TBAI-PbSe QDs/MoO3/Ag, respectively. Figure 5a 1

presents 𝐽2 verse 𝑉 ― 𝑉𝑏𝑖 characteristics of the electron-only device and the hole-only device (where J is current density, V is applied voltage and Vbi is the built-in potential). Based on space charge limited current (SCLC) method,39-41 charge carrier mobilities of a 5-thin-layer EDT-PbSe QDs thin film and a 4-thin-layer EDT-PbSe QDs/1-thin-layer TBAI-PbSe QDs thin film are estimated. The electron mobilities (𝜇𝑒) of 2.48 × 10-3 cm2/Vs and the hole mobilities (𝜇ℎ) of 3.89 × 10-4 cm2/Vs are estimated from a 5-thin-layer EDT-PbSe QDs thin film (Figure 5b). The electron mobility is one order magnitude larger than that of hole mobility, which demonstrates that the EDT-PbSe QDs thin film possesses large amount of trap-states. The 𝜇𝑒 of 2.51 × 10-3 cm2/Vs and the 𝜇ℎ of 7.1 × 10-6 cm2/Vs are estimated from a 4-thin-layer EDT-PbSe QDs/1-thinlayer TBAI-PbSe QDs thin film. The electron mobility is 3 orders of magnitude higher than that of hole mobility, which indicates that hole can be hardly transported in PbSe QDs active layer due to deeper hole-trap state on its surface. Therefore, through the PM effect, the PDs by the 4thin-layer EDT-PbSe QDs/1-thin-layer TBAI-PbSe QDs thin film exhibits boosted EQEs. CONCLUSIONS


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In summary, in this study, we reported a novel design of photodetectors for obtaining high photoresponsivity and detectivity from the ultraviolet to the middle infrared region (350 nm2500 nm). By formation of n-type TABI capped PbSe QDs thin layer on the top of p-type EDT capped PbSe QDs thin layer, low absorption coefficient issue of PbSe QDs from the near infrared (IR) to middle IR (MIR) region was circumvented. Moreover, the hole-trap assisted photomultiplication effect was successfully applied to boost external quantum efficiency (EQE) of PbSe ranging from NIR to MIR region for PbSe QDs based photodetectors (PDs). As a result, EQE over 450% in the visible region and 120% in the NIR region were observed from PbSe QDs based PDs. To further suppress the dark current, a layer of conjugated polyelectrolyte, which offers significant hole transport resistance in dark, but high electrical conductivity under illumination by photo-induced self-doping process, was used to reengineer the ZnO electron extraction layer. As a result, dramatically improved detectivity from ultraviolet to the middle infrared region was observed from PbSe QDs based PDs. Our work opened a workable route to achieve uncooled, inexpensive broadband PDs.

ACKNOWLEDGEMENTS The authors acknowledge Air Force Office of Scientific Research (AFOSR) (through the Organic Materials Chemistry Program, Grant Number: FA9550-15-1-0292, Program Manager, Dr. Kenneth Caster) for financial supports. Support Information The crystalline orientation of PbSe QDs thin films, the J-V characteristics of the PDs fabricated by 4-thin-layer EDT-PbSe QDs/1-thin-layer TBAI-PbSe QDs thin film W/O PNDIT-F3N spectra, EQE of the PDs fabricated by 4-thin-layer EDT-PbSe QDs/1-thin-layer TBAI-PbSe QDs thin



film that incorporated with and without PNDIT-F3N, measured under an external bias of 0V and -1 V, respectively and the absorption spectra of PNDIT-F3N.

Associated information Supporting information is available in the online version of the paper.

Acknowledgements The authors at The University of Akron acknowledge Air Force Office of Scientific Research (FA9550-15-1-0292) for financial support. The authors thank Drs. Zhang Jiang and Joseph W. Strzalka at Argonne National Laboratory for assistance with the GIWAXS measurements through the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357.

CONFLICT OF INTEREST The authors declare no conflict of interest.


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Figure 1. (a) Molecular structure of OA, EDT and TBAI. (b) The azimuthally integrated intensity plots of the GIWAXS patterns of PbSe QDs. (c) The J-V characteristics of diodes fabricated by the EDT-PbSe QDs thin film, the TBAI-PbSe QDs thin film, the OA-PbSe QDs thin film. (d) The TEM image of PbSe QDs thin film. (e) Absorption spectra of PbSe QDs (single layer and double layers).


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Figure 2. (a) Device architecture of PbSe QDs based PDs and (b) the LUMO and HOMO energy levels of ZnO, EDT-PbSe QDs, TBAI-PbSe QDs. MoO3 and the work function of FTO and Ag. (c) The EQE spectra and (d) the J-V characteristics of PDs fabricated by either a 5-thin-layer EDT-PbSe QDs thin film, or a 4-thin-layer EDT-PbSe QDs/1-thin-layer TBAIPbSe QDs thin film, or a 3-thin-layer EDT-PbSe QDs/2-thin-layer TABI-PbSe QDs thin film, in dark, respectively.



Figure 3. (a) The molecular structure of PNDIT-F3N. (b) The J-V characteristics of the PDs measured in dark and under white light with light intensity of 100 mW/cm2, and the monochromatic light at λ=1300 nm with a light intensity of 0.40 mW/cm2 and at λ=2400 nm with a light intensity of 0.1 mW/cm2, respectively. (c) Dark current density of PDs as a function of time after shutting off the light, (d) R and D* of PbSe QDs based PDs as a function of wavelength, under both external biases of -0.5 V and -1 V. (e) The linear dynamic range (LDR) at λ=1300 nm and (f) the response times of PbSe QDs based PDs at λ=640 nm. Note the PDs with a device structure of FTO/ZnO/PNDIT-F3N/4-thin-layer EDT-PbSe QDs/1-thin-layer TBAI-PbSe QDs/MoO3/Ag.


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Figure 4. (a) The dC/dV as a function of Voc and (b) the chemical capacitance as a function of Voc. (c) the t-DOS spectra of the photodiodes by either a 5-thin-layer EDT-PbSe QDs thin film or a 4-thin-layer EDT-PbSe QDs/1-thin-layer TBAI-PbSe QDs thin film, and (d) the schematic illustration of the trap density distribution of the photodiodes by either a 5-thinlayer EDT-PbSe QDs thin film or a 4-thin-layer EDT-PbSe QDs/1-thin-layer TBAI-PbSe QDs thin film.



a

b

350

350

5-thin-layer EDT-PbSe

4-thin-layer EDT-PbSe/1-thin-layer TBAI-PbSe

300

1/2

250

1/2

Electron

200

1/2

1/2

J (A /m)

300

J (A /m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

150

250 Electron

200 150 100

100

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1

1.5

2

V

appl

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-V -V (V) bi

3.5

0

4

Hole

1

1.5

2

V

appl

s

2.5

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-V -V (V) bi

3.5

4

s

Figure 5. The J1/2 vs V characteristics of the hole-only diode and the electron-only diode by (a) a 5-thin-layer EDT-PbSe QDs thin film or (b) a 4-thin-layer EDT-PbSe QDs/1-thin-layer TBAI-PbSe QDs thin film, respectively.


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Table of Contents Graphic


Ultrasensitive Solution-Processed Broadband PbSe Photodetectors

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