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Abstract
The recent success of the surface passivation of lead chalcogenide quantum dots for enhancing ambient stability offers further investigation of surface states in air. Here we studied the photoresponses due to surface passivation and oxygen, thus providing the relations of surface states and the photoresponse of PbSe QD films by monitoring the changes in QD film with air exposure. A dramatic near-infrared photoresponse was observed when the PbSe film stabilized through surface passivation was exposed to air. As a result of estimating the density of trap states from the FET characteristics, it was confirmed that the iodide-passivated PbSe film exposed to the air had more trap states than the nitrogen atmosphere. The increase of the trap state due to oxygen adsorption led to the increase of the trap-captured photogenerated electrons, which increased the photoconduction gain. Even though some trap states have a negative impact on device parameters such as charge mobility and response time, controlling the oxygen-related trap states on QD surface is expected to enhance the photoresponsivity on the QD-based photoconductive films without performance degradation in air.
© 2017 Optical Society of America
1. Introduction
The use of colloidal quantum dots (QDs) with near-infrared (NIR) band gap has been expanded beyond the biological applications including labeling and imaging to photovoltaic devices [1–3], because of its size tunable energy gaps. Among the QDs based on the lead chalcogenides, PbSe has a large Bohr exciton radius of approximately 46 nm, narrow bulk band gap of 0.28 eV, and high optical and static dielectric constants of ε∞ = 24 and ε0 = 250, respectively [4]. Furthermore, the efficient emission of PbSe QDs in the IR spectral range makes it suitable for optoelectronic applications, such as IR photodetectors [5–8]. These applications, however, have been limited by the instability of PbSe QD films when exposed to ambient conditions. Therefore, various methods have been reported to improve the stability of QDs, including covering the shell with a more stable material having a wider band gap, passivating the surface of QDs with halide molecules [9–11], and depositing a metal oxide by atomic layer deposition [12,13]. Although the stability of QDs is guaranteed by various surface passivations, additional treatments can lead to unforeseen results. In the case of PbS film deposited with Al2O3, we have already reported that Al2O3 layer provides stability for more than two months, but the substituted Al atoms form the electron transport channels of the PbS film [13]. In addition, the surface states of QDs greatly affect the photophysical properties. For example, Sargent et al. reported the PbS photodetector with increased responsivity by oxidation [14]. They have shown that the long trap state lifetime required for high photoconductive gain can also be achieved via the oxidation of semiconductor particles. However, since this oxidation occurs spontaneously unless specific post-treatment is performed, it is difficult to distinguish the role of oxidation in the photoresponse. Because the excessive oxidation not only reduces the particle size by forming oxide layers on the surface of the QDs but also reduces the conductivity and responsivity, the air stability is critical to a more accurate understanding of the photoresponse. Here, we investigated the effect of oxygen on the photoresponse by measuring the photoresponse using air-stable PbSe QDs. Preliminary experiments on the oxidation revealed that the surface-passivated films have more stable and better optical properties than the as-grown PbSe films in air. Compared with in a nitrogen atmosphere, the photoresponse of the iodide-passivated PbSe film was greatly improved in the air in which oxygen was adsorbed. Finally, we demonstrated the correlations between photoresponse and trap states by estimating the trap density of the air-exposed, iodide-passivated PbSe film. Our studies of photoresponse of PbSe film revealed that controlled trap states and superior surface passivation will expand the use of unstable PbSe films for electronic and optoelectronic applications under ambient conditions.
2. Experimental
2.1 PbSe film preparation
As-grown and surface passivated PbSe QDs with tetrabutylammonium iodide (TBAI, C16H36NI) were synthesized according to previously published procedures [9]. The PbSe QD film was deposited using a layer-by-layer (LbL) coating method in a nitrogen box. First, a PbSe QDs solution in octane (10 mg/ml) was spin-coated onto quartz substrate. This was followed by treatment with 3-mercaptopropionic acid (1% MPA) in acetonitrile, and a washing procedure with acetonitrile and octane was conducted to remove residual impurities introduced during ligand exchange process. After repeating this process five times, the sample was then annealed at 70°C for 5 min.
The film preparation process for the FET was identical to the above mentioned LbL method except for the substrate. For FET, the interdigitated Cr/Au electrodes (300 μm width, 5 μm spacing, 16 finger pairs) on a SiO2/Si substrate were fabricated by photolithography and lift-off processes. The heavily doped Si substrate with a 500 nm oxide layer was used as a back-gate electrode.
2.2 Characterization
Absorption and photoluminescence spectra of the PbSe films were obtained on a UV-VIS-NIR 3600 spectrophotometer (Shimadzu) and a fluorescence spectrometer (Fluorolog, Horiba Scientific), respectively. FET measurements were carried out with a semiconductor parameter analyzer in N2 and air. To measure the NIR photoresponse, we used a tungsten lamp (SLS202, Thorlabs) as the IR light source. And the specific wavelength filtered through the optical band pass filter was directly incident on the sample through the bifurcated fiber bundle (Thorlabs). The sensing area was about 300 × 400 μm2. The power of the incident light was measured using an InGaAs photodetector connected to one end of the fiber. The optical power density was measured to be about 130 nW/cm2 at λ = 1500 nm. The photocurrent was measured as a function of time with the shutter on and off. For N2 photoresponse measurement, one sample was encapsulated with a glass lid by conventional encapsulation techniques in a glove box. After all processes were finished in a nitrogen box, the measurements were carried out in air.
3. Results and discussion
First, the conductive PbSe films substituted with MPA were prepared by LbL process using two PbSe QDs, as-grown and iodide-passivated, to measure the optical properties of the PbSe film according to the air exposure. The PbSe QDs used in this study have a mean diameter of 4.4 nm, which corresponds to the first absorption peak at 1440 nm. Figure 1(a) shows the absorption spectrum of the PbSe films with air exposure time. Unlike the pristine PbSe film without any passivation layers, no obvious shift in the peak position of the iodide-passivated PbSe film was observed despite the 4 days of air exposure. The emission spectra of these PbSe films, as shown in Fig. 1(b), were similar to their absorption spectra. The PL peak of the pristine film nearly disappeared as soon as the film was exposed to air. However, despite several days of air exposure, the emission intensity of the iodide-passivated film was weak but persistent. This was consistent with previous results, in which the surface passivation with ammonium halides greatly hinders oxidation on the under-coordinated (100) surface and improves the air stability of PbSe QDs [9–11].
Fig. 1 (a) Optical absorption spectra and (b) PL spectra of pristine and iodide-passivated, MPA-capped PbSe QD film with air exposure time.
In general, the as-grown lead chalcogenide QDs without a surface passivation layers are more rapidly oxidized and the film tends to degrade when exposed to air, resulting in poor optoelectrical characteristics of the films. Figure 2(a) shows the photocurrent responses of the two films at λ = 1450 nm. To minimize the dark current, a low bias of 0.1 V was applied to the films. In both films, the current was increased under illumination. When the shutter was closed, the current decreased and became almost equal to the dark current value within 1 second. However, the rise time of the iodide-passivated film was much faster and more sensitive than the pristine PbSe film. The photocurrent sensitivity, defined as the ratio of the generated photocurrent to the dark current, was increased to 50% for the iodide-passivated PbSe film as shown in the Fig. 2(b). The spectral response characteristics were very similar to the photocurrent sensitivity. At a low bias of 0.1 V, the photoresponsivity of the iodide-passivated PbSe film at 1400 nm was 0.71 A/W. The high dark current, low photosensitivity and responsivity of the pristine film were believed to be due to the oxidation of QDs.
Fig. 2 Photoresponses measured in air for the pristine and iodide-passivated PbSe film. (a) Current as a function of time with shutter on/off at an applied bias of 0.1 V under the wavelength of 1450 nm, which the first excitonic peak. (b) Spectral photosensitivity and responsivity of two PbSe QD Films in air. The photosensitivity of pristine PbSe film was multiplied by a factor 10, for an easier comparison.
The electrical properties of two PbSe films were measured by fabricating the FETs. The current in the passivated films was generally lower than that of the non-passivated one because the electron transfer could be blocked by type 1 band alignment between passivated PbI2 and PbSe. Figure 3 shows the changes of I-Vbg curves of the two PbSe FETs with air exposure. Before the air exposure, both PbSe films exhibited n-type dominant ambipolar transistor behavior in which electrons are injected into the PbSe film under a positive gate bias. The electron and hole mobility of the iodide-passivated PbSe film in the nitrogen atmosphere were 2.8 × 10−3 cm2 V−1s−1 and 2.0 × 10−3 cm2 V−1s−1, respectively. However, when exposed to air, both films were changed to p-type, but there was a difference in degree. For the pristine PbSe FET, the dependence of the drain current on the gate voltage after air exposure was relatively low. Unlike the pristine PbSe film, the iodide-passivated PbSe FET was converted to a perfect p-type with an on/off ratio of approximately 103 when exposed to air. The hole mobility of the iodide-passivated film exposed in air was 1.8 × 10−3 cm2 V−1s−1. The lead chalcogenide QDs exposed to the air have been reported to exhibit p-type characteristics by oxygen doping [15–18]. Woo et al. reported that NH4Cl-treated PbSe FET retained their original n-type behaviors when exposed to air within 2 hours [9]. If the air exposure is prolonged or the sample is exposed to high O2 pressure, the film is converted to p-type solid and does not return to the initial state. Leschkies et al. showed that the doping effect was reversible at low O2 but irreversible at high O2 pressures [18].
Fig. 3 I-Vbg curves of (a) pristine and (b) iodide-passivated PbSe QD FETs, measured over time in N2 and air at Vds = 10 V.
The photoresponse of QDs can be enhanced by molecules adsorbed on the surface such as oxygen and iodine as well as the surrounding environment [19,20]. Qiu, based on photoluminescence and responsivity studies of polycrystalline PbSe, pointed out that iodine plays an important role in the inducing of photoresponse and oxygen acts as sensitization improver [19]. The photoresponse of the iodide-passivated PbSe film was measured in N2 and oxygen atmosphere to confirm the determinants of increased sensitivity. Figure 4(a) shows the photocurrent responses of the iodide-passivated PbSe films measured in N2 and air. The photoresponse of the film in N2 atmosphere was similar to that measured in an oxygen atmosphere, but the photocurrent was increased further in air. The photoresponse at the wavelength below the bandgap of the PbSe QDs was negligible and the photocurrent sensitivity in air was much better than that in N2 (Fig. 4(b)). The photocurrent peak of the iodide-passivated PbSe film measured in N2 was clearly consistent with the absorption peak of PbSe QDs, but the peak in air was shifted slightly toward the shorter wavelength. Increased photosensitivity and the blue shift were clearly associated with oxygen adsorption and doping on the surface of nanocrystal. The adsorbed oxygen on the QD surface creates the surface traps that enhance the lifetime of the photogenerated holes [21]. Of course, the surface trap states can be also caused by stoichiometric imbalances [22], incomplete ligand passivation, and impurities in addition to oxygen adsorption. Trap states are known to act as a recombination center for the photogenerated carrier, thereby reducing the efficiency of the optoelectronic device. However, it also positively affects the photocurrent gain of the photoconductive detectors. The extraordinarily high photoconductive gain of recently reported nanostructured photoconductors was made possible by the trap states [14, 23–26]. For semiconductor in contact with metal electrodes, the photoconductive gain can be obtained not only by the time domain of the trapped carrier (carrier lifetime), but also by the spatial distribution of trap states (density of trap states) [23]. Therefore, we have calculated the density of trap states from the transfer characterization curve of iodide-passivated PbSe FET to account for the increase in photosensitivity [27–29]. The subthreshold swing SS of a device is defined as the change in gate voltage needed to induce a decade increase in the output current. If we assume that both the density of deep bulk traps Nbulk and the density of interface traps Nint are independent of energy and that both traps contribute to the density of trap states N□ (per unit area and unit energy), then SS may be written as
where kT is the thermal energy and Ci is the capacitance per unit area [27]. The SS of the film when measured in N2, depicted in Fig. 4(c), was about 5.01 V/dec. When Ci = 6.9 nF/cm2, the calculated density of trap states N□ from the SS was 3.58 × 1012 eV−1cm−2. However, in air, the calculated N□ was 6.12 × 1012 eV−1cm−2, which was 1.7 times greater than that measured in N2.Fig. 4 (a) NIR photoresponse recorded for a wavelength ranging from 1150 nm to 1650 nm, and (b) Spectral photocurrent sensitivity and responsivity of the iodide-passivated PbSe film in N2 and air. A 0.1 V bias was applied for photoresponse measurement. The sensitivity and responsivity peak of the sample measured in air has shifted toward the shorter wavelength. It should be noted that the spectral resolution of our system is 100 nm, which is a fairly large value. (c) Transfer curves of the iodide-passivated PbSe QD FETs measured in N2 and air. Inset: optical image of device. The air-exposed sample was prepared by exposing the film coated in N2 to air for 3 days.
4. Conclusion
We have demonstrated that the photoresponse of PbSe QDs films could be greatly enhanced by thin iodide-surface-passivating layers and oxygen-related trap states. The thin iodide-passivating layers improved the stability of the films by preventing the formation of oxide layers on PbSe QDs in air, and ensured greater photoresponsivity in the air than non-passivated samples. The optical properties of the iodide-passivated PbSe film seemed to be unchanged in the air thanks to successful surface passivation, but the electrical properties changed from n-type dominant ambipolar to p-type after exposure to air. Air exposure has resulted in improved photoresponse of iodide-passivated PbSe film. The photoresponse improvement of films could be explained by the increase of trap states due to oxygen adsorption, and the increase of trap states was confirmed by estimation of the density of trap states. The combination of atmospheric stability and controllable surface trap is expected to enable the realization of high-performance optoelectronic devices.
Funding
Global Frontier Research Program of Ministry of Science, ICT and Future Planning (CASE-2016M3A6A5929198); Nano Material Technology Development Program of MSIP/NRF (2014M3A7B6020163); Industrial Core Technology Program of MOTIE/KEIT (10062837); Global R&D program of KIAT (1415134409); Basic Research Fund of Korea Institute of Machinery and Materials (SC1170).
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