Highly Photosensitive Vertical Phototransistors Based on a Poly(3

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Highly Photosensitive Vertical Phototransistors Based on a Poly(3hexylthiophene) and PbS Quantum Dot Layered Heterojunction Haiting Zhang,†,‡ Yating Zhang,*,†,‡ Xiaoxian Song,†,‡ Yu Yu,†,‡ Mingxuan Cao,†,‡ Yongli Che,†,‡ Zhang Zhang,†,‡ Haitao Dai,§ Junbo Yang,⊥ Guizhong Zhang,†,‡ and Jianquan Yao†,‡ †

Institute of Laser and Optoelectronics, College of Precision Instruments and Optoelectronics Engineering, ‡Key Laboratory of Optoelectronics Information Technology, Ministry of Education, and §Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, School of Science, Tianjin University, Tianjin 300072, China ⊥ Center of Material Science, National University of Defense Technology, Changsha 410073 China ABSTRACT: We fabricated a vertical field effect phototransistor with Au/Ag nanowires as the transparent source electrode and with vertically stacked layers of poly(3-hexylthiophene) (120 nm) and lead sulfide quantum dots (380 nm), which formed heterojunctions. The built-in electric field in the layered heterojunction aids the separation of photoinduced excitons, while the short channel enables efficient carrier transport across the active region. Both of these benefits enable a high photoperformance and fast photoresponse. This vertical phototransistor can be operated at room temperature with a low operation voltage of −1 V and is therefore energy-efficient. Further, it has a wide response spectrum from 400 to 2100 nm, a high photoresponsivity of more than 9 × 104 AW−1, and a high detectivity of up to 2 × 1013 Jones (cm Hz1/2 W−1) under infrared illumination. Additionally, this vertical phototransistor had a response time of 9 ms, which is faster than a previously reported lateral field effect phototransistor based on poly(3hexylthiophene)/lead sulfide quantum dots. The vertical architecture combined with the layered heterojunction approach provides a new, facile way of fabricating high performance devices. KEYWORDS: infrared, poly(3-hexylthiophene), lead sulfide quantum dot, vertical phototransistor, layered heterojunction

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In contrast to photoconductors with two electrodes, FEpTs have three electrical contacts (source, drain, and gate electrodes). The gate voltage can control the conductivity of the active layers as well as the source−drain voltage and the incident optical power. Significant progress has been made with lateral FEpTs (LFEpTs) with polymer/CQD composite bulk heterojunctions (BHs; CQDs are embedded in the polymer matrix).22−26 However, the large separation between the source and drain contacts hinders efficient carrier transport across the active region. The photoperformances are limited in terms of the photoresponsivity, photoresponse speed, specific detectivity, and operation voltage. Yang et al. have studied infrared FEpTs based on a poly(3hexylthiophene) (P3HT) and lead sulfide (PbS) QD composite BH; they obtained a photoresponsivity of 500 AW−1 with a source−drain bias (VSD) of −40 V.27 Sun’s group has reported a P3HT and PbS QD composite LFEpT with a photoresponsivity of 2 × 104 AW−1 and long rise and fall relaxation times on the scale of seconds. However, in such devices with lateral structures a large driving voltage of up to 100 V is required, which is too high to be compatible with conventional electronic driver circuitry.22 We recently compared LFEpTs with layered heterojunctions (LHs) and BHs; our results

n recent years, solution processed materials have attracted considerable attention owing to their physical flexibility, inexpensiveness, and innate compatibility with large area fabrication techniques.1−6 Photosensitive materials, including quantum dots,7,8 organic molecules,9,10 and Perovskites,11,12 can be integrated into a variety of optoelectronic devices for practical applications in imaging techniques, optical communications, and optoelectronic integrated circuits. Photodetectors play a key role in such photoelectrical systems, because they can convert photons into current. The appropriate selection of photosensitive materials and the design of the device architecture are important for achieving high performance photodetectors. Solution-processed colloidal quantum dots (CQDs) have excellent properties such as tunable band gaps, small exciton binding energies, and high photoluminescence quantum yields;6,13,14 they have therefore emerged as an interesting class of new materials for photodetectors (PDs) owing to their advantages of having low manufacturing costs, tunable spectral sensitivity, and being compatible with flexible substrates.1 The sensitivity of photodetectors that integrate CQDs with polymers as photosensitive materials can be tuned over a wide spectral range from the visible to the infrared (IR).15−19 Polymer/CQD hybrid photodetectors have been designed for photoconductors (PCs),15 photodiodes (PDs),20,21 and field effect phototransistors22,23 (FEpTs). © 2017 American Chemical Society

Received: November 12, 2016 Published: February 16, 2017 584

DOI: 10.1021/acsphotonics.6b00896 ACS Photonics 2017, 4, 584−592

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Figure 1. (a) Architecture of the P3HT/PbS QD composite VFEpT. The channel length (L) is defined by the thickness of the active layers. (b) Side view of the device geometry and measurement setup. (c) SEM image of the Au/Ag nanowire. (d) TEM image of PbS QDs. The QDs typically had diameters of 4.1 nm. (e) Cross-sectional SEM image of a NW-VFEpT with a 120 nm P3HT layer and a 380 nm PbS QD layer.

a high photoresponsivity (R) of more than 9 × 104 AW−1 and a high specific detectivity (D*) of 2 × 1013 Jones (cm Hz1/2 W−1) at a low bias voltage of −1 V under illumination with a 808 nm laser with a light irradiance of 1.6 μW cm−2. It is notable that the VFEpT showed a photoresponse time of 9 ms, which is one order magnitude faster than that of the PbS/P3HT BH-based LFEpT reported by Zhenhua Sun et al.22 Combining a vertical architecture with a layered heterojunction thus provides a simple and low cost scheme for phototransistors with high photo performances that are easy to integrate and have a low power consumption.

showed that LH phototransistors had higher photoresponsivities because they had highly ordered channels.28 Thus, using a lateral device configuration with a BH does not take full advantage of these composites’ properties. Compared with a LFEpT, a FEpT with a vertical architecture (VFEpT) has the advantage that the photoexcited carriers only have to travel a short distance before reaching a contact. Photoexcited carriers can circulate in the short channel many times before recombining and are scattered fewer times by structural defects and grain boundaries before they reach the electrodes. Both these advantages increase the photoconductivity. When light illuminates a layered heterojunction, photoinduced carriers can be effectively separated by the built-in potential or the source−drain voltage, before being quickly transferred to the electrodes (short transit time). This photosensing mechanism is expected to aid the development of photodetectors with fast photoresponse times. Recently, Arial J. Besson et al. developed vertical organic field effect transistors (VOFETs) by applying Au/Ag nanowires (NWs) as the transparent source electrode.29 Their VOFETs have a vertically stacked source and drain electrode architecture, which enables precise control and allows for facile downscaling the device’s critical dimensions without substantial increase in cost or fabrication complexity. VOFETs have nanoscale short channel lengths that can be operated under low source−drain bias voltages without catastrophic shorts between the top and bottom contacts. By applying photosensitive materials such as polymer/CQD composites as active layers the NW-based VOFET can be used as photodetectors. However, polymer/CQD composite-based vertical field effect phototransistors (VFEPTs) have never been reported before. In this work, we fabricated a short channel (500 nm) VFEPT with P3HT and PbS QDs in a LH; we used Au/Ag nanowires (NWs) as the transparent source electrode. The device can be operated at −1 V at room temperature. We found that the LH assisted both the separation of the photoinduced electron−hole pairs and the free carrier transport. The device has a broad spectral bandwidth from the visible to the infrared and exhibits

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EXPERIMENTAL SECTION PbS QDs were synthesized using a wet chemical method that was similar to methods described in previous work30,31 (see Methods). The Au/Ag nanowires were fabricated in situ using self-assembly and solution-processing, following the method in references.32,33 The details of the Au/Ag nanowires synthesis process are given in the Methods section. The fabrication procedures for the Au/Ag nanowire-based vertical P3HT/PbS QD composite FEpT are as follows. Au/Ag nanowires were deposited on the surface of an n-doped Si/SiO2 (300 nm) substrate to serve as the transparent source electrode. Then the Au source electrode was thermally evaporated onto the SiO2 to connect with the Au/Ag nanowires. In the following step, one drop of P3HT trichloromethane solution with a concentration of 3 mg/mL was deposited on top of the Au/Ag nanowire film, followed by three drops of a PbS QDs toluene solution with a concentration of 10 mg/mL. To achieve a high photoconductivity and thereby a high photosensitivity of the device, it is crucial to reduce the highly insulating barriers created by the ligand shells around each PbS QD. We therefore followed the ligand exchange process of PbS QDs in ref 4 and changed the ligand shell from oleylamine to 1,2-ethanedithiol (EDT), which has shorter carbon chains. Each PbS QD layer was deposited as follows: one drop of PbS QDs solution was spincoated at a speed of 2000 rpm and left to dry for 15 s while still being rotated. Three drops of 2% EDT solution were then 585

DOI: 10.1021/acsphotonics.6b00896 ACS Photonics 2017, 4, 584−592

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Figure 2. (a) IV curve (ID−VSD) of a P3HT/PbS QD composite VFEpT at increasing gate biases of −0.02, −0.2, −0.4, −0.6, −0.8, and −1 V in the dark. (b) Transfer curve (ID−VG) at increasing source−drain biases of −0.02, −0.2, −0.4, −0.6, −0.8, and −1 V in the dark.

Figure 3. (a) Absorption spectra of P3HT in a trichloromethane solution and of PbS QDs in toluene solvent. (b) Photocurrent spectral response with a source−drain bias of −1 V. Panels (c) and (d) show the band diagrams under visible and infrared illumination.

deposited on the still rotating film, followed by two drops of acetonitrile and two drops of toluene. Finally, the Au drain (thickness of 150 nm) was deposited through a shadow mask on top of the patterned P3HT/PbS QD nanocomposite film via thermal evaporation. The overlap between the three electrodes defined the device area, which was about 0.04 cm2.

distances between the Au/Ag nanowire bundles (referred to as the diameter (D) of the perforations between the Au/Ag nanowires (blue lines)) ranged from 150 to 500 nm. The low H/D (electrode thickness to perforation size) aspect ratio achieved here is beneficial for achieving good conductivities.29,34 Figure 1e presents a cross-sectional SEM image of the vertical phototransistor. The structure of the stacked layers is clearly visible, including a bottom Si n+ substrate, SiO2 layer (300 nm), P3HT layer (120 nm), PbS QDs layer (380 nm), and Au drain electrode (150 nm). The physical mechanism underlying the observed current response of the P3HT/PbS QD composite VFEpT is similar to that of the transparent electrode based vertical organic field effect transistors reported by Ben-Sasson et al.34,35 Traditional metal source electrodes introduce significant shielding effects and separate VFEpT into two functional structures: (1) a diode containing a transparent electrode, P3HT, PbS QDs, and the drain and (2) a capacitor containing the gate, SiO2, and the source. For the two parts to interact, an Au/Ag nanowire electrode was used as the source electrode, which is transparent to the low-frequency or DC vertical gate electric fields. A builtin field forms at the junction owing to the diffusion and drift of

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RESULTS AND DISCUSSION The architecture of the vertical phototransistor with the layered heterojunction and its cross section are shown schematically in Figure 1a,b. The device consists of overlapped components including, from bottom to top, the gate, SiO2 gate dielectric layer, Au/Ag nanowire transparent source electrode, P3HT layer, PbS QD layer, and Au drain. The channel length (L) was precisely controlled by the thickness of the composite layer heterojunction. A transmission electron microscope (TEM) image of the PbS QD layer is shown in Figure 1c. PbS QDs have an average diameter of about 4.1 nm. The Au/Ag nanowire film was analyzed using scanning electron microscopy (SEM) (Figure 1d). A typical diameter (referred to here as height, H, as it corresponds to the electrode thickness) of an Au/Ag nanowire bundle (yellow lines) was about 20 nm. The 586

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Figure 4. (a) Channel current of a phototransistor as a function of the gate voltage under NIR light illumination (wavelength: 808 nm) at VSD = −1 V. From bottom to top the light irradiance corresponding to each curve is 0, 1.6 μW cm−2, 0.1 mW cm−2, 4.8 mW cm−2, 49 mW cm−2, 335 mW cm−2, and 480 mW cm−2, respectively. Panel (b) shows ID as a function of the laser intensity at VSD = −1 V and VG = −1 V. Panel (c) shows the photoresponsivity (R) as a function of gate voltage (VG) for an irradiance of 1.6 μW cm−2 with infrared illumination from a 808 nm laser. Panel (d) shows the photoresponsivity as a function of the light irradiance of the vertical P3HT/PbS QD composite phototransistor at VSD = −1 V and VG = −1 V. (e) A typical noise level of the vertical phototransistor over 1 s with sampling rate of 5 ms. Panel (f) shows the specific detectivity as a function of the light irradiance of the device at VSD = −1 V and VG = −1 V.

as the lower potential barrier for hole injection.29,35 The hole and electron mobilities of a PbS/P3HT VFEpT35 device are μH = 0.43 cm2 V−1 s−1 and μE = 0.06 cm2 V−1 s−1 in the linear region (which is higher than for an equivalent LFEpT with a bulk heterojunction22) owing to the short carrier transit time and strong electric field.36 Figure 3a shows the absorption spectra of photosensitive solutions of P3HT in a trichlorobenzene solution; the solution shows a strong absorption peak at 465 nm and negligible absorption in the near-infrared, while toluene solutions of the PbS QDs have a remarkable absorption peak at 1170 nm. The energy levels of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of P3HT are 3.2 and 5.0 eV, respectively, and the energy levels of the valence band and the conduction band of a PbS QD37,38 are estimated to be 4.21 and 5.27 eV, respectively. Following the absorption of photons, the photoexcited electron−hole pairs that are generated in the active layers are separated and flow to the drain owing to the source−drain bias or built-in field, leading to the generation of photocurrent. As shown in the Figure 3b, the device is photoresponsive from 300 to 1350 nm and has two peaks at 465 and 1170 nm that closely match the P3HT and PbS QD absorption peaks. This result demonstrates that the photocurrent is caused by the effective

carriers in the active layers. As shown in Figure 2a, the device works as a diode before a gate voltage is applied (black lines). In this situation, the source−drain current originates from the field-enhanced thermionic emission of the Au/Ag nanowires. When a gate voltage is applied, the device works as a transistor (colored lines) and the source electrode film forms an ohmic contact. The gate electric field penetrates into the openings in the source electrode and lowers the Schottky barrier at the interface between the Au/Ag nanowires and the P3HT layer. A great number of holes are injected and accumulate in the perforations; as a result, the perforations are saturated with holes and then these mobile charge carriers flow toward the drain. This phenomenon is referred to as a “virtual contact”. Figure 2b describes the transfer characteristics (ID−VG) for different source−drain voltages. A strong electric field of up to 2 × 104 V cm−1 is created by the source−drain bias in the short vertical channel (∼500 nm), which results in a low operating voltage of 1 V. The barrier’s height and width can both be reduced by increasing the reverse source−drain bias. The channel current increases and the electrical neutral points shift to a negative voltage. The EDT treated PbS/P3HT device exhibited ambipolar characteristics. Simultaneously, hole transport dominated over electron transport in the composite device, owing to the higher hole concentration in P3HT as well 587

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network dominates. Above a laser intensity of 5 mW cm−2, the current increases at a low rate. This approximate photocurrent saturation effect indicates the maximum separation of in PbS QDs under large light irradiance. Along with more photoinduced carriers generated by the absorption of photons at high light irradiances, the recombination between free holes and electrons occurs in addition to the recombination at electron trap centers, which reduces the number of photoexcited carriers. Two crucial parameters are used to evaluate the photoperformances of the devices: photoresponsivity (R) and external quantum efficiency (EQE).40 The photoresponsivity22 of a photodetector is defined as the ratio of the light-induced current over the power of the incident light, Iph/P, where Iph and P are the photocurrent and incident illumination power, respectively. Figure 4c shows R as a function of the gate voltage for a low light irradiance of 1.6 μW cm−2 (in the third quadrant) under a source−drain bias of −1 V. We found that higher photoinduced hole current responsivities were achieved for a gate voltage of −1 V, which we attributed to the lower potential barrier for hole transport and the high hole concentration in P3HT. Meanwhile, a lower photoelectron current was generated under zero gate voltage, which we ascribed to the high potential barrier for photoinduced electron transport. The responsivity R reached its maximum value at over 9 × 104 AW−1 at the largest negative gate voltage (VG = −1 V). This value of R is higher than for the LFEpT based on a P3HT/PbS QD BH reported by Sun’s group.22 Photoresponsivity is related to EQE according to R × E × 100, where R and E are the photoresponsivity and the incident photon energy, respectively. As shown in Figure 4d, the photoresponsivity and external quantum efficiency (EQE) of the P3HT/PbS QD composite VFEPTs decrease linearly with increasing incident light irradiance on a double-logarithmic plot. A large responsivity of over 9 × 104 AW−1 was measured at VSD = VG = −1 V, corresponding to an external quantum efficiency above 1.38 × 107%. Photoconductive gain4 is defined by the following expression:

absorption of visible light by P3HT/PbS QDs and infrared light by PbS CQDs. To understand the photosensing mechanism of the device under visible and infrared illumination, the corresponding band diagrams are depicted in Figure 3c and d, respectively. The differences between the work functions of the PbS QDs and P3HT produce a built-in potential in the depletion region near the junction, which propels electrons and holes in opposite directions. When visible light is laterally incident on the device, a photoexcited electron−hole pair is generated in P3HT or in the PbS QDs by absorption of a photon and then dissociates into an electron and a hole driven by the built-in potential or the source−drain bias. Under reverse bias, the photoinduced holes are captured by the drain (cathode), while the photoinduced electrons are trapped in the active layers by a high potential barrier. Under infrared illumination, e−h pairs are only generated in the PbS QDs. In the short channel, photoinduced holes generated by the dissociation of e−h pairs are transferred to the drain (cathode) owing to the reverse source drain bias or the built-in potential with a short transit time on the nanosecond scale. As soon as a hole reaches the drain, a hole replenishes the supply in the device from the Au/Ag nanowire source electrode and subsequently moves through the active layers (P3HT layer and PbS QD layer). Accordingly, multiple holes circulate in the short channel following a single electron−hole pair generation, which increases the photoconductive gain. As the number of photoexcited electrons trapped in the PbS QD layer owing to the high potential barrier increases, an additional built-in potential is simultaneously induced at the interface of the two materials. As a result, the electron and hole recombination rate at the interface is increased.24 The photosensitivity of the P3HT/PbS QD vertical photodetector was estimated by measuring the transfer characteristics under the illumination of 808 nm laser with different light irradiances. Under NIR light illumination, the density of electrons accumulated in the PbS QDs increased along with that of trapped photoinduced charges. The electrostatic potential of P3HT close to the dielectric layer also changed.24 In lateral hybrids FEPTs, the shift of the threshold voltage under illumination is related to the electrostatic potential of the active layer. However, in a vertical transistor, the threshold voltage with fixed source−drain biases can be expressed using the following equation.39

eN (c)L Vth = VFB + A COX

G = Rhν /q = τlifetime/τtransit time

(2)

where hν is the photon energy and q is the charge of an electron. The transit time35 is proportional to the second power of the channel length and can be expressed by the following equation: τtransit time = L2 /μVSD

(3)

A long lifetime and short transit time are important for obtaining a high gain. Propelled by the built-in field or source− drain bias, photoexcited carriers travel in the short channel and reach a contact in a short carrier transit time of 1.3 ns, which is nearly 5 orders of magnitude shorter than the lifetime of the carriers (about 0.18 ms). Therefore, the photoexcited carriers can circulate in the short channel many times before recombining. Simultaneously, light-induced carriers are scattered fewer times by structural defects and grain boundaries before reaching the electrodes. These benefits related to having a short channel length lead to a high photoconduction and enable high gains. A gain of over 1.38 × 105 electrons per photon under a light irradiance of 1.6 μW cm−2 was measured in the device, which is, to the best of our knowledge, the highest result for a P3HT/PbS QD composite phototransistor to date. This result demonstrates the excellent photoelectrical properties of the device.

(1)

where VFB is the transistor’s flat-band voltage, c is the doping concentration, e is the elementary charge, NA(c) is the density of activated dopants, and L is the channel length. The threshold voltage is therefore a constant and unrelated to the electrostatic potential in the active layer. As shown in Figure 4a, the current increased significantly as the light irradiance increased. The threshold voltage of the P3HT/PbS QD composite VFEPTs does not change under illumination, which differs from the behavior of lateral hybrid FEPTs.22 The photocurrent under different laser intensities with a fixed source−drain bias and a gate voltage of VSD = VG = −1 V is summarized in Figure 4b. Under the illumination of an 808 nm laser, the photocurrent increases dramatically as the intensity increases from zero to 5 mW cm−2. At low light intensities, the recombination of free holes with trapped electrons in the QD 588

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Figure 5. Panels (a) and (b) show the current response of a phototransistor under alternating dark conditions and 808 nm light illumination (irradiance: 120 mW cm−2) with VSD = −0.02 V and VG = −0.2 V.

Table 1. Progress in PbX (X = S, Se)/Polymer Composite-Based Photodetectors and Other QDs Hybrids Based Solution Processed Photodetectors PD structure

materials

PC PC

PbS QDs/MEH-PPV PbSe QDs/MEH-PPV

LFEpT with BH LFEpT With BH LFEpT with BH LFEpT with BH VFEpT with LH LFEpT PC LFEpT LFEpT LFEpT PD

PbS QDs/P3HT

PD

responsivity/ EQE/gain

response time

0.23 s/0.21 s

PbSe QDs/P3HT

R = 500 AW−1

PbS QDs/P3HT/PCBM

R = 0.391 mAW−1

PbS0.4Se0.6/P3HT

R = 55.98 mAW−1

measure conditions 15

VG = VSD = −100 V, λ = 875 nm

22

5.02 × 1012 Jones 1.31 × 1011 Jones 1.02 × 1010 Jones 2 × 1013 Jones

VG = VSD = −40 V, λ = 980 nm

39

VG = 1 V, VSD = 2 V, λ = 600 nm

27

VDS = −10 V, VG = 3 V, λ = 980 nm VG = VSD = −1 V, λ = 808 nm

23

3 × 109 Jones 1.8 × 1013 Jones

7

43

9 ms/9.4 ms

R = 9 × 104 AW−1

PbSe QDs PbS QDs PbS QDs graphene/PbS QDs graphene/PbS QDs PCBM/CH3NH3PbI3−xClx/ PEDOT:PSS PbS QDs/CH3NH3PbI3

0.16 s

R = 20 mAW−1 R = 103 AW−1 gain = 10 R = 109 AW−1 R = 107 AW−1 EQE = 80%

7 × 10 Jones 1014 Jones

5 V, λ = 1.6 μm λ = 1.3 μm VSD = 30 V, λ = 450 nm λ = 532 nm λ = 500 nm λ = 550 nm

R = 132 mAW−1

5.1 × 1012Jones

λ = 900 nm

10 μs 0.8 s 10−20 ms 180 ns/160 ns

AΔf (4)

this work

14 41 37 4 42

R (2qJD)1/2

(6)

where R is the responsivity and JD is the dark current density. A high photodetectivity (D*) up to 2 × 1013 Jones can be derived from the inset of Figure 4f, which is close to the maximum value of the measured detectivity. This value is comparable with the most sensitive organic/inorganic detectors presented in the literature,22 demonstrating the hyper photosensitivity of the P3HT/PbS QD composite VFEPTs. Although the specific detectivity in our vertical composite phototransistors is impressive, there is still room for improvement. A promising way to suppress the dark current and improve the photodetectivity further would be to select more appropriate polymers, QDs, and metal drain electrode materials to introduce a higher injection barrier.35 Another way to block hole injection in the VFEpT would be to deposit materials such as 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, which has a deep highest occupied molecular orbital level of −6.7 eV on PbS QDs layer.42

where A is the effective area of the detector, Δf is the electrical bandwidth, and the noise equivalent power (NEP)14 represents the minimum impinging optical power that a detector can distinguish from noise, which can be expressed as NEP = ⟨In2⟩1/2 /R

13

40

Considering the shot noise coming from the dark current as the dominant source of noise, D* can be expressed by the following equation:41

D* = NEP

ref

Vbi = −5 V, λ = 975 nm Vbi = −7 V, λ = 1550 nm

PbS QDs/P3HT

Another figure of merit employed to characterize the sensitivity of a detector is normalized detectivity D*.14 The normalized detectivity in units of Jones (cm Hz1/2 W−1) is given by the following equation D* =

detectivity

EQE = 0.38% R = 17.5 × 10−5 AW−1 R = 2 × 104 AW−1

(5)

where R is the photoresponsivity measured under the same conditions as the root-mean-square dark noise current. We measured the background noise level of the phototransistor at a modulation frequency of 2 Hz following the method in ref 4 (see Methods for details) as shown in Figure 4e. The noise current of the device has a typical value of 21.7 pA Hz−1/2, which yields a NEP of 3.4 × 10−16 W and a measured detectivity of 1.35 × 1013 Jones. 589

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current, for example, by increasing the potential barriers for carrier injection. The photoresponse speed of our PbS/P3HT composite VFEpT was faster than that of both PbSe QDs LFEpTs and graphene/QD hybrid PDs, and slower than that of PCBM/CH3NH3PbI3−xClx/PEDOT:PSS PDs.42

Figure 5a shows the temporal photocurrent responses of the devices under light irradiance of 120 mW cm−2 from an 808 nm laser. The photodetector exhibits good on/off photoswitching and a fast photoresponse. When the light is switched on, photoinduced electron−hole pairs in the PbS QDs are dissociated owing to the built-in potential or the source− drain bias. Then photoinduced holes flow to the drain driven by the reverse source−drain bias, while photoinduced electrons remain trapped in the PbS QD layers because of the high potential barrier. As soon as a hole reaches the drain, another hole replenishes the supply from the source, and so they sequentially move through the P3HT and PbS QD layers. The channel current increased with the illumination time and could be fitted with an exponential function.22 ΔID = ΔI1 exp(1 − t /τ1) + ΔI2 exp(1 − t /τ2)

CONCLUSIONS

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METHODS

An Au/Ag nanowire based lead sulfide quantum dot (PbS QD) and poly(3-hexylthiophene) (P3HT) layer heterojunction vertical field effect phototransistor has been demonstrated in this study; it showed a broadband photoresponse from the visible to the infrared regions. By combining a highly ordered layered heterojunction with a vertical architecture, the P3HT/ PbS QD phototransistor was able to achieve a low power consumption and high photoperformance, displaying a high photoresponsivity of more than 9 × 104 AW−1 and a high detectivity of up to 2 × 1013 Jones with a supply of −1 V under illumination by a 808 nm laser. Additionally, the device showed a fast photoresponse with short rise and fall relaxation times, superior to what has been achieved previously with a lateral phototransistor architecture. The layered heterojunction with the vertical architecture is expected to contribute to the development of large area integrated high-sensitivity IR photodetectors.

(7)

where τ1 and τ2 are the two rise times and ΔI1 and ΔI2 are the changes in the channel current increments. The time constants τ1 and τ2 were 9.4 and 153 ms, respectively. The short relaxation time τ1 corresponds to the carriers in the PbS QD sheet. Meanwhile the longer response time τ2 corresponds to carrier transport through the active layers. Similarly, the channel current decreased when the light was switched off; this current thus represents the recombination process of the electrons and holes in the PbS QDs. This current can be fitted by an exponential equation with two relaxation times (τ3 and τ4). ΔID = ΔI3( −exp( −t /τ3)) + ΔI4( −exp( −t /τ4))

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P3HT was purchased from Rieke Metals, Incorporation (No. 4002-E). PbS QDs were synthesized via a wet chemical method.31 Synthesis of PbS Quantum Dots. Chemicals. Lead oxide (PbO, 99.99% pure from Aladdin), bis(trimethylsilyl) sulfide ((TMS)2S 95% pure from Acros organics), oleic acid (OA, 80−90% pure from Aladdin), octadecene (ODE, 90% pure from Acros organics), methanol, acetone, and n-hexane were used during the synthesis process. Synthesis of PbS QDs. A total of 37.5 mL of octadecylene, 3.8 mL of oleic acid (OA), and 1.338 g lead oxide were loaded into a 100 mL four-neck flask, degassed at 90 °C for 1 h, and heated to 120 °C in 10 min under Ar flow. Eighteen mL of bis(trimethylsilyl) sulfide solution (630 μL in ODE) was quickly injected. After 5 s, the reaction mixture was cooled in room temperature. The reactant was washed three times with methanol, n-hexane, and acetone. Synthesis of Au/Ag Nanowires. Chemicals. The surfactant cetyltrimethylammonium bromide (CTAB; 99% pure from Energy Chemical), chloroauric acid (98% pure with Au content of 47.8% from Energy Chemical), silver nitrate (99.99% pure from Aladdin), sodium ascorbate (98% pure from Energy Chemical), sodium borohydride (99.99% pure from Aladdin), alcohol, and deionized water were used in this synthesis procedure. Synthesis of Au/Ag Nanowires. A total of 20 mL of deionized water, 0.91 g CTAB, 0.00085 g chloroauric acid, 0.0297 g sodium ascorbate, and 0.00085 g silver nitrate were successively loaded into a 50 mL glass beaker in a water bath maintained at 35 °C. The yellowish Au(III) ion solution oxidized into a colorless Au(I) solution. A total of 5.7 × 10−8 g sodium borohydride (NaBH4) was added to the above solution to initiate a metal reduction in the solution. Immediately, the ∼20 μL drops of the solution were deposited on 1.4 cm × 1.4 cm Si/SiO2 substrates and left to dry in the beaker in the water bath. A water and alcohol mixture with a volume ratio of 3:7

(8)

The time constants τ3 and τ4 were 9 and 157 ms, respectively. The longer decay time τ4 may represent the carriers transferred through the active layers, as it is similar to τ2. The short recombination time τ3 is one order magnitude greater than the carrier lifetime (∼0.18 ms) in the PbS QD layer, which we attribute to the trapped electrons in the PbS QDs, which hinder the fast recovery process and extend the carrier recombination time. The device shows a faster photoresponse than the lateral P3HT/PbS QD blend phototransistors22 reported by Sun’s group, which we ascribe to the fast transport of carriers in the short channel and the wellordered, layered heterojunction. Figure 5b shows the on/off photoswitching behavior for multiple cycles, demonstrating the robustness and reproducibility of our photodetectors. We have summarized the characteristics of various common types of solution processed photodetectors in Table 1. Compared with the composite phototransistors with other PbX (X = S, Se)/polymer composite based LFEpTs with bulk heterojunctions, the vertical phototransistor with the PbS/ P3HT composite layer heterojunction is superior owing to its high photoperformance, fast photoresponse, lower operating voltage (owing to the shorter electrode spacing), and flexible channel length modulation. The photoresponsivity of our device was higher than pure QD-based PDs and CH3NH3PbI3-based PDs; however, it was lower than that of graphene/QD hybrid LFEpTs, owing to the ultrahigh carrier mobility of graphene and the low carrier mobility of both QDs and polymers. The detectivity of our PbS/P3HT composite VFEpT was slightly lower than that of graphene/PbSQD LFEpTs4 and PCBM/CH3NH3PbI3−xClx/ PEDOT:PSS PDs42 reported in ref 42; we ascribe this lower detectivity to the large dark current in our device. Therefore, appropriate measures should be taken to reduce the dark 590

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Infrared Remote Sensing and Instrumentation Xxiii; Scholl, M. S., Paez, G., Eds.; Spie-Int Soc Optical Engineering: Bellingham, 2015; Vol. 9608, pp 960813−9. (9) Han, H.; Nam, S.; Seo, J.; Jeong, J.; Kim, H.; Bradley, D. D. C.; Kim, Y. Organic Phototransistors With All-Polymer Bulk Heterojunction Layers of p-LEType and n-Type Sulfur-Containing Conjugated Polymers. IEEE J. Sel. Top. Quantum Electron. 2016, 22, 6000107. (10) Kim, K. H.; Bae, S. Y.; Kim, Y. S.; Hur, J. A.; Hoang, M. H.; Lee, T. W.; Cho, M. J.; Kim, Y.; Kim, M.; Jin, J. I.; Kim, S. J.; Lee, K.; Lee, S. J.; Choi, D. H. Highly Photosensitive J-Aggregated SingleCrystalline Organic Transistors. Adv. Mater. 2011, 23, 3095−3099. (11) Saidaminov, M. I.; Adinolfi, V.; Comin, R.; Abdelhady, A. L.; Peng, W.; Dursun, I.; Yuan, M.; Hoogland, S.; Sargent, E. H.; Bakr, O. M. Planar-integrated single-crystalline perovskite photodetectors. Nat. Commun. 2015, 6, 8724. (12) Li, F.; Ma, C.; Wang, H.; Hu, W.; Yu, W.; Sheikh, A. D.; Wu, T. Ambipolar solution-processed hybrid perovskite phototransistors. Nat. Commun. 2015, 6, 8238. (13) Konstantatos, G.; Clifford, J.; Levina, L.; Sargent, E. H. Sensitive solution-processed visible-wavelength photodetectors. Nat. Photonics 2007, 1, 531−534. (14) Konstantatos, G.; Howard, I.; Fischer, A.; Hoogland, S.; Clifford, J.; Klem, E.; Levina, L.; Sargent, E. H. Ultrasensitive solutioncast quantum dot photodetectors. Nature 2006, 442, 180−183. (15) McDonald, S. A.; Konstantatos, G.; Zhang, S. G.; Cyr, P. W.; Klem, E. J. D.; Levina, L.; Sargent, E. H. Solution-processed PbS quantum dot infrared photodetectors and photovoltaics. Nat. Mater. 2005, 4, 138−142. (16) Kahmann, S.; Mura, A.; Protesescu, L.; Kovalenko, M. V.; Brabec, C. J.; Loi, M. A. Opto-electronics of PbS quantum dot and narrow bandgap polymer blends. J. Mater. Chem. C 2015, 3, 5499− 5505. (17) McDonald, S. A.; Cyr, P. W.; Levina, L.; Sargent, E. H. Photoconductivity from PbS-nanocrystal/semiconducting polymer composites for solution-processible, quantum-size tunable infrared photodetectors. Appl. Phys. Lett. 2004, 85, 2089−2091. (18) Zhang, S.; Cyr, P. W.; McDonald, S. A.; Konstantatos, G.; Sargent, E. H. Enhanced infrared photovoltaic efficiency in PbS nanocrystal/semiconducting polymer composites: 600-fold increase in maximum power output via control of the ligand barrier. Appl. Phys. Lett. 2005, 87, 233101−3. (19) Qi, D. F.; Fischbein, M.; Drndic, M.; Selmic, S. Efficient polymer-nanocrystal quantum-dot photodetectors. Appl. Phys. Lett. 2005, 86, 093103−3. (20) Cui, D. H.; Xu, J.; Xu, S. Y.; Paradee, G.; Lewis, B. A.; Gerhold, M. D. Infrared photodiode based on colloidal PbSe nanocrystal quantum dots. IEEE Trans. Nanotechnol. 2006, 5, 362−367. (21) Pichler, S.; Rauch, T.; Seyrkammer, R.; Boberl, M.; Tedde, S. F.; Furst, J.; Kovalenko, M. V.; Lemmer, U.; Hayden, O.; Heiss, W. Temperature dependent photoresponse from colloidal PbS quantum dot sensitized inorganic/organic hybrid photodiodes. Appl. Phys. Lett. 2011, 98, 053304−3. (22) Sun, Z. H.; Li, J. H.; Yan, F. Highly sensitive organic nearinfrared phototransistors based on poly (3-hexylthiophene) and PbS quantum dots. J. Mater. Chem. 2012, 22, 21673−21678. (23) Song, T. J.; Cheng, H. J.; Fu, C. J.; He, B.; Li, W. L.; Xu, J. F.; Tang, Y.; Yang, S. Y.; Zou, B. S. Influence of the active layer nanomorphology on device performance for ternary PbSxSe1-x quantum dots based solution-processed infrared photodetector. Nanotechnology 2016, 27, 165202−9. (24) Yan, F.; Li, J. H.; Mok, S. M. Highly photosensitive thin film transistors based on a composite of poly(3-hexylthiophene) and titania nanoparticles. J. Appl. Phys. 2009, 106, 074501−7. (25) Mok, S. M.; Yan, F.; Chan, H. L. W. Organic phototransistor based on poly(3-hexylthiophene)/TiO(2) nanoparticle composite. Appl. Phys. Lett. 2008, 93, 023310−3. (26) Sun, Z. H.; Li, J. H.; Liu, C. M.; Yang, S. H.; Yan, F. Enhancement of Hole Mobility of Poly(3-hexylthiophene) Induced by

was used to wash off the residual CTAB, and the prepared substrates were dried in a vacuum oven at 50 °C for 12 h. Device Characterization. The absorption spectra of P3HT in trichloromethane and of PbS CQDs in toluene were measured with a Zolix Omni-λ300 spectrometer excited by a continuous-wave laser with a wavelength of 532 nm. HRTEM measurements were performed using a FEI Co., Tecnai G2 F20 TEM system at 200 kV. Current−Voltage Characterization. During the measurement, the samples were placed on a probe station in a glovebox and kept at room temperature. A bias voltage (VSD) was applied across the source (ground connection) and drain electrodes using a Keithley 2400 source meter. Channel current flowing into the drain is denoted as ID and was also detected using a Keithley 2400. A gate voltage (VG) was applied from the gate electrode to the ground connection using a HP6030A power supply. Photoresponse Characterization. The light source was a laser with the wavelength of 808 nm and its light irradiance was controlled by a DC power source. The broadband spectral response is measured using a Zolix Omni-λ300 spectrometer. Noise Current Characterization. We measured the background noise level of the phototransistor by taking a 1 s time trace with no illumination at a sampling rate of one measurement every 5 ms4. A modulation frequency of 2 frames per second was adopted to match the photoresponse time (0.6 s).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Haiting Zhang: 0000-0001-8046-710X Yu Yu: 0000-0001-6712-6191 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 61675147 and 61605141) and the Foundation of Independent Innovation of Tianjin University (Grant No. 0903065043).

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