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Most organic photodetectors utilize a bulk heterojunction (BHJ) photo-active film due to its high exciton dissociation efficiency. However, the low dark current density, a key role in determining the overall performance of photodetectors, is hardly achieved in the BHJ structure since both the donor and acceptor domains are in contact with the same electrode. The most popular strategy to overcome this problem is by fabricating bilayer or multilayer devices. However, the complicated fabrication process is a challenge for printing electronics. In this work, we demonstrate a solution processed polymer photodetector based on a poly (3-hexylthiophene) (P3HT): (phenyl-C61-butyric-acid-methyl-ester) (PC61BM) blend film with polyethylenimine ethoxylated (PEIE) modified ITO electrode. The transparent PEIE efficiently blocks the unnecessary electronic charge injection between the active film and the electrode, which dramatically decrease the dark current. Under illumination, the photoexcited charges accumulated in the PEIE modified ITO region finally can tunnel through the barrier with the help of the applied reverse bias, leading to a large photocurrent. Therefore, the resulting polymer photodetector shows a 2.48 × 104 signal-to-noise ratio (SNR) under −0.3 V bias and an 11.4 MHz bandwidth across the visible spectra under a small reverse bias of 0.5 V. The maximum EQE of 3250% in the visible wavelength is obtained for the polymer photodetector at −1 V under 370 nm (3.07 μW/cm2) illumination. This solution processed polymer photodetector manufacturing is highly compatible with the flexible, low-cost, and large area organic electronic technologies.
© 2017 Optical Society of America
1. Introduction
Organic photodetectors (OPDs) have attracted considerable attention because of their potential for lightweight and flexible photodetector in the coming flexible electronics era. The optoelectronic properties (i.e., charge generation, transport and recombination) of organic semiconductors can be relatively easily tunable by controlling their molecular structures [1]; these semiconductors generally exhibit excellent photodetection ability [1,2]. OPDs also have the strong benefit of facile control of the detection band (wavelength range) by modifying organic active materials. High external quantum efficiency, broad operation bandwidth, and large signal to noise ratio are essential for a high performance photodetector [2]. To achieve good device characteristics, various organic photodetectors have been developed. Compared with other devices, the photoconductive detector has attracted more attention because its quantum efficiency can exceed 100% [3]. To date, most photoconductive detectors utilize a bulk heterojunction (BHJ) structure, which can be simply prepared by the one-step solution process of polymer (donor):fullerene (acceptor) blend. Although, the BHJ structure can dramatically improve the exciton dissociation efficiency, it also leads to an undesirable high dark current, which greatly degrade the device performances. In the BHJ devices, both holes and electrons can inject from either low work function cathode or high work function anode because the donor domains and the acceptor domains are in contact with the same electrode [4]. Recently, highly sensitive organic photodetectors have been realized with a bilayer or a multilayer structure to break the two-way channels for charge transport. Several research groups have demonstrated that organic bilayer photodetectors consisting of an electron donor layer and an electron acceptor layer can adequately reduce undesirable charge injection [4–6]. Shafian et al. reported a P3HT:PC61BM photodetector utilizing the polymer/interdiffusion/fullerene photoactive layer prepared by the sequential solution deposition process to achieve the reduced dark current [4]. Ma et al. reported a fast response organic photodetector based on rubrene as donor and C60 as acceptor with the bilayer structure [5,6]. Besides the bilayer devices, the electric charge blocking layers or trapping layers have been widely applied to achieve high performance organic photodetectors. Hammond et al. reported the photoconductive detectors achieved photo current gains up to 500 across the visible spectrum and bandwidths on the order of 1 kHz by trap-controlled charge injection based on the blocking layer [7]. Melancon et al. reported the P3HT:PCBM substantial broadband photoconductive gain photodetector in NIR response with a semicontinuous Au blocking layer [8] which show the highest EQE about 1500% in the visible wavelength. Huang’s group demonstrated the high EQE photodetectors about 10000% by achieving interfacial trap charges with ZnO [9] and CdTe [3] nanoparticles dispersed throughout the device-active layer. Chen et al. reported the photoconductive gain (also called photomultiplication, PM) achieved high EQEs (>7000%) response from the ultraviolet to the near-infrared (NIR) region due to the organic NIR dye doping in P3HT:PC61BM bulk heterojunction layers [10].
Although the above mentioned methods have achieved significant improvements in device performances, their complicated fabrication process, such as vacuum thermal evaporation [5–8], high annealing temperatures [11], environment-friendly process [3] and so on, is a challenge for all-solution manufacturing and thus limits the extensive application of flexible printing electronics. In this work, a sensitive and fast photoconductive detector has been demonstrated by sandwiching a single P3HT:PC61BM film with polyethylenimine ethoxylated (PEIE) modified ITO electrode. It is known that PEIE coating could shift the work function and leads to an electron selective contact [12] with a one-step, low temperature solution-process on various different materials. Therefore, the scheme with PEIE coating in our work alleviates the complexity of the fabrication process compared with the other kinds of blocking layers. Although, in this BHJ device, the P3HT domains and the PCBM domains are in contact with the same electrode, the low work function transparent PEIE modified ITO electrode is expected to reduce the undesirable electric current due to the formed barrier between the active film and the electrode. Therefore, this solution processed polymer photodetector fabrication may pave the way for the applications in the flexible, low-cost, high performance, and large area organic electronic technologies.
2. Experimental
2.1 Device fabrication
The ITO-modified glass substrates (sheet resistance:15 Ω/□) were cleaned with detergent, followed by sequential sonication in deionized water, acetone, ethanol, and then dried by dry nitrogen and treated by UV-ozone for 15 min. Polyethylenimine, 80% ethoxylated (Mw = 70 000 g mol−1) was dissolved in H2O with a concentration of 35–40 wt. % and then, the PEIE was spin coated on the top of ITO from 0.05 wt. % 2-methoxyhanol with 4000 rpm for 1 min and was subsequently dried in ambient at 100 °C for 30 min, resulting in the PEIE modified ITO (ITO + PEIE) substrate. The active films were spin-coated on top of either bare ITO or PEIE modified ITO substrate from a P3HT and PCBM (1:1 by weight) solution in 1,2-dichlorobenzene by stirring at 50 °C for 24 hrs. Then, the active films were then annealed at 110 °C for 10 min and the obtained film is 270 nm thick. Finally, the aluminium electrodes (100 nm-thick) were thermally deposited onto the active film through a shadow mask in high-vacuum (10−7 mbar). Herein, P3HT and PC61BM were purchased from Rieke Metals and Nano-C, respectively. PEIE and 1, 2-dichlorobenzene were obtained from Sigma-Aldrich. All materials were used as received without further purification.
2.2 Characterization techniques and measurements
The Current density versus voltage (J-V) characteristics were measured using a Keithley 6430 source-power unit. An AM 1.5 solar simulator (ABET Technologies) at 100 mW/cm2 intensity was used to provide illumination. The absorption spectra of the active layers were measured by a SHINADZU UV-3101 PC spectrophotometer. Ultraviolet photoelectron spectroscopy (UPS) images were recorded on a Kratos Axis Ultra DLD spectrometer using He (I) ultraviolet light (21.22 eV) as the excitation source. The capacitance–voltage (C–V) measurements were conducted using a Keithley 4200-SCS with an AC bias of 25 mV and the data were recorded at a frequency of 3 kHz. For the transient short-circuit photocurrent measurement, a square-pulsed optical excitation of 1 ms was generated from a 530 nm high-brightness LED driven by a function generator, and a 50 Ω input termination of the oscilloscope was used. The thickness of active layers was determined by a AMBIOS technology XP-2 stylus profilometer. To obtain a modulated excitation light at various frequencies, a WF 1946B multifunction synthesizer (NF Corporation) was utilized to control the light. The bandwidth of the photodetector was calculated with fast Fourier transform (FFT) algorithm. The monochromatic light used in all these measurements was provided by a 150 W xenon lamp coupled with a monochromator. All the measurements were carried out at room temperature in the ambient condition.
2.3 Calculations
where Jph is photocurrent density, Jlight is current density under light, Jdark is dark current density, Pin is the incident light intensity, e is absolute value of electron charge, and hυ is the energy of incident photon. R is responsibility, D* is detectivity.
3. Results and discussion
The transmittance of the electrode is essential for fabrication of high-performance organic photodetectors since the light need to pass through the electrode being absorbed by the BHJ photo-active film. Compared with the transmittance of the device based on bare ITO substrate shown in Fig. 1(a), nearly same transmittance from the visible region is observed for the device based on PEIE modified ITO substrate. Fig. 1(a) indicates that the incident light can pass through the PEIE layer into the P3HT:PC61BM layer almost without absorption loss. The work functions of the PEIE modified ITO and the pristine ITO were measured by ultraviolet photoelectron spectroscopy (UPS). The wavelength as the ultraviolet is 58.13 nm and energy is 21.22 eV. Fig. 1(c) shows that the binding energy EB (the intercept of binding energy at the linear area of the UPS curve) is 17.51 eV for PEIE modified ITO, and EB is 16.76 eV for pristine ITO. By subtracting the excitation energy (He Iα source with 21.22 eV) the work function (WF) of the modified ITO is calculated to be approximately 0.75 eV lower than that of the bare ITO electrode. Therefore, the energy barrier formed between the WF of the PEIE modified ITO and the highest occupied molecular orbital (HOMO) of the donor polymer (P3HT) is 0.75 eV larger than that between the bare ITO and P3HT. It is well known that the higher energy barrier causes the enhanced interfacial accumulation of photogenerated charges, which is preferred to the increased photocurrent gain [7, 8], as shown in Fig. 1(b). Therefore, the photocurrent gain could be improved due to the enlarged barrier height presented by the PEIE modified ITO electrode.
Fig. 1 (a) Absorption spectra of the P3HT:PC61BM layer based on ITO and PEIE modified ITO substrates (b) The energy level diagram of the OPD (c) UPS spectra of the pristine ITO and PEIE modified ITO electrodes
Fig. 2(a) shows the current density versus voltage (J-V) curves of the devices based on ITO and PEIE modified ITO in the dark and under illumination. The dark current of device based on PEIE modified ITO in the forward bias region is significantly suppressed. It is due to the formed barrier between the active film and the electrode owing to the transparent PEIE modified ITO electrode. Fig. 2(b) shows the J-V characteristics of the device on PEIE modified ITO demonstrate a difference in the magnitude of currents in the presence and absence of illumination under the same applied voltage bias. In the dark, the device on PEIE modified ITO has a lower current density about 8.79 × 10−7 A/cm2 at −0.5 V and 1.20 × 10−6 A/cm2 at 0.5 V while the device on bare ITO shows 2.25 × 10−5 A/cm2 at −0.5 V and 1.92 × 10−3 A/cm2 at 0.5 V. The forward and reverse bias dark current densities of the device on PEIE modified ITO are almost four and three orders of magnitude lower than those of the device on ITO, respectively. Apparently, the device on PEIE modified ITO can avoid parasitic injection of holes and/or electrons from the electrodes more effectively [13]. The photocurrent density under reverse bias voltage maintain the range from 17.76 mA/cm2 at −0.5 V up to 51.41 mA/cm2 at −2 V and 7.66 mA/cm2 at −0.5 V up to 8.26 mA/cm2 for the PEIE modified ITO and ITO devices respectively. The signal-to-noise ratio (SNR) given as the ratio of photo to dark current. The device on PEIE modified ITO shows the SNR of 2.02 × 104 which is much higher than that of the device on ITO (3.40 × 102). The higher photo current and SNR could be attributed to the introducing of PEIE modified ITO. EQE in excess of 100%, can be observed from the OPD based on PEIE modified ITO, as shown in Fig. 2(c). The EQE of the OPD based on ITO just can be lower than 100% because of the typical organic photovoltaic structure. The EQE values of OPD based on PEIE modified ITO arrive to about 3250% and 3200% at −1 V for the device under 370 nm and 550 nm illumination respectively. It even can reach about 1300% and 1000% at −0.3 V under 370 nm and 550 nm illumination. Light intensity spectrum of the monochromatic lights through a monochromator for EQE test was measured and is shown in Fig. 2(d). Figs 2(b) and 2(c) indicate the EQE value and Jph are highly dependent on light intensity (mW/cm2 and μW/cm2). The devices have a higher EQE in the low intensity (μW/cm2) than the one in the high intensity (mW/cm2). The detailed explanation will be presented in Fig. 5. The EQE value above 100% is because of the extra charges injected from the electrodes under an applied bias and these charges can be only injected under illumination due to the large energy barrier preventing the charge injection in the dark. These injected charges plus the photogenerated charges contributes an EQE value greater than 100% [3]. The detailed explanation will be presented in Fig. 3.
Fig. 2 (a) J-V curves of the OPD on ITO and ITO + PEIE (under AM 1.5 100 mW/cm2) (b) The J-V curves of the OPD on ITO + PEIE and ITO (inset) in logarithmic scale (under AM 1.5 100 mW/cm2) (c) EQE curves of the photodetector on ITO + PEIE under different bias conditions, inset EQE curves of the device on ITO without bias conditions (d) Light intensity spectrum of the monochromatic lights for the EQE test of the PEIE treated device
Fig. 3 (a) The capacitance-voltage characteristics of the OPD on ITO and PEIE modified ITO under dark and light conditions; (b) transient photocurrent rise and fall of the OPD on ITO (c) transient photocurrent rise and fall of the OPD on PEIE modified ITO (d) Transient photocurrent rise and fall of the OPD on PEIE modified ITO under different bias.
It is known that the responsivity (R) and detectivity (D*) of photodetectors strongly depend on the bias and photon wavelength. The responsivity and detectivity values of OPDs on PEIE modified ITO with P3HT:PC61BM as the active layers arrive to 5.61 A/W, 7.91 A/W, 14.25 A/W and 1.42 × 1012 Jones, 1.23 × 1012 Jones, 1.04 × 1012 Jones corresponding to 550 nm light illumination under −0.3 V, −0.5 V and −1 V bias, respectively. The responsivity improvement and the detectivity decrease should be attributed to the increased reverse bias induced photo current enhancement and dark current increase, respectively (Table 1).
Table 1. Figures of merit for PEIE modified ITO OPDs under different biasa
In order to explore the photogenerated charge accumulation existed at the PEIE modified ITO region of the device, the light-assisted capacitance measurements were carried out to study the charge interfacial confinement at the interface between the active film and PEIE modified ITO electrode. It is known that the device capacitance can be directly determined by the charge densities at the electrode interfaces in organic solar cell under illumination [14,15]. Fig. 3(a) shows the C-V characteristics of the devices on ITO and PEIE modified ITO in the dark and under white LED illumination (64 mW/cm2). The capacitances at zero bias increase from 0.54 nF to 3.42 nF and from 1.65 nF to 1.99 nF for the devices on PEIE modified ITO and on ITO, respectively. It implies that the photogenerated holes do accumulate at the interface between the active film and the electrode for the both devices under illumination. The capacitance increments are 2.88 nF and 0.34 nF for devices on PEIE modified ITO and ITO respectively, which indicates that the interface between the PEIE modified ITO and the active film can accumulate much more photogenerated holes than that between ITO and the active film due to the enhanced barrier. In other words, the increase in capacitance is due to photogenerated charges that are distributed throughout the active layer, probably near the interface between the PEIE-modified ITO and active layer, as a result of charge accumulation at the interface. Therefore, the details about working process of the OPD on PEIE modified ITO can be proposed as following: when the device is illuminated, the photogenerated excitons are created by photo absorption of the P3HT:PC61BM film, and then dissociate at the P3HT-PC61BM interface, generating holes on the P3HT HOMO level and electrons on the PC61BM LUMO level. In the PEIE modified ITO device, more efficient interfacial photo-induced charge trapping is achieved as a result of the increased barrier near the interface between the PEIE-modified ITO and active layer. This high density of trapped charge will reduce the electron injection barrier from PEIE and will eventually lead to strong electron injection from PEIE to P3HT. When a reverse bias is applied, even under a very low light intensity illuminated, efficient electric charge tunneling through the interface between the active film and the electrode occurs with the help of the accumulated electric charges, leading to a higher photocurrent [16–18].
The transient short-circuit photocurrent is an important parameter to study the operating mechanism of the photodetectors. The measurement cannot only demonstrate the time required for establishing the steady-state condition, but also reveal the transient-state processes such as carriers trapping and detrapping from the turn-on and turn-off dynamics. To investigate charge accumulation and transportation in the OPDs on ITO and PEIE modified ITO substrates, the transient short-circuit photocurrent measurements were carried out to study rise and fall transient responses. Fig. 3(b) and 3(c) show the transient photocurrent responses of the devices on ITO and PEIE modified ITO substrates on an illumination square-pulse of 1 ms. The rise and fall times of the device are defined as the time taken to go from 10% to 90% of the full response and 90% to 10% of the full response, respectively [19,20]. For the OPDs on ITO and PEIE modified ITO, both devices show fast turn on/turn off dynamics. The device with ITO shows a rise time of 18 μs and a fall time of 20 μs. The overshoot [Fig. 2(b)] from the device on ITO is observed and it has been successfully explained by charge trapping model [21,22]. By contrast, Fig. 3(c) shows that the photocurrent of the PEIE modified ITO device rises slowly to reach steady state without experiencing overshoot. The similar phenomenon has been reported and explained in terms of the enhanced injection barrier [23,24]. This explanation is consistent with our case since the PEIE coating does introduce a larger injection barrier between the active film and the electrode. The injecting barrier introduced electric field decreases the electric field in the bulk since the total applied voltage is constant. Therefore, the photocurrent of the ITO + PEIE device rises more slowly than the one of the ITO device until the charge tunneling occurs between the electrode and the active film [25]. The PEIE modified ITO device exhibits a rise time of 78 μs and a fall time of 87 μs which is a little longer than that of the ITO device. It is worth noting that the PEIE modified ITO photodetector demonstrates a response time of the same order compared with the colloidal quantum-dot photodetectors [26,27] and organic hybrid photodetectors [9]. In addition, the response time keeps decreasing with continuously increasing the reverse bias, which is caused by the decreased device capacitance due to the increased reverse bias. [Fig. 3(d)] This phenomenon supports the above discussion: the decreased electric field induced by the injection barrier at the PEIE modified ITO leads to a slower response of the PEIE modified ITO device comparing with the ITO device.
The cyclical photoresponses of device on ITO and PEIE modified ITO under white LED illumination (30.56 mW/cm2) at a bias voltage of −0.5 V are shown in Fig. 4(a). The each cycle is for 5 s of exposure time. The response current (defined as Ilight-Idark) in the device on PEIE modified ITO is about 0.22-0.26 mA, which is almost 2.5 times as compared to the current of the device on ITO. The enhancement in response current is because that the electron tunneling induced by PEIE can generate more current comparing with the ITO device. It is noted that a continuous increase of the maximum response current occurs in the PEIE modified ITO device, which was reported in the previous research and attributed to the self-generation of heat in the electrode/active film interface under illumination [28]. The cycle response currents reveal that the device on PEIE modified ITO shows stable photoresponse under illumination. Fig. 4(b) shows the optoelectronic small signal frequency response of the PEIE modified ITO device at −0.5 V and the results have been normalized to the value measured at 50 Hz. The frequency dependent normalized gain indicates that the device bandwidth (the −3 dB cut-off frequency) is 11.4 ± 0.3 MHz at the bias of −0.5 V. This bandwidth is higher than that reported on the P3HT:PC61BM devices [29–31] and in the same order of magnitude as those of the organic bulk heterojunction photodetector [32] and small molecular organic photodetector [5,33].
Fig. 4 (a) Cyclical photoresponses of the OPDs based on ITO and PEIE modified ITO. (b) Optoelectronic small signal frequency response of the OPD with the PEIE modified ITO at −0.5 V under modulated green-LED (530 nm) illumination. (c) Optoelectronic small signal frequency response of the OPD with the PEIE modified ITO at −0.5 V under modulated green-LED (530 nm) illumination with different light intensity.
The frequency response of the OPD with the PEIE modified ITO at −0.5 V under modulated green-LED (530 nm) illumination with different light intensity are shown in Fig. 4(c), it indicates that the frequency response is strongly dependent on the light intensity irradiated. As shown in the I-V curves [Figs. 5(a)-5(c)], (468 nm, 530 nm, 625 nm), the current was greatly increased (with bias voltage) as incident light intensity increased irrespective of wavelength. It can be observed that the current becomes larger at higher voltages than the one at lower voltages, leading to the noticeable steep slopes in the I–V curves. In particular, as the intensity of incident light increases, the current increases in the same tendency, resulting in 0.2-0.4 mA at a reverse bias of 2 V. The results indicate that the ITO + PEIE photodetectors can properly work under the various visible light illuminations.
Fig. 5 (a) (b) (c)Light I-V characteristics of the OPD based on the PEIE modified ITO under various wavelength (468 nm, 530 nm, 625 nm) illuminations with increasing incident light intensity, (d) (e) (f)Jph of the OPD at −0.5 V based on the PEIE modified ITO as a function of light intensity (468 nm, 530 nm, 625 nm).
Figs. 5(d)-5(f) show the Jph as a function of the light intensity at the specific wavelengths (468 nm, 530 nm, 625 nm) under −0.5 V. The Jph increases only in two or three orders of magnitude while the Pin increases in four or five orders of magnitude (from μW/cm2 to mW/cm2), resulting in a higher Jph/Pin (or EQE) in the low intensity (μW/cm2) than in the high intensity (mW/cm2). This observation is consistent with the one obtained in Fig. 2. This behavior has been reported in some organic and inorganic photoconductive photodetectors [34,35]. The decreased EQE of the PEIE based photodetector with the dramatically increased illumination intensity under the same electric field can be explained by the following mechanism: the great enhanced illumination induced excess excitons result in less effective trapping due to the saturation of the electron trap states and reducing the hole mobility [34,35].
From the practical perspectives, presence of the severe variation in EQE over light intensity can be a limiting factor for the proposed devices. The high EQE of a few thousand % is available only in the low-intensity region. At mid-to-high intensity region, EQE is degraded significantly. For example, EQE and responsivity are 147.91% and 0.67 A/W at −0.5 V bias for Pin = 13.35 mW/cm2 at the wavelength of 530 nm. Potential users should check EQE or the responsivity at a desired intensity level before using the proposed devices. The origin of such a large degree of variation in EQE remains elusive.
4. Conclusion
In summary, we have demonstrated a solution processed polymer photoconductive detector with PEIE modified ITO electrode. With PEIE modified ITO electrode, a sensitive and fast polymer photoconductive detector is demonstrated for the light signals detection with a high SNR of 2.48 × 104 and a high bandwidth frequency of 11.4 MHz. The maximum EQE of 3250% in the visible wavelength is obtained for the polymer photodetector at −1 V under 370 nm illumination with the intensity of 3.07 μW/cm2. The effect of PEIE modified ITO electrode on the polymer photoconductive detector is investigated and explained. The combination of SNR, bandwidth, photo stability and easy fabrication of the photodetectors pave the way for the applications in flexible printing electronics.
Funding
The Fundamental Research Funds for the Central Universities (2014JBZ009); National Natural Science Foundation of China (61377028, 61475014, 61475017, 61674012, 61675018).
Acknowledgments
We gratefully thank Dr. Ye Zou for the UPS test.
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