A near infrared organic photodiode (OPD) utilizing a double electron blocking layer (EBL) fabricated by the sequential deposition of molybdenum (VI) oxide (MoO3) and poly(3,4ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) is reported. The double EBL improves the on/off current ratio of OPD up to 1.36 x 104 at −1V, which is one order of magnitude higher than PEDOT:PSS single EBL (2.45 x 103) and three orders of magnitude higher than that of MoO3 single EBL (7.86). The detectivity at near infrared (800 nm) at −1V is 4.90 x 1011 Jones, which is 2.83 times higher than the PEDOT:PSS single EBL and 2 magnitudes higher compared to the MoO3 single EBL.
© 2016 Optical Society of America
The most widely used photoactive layer type in organic photodetectors (OPD) is a bulk heterojunction (BHJ) photoactive layer. The BHJ photoactive layer consists of a blended mixture of donor and acceptor materials. Usually an efficient BHJ layer has an interpenetration network of donor and acceptor domains with a size of 10~20 nm, which is below the exciton diffusion length of many organic semiconductors. Due to the simple and room temperature process, the BHJ is mainly used for the photoactive layer in organic solar cells (OSC) [1,2]. In addition, the BHJ type photoactive layers exhibit the relatively high photon to electron conversion quantum yield due to the high exciton dissociation efficiency and absorption coefficient.
Usually, a photodiode is required to have a high on/off current ratio under high electric field to maintain a high photo-responsivity and detectivity at low light intensity. However, considering that the BHJ structure consists of a mixture of donor and acceptor in one single layer, large dark current under reverse bias is expected due to the undesirable charge transfer from electrode to active layer. To overcome this problem, many methods have been employed such as using a thicker active layer , utilizing a spray coated technique  and sequential deposition of the donor/acceptor layer .
Poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b’]dithiophene-2,6-diyl]] (PCPDTBT) is a promising organic semiconductor for near IR detective OPD since it has a broad absorption wavelength λ = 300 - 950 nm, with a cutoff at λ ~1000 nm . However, the low solubility of the PCPDTBT makes it difficult to obtain the desired active layer thickness, which leads to a large dark current. Due to this limitation, it is necessary to introduce an interlayer between the electrode and photoactive layer to improve the charge transfer under forward bias and to block charge injection under reverse bias.
Among the electron blocking (EBL) or hole transporting layer (HTL) materials, poly(3,4ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) is the most commonly used . However, degradation of these devices can occur due to the acidic PEDOT:PSS corroding the ITO electrode. This weakness means that PEDOT:PSS cannot be deposited in a sufficient thickness on top of the ITO electrode . Alternatively, metal oxides including molybdenum (VI) oxide (MoO3) have been utilized as an EBL or HTL for OPD and organic photovoltaics (OPV) [7,9].
As seen in the comparison of the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) in Fig. 1, the LUMO of MoO3 is much higher than that of the PCPDTBT and PCBM, which implies that electron injection from the anode through the active layer can be blocked. Although the MoO3 functions as an EBL, adhesion between the inorganic MoO3 and the organic photoactive layer is expected to be poor, which might increase the series resistance of the device. This poor adhesion can be improved by inserting a PEDOT:PSS layer between the MoO3 and photoactive layer because the PEDOT:PSS layer is known to improve the adhesion between inorganic ITO and organic photoactive layers in organic opto-electronic devices.
In this work, we present a study on the performance of an OPD utilizing a double EBL of PEDOT:PSS and MoO3 and its comparison to a single EBL. The photodetector parameters including on/off current ratio, responsivity, detectivity and spectral response were investigated for both the double and single electron blocking layers.
2.1 Device fabrication
Commercial ITO coated glass (20 Ohm/sq) was cleaned sequentially by isopropyl alcohol and acetone in an ultrasonic bath. Substrates were then dried in a convection oven for 30 min. UV/ozone treatment for 20 min was used to clean and remove any organic residues present on the surface. PEDOT:PSS (Clevious P VP AI 4083, Germany) was spin coated on top of the clean ITO anode and dried at 110 °C for 10 min in a vacuum oven. For the deposition of the MoO3 layer, MoO3 powder (Aldrich) was evaporated by thermal evaporation at a pressure of ~3 x 10−6 torr. For the fabrication of PEDOT:PSS/MoO3 or MoO3/PEDOT:PSS double EBL, each layer was sequentially deposited with its corresponding deposition method. The photoactive layer solution, PCPDTBT (Organic Nano Electronic) and the [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) (Nano-C, USA) (1:2) 20 mg/ml in chlorobenzene (CB) with 3% 1,8-diiodooctane (DIO) was stirred overnight at 60 °C. The prepared solution was spin-coated onto the EBL at a speed of 800 rpm for 30 seconds to give a 90 nm thickness and dried at 110 °C for 15 min inside a vacuum oven. Finally, lithium fluoride (LiF) (Aldrich) and aluminium (Al) were deposited onto the ITO/hole transport layer/photoactive layer by thermal evaporation through a shadow mask at a pressure of ~3 x 10−6 torr. The fabricated OPDs were encapsulated with a glass cap and a UV curable resin (Raynics, Japan) in a nitrogen (N2) filled glove box for further analysis.
2.2 Characterization techniques and measurements
The absorption spectra of the photoactive layers were measured using a UV-2450 ultraviolet-visible spectrophotometer (SHIMADZU, Japan). Current density vs. voltage (J-V) curves of the OPDs under various light intensities were measured by using an AM 1.5G solar simulator (McScience K201 LAB50, Korea) and Keithley 2400 source meter. The external quantum efficiency (EQE) of the OPDs, which shows the photon-to-electron conversion efficiency as a function of wavelength, were obtained under short-circuit conditions with a lock-in amplifier (SR830, Stanford Research System) at a chopping frequency of 20 Hz during illumination with monochromatic light from a Xenon lamp (McScience K3100 EQX, Korea). The thickness of the devices was determined by using Atomic Force Microscopy (Hitachi High-Tech Science Corporation, Japan).
3. Results and discussions
The molecular structure of the active layer and the energy levels of the materials used in the OPDs are shown in Fig. 1. The photoactive layer solution exhibited broad absorption in near infrared (IR) wavelengths of 900 nm due to the low band gap of PCPDTBT. As shown in Figs. 1(a)-1(d), OPDs with four different types of EBLs were tested; PEDOT:PSS (P) in Fig. 1(a), PEDOT:PSS/MoO3 (P/M) in Fig. 1(b), MoO3/PEDOT:PSS (M/P) in Fig. 1(c) and MoO3 (M) in Fig. 1(d). The thermally evaporated MoO3 with thicknesses of 2.5 nm, 5 nm, 7.5 nm and 10 nm were used to construct the P/M and M/P double EBLs while the thickness of the PEDOT:PSS layer was kept constant at ~30 nm. From the energy levels of the materials, excitons would dissociate into holes and electrons at the interface of PCPDTBT and PC71BM. At 0V and under illumination, holes would be transferred from the photoactive layer to the ITO anode through the HOMO of the EBL, and electrons would be collected at the Al electrode from the photoactive layer.
Under reverse bias (negative voltage) and illumination, the hole and electron transport would be accelerated because carrier drift velocity (υd) is proportional to the electric field,
An increase in the dark current by the electron injection from the ITO to the PC71BM layer is expected under reverse bias, which is inevitable in a BHJ photoactive layer. The undesirable electron injection in the dark and under reverse bias can be prevented by inserting an EBL, such as PEDOT:PSS and MoO3, between the ITO and photoactive layer. The measured properties of OPDs with various EBLs are listed in Table 1, and the semi log J-V curves for the representative OPDs are shown in Fig. 2(a). Figures 2(b) and 2(c) shows the on/off current ratio of the corresponding OPDs at a bias of −1V. For the OPD utilizing a P/M double EBL, Fig. 2(b) the on/off current ratio decreased as the MoO3 thickness increased, and the current value for the thickest P/M double EBL (~30 nm PEDOT:PSS / 10 nm MoO3) was similar to that of the M EBL (10 nm MoO3 only). For the OPD utilizing an M/P double EBL, Fig. 2(c) the on/off current ratio increased as the thickness of the MoO3 layer increased in the M/P double EBL, and these value are higher than that of OPDs utilizing single PEDOT:PSS or MoO3 EBLs. The highest ratio of 1.36 x 104 was observed for the M (10 nm)/P EBL which is one order of magnitude greater than the single PEDOT:PSS blocking layer (2.45 x 103) and four orders of magnitude greater than the single MoO3 blocking layer (7.86).
Comparing the Rs values of OPDs with a single layer EBL, the OPD with a P EBL exhibited lower Rs value than the OPD with an M EBL. This implies that the interfacial resistance at the PEDOT:PSS/photoactive layer is lower than that at the MoO3/photoactive layer because the organic photoactive layer would form a better contact with the PEDOT:PSS layer than the inorganic MoO3 layer. The same explanation can be applied to the significant difference in Rs values between OPDs with P/M and M/P double EBLs. Based on the results that the OPDs with P and M/P EBLs exhibited similar Rs values and the largest Jph at −1 V, it seems that the contact resistance at photoactive layer/EBL interface plays a significant role in determining the performance of OPDs.
Under dark conditions, the M/P double EBL most effectively blocked electron injection to the photoactive layer from the ITO under reverse bias. The Jd values measured at −1V were 5.10 x 10−6 A/cm2, 5.20 x 10−4 A/cm2, 9.40 x 10−7 A/cm2 and 6.08 x 10−4 A/cm2 for OPDs with P, P/M, M/P and M EBLs, respectively. Consequently, the significant reduction in Jd by maintaining a high Jph enabled the OPDs with an M/P EBL to have the highest on/off current ratio.
In order to study the spectral response of OPD, external quantum efficiency (EQE) spectra were plotted under a bias of 0V and −1V. The EQE of all OPDs increased as the reverse bias increased. Due to the unsymmetric electrode work function, a built-in field exists in the device and drives the charges to the corresponding electrodes even at 0V . At reverse bias, drift current is dominant due to the presence of a strong electric field. Applied bias reinforces the built-in electric field, enhancing the charge transport and resulting in a large photocurrent. As a BHJ consists of a mix of the donor and acceptor, under a bias of 0V, the weak electric field allowed for non-geminate recombination at the inside of the photoactive layer. Whereas, applying reverse bias would enhance the charge carrier velocity resulting in a reduction in the charge recombination of the OPDs. As is expected from the Jph values, OPDs with P in Fig. 3(a) and M/P in Fig. 3(c) EBLs showed similar EQE spectra whereas OPDs with M in Fig. 3(d) and P/M with 10 nm MoO3 in Fig. 3(b) EBLs showed a similar pattern and significantly lower EQE values for all wavelengths. The EQE values at 800nm were 33.0% and 41.8% for the OPDs with P and M/P EBLs, respectively.
The responsivities were obtained from the equationTable 2 and Fig. 4(a), P and M/P EBLs exhibited almost similar responsivity, and the value was greater than that of OPDs with M and P/M EBLs. Based on the EQE spectrum, the OPD with M/P EBL exhibited better spectral response at the shorter wavelength (480 nm) than near IR range due to the efficient exciton generation from the PCBM. However, the responsivity of the OPD at near IR (800 nm) was higher than that of visible (480 nm). There is significant One of the important factors determining the performance of the photodetector is the detectivity (D*). High D* can reduce the noise equivalent power (NEP) - the minimum optical power for the detector to distinguish from the noise . There are three types of noise influencing the D*: shot noise from dark current, Johnson noise, and thermal fluctuation “flicker” noise [12,13]. According to the previous reports on the OPDs, the shot noise from the dark current is considered to be the major contributor to the D* and can be express as
The detectivities of OPDs at the near IR wavelength of 800 nm were calculated and their results were plotted in Fig. 4(b) and listed in Table 2. The calculated detectivity of OPDs with an M/P EBL was 4.90 x 1011 Jones, which was 2.83 times higher than the OPD utilizing a single PEDOT:PSS blocking layer and two orders of magnitude higher compared to the OPD with a single MoO3 EBL. It was clear that the low dark current (noise) with high photocurrent was essential to obtaining high detectivity . Recently, interesting reports on the near IR photodetector with plasmon-induced hot electron device [15,16]. The device exhibited narrowband photodetection with the responsivity of 0.6 mAW−1. Although the spectral response range of this OPD was broader, the responsivity of this OPD was ~450 times higher than that of plasmon-induced hot electron device.
The performance of near IR detecting OPDs has been enhanced by applying a MoO3/PEDOT:PSS double EBL between the ITO and the photoactive layer. The double EBL reduced the Jd by suppressing electron injection from the ITO and maintained a high Jph via the formation of good adhesion with the photoactive layer.
The detectivity of this double EBL OPD was 2.83 times and 2 orders of magnitude higher than that of the OPDs utilizing single PEDOT:PSS and MoO3 EBLs, respectively. Additionally, in term of preventing the ITO from coming into direct contact with the acidic PEDOT:PSS, the double EBL is expected to have improved stability and efficiency.
National Research Foundation of Korea (NRF) (NRF-2015M1A2A2057506); Ministry of Trade, Industry & Energy, Republic of Korea (No. 20123010010140, 20133030011330 and 20133030000210).
We thank Dr. Kris Rathwell for the valuable discussions.
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