Incorporation of MoS2 nanoflakes into poly(3-hexylthiophene)/n-type Si devices to improve the rectification behavior and optoelectronic performance
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This study determines the effect of incorporating MoS2 nanoflakes into poly(3-hexylthiophene) (P3HT) on the electrical conduction mechanisms using the rectification current–voltage characteristics of P3HT/n-type Si devices. It is shown that the forward-voltage current for P3HT/n-type Si devices is limited by the combined effect of thermionic emission and space-charge-limited current conduction. However, carrier transport for P3HT:MoS2/n-type Si devices in the forward-voltage region is almost dominated by thermionic emission. Incorporation of MoS2 nanoflakes into P3HT modifies the P3HT-Si interface and the values for the carrier mobility in the P3HT layer and the external quantum efficiency of the P3HT/n-type Si devices are significantly increased, which improves the rectification and optoelectronic performance of P3HT:MoS2/n-type Si devices.
KeywordsHeterojunction Polymer Electrical properties Thin films Two-dimensional materials
PACS Nos.72.80.Cw Elemental semiconductors 72.80.Tm Composite materials 73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions 73.90. + f Other topics in electronic structure and electrical properties of surfaces, interfaces, thin films, and low-dimensional structures
The fabrication of organic/inorganic semiconductor heterojunctions allows the development of other classes of semiconductor devices that have applications in low cost electronics, optoelectronics and photovoltaics. Various device structures and experimental methods have been developed to study the electronic and optoelectronic properties of p-type conjugated polymer/n-type inorganic semiconductor devices [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. The high power conversion efficiency, excellent stability and high flexibility of poly(3-hexylthiophene) (P3HT) mean that it has applications in organic/inorganic heterojunction devices [1, 2, 3, 4, 5, 6, 12]. A heterojunction that is composed of P3HT and Si has many potential applications in electronic and optoelectronic devices and allows study of the effect of the interface on the nanoscale and the evolution of the electrical transport mechanisms [1, 5, 6, 13, 14]. The integration of P3HT with Si has the potential to lower the cost of electronic and optoelectronic devices and multifunctional devices. However, P3HT/n-type Si (n-Si) devices exhibit non-ideal rectification behavior because of the bulk effect of the P3HT layer . P3HT exhibits a low hole mobility of 10−5–10−4 cm2 V−1 s−1 [15, 16, 17] because there is strong electron–phonon coupling [5, 18]. Incorporating inorganic nanomaterials into P3HT is of use in electronics and optoelectronics [11, 12, 18, 19, 20, 21]. So-called hybrid polymer devices use a blend of P3HT and inorganic nanomaterial to improve the interfacial junction between P3HT and Si and to increase the carrier mobility in the P3HT layer. Recent years have witnessed significant progress in the application of two-dimensional (2D) materials in transistors and for energy conversion [22, 23, 24, 25]. 2D MoS2 has a high carrier mobility at room temperature [22, 26], so it is used to dope P3HT to improve the performance of P3HT/n-Si devices. This study uses blends of MoS2 nanoflakes and P3HT (referred to as P3HT:MoS2) to construct hybrid P3HT:MoS2/n-Si devices for which the space-charge-limited current (SCLC) conduction is decreased and the rectification current–voltage (I–V) characteristics of the P3HT/n-Si devices are improved. To the authors’ best knowledge, there have been no reports on the fabrication and characterization of P3HT:MoS2/n-Si devices.
2 Experimental details
P3HT was purchased from Luminescence Technology Corp. Four-inch 525-μm thick n-Si (100) wafers with an electrical resistivity of 3 Ω cm (Guv Team International Co., Ltd.) were used in the experiment. MoS2 flakes were grown on a SiO2 layer using chemical vapor deposition (CVD). A SiO2 layer was grown on the n-type Si wafer using a dry oxidation process. The SiO2/Si substrates were ultrasonically cleaned for 10 min in acetone, then in methanol and then in de-ionized water and dried in nitrogen. The growth process was performed in a CVD system, using MoO3 (10 mg) and sulfur (30 mg) powder as the precursor. The detailed CVD-grown process is shown in [27, 28]. The MoS2 flakes exhibit p-type behavior . Raman spectroscopy was used to determine the structural properties of the MoS2 flakes. A 532-nm laser was used for excitation. Composite samples were prepared by adding P3HT (200 mg) to 1,2-dichlorobenzene solutions (10 mL), with and without MoS2 (10 mg), in a nitrogen-filled glove box. These solutions were stirred for 24 h at 40 °C using a magnetic stirrer. The P3HT and P3HT:MoS2 solutions were then deposited on n-Si surfaces by spin coating in a nitrogen-filled glove box. Spin coating was performed at 600 rpm for 30 s. The n-Si samples were cleaned in chemical cleaning solutions of acetone and methanol. The n-Si sample was then chemically etched for 1 min using a diluted HF solution, rinsed with de-ionized water and blow-dried with N2. After deposition by spin coating, the films were baked at 55 °C for 25 min on a hotplate in a nitrogen-filled glove box. The thickness of the P3HT (P3HT:MoS2) film was approximately 105 nm. The effect of doping with MoS2 on the structural properties of P3HT films was determined using Raman spectroscopy. Using sputter coater, gold (Au) ohmic contacts with interdigitated patterns (0.482 cm2) were deposited onto the P3HT (P3HT:MoS2) surface (4 cm2) and indium (In) ohmic contacts with a square pattern (4 cm2) were deposited onto the n-Si back surface. The current–time (I–t) and I–V curves were measured at room temperature using a Keithley Model-4200 semiconductor characterization system. The relationship between I and V at high voltages for the measured P3HT/n-Si and P3HT:MoS2/n-Si devices shows a transition from super-linear to ohmic. This super-linear or power-law dependence between I and V is a characteristic of SCLC [1, 29]. For SCLC conditions the relationship between I and V is defined by I ~ Vβ where β is a constant. The photo-response for the device was measured under an illumination intensity of 100 mW/cm2 using a 150 W solar simulator with an AM 1.5G filter. The photo-response was measured by recording the current over time while sunlight illumination was turned on and off using a shutter. Van der Pauw–Hall measurements (Ecopia HMS-3000) were performed at room temperature to determine the carrier mobility and carrier concentration for P3HT (P3HT:MoS2) films. The P3HT and P3HT:MoS2 thin films were deposited on glass surfaces. The electrodes were fabricated by depositing Au metal onto the P3HT (P3HT:MoS2) layer through a shadow mask. The HMS-3000 includes software with an I–V curve that determines the ohmic integrity of sample contacts that are made by a user. The Hall effect was discovered by Hall in 1879 when he investigated the nature of the force that acts on a conductor that carries a current in a magnetic field . Discussions of the Hall effect can be found in solid-state and semiconductor literature [31, 32].
3 Results and discussion
In this case, the organic and inorganic phases contribute differently to charge carrier transport. Two conduction mechanisms (that is, TE and SCLC)  that affect the I–V characteristics are considered. In order to understand this phenomenon, two fitted curves at high voltages for ohmic (I ~ V1) and SCLC (I ~ V1.5) behaviors are plotted in Fig. 3. The value for β for I ~ Vβ is derived to be 1.0 for P3HT:MoS2/n-Si devices, which demonstrates that the current is limited by series resistance. However, the value for β for I ~ Vβ is derived to be 1.5 for P3HT/n-Si devices. This super-linear or power-law dependence between current and voltage is a characteristic of SCLC [1, 29]. It is found that the transport mechanism for the P3HT/n-Si device does not solely rely on TE. The forward-voltage current is limited by the combined effect of TE and SCLC conductions in P3HT/n-Si devices. It is worthy of note that incorporating MoS2 nanoflakes into P3HT decreases SCLC conduction, which improves the rectification performance for P3HT:MoS2/n-Si devices. In order to determine how SCLC conduction is decreased and the correlation between doping with MoS2 and performance, the values for the carrier mobility and carrier concentration were derived from the Hall measurements in the Van der Pauw configuration. For P3HT and P3HT:MoS2 samples, the respective derived values for the hole mobility are 2 × 10−4 and 0.8 cm2 V−1 s−1. For P3HT and P3HT:MoS2 samples, the respective derived values for the hole concentration are 5 × 1014 and 4 × 1014 cm−3. The hole mobility increases significantly but the hole density shows no significant change. It is postulated that incorporating MoS2 nanoflakes into P3HT results in a significant increase in hole mobility, so there is a decrease in SCLC conduction. It is reasonable to conclude that the increased carrier mobility in the P3HT layer is associated with the arrangement of MoS2 nanoflakes, which improves the rectification performance for P3HT:MoS2/n-Si devices.
Figure 6e, f show the reflectance of the P3HT/n-Si and P3HT:MoS2/n-Si samples. Incorporating MoS2 into the P3HT layer results in a reduction in the value of the reflectance, so the external quantum efficiency of a P3HT:MoS2/n-Si device is increased. P3HT:MoS2/n-Si devices have a high responsivity value because of the internal power conversion efficiency, the external quantum efficiency and carrier collection.
The efficiency of the internal power conversion is increased because of junction effects (that is, η = 1.3 and charge separation for the photo-generated electron–hole pairs at the P3HT:MoS2/n-Si interface). The improvement in carrier collection is mainly due to the presence of a more efficient carrier transport through the MoS2 percolation paths. In a P3HT/n-Si device, the photo-generated excitons are dissociated at the P3HT/n-Si interfaces and these electrons are transported to the n-Si by hopping in the P3HT layer. Incorporating MoS2 nanoflakes in the P3HT layer provides additional paths through the dispersed MoS2 percolation network for these electrons, so there is decreased charge recombination and enhanced carrier transport. The photo-carriers that are generated as a result of the dissociation of excitions at the P3HT/MoS2 interface also constitute the current in the P3HT:MoS2/n-Si device , so the value of IP isincreased. Jariwala et al.  demonstrated the photovoltaic effect for a twin-layer organic–inorganic heterostructure of pentacene and MoS2. However, the photosensitivity (PS) is defined as PS = IP/ID . The respective derived values for PS for P3HT/n-Si and P3HT:MoS2/n-Si devices are 9.3 × 103 and 4.6 × 104. A P3HT:MoS2/n-Si device has a much greater PS value than a P3HT/n-Si device. A PS value for graphene oxide-doped methylene blue/p-type Si diodes of 8.7 × 103 has been reported  and the derived values of PS for In/Ti0.05Zn0.95O nanoparticles/In devices are between 700 and 10,000 . The P3HT:MoS2/n-Si device that is fabricated by this study exhibits high photosensitivity, so a P3HT:MoS2/n-Si device can be used as a sensor for optoelectronic applications.
This study fabricates and determines the electrical and optoelectronic properties of heterojunction diodes that use n-Si and P3HT with and without the addition of MoS2. The P3HT/n-Si device displays non-ideal rectification behavior, but the P3HT:MoS2/n-Si device displays near-ideal rectification behavior. The I–V characteristics for P3HT/n-Si devices exhibit a combination of TE and SCLC conduction. However, incorporating MoS2 nanoflakes into P3HT results in the modification of the P3HT-Si interface and a significant increase in the values for the carrier mobility in the P3HT layer and the external quantum efficiency of the P3HT/n-Si devices, so the rectification and optoelectronic performance for P3HT/n-Si devices is improved. A P3HT:MoS2/n-Si device also exhibits a high degree of photosensitivity, so a P3HT:MoS2/n-Si device can be used as a sensor for optoelectronic applications.
The authors acknowledge the support of the Ministry of Science and Technology, Taiwan (Contract No. 106-2112-M-018-001-MY3) in the form of Grants.
- C Y Liu, Z C Holman and U R Kortshagen Nano Lett. 9 449 (2009)Google Scholar
- A Tsumura, H Koezuka and T Ando Appl. Phys. Lett. 49 1210 (1986)Google Scholar
- Y J Lin, T H Su and S M Chen J. Mater. Sci.: Mater. Electron. 28 14430 (2017)Google Scholar
- Y J Lin and T H Su J. Mater. Sci.: Mater. Electron. 28 10106 (2017)Google Scholar
- D K Schroder Semiconductor Material and Device Characterization (New York: John Wiley & Sons) (1998)Google Scholar
- D A Neamen Semiconductor Physics & Devices (Boston: McGraw-Hill) (2002)Google Scholar
- A A El-Shazly, H S Metwally, A M Farid, H A Hussainey and A A M Farag Indian J. Pure Appl. Phys. 36 753 (1998)Google Scholar
- M Ilhan J. Mater. Electron. Dev. 1 15 (2015)Google Scholar