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Indian Journal of Physics

, Volume 92, Issue 12, pp 1533–1539 | Cite as

Incorporation of MoS2 nanoflakes into poly(3-hexylthiophene)/n-type Si devices to improve the rectification behavior and optoelectronic performance

  • Chang-Lin Wu
  • Yow-Jon Lin
Original Paper
  • 104 Downloads

Abstract

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.

Keywords

Heterojunction 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 

1 Introduction

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 [1]. 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 [28]. 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 [30]. Discussions of the Hall effect can be found in solid-state and semiconductor literature [31, 32].

3 Results and discussion

Figure 1a shows the Raman spectra for MoS2 nanoflakes. Figure 1a shows A1g and E 2g 1 Raman modes that are respectively located at approximately 408 and 384 cm−1. In the E 2g 1 mode, both S and Mo atoms vibrate along the direction of the plane, but the S atoms vibrate perpendicular to the plane in the A1g mode. The Raman spectrum result is consistent with the result that is shown in Ref. [33]. Figure 1b, c show the Raman spectra for P3HT and P3HT:MoS2 films from 600 to 1700 cm−1. Figure 1b, c show six peaks that are centered at 724 cm−1 (corresponding to the Cα–S–Cα’ deformation), at 1000 cm−1 (corresponding to the Cβ–Calkyl stretching), at 1088 cm−1 (corresponding to the Cβ–H bending), at 1204 cm−1 (corresponding to the Cα–Cα, + Cβ–H bending), at 1376 cm−1 (corresponding to the Cβ–Cβ’ stretching) and at 1446 cm−1 (corresponding to the C=C stretching vibration for thiophene ring) [34]. No differences are seen between the Raman spectra for P3HT and P3HT:MoS2 films, which demonstrates that the polymer structure is not affected by the addition of MoS2.
Fig. 1

Raman spectra for (a) MoS2, (b) P3HT and (c) P3HT:MoS2 films and (d) schematic view of a P3HT/n-Si (P3HT:MoS2/n-Si) device with Au/In contacts. (e) The flat band diagram for Au/P3HT:MoS2/Si/In devices. For P3HT and MoS2 samples, the valence band maximum (highest occupied molecular orbital level) and the conduction band minimum (lowest unoccupied molecular orbital level) are calculated with respect to the vacuum level. The Fermi levels for Au and In are calculated with respect to the vacuum level

A schematic diagram of the P3HT/n-Si (P3HT:MoS2/n-Si) device with Au/In contacts is shown in Fig. 1d. An energy level diagram for the Au/P3HT:MoS2/Si/In devices is shown in Fig. 1e. The valence band maximum (highest occupied molecular orbital level), the conduction band minimum (lowest unoccupied molecular orbital level) and the Fermi level are calculated with respect to the vacuum level. Previously reported values [35, 36] are used for the energy levels in the figures and adjusted for this study. Figure 2 shows the |I|–V characteristics for the P3HT/n-Si and P3HT:MoS2/n-Si devices at room temperature and in darkness. The rectification I–V characteristics show that the depletion regions are formed at the P3HT/n-Si (P3HT:MoS2/n-Si) interfaces. For the P3HT/n-Si and P3HT:MoS2/n-Si devices, the respective derived values for the rectification factor at ± 2 V are 63 and 313. The P3HT:MoS2/n-Si devices exhibit good rectification behavior. El-Shazly et al. [37] noted that thermionic emission (TE) is the predominant mechanism for conduction across the organic/inorganic barrier in the organic thin films. Using TE theory, the I–V characteristic is defined as [31, 32, 38, 39, 40, 41, 42, 43]
$$I = I_{s} \left[\exp \left(\frac{qV}{\eta kT}\right) - 1\right]$$
(1)
where Is is the reverse-bias saturation current, η is the ideality factor, T is the absolute temperature and k is the Boltzmann constant. η is determined from the gradient of the linear region of the forward bias ln(I)–V characteristics at low voltages. This method is used to determine the transport mechanisms in ideal and in non-ideal diodes. For the P3HT/n-Si and P3HT:MoS2/n-Si devices, the respective derived values for η are 1.8 and 1.3. Figure 3 shows the forward I–V characteristics for P3HT/n-Si and P3HT:MoS2/n-Si devices at room temperature in the dark and the two fitted curves for TE conduction (η = 1.8 and η = 1.3). The deviation is attributed to either the recombination of electrons and holes in the depletion region, the presence of a large number of surface states and/or an increase in the diffusion current [44, 45]. It is also possible that the transport mechanism for these devices does not solely rely on TE. Nolasco et al. [1] found that carrier transport occurs because there is hopping between localized states at the P3HT/Si interface at low voltages, via multi-tunneling capture emission, and at medium voltages, via tunneling-enhanced recombination. The high value for the ideality factor demonstrates that the I–V characteristics for these devices are affected by the carrier transport through the organic material [46]. This demonstrates that the charge traps in P3HT affect electronic conduction through the device. This is called SCLC conduction, which is a conduction model that is commonly used when there is a low free carrier density and a high trap density in conducting materials [46]. In this case, the organic and inorganic phases contribute individually to charge carrier transport. The extent to which these two highly dissimilar conduction mechanisms (that is, TE and SCLC) influence the I–V characteristics is of interest. It is deduced that the value of η is affected by SCLC conduction, so a TE model is used to determine larger values for η in this case.
Fig. 2

|I|–V curves for P3HT/n-Si and P3HT:MoS2/n-Si devices in the dark

Fig. 3

Forward I–V curves for P3HT:MoS2/n-Si and P3HT/n-Si devices in the dark, showing two fitted curves for TE conduction (η = 1.3 and η = 1.8) and two fitted curves for ohmic (I ~ V1) and SCLC (I ~ V1.5) conduction

In this case, the organic and inorganic phases contribute differently to charge carrier transport. Two conduction mechanisms (that is, TE and SCLC) [1] 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.

It is worthy of note that incorporating MoS2 nanoflakes into P3HT results in a reduction in the leakage current density for the P3HT:MoS2/n-Si device at negative voltages. The reverse current occurs because electron–hole pairs are thermally generated in the depletion region. If there is lattice mismatch or if defects are segregated, there is a high density of defects in the interface between the two hetero-partners [47]. Figure 4 shows AFM images for P3HT films that are not doped with MoS2. Figure 5 shows AFM images for P3HT films that are doped with MoS2. The root-mean-square surface roughness (Rrms) is the root mean square average of the roughness profile ordinates. The value of Rrms for P3HT:MoS2 films is 1.3 nm and the value for P3HT films is 1.7 nm of. It is shown that incorporating MoS2 results in a reduction in the value of Rrms. Incorporating MoS2 into P3HT results in the formation of a good P3HT/Si heterojunction, so the density of the leakage current is reduced.
Fig. 4

An AFM image of a P3HT film

Fig. 5

An AFM image of a P3HT:MoS2 film

Figure 6a, b show the I–V characteristics for P3HT/n-Si and P3HT:MoS2/n-Si devices at room temperature under illumination. The important figures for the photovoltaic devices are the open circuit voltage (Voc) and the short circuit current (Isc). The values for Voc and |Isc| increase when MoS2 nanoflakes are added. Figure 6c, d show the time-resolved response of the current to repeated light switching for durations of between 0 and 100 s and for voltages from 0 to 1 μV. The increase in the photocurrent (IP) was measured by switching on the white light. When the white light is switched off, the photocurrent decreases very rapidly, almost to the value of the dark current (ID). The devices that are fabricated for this study exhibit reversible switching between high and low current densities when light illumination is turned on and off. The value of IP is significantly affected by the addition of MoS2. The P3HT:MoS2/n-Si device exhibits a much greater IP value than the P3HT/n-Si device. In this case, the efficiency with which external light is injected, the internal power conversion efficiency and carrier collection contribute differently to the value of IP. This phenomenon is understood by observing the reflectance of the P3HT/n-Si (P3HT:MoS2/n-Si) sample.
Fig. 6

I–V curves for (a) P3HT/n-Si and (b) P3HT:MoS2/n-Si devices at room temperature in the light and time-resolved photocurrent measurement for (c) P3HT/n-Si and (d) P3HT:MoS2/n-Si devices (sunlight illumination is switched on and off) and the reflectance as a function of wavelength for (e) P3HT/n-Si and (f) P3HT:MoS2/n-Si samples

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 [48], so the value of IP isincreased. Jariwala et al. [49] 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 [50]. 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 [49] and the derived values of PS for In/Ti0.05Zn0.95O nanoparticles/In devices are between 700 and 10,000 [51]. 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.

4 Conclusion

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.

Notes

Acknowledgements

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.

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Copyright information

© Indian Association for the Cultivation of Science 2018

Authors and Affiliations

  1. 1.Department of PhysicsNational Changhua University of EducationChanghuaTaiwan, ROC
  2. 2.Institute of PhotonicsNational Changhua University of EducationChanghuaTaiwan, ROC

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