A positive synergetic effect observed in the P3HT–SnO₂ composite semiconductor: the striking increase of carrier mobility

Abstract

Although the mobility of poly-3-hexylthiophene (P3HT) (10⁻³ to 10⁻² cm² V⁻¹ s⁻¹) is far lower than that of SnO₂ (10² cm² V⁻¹ s⁻¹), a novel P3HT–SnO₂ composite semiconductor, exhibiting a strikingly positive synergetic effect between organic and inorganic moieties, was prepared by combining P3HT with SnO₂ porous nanosolid (SnO₂ PNS). The experimental results indicate that, after the formation of the composite semiconductor, the electron concentration increased by two orders and the mobility was up to 37 times as high as that of pristine SnO₂ PNS. Furthermore, it was found that the electron concentration of the composite semiconductor was determined by both the content of P3HT and the number of chemical bonds at the P3HT–SnO₂ interface, while mobility was mainly determined by the mobility of SnO₂ PNS. Besides, thermal activation could increase the concentration and mobility of this composite semiconductor to some extent. By analyzing the experimental results, it is reasonable to believe that the electron transfer from P3HT to SnO₂ PNS, together with the continuous connected structural characteristic of the latter, are responsible for the above novel phenomenon. This phenomenon may be of great helpful in the preparation of extra-high mobility organic–inorganic composite semiconductors, thus greatly improving the performance of optoelectronic devices.

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1. Introduction

Due to the prospective of obtaining composite semiconductors with high carrier mobility and stability, excellent processability, low cost and structural diversities by utilizing the synergetic effect between organic and inorganic moieties, many efforts have been devoted to the design and preparation of organic–inorganic composite semiconductors. Among the important parameters of composite semiconductors, carrier mobility is the dominant one in determining the major performances of optoelectronics devices, thus strikingly increasing the carrier mobility of composite semiconductors is of great importance. Up to now, many kinds of organic–inorganic composite semiconductors have been prepared,^1,2 and the field effect transistor (FET) fabricated from (PEA)₂SnI₄ thin film exhibited a mobility of 2.6 cm² V⁻¹ s⁻¹.³ In contrast, many organic semiconductors exhibited much higher mobility.^4,5 For example, the mobility of C13-BTBT reached 17.2 cm² V⁻¹ s⁻¹,⁶ and rubrene exhibited a mobility of 15–20 cm² V⁻¹ s⁻¹.^7–9 Furthermore, it was reported that the mobility of pentacene single crystal exceeded 35 cm² V⁻¹ s⁻¹.^10,11 Obviously, the mobility of organic–inorganic composite semiconductors is lower than that of organic semiconductors, not to mention the inorganic semiconductors possessing much higher mobility.¹² In other words, the anticipated synergetic effect has not been realized in the organic–inorganic composite semiconductors.

Similar phenomenon was also observed in organic–inorganic composite solar cells. It was found that most of the organic–inorganic composite solar cells exhibited rather low efficiency,^13–17 although a few of them fabricated from Si- and GaAs-based composites exhibited efficiencies of 9–11%.^18–21 One of the crucial reasons is the poor interconnection of inorganic nanostructures, thus it is inevitable for the carriers to encounter large amount of “dead ends (or corners)” on the pathway to their respective electrodes.²² In order to overcome this difficulty, several strategies including surface ligand-exchange,^23–25 thermal treatment²⁶ and in situ formation of inorganic nanostructures²⁷ were utilized. As a result, the connectivity among inorganic nanostructures was greatly improved, and solar cells with obviously increased efficiencies were fabricated.^15,16

All the above results verified that, constructing more paths for the carriers and reducing their transportation barrier will be the most effective route to increase the mobility, hence improving the performance of composite semiconductor solar cells. Further proofs for this conclusion come from the perovskite composite semiconductor solar cells. Due to their strikingly improved performance, the perovskite composite semiconductor solar cells attracted many interests of material scientists and chemists in recent years.^28–34 Among these efforts, Eran Edri et al. prepared solar cells with an open-circuit voltage of 1.3 V by optimizing the hole-transporting materials, and found that the high V_OC and efficiency were closely related to the high mobility of perovskite semiconductor.^35,36 Besides, it was reported that the superior performance of perovskite solar cells were mainly resulted from the fact that both the porous TiO₂ and CH₃NH₃PbI₃ respectively formed continuous phases, thus the carriers can be quickly transported to their respective electrodes.³⁷ Furthermore, it was also reported that the high conductivity of perovskite composite semiconductors mainly come from the high mobility of tin-iodide skeleton, thus the carriers exhibited super-long diffusion lengths and strikingly lower recombination rate.^{32–34,38–40} These results also proved that, greatly increasing the carrier mobility by constructing continuous transportation paths was the most effective way to realize high performance of composite solar cells.

Moreover, the “interfacial engineering” was proved to be another effective route for realizing the same purpose. In fact, besides the inorganic high electron mobility transistors (HEMT),¹² some other important results obtained via “interfacial engineering” were also reported in recent years.⁴¹ For example, both the mobility and on–off ratio of graphene FET was greatly improved by grafting C–F bond-containing organic molecules onto the surface of graphene.⁴² Furthermore, the mobility of pentacene FET increased from 0.79 cm² V⁻¹ s⁻¹ to 4.52 cm² V⁻¹ s⁻¹ by combining some Hex-4-TFPTA “nano-patches” on the surface of pentacene film,⁴³ and metallic conductivity was observed at the interface of TTF and TCNQ⁴⁴ etc.

Here by combining these two strategies, namely, constructing more continuous transportation paths and utilizing “interfacial engineering”, we prepared an organic–inorganic composite semiconductor from SnO₂ porous nanosolid (PNS) and poly-3-hexylthiophene (P3HT). In this composite semiconductor, a novel positive “synergetic effect” was observed for the first time, i.e., it exhibited strikingly higher mobility than both the organic and inorganic moieties, especially the latter. To the best of our knowledge, such a result has never been reported before, and it is reasonable to believe that this phenomenon will provide helpful clues for designing and preparing organic–inorganic composite semiconductors with extra-high carrier mobility. It can be expected that these high mobility composite semiconductors will find extensive applications in the fabrication of highly sensitive gas sensors and photodetectors, normally-on type field effect transistors etc.

2. Experimental

SnO₂ nanoparticles with size of 50–70 nm were purchased from Luoyang Institute of Materials, Henan province, China. Dioxane (AR) and chloroform (AR) were used as solvents and obtained from Sinopharm Chemical Reagent Co. Ltd (China). P3HT with regioregularity of 97% was purchased from Rieke Metals Company (USA). All the materials and reagents were used as received without further purification.

In order to construct a well connected SnO₂ nanoparticles network while maintaining the porous characteristic, a solvothermal hot-press (SHP) strategy was utilized in the fabrication of SnO₂ PNS.⁴⁵ A typical experimental process was: firstly, 4 g SnO₂ nanoparticles were mixed with 5 ml dioxane and ground in a planet-type ball mill at a speed of 180 rpm for 4 hours. Secondly, the resultant mixture was transferred into a SHP autoclave⁴⁶ and heated at a rate of 2.5 °C min⁻¹ to 100 °C. Keeping constant at 100 °C for 0.5 hours, then a pressure of 60 MPa was applied on the mixture and maintained constant during the whole experimental process. Thirdly, the temperature was further increased with the same speed of 2.5 °C min⁻¹ to 200 °C and kept for 3 hours, followed by cooling naturally to room temperature. After releasing the pressure and calcining the resultant sample in air at 500 °C for 2 hours, a SnO₂ porous nanosolid was obtained. Finally, the SnO₂ porous nanosolid was cut into square-shaped pellet with dimensions of 7.5 mm × 7.5 mm × 0.5 mm, and gold electrodes of 2 mm × 2 mm were sputter-coated on four corners of the pellet.

The P3HT solutions were prepared by dissolving appropriate amounts of P3HT into 10 ml chloroform, and P3HT–SnO₂ composite semiconductors were prepared by two routes: (1) in the conventional composite route (CCR), SnO₂ PNS pellet coated with gold electrodes was directly immersed into P3HT solutions with different concentrations. After immersing for 24 hours, the pellets were dried at room temperature for 2 hours. Finally, the resultant pellets were annealed at 140 °C in argon for 20 minutes. (2) In the vacuum pretreatment in situ composite route (VPISCR), SnO₂ PNS pellets were firstly heated at 150 °C in a three-necked flask with a vacuum of 2.3 × 10⁻³ Pa, and the temperature was kept constant for 3 hours. After that, the pellets were cooled naturally in the vacuum to room temperature, and P3HT solution of 10 mg ml⁻¹ was injected into the three-necked flask to immerse the pretreated SnO₂ PNS pellets. After immersing for 24 hours, the SnO₂ PNS pellets were recovered and annealed at 140 °C in argon for 20 minutes, thus P3HT–SnO₂ composite semiconductors were obtained.

Morphology of the samples was observed using a JSM-6700F field emission scanning electron microscope (SEM). Distribution of pore size of the SnO₂ PNS was determined by nitrogen adsorption–desorption isotherms, the details were presented in ESI S1.† Infrared absorption and Raman scattering spectra (excitation wavelength: 1064 nm; density power: 1 W mm⁻²; acquisition time: 30 minutes; spectroscopic resolution: 4 cm⁻¹; probed area: 1 mm × 1 mm) were collected on a Nicolet NEXUS 670 Fourier Transformation Infrared-Raman (FTIR-Raman) spectrometer. The XPS spectra were recorded by using a PHI 5300 X-ray photoelectron spectrometer with Al Kα radiation. The carrier concentration and mobility of both the SnO₂ PNS and composite semiconductors were determined by Hall effect measurement, which was carried out on a HMS3000 Hall effect testing system. Gold electrodes were sputter-coated on the corners of square-shaped samples in a four-point van der Pauw configuration.

3. Results and discussion

Just as verified by the reported results, one of the effective routes to realize high carrier mobility is to construct continuous pathways for the carriers. Here we prepared SnO₂ PNSs in which SnO₂ nanoparticles interconnected with each other, forming a porous network with high surface area and reactivity. This structural characteristic was confirmed by scanning electron microscope (SEM), high resolution transmission electron microscope (HRTEM) images and nitrogen adsorption–desorption isotherms. The SEM image presented in Fig. 1(a) indicates that SnO₂ PNS was constructed by interconnected SnO₂ nanoparticles with large amount of pores existing in it. A specific surface area of 7.6 m² g⁻¹ of a typical SnO₂ PNS was deduced from the corresponding nitrogen adsorption–desorption isotherms by using BET model (ESI S1†). Because the inner surface of the pores in SnO₂ PNS was constructed by the surface of SnO₂ nanoparticles (see Fig. 1(a) and 4(a)), together with the fact that large amount of dangling bonds (serving as the active sites) exist on the surface of nanoparticles, the surface energy of SnO₂ PNS should be quite high. During the preparation of organic–inorganic composite semiconductors, the highly reactive dangling bonds are prone to combine with other atoms to decrease their energy, which will inevitably facilitate the formation of chemical bonds between organic and inorganic moieties.^47–49 Besides, the large specific surface area provides more active sites for forming chemical bonds. Therefore, SnO₂ PNS should be an appropriate candidate for preparing composite semiconductors, and P3HT–SnO₂ composite semiconductors were prepared by combining P3HT and SnO₂ PNSs.

Fig. 1 (a) SEM image of a SnO₂ PNS, (b) HRTEM image of a SnO₂ nanocrystal covered with P3HT thin layer, (c) HRTEM image showing the lattice fringes of both the SnO₂ nanocrystals and their interfacial zone. The fringes marked with red circle indicate that interfacial zone with rather high crystalline perfection between adjacent SnO₂ nanoparticles was formed during the preparation of SnO₂ PNS.

The HRTEM image shown in Fig. 1(b), which was taken from a SnO₂ nanoparticle in P3HT–SnO₂ PNS composite semiconductor, clearly indicates that SnO₂ nanoparticles possess quite high crystalline perfection. P3HT was uniformly coated on the SnO₂ nanoparticle with a thickness of ∼2 nm, revealing that composite semiconductor was uniformly formed. Furthermore, the interfacial zone indicated by the red circle in Fig. 1(c) also exhibits rather clear lattice fringes, indicating that SnO₂ nanoparticles interconnected via crystalline interfacial regions in SnO₂ PNS. This phenomenon reveals that continuous and “smooth” transportation pathways for electrons were formed in P3HT–SnO₂ PNS composite semiconductors.

The formation of chemical bonds between P3HT and SnO₂ was verified by both the Raman and XPS spectra of the composite semiconductor. The Raman spectra shown in Fig. 2(a) indicate that the band of C=C bonds in P3HT red-shifted from 1447 cm⁻¹ to 1435 cm⁻¹ when it was combined with SnO₂ PNS, revealing an electron transfer process from P3HT to SnO₂. On the other hand, it is well known that the binding energy of an element usually increases with the increase of its valence state, thus the XPS spectroscopy can be used to distinguish different binding states of a specific element. When P3HT was introduced into the pores of SnO₂ PNS, there must be an interaction between them via the S → Sn⁴⁺ coordination bond. As a result, part of the electrons on S atoms should have transferred to Sn⁴⁺, resulting in the increase of binding energy of S element. By analyzing the XPS spectra presented in Fig. 2(b), it is found that the binding energy of S increased by about 0.6 eV when P3HT was combined with SnO₂ to form the composite semiconductor. This result directly verifies the formation of chemical bonds between P3HT and SnO₂ and the electron transfer from the former to the latter.

Fig. 2 (a) Raman spectra of C=C bonds in P3HT before (blue line) and after (red line) being combined with SnO₂. This band red-shifted by ∼25 cm⁻¹ after P3HT–SnO₂ composite semiconductor formed, indicating that some electrons transferred from thiophene rings to SnO₂. (b) XPS spectra of S_2p in P3HT and P3HT–SnO₂ composite semiconductor.

Fig. 3(a) and (b) present the results of Hall effect measurement for SnO₂ PNS and P3HT–SnO₂ composite semiconductor, respectively. The I–V curves reveal that both SnO₂ PNS and P3HT–SnO₂ composite semiconductor possess rather uniform texture. Calculated results (Fig. 3(c)) indicate that the resistance of P3HT–SnO₂ composite semiconductor is lower than that of SnO₂ PNS by 3 orders of magnitude. The insets in Fig. 3(c) show that the carrier concentration and mobility of P3HT–SnO₂ composite semiconductor are 29 and 37 times higher than that of pristine SnO₂ PNS under optimized conditions, up to 3.2 × 10¹⁸ cm⁻³ and 24.3 cm² V⁻¹ s⁻¹, respectively.

Fig. 3 HALL effect measurement results of (a) SnO₂ PNS and (b) P3HT–SnO₂ composite semiconductor. (c) Resistances of SnO₂ PNS and P3HT–SnO₂ composite semiconductor calculated by using the data in (a) and (b).

In fact, it was reported that the electron mobility of P3HT was ∼10⁻³ cm² V⁻¹ s⁻¹, much lower than that of the P3HT–SnO₂ composite semiconductor. Furthermore, even the SnO₂ PNS also exhibited obviously lower mobility than the composite semiconductor. Thus, the striking increase of both carrier concentration and mobility of P3HT–SnO₂ composite semiconductor should come from a “synergetic effect” of the organic and inorganic moieties. Fortunately, similar phenomenon was observed in both inorganic semiconductor high electron mobility transistor (HEMT)¹² and organic field effect transistors (OFETs).^43,44 Herein, a preliminary model was proposed to illuminate the synergetic effect (Fig. 4). Firstly, the chemical bonds formed between P3HT and SnO₂ PNS, which was already verified by Raman and XPS spectra of the composite semiconductor (Fig. 2), provided a path for the electron transfer from P3HT to SnO₂. Secondly, the conduction band (CB) of SnO₂ is quite near the highest occupied molecular orbital (HOMO) level of P3HT, thus electrons could transfer from P3HT to the localized states and CB levels of SnO₂ by the assistant of thermal activation,⁵⁰ thus excessive electrons appeared in SnO₂. These excessive electrons firstly occupied the localized levels (contributed by crystal defects, oxygen vacancies etc.) within the band gap of SnO₂, resulting in the significant reduction of its activation energy. Thereafter, more electrons gradually occupied the states in conduction band, possessing much higher mobility. The higher the concentration of excessive electrons is, the larger the number of electrons in conduction band is. In other words, the excessive electrons will fill the band-tail trap states, resulting in the movement of Fermi level towards the conduction band of SnO₂, thus more electrons occupied the extended states in conduction band. As a result, the P3HT–SnO₂ composite semiconductor exhibited strikingly higher mobility. So, constructing well-connected network and the energy level matching played the key roles in the enhancement of carrier concentration and mobility in P3HT–SnO₂ composite semiconductor.

Fig. 4 (a) Schematic structure of P3HT–SnO₂ composite semiconductor. (b) Energy level alignment of P3HT and SnO₂.

According to the above model, it is reasonable to believe that the following parameters, including the content of P3HT, the mobility and surface state of SnO₂ PNS, and thermal activation, should dominate the performance of P3HT–SnO₂ composite semiconductors.

Firstly, since the excessive electrons in composite semiconductor come from P3HT, the loading amount of P3HT on the surface of SnO₂ should determine the concentration of these electrons. Here we conducted a series of experiments of changing the content of P3HT by varying the concentration of P3HT solutions. The curves shown in Fig. 5(a) reveal that, both the carrier concentration and mobility of the composite semiconductors increased monotonically with the increase of P3HT concentration. However, a saturation trend appeared at a high P3HT concentration of 10 mg ml⁻¹, which may correspond to the complete coordination of active sites on the surface of SnO₂. In fact, the electron transfer process can only happen between SnO₂ and the directly bonded P3HT molecules.

Fig. 5 Effects of some key parameters on electron concentration and mobility of P3HT–SnO₂ composite semiconductors. (a) Electron concentration and mobility as a function of P3HT concentration. (b) A comparison between SnO₂ PNS and P3HT–SnO₂ composite semiconductor during a heating–cooling process. (c) Electron concentration and mobility of SnO₂ PNSs and the corresponding P3HT–SnO₂ composite semiconductors. The SnO₂ PNSs were annealed at 350 °C in oxygen with different pressures prior to the measurement, and the composite semiconductors were prepared by immersing the annealed SnO₂ PNS into 10 ml P3HT solutions with concentration of 10 mg ml⁻¹. (d) Comparison between two P3HT–SnO₂ composite semiconductors prepared by CCR and VPISCR, respectively.

Secondly, the model shown in Fig. 4(b) reveals that the electron transfer from P3HT to SnO₂ might be promoted by thermal excitation. In fact, both the concentration and mobility of electrons in P3HT–SnO₂ composite semiconductor obviously increased upon heating. The data presented in Fig. 5(b) reveal a quite interesting phenomenon, i.e., the mobility slightly increased from 14.4 cm² V⁻¹ s⁻¹ to 15.6 cm² V⁻¹ s⁻¹ when the sample was cooled from 140 °C to 30 °C, although the concentration decreased accordingly. Two factors may be responsible for this phenomenon, namely, the irreversible electron transfer from P3HT to SnO₂ due to the formation of chemical bonds between them (transfer (1) in Fig. 4(b)), and the P3HT molecules arranged in a more ordered way after the heat treatment.

Thirdly, as the major transportation path of electrons, SnO₂ PNS plays a key role in determining the mobility of composite semiconductor. Besides, it is well known that the mobility increases with the improvement of crystalline perfection, and oxygen vacancy is the dominant defect in SnO₂ crystal. Therefore, decreasing the concentration of oxygen vacancies in SnO₂ (hence reducing the local trap states within the band gap) will increase the carrier mobility of both SnO₂ PNS and composite semiconductor. Here the SnO₂ PNS was thermal treated at 350 °C in high-pressure oxygen before the preparation of composite semiconductor. The curves shown in Fig. 5(c) indicate that both the carrier concentration and mobility of SnO₂ PNS reached their maximum values when being annealed in 4.0 MPa oxygen (also see the Fig. S2 of ESI†). Correspondingly, the P3HT–SnO₂ composite semiconductors also exhibited maximum carrier concentration and mobility under the same oxygen pressure. This result verifies that SnO₂ PNS is the major transportation path for electrons, and it determines the mobility of the P3HT–SnO₂ composite semiconductor.

Fourthly, as a porous material, SnO₂ PNS possesses high surface reactivity and large amount of pores, thus many kinds of gas molecules were adsorbed on its surface. These adsorbed molecules will occupy the active surface sites, frustrating the formation of chemical bonds between SnO₂ and P3HT. For facilitating the electron transfer from the latter to the former, the adsorbed molecules must be completely removed. Here two SnO₂ PNS samples were heated at 150 °C for 3 hours in a vacuum of 2.3 × 10⁻³ Pa prior to the preparation of composite semiconductor. Afterwards, P3HT solution was introduced in an in situ way, thus two P3HT–SnO₂ composite semiconductors were prepared. This route was called Vacuum Pretreatment In situ Composition Route (VPISCR). For comparison, a control experiment was conducted, in which another two P3HT–SnO₂ composite semiconductors were prepared by using Conventional Composition Route (CCR). In these experiments, the concentrations of P3HT solutions were all the same, i.e., 10 mg ml⁻¹. The data presented in Fig. 5(d) clearly indicated that, in comparison with the situation of CCR route, the increment of both concentration and mobility of electrons became much higher if the composite semiconductors were prepared by VPISCR route. This result verifies that the formation of chemical bonds between SnO₂ and P3HT was promoted by eliminating the adsorbed gas molecules before the hybridization process, thus more free electrons appeared in SnO₂ via the electron transfer from P3HT to SnO₂.

4. Conclusions

In summary, by constructing continuous interconnected SnO₂ porous nanosolid, together with utilizing the “interfacial engineering” route, P3HT–SnO₂ composite semiconductor with carrier mobility much higher than both the organic and inorganic moieties was prepared. The results reveal us that the merits of both organic and inorganic moieties could be merged into the composite semiconductor via the positive synergetic effect, if a continuous transport pathway were constructed for the carriers and the interfacial energy levels were properly matched. It is reasonable to believe that this phenomenon may find variety of applications in exploiting composite semiconductors with strikingly improved properties. Besides, it can also be prospected that composite semiconductors with high hole mobility would be obtained via a similar route.

Acknowledgements

The authors want to express our faithful thanks to Prof. Duo Liu and Prof. Baoquan Sun for their helpful discussions and suggestions. This work was supported by the Natural Science Foundation of China (NSFC 51372143, 51102151, 50990061) and Natural Science Foundation of Shandong Province (2013GGX10208).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra21762h

‡ Present address: Faculty of Sciences, National University of Singapore, Singapore 119617.

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