Charge Transport in Ordered and Disordered Regions in Pristine and

Jan 6, 2014 - Spectral trap density of states (DOS) and intrinsic mobilities in pristine and sonicated P3HT films were calculated and compared using t...
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Charge Transport in Ordered and Disordered Regions in Pristine and Sonicated-Poly(3-hexylthiophene) Films Byoungnam Park*,† and Doo-Hyun Ko*,‡ †

Department of Materials Science and Engineering, Hongik University, 72-1, Sangsu-dong, Mapo-gu, Seoul 121-791 Korea Center for Opto-Electronic Convergence Systems, Korea Institute of Science and Technology (KIST), Seoul 136-791 Korea

‡

ABSTRACT: Spectral trap density of states (DOS) and intrinsic mobilities in pristine and sonicated P3HT films were calculated and compared using temperature-dependent field-effect charge transport measurements. With sonication of P3HT solution, the field effect transistor (FET) hole mobility in its film state was improved remarkably in comparison with that in a pristinepoly(3-hexylthiophene) (P3HT) film. Spectral trap DOS were calculated through gate voltage-dependent activation energy measurements using the bottom-contact field effect transistors (FETs). For the pristine-P3HT films, the width of the shallow trap DOS was increased, indicating increased disorder in comparison with that for the sonicated-P3HT films. Further, the intrinsic mobility, 0.155 cm2/(V s), in the sonicated-P3HT film was far larger than that, 0.018 cm2/(V s), in the pristine one, suggesting that the structural properties of the P3HT films in the ordered region close to the gate dielectric are different. This reveals that a higher mobility in the sonicated-P3HT film originates from more efficient hole transport in the ordered P3HT region as well as less trap densities in the disordered region.

1. INTRODUCTION π-Conjugated polymers have been focused as component layers in a variety of electronic devices including organic field effect transistors and organic sensors because of their relatively high conductivity and semiconducting properties.1−4 Particularly, their luminescence properties have attracted much attention in the research field of organic light emitting diodes.5,6 More importantly, conjugated polymers are solution processable which enables coupling with other materials to form a homogeneous morphology in its solid state.7−9 In assembling optoelectronic devices such as photovoltaic devices, they are also compatible with inorganic materials either as electron donors or as electron acceptors under illumination.8,10,11 Charge transport properties in conjugated polymers have been extensively researched because carrier mobility in conjugated polymers is a key parameter in determining performance of devices in which conjugated polymers are incorporated.12,13 Among various π-conjugated polymers, poly(3-hexylthiophene) (P3HT) has shown high field effect transistor (FET) hole mobilities with a wide range between 10−5 and 10−1 cm2/(V s), depending on the formation of structurally ordered microcrystalline domains.14−18 The degree of structural ordering has been demonstrated to be governed by the physical properties of thiophene polymers including regioregularity and molecular weight.19−21 Carrier trapping in the disordered region coupled with the formation of ordered microcrystalline domains plays crucial roles in determining charge transport properties, particularly carrier mobility.22,23 According to Noriega et al.,23 short-range intermolecular aggregation dominates long-range charge transport in the © XXXX American Chemical Society

conjugated polymers. They demonstrated that carrier trapping induced by lattice disorder arising from structural disorder is critical in determining the carrier mobility. In the context, charge transport properties have been improved by optimizing structural properties of P3HT using various ways.14,15,24−26 Recently, a new facile method to improve carrier mobility was developed through increasing the degree of the conformational freedom of polymer chains in its solution state of P3HT conjugated polymers.27 According to Aiyar et al.,27 FET mobilities in P3HT films sonicated in their solution states before spincoating (sonicated-P3HT film) were remarkably enhanced. The enhancement of the FET mobility was attributed to the overall increase in crystallinity allowing for efficient hole transport along the π-stacked polymer chains. Xray diffraction (XRD) results for the pristine and sonicated P3HT samples confirmed that the number of crystallites increased in the sonicated-P3HT samples, increasing the (100) peak intensity.27 Mobility enhancement due to crystallinity improvement is not surprising because it has been known that carrier transport in an ordered region determines the charge transport properties. However, according to Noriega et al.,23 shortrange intermolecular aggregation is sufficient enough to transport carriers. Consequently, in the framework that P3HT consists of ordered and disordered regions in which carriers must overcome the energy barrier imposed by the disordered Received: November 26, 2013 Revised: December 29, 2013

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Figure 1. (a) Schematic diagram of a bottom-contact P3HT FET. (b) Plots of drain current as a function of gate voltage for pristine and sonicatedP3HT FETs. The channel length and width were 200 and 500 μm, respectively. The drain voltage was fixed at −5 V. (c) Transistor output characteristic curves for a pristine P3HT FET. (d) Transistor output characteristic curves for a sonicated P3HT FET.

After drying, the FETs were transferred to an ozone cleaner for further cleaning. Pristine P3HT films were prepared by spincoating of a P3HT solution at a concentration of 3 mg/mL in anhydrous chloroform solvent. P3HT and anhydrous chloroform were purchased from Sigma Aldrich (CAS Number 156074−98−5) and used without further purification. The physical properties of P3HT were examined prior to film deposition. The P3HT had an Mn of 24 kD and Mw of 47.7 kD, as obtained from gel permeation chromatography in tetrahydrofuran calibrated with polystyrene standards and a head to tail regioregularity of 92− 94% (as estimated from the 1H NMR spectrum). To fabricate sonicated-P3HT films, a P3HT solution (3 mg/ mL) was sonicated for 20 min, followed by spincoating for 60 s at 2000 rpm, producing a thickness of ∼90 nm. After spincoating, the P3HT FET devices were dried for hours. All fabrication and electrical characterizations were performed in a nitrogen-filled glovebox. For temperature dependent FET transport measurements, P3HT FET samples were transferred to a vacuum chamber equipped with a cryostat at a working pressure of 1 × 10−6 Torr. A bottom-contact FET in Figure 1a serves as an electrical probe to investigate the difference in the electronic properties of pristine and sonicated-P3HT films. In the linear regime of transistor operation (VG ≫ VD), the drain current in the P3HT layers, ID, is determined by the mobility of holes, μ, the capacitance per unit area of the interface Ci, and the width and length of the device, Z and L, respectively, as given by following eq 1.

region, electronic, and structural changes in the ordered and disordered P3HT regions should be addressed separately to fully understand the effect of sonication in a solution state on the charge transport properties in a film state. Here, a question arises that how the electronic trap DOS in the disordered region in sonicated P3HT films differs from the P3HT films without any treatment (pristine P3HT film) in the film state. Considering a significant improvement in FET mobility for sonicated P3HT films, significant changes in the electronic trap DOS in the disordered region are expected through ultrasoundinduced aggregation of polymer chains in its solution state. Another important subject to address is to compare the intrinsic carrier mobility in the ordered regions for the pristine and sonicated-P3HT samples which can provide insights into structural arrangement in the ordered P3HT microcrystalline region. In other words, addressing the spectral trap DOS and the intrinsic mobility in the disordered and ordered regions, respectively, is essential in understanding the origins of improved charge transport properties in sonicated-P3HT films. Here, the spectral trap DOS and intrinsic mobilities for pristine and sonicated P3HT films were approximated using FET transport measurements. Temperature-dependent ID−VD measurements at different gate voltages allowed for estimation of the activation energy, which corresponds to the difference between the valence band edge and the trap energy.

2. EXPERIMENTAL SECTION A bottom-contact FET was fabricated to explore charge transport properties in P3HT films. The source and drain electrodes [Au(80 nm)/Cr (3 nm)] were photolithographically patterned onto a 200 nm thick SiO2 gate dielectric. Highly doped p-type silicon substrate served as a gate electrode. Prior to P3HT deposition, FET devices were cleaned in acetone, methanol, and deionized water sonication baths sequentially.

Z ID μC i(VG − VT)VD L

(1)

Here, the threshold voltage, VT, the voltage beyond which mobile carriers are induced in the channel, was extracted from B

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the plots of gate voltage versus drain current in the linear regime of transistor operation.

3. RESULTS Enhanced Fet Hole Mobility in Sonicated P3HT Film. Sonicated-P3HT films exhibited far larger FET hole mobilities than those in pristine P3HT films, consistent with a previous study.27 Figure 1b shows ID−VG plots for pristine and sonicated-P3HT FETs. The FET hole mobilities for the pristine and sonicated-P3HT films were 0.005 and 0.042 cm2/(V s), respectively. For all devices, the threshold voltages, VT, were similar at −12 V. The FET output characteristic curves in Figure 1c and d demonstrate that the number of free hole carriers in the P3HT channel is modulated by the gate voltage, as evidenced by the appearance of the linear and saturation regimes with the drain voltage increasing. At a gate voltage of −60 V, the drain current in the sonicated-P3HT film was larger than that in the pristine film by 1 order of magnitude due to enhanced FET hole mobility. To ensure the measurement of charge transport properties in the P3HT channel between the source and drain electrodes, we calculated the contact resistance between the metal electrodes and the semiconductor. The total resistance was obtained through the ID−VD curves at a particular gate voltage. The channel resistance, Rch = R□(L/Z), was estimated by measuring the channel sheet resistance given by equation, R□ = (|V1 − V2|/ID)(Z/d), where |V1 − V2| is the potential difference at two points between the source and drain electrodes and d is the distance between the points V1 and V2. The contact resistance was calculated by deducting the channel resistance from the total resistance. For a number of samples we measured, the ratio of the contact resistance to the channel resistance was less than 0.1 at a P3HT thickness of ∼80 nm, concluding that the FET mobility and the field effect conductivity are dominated by the charge transport properties in the channel not the metal/ semiconductor contact properties. Temperature-Dependent Field-Effect Charge Transport Measurements for Trap DOS Calculation. Temperature-dependent charge transport measurements in the pristine and sonicated-P3HT films were carried out for three P3HT samples to calculate the spectral trap DOS in the band gap. Figure 2a,b exhibit the temperature dependent ID−VG transfer characteristic curves for pristine and sonicated-P3HT samples. At the temperature range between 293 and 77 K, the FET hole mobility, proportional to the slope in the ID−VG plot, increased with temperature increasing which indicates that hole carriers transport by thermal activation. For the sonicated-P3HT FET, the mobility decreased from 0.042 ± 0.015 to 0.002 ± 0.001 cm2/(V s), while the mobility decreased, for the pristine P3HT, from 0.005 ± 0.002 to 0.0001 ± 0.0001 cm2/(V s). The threshold voltages for the samples were similar at −12 V at room temperature and decreased with temperature decreasing. The threshold voltage varied between −12 and −20 V at the temperature range. Calculation of the Spectral Trap DOS Using Mobility Edge (ME) Transport Model. In estimating the spectral trap DOS, the charge transport mechanism in conjugated polymers should be addressed. As reviewed by numerous studies, it is still debatable to claim the charge transport mechanism that can be applied to all conjugated polymers. However, the presence of the energy boundary that separates the transport band and the trap energy band, has been generally accepted for inorganic amorphous materials and polycrystalline materials.22,28−30 The

Figure 2. (a) Temperature-dependent ID−VG measurements for a sonicated P3HT. Drain voltage was fixed at −5 V. (b) Temperaturedependent ID−VG measurements for a pristine P3HT. Drain voltage was fixed at −5 V.

concept has been widely extended to organic semiconductors such as pentacene and oligothiophene featuring microsized crystalline domains with high carrier mobility, demonstrated by FETs and XRD results. However, it was not clear that the existence of the energy boundary beyond which carriers contribute to charge transport is valid in explaining the carrier transport in P3HT until the transport study by Salleo.29 In polymer films, carriers are confined in the polymer chains and are dominated by π−π intrachain coupling. Therefore, a hopping mechanism between localized states has been considered to be crucial in dominating charge transport. However, for regioregular poly(thiophenes) including P3HT, the presence of the transport edge is validated because regioregular P3HT polymer chains are linked by strong π−π coupling forming polycrystalline domains demonstrated by XRD results coupled with FET measurements with a fairly high mobility.29 Interchain delocalization of carriers in thiophene films validates involvement of the energy boundary in transporting carriers in regioregular P3HT polymer films.31,32 To calculate the spectral trap DOS in P3HT, we assumed the mobility edge (ME) model in which the carriers transport in the mobile states above the energy boundary called mobility edge. Below the mobility edge, carriers are trapped in the localized states. Thermal excitation enables the trapped carriers to participate in the charge transport in the mobile states above the mobility edge. It is assumed that direct hopping between localized states is negligible as justified above. The ME model C

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has been successfully applied to regioregular poly(thiophenes) films with strong π−π coupling within the crystallites.29 To calculate the trap DOS from FET measurements, the field effect conductivity at particular gate voltages was measured at different temperatures. The field effect conductivity is calculated from the ID−VG transfer characteristic curves at the linear region in transistor operation in which VG is far larger than VD. In the bias condition, a small voltage difference between the applied gate voltage and the potential at the location in the channel is achieved, as given in eq 2: ID =

Z σVD L

(2)

Therefore, the carrier concentration in the horizontal channel region remains same ensuring gradual channel approximation. Temperature-dependent field effect conductivity σ is measured to calculate the activation energy for thermal release from localized states below the mobility edge to the transport band above the mobility edge. The activation energy is calculated from eq 3: ⎛ E ⎞ σ(VG) = σ0 exp⎜ − a ⎟ ⎝ kT ⎠

(3)

where activation energy, Ea, is estimated from the plot of ln σ versus 1/T for different gate voltages. The activation energy is approximated to be the difference between the Fermi level, EF, and the valence band edge of the bulk P3HT channel positioned far from the gate dielectric/P3HT interface. The effective channel thickness in P3HT, that is, the accumulation layer thickness, is assumed to be 5 nm. Further, the effective channel thickness is assumed to be independent of the gate voltage applied. The activation energy, Ea, can be expressed as

Figure 3. (a) Gate voltage dependent activation energy for pristine and sonicated P3HT FETs. (b) Plots of density of trap states as a function of energy above valence band edge for pristine and sonicated P3HT FETs.

Ea = Ev − E F − eVi

where Vi is the potential at the gate dielectric and the semiconductor, which is close to zero (the flat band voltage is close to zero). The activation energy is a function of the gate voltage. As the gate voltage increases, the two-dimensional hole density, equivalent to n2D = (CiΔVG)/q, increases filling the localized states at a lower energy level. Therefore, more hole carriers induced in the channel decrease the activation energy. The assumption here is that the charge density is constant within the accumulation layer thickness of t from the gate dielectric/semiconductor interface. The volume density, n3D, can be obtained by dividing n2D by the accumulation layer thickness t as shown in the relation, n3D = n2D/t. Consequently, the spectral trap DOS N(E) is estimated to be −1 n 3D C i ⎛ ΔEa ⎞ N (E ) = = ⎜ ⎟ ΔEa qt ⎝ ΔVG ⎠

shallow trap states within less than 50 meV was larger than that of the sonicated-P3HT sample. Furthermore, the activation energy for holes to reach the mobility edge for the pristine and sonicated-P3HT films was calculated from the temperature dependent FET mobility in Figure 4. The activation energy, calculated from Arrhenius equation, was 29 and 22 meV for the pristine and sonicatedP3HT films, respectively. The intrinsic mobilities from temperature-dependent mobility measurement for the pristine

(4)

From eq 4, the spectral trap DOS, N(E) as a function of the activation energy at a particular gate voltage, can be obtained. Figure 3a shows the plot of activation energy as a function of gate voltage for pristine and sonicated-P3HT films. The activation energy was calculated from temperature dependent field effect conductivity at a drain voltage of −5 V as described above. As the gate voltage decreases the activation energy decreased because more carriers induced by a negative gate voltage fill the localized states at a lower energy level. The spectral trap DOS as a function of energy above the valence band in Figure 3b was calculated based on eq 4 and the plot in Figure 3a. For the pristine P3HT sample, the width of the

Figure 4. Plots of FET hole mobility as a function of temperature for pristine and sonicated P3HT FETs. The FET mobilities were measured at a drain voltage of −5 V. D

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temperature-dependent field effect conductivity in Figure 2 was 0.155 cm2/(V s), which is approximately 1 order of magnitude larger than, 0.018 cm2/(V s), of the pristine-P3HT film. This is consistent with the value of the intrinsic mobility estimated from the temperature dependent mobility measurement in Figure 4, in which approximately 1 order of magnitude difference in the FET mobility was observed. The difference in the intrinsic mobility between the two samples is not limited to the ME transport model. The intrinsic mobility difference is also consistent with the multiple trapping and release model in which carrier transport is determined by the tail states approximated by an exponential distribution of DOS. As the gate voltage increases more carriers induced fill the localized states, shifting the Fermi level toward the transport band edge. Consequently, more carriers are located closer to the transport band edge, facilitating thermal release of the trapped carriers.34,35 In the model, the free carrier density, pf, can be expressed by pf = exp(−Ea/kBT)CiVG.35 The total number of carriers is represented by CiVG. Estimation of Ea allows for calculation of the ratio of free carriers to the total number of carriers, pf /CiVG. The ratio allows for the estimation of a free carrier mobility (intrinsic mobility) based on the fact that the free carrier mobility is determined by the ratio between the free carrier density (pf) and the total carrier density (pf + ptr), as described in the relation μ = μe(pf/(pf + ptr)). Therefore, the free carrier mobilities for the pristine and sonicated-P3HT films, corresponding to the intrinsic mobility, were 0.018 and 0.155 cm2/(V s), respectively. The different intrinsic mobility for the P3HT films can be interpreted that the structure of P3HT domains in the ordered region close to the gate dielectric is different. Indeed, according to Aiyar et al., the d spacing from XRD data was different for the two samples.27 As the sonication time increases, d spacing decreased. This result is consistent with our result in which the intrinsic mobility in the ordered P3HT region close to the gate dielectric does not converge because the structural arrangement including d spacing is different. The change in the structural arrangement originates from increase in the degree of conjugation of P3HT chains due to sonication. Spectral evidence has been provided that sonication induces aggregates of the individual polymer chains with more enhanced planarity of the conjugated backbone in its solution states.27,36−38 In other words, order−disorder transformation in the individual polymer chains occurs forming π-stacked conjugated polymers. To summarize, the mobility difference between the pristine and sonicated-P3HT films arose from the structural difference in the ordered region and the trap DOS distribution in the disordered region. The sonicated P3HT film features more ordered structure near the gate dielectric and less trap density with a narrow trap DOS distribution.

and sonicated P3HT films were 0.015 ± 0.005 and 0.098 ± 0.025 cm2/(V s), respectively.

4. DISCUSSION For the pristine-P3HT films in Figure 3, the band tail in the localized states is wider than that in the sonicated-P3HT film. In other words, the width of the shallow traps less than 50 meV within the valence band edge in the pristine P3HT is wider in comparison with that in the sonicated-P3HT film. The traps distributed within the band gap of the P3HT films are characterized by donor-like localized states because the threshold voltage in the P3HT FETs shifted to a more negative value with temperature decreasing. As the temperature decreases, a decrease in the number of events for thermal release of holes trapped in the trap states (donor-like localized states) shifts the threshold voltage to a more negative value. In other words, more hole carriers are required to fill the deep traps (donor-like states) to achieve the threshold voltage.33 According to Noriega et al.,23 short-range disorder including lattice disorder is found to be crucial in governing the charge transport as mentioned earlier. Indeed, in a previous study,29 regioregular poly(thiophene) films were differently treated through various thermal processing exhibiting a wide range of FET mobilities between 0.004 and 0.1 cm2/(V s). The differences in the FET mobility were explained by the extent of structural disorder, evidenced by the different widths of the shallow trap state distribution close to the valence band edge. From temperature-dependent charge transport measurements, the authors found that the depth and the amount of the traps were different from sample to sample, explaining the mobility difference. In our experiments, the FET mobility difference between the pristine and sonicated-P3HT films can be understood in the context of energetic difference in the disordered P3HT region. As shown in Figure 3b, the difference in the distribution of trap DOS can explain the mobility difference. The enhanced mobility in the sonicated-P3HT sample originates from reduced structural disorder evidenced by the width of the shallow traps and the amount of traps in Figure 3, consistent with previous studies.23,29 Considering that hole carriers transport in polycrystalline P3HT regions in which ordered and disordered P3HT domains coexist,23 however, reduced structural disorder in the disordered P3HT region may not be a sole reason for the enhanced FET hole mobility in sonicated-P3HT films. Assuming the ME model as the main charge transport mechanism in regioregular P3HT films, the intrinsic mobility, μ0, can be estimated from the relation, μ0 = μ exp(Ea/kT). The intrinsic mobility features the FET mobility of free carriers above the mobility edge. In other words, all carriers induced by a gate voltage are located above the mobility edge assuming the absence of trap states below the mobility edge. Consequently, the intrinsic mobility reflects hole transport properties in the ordered P3HT region. In estimating the intrinsic mobility experimentally from ID−VG measurements, the activation energy, Ea, can be calculated from a sufficiently large gate voltage so that it remains almost constant with further increase in the gate voltage. Importantly, the intrinsic mobilities for the two samples were different. This is in contrast to the results from the previous studies in which the intrinsic mobilities in differently treated thiophene films converge to a certain value,29 indicating that the structure in the ordered region is identical. For the sonicated-P3HT sample, the intrinsic mobility estimated from

5. CONCLUSIONS In conclusion, ultrasound treatment in solution state is clearly different from other postfabrication thermal treatment in that it not only decreases the shallow trap density in the disordered region but also leads to structural difference in the ordered region. Most of the works done to improve the carrier mobility is related to either decreasing the structural disorder or enlarging the crystalline area by increasing the extent of the degree of freedom of conjugated polymer chains in its solid state. Providing more conformational freedom of polymer chains in its solution state offers a new way to improve the ordered region as well as the disordered region. E

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (+82)-2-320-1631 (B.P.); (+82)-2-958-6836 (D.-H.K.). E-mail [email protected] (B.P.); [email protected] (D.H.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work (B.P.) was supported by 2013 Hongik University Research Fund. D.-H.K. was supported by the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (2013M3C1A3065040).

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