Exciton Dissociation and Charge Transport Properties at a Modified

Jan 17, 2012 - School of Chemical & Biomolecular Engineering, Georgia Institute of ... treated with thiol molecules and charge-transport properties in...
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Exciton Dissociation and Charge Transport Properties at a Modified Donor/Acceptor Interface: Poly(3-hexylthiophene)/Thiol-ZnO Bulk Heterojunction Interfaces Byoungnam Park,*,† Jung-Hyun Lee,† Mincheol Chang,† and Elsa Reichmanis*,†,‡,§ †

School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100, United States School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30126, United States § School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ‡

ABSTRACT: Engineering the electron donor/acceptor interface provides not only flexibility in designing optoelectronic devices but also new opportunities to optimize device performance. However, significant challenges arise from the difficulty of examining the interface, which is buried in a composite film. In this paper, we probed exciton dissociation at a poly(3-hexylthiophene) (P3HT)/ZnO bulk heterojunction interface treated with thiol molecules and charge-transport properties in P3HT of the composite film using field effect transistor (FET) devices. The role of thiol modification of ZnO in charge transfer and charge transport within the bulk heterojunction interface was addressed through comparison of the threshold voltage and the FET mobility between P3HT/thiol-ZnO and P3HT/ZnO FETs. Attachment of thiol molecules onto the ZnO surface induced a larger photoinduced threshold voltage shift than that for an FET with pristine ZnO, indicating that more excitons were dissociated within the P3HT/ZnO composite film. The origin of this effect is discussed through the relationship between the increased interfacial area between P3HT and thiol-ZnO, inferred from surface morphology, and structural ordering of P3HT at the interface. The charge carrier transport properties of P3HT in the composite film are separated from charge transfer, and found to depend on the presence of thiol, offering information on carrier scattering/trapping at the interface. The integrated information of charge transfer and charge transport properties probed with an FET device would benefit efforts aimed at the optimization of solar cell devices.

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INTRODUCTION Solar cell technologies based upon organic/inorganic hybrid materials are emerging as an attractive area of research because the combination of “soft” and “hard” materials provides flexibility for the design of materials and processes necessary for desired device configurations and performance.1−4 Organic components are readily incorporated into the active layer via solution based processes, while inorganic counterparts such as metal oxides can be functionalized with an array of selfassembled-monolayers for optimizing solar cell devices through changes in the interfacial energy band offset that governs charge transfer.3,5,6 A range of solar cells using hybrid organic−inorganic materials for the active layer have been reported; typically conjugated polymers are used in conjunction with metal oxide nanowires, nanorods, and nanoparticles.7−10 The combination of poly(3-hexylthiophene) (P3HT) and ZnO is of particular interest due to the high hole mobility, of up to ∼0.3 cm2/(V s),11−14 reported for P3HT vs other electron-donating semiconducting polymers, coupled with the ambient stability and low cost of ZnO. The high electron mobility of ZnO and large band offset relative to the conjugated polymer are desirable properties for an electron acceptor.5,15 Several classes of ZnO nanostructures, including nanoparticles, nanorods, and © 2012 American Chemical Society

nanowires, were, therefore, employed to fabricate solar cell devices. While the power conversion efficiency (PCE) of less than 2% reported for the hybrid cells is low compared to the relatively larger PCE of ∼5% achieved for conjugated polymer− fullerene blends,1,16−20 the hybrid system provides an abundance of available options for optimization due to the availability of alternative organic and inorganic components. The central challenge for the design of high performance hybrid solar cell devices is to realize a balance between having sufficient band offset for charge transfer between the organic semiconductor and its inorganic counterpart and an optimum open circuit voltage through interface engineering. A representative organic semiconductor is P3HT, and ZnO is often explored for solar cell applications because of its aptitude as an electron transporter. Many research groups have tailored ZnO surfaces using a variety of molecules to enhance the power conversion efficiency.6,21−23 For example, phenyl-C61-butyric acid was attached to ZnO and enhanced the system’s shortcircuit current and power conversion efficiency.23 The improvement originated from the increased work function of Received: September 15, 2011 Revised: January 17, 2012 Published: January 17, 2012 4252

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ZnO by 0.5 eV arising from dipole formation at the P3HT/ ZnO interface. Further, alkanethiol treatment of ZnO was successful in increasing the photocurrent by decreasing the charge carrier recombination rate within P3HT resulting from the more ordered microstructure of P3HT in immediate contact with the thiol-treated ZnO surface.6,24 However, despite the progress associated with increased power conversion efficiency through interface engineering, a comprehensive study of the organic/inorganic bulk heterojunction interfaces, including studies related to interfacial charge transfer and charge transport within the component layers using FET structures, is not available. Information derived from such investigations will provide fundamental understanding of the role of interface modification in enhancing photoinduced charge separation; understanding of which is an imperative for the design of hybrid materials archtictures for solar cell applications. Here, we investigated exciton dissociation at the electron acceptor/donor interface of a hybrid ZnO/P3HT material, through measurement of the threshold voltage in an FET prepared with a blend of ZnO and P3HT, under illumination. Threshold voltage is a measure of the electrostatic potential in the composite film and scales with the number of carriers separated at the interface. This approach can provide a means to address the effect of interface modification on charge transport and transfer properties. For comparison, two bulk heterojunction interfaces, P3HT/ZnO and P3HT/thiol-ZnO, were chosen for this study. Charge transport properties of the carriers after charge separation are discussed in relation to the FET mobility values obtained from transfer characteristic curves of the P3HT/ZnO- and P3HT/thiol-ZnO-based FETs. This approach enables us to probe charge transfer at the interface and charge transport within the individual component layers separately, which benefits optimization of solar cells in which a series of processes that result from the solar cell configuration having vertically stacked component layers severely complicate elucidation of the role of interface modification in altering photoinduced charge transfer and charge transport properties in the hybrid structure. Additionally, we used large ZnO nanoparticles to minimize any morphological difference between the surface vs bulk in a composite film, enabling a straightforward correlation between the morphology and electrical properties, including exciton dissociation and charge transport properties.

Figure 1. Schematics of (a) a bottom-contact FET device, the molecular structure of thiol-treated ZnO with FT-IR and P3HT, and (b) experimental setup for FET measurements in vacuum.

composite film. A control sample using ZnO nanoparticles that had not undergone thiol-treatment were blended with the P3HT chloroform solution (5 mg/mL). For both composites, all compositions of P3HT and ZnO were approximately 1:1 in weight. After formation of the P3HT/ZnO and P3HT/thiolZnO bulk heterojunction interfaces within the FET device structures, the devices were immediately inserted into a vacuum chamber, which was then evacuated to a base pressure of 10−8 Torr to minimize oxidation of P3HT. The devices remained in the vacuum environment for several days to allow for dedoping of oxygen molecules from the P3HT. The threshold voltage and FET mobility were measured from the FET transfer characteristic curves of the devices operated in the linear regime of transistor operation under vacuum. For the study of photoinduced charge transfer within the films, the devices were illuminated through a quartz window on the vacuum chamber using a laser diode having a wavelength of 520 nm, as shown in Figure 1b.

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EXPERIMENTAL METHODS The bottom-contact FET device configuration and molecular structure of ZnO modified with 1-decanethiol are shown in Figure 1a. Thiol-capped ZnO nanoparticles with a diameter of ∼20 nm were formed via immersion of pristine ZnO nanoparticles in 1 mM 1-decanethiol solution in ethanol for several days in air. The thiol-treated ZnO was washed with ethanol several times and dried with N2 followed by thermal annealing at 120 °C in air. The thiol-treated material was characterized using Fourier transform infrared spectroscopy in which two peaks at 2852 and 2923 cm−1, arising from the symmetric and asymmetric CH2 stretching modes, were observed and are due to the presence of thiol on the ZnO nanoparticle surface. P3HT, in chloroform solution (3 mL, at a concentration of 5 mg/mL), was blended with the thiol-treated ZnO nanoparticles (∼20 mg). The blend was spin-coated onto an SiO2 substrate having prepatterned source and drain (Au/ Cr) electrodes at 1500 rpm for 30 s to form a P3HT/thiol-ZnO

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RESULTS AND DISCUSSION The photoinduced threshold voltage shift for FETs fabricated with active layers comprising P3HT/ZnO and P3HT/thiolZnO interfaces is a measure of photoinduced charge separation at the electron donor/acceptor interfaces.25 The magnitude of the threshold voltage in an FET with a hole conducting channel is determined by the flat band voltage VFB and the electrostatic potential ψB within the conducting channel, which is the potential difference between the highest occupied molecular orbital and the Fermi energy of the conducting channel at the interface as expressed by eq 125 VTH = VFB − ψB − 4253

2εsqNDψB Ci

(1)

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where VTH is the threshold voltage, εs the dielectric constant of the conducting channel, q the elementary charge, ND the dopant density within the conducting channel, and Ci the capacitance of the gate dielectric per unit area. For the devices fabricated with a P3HT/ZnO composite film under illumination, excitons from P3HT created within their diffusion length of 3−10 nm3,26−28 are dissociated at the interface, as seen in the schematic of Figure 2. As the number of charge carriers

Figure 2. Schematic of photoinduced charge transfer at the interface between P3HT and ZnO nanoparticles and the energy band diagrams of P3HT and ZnO at short-circuit conditions during illumination with a laser diode with a wavelength of 520 nm.

separated at the interface increases, ψB decreases, shifting the threshold voltage in a positive direction following the above equation. In other words, more electrons transferred to ZnO results in more holes being left in the P3HT side of the interface, which shifts the Fermi energy level to that of the highest-occupied molecular orbital level, resulting in a more positive threshold voltage for the FET due to the reduced value of ψB. Attachment of thiol molecules to ZnO nanoparticles interfacing with P3HT enabled more electrons to transfer to ZnO during illumination causing a larger threshold voltage shift in the positive direction. Figure 3 shows the output and transfer characteristic curves from FETs prepared with P3HT/ZnO and P3HT/thiol-ZnO composite films. The light intensity of a green laser diode varies between 0.06 and 180 mW cm−2. Drain current, ID, in an FET transistor operated at a low drain voltage, VD, can be expressed by ID =

W μCi(VG − VTH)VD L

(2)

where L is the channel length and μ is the FET mobility. For the control sample with pristine ZnO [Figure 3a], the threshold voltage, defined as the gate voltage over which mobile charge carriers flow through the channel, shifted by 2.3 V when the device was illuminated with a light intensity of 180 mW cm−2, which is comparable with the shift observed for native P3HT.29 In the subthreshold region in Figure 3a, the drain current gradually increased after current onset at a gate voltage of about 30 V. After mobile charge carriers are induced in the channel, the drain current began to increase linearly with gate voltage, allowing for measurement of a threshold voltage via a linear fit in the ID−VG curves, as described in the above eq 2. Upon treatment of the ZnO with thiol [Figure 3b], the threshold voltage shift remarkably increased by 23 V, shifting from 14 to 37 V with illumination under the same conditions used for the untreated system. The increase in the threshold voltage shift that occurs with thiol treatment was reflected in the output characteristic curves as shown in the inset of Figure 3b, in which the relatively large increase in drain current during illumination is consistent with a large threshold voltage shift,

Figure 3. Transfer characteristic curves of (a) P3HT/ZnO and (b) P3HT/thiol-ZnO FETs in the linear region of transistor operation (VD = −3 V). Insets show the output characteristic curves for the devices. (c) Threshold voltage variations as a function of light intensity for both FET devices. The inset shows a magnified plot for the P3HT/ thiol-ZnO-based device in the low light intensity region. The channel length and width of the FETs were 20 μm and 2 mm, respectively.

unlike the rather small increase in drain current observed for the system having no thiol treatment, as can be seen in the inset of Figure 3a. The significantly larger current observed at a low drain voltage for the untreated device compared with the thiol analog is attributed to a much higher mobility in the device fabricated without thiol since the current is proportional to the product of mobile charge carrier concentration and mobility in the linear region of transistor operation, a factor that will be 4254

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that no substantial improvement in molecular ordering of bulk P3HT on ZnO by thiol treatment occurs. In the case where a more ordered phase of P3HT at the thiol-ZnO interface may be present, it is likely to be confined to a few nanometers from ZnO surface; the bulk phase of P3HT dominates the optical absorption spectra, making it difficult to detect a peak emanating from the buried, ordered P3HT phase. Were this to be the case, the increased threshold voltage shift observed using thiol-treated ZnO is partly explained by fewer recombination centers on P3HT in close proximity to the thiol-treated inorganic oxide. Alternatively, the larger threshold voltage shift observed under illumination for the FET device having a thiol modified ZnO surface may arise from the difference in the interface area between P3HT and ZnO induced by addition of thiol (vide infra) because the photocurrent in solar cell devices can be affected by the morphology of electron donor/acceptor blends, as demonstrated in Oosterhout et al.31 The presence of the shoulder at 610 nm for the P3HT/ZnO prepared in the absence of thiol, a signature of enhanced molecular ordering in P3HT, can be interpreted to mean that the contribution of the thiol treatment in reducing trap density in P3HT due to enhanced twodimensional organization is negligible. It should be noted that the possibility of dipole formation at the P3HT/ZnO interface resulting from introduction of thiol self-assembled layers onto the ZnO surface,23 altering the electron transfer efficiency to ZnO, cannot be ruled out. The possible presence or absence of interface dipoles could be elucidated through the use of ultraviolet photoemission spectroscopy and is a subject outside the scope of the present study. The surface morphology of the composite films was studied to gain insight into the effects of the presence of thiol on interfacial area. AFM height images of P3HT/ZnO and P3HT/ thiol-ZnO are shown in Figure 5. The P3HT film spin-coated onto an SiO2 surface, as shown in Figure 5a, has a very smooth surface with a rms roughness of ∼1.46 nm evaluated from a 4.0 × 4.0 μm2 area. The pinhole voids are believed to be due to poor wetting of the P3HT solution on the substrate. Without thiol treatment, ZnO nanoparticles formed clusters, creating ZnO-rich (left) and P3HT-rich (right) regions, as seen in Figure 5b. In the ZnO-rich region, ZnO nanoparticles aggregate to form large agglomerates with an rms roughness of ∼565 nm, while large islands of ZnO nanoparticle clusters are surrounded by P3HT in the P3HT rich region. A significant effect of thiol treatment on the morphology is shown in Figure 5c. The thiol treated ZnO nanoparticles are relatively uniformly distributed throughout the P3HT over the entire area. This phenomenon arises because thiol modification tends to promote compatibility between the ZnO surface and P3HT, preventing the formation of aggregates within the polymer matrix.32,33 The large threshold voltage shift observed in Figure 3, thus, is attributed in part to the large interface area between P3HT and thiol-ZnO, assuming the surface morphology is representative of the buried P3HT/ZnO interfaces. Introduction of thiol into the system influenced not only charge transfer at the P3HT/ZnO interface, but also the charge transport properties of P3HT. Under illumination and in the dark, thiol treatment decreased the FET mobility by about an order of magnitude compared to the mobility obtained for a device fabricated with the pristine hybrid material. The FET mobility values, determined from transfer characteristic curves (ID−VG) as described in eq 2, are summarized in Table I. The observed differences can be ascribed to an increase in carrier

discussed below. The variations in threshold voltage with respect to light intensity for the two types of devices are investigated in the plots depicted in Figure 3c, where the threshold voltage for a FET fabricated with P3HT/thiol-ZnO increases linearly in the low intensity regime [Figure 3c inset], followed by saturation at higher light intensities. The saturation results from electrons becoming trapped onto the ZnO, suppressing further transfer by the local electric field that points toward ZnO. The incomplete conducting path between the metallic source and drain electrodes for electrons trapped onto ZnO, confirmed by the negligible drain-source current observed for FET devices fabricated with P3HT/ZnO both in the dark and under illumination, allowed for a build-up of electrons at the interface, preventing further photoinduced electron transfer to ZnO. In addition, an increase of the carrier recombination rate at the interface resulting from the increase in the electron and hole concentrations at the higher light intensities can lead to saturation in threshold voltage. This effect explains the nonlinear relationship between the threshold voltage (VTH) and light intensity (I) which can be fitted by a power law, VTH = AIβ, producing β = 0.08.30 The results described here are consistent with earlier reports in which the short circuit current of hybrid solar cells with P3HT/ZnO planar−heterojunction interfaces was enhanced by surface treatment of ZnO with alkanethiol.6,24 In those studies, the increased short circuit current was attributed to reduced charge recombination in P3HT arising from a more ordered microstructure of P3HT in contact with thiol-treated ZnO.24 In-plane grazing incidence X-ray diffraction data showed that P3HT deposited on thiol-treated ZnO is more ordered with a preferential in-plane orientation compared with P3HT on native ZnO.24 This result was supported by UV−visible absorption data where the absorption peak of P3HT associated with thiol-treated ZnO was red-shifted by ∼50 nm compared to P3HT with pristine-ZnO.24 Our UV−vis spectral data in Figure 4, however, show that all the P3HT films examined in the

Figure 4. UV−vis absorption spectra for P3HT, P3HT/ZnO, and P3HT/thiol-ZnO films.

present study have the same peak position of 560 nm, independent of the presence of ZnO or thiol-ZnO. One interpretation of the apparent lack of a shift in P3HT λmax is 4255

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Figure 5. Tapping mode AFM height images of (a) P3HT, (b) P3HT/ZnO, and (c) P3HT/thiol- ZnO.

is somewhat puzzling in light of reports noting that an increase in charge carriers within disordered semiconductors causes enhancement of the carrier mobility,36,37 as justified by a multiple trapping and thermal release model.38 The limited increase in carrier mobility, even though the carrier concentration undergoes a relatively large increase for the P3HT/thiol-ZnO based FET, can be explained by the fact that an increase in carrier concentration within doped organic semiconductors broadens the Gaussian density of states (DOS) distribution.39,40 In other words, as the carrier concentration increases by photodoping up to a certain critical value, broadening of the DOS dominates over filling the localized states, thus suppressing any increase in carrier mobility. To summarize, we studied photoinduced charge transfer at the bulk heterojunction interfaces of P3HT/ZnO using FET device architectures. The P3HT/ZnO interface was modified with thiol and the effect of thiol treatment was investigated through examination of the threshold voltage shifts observed for the FETs. The total number of photoinduced charge carriers transferred to ZnO from P3HT for the thiol-modified structure was larger than that for the untreated alternative due to increased interface area, evidenced by a remarkable difference in morphology observed through comparison of AFM images, resulting in a large threshold voltage change arising from an electrostatic potential change within the film. Thiol treatment effects an increase in interface area which is conducive to increasing the photocurrent in bulk heterojunction solar cell devices, however, it is not beneficial for charge transport of holes within P3HT because of the increased number of localized states at the P3HT/ZnO interface limiting hole transport.

Table I. P3HT FET Hole Mobilities in the Dark and Under Illumination (180 mW cm−2)a

P3HT/ZnO P3HT/ZnOthiol

P3HT mobility, dark (cm2/(V s))

P3HT mobility, light (cm2/(V s))

9.6 × 10−4 7.1 × 10−5

1.1 × 10−3 9.0 × 10−5

a

The FET mobility was calculated in the linear regime of transistor operation at VD = −3 V.

scattering/trapping centers arising from the large interface area between P3HT and the thiol-ZnO. It is known that localized electronic states that serve as charge carrier trapping sites exist on ZnO nanoparticle surfaces.15,34,35 The presence of surface traps predicts a larger mobility for devices prepared with parent P3HT than with a hybrid P3HT/ZnO material. Indeed, the FET mobility for the device fabricated with the hybrid of P3HT and pristine ZnO is significantly lower than that for pure P3HT, which is typically in the range of 10−2 to 10−3 cm2/(V s). In the case where passivation of surface traps associated with ZnO by the thiol molecules is not efficient, the large interface area between P3HT and the thiol-treated ZnO would be a critical factor in limiting charge transport of holes in P3HT. Under illumination, the greatly increased hole density induced in P3HT in the presence of thiol-ZnO by photodoping, as confirmed by the large threshold voltage shift shown in Figure 3, did not result in a significant mobility enhancement in comparison with that obtained in the dark. Upon illumination, the increase in carrier concentration from 4.9 × 1017 to 1.3 × 1018 cm−3 in P3HT with thiol-ZnO, corresponding to the threshold voltage shift of 23 V, led to an increase in mobility by a factor of 1.3 when the light intensity was 180 mW cm−2, while only a small increase in carrier concentration from 1.5 × 1017 to 2.2 × 1017 cm−3 in P3HT with pristine-ZnO, estimated from carrier mobility and conductivity in the linear region, was necessary to produce a similar increase in mobility. This result

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CONCLUSIONS In conclusion, the results presented here demonstrate that surface modification, as demonstrated by the thiol treatment presented here, can be an alternative methodology to increase 4256

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photocurrent in hybrid semiconducting systems via engineering interface area, an essential step for the realization of optimized solar cell devices. To maximize the photocurrent through thiol treatment, elimination of localized electronic states resulting from an increased interface is required for enhanced carrier mobility. Use of relatively large nanoparticles with a diameter comparable in thickness to that of a polymer matrix allowed for observation of the remarkable difference in dispersivity of nanoparticles in such a matrix that can arise from interface modification. Such differences cannot be achieved in a composite prepared with very small size nanoparticles in which a significant fraction of the interface is buried, thus limiting direct correlation between interface modification and the relevant electrical properties of bulk heterojunction interfaces. Our strategy afforded the opportunity to clarify the role of interface modification in changing charge transport properties as well as interfacial morphology relating to charge separation in an electron donor/acceptor composite film. Further, the obvious difference in morphology observed after thiol modification that leads to a remarkable difference in the interface area was consistent with the large threshold voltage shift observed during illumination. This result, obtained through comparison of the threshold voltage in an FET configuration, demonstrates that our approach that uses an FET device as a test structure to evaluate an electron donor/ acceptor interface is valid and potentially useful for the optimization of solar cell diode structures. To achieve a good balance between charge transfer and charge transport to electrodes, the interfacial area must be taken into consideration. In the case of P3HT/ZnO based systems, fine control of the composition would offer an opportunity to optimize both charge transfer and charge transport properties. The role of thiol molecules as an ultrathin passivation layer for the ZnO surface in enhancing photoinduced charge separation at the interface has not been identified in our study. Further, charge transfer efficiency, depending on the interface modification used, can be studied by either structuring an electron donor/acceptor bilayer in an FET device configuration or fabricating a homogeneous composite film with bulk heterojunction interfaces. The use of an FET structure allowed for the independent study of photoinduced charge transfer and charge transport properties, providing fundamental insights into the electrical and structural properties of the buried electron donor/acceptor interfaces.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (E.R.); metalpbn@ gmail.com (B.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded in part by Georgia Tech, the Center for Organic Photonics and Electronics (COPE), and by the CMDITR STC Program of the National Science Foundation (DMR-0120967).

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