SEARCH

SEARCH BY CITATION

Keywords:

  • charge transfer;
  • doping;
  • organic field-effect transistors;
  • carrier mobility;
  • excitons;
  • interfaces

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental methods
  5. Results and discussion
  6. Summary and conclusions
  7. Acknowledgements
  8. References

We studied various aspects relating to surface charge-transfer-induced doping at an organic/organic interface using in situ electrical measurements with a field-effect transistor (FET) during the formation of the electron donor/acceptor interface. Adsorption of the electron-accepting molecules (C60) on top of the electron donating molecules (α-6T) led to an increase in the FET hole mobility in an α-6T film. Under illumination, the FET hole mobility in the α-6T film with C60 deposition was significantly increased in comparison with that in the dark due to exciton dissociation at the C60/α-6T interface, resulting in a large threshold voltage shift. The origin of the mobility increase is explained by the multiple trapping and release (MTR) model in which the mobility is determined by the carrier density. Various phenomena relevant to charge transfer and charge transport at the organic/organic interface are reported and their origins are discussed. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental methods
  5. Results and discussion
  6. Summary and conclusions
  7. Acknowledgements
  8. References

Organic semiconducting materials have been extensively researched due to their various applications in organic transistors 1–3, organic photovoltaics (OPVs) 4 and organic light-emitting diodes (OLEDs) 5. To optimize device performance, understanding of their electronic and structural properties at organic/organic interfaces in the electronic devices is essential 6. In particular, charge transfer and transport at the interface play crucial roles in operating the devices 7. In OLEDs 8, for example, injected electrons or holes into an electron-transport layer or a hole-transport layer, respectively, are required to transport to the recombination zone to emit the light. In OPVs 9–11, exciton separation at the electron donor/accepter interface is a key process in optimizing the photocurrent. In the process, electrons in the electron-donating layer are transferred into the electron-accepting layer and the holes left in the electron-donating layer transport to the charge-collection electrodes to produce an electric potential between the anode and cathode electrodes in OPVs. Therefore, addressing electronic states associated with charge transfer and transport at the interfaces is crucial in correlating device performance with individual electronic processes for device operation.

Recently, the study of charge transfer and transport at the organic/organic interfaces has attracted much interest because surface charge-transfer-induced doping is emerging as a promising technique in controlling carrier concentration and electrical mobility of component layers in- corporated in electronic devices 12, 13. To control the doping process, change in the electrical properties associated with charge transfer between the adjacent organic layers should be addressed during the formation of the organic/organic interfaces.

For the study of the organic/organic interfaces, energy-level alignment between the two contacting organic materials has been researched using a variety of spectroscopic techniques including ultraviolet photoelectron spectroscopy (UPS). The origins of the vacuum-level offset have been attributed to charge transfer arising from chemisorption or physisorption or the presence of interface states. Despite the extensive information acquired from the spectroscopic techniques, however, there is still a lack of direct connection between spectroscopic information including the vacuum-level offset and the change in the electrical properties due to charge transfer and transport responsible for device operation. In other words, understanding of the change in the electrical properties resulting from interfacial charge transfer remains elusive because not only is the relevant interface buried in a composite material in the electronic devices but also localized states are present at the interface, modifying the electrical contact properties.

Here, to probe the change in the electrical properties as a result of charge transfer at an organic/organic interface, we structured a field-effect transistor (FET) embedding a structurally well-defined electron donor/acceptor interface where charge transfer occurs in the dark and under illumination. Transistor parameters including the FET mobility and the threshold voltage are extracted to interpret various phenomena relating to charge transfer and transport at the interface. For a systematic analysis, the formation of the organic/organic interface is monitored using an in situ electrical measurement system combined with an ex situ atomic force microscopy (AFM) study.

α-6T and C60 were chosen as electron donor/acceptor materials, respectively, because they are representative organic semiconductors applied to optoelectronic devices. Thiophene oligomers including α-6T exhibits high crystalline ordering in their solid state and feature a favorable band structure coupled to C60. Optoelectronic devices consisting of α-6T and C60 have been studied and showed potential interests because they can easily be chemically functionalized providing various ways for device optimization. However, the low solubility of thiophene oligomers compared to thiophene polymers including poly(3-hexyl-thiophene) hampers solution processing that enables cost-effective fabrication. The advantage of using thiophene oligomers is that they can be vacuum deposited providing an ordered and sharp interface that enables correlation between the electronic and structural properties. This allows for in situ electrical measurements in which charge-transfer-dependent electrical properties are explored during the interface formation. Additionally, it offers insights into understanding thiophene polymers because of their analogous structural properties.

Experimental methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental methods
  5. Results and discussion
  6. Summary and conclusions
  7. Acknowledgements
  8. References

α-6T and C60 were purchased from Sigma Aldrich and used without further purification. In situ current–voltage measurements in the dark and under illumination during α-6T/C60 interface formation were performed in a vacuum chamber. α-6T molecules, as depicted in Fig. 1a, were thermally evaporated onto a bottom-contact FET device in which the source and drain metal electrodes [Au(80 nm)/Cr(3 nm)] were prepatterned on a 200 nm thermally grown SiO2 gate dielectric using conventional photolithography. A highly doped silicon substrate with a resistivity of less than 0.01 Ω cm served as a gate electrode. The channel length and width of 100 μm and 600 μm, respectively, were used. FET devices were transferred to a vacuum chamber after sonication for cleaning, sequentially in acetone, isopropanol and deionized water for several minutes. During C60 deposition, IDVG curves at a low drain voltage of –3 V were acquired in situ through the electrical feed-throughs installed on the vacuum chamber, as shown in Fig. 1b. The photocurrent in α-6T and C60 films was obtained by illuminating the FET devices using a green laser diode with a wavelength of 520 nm through a quartz window in the vacuum chamber. The light intensity was measured using an optical power meter. To investigate the growth mode of C60 on top of α-6T, tapping mode AFM was used. The chamber pressure was maintained at ∼10–7 Torr during deposition and electrical characterizations. The FET mobility, μ, and the threshold voltage, VT, in the linear regime in transistor operation were calculated by the IDVG transfer characteristic curves based on Eq. (1):

  • equation image((1))

where Z is the channel width, L the channel length and Ci the capacitance per unit area of the SiO2 gate dielectric.

thumbnail image

Figure 1. (a) Molecular structures of α-6T and C60. (b) In situ electrical measurement set-up for C60/α-6T field-effect transistors.

Download figure to PowerPoint

Results and discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental methods
  5. Results and discussion
  6. Summary and conclusions
  7. Acknowledgements
  8. References

To identify the majority carriers in C60 and α-6T films, a FET containing a thick C60/α-6T bilayer film was fabricated. C60 is a very well known n-type semiconductor, while α-6T is a p-type semiconductor 14, 15. As expected, ambipolar transport behavior was observed for a C60 (∼40 nm)/α-6T (∼9 nm) bilayer FET in Fig. 2a and b. An α-6T film exhibited p-type behavior in which a more negative gate voltage at a constant drain voltage of –3 V increased the drain current due to an increased hole density, n = Ci(VGVT), in Eq. (1). Electron current was observed at the positive gate voltage range between 20 V and 40 V in Fig. 2a. It is noted that no electron transport was observed with only α-6T film in the gate voltage range up to 100 V from 0 V. In the IDVD transistor output characteristic curves in Fig. 2b, the drain current exhibited linear and saturation regions at a negative gate voltage of –40 V. However, as the gate voltage increases to a more positive value, the drain current started to increase at a high drain voltage without saturation. At a gate voltage of zero, for example, the drain current due to mobile electrons began to increase rapidly at a drain voltage of –30 V. This arises because electrons begin to accumulate in the channel region near the drain electrode at a more positive gate voltage in comparison with the applied drain voltage 14.

thumbnail image

Figure 2. (a) IDVG and (b) IDVD plots for a C60/α-6T bilayer FET. The drain voltage is fixed at –3 V.

Download figure to PowerPoint

To investigate the change in the charge transport properties in an α-6T film during C60 film growth on top of it, an in situ electrical measurement was performed, see Fig. 3. As seen in a schematic diagram in the inset of Fig. 3a, a fairly thick α-6T film at a thickness of 9 nm was deposited on a SiO2 gate dielectric followed by C60 deposition. During C60 growth the FET hole mobility in the α-6T film was measured as a function of C60 thickness in Fig. 3b. Until the C60 thickness of 23 nm in Fig. 3a, the drain current at the positive gate voltage range between 0 V and 40 V was not observed, indicating that the electron conducting path in the C60 film has not been formed, while the hole current at a gate scan from 0 V to –40 V kept increasing. Increasing the thickness of the C60 film up to 83 nm, the FET hole mobility in the α-6T film, proportional to the slope in the IDVG curves, increased as observed in Fig. 3a. At the C60 thickness of 30 nm, the conducting path in the C60 film is formed, evidenced by the increase in the electron current at a gate scan from 0 V to 40 V in Fig. 3a. More interestingly, the hole current in the underlying α-6T film kept increasing during deposition of the C60 film. Increase in the C60 thickness enhanced the FET hole mobility as well as the hole current in the α-6T film as seen in Fig. 3a and b. Surprisingly, the FET hole mobility in the α-6T film increased by approximately an order of magnitude during growth of the C60 film up to 80 nm. The origin of the enhanced hole-transport properties in the underlying α-6T film, independent of the formation of the conducting path in the C60 film, will be discussed later.

During C60 growth, enhancement in the FET hole mobility in the underlying α-6T film is significant under illumination. Under illumination using a green laser diode with a wavelength of 520 nm in Fig. 3c and d, the increased FET hole mobility in the α-6T film during C60 growth is consistent with that in the dark, except that its mobility is far larger than that in the dark. It is noted that the threshold voltage for hole conduction in the α-6T film under illumination, 20 V, is more positive than that, –4 V, in the dark, indicating that more holes are induced in the underlying α-6T film in comparison with those in the dark, consistent with a marked increase in the FET hole mobility under illumination.

thumbnail image

Figure 3. C60 thickness dependent FET hole mobility increase in the dark and under illumination. (a) IDVG plots for different C60 thicknesses in the dark. (b) Plot of FET hole mobility of α-6T as a function of C60 thickness in the dark. (c) IDVG plots for different C60 thicknesses under illumination with a green laser diode. (d) Plot of FET hole mobility of α-6T as a function of C60 thickness under illumination with a green laser diode. The light intensity was 80 mW/cm2. The FET mobilities were acquired in the linear regime of transistor operation.

Download figure to PowerPoint

To provide insights into the origin of enhanced hole conduction in the underlying α-6T film during C60 growth, morphology of C60 film at the interface was studied using AFM images. C60 molecules form three-dimensional islands on the α-6T layer and grow both laterally and vertically, forming a continuous film, as seen in the AFM images in Fig. 4a and b. C60 molecules are bound to each other rather than to the surface of the underlying α-6T film because the surface energy of α-6T is lower than that of C60 18, 19. Consequently, at the initial stage of C60 growth, C60 molecules form blobs with a height of ∼7 nm (Fig. 4a) followed by an increase in the area coverage of C60 on α-6T (Fig. 4b).

thumbnail image

Figure 4. AFM images of C60 islands deposited on an α-6T layer: (a) before and (b) after conducting channel formation in C60.

Download figure to PowerPoint

An increase in the FET hole mobility in the underlying α-6T film during C60 growth is suggested to be due to surface layer (C60)-induced doping where electron transfer from α-6T to C60 takes place, as shown in the energy band diagrams of Fig. 5a and b. Ge et al. investigated the energy-level alignment between α-6T and C60 using UPS 20. In the study, electron transfer from α-6T from C60 is confirmed by a vacuum-level offset of 0.6 eV, as displayed in Fig. 5b. In elucidating the origin of the vacuum-level offset, transfer of electrons positioned at the highest occupied molecular orbital (HOMO) of α-6T to the lowest unoccupied molecular orbital (LUMO) of C60 is not feasible due to a large energy level difference of 1.2 eV. The specific interfacial resistivity at the C60/α-6T interface is defined as ρi = (dV /dJ)V =0, where J is the current density through the interface 21. Despite the fact that the thermionic emission has been applied to inorganic crystalline materials, we assumed thermionic emission mechanism for electron transfer from α-6T to C60 interface to estimate the minimum energy required for the electron transfer because the binding energy in organic materials is far larger than that in inorganic crystalline materials 22. Based on this assumption, the specific interfacial resistivity can be expressed as ρi = ρ0 exp (B/kT) with ρ0 = k /qA*T, where k is the Boltzmann constant, φB is the energy barrier, and A* is the Richardson's constant. Therefore, the specific resistivity for an energy barrier of 1.2 eV is tens of orders of magnitude larger than that for 0 eV in which the HOMO of α-6T and the LUMO of C60 are aligned. For an effective doping, the LUMO of the surface adsorbates (C60) should be close to the HOMO of the underlying layer (α-6T) 12.

thumbnail image

Figure 5. Energy-level diagrams for a C60/α-6T interface: (a) before contact, (b) after contact. (c) Schematic of hole doping in α-6T due to electron transfer to midgap states in C60. (d) Schematic of the MTR transport model.

Download figure to PowerPoint

Considering the large energy-level difference for electron transfer, we propose a surface-states-induced doping mechanism as a possible mechanism. It is suggested that midgap states between the LUMO and HOMO of the C60 film can serve as surface acceptors, as described in Fig. 5c. Indeed, the presence of midgap states in fullerene derivatives has been evidenced by realization of a charging effect in a 6, 6-phenyl C61 buttyric acid methyl ester (PCBM)/pentacene 23 bilayer and pentacene/C60 self-assembled monolayers (SAMs) FETs 24. In the study, electrons were trapped in the underlying molecules, i.e. PCBM and C60 SAMs, resulting in a large threshold voltage shift in the FETs 23, 24. Additionally, the presence of surface states at the α-6T/C60 interface can be inferred from the values of the open-circuit voltage in organic photovoltaic devices in which α-6T and C60 layers work as electron donor and acceptor layers, respectively 16, 17. According to Saito and coworkers, the open-circuit voltage of 0.4 V measured from the photovoltaic devices using C60 and α-6T layers as photoactive materials was far lower than the 0.8 eV estimated from the difference between the HOMO of α-6T and the LUMO of C60 16. It has been generally accepted that the open circuit voltage is deterined by the LUMO of the electron acceptor and the HOMO of the electron donor 25. The low open-circuit voltage compared to the LUMO–HOMO difference im-plies two possible cases. One is the LUMO–HOMO difference is lower than that estimated from Fig. 5a and b. For example, the interface dipoles may be induced leading to a change in the vacuum-level offset in the way that the energy barrier for electron transfer is lowered depending on the orientation of the molecules at the interface 26. According to Duhm et al. the ionization potential of conjugated polymers including α-6T can be modified by up to 0.6 eV depending on their molecular orientations 26. The schematic energy band diagrams presented in Fig. 5a and b were derived from the arrangement in which α-6T molecules are deposited on a metal substrate. In the arrangement, α-6T molecules tend to lie with the long molecular axis parallel to the surface normal on the oxide surface. Even if the molecular orientation at the interface is considered, however, a barrier lowering of up to 0.6 eV due to interface dipole is still not large enough to cause electron transfer from α-6T to C60. This possibility should be investigated more using spectroscopic measurements.

The other possibility is the presence of midgap states in the C60 film working as carrier-recombination centers for electrons separated at the α-6T and C60 interface. The recombination centers can lower the open-circuit voltage in organic photovoltaic devices, decreasing the number of carriers dissociated at the electron donor/acceptor interface 27. Indeed, at the C60/α-6T interface, structural defects in the C60 film are expected due to a rough surface of the α-6T film, creating localized states in the midgap of the C60 film 28.

The origin of the enhanced FET mobility due to charge transfer can be explained by the multiple trapping and release (MTR) model 29, 30. In the charge-transport model, charge carriers trapped in the localized states such as grain boundaries between small crystallites can be thermally activated to the transport band edge, contributing to charge-carrier transport under an electric field, as described in Fig. 5d 31. In the model, charge carriers fill the deep traps followed by occupying the shallow traps. The carriers trapped in the shallow traps can participate in charge transport, the mechanism of which is similar to hopping transport through localized states near the charge-transport energy level.

According to the MTR model, as the number of charge carriers increases, the number of carriers trapped in the shallow traps increases, enhancing the FET mobility due to a lowered energy barrier for transport in the transport band edge in Fig. 5d. According to the model, the mobility can be expressed by Eq. (2):

  • equation image((2))

where Nc is the effective density of states at the transport band edge, T0 is the width of the distribution, and Nt is the total density of localized states. Assuming that the charge transport is determined by the tail states approximated by an exponential distribution of density of states (DOS), an increase in the gate voltage induces more carriers and fills the localized states, shifting the Fermi level toward the transport band edge 32. Therefore, more carriers are positioned closer to the transport band edge, facilitating thermal release of the trapped carriers. Electron transfer to localized states of C60 from α-6T produces holes in α-6T. With further deposition of C60 molecules onto the α-6T layer, the interface area between C60 and α-6T increases, as evidenced by the AFM images in Fig. 4, increasing the number of electrons transferred to C60, i.e. the number of holes left in α-6T. This process increases the number of holes, n = Ci(VG – VT)/e, available for charge transport in the transport band edge, resulting in an increase in the FET hole mobility in the α-6T film, as described in Eq. (2). Importantly, an increase in the FET hole mobility in the underlying α-6T layer was accompanied by an increase in the FET electron mobility in the C60 film during C60 deposition. This indicates that the deposition of C60 molecules filled the voids between the C60 islands on the α-6T enhancing both the structural and electrical connectivity leading to the increase in the FET electron mobility in the C60 film. With the increased connectivity between C60 islands, the C60/α-6T contact area increases, enabling more electrons to transfer to the C60 film.

Interestingly, in the in situ measurements, before the conducting path in the C60 film is formed, confirmed by AFM images, the electron current modulated by a positive gate voltage was observed, implying that the α-6T film is involved in the electron transport. To address the origin of the electron current, we tested FETs with only α-6T layers. In the FETs, no electron transport was observed. It is known that the carrier type in the FETs including organic semiconducting materials is determined by the presence of the carrier-trapping centers 33. Chua et al. showed that the gate dielectric layer can serve as the main trapping centers for carriers in an active transport layer determining the carrier type in the FETs including organic semiconducting materials 33. In our case, we speculate that C60 islands prefer to form in the region of the α-6T with a high surface energy due to the presence of structural defects that can serve as electron-trapping centers. Therefore, the C60 islands can partially passivate the region in the α-6T film where electron trapping centers are concentrated, enabling the electron transport in the α-6T layer. However, more controlled work is needed to address the origin of the electron transport.

Under illumination, an increase in the FET hole mobility in the α-6T layer with an increase in the C60 thickness is consistent with that in the dark. Excitons are created in α-6T and separated at the C60/α-6T interface, increasing the number of holes that can occupy the shallow traps in α-6T. It is also feasible that the excitons created in the C60 layer are dissociated, producing more hole carriers in the underlying α-6T. The shift in the threshold voltage from –3 V to 20 V at a thickness of about 80 nm upon illumination shows that the number of electrons, N = 2.5 × 1012 cm–2, equal to a threshold voltage difference of ΔVT = 23 V, by N = (CiΔVT)/e, were transferred to C60 molecules. This is consistent with the mechanism in which more holes induced in α-6T lead to mobility improvement, as described in Eq. (2).

Summary and conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental methods
  5. Results and discussion
  6. Summary and conclusions
  7. Acknowledgements
  8. References

To summarize, we explored surface charge-transfer-induced doping at a C60/α-6T interface using in situ electrical measurements and elucidated phenomena relating to charge transfer electrically during the formation of the interface that has not been addressed before with spectroscopic tools. To maximize the FET hole mobility in α-6T, particularly, the interface between α-6T and C60 needs to be optimized to increase the number of electrons transferred to C60 by increasing the interface area. To control the growth mode of the C60 film on the underlying α-6T film can be a solution to increasing the doping concentration (hole) in α-6T.

The in situ measurement system using a structurally well-defined bilayer can provide a complementary method to understanding interfacial charge transfer with spectroscopic methods including UPS through correlating the change in the structural properties during interface formation with the electrical device parameters including the mobility and threshold voltage that can be used to control charge-carrier doping at the nanoscale.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental methods
  5. Results and discussion
  6. Summary and conclusions
  7. Acknowledgements
  8. References