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Healing contact
Traps in organic semiconducting crystals are healed when a perfluoropolyether oil is deposited on the surface of these materials, thus making possible the detection of intrinsic features of charge-carrier transport in rubrene and tetracene.
Chad Risko and Jean-Luc Brédas
E
lectronic devices based on π-conjugated molecular or polymer semiconductors have moved out of the realm of academic curiosity and are now regarded as high-value commercial technologies. Displays derived from organic light-emitting diodes have made a tremendous impact in mobile devices and are now being brought to the television market; moreover, organic photovoltaics and field-effect transistors (OFET) are also starting to show promise. However, the presence of chemical and physical imperfections — such as chemical defects and reaction byproducts, environmental impurities, or energetic and structural disorder — still hampers the performance of these soft materials and limits the understanding of their intrinsic electronic and optical properties. Because these imperfections usually escape precise characterization, they generally go by one name: traps. Writing in Nature Materials, Lee and co-workers1 demonstrate that the simple application of a polymer oil based on a perfluoropolyether (PFPE) to the surface of an organic semiconducting crystal markedly reduces the influence of traps and results in the ability to measure the chargecarrier transport properties of π-conjugated materials with high accuracy. The efficiency of hole or electron transport in π-conjugated organic semiconductors is dependent on the conduction pathways present in these materials and on the existence of scattering centres and traps. In such systems, where non-covalent interactions between the organic units drive structural packing, the strength of the electronic communication (electronic coupling) among neighbouring molecules or polymer chains, and the interactions between charge carriers and vibrations play crucial roles in defining the charge-carrier mobilities. Current understanding suggests two extremes to describe how charges move through these media. In the case where electronic coupling is weak, the charge carriers (often referred to as polarons) localize on a molecule or chain segment and diffuse through the material
h+ +
h
h+
h+
h+
Figure 1 | The healing effect of PFPE on charge transport in organic single crystals as proposed by Lee and co-workers1. On deposition of a PFPE oil drop (yellow) on the organic crystal surface (molecules shown as blue bars), dipole moments (black arrows, where the tips and plus signs represent the negative and positive ends of the dipole, respectively) along the polymer chains (light green) align and electrostatically attract positive charges (red circles) to fill deep traps (grey molecules) near the crystal surface. On operation of the transistor, the positive charges (green circles) injected into the conduction channel are then allowed to migrate freely.
by means of a series of incoherent hops assisted by electron–vibration interactions. When the electronic coupling is strong and significantly larger than the electron– vibration interactions, the charges move through delocalized electronic states (that is, band regime), in a fashion similar to what is known in inorganic semiconductors. Numerous experimental studies, including angle-resolved ultraviolet photoelectron spectroscopy2, electron spin resonance3 and Hall effect measurements4, as well as theoretical developments5 have recently increased our understanding of how thinfilm or crystalline packing impacts the
NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials
© 2013 Macmillan Publishers Limited. All rights reserved
charge-carrier transport characteristics. However, realizing trap-free, perfectly ordered materials where the intrinsic transport properties can be measured remains a difficult task. It is in this context that the work of Lee et al.1 provides an important step forward. These researchers investigated the hole mobility in single crystals of rubrene and tetracene, two of the highest-performing hole transporters in OFETs. They deposited on the surface of the crystals a thin layer of a non-conjugated polymer, PFPE — an inert, insulating lubricant typically used in strongly oxidizing or reactive environments 1
news & views to prevent corrosion. Different fluorinated polymers have sometimes served as the dielectric material between the gate electrode and the organic semiconductor in OFETs, with minor effects on the intrinsic conduction properties of the device. In contrast, the impact of surface functionalization with PFPE on chargecarrier transport is remarkable: this polymer is seen to ‘heal’ the traps present in the OFET channel (that is, the ultrathin layer at the organic semiconductor surface within which charge carriers are transported), so that the charges moving in the channel itself experience a trap-free conduction regime. To explain the nature of the PFPE−crystal interaction, Lee and colleagues postulate that dipole moments associated with the pendant trifluoromethyl groups protruding from the PFPE backbone (black arrows in Fig. 1) align and induce a local electric field at the interface, thereby creating a holeaccumulation layer (a process equivalent to the application of a negative gate bias in a p-channel field-effect transistor). These holes then fill the deep traps present in the channel and allow the charges induced by switching on the transistor to flow freely (Fig. 1). This surface functionalization allows the accurate measurement of the anisotropic charge-carrier transport in the two crystals, an intrinsic feature (due to the asymmetric
2
shape and packing of the organic molecules) that is easily disrupted in the presence of even small degrees of disorder.6,7. Also, Lee et al. find that the conductance characteristics of rubrene crystals that were first photo-oxidized — a process that is known to introduce both shallow and deep electronic traps — and then treated with PFPE are nearly identical to that of a pristine rubrene crystal coated with PFPE, another significant manifestation of the ‘trap healing’ effect arising from this treatment. Additionally, the decrease of the electronic noise that is usually associated with charge traps leads to an exceptional improvement of the signal-to-noise ratio in Hall effect measurements — a technique used to investigate independently chargecarrier mobility and density in materials. The increased sensitivity of the Hall effect measurements allows the researchers to confirm that hole transport in rubrene is indeed in the band regime, whereas holes in tetracene seem to move by means of an incoherent hopping mechanism. The results presented by Lee et al. open a wide range of further device investigations that will explore the applicability of PFPE or similar surface modifiers to other organic materials. These findings also provide a new test bench for charge-transport theories; indeed, a broader range of experimental data on intrinsic charge
conduction — not affected by spurious effects due to traps — will be available to probe the predictions of the current models and to refine the theoretical underpinnings of charge-carrier transport in organic semiconductors. Furthermore, confirming the proposed mechanism of trap healing will require molecular-scale details that can currently only come from theoretical modelling. Hence, there is no doubt that the work reported by Lee and co-workers will spur considerable efforts on a number of fronts. ❐ Chad Risko and Jean-Luc Brédas are at the School of Chemistry and Biochemistry, and the Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA. e-mail: [email protected]; [email protected] References 1. Lee, B. et al. Nature Mater. http://dx.doi.org/10.1038/ nmat3781 (2013). 2. Machida, S.-i. et al. Phys. Rev. Lett. 104, 156401 (2010). 3. Marumoto, K., Kuroda, S.-i., Takenobu, T. & Iwasa, Y. Phys. Rev. Lett. 97, 256603 (2006). 4. Podzorov, V., Menard, E., Rogers, J. A. & Gershenson, M. E. Phys. Rev. Lett. 95, 226601 (2005). 5. Coropceanu, V., Li, Y., Yi, Y. P., Zhu, L. Y. & Bredas, J. L. Mater. Res. Soc. Bull. 38, 57–64 (2013). 6. Sundar, V. C. et al. Science 303, 1644–1646 (2004). 7. Xia, Y., Kalihari, V., Frisbie, C. D., Oh, N. K. & Rogers, J. A. Appl. Phys. Lett. 90, 162106–162103 (2007).
Published online: 27 October 2013
NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials
© 2013 Macmillan Publishers Limited. All rights reserved
Healing contact
Traps in organic semiconducting crystals are healed when a perfluoropolyether oil is deposited on the surface of these materials, thus making possible the detection of intrinsic features of charge-carrier transport in rubrene and tetracene.
Chad Risko and Jean-Luc Brédas
E
lectronic devices based on π-conjugated molecular or polymer semiconductors have moved out of the realm of academic curiosity and are now regarded as high-value commercial technologies. Displays derived from organic light-emitting diodes have made a tremendous impact in mobile devices and are now being brought to the television market; moreover, organic photovoltaics and field-effect transistors (OFET) are also starting to show promise. However, the presence of chemical and physical imperfections — such as chemical defects and reaction byproducts, environmental impurities, or energetic and structural disorder — still hampers the performance of these soft materials and limits the understanding of their intrinsic electronic and optical properties. Because these imperfections usually escape precise characterization, they generally go by one name: traps. Writing in Nature Materials, Lee and co-workers1 demonstrate that the simple application of a polymer oil based on a perfluoropolyether (PFPE) to the surface of an organic semiconducting crystal markedly reduces the influence of traps and results in the ability to measure the chargecarrier transport properties of π-conjugated materials with high accuracy. The efficiency of hole or electron transport in π-conjugated organic semiconductors is dependent on the conduction pathways present in these materials and on the existence of scattering centres and traps. In such systems, where non-covalent interactions between the organic units drive structural packing, the strength of the electronic communication (electronic coupling) among neighbouring molecules or polymer chains, and the interactions between charge carriers and vibrations play crucial roles in defining the charge-carrier mobilities. Current understanding suggests two extremes to describe how charges move through these media. In the case where electronic coupling is weak, the charge carriers (often referred to as polarons) localize on a molecule or chain segment and diffuse through the material
h+ +
h
h+
h+
h+
Figure 1 | The healing effect of PFPE on charge transport in organic single crystals as proposed by Lee and co-workers1. On deposition of a PFPE oil drop (yellow) on the organic crystal surface (molecules shown as blue bars), dipole moments (black arrows, where the tips and plus signs represent the negative and positive ends of the dipole, respectively) along the polymer chains (light green) align and electrostatically attract positive charges (red circles) to fill deep traps (grey molecules) near the crystal surface. On operation of the transistor, the positive charges (green circles) injected into the conduction channel are then allowed to migrate freely.
by means of a series of incoherent hops assisted by electron–vibration interactions. When the electronic coupling is strong and significantly larger than the electron– vibration interactions, the charges move through delocalized electronic states (that is, band regime), in a fashion similar to what is known in inorganic semiconductors. Numerous experimental studies, including angle-resolved ultraviolet photoelectron spectroscopy2, electron spin resonance3 and Hall effect measurements4, as well as theoretical developments5 have recently increased our understanding of how thinfilm or crystalline packing impacts the
NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials
© 2013 Macmillan Publishers Limited. All rights reserved
charge-carrier transport characteristics. However, realizing trap-free, perfectly ordered materials where the intrinsic transport properties can be measured remains a difficult task. It is in this context that the work of Lee et al.1 provides an important step forward. These researchers investigated the hole mobility in single crystals of rubrene and tetracene, two of the highest-performing hole transporters in OFETs. They deposited on the surface of the crystals a thin layer of a non-conjugated polymer, PFPE — an inert, insulating lubricant typically used in strongly oxidizing or reactive environments 1
news & views to prevent corrosion. Different fluorinated polymers have sometimes served as the dielectric material between the gate electrode and the organic semiconductor in OFETs, with minor effects on the intrinsic conduction properties of the device. In contrast, the impact of surface functionalization with PFPE on chargecarrier transport is remarkable: this polymer is seen to ‘heal’ the traps present in the OFET channel (that is, the ultrathin layer at the organic semiconductor surface within which charge carriers are transported), so that the charges moving in the channel itself experience a trap-free conduction regime. To explain the nature of the PFPE−crystal interaction, Lee and colleagues postulate that dipole moments associated with the pendant trifluoromethyl groups protruding from the PFPE backbone (black arrows in Fig. 1) align and induce a local electric field at the interface, thereby creating a holeaccumulation layer (a process equivalent to the application of a negative gate bias in a p-channel field-effect transistor). These holes then fill the deep traps present in the channel and allow the charges induced by switching on the transistor to flow freely (Fig. 1). This surface functionalization allows the accurate measurement of the anisotropic charge-carrier transport in the two crystals, an intrinsic feature (due to the asymmetric
2
shape and packing of the organic molecules) that is easily disrupted in the presence of even small degrees of disorder.6,7. Also, Lee et al. find that the conductance characteristics of rubrene crystals that were first photo-oxidized — a process that is known to introduce both shallow and deep electronic traps — and then treated with PFPE are nearly identical to that of a pristine rubrene crystal coated with PFPE, another significant manifestation of the ‘trap healing’ effect arising from this treatment. Additionally, the decrease of the electronic noise that is usually associated with charge traps leads to an exceptional improvement of the signal-to-noise ratio in Hall effect measurements — a technique used to investigate independently chargecarrier mobility and density in materials. The increased sensitivity of the Hall effect measurements allows the researchers to confirm that hole transport in rubrene is indeed in the band regime, whereas holes in tetracene seem to move by means of an incoherent hopping mechanism. The results presented by Lee et al. open a wide range of further device investigations that will explore the applicability of PFPE or similar surface modifiers to other organic materials. These findings also provide a new test bench for charge-transport theories; indeed, a broader range of experimental data on intrinsic charge
conduction — not affected by spurious effects due to traps — will be available to probe the predictions of the current models and to refine the theoretical underpinnings of charge-carrier transport in organic semiconductors. Furthermore, confirming the proposed mechanism of trap healing will require molecular-scale details that can currently only come from theoretical modelling. Hence, there is no doubt that the work reported by Lee and co-workers will spur considerable efforts on a number of fronts. ❐ Chad Risko and Jean-Luc Brédas are at the School of Chemistry and Biochemistry, and the Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA. e-mail: [email protected]; [email protected] References 1. Lee, B. et al. Nature Mater. http://dx.doi.org/10.1038/ nmat3781 (2013). 2. Machida, S.-i. et al. Phys. Rev. Lett. 104, 156401 (2010). 3. Marumoto, K., Kuroda, S.-i., Takenobu, T. & Iwasa, Y. Phys. Rev. Lett. 97, 256603 (2006). 4. Podzorov, V., Menard, E., Rogers, J. A. & Gershenson, M. E. Phys. Rev. Lett. 95, 226601 (2005). 5. Coropceanu, V., Li, Y., Yi, Y. P., Zhu, L. Y. & Bredas, J. L. Mater. Res. Soc. Bull. 38, 57–64 (2013). 6. Sundar, V. C. et al. Science 303, 1644–1646 (2004). 7. Xia, Y., Kalihari, V., Frisbie, C. D., Oh, N. K. & Rogers, J. A. Appl. Phys. Lett. 90, 162106–162103 (2007).
Published online: 27 October 2013
NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials
© 2013 Macmillan Publishers Limited. All rights reserved
