Research Article www.acsami.org
Polarization-Dependent Photoinduced Bias-Stress Effect in SingleCrystal Organic Field-Effect Transistors Hyun Ho Choi,†,§ Hikmet Najafov,†,# Nikolai Kharlamov,∥ Denis V. Kuznetsov,∥ Sergei I. Didenko,∥ Kilwon Cho,§ Alejandro L. Briseno,⊥ and Vitaly Podzorov*,†,‡,∥ †
Department of Physics and ‡Institute for Advanced Materials and Devices for Nanotechnology, Rutgers University, Piscataway, New Jersey 08854, United States § Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, South Korea ∥ National University of Science and Technology MISiS, Moscow 119049, Russia ⊥ Department of Polymer Science & Engineering, University of Massachusetts, Amherst, Massachusetts 01002, United States S Supporting Information *
ABSTRACT: Photoinduced charge transfer between semiconductors and gate dielectrics can occur in organic field-effect transistors (OFETs) operating under illumination, leading to a pronounced bias-stress effect in devices that are normally stable while operating in the dark. Here, we report an observation of a polarization-dependent photoinduced biasstress effect in two prototypical single-crystal OFETs, based on rubrene and tetraphenylbis(indolo{1,2-a})quinolin. We find that the decay rate of the source−drain current in these OFETs under illumination is a periodic function of the polarization angle of incident photoexcitation with respect to the crystal axes, with a periodicity of π. The angular positions of maxima and minima of the bias-stress rate match those of the optical absorption coefficient of the corresponding crystals. The analysis of the effect shows that it stems from a charge transfer of “hot” holes, photogenerated in the crystal within a very short thermalization length (≪μm) from the semiconductor−dielectric interface. The observed phenomenon is a type of intrinsic structure−property relationship, revealing how molecular packing affects parameter drift in organic transistors under illumination. We also demonstrate that a photoinduced charge transfer in OFETs can be used for recording rewritable accumulation channels with an optically defined geometry and resolution, which can be used in a number of potential applications. KEYWORDS: organic semiconductor, rubrene, molecular crystal, organic transistor, bias-stress effect, photoinduced charge transfer, memory, mobility
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devices (contact resistance,9,10 ionic motion and leakage in the gate dielectrics,8,11 etc.) but also more importantly by the intrinsic phenomena, such as a charge transfer at semiconductor−dielectric interfaces,12 and/or charge carrier trapping in the semiconductor’s channel itself.13 Identification of these intrinsic mechanisms of bias-stress effect is very important, as even perfect devices operating under ideal environmental conditions will suffer from this detrimental instability, unless proper interface engineering has been undertaken. Single-crystalline organic semiconductors, where structural disorder is minimized, are the best candidates for experimentally addressing the intrinsic mechanisms of bias-stress effect. These devices have already contributed to our basic understanding of many intrinsic physical properties of organic semiconductors, including, for instance, the dependence of
INTRODUCTION Recent progress in understanding charge carrier transport in organic semiconductors has been greatly facilitated by advances in organic field-effect transistors (OFETs) (see, e.g., refs1, 2), including synthesis of several types of high-performance organic semiconductors (see, e.g., ref 3), as well as development of novel techniques for device fabrication and powerful measurements, such as high-resolution ac-Hall effect,4 photocurrent excitation spectroscopy,5 or ac-admittance spectroscopy.6 As a result of these advances, OFETs with high field-effect mobilities sufficient for practical electronic devices are now achievable. Given the availability of a number of organic semiconductors for applications in organic electronics, understanding mechanisms of operational instabilities and parameter drift in OFETs based on these materials becomes of paramount importance. Indeed, it is known that even relatively high-performance OFETs can suffer from issues such as bias-stress effect.7 It was recently shown that this effect may occur not only due to extrinsic factors, such as environmental issues (humidity, interfacial contamination, etc.),8 or technical problems with © 2017 American Chemical Society
Received: July 27, 2017 Accepted: September 15, 2017 Published: September 15, 2017 34153
DOI: 10.1021/acsami.7b11134 ACS Appl. Mater. Interfaces 2017, 9, 34153−34161
Research Article
ACS Applied Materials & Interfaces charge carrier mobility, μ, on the crystal structure and molecular packing,1,14 anisotropic exciton diffusion,15,16 and the effect of pressure and strain on charge carrier transport,17−19 to name a few. By using vacuum-gap single-crystal OFETs, in which a material gate dielectric is replaced with a narrow air or vacuum gap, it was experimentally shown that at least one fundamental mechanism of the bias-stress effect is a slow charge carrier trapping in the organic semiconductor.13 Although this mechanism has been envisioned before,20 it was difficult to separate different contributing factors experimentally, until the development of vacuum-gap OFETs. The studies of vacuum-gap OFETs showed that even in this case the devices exhibit a bias-stress effect, although much weaker compared to that in OFETs with material gate dielectrics, thus pointing to additional contributing mechanisms in conventional OFETs. Studies of single-crystal OFETs with material gate dielectrics have shown that the dominant contribution to the bias-stress effect is due to a charge transfer across the interface (in the dark).12 In this case, thermalized (dark) polarons, holes in ptype and electrons in n-type OFETs, tunnel from the transistor’s channel, where they are accumulated under an applied VG, to the gate dielectric. The probability of tunneling through the interface strongly depends on the energy difference between the states occupied by the mobile carriers in the semiconductor and unoccupied states available in the gate dielectric. Thus, as confirmed experimentally, the rate of the dark bias-stress effect in p-type OFETs systematically depends on the density of tail states of the gate insulator occurring at the energies comparable to the highest occupied molecular orbital (HOMO) edge of the organic semiconductor.12 It was found that the deeper the HOMO of the organic semiconductor (or the greater its ionization potential), the stronger the bias-stress effect in p-type OFETs with a particular gate dielectric. Conversely, the greater the ionization potential of the gate insulator, the lower the bias-stress rates in OFETs. This behavior is consistent with an interfacial charge transfer mechanism, as the leading cause of the bias-stress effect in OFETs. This concept has been further utilized in developing OFETs more resilient against bias stress, for instance, by using gate dielectrics with high ionization potentials (such as a fluoropolymer Cytop).10,21 Alternatively, incorporating selfassembled monolayers at the interface between the gate dielectric and organic semiconductor in OFETs can either facilitate the charge transfer and enhance the so-called memory effect or provide a nanoscale physical barrier and reduce the charge tunneling from the semiconductor to the gate dielectric.22−24 Interestingly, charging of OFETs’ gate dielectrics during the gate stressing has been later observed directly, when an organic semiconductor layer was mechanically stripped off the gate dielectric’s surface of an OFET after gate bias stressing, revealing a charged surface of the dielectric.25 A similar conclusion has been also reached in another study of the socalled lateral OFETs, in which a local potential distribution along a “cross-sectional slice” of a device could be investigated by scanning Kelvin-probe microscopy before and after stressing.26 These studies have suggested strategies for improving the stability of OFETs, based on (a) minimization of the charge transfer by engineering the energetic structure of semiconductor−dielectric interfaces in such a way as to reduce the overlap between the HOMO band of the organic semiconductor and the tail states of the gate insulator and
(b) reduction of the trap density in the organic semiconductor itself. An example of devices with very small bias-stress effect is rubrene single-crystal OFETs with parylene-N gate insulator: in the dark, these OFETs exhibit source−drain current, ISD, decay rates as small as 1%/h of continuous stressing at a high gate voltage corresponding to the hole densities >1012 cm−2.12 However, under illumination with a visible light, the same type of devices exhibits a substantial decay of ISD. For this effect to occur, OFETs must be illuminated through the (semitransparent) gate electrode with photons of energy above the semiconductor’s band gap (that is, when VG is applied simultaneously with photoexcitation of charge carriers in the semiconductor).27,28 The dependences of the photoinduced bias-stress rate on the photoexcitation wavelength, λ, light intensity, P, and gate voltage, VG, suggest that a charge transfer of photogenerated carriers from the semiconductor to the gate dielectric, leading to the dielectric’s charging, is responsible for this effect.27 Depending on the polarity of VG applied during illumination, either electrons or holes are transferred to the dielectric, thus resulting in a positive or negative shift of OFET’s onset voltage, respectively.27 The new, optically “programmed” onset voltage remains rather stable in further measurements in the dark but can be reprogrammed by illuminating the OFET while a different VG is applied. Since the first demonstration of such optically programmable OFET memory devices,27 many similar “memories” were suggested, some using various polymeric or amorphous oxide gate dielectrics29−39 and some using inorganic oxide nanoparticles to enhance charging and widen the memory window.40,41 Furthermore, observation and understanding of this lightinduced bias-stress effect are very important for future flexible and transparent organic optoelectronics, where some circuit components might be inevitably exposed to light either from external sources or from organic light-emitting diode (OLED) pixels integrated as a part of the circuits. For instance, in using OFETs as switches controlling OLED pixels, one would face the problem of bias stress under illumination. Even though we have suggested an interpretation of this phenomenon based on a charge transfer of “hot” photocarriers, further studies are necessary to verify this mechanism and understand its dependence on various parameters, such as light polarization, energy, angle of incidence, crystal facets, and molecular packing of the semiconductor. Here, we performed a comparative study of photoinduced bias-stress effect in organic crystals that exhibit different packing motifs, rubrene and tetraphenylbis(indolo{1,2-a})quinolin (TPBIQ). We have discovered that the rate of bias-stress decay of ISD under illumination in the corresponding OFETs is a periodic function of the polarization angle of incident light with respect to the crystal axes, with maxima and minima in the bias-stress rate reflecting the specific molecular packing of the crystals. In addition, we have observed that the rate of this effect also depends on the angle of incidence and this dependence is qualitatively different for s- and p-wave polarized photoexcitations. These new observations support the mechanism of photoinduced bias-stress effect based on a charge transfer of hot photocarriers and also reveal how molecular packing affects this instability in OFETs. Our results also bear practical importance, as they suggest simple strategies for minimizing this undesirable parameter drift in applied organic optoelectronics, for instance by controlling the angle of incidence or polarization of light emanating from OLED pixels and incident on OFETs. 34154
DOI: 10.1021/acsami.7b11134 ACS Appl. Mater. Interfaces 2017, 9, 34153−34161
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ACS Applied Materials & Interfaces
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polarizer had a resolution of about 1−2°. To ensure that all of the light reaching the crystal/insulator interface in our OFETs is absorbed in the semiconductor, the single crystals used in our devices were chosen to be much thicker than the light penetration length, α−1, for all of the used wavelengths and polarizations (α is the absorption coefficient). Typical absorption lengths (α−1) for visible light in these organic crystals are in the range ∼1−20 μm. All device measurements were performed in air, at room temperature, using Keithley K2400 source meters and K6514 electrometers. Optical “Programming” of the Accumulation Layers in OFETs. Optical recording was achieved by first illuminating the whole channel of a rubrene OFET with a white light at VG = −50 V, which sets the device in a depleted state, and then illuminating the device at VG = +50 V through a shadow mask consisting of a narrow (150 μm wide) slit. This creates a laterally patterned hole accumulation channel (due to photoinduced electron transfer to the gate dielectric), in the form of a narrow stripe, conducting at VG = 0 in the dark. The shadow mask was suspended above the OFET and could be moved along the ITO gate to “record” multiple conducting stripes at chosen locations. Typical times of erasing (depleting) and recording the OFETs at VG = ±50 V were 5−10 min. Typical retention times of the recorded structures were at least several days.
EXPERIMENTAL SECTION
Device Fabrication. We used top-gated single-crystal OFETs with a polymer gate insulator (parylene-N) and a transparent indium-tin oxide (ITO) gate to study the response of the source−drain current of these devices to a linearly polarized light. Single crystals of rubrene and tetraphenylbis(indolo{1,2-a})quinolin (TPBIQ) were grown by physical vapor transport in a stream of ultrahigh-purity He and Ar gases, respectively.42 A sublimed grade rubrene (Sigma-Aldrich) and TPBIQ synthesized and purified by sublimation as described in ref 43 were used for the crystal growth. Temperatures in the sublimation zone during the crystal growth were kept at 320 and 360 °C for rubrene and TPBIQ, respectively. The temperature gradient along the growth tube was ∼5 °C/cm, and the He (Ar) flow rate was 150 (100) cc/min. The details of device fabrication can be found elsewhere.44 In brief, graphite source and drain contacts were painted onto the (a, b) facet of rubrene and (a, c) facet of TPBIQ crystals, defining a channel with length L = 1−3 mm and width W = 50−200 μm (for TPBIQ) and 1−2 mm (for rubrene). This was followed by a deposition of 1− 1.6 μm thick parylene film that served as a gate insulator, topped with a transparent 30−40 nm thick ITO gate sputtered through a shadow mask. Electrical Measurements. The devices were exposed to a linearly polarized light incident on the smooth flat surface of the ITO gate electrode (inset in Figure 1). We used a 20 W halogen lamp with a
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RESULTS AND DISCUSSION Prior charge-transfer studies in rubrene OFETs have shown that these devices exhibit extremely small bias-stress effects in the dark,12 the fact that has driven us to conclude that thermalized gate-induced polarons can hardly cross the rubrene/parylene interface. A significant bias-stress effect in these OFETs can only be observed under illumination with photons of energy above the band gap of rubrene.27 In this process, nonequilibrium charges photogenerated in the crystal next to the interface have sufficiently high energy to overcome the tunneling barrier at the rubrene/parylene interface and be driven into the gate insulator by the applied transverse gate electric field, EG⊥ ≡ (VG − Vonset)/d, where d is the thickness of the gate dielectric (inset in Figure 1). Note that Vonset in this expression corresponds to the onset (or threshold) voltage of the OFET determined from its linear-regime transconductance characteristics, ISD(VG). Nonzero Vonset is usually due to screening of the gate electric field by the charges transferred and localized in the gate insulator (here, we assume that the contribution of deep traps to the threshold voltage is negligible, as is typically the case in pristine single-crystal OFETs1). The Vonset in single-crystal p-type OFETs can be negative or positive, depending on whether the gate insulator is charged with holes or electrons, respectively, and it is zero in pristine devices that were not subjected to any gate bias stressing (under illumination or in the dark). The screening or enhancement of the gate electric field in the transistor channel leads to a positive (electron transfer) or negative (hole transfer) shift of the p-type OFET’s onset and correspondingly to an increase or a decrease of the source−drain current, ISD, in realtime measurements at fixed gate, VG, and source−drain, VSD, voltages under illumination. The source−drain current of an operating transistor at fixed VSD and VG in the linear regime can be expressed as
Figure 1. Demonstration of the polarization-dependent photoinduced bias-stress effect in OFETs. ISD(t) of a rubrene OFET recorded at VSD = 1 V and VG = −50 V, while the device is illuminated with a monochromatic (λ = 460 nm) linearly polarized light. Illumination intensity is 0.8 mW·cm−2. The thickness of parylene gate dielectric is 1.63 μm, corresponding to the capacitance Ci = 1.44 nF·cm−2. The channel length/width (L/W) = 1.8. The light polarization is intermittently varied between the crystal’s a and b axes by rotating the polarizer (inset). It is clear that the bias-stress rate, dISD/dt, under illumination depends on the polarization angle of light with respect to the crystal axes. The energetic structure of the semiconductor− insulator interface is schematically shown in the inset (note: the case of electron transfer is shown for convenience, whereas the data actually correspond to the hole transfer). Whereas hot photocarriers (red dots) can cross the interface above the barrier and occupy the localized states available in the insulator, thermalized (dark) band-edge carriers (black dots) remain in the semiconductor.
ISD =
smooth emission spectrum in the visible range or a blue (460 nm) LED as the light sources.45 The emission of the lamp passed through a band-pass filter, selecting an excitation wavelength of 500 nm, close to the maxima of absorption in TPBIQ and rubrene. A linear polarizer was then used to polarize the light. Thus, the devices were illuminated with a monochromatic, linearly polarized light. The bandwidth of the band-pass filters was Δλ = 10 nm. The calibrated angular scale of the
W μC iVSD(VG − Vonset) L
(1)
In this expression, μ is the longitudinal FET carrier mobility, Ci is the gate-channel capacitance per unit area, and the onset voltage, Vonset(t), is changing with time due to the charge transfer to the gate insulator: Vonset(t) = en(t)/Ci, where n(t) is the projected (areal) density of the transferred charge (not to 34155
DOI: 10.1021/acsami.7b11134 ACS Appl. Mater. Interfaces 2017, 9, 34153−34161
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ACS Applied Materials & Interfaces be confused with the density of mobile polarons in the accumulation channel, n2D = e−1Ci(VG − Vonset)), and e is the elementary charge. Therefore, the charge transfer rate, dn/dt, can be expressed through the measured decay rate of the source−drain current, dISD/dt, as ⎛ L ⎞ dISD dn = −⎜ ⎟ dt ⎝ eμWVSD ⎠ dt
(2)
An example of such a measurement under a linearly polarized photoexcitation normally incident on a rubrene OFET is shown in Figure 1. The polarizer is set intermittently along the a or b axis of the crystal, whereas ISD(t) at fixed VSD and VG is recorded. The overall decrease of the source−drain current with time is due to the transfer of holes to parylene that occurs in this measurement at VG < 0 under illumination. We note that performing these measurements in the linear regime of OFET’s operation simplifies data analysis because of a nearly constant transverse (gate) electric field and a uniformly distributed (that is, without a pinch-off) longitudinal potential drop along the channel (because |VSD| ≪ |VG|). The key observation in this figure is that the rate of the current decay, dISD/dt, systematically depends on the orientation of light polarization with respect to the crystal’s axes. This seems to be related to the fact that the absorption coefficient of rubrene crystals, α, is a function of polarization angle, θ. In rubrene, the strongest absorption of light normally incident on the (a, b) facet occurs for polarization along the b axis of the crystal (the high-mobility axis of rubrene) and the weakest along the a axis. In a similar manner, one can obtain a full 360°-range angular dependence of the photoinduced bias-stress (or chargetransfer) rate with a finer angular resolution. The results of such measurements for TPBIQ and rubrene OFETs are summarized in Figure 2, showing the dependence of the charge-transfer rate, dn(θ)/dt, on the angle θ between the light polarization and the c axis of TPBIQ or the b axis of rubrene (angle θ is defined in the insets). These axes correspond to the direction of natural elongation of these crystals. The molecular packing motifs of the illuminated facets of the crystals, corresponding to the accumulation channel in our OFETs, are also schematically shown in the insets. Note that dn/dt is measured purely electrically, from ISD(t) following eq 2. Measurements in Figure 2 show that the photoinduced biasstress rate is a periodic function of polarization angle, with a periodicity of π, and the maxima and minima in the chargetransfer rate, dn(θ)/dt, correspond to the maxima and minima of the absorption coefficient, α(θ), of the corresponding crystals. Indeed, for instance, in rubrene, α is a periodic function of polarization angle θ, with a periodicity of π, for light normally incident on the (a, b) facet of the crystal,5 which is due to the fact that molecular packing is invariant over rotation by π around the c axis (inset in Figure 2b).46 The crystal structure of rubrene is orthorhombic,46 with molecules of the (a, b) facet forming two complementary slip-stacks, both along the b axis of the crystal (the high-mobility axis of rubrene), but with the neighboring stacks tilted with respect to this axis by θR = ±31.2° (in the opposite directions), thus forming a herringbone structure (inset in Figure 2b). TPBIQ crystallizes in a monoclinic structure with molecules stacked along the c axis of the crystal.43 This axis corresponds to the direction of natural elongation of the crystals, along which the source−drain accumulation channel is oriented in our TPBIQ OFETs. The (a, c) facet of TPBIQ, used in our OFETs, is composed of a
Figure 2. Dependence of the photoinduced charge-transfer rate, dn/dt, on the polarization angle of incident light (λ = 500 nm for TPBIQ and 460 nm for rubrene) in single-crystal OFETs. The rate is calculated from the measured ISD(t) dynamics under illumination according to eq 2 and plotted for (a) TPBIQ and (b) rubrene OFETs. Polarization angle θ of the linearly polarized light relative to the crystals’ axis of elongation (c axis in TPBIQ and b axis in rubrene) is varied in real time, as ISD(t) of the operating OFETs is recorded. The insets schematically show the molecular packing in TPBIQ and rubrene crystals for the facets, on which the FET structures are fabricated. Molecular axes L, M, and N are also shown for rubrene. It is clear that the bias-stress rate in both cases is a periodic function of polarization angle with a periodicity of π, and the maxima (minima) correspond to the maxima (minima) of the absorption coefficient of the crystals. The overall decay of the rates and oscillation amplitudes is due to the fact that both dark and illumination-induced bias-stress rates diminish with measurement time at fixed VG.
single stack, in which all of the molecules are tilted relative to the c axis by θT = 32°, as schematically shown in the inset in Figure 2a. Thus, optical absorption of TPBIQ reaches maxima for light polarized along the long axes of the molecules (that is, at about θ = 30° off the c axis of the crystal). These differences in α are directly related to the different molecular packing of these systems. Assuming that absorption of linearly polarized light normally incident at the crystal’s facet is dominated by the dipole moment of the molecules along their long molecular axis (in rubrene, the so-called L axis), although the absorption along the normal axis (N axis) is negligible, we can treat the situation as if each molecule absorbed only the component of the electric field of the incident electromagnetic wave parallel to its core backbone, EL. All three molecular axes of rubrene (L, N, and M) are shown in the inset in Figure 2b: it is worth noting that 34156
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Therefore, the modulation contrast, η, of the bias-stress rate with polarization angle is
absorption of light polarized along the M axis is, in fact, the strongest in rubrene.47,48 This transition dipole however is inaccessible when light is incident normally on the (a, b) facet of the crystal but can be activated by changing the angle of incidence (see below). For polarization in the (a, b) facet of rubrene (normal incidence), the absorption coefficient (α), proportional to the square of the electric field component along the molecular core, α ∝ EL2, would thus be composed of the two parts, corresponding to the two complementary molecular stacks (tilted at ±31.2°)
η≡
= 1 + (1/2) cos(2θ − 2θR ) + (1/2) cos(2θ + 2θR ) (3)
In TPBIQ, where only one type of stack is present α(θ ) ∝ cos2(θ − θT) = 1/2 + (1/2) cos(2θ − 2θT) (4)
Angles θR and θT are the tilt angles described above. Both functions 3 and 4 are periodic functions with a period of π. In rubrene, maxima occur at θ = nπ, where n is an integer, whereas in TPBIQ, maxima occur at θ = θT + nπ. We emphasize that eqs 3−4 are good only for evaluating the angular positions of the extrema in α(θ) dependence and they do not correctly represent α at an arbitrary angle (see Figure S1, Supporting Information, for more details). Figure 2 shows that indeed the photoinduced charge-transfer rate in OFETs “parallels” the angular dependence of absorption coefficient α(θ) given by eqs 3 and 4 (see also Figures S2−S5 of the Supporting Information). We can thus conclude that this effect is governed by the light absorption in the crystals. The data in Figures 1 and 2 also reveal that the modulation contrast of the bias-stress rate with polarization is quite substantial: dn(θ)/dt changes with θ by more than 50% both in TPBIQ and rubrene. This observation suggests that only a very thin layer of organic semiconductor adjacent to the interface (a charge-transfer layer) contributes to the effect. Here, we introduce the quantum efficiency, χ0, of the generation of species of interest (that is, the species responsible for the charge transfer at the interface). Most likely these species are hot photocarriers that can reach the interface via diffusion before being thermalized. The characteristic thermalization length, l, of these carriers will be an important parameter in our consideration. It defines the depth scale of the photoexcited crystal that contributes to the observed effect. The probability for such a carrier, generated at a depth z below the interface, to reach the interface is exp(−z/l). According to the Beer− Lambert law of the exponential decay of light intensity in a medium, the portion of the photon flux absorbed within a layer of the crystal from z to z + dz is dΦ = α·Φ0 exp(−αz) dz, where Φ0 is the incident photon flux (z = 0 corresponds to the crystal/dielectric interface). Hence, the number of hot photocarriers reaching the interface before thermalization (and thus the rate of photoinduced bias-stress effect) must be proportional to an integral of α·Φ0 exp(−αz) exp(−z/l) dz from z = 0 to ∞ (because the crystal thickness ≫α−1, we consider that the upper integration limit can be ∞) dISD ∝ n′(α) = χ0 Φ0α dt αl = χ0 Φ0 · αl + 1
∫0
(6)
where αa and αb are the absorption coefficients for light polarized along the a and b axes of rubrene. If the photocarrier thermalization length is much shorter than the light penetration length, l ≪ α−1, the modulation contrast given by eq 6 reduces to (αb − αa)/αb, that is, η approaches the modulation contrast of absorption coefficient α. In the opposite limit of a very long thermalization length, l ≫ α−1, the modulation contrast becomes negligible, η ≈ 0, corresponding to the experimental situation in which we would not see any difference in the biasstress rate while rotating the polarizer. We now compare the modulation contrast of the photoinduced bias-stress rate, dISD(θ)/dt, in rubrene OFETs (Figures 1 and 2) to that of absorption coefficient α(θ) of the crystal. For light normally incident on the (a, b) facet, the absorption spectra of rubrene for the two orthogonal polarizations, along b and a axes, can be found in ref 5. For photoexcitation with a green (500 nm) light used here, the modulation contrast of α is about 41% (the corresponding light penetration lengths are αb−1 = 1 and αa−1 = 1.7 μm). Figure 1 shows that the measured modulation contrast of rate dISD(θ)/dt is about 50%, which is close to the modulation of α. Here, it must be noted that oscillations of the bias-stress rate with θ are gradually diminishing, as θ is varied, simply because a negative gate voltage (VG < 0) is constantly applied under a continuous-wave illumination during this measurement, and a substantial charge is being transferred to the gate insulator, as the experiment proceeds, causing the bias-stress rate to gradually decrease with time. This effect can be seen in Figure 2, where the overall rates and the oscillation amplitude are gradually diminishing with θ (see also measurements of other devices in Figures S2−S5 of Supporting Information). For this reason, the contrast of the bias-stress rate oscillations, determined in such a measurement, might vary depending on the sample’s stressing history. Nevertheless, we can see that the contrast of oscillations of dISD(θ)/dt is comparable to the contrast in α(θ), which suggests that the length scale relevant to this effect is rather small (l ≪ α−1). This observation is consistent with nanoscale thermalization lengths typical of photocarriers in organic semiconductors.49 This is also consistent with the observed one-to-one correspondence between the maxima and minima in the bias-stress rate and those in α (Figure 2). Indeed, according to eq 5, when the carrier thermalization length is very short (l ≪ α−1), the photoinduced bias-stress rate becomes proportional to α(θ): −dISD/dt ∝ χ0Φ0l·α(θ). To further validate our model, we look at the experimental arrangement that allows us to optically excite the transition dipole moment along the M molecular axis of rubrene (corresponding to the crystal’s c axis), which has a stronger absorption coefficient than that of L and N axes.47,48 This dipole is perpendicular to the (a, b) facet of the crystal and cannot be excited by light normally incident on the (a, b) facet. However, as shown in Figure 3, if one uses an a-polarized light, initially normally incident at the (a, b) facet of rubrene, and then increases the angle of incidence, φ, in the (a, c) plane of incidence, this will produce an increasing component of the electric field along the M axis of the molecules (c axis of the crystal): Ec = E0 sin(φ), where E0 is the electric field amplitude
α(θ ) ∼ cos2(θ − θR ) + cos2(θ + θR )
−
α (α l + 1) n′(b) − n′(a) = 1 − a· b n′(b) αb (αal + 1)
∞
exp(−αz) exp(−z /l)dz (5) 34157
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changes in the electric field components, the effective density of the (a, b) molecular planes passed by the refracted beam in the crystal is reduced by a factor of cos(φ′), thus leading to an additional factor (1 − ε−1 sin2(φ))1/2 on the right-hand side of eq 7. The obtained effective absorption coefficient αp‑wave given by eq 7 is an increasing function of the angle of incidence, φ, in the entire range of experimentally accessible angles, 0 ≤ φ < π/ 2, as long as the absorption coefficient of c-polarized light is greater than that of a-polarized light, αc > αa. According to the careful spectroscopic measurements and calculations by Irkhin et al.,48 this is the case in rubrene in the wide range of excitation wavelengths. The situation is qualitatively different when a b-polarized light (s-wave) is used in this experiment. The M molecular dipole (c-axis absorption) should not be excited at all (for any φ) and thus the effective absorption coefficient will be a decreasing function of φ in the entire experimental range, 0 ≤ φ < π/2 αs ‐ wave(φ) = αb· 1 − ε−1 sin 2(φ)
Figure 3 shows measurements of the photoinduced biasstress rate, dISD/dt, performed in these two modes in a rubrene single-crystal OFET (symbols). It can be seen that the data are qualitatively well-described by eqs 7 and 8 with ε = 2.94, αc/αa = 8.1, and αb/αa = 1.26 (dotted lines). These ratios of the absorption coefficients are within qualitative agreement with the report of Irkhin et al., where αc/αa = 5 and αb/αa = 1.5 were determined via optical spectroscopy.48 Note that the large increase of dn/dt for a p-wave excitation (blue squares) cannot be ascribed to the suppression of p-wave reflection (increased transmission) due to φ approaching the Brewster angle (the Brewster angle for rubrene is φB ≈ 66°). Indeed, according to the Fresnel equations, the reflection coefficient of a p-wave is not greater than 5% at any angle in the range 0 < φ < φB. Thus, the reflection losses cannot account for the effects shown in Figure 3. We emphasize that this method yields only approximate ratios of optical constants αb/αa because of several possible sources of error, including the dependence of photoinduced bias-stress rate on the OFET’s stressing history, measurement sequence, and duration at each polarization, as well as contribution from dark bias stress. These measurements show that the photoinduced bias-stress effect in OFETs parallels the polarization and angular dependence of the absorption coefficient of the crystal, α(θ, φ), thus confirming the relevance of a thin charge-transfer region, with the thickness l ≪ α−1 ∼ μm, that occurs right next to the semiconductor/insulator interface under photoexcitation. Hot, nonthermalized photocarriers transferred across the interface to the gate dielectric are thus essential for the explanation of the observed photoinduced instabilities and parameter drift in OFETs. On the applied side, by illuminating OFETs through an optical shadow mask, while VG is applied, one can create laterally patterned accumulation channels in the semiconductor with optically defined shapes corresponding to the area of the channel exposed to light. Such an optical recording can be used for creating reconfigurable conducting patterns of arbitrary complexity (with a spatial resolution limited, in principle, only by diffraction) that can be optoelectronically erased and reprogrammed in the same device by using various optical masks or other optical means, including interference patterns or a microscopic image projection.
Figure 3. Photoinduced charge-transfer rate, dn(φ)/dt, measured in a single-crystal rubrene OFET as a function of the angle of incidence, φ, of a linearly polarized monochromatic light (λ = 460 nm). The rate is determined from the decay dynamics of the source−drain current, dISD/dt, at VG = −50 V and VSD = 1 V under illumination (eq 2). The cartoon shows the experimental arrangement for two types of polarization (the FET structure is omitted for clarity): light is incident through the transparent ITO gate on the (a, b) facet of rubrene in the (a, c) plane of incidence. The blue squares correspond to a p-wave excitation with light polarized along the a axis of the crystal (at φ = 0). The red circles correspond to an s-wave excitation with light polarized along the b axis of the crystal (at any φ). The dotted lines are the fits with eqs 7 and 8 with ε = 2.94, αc/αa = 8.1, and αb/αa = 1.26.
of the incident wave. This should result in an increase in the effective absorption of this p-wave photoexcitation for φ > 0. Of course, one also needs to take into account that increasing φ will lead to a decreasing component of the electric field along the a axis of the crystal, Ea = E0 cos(φ), whereas the b axis component remains zero, Eb = 0. Calculations done in this manner lead to the following expression for the total absorption coefficient for such a p-wave in the (a, c) plane of incidence illuminating the (a, b) facet of rubrene (this arrangement is shown in Figure 3) ⎡ ⎤ ⎛α ⎞ αp ‐ wave(φ) = αa⎢1 + ε−1·⎜ c − 1⎟ sin 2(φ)⎥ · ⎢⎣ ⎥⎦ ⎝ αa ⎠ 1 − ε−1 sin 2(φ)
(8)
(7)
In this expression, we took into account the fact that for the calculation of the above electric field components, Ea, Eb, and Ec, in the crystal, one has to use the refraction angle, φ′, instead of the angle of incidence, φ, and that these angles are related via Snell’s law: sin(φ) = ε1/2 sin(φ′), where ε is the dielectric permittivity of rubrene. In addition, we also took into account that with an increasing angle of incidence φ, irrespective of the 34158
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Figure 4. Demonstration of optically recorded, laterally patterned accumulation layers in OFETs. (a) Schematics of the optical recording process: OFET with a transparent ITO top gate is illuminated through a narrow slit, while VG = +50 V is applied. Electrons transferred and trapped in the parylene gate insulator lead to an electrostatic accumulation of mobile holes in the semiconductor in the dark (at VG = 0), occurring in the exposed areas, thus creating a laterally patterned conducting channel. The shadow mask (a 150 μm wide slit) can be moved along the gate. Multiple conducting “lines”, about 0.4 mm apart, were sequentially recorded. (b) ISD(VSD) characteristics recorded at VG = 0 in the dark after each conducting line was added (numbered N = 0−6). (c) Conductance σ = (L/W)(dISD/dVSD) calculated at VSD = 0 (left panel), and −ISD at VSD = −50 V (right panel), both plotted as a function of N.
shows a linear dependence on the number of programmed lines (Figure 4c). This demonstrates an optical recording of reconfigurable conducting channels in OFETs. We envision that in the future this method can be used for various purposes, including studies of unconventional OFET channel geometries (very long or narrow structures, meanders, channels with microconstrictions, etc.), avoiding (going around) defect sites, or even studies of mobility anisotropy, all performed in a rewritable manner using optical masks.
Here, to demonstrate such an optically recorded, laterally patterned accumulation channel, we “drew” discrete narrow conducting lines, connecting the source and drain contacts in a rubrene OFET (the device structure and FET characteristics are shown in the Supporting Information, Figures S6−S8). The patterning was achieved by illuminating the device with white light through a narrow 150 μm wide slit (Figure 4a). The slit, which was suspended above the FET, could be moved along the surface of the ITO gate, so that multiple conducting lines could be recorded at desired locations. To prepare the FET for recording, the whole channel area has been first exposed to white light at VG = −50 V for 10 min, resulting in transfer of holes to the insulator, thus driving the device to a complete depletion (Supporting Information, Figure S9). After this, the device has been illuminated through the slit at VG = +50 V for 5 min (at each location of the slit), which resulted in an electron transfer to the dielectric (occurring with a greater quantum efficiency than that of holes27) and the formation of the corresponding p-type accumulation channel in the exposed area. After adding each such conducting line, the conductivity of the device was measured in the dark at VG = 0 by recording ISD(VSD) characteristics (Figure 4b). In total, we have sequentially recorded six parallel lines (in 25 min time intervals), placed about 0.4 mm apart, all connecting the source−drain contacts. The conductivity of the device measured in the dark at VG = 0 after recording each line
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CONCLUSIONS In conclusion, we report on a polarization-dependent photoinduced bias-stress effect in OFETs that occurs due to a charge transfer of nonequilibrium (hot) photogenerated charges from the semiconductor to the gate insulator. Only a thin layer of the semiconductor (l ≪ α−1) next to the interface contributes to the effect, leading to the rate of the source−drain current decay being proportional to the anisotropic absorption coefficient of the crystals, α(θ, φ). The observed effect and its interpretation provide a deeper understanding of the microscopic processes leading to operational instabilities in organic electronic devices, which might be important for such practical applications as transparent and flexible electronics, as well as OLED displays, where transistor switches might be exposed to illumination from external or internal sources. The demonstration of 34159
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optically recorded, laterally patterned conducting channels, on the other hand, can lead to potential applications in optically programmable OFET memories.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b11134. Modeling of the angular dependence of the absorption coefficient in organic crystals, basic OFET characterization, and additional measurements of the polarizationdependent bias-stress rate in OFETs (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kilwon Cho: 0000-0003-0321-3629 Alejandro L. Briseno: 0000-0003-2981-9143 Vitaly Podzorov: 0000-0001-8276-882X Present Address #
Coherent Advanced Crystal Group, 31 Farinella Drive, East Hanover, New Jersey 07936, United States (H.N.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Eilaf Ahmed, Samson Jenekhe, and Senia Katalinic for their technical assistance in this experiment. The authors are grateful to the National Science Foundation for financial support of this work under the grants DMR-1506609 and DMR-1508627, as well to the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST “MISiS” (No. K3-2016004), implemented by a governmental decree dated March 16, 2013, N 211.
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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on September 21, 2017, with an error in equation 5. The corrected version was reposted on September 22, 2017.
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