Photoswitching of an n-Type Organic Field Effect Transistor by a

ARTICLE pubs.acs.org/JPCC

Photoswitching of an n-Type Organic Field Effect Transistor by a Reversible Photochromic Reaction in the Dielectric Film Petro Lutsyk,* Krzysztof Janus, and Juliusz Sworakowski Institute of Physical and Theoretical Chemistry, Wroczaw University of Technology, Wyb. S. Wyspianskiego 27, 50-370 Wroczaw, Poland

Gianluca Generali, Raffaella Capelli, and Michele Muccini Institute for Nanostructured Materials (ISMN), CNR Division in Bologna, via P. Gobetti 101, I-40129 Bologna, Italy ABSTRACT: We report on an organic field effect transistor (OFET) with a photochromic dielectric layer, operating as an opto-electrical switch device. The structure contained a photochromic material dissolved in the polymer dielectric layer. The photochromic material was spiropyran exhibiting a large difference of the dipole moments of the stable and metastable forms; poly(methyl methacrylate) was a polymeric insulator; and an n-type perylene derivative was used as the organic semiconductor. Illumination of the structure with UV light resulted in a reversible increase of the source-drain current, accompanied by a reversible decrease of the threshold voltage. The initial parameters were restored by a thermal relaxation in the dark or by illumination with visible light. The photoswitching ratio was found to be dependent on the gate voltage ranging between ca. 2 just above the threshold voltage and ca. 1.3 at the highest voltage employed (90 V). The switching has been attributed to reversible changes of dielectric properties of OFET's insulator (dielectric layer) due to a reversible light-triggered reaction of polar photochromic species, dissolved in the bulk of the dielectric layer. The contribution of dipoles aggregated on the semiconductordielectric interface was estimated to be negligible at gate voltages exceeding ca. 10 V.

’ INTRODUCTION All-optical or opto-electric molecular switches performing by virtue of a photochromic reaction have been of keen interest for the past decades due to fast emerging applications of organic materials in electronics.1-4 The switch may be used as a logic or memory element in future optical computer chips and other circuits. One of the most challenging ideas has been the development of opto-electrical switches or photoswitchable transistors, where the current can be controlled by illumination. The use of photochromic bistable molecules may allow one to control the current by illumination with light, activating a reversible photochemical reaction. The light of a given wavelength triggers the reaction, hence changing the current, while an exposure of the device to radiation of a different wavelength results in a reverse reaction, resulting in the system return to its initial state. Suitably selected thermally stable photochromic molecules are able to remain in their metastable form for a sufficiently long time, hence the current does not drop down after turning off the illumination. The performance of the switch is determined by the ratio of highcurrent (on) and low-current (off) states.2-4 One may envisage various architectures of opto-electrical switches based on reversible photochromic reactions, one of the options making use of systems with large changes of the dipole moment of their components.5,6 The photochromic r 2011 American Chemical Society

materials, dissolved in an organic semiconductor matrix, may reversibly modify the mobility of charge carriers due to formation and annihilation of charge carrier traps in the vicinity of highly polar molecules of the photochromic system. To form traps sufficiently deep for efficient trapping of charge carriers (deeper than ca. 0.5 eV), the dipole moment should be higher than ca. 10 D.5-7 According to quantum-chemical calculations, the onoff ratio of the dipolar photochromic switch may be over 5 orders of magnitude.6,8 However, experimental performance of the photoswitching has been much lower: the on-off ratio has been ca. 1 order of magnitude at most.9-12 Another possibility is a reversible formation and annihilation of chemical traps due to differences of ionization energies (in the case of hole traps) or electron affinities (electron traps) between the two forms of the photochromic system.6 The concept of using a polar photochromic material as an element of field effect transistor (FET) was put forward in earlier papers.13,14 The photochromic material was deposited on the surface of an organic semiconductor or dissolved in its bulk affecting the current in the FET’s channel. The modulation of the current has been explained by formation of dipolar traps for Received: September 20, 2010 Revised: December 6, 2010 Published: January 28, 2011 3106

dx.doi.org/10.1021/jp108982x | J. Phys. Chem. C 2011, 115, 3106–3114

The Journal of Physical Chemistry C

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Figure 1. Schematic architecture of an OFET. The acronyms in the parentheses refer to the names of materials used in the OFET employed in the present study. The chemical formulas of the materials employed in the present study and the scheme of the SP-MR photochromic reaction are shown on the right side of the figure.

In this work, we present results of measurements of lightdriven electrical switching in an organic FET built of an n-type organic semiconductor and a photoactive gate insulator consisting of a photochromic material dissolved in a polymer (Figure 1). We will qualitatively estimate contributions to the total effect of photoswitchable dipoles aggregated on the semiconductorinsulator interface and of those dispersed in the bulk of the insulator.

Figure 2. Absorption spectra of PMMA (1), PMMA þ SP (10 wt %) (2), and PMMA þ MR (10 wt %) (3) films. Thicknesses of the films were ca. 630 nm.

charge carriers13 or by a modification of the contribution to the gate field by the dipoles.14 Shen et al.14 studied structures in which a photochromic spiropyran (SP) derivative was deposited over the semiconductor surface. According to ref 14, SP molecules deposited on the pentacene (p-type) surface in FETs increased the source-drain current after transformation of SP into metastable photomerocyanine (MR) and decreased it after MR returned back to SP, whereas an opposite effect was observed for fluorinated copper phthalocyanine (n-type): the current decreased upon transformation of SP into MR. The effect was explained with creation of a local negative gate voltage leading to an increase of electrical conductivity in p-type FETs and to a decrease of the current in n-type devices.14 A similar phenomenon was also observed in single-walled carbon nanotubes with photochromic spiropyran molecules assembled on the CNT surface.15 Also in this case, the switching was attributed to the reversible SP-MR chemical reaction. Recently, Shen et al.16 reported on reversible switching in an OFET in which p-type pentacene was employed as the semiconductor and PMMA doped with SP was the gate insulator (cSP was ca. 0.07 M, i.e., ca. 2 wt %). Upon UV illumination, the drain current increased by a factor of ca. 2; subsequent illumination with visible light resulted in a return to the initial state. The effect was attributed to changes of electric permittivity of the gate insulator resulting from the SP-MR reversible photochromic reaction. The authors reported also on a similar effect observed in an n-type organic semiconductor perfluorinated copper phthalocyanine.16

’ EXPERIMENTAL DETAILS Photochromic systems with a dramatic change of the dipole moment and that are photochemically stable and highly reversible (i.e., capable of undergoing many switching cycles) are scarce. Spiropyrans (SP), and among them10 ,30 -dihydro10 ,30 ,30 trimethyl6-nitrospiro[2H-1-benzopyran-2,2 112 0-(2H)-indole], are frequently studied photochromic materials. Under UV illumination, SP with a dipole moment of 5.4 D transforms into metastable colored photomerocyanine (MR) with a dipole moment about 11 D or even higher6 (Figure 1). Poly(methyl-methacrylate) (PMMA) is a suitable dielectric material for OFETs: PMMA films are transparent for visible light (Figure 2), and form good matrices for photochromic molecules allowing fast SP-MR photochemical transformation. A solution of SP in PMMA was employed as the dielectric layer in the FETs under study. Occasionally, we also used neat PMMA for reference. n-type organic semiconductor17-19 N,N0 -ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13H27, hereafter referred to as P13, Figure 1) was used as the semiconductor. The films of neat PMMA (Allresist) or of 10 wt % of SP (Aldrich) in PMMA were formed by spin coating of 6 wt % solutions in ethyl lactate, at 6000 rpm. The films were thermally annealed in a glovebox at 120 °C (near the glass transition temperature of PMMA) for 15 h in N2 atmosphere. Annealing of PMMA with SP films at that temperature did not affect the ability of reversible photochromic transformation of SP to MR. The thickness of the films was 450 nm. The surfaces of the films were very smooth with rms roughness less than 1 nm. P13 from Fluka was thermally deposited on the dielectric film at a base pressure (3-5) 10-6 mbar, with the growth rate 0.01 nm/s, at the sublimation temperature 300 °C. The thickness of P13 was ca. 10 nm. The source and drain electrodes were made of gold sublimed with a growth rate of 0.1 nm/s. The thickness of gold was 50 nm. The devices were fabricated in a top-electrode configuration (Figure 1) with the channel length, L, and width, W, amounting to 70 μm and 1.5 cm, respectively. The bottom ITO layer was used as the gate electrode. The insulator capacitance, Cins (capacitance per unit area between the gate and 3107

dx.doi.org/10.1021/jp108982x |J. Phys. Chem. C 2011, 115, 3106–3114


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Figure 3. (a) Output current-voltage characteristics for (PMMA þ SP)(10 wt %)/P13 FET. The characteristics are plotted in the double-linear coordinates with UG displayed on the right margin. (b) Expanded section of (a) for UG = 90 V showing the switching cycle. (1) Virgin sample; (2) sample irradiated with UV for 1 min; (3) relaxed for 16 h at ambient temperature; (4) irradiated with UV for 2 min.

source-drain electrodes), determined for the neat PMMA film was 7 nF/cm2. The current-voltage characteristics of (PMMA þ SP)/P13 and PMMA/P13 FETs were measured with an Agilent B1500A Semiconductor Analyzer connected to a Suss PM5 Professional Probe Station, keeping the samples in a glovebox (N2 atmosphere). An IUV250 (Adhere) lamp with maximum emission at 375 nm was used for illumination of the FETs.

’ RESULTS The current-voltage characteristics of FETs were measured for gate voltages (UG) ranging from 0 to 90 V (Figure 3). The FET on-off ratio of the source-drain current (ISD) in the saturation regime was over 106. The mobility of charge carriers (μFET), threshold voltage (UT), on-off ratio of the FETs, as well as the photoswitching ratio have been calculated from the current-voltage characteristics. According to the conventional FET theory, ISD obeys the following dependencies WCins USD UG -UT ISD ¼ μFET ð1Þ USD L 2 ISD ¼ μFET

WCins ðUG -UT Þ2 2L

ð2Þ

where USD stands for source-drain voltage. Equations 1 and 2 apply to the linear section and to the saturation regime of the current-voltage characteristics, respectively. μFET and UT were determined in the saturation regime using transfer (USD = const.) and locus (USD = UG) characteristics. In the latter one, ISD is not limited by the contact resistance at the metal/organic interfaces. Although the results thus obtained were similar, the locus characteristics were exploited to determine μFET and UT. The parameters used to calculate the mobilities were the slopes of the (ISD)1/2 vs UG dependencies, in the regime of ISD saturation, β = [(∂(ISD1/2))/(∂UG)]UG>UT. As follows from eq 2 μFET ¼

2L 2 β WCins

ð3Þ

Upon UV illumination, the source-drain current for (PMMA þ SP)/P13 FETs increased (Figure 3, curves 1 and 2), apparently

Figure 4. Output current-voltage characteristics for neat PMMA/P13 FET. (1) Virgin sample; (2) after UV irradiation.

due to the conversion of SP to MR. Metastable MR can be reconverted into the stable SP form by illumination with visible light, but it can also return to its initial form by a slow thermal relaxation in the darkness. It is important to note that, in the present paper, we do not report on the kinetic parameters of photoswitching. Thus, although the reverse photoreaction is much quicker than the thermal one, we used the thermal relaxation as it allowed us to avoid an influence of light on charge carrier transport properties of P13. The relaxation at ambient temperature for 16 h was sufficient for transforming MR back into SP and for turning back the source-drain current in (PMMA þ SP)/P13 FETs to its initial value (Figure 3, curve 3). The process of photoswitching in the system under study was reversible (Figure 3, curve 4). To make sure whether the effect of photoswitching originated from the photochromic SP-MR reaction in the PMMA matrix, current-voltage characteristics of neat PMMA/P13 FETs were measured before and after UV illumination. As can be seen in Figure 4, the preillumination and postillumination currentvoltage characteristics for PMMA/P13 FETs are nearly identical, thus confirming the photochromic origin of the switching of the current in (PMMA þ SP)/P13 FETs. A family of the locus current-voltage characteristics measured in a (PMMA þ SP)/P13 FET are shown in Figure 5, in the 3108



Figure 5. Locus (USD = UG) current-voltage characteristics for PMMA/P13 (curve 1) and (PMMA þ SP)(10 wt %)/P13 FETs (curves 2-6). (a) The data plotted in the coordinates resulting from eq 2. Bottom and top curves were obtained at increasing and decreasing voltages, respectively. The broken lines show linear fits to the experimental curves above the threshold voltages. (b) The subthreshold sections of the curves plotted in the semilog scale (only the characteristics measured with increasing voltages are presented). The broken lines show linear fits to the sections below the threshold voltages with constant subthreshold swings. (2) Virgin sample; (3) after UV irradiation for 1 min; (4) relaxed for 16 h, (5) after UV irradiation for 2 min; (6) relaxed for 16 h. The curves 2-6 have been vertically displaced for the sake of clarity.

coordinates resulting from eq 2 (Figure 5a) and in the semilog scale showing the subthreshold region (Figure 5b). The threshold voltage, UT, for the (PMMA þ SP)/P13 FET was 34 V, and the UV illumination (and the resulting conversion of SP into MR) shifted UT to 29 V. The thermal relaxation (resulting in the reverse MR f SP reaction) restored the previous level of UT (35 V).20 These changes can be attributed to the modulation of the capacitance of the insulator as will be discussed in the next section. The threshold voltage in the reference system, containing neat PMMA as the insulator, amounted to 22 V.

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Figure 6. (a) Gate voltage dependence of the subthreshold swing. The voltage range covered by the figure corresponds to the low voltage section in Figure 6b. (b) Gate voltage dependence of the photoswitching ratio. The parameter has been defined as the ratio of the currents measured in the UV irradiated sample and in the thermally relaxed sample. The data represented by open symbols have been taken from the locus characteristics and those represented by filled symbols from the output characteristics.

Current-voltage dependencies in the subthreshold range of voltages are commonly characterized by the subthreshold swing: S = [(∂(log10 ISD))/(∂UG)]UG < UT-1. The latter parameter was found to be voltage dependent, being of the order of 1 V/decade at the lowest voltages and increasing to ca. 15 V/decade for UG > 15 V. The dependencies for two exemplary curves are shown in Figure 6a. It should be noted that at low voltages (below ca. 15 V) S reversibly changes upon illumination (cf. Table 1), whereas it remains practically unaffected by the illumination at higher voltages. The slope of the current-voltage characteristics in the voltage range above the threshold, β, (cf. eq 3) in the neat PMMA/P13 FETs amounted to 5.15 10-4 A1/2/V. Taking the literature value of εPMMA (εPMMA = 3.6 at 100 Hz),21,22 the latter value translates into the mobility equal to 0.35 cm2/(V 3 s), being on the same level as the values reported previously for P13 FETs on SiO2 substrates17,18 and 1 order of magnitude higher than those for P13 FETs on PMMA.19 Addition of SP into PMMA increased the slope by ca. 8%; the irradiation with UV light resulted in a further increase of the slope by ca. 2%. The latter change, small 3109



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Table 1. Threshold Voltages, Subthreshold Swings, Source-Drain Currents, Slopes of the Current-Voltage Characteristics above the Threshold Voltages, and FET Mobilities Calculated Assuming (i) a Constant Electric Permittivity and (ii) Permittivities Calculated from Equations 7 and 8, Employing the Procedure Described in the Appendixa FET mobility [(∂(ISD1/2))/ S@USD= UG = no.

dielectric layer

UT [V]

6V[V/dec]

ISD@USD =

(∂UG)]UG>UT

UG= 90V[mA] [10-4 A1/2/V]

FET mobility

calculated assuming electric permittivity

calculated taking

ε = const = 3.6

calculated from

ε calculated from

[cm2/(V 3 s)]

eqs 7 and 8

eqs 7 and 8 [cm2/(V 3 s)]

1

neat PMMA

22

2.8

1.13

5.15

0.35

3.6

0.35

2 3

virgin (PMMA þ SP) (PMMA þ SP) UV

34 29

1.9 4.6

1.04 1.27

5.56 5.67

0.41 0.43

4.1 5.4

0.36 0.29

4

(PMMA þ SP) thermally

34.5

3.6

1.03

5.56

0.41

4.1

0.36

29

5.9

1.35

5.68

0.43

5.4

0.29

35

3.5

1.03

5.57

0.41

4.1

0.36

irradiated for 1 min relaxed for 16 h 5

(PMMA þ SP) UV irradiated for 2 min

6

(PMMAþ SP) thermally relaxed for 16 h

a

The numbers refer to the characteristics shown in Figure 5.

but discernible, was found reversible: the slope returned to its preirradiation value after the thermal relaxation. The results are collected in Table 1. A negligibly small hysteresis of the ISD vs UG characteristics in the neat PMMA/P13 FETs was similar to that reported earlier.18 Addition of SP into PMMA slightly increased the hysteresis (Figure 5a), and UV illumination of (PMMA þ SP) films resulted in an additional small broadening of the hysteresis loop between ISD vs UG curves measured on increasing and decreasing voltage. The on-off ratio of the photoswitching, evaluated from the current-voltage characteristics of (PMMA þ SP)/P13 FETs, is shown in Figure 6b. The ratio was found to depend on voltage: at the lowest voltages, of the order of a few volts (i.e., below UT), the ratio was of the order of 10, decreasing to ca. 2 just above the threshold voltage. At the highest applied voltages, it amounted to 1.3.

’ DISCUSSION Our experiments confirmed that the switching described in the preceding section is associated with the presence of the highly polar photochromic molecules dissolved in the FET’s dielectric layer (PMMA) and with light-triggered changes of their dipole moment. The main results can be summarized as follows: (i) UV illumination of the structures containing SP in the dielectric layer resulted in an increase in source-drain currents due to conversion of SP into metastable MR. The ratio of the pre- and postillumination currents is dependent on the gate voltage: at the lowest voltages it is close to 1 order of magnitude, just above the threshold voltage it amounts to ca. 2, and at the highest voltages it equals 1.3. (ii) The reversible changes in the source-drain currents are accompanied by small changes in the slopes of the currentvoltage characteristics, β = [(∂(ISD1/2))/(∂UG)]UG UT. The increase of the subthreshold current was found dependent on the density and depth of the interface states.26 In our experiments, changes of the subthreshold current during reversible conversion of SP into MR can be explained by the transformation of shallow dipolar traps associated with SP molecules into deep interfacial states associated with MR molecules. One can thus conclude that, in the structures investigated in this paper, a contribution of interface dipoles to the modulation of source-drain current may be noticed at gate voltages below the threshold voltage. Our experiments show, however, that at high UG (for example, 90 V) the photoswitching on-off ratio was still as high as 1.3. Thus, one should look for another mechanism, operating at moderate and high fields. Let us now consider a possible mechanism involving the bulk dipoles, similar to that assumed in ref 16. The insulator in our FET structures contains the photochromic SP/MR system dissolved in PMMA. Upon illumination with UV, moderately polar SP (m = 5.4 D) converts into highly polar MR (m = 11 D). It is obvious that the process affects the electric permittivity (and hence the capacity) of the insulator, resulting in changes of the parameters characterizing the FET: threshold voltage, sourcedrain current, and mobility. The threshold voltage in a FET is related to the parameters of the semiconductor and insulator by the equation25,27 Qs UT ¼ Φs þ Cins

difference can be rationalized taking into account the effect of the interface states created due to the presence of the SP/MR system at the semiconductor/dielectric interface. Consequently, the value of Qs in (PMMA þ SP) would exceed that in PMMA. If the difference is high enough to compensate the simultaneous difference in the capacitance of the insulator (vide infra), one should observe UT in the (PMMA þ SP)/P13 FET higher than that in the reference structure as was indeed found in our experiments. Another feature that needs a comment are changes in the threshold voltage accompanying the photochromic reaction in the insulator. It should be stressed that the cycling of UT was measured in the experiment in which the only change was the SP f MR f SP reaction. One may thus assume that changes of both Φs and Qs are small, and the effect is controlled by changes in the insulator capacitance (i.e., by changes in the electric permittivity). Moreover, surface potential should typically be of the order of single volts, much less than the threshold voltages in our experiments. Thus the values of UT are primarily controlled by the second right-hand term in eq 6. Consequently, assuming that (i) Φs is small compared to the other right-hand term and (ii) Qs does not change much during the photochromic cycle, one obtains an approximate relation UT, SP -UT, MR εMR -εSP UT, MR εSP

where the subscripts “SP” and “MR” refer, respectively, to the insulator before and after UV illumination, i.e., PMMA containing mostly spiropyran or merocyanine. Thus the reversible changes in UT may be linked to changes of the electric permittivity associated with the photochromic reaction: an increase in ε should result in a decrease in UT. It follows from eqs 2 and 3 that, in the saturation region ISD ¼ β2 ðUG -UT Þ2

ð2aÞ

μFET1/2.

Thus the reversible changes of the with β µ ε source-drain current result from a modulation of the electric permittivity of the insulator and/or mobility of carriers in the semiconductor, associated with the photochromic reaction. These two effects cannot be decoupled unless ε is known from an independent source. At present, no experimental data are available on the electric permittivity of spiropyran-doped PMMA and its changes during the photochromic cycle. One may, however, estimate the changes employing the classical model of dielectrics. Our estimation will be based on the Onsager equation (ref 28, chap. 5). The model system mimicking the insulator consists of isotropic photochromic molecules dispersed in a nonpolar dielectric matrix built of isotropic molecules. The calculations (shown in the Appendix) lead to the following equation 1/2

ðε-1Þð2εþ1Þ ¼ AþxðBþDm2phot Þ ε

ð6Þ

where Φs is the surface potential of the semiconductor and Qs is a sum of the space charge in the semiconductor and a charge accumulated in the interface states. In our experiments, UT in the (PMMA þ SP)/P13 FET was significantly higher than that in the reference structure containing neat PMMA as insulator. This

ð7Þ

ð8Þ

where x is the mole fraction of the photochromic molecules and mphot stands for their dipole moment. A, B, and D are parameters independent of the concentration of polar molecules and their dipole moment whose values have been calculated in the Appendix. Expressing mphot in Debye units, we arrive at A = 5.9, B = 14.7, D = 5.6. 3111



Figure 7. Dependencies of relative electric permittivity of the dielectric layer on dipole moment of the photochromic component, calculated from eq 8 for x = 0.033 (nominal concentration of SP in PMMA in the samples used in measurements reported in this paper) and x = 0.006 (concentration of SP in PMMA reported in ref 16).

Relative changes of the electric permittivity, calculated for x = 0.033 (corresponding to the mass fraction of spiropyran equal to 0.1, used in our experiments), are shown in Figure 7. It follows from the figure that one should expect a 2-fold increase in the permittivity after a complete conversion of SP into MR. Similar calculations performed for x = 0.006 (the concentration used in the experiments reported in ref 16) yielded an expected increase by ca. 37% (the expected value obtained by Shen et al.16 from the molecular dynamics calculations is 28%). Thus, our simple model yields the result reasonably compared with the predictions of Shen et al.16 As expected, the electric permittivity of an insulator containing polar molecules exceeds that of the neat insulator and should increase with increasing dipole moment of the dopant and its concentration. It should be noted, however, that in both cases the predicted values significantly exceed the experimental ones. The capacitance of the gate insulator experimentally determined in ref 16 (PMMA with dissolved SP, the concentration of SP equal to ca. 0.07 M, i.e., to ca. 2 wt %) increased by ca. 2% upon illumination, over 1 order of magnitude less than the predicted value. Our model also overestimates the changes, and the results obtained from the model presented above can, however, be calibrated using the values of the threshold voltages obtained in our experiments on the (PMMA þ SP)/P13 sample and employing eq 7. The left-hand term, calculated from the data listed in Table 1, amounts to 0.19, whereas the relative change of the electric permittivity calculated from the results obtained in the model calculation (cf. Figure 7) is equal to 1.27. Thus the calibration factor obtained from the comparison of these two numbers amounts to 0.15. Such a low value of the calibration factor may result from several experimental factors such as incomplete conversion of spiropyran, crystallization of the solute, etc., not to mention simplifications introduced in the model. With the values so obtained, we assessed limits of the effect of changes of the electric permittivity on the current-voltage characteristics in the system under study and in particular on the mobilities of electrons. On one hand, one may assume that changes of the electric permittivity of the insulator upon incorporation of SP and its further conversion into MR are insignificant. On the other hand, one may take the values of ε based on the reasoning presented above. The apparent FET mobilities thus obtained are listed in Table 1. At this point, one should recall earlier papers on the dependence of the FET mobility on the electrical permittivity of the

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dielectric layer.29,30 Stassen et al.29 reported on the μFET µ ε-1 dependence observed in rubrene single crystals, whereas a stronger dependence may be deduced from the results reported by Veres et al.30 for amorphous poly(triarylamines). The latter authors attribute the dependence to changes in the width of the distribution of local states due to contribution of dipoles (for the discussion of the effect, see also refs 31-34). It should be noted that the μFET µ ε-1 dependence implies independence of the slope of the current-voltage characteristics, β, of the electric permittivity of the dielectric layer (cf. eq 2a). Our results, with changes of β of the order of a few percent, seem to agree with the relation established by Stassen et al.29 The paper does not contain information on kinetic parameters of photoswitching. This problem will be studied and reported in more detail in the near future. One may expect, however, that the rate of switching should be the same as the rate of the chemical reaction between the components of the photochromic system. Consequently, the rate should depend on the light intensity triggering the reaction and, under identical experimental conditions, should be similar to the rates reported in, e.g., refs 14-16.

’ FINAL REMARKS An organic opto-electrical device whose action is based on photochromic isomerization was described in the paper. The architecture of the FET under study was similar to that reported recently by Shen et al.16 In both cases, the devices contained an insulator with photochromic spiropyran exhibiting a dramatic change of the dipole moment. The current in studied FET increased after UV irradiation and merocyanine formation and returned to a lower value when metastable merocyanine was converted to the previous spiropyran form. The on-off ratio of photoswitching was found to depend on the gate voltage, ranging between 1.3 at the highest voltages to ca. 2-3 around the threshold voltage and significantly increasing below the threshold. To explain the behavior observed, the effect of the presence of dipolar photochromic molecules was considered, either aggregated on the organic semiconductor-polymeric insulator interface or dispersed in the bulk of the insulator. The contribution of surface and bulk dipoles to the photoswitching has been estimated: one may expect its noticeable contribution to the total effect only below the threshold voltage. At higher voltages, the dominant mechanism seems to be associated with changes in the electric permittivity resulting from changes of the dipole moment during the photochromic cycle. These changes affect the sourcedrain current both indirectly, by changes in the FET mobility, and directly, since according to eqs 1 and 2 the current is proportional to the capacitance of the dielectric layer. The results presented in this paper supplement and corroborate those reported in ref 16. In both cases, the FET current was found to increase on increasing the electric permittivity of the insulator, irrespective of the sign of carriers. This seems to rule out the mechanism put forward in ref 14. A direct confirmation of the mechanism put forward in this paper would come from a direct comparison of the current-voltage characteristics and electric permittivity and the kinetic behavior of these two parameters during exposition to UV light. These experiments will be carried out and reported in the near future. Finally, it should be noted that one may attribute a more general significance to the results obtained: our results indicate that any reversible and controllable changes of the dielectric 3112



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properties of the FET’s insulating layer may be employed in the construction of switches. The input would be a stimulus changing the dielectric properties of the insulator, the source-drain current being the output.

’ AUTHOR INFORMATION

’ APPENDIX Let the molecular polarizabilities of the photochromic molecules and the dielectric amount to Rphot and Rdiel and their radii to aphot and adiel, respectively. For the sake of simplicity, we will additionally assume that neither the size nor the polarizability of the photochromic molecules change upon conversion, the only variable parameter being the dipole moment. Under these conditions, the Onsager equation (cf. ref 28 eq 5.60) can be written in the form " ðε-1Þð2εþ1Þ ð1-xÞRdiel x ¼N þ 12πε 1-fdiel Rdiel 1-fphot Rphot !# m2phot Rphot þ ðA1Þ 3kTð1-fphot Rphot

Notes

where ε is the relative electric permittivity of the two-component insulator; N is the number of molecules per unit volume; x stands for the mole fraction of the photochromic molecules; and fi (i = diel, phot) are the factors of reaction field of the components. The above equation should also hold for pure dielectric (i.e., for x = 0) ðεdiel -1Þð2εdiel þ1Þ NRdiel ¼ 12πεdiel 1-fdiel Rdiel

ðA2Þ

where εdiel is the relative permittivity of the pure dielectric. Combining A1 and A2, one obtains ðε-1Þð2εþ1Þ εdiel ε ðεdiel -1Þð2εdiel þ1Þ ¼ ð1-xÞþx þx

ð1-fdiel Rdiel ÞRphot ð1-fphot Rphot ÞRdiel

ð1-fdiel Rdiel Þ m2phot 3kT ½ð1-fphot Rphot Þ2 Rdiel

ðA3Þ

The parameters Ri and fi can be estimated from the formula28 Rk 2ε-2 ðA4Þ fk R k ¼ 3 ak 2εþ1 Since the ratio (Rk)/(a3k) usually ranges between 0.4 and 0.6,28 we will arbitrarily assume (Rk)/(a3k) ≈ 0.5, hence ε-1 ðA5Þ fk Rk 2εþ1 After simple transformations, eq A5 can be rewritten in the form ðε-1Þð2εþ1Þ ¼ AþxðBþDm2phot Þ ðA6Þ ε where A, B, and D are parameters which can be determined from eqs A3 - A5. Taking εdiel = 3.6,21,22 Rdiel = 16.7 Å3 (value estimated from the molecular volume of the molecular segment of PMMA), and Rphot = 3Rdiel (estimated from the ratio of molecular volumes of PMMA segment and SP) and expressing mphot in Debye units, we arrive at A = 5.9; B = 14.7; and D = 5.6.

Corresponding Author

*E-mail: [email protected]. Permanent Address: Institute of Physics, National Academy of Sciences of Ukraine, Kyiv, Ukraine.

’ ACKNOWLEDGMENT This work was supported by the European Commission through the Human Potential Programme (Marie-Curie RTN BIMORE, Grant No. MRTN-CT-2006-035859) and, in part, by the Wroclaw University of Technology. Special thanks to Catherine Kitts for her assistance in the experimental part of the work. ’ REFERENCES (1) Feringa, B. L., Ed. Molecular Switches; Wiley-VCH: Weinheim, 2001. (2) Bao, Z.; Locklin, J. J., Eds. Organic Field Effect Transistors; CRC Press: Boca Raton, 2006. (3) Klauk, H., Ed. Organic Electronics. Materials, Manufacturing and Applications; Wiley-VCH: Weinheim, 2007. (4) Hadziioannou, G.; Malliaras, G. G., Eds. Semiconducting Polymers, 2nd ed.; Wiley-VCH: Weinheim, 2007. (5) Nespurek, S.; Sworakowski, J. Molecular Current Modulator Consisting of Conjugated Polymer Chain with Chemically Attached Photoactive Side Groups. Thin Solid Films 2001, 393, 168–176. (6) Toman, P.; Bartkowiak, W.; Nespurek, S.; Sworakowski, J.; Zalesny, R. Quantum-Chemical Insight into the Design of Molecular Optoelectrical Switch. Chem. Phys. 2005, 316, 267–278. (7) Jakobsson, F. L. E.; Marsal, Ph.; Braun, S.; Fahlman, M.; Berggren, M.; Cornil, J.; Crispin, X. Tuning the Energy Levels of Photochromic Diarylethene Compounds for Opto-Electronic Switch Devices. J. Phys. Chem. C 2009, 113, 18396–18405. (8) Toman, P.; Nespurek, S. Modeling of Hole Transport in Poly[2Methoxy-5-(20 -EthyIhexyloxy)-p-Phenylene Vinylene] Doped with Polar Additives. Mol. Cryst. Liq. Cryst. 2008, 496, 25–38. (9) Nespurek, S.; Sworakowski, J.; Combellas, C.; Wang, G.; Weiter, M. A Molecular Device Based on Light Controlled Charge Carrier Mobility. Appl. Surf. Sci. 2004, 234, 395–402. (10) Nespurek, S.; Wang, G.; Toman, P.; Sworakowski, J.; Bartkowiak, W.; Iwamoto, M.; Combellas, C. Charge Mobilities in Molecular Materials Reversibly Modified by Light: Towards a Molecular Switch. Mol. Cryst. Liq. Cryst. 2005, 430, 127–133. (11) Vala, M.; Weiter, M.; Zmeskal, O.; Nespurek, S.; Toman, P. Light Induced Change of Charge Carrier Mobility in Semiconducting Polymers. Macromol. Symp. 2008, 268, 125–128. (12) Andersson, P.; Robinson, N. D.; Berggren, M. Switchable Charge Traps in Polymer Diodes. Adv. Mater. 2005, 17, 1798–1803. (13) Kratochvílova, I.; Nespurek, S.; Sebera, J.; Zalis, S.; Pavelka, M.; Wang, G.; Sworakowski, J. New Organic FET-like Photoactive Device, Experiments and DFT Modeling. Eur. Phys. J. E 2008, 25, 299–307. (14) Shen, Q.; Cao, Y.; Liu, S.; Steigerwald, M. L.; Guo, X. Conformation-Induced Electrostatic Gating of the Conduction of Spiropyran-Coated Organic Thin-Film Transistors. J. Phys. Chem. C 2009, 113, 10807–10812. (15) Guo, X.; Huang, L.; O’Brien, S.; Kim, P.; Nuckolls, C. Directing and Sensing Changes in Molecular Conformation on Individual Carbon Nanotube Field Effect Transistors. J. Am. Chem. Soc. 2005, 127, 15045–15047. (16) Shen, Q.; Wang, L.; Liu, S.; Cao, Y.; Gan, L.; Guo, X.; Steigerwald, M. L.; Shuai, Z.; Liu, Z.; Nuckolls, C. Photoactive Gate Dielectrics. Adv. Mater. 2010, 22, 3282–3287. (17) Puigdollers, J.; Della Pirriera, M.; Marsal, A.; Orpella, A.; Cheylan, S.; Voz, C.; Alcubilla, R. N-type PTCDI-C13H27 Thin-Film 3113



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Photoswitching of an n-Type Organic Field Effect Transistor by a

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