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Effect of UV irradiation on solution processed low voltage flexible organic field-effect transistors
Superlattices and Microstructures xxx (2017) 1e7
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Effect of UV irradiation on solution processed low voltage flexible organic field-effect transistors Deepak Bharti, Vivek Raghuwanshi, Ishan Varun, Ajay Kumar Mahato, Shree Prakash Tiwari* Department of Electrical Engineering, Indian Institute of Technology Jodhpur, Jodhpur, Rajasthan 342011, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 April 2017 Received in revised form 18 May 2017 Accepted 19 May 2017 Available online xxx
Effect of ultra-violet (UV) irradiation (lpeak ¼ 365 nm) on the electrical characteristics of solution processed flexible TIPS-pentacene organic field-effect transistors (OFETs) has been investigated. Pristine TIPS-pentacene OFETs demonstrated average field-effect mobility of 0.1 cm2 V1 s1, with near zero threshold voltage and current on-off ratio of ~104. On UV irradiation, OFETs displayed a joint photoconductive and photovoltaic effect due to photogenerated excitons. The maximum current modulation and photo-responsivity obtained from these OFETs were ~500 and ~43 mA/W respectively at an intensity of 1.8 mW/cm2 while operating at a low voltage of 5 V. On increasing the irradiation time, a positive shift in the threshold voltage was observed. At larger values of irradiation time, a roll-off in maximum drain current and mobility values were observed, which was attributed to slight deterioration in crystallinity due to prolonged UV exposure, as confirmed from X-ray diffraction studies. Similar trend was observed for mobility and threshold voltage values, when gate bias during UV irradiation was increased. In addition, transistors exhibited a repeatable dynamic response to periodic pulses of UV irradiation. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Photo-sensitive organic field-effect transistors (photo-OFETs) Flexible TIPS-pentacene crystal UV-irradiation
1. Introduction An extensive research advancement in the field of organic field-effect transistors (OFETs) has already lead to their successful integration in many applications such as flexible displays [1,2] radio frequency identification (RFID) tags [3], wearable devices [4], and various types of sensors including chemical sensors [5], bio-sensors [6], gas sensors [7], and pressure sensors [8]. Use of OFETs in various optoelectronic applications like photo-sensing [9e11], optical memory elements and photoswitches [12e14] has also been very widely explored due to photo-sensitive nature of organic semiconductors. However, majority of these reports along with a recent study by our group [15] deal with photo-sensitive properties of organic semiconductors in the visible range of electromagnetic spectrum. Effect of ultra-violet (UV) irradiation on organic semiconductors largely remains under-explored with very less reports available. Nonetheless, a detailed study of the effects of UV irradiation on organic semiconductors and corresponding devices is highly imperative for development of several low cost, low power and high performance civil and military applications including smoke and fire detection, missile warning, combustion monitoring and ozone sensing [16,17]. To the best knowledge of authors, for most of the organic semiconductors, which have been explored as UV detectors, corresponding devices are in the diode architecture and have been fabricated on
* Corresponding author., E-mail address: [email protected] (S.P. Tiwari). http://dx.doi.org/10.1016/j.spmi.2017.05.041 0749-6036/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: D. Bharti et al., Effect of UV irradiation on solution processed low voltage flexible organic fieldeffect transistors, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.05.041
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D. Bharti et al. / Superlattices and Microstructures xxx (2017) 1e7
rigid substrates [18e23]. However, a photo-OFET is always a better choice over a photo-diode due to simultaneous photodetection and amplification along with higher sensitivity. Despite this fact, reports on UV sensitive photo-OFETs are unfortunately obscure and effect of UV irradiation on the performance of OFETs remain under-addressed. For this reason, it is highly essential to study the effect of UV irradiation on the electrical characteristics of OFETs. In this study, we report the effect of UV irradiation on the electrical performance of low voltage, flexible, solution processed OFETs. Organic semiconductor TIPS-pentacene was selected for this study due to its high performance and air stability [24e26]. Pristine OFETs function at moderate operating voltage of 10 V and show a mobility of 0.11(±0.08) cm2 V1 s1 with current on-off ratio of ~104. UV irradiation was found to increase the off currents and threshold voltage (VTH) in positive VGS direction due to photo-generation of excitons. Photo-OFETs yielded a maximum drain current modulation (ratio of irradiated and dark currents) of ~500, a maximum photo-responsivity of ~43 mA/W for UV irradiation with an intensity of 1.8 mW/cm2. A Higher exposure time under UV irradiation led to a positive shift in VTH and reduction in the saturation current and mobility. This reduction was attributed to decrease in the degree of crystallinity of organic semiconductor due to prolonged UV exposure. On increasing the gate bias during irradiation, similar positive shift in VTH and reduction in mobility was observed. In addition, these OFETs showed a repeatable switching response to periodic illumination pulses, signifying their capability to be used as UV switches.
2. Experiments Flexible polyethylene terephthalate (PET) substrates (thickness: 127 mm) having a 130 nm thick layer of indium tin oxide (ITO) with surface resistivity of 60 U/sq, were used to fabricate bottom-gate top-contact OFETs. Procedures of substrate cleaning and deposition of bi-layer gate dielectric consisting of high-k HfO2 and poly(4-vinylphenol) (PVP) on cleaned substrates were similar as described in our previous reports [15,27]. A 0.5 wt % solution of TIPS-pentacene solution was prepared in toluene by stirring for 3 h at 70 C, which was cast on dielectric deposited substrates. Au source-drain contacts (200 nm thick) were deposited by thermal evaporation under a high vacuum of 106 Torr using metal shadow masks. The schematic of the device architecture and digital image of the fabricated devices are shown in Fig. 1(a) and (b). All processing and characterization steps were performed in dark and ambient conditions. Electrical characterizations of the devices were performed using Keithley 4200 SCS. Saturation regime field-effect mobility of the devices (msat) and VTH were calculated from transfer characteristics of the devices, as in our previous reports [27,28]. Capacitance density (Ci) of the HfO2/PVP hybrid dielectric layer was found to be 24.46 ± 0.71 nF/cm2 at 1 KHz. To investigate the steady-state response of the devices to UV irradiation, a 2 terminal low power UV LED with a peak wavelength of 365 nm was used as a UV light source. This light source was adjusted to shine above the sample with maximum intensity of 1.8 mW/cm2. The figures of merit of a photo-OFET, the current modulation or the ratio of current under irradiated and dark conditions current (P) and photo-responsivity (R) have been evaluated using following equations,
Fig. 1. (a) Device structure of a bottom-gate top-contact TIPS-pentacene OFET. (b) Digital image of fabricated flexible OFETs. (c) UV absorption spectrum of TIPSpentacene. (d) Transfer characteristics of a representative TIPS-pentacene OFET.
Please cite this article in press as: D. Bharti et al., Effect of UV irradiation on solution processed low voltage flexible organic fieldeffect transistors, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.05.041
D. Bharti et al. / Superlattices and Microstructures xxx (2017) 1e7
3
P¼
Iirr Idark ; Idark
(1)
R¼
Iirr Idark ; APi
(2)
where Iirr and Idark are the currents under irradiated and dark conditions, A is the effective device area, Pi is the power of the incident illumination per unit area and e is the electronic charge [29]. 3. Results and discussion A uniform PVP layer and terraced TIPS-pentacene crystals as obtained in our previous reports [28], were verified in this study also. Fig. 1(c) shows the absorption spectrum of TIPS-pentacene, which indicates higher absorbance in the ultra-violet region of the spectrum. Transfer characteristics of a representative TIPS-pentacene device is shown in Fig. 1(d). An average field effect mobility of 0.11(±0.08) cm2 V1 s1 was obtained with on-off current ratio of ~104, as also recently reported by our group [28]. This performance was obtained at moderate voltage of 10 V, which is lower than that of the most of the reported TIPS-pentacene flexible devices. Relatively slow rate of solvent evaporation in the drop cast method improves the molecular arrangement and crystalline quality of semiconductor layer, which ultimately results in high performance in these devices [28]. Fig. 2 shows the effect of UV irradiation on the transfer characteristics of a representative device on logarithmic and linear scales. Incident UV photons cause generation of excitons across the semiconductor film, which eventually dissociate into free electrons and holes and induce a combined photoconductive (an increase in the drain current) and photovoltaic effect (a shift in VTH). Since holes have higher mobility in a hole transport semiconductor, they escape swiftly towards the drain electrode increasing the levels of drain current. Less mobile electrons are trapped on PVP/TIPS-pentacene interface, reducing the potential barrier between source and channel and leading finally to a decreased VTH [30,31]. It is noteworthy that if the light is shined from the top of the film, the light with wavelengths having large absorption coefficients would be largely absorbed in the upper few monolayers of the semiconductor and density of photo-generated excitons will be higher in these regions. However, the light with wavelengths having smaller absorption coefficients traverse down in the semiconductor film up to dielectric-semiconductor interface (the internal filter effect in organic semiconductors [32]). Since excitons have short diffusion lengths (of the order of few nm), large number of excitons of the upper monolayers of the semiconductor are unable to produce any photo-current in the device. Nonetheless the excitons, which are generated in the proximity of dielectricsemiconductor interface, only would contribute towards effective photo-current in the device. Table 1 lists device parameters for a set of five devices undergone UV irradiation. It can be observed from the table that average mobility of OFETs is reduced, whereas the average threshold voltage has been shifted towards positive values after UV exposure. Maximum values of photo-responsivity (Rmax) and current modulation (Pmax) of ~43 mA/W and ~500 respectively were obtained for UV illumination intensity of 1.8 mW/cm2 at VDS ¼ 5 V, from a set of 5 devices. It is to be noted that this performance was obtained with solution processed flexible photo-OFETs while operating at lower voltages, which is comparable to some of the previously reported diode UV detectors based on organic and hybrid materials [18e23]. Moreover, most of the photo-OFETs reported in the literature have been shown to operate in a voltage range of 40e80 V in the visible region of the electromagnetic spectrum and have been fabricated on rigid substrates [33e36]. However, UV sensitive photo-OFETs reported in this study are on flexible substrates and operate at lower voltages (5e10 V). Fig. 3(a) shows the effect of increasing irradiation time on the transfer characteristics of a representative OFET which were measured in dark conditions after illuminating the device with UV irradiation for a particular irradiation time and at constant
Fig. 2. Transfer characteristics of TIPS-pentacene OFET on logarithmic (a), and linear scales (b), under UV irradiation.
Please cite this article in press as: D. Bharti et al., Effect of UV irradiation on solution processed low voltage flexible organic fieldeffect transistors, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.05.041
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Table 1 Summary of electrical parameters for a set of 5 devices undergone UV irradiation. Mobility (cm2 V1 s1)
Threshold Voltage (V)
Before
After
Before
After
0.031 ± 0.009
0.019 ± 0.006
0.63 ± 0.48
1.24 ± 0.68
Pmax
Rmax (mA/W)
~500
~43
Fig. 3. (a) Effect of UV illumination time on the transfer characteristics of the OFET. (b) Variation of msat and VTH with increasing illumination time. (c) Trend of maximum drain current at VGS ¼ 10 V with increasing illumination time. (d) X-ray diffractogram of a pristine and 10 min UV irradiated sample.
biasing conditions of VGS ¼ 10 V. It can be observed that with increasing illumination time, transfer curves shift towards positive values of VGS, ultimately resulting in a positive shift in VTH. This shift in VTH can be attributed to the increased exciton generation and subsequent minority charge carrier trapping under increasing illumination time, which is simultaneously enhanced with applied positive gate bias. Fig. 3(b) shows the variation in msat and VTH as a function of illumination time. With increasing illumination time, msat follows a decreasing trend, while VTH increases linearly towards positive voltages. Not only the msat and VTH, the magnitude of drain current also varies with UV illumination time. Variation in the drain current at biasing conditions of VGS ¼ 10 V and VDS ¼ 5 V is plotted with UV illumination time and shown in Fig. 3(c). Drain current shows a little increment at smaller illumination time after which it rapidly falls at higher illumination time. At smaller values of illumination time, the photoconductive effect is prevalent which causes a small increment in the drain current. To ascertain the reason of current roll-off at higher values of illumination time, X-ray diffraction studies were performed. Fig. 3(d) shows the X-ray diffractogram of a pristine and 10 min UV irradiated sample. A slight increase in full width at half maximum (FWHM) was observed on UV irradiation signifying a decrease in crystallinity. The crystallinity was reduced due to destructive nature of prolonged UV irradiation, which has a tendency to break bonds in crystals and generate defects [37]. Another reason for the impaired conduction can be the UV induced irreversible photo-chemical degradation of the organic semiconductor which is further enhanced in the presence of oxygen and moisture. This degradation can also generate permanent defects in semiconductor layer [38e40]. Increased number of defects sites and decreased crystallinity due to prolonged UV exposure lead to permanently deteriorated quality of charge transport and therefore decreased drain current and charge carrier mobility, which renders the UV induced degradation irreversible. Fig. 4(a) shows the effect of applied gate bias during UV illumination on the transfer characteristics of the OFET for a constant illumination time of 100 s. An increasing VGS,bias during illumination causes a shift in the transfer curve towards positive gate voltages, resulting in a positive VTH shift. Fig. 4(b) shows the variation in msat and VTH as a function of VGS,bias. msat decreases whereas VTH shifts to positive values in proportion to the applied VGS,bias. A higher value of positive VGS,bias causes higher degree of trapping of photo-generated electrons at the dielectric-semiconductor interface, which results in larger VTH shift. However, decrease in the mobility may be attributed to the deteriorated quality of charge transport under cumulative effect of extended UV irradiation as discussed earlier. Please cite this article in press as: D. Bharti et al., Effect of UV irradiation on solution processed low voltage flexible organic fieldeffect transistors, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.05.041
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Fig. 4. (a) Effect of increasing gate bias during illumination on the transfer characteristics of an OFET for a constant illumination time of 100 s. (b) Dependence of msat and shift in VTH on the applied gate bias during illumination.
Fig. 5(a) shows the dynamic response of the OFETs under UV illumination. Under this study, the devices were irradiated with periodic UV illumination pulses (ON and OFF period ¼ 20 s), and the drain current was measured at VGS ¼ 5 V, VDS ¼ 5 V in the interval of 0.5 s. A repeatable switching phenomenon was observed with these irradiation pulses. On turning on the UV illumination, drain current begins to increase due to photo-generated holes, which reach quickly to drain terminals due to their higher mobility. After turning the illumination off, drain current starts to decrease due to de-trapping of electrons and their recombination with holes. Though the drain current responds quickly (with a response time of ~1 s) to UV illumination ON/OFF events, it does not attain a steady state value even for large illumination durations, making the response time apparently large of the order of a few tens of seconds. Response rate (rate of change in drain current) to UV irradiation was found to be ~1.5 nA/s. Trapping and release rates during illumination ON and OFF states can be calculated using following equation [41],
Rate ¼
dQ dV ðtÞ 2LC 1=2 dfIDS ðtÞg1=2 ¼ C TH ¼ ; dt dt msat W dt
(3)
Fig. 5. (a) UV Switching response of TIPS-pentacene OFETs. (b) Variation in {-IDS(t)}1/2 for a single cycle of UV illumination. Slopes have been extracted from the linear fit in the ON and OFF regions.
Please cite this article in press as: D. Bharti et al., Effect of UV irradiation on solution processed low voltage flexible organic fieldeffect transistors, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.05.041
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D. Bharti et al. / Superlattices and Microstructures xxx (2017) 1e7
where Q is the charge density and C is the dielectric capacitance density. For p-type devices, -IDS(t) is used in place of IDS(t) to take square root, since IDS is negative during operation. Fig. 5(b) shows the plot of {-IDS(t)}1/2 for a single cycle of UV illumination. To estimate the trapping and release rates, the linear fit was applied on {-IDS(t)}1/2 in the ON and OFF regions in Fig. 5(b), and slope was extracted [42]. Trapping and release rates in UV ON and OFF regions were found to have values of 5.7 109 cm2 s1 and 12.1 109 cm2 s1 respectively. A higher release rate than the trapping rate signifies that de-trapping of electrons is faster in the absence of UV illumination than their trapping at PVP/TIPS-pentacene interface during illumination. 4. Conclusion Effect of UV irradiation (lpeak ¼ 365 nm) on solution processed, TIPS-pentacene flexible OFETs has been analyzed. Due to trapping of photo-generated electrons, a positive shift in VTH was observed, whereas a quick collection of photo-generated holes resulted in an increase in off current. Maximum current modulation of ~500 and maximum photo-responsivity of ~43 mA/W were obtained from these OFETs for UV illumination intensity of 1.8 mW/cm2 while operating at low voltage of 5 V. Increasing UV irradiation time resulted in an enhanced VTH shift and reduced mobility. The drain current at VDS ¼ 5 V and VGS ¼ 10 V was found to rise slightly for smaller values of irradiation time, however decreased for higher values of illumination time. This reduction in the drain current was associated with decrease in crystallinity of the semiconductor due to prolonged UV exposure. Similar trend of positive shifting of VTH and mobility roll-off was observed when gate bias during irradiation was increased. In addition, devices demonstrated repeatable switching response to periodic UV illumination pulses. 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Effect of UV irradiation on solution processed low voltage flexible organic field-effect transistors Deepak Bharti, Vivek Raghuwanshi, Ishan Varun, Ajay Kumar Mahato, Shree Prakash Tiwari* Department of Electrical Engineering, Indian Institute of Technology Jodhpur, Jodhpur, Rajasthan 342011, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 April 2017 Received in revised form 18 May 2017 Accepted 19 May 2017 Available online xxx
Effect of ultra-violet (UV) irradiation (lpeak ¼ 365 nm) on the electrical characteristics of solution processed flexible TIPS-pentacene organic field-effect transistors (OFETs) has been investigated. Pristine TIPS-pentacene OFETs demonstrated average field-effect mobility of 0.1 cm2 V1 s1, with near zero threshold voltage and current on-off ratio of ~104. On UV irradiation, OFETs displayed a joint photoconductive and photovoltaic effect due to photogenerated excitons. The maximum current modulation and photo-responsivity obtained from these OFETs were ~500 and ~43 mA/W respectively at an intensity of 1.8 mW/cm2 while operating at a low voltage of 5 V. On increasing the irradiation time, a positive shift in the threshold voltage was observed. At larger values of irradiation time, a roll-off in maximum drain current and mobility values were observed, which was attributed to slight deterioration in crystallinity due to prolonged UV exposure, as confirmed from X-ray diffraction studies. Similar trend was observed for mobility and threshold voltage values, when gate bias during UV irradiation was increased. In addition, transistors exhibited a repeatable dynamic response to periodic pulses of UV irradiation. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Photo-sensitive organic field-effect transistors (photo-OFETs) Flexible TIPS-pentacene crystal UV-irradiation
1. Introduction An extensive research advancement in the field of organic field-effect transistors (OFETs) has already lead to their successful integration in many applications such as flexible displays [1,2] radio frequency identification (RFID) tags [3], wearable devices [4], and various types of sensors including chemical sensors [5], bio-sensors [6], gas sensors [7], and pressure sensors [8]. Use of OFETs in various optoelectronic applications like photo-sensing [9e11], optical memory elements and photoswitches [12e14] has also been very widely explored due to photo-sensitive nature of organic semiconductors. However, majority of these reports along with a recent study by our group [15] deal with photo-sensitive properties of organic semiconductors in the visible range of electromagnetic spectrum. Effect of ultra-violet (UV) irradiation on organic semiconductors largely remains under-explored with very less reports available. Nonetheless, a detailed study of the effects of UV irradiation on organic semiconductors and corresponding devices is highly imperative for development of several low cost, low power and high performance civil and military applications including smoke and fire detection, missile warning, combustion monitoring and ozone sensing [16,17]. To the best knowledge of authors, for most of the organic semiconductors, which have been explored as UV detectors, corresponding devices are in the diode architecture and have been fabricated on
* Corresponding author., E-mail address: [email protected] (S.P. Tiwari). http://dx.doi.org/10.1016/j.spmi.2017.05.041 0749-6036/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: D. Bharti et al., Effect of UV irradiation on solution processed low voltage flexible organic fieldeffect transistors, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.05.041
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rigid substrates [18e23]. However, a photo-OFET is always a better choice over a photo-diode due to simultaneous photodetection and amplification along with higher sensitivity. Despite this fact, reports on UV sensitive photo-OFETs are unfortunately obscure and effect of UV irradiation on the performance of OFETs remain under-addressed. For this reason, it is highly essential to study the effect of UV irradiation on the electrical characteristics of OFETs. In this study, we report the effect of UV irradiation on the electrical performance of low voltage, flexible, solution processed OFETs. Organic semiconductor TIPS-pentacene was selected for this study due to its high performance and air stability [24e26]. Pristine OFETs function at moderate operating voltage of 10 V and show a mobility of 0.11(±0.08) cm2 V1 s1 with current on-off ratio of ~104. UV irradiation was found to increase the off currents and threshold voltage (VTH) in positive VGS direction due to photo-generation of excitons. Photo-OFETs yielded a maximum drain current modulation (ratio of irradiated and dark currents) of ~500, a maximum photo-responsivity of ~43 mA/W for UV irradiation with an intensity of 1.8 mW/cm2. A Higher exposure time under UV irradiation led to a positive shift in VTH and reduction in the saturation current and mobility. This reduction was attributed to decrease in the degree of crystallinity of organic semiconductor due to prolonged UV exposure. On increasing the gate bias during irradiation, similar positive shift in VTH and reduction in mobility was observed. In addition, these OFETs showed a repeatable switching response to periodic illumination pulses, signifying their capability to be used as UV switches.
2. Experiments Flexible polyethylene terephthalate (PET) substrates (thickness: 127 mm) having a 130 nm thick layer of indium tin oxide (ITO) with surface resistivity of 60 U/sq, were used to fabricate bottom-gate top-contact OFETs. Procedures of substrate cleaning and deposition of bi-layer gate dielectric consisting of high-k HfO2 and poly(4-vinylphenol) (PVP) on cleaned substrates were similar as described in our previous reports [15,27]. A 0.5 wt % solution of TIPS-pentacene solution was prepared in toluene by stirring for 3 h at 70 C, which was cast on dielectric deposited substrates. Au source-drain contacts (200 nm thick) were deposited by thermal evaporation under a high vacuum of 106 Torr using metal shadow masks. The schematic of the device architecture and digital image of the fabricated devices are shown in Fig. 1(a) and (b). All processing and characterization steps were performed in dark and ambient conditions. Electrical characterizations of the devices were performed using Keithley 4200 SCS. Saturation regime field-effect mobility of the devices (msat) and VTH were calculated from transfer characteristics of the devices, as in our previous reports [27,28]. Capacitance density (Ci) of the HfO2/PVP hybrid dielectric layer was found to be 24.46 ± 0.71 nF/cm2 at 1 KHz. To investigate the steady-state response of the devices to UV irradiation, a 2 terminal low power UV LED with a peak wavelength of 365 nm was used as a UV light source. This light source was adjusted to shine above the sample with maximum intensity of 1.8 mW/cm2. The figures of merit of a photo-OFET, the current modulation or the ratio of current under irradiated and dark conditions current (P) and photo-responsivity (R) have been evaluated using following equations,
Fig. 1. (a) Device structure of a bottom-gate top-contact TIPS-pentacene OFET. (b) Digital image of fabricated flexible OFETs. (c) UV absorption spectrum of TIPSpentacene. (d) Transfer characteristics of a representative TIPS-pentacene OFET.
Please cite this article in press as: D. Bharti et al., Effect of UV irradiation on solution processed low voltage flexible organic fieldeffect transistors, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.05.041
D. Bharti et al. / Superlattices and Microstructures xxx (2017) 1e7
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P¼
Iirr Idark ; Idark
(1)
R¼
Iirr Idark ; APi
(2)
where Iirr and Idark are the currents under irradiated and dark conditions, A is the effective device area, Pi is the power of the incident illumination per unit area and e is the electronic charge [29]. 3. Results and discussion A uniform PVP layer and terraced TIPS-pentacene crystals as obtained in our previous reports [28], were verified in this study also. Fig. 1(c) shows the absorption spectrum of TIPS-pentacene, which indicates higher absorbance in the ultra-violet region of the spectrum. Transfer characteristics of a representative TIPS-pentacene device is shown in Fig. 1(d). An average field effect mobility of 0.11(±0.08) cm2 V1 s1 was obtained with on-off current ratio of ~104, as also recently reported by our group [28]. This performance was obtained at moderate voltage of 10 V, which is lower than that of the most of the reported TIPS-pentacene flexible devices. Relatively slow rate of solvent evaporation in the drop cast method improves the molecular arrangement and crystalline quality of semiconductor layer, which ultimately results in high performance in these devices [28]. Fig. 2 shows the effect of UV irradiation on the transfer characteristics of a representative device on logarithmic and linear scales. Incident UV photons cause generation of excitons across the semiconductor film, which eventually dissociate into free electrons and holes and induce a combined photoconductive (an increase in the drain current) and photovoltaic effect (a shift in VTH). Since holes have higher mobility in a hole transport semiconductor, they escape swiftly towards the drain electrode increasing the levels of drain current. Less mobile electrons are trapped on PVP/TIPS-pentacene interface, reducing the potential barrier between source and channel and leading finally to a decreased VTH [30,31]. It is noteworthy that if the light is shined from the top of the film, the light with wavelengths having large absorption coefficients would be largely absorbed in the upper few monolayers of the semiconductor and density of photo-generated excitons will be higher in these regions. However, the light with wavelengths having smaller absorption coefficients traverse down in the semiconductor film up to dielectric-semiconductor interface (the internal filter effect in organic semiconductors [32]). Since excitons have short diffusion lengths (of the order of few nm), large number of excitons of the upper monolayers of the semiconductor are unable to produce any photo-current in the device. Nonetheless the excitons, which are generated in the proximity of dielectricsemiconductor interface, only would contribute towards effective photo-current in the device. Table 1 lists device parameters for a set of five devices undergone UV irradiation. It can be observed from the table that average mobility of OFETs is reduced, whereas the average threshold voltage has been shifted towards positive values after UV exposure. Maximum values of photo-responsivity (Rmax) and current modulation (Pmax) of ~43 mA/W and ~500 respectively were obtained for UV illumination intensity of 1.8 mW/cm2 at VDS ¼ 5 V, from a set of 5 devices. It is to be noted that this performance was obtained with solution processed flexible photo-OFETs while operating at lower voltages, which is comparable to some of the previously reported diode UV detectors based on organic and hybrid materials [18e23]. Moreover, most of the photo-OFETs reported in the literature have been shown to operate in a voltage range of 40e80 V in the visible region of the electromagnetic spectrum and have been fabricated on rigid substrates [33e36]. However, UV sensitive photo-OFETs reported in this study are on flexible substrates and operate at lower voltages (5e10 V). Fig. 3(a) shows the effect of increasing irradiation time on the transfer characteristics of a representative OFET which were measured in dark conditions after illuminating the device with UV irradiation for a particular irradiation time and at constant
Fig. 2. Transfer characteristics of TIPS-pentacene OFET on logarithmic (a), and linear scales (b), under UV irradiation.
Please cite this article in press as: D. Bharti et al., Effect of UV irradiation on solution processed low voltage flexible organic fieldeffect transistors, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.05.041
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Table 1 Summary of electrical parameters for a set of 5 devices undergone UV irradiation. Mobility (cm2 V1 s1)
Threshold Voltage (V)
Before
After
Before
After
0.031 ± 0.009
0.019 ± 0.006
0.63 ± 0.48
1.24 ± 0.68
Pmax
Rmax (mA/W)
~500
~43
Fig. 3. (a) Effect of UV illumination time on the transfer characteristics of the OFET. (b) Variation of msat and VTH with increasing illumination time. (c) Trend of maximum drain current at VGS ¼ 10 V with increasing illumination time. (d) X-ray diffractogram of a pristine and 10 min UV irradiated sample.
biasing conditions of VGS ¼ 10 V. It can be observed that with increasing illumination time, transfer curves shift towards positive values of VGS, ultimately resulting in a positive shift in VTH. This shift in VTH can be attributed to the increased exciton generation and subsequent minority charge carrier trapping under increasing illumination time, which is simultaneously enhanced with applied positive gate bias. Fig. 3(b) shows the variation in msat and VTH as a function of illumination time. With increasing illumination time, msat follows a decreasing trend, while VTH increases linearly towards positive voltages. Not only the msat and VTH, the magnitude of drain current also varies with UV illumination time. Variation in the drain current at biasing conditions of VGS ¼ 10 V and VDS ¼ 5 V is plotted with UV illumination time and shown in Fig. 3(c). Drain current shows a little increment at smaller illumination time after which it rapidly falls at higher illumination time. At smaller values of illumination time, the photoconductive effect is prevalent which causes a small increment in the drain current. To ascertain the reason of current roll-off at higher values of illumination time, X-ray diffraction studies were performed. Fig. 3(d) shows the X-ray diffractogram of a pristine and 10 min UV irradiated sample. A slight increase in full width at half maximum (FWHM) was observed on UV irradiation signifying a decrease in crystallinity. The crystallinity was reduced due to destructive nature of prolonged UV irradiation, which has a tendency to break bonds in crystals and generate defects [37]. Another reason for the impaired conduction can be the UV induced irreversible photo-chemical degradation of the organic semiconductor which is further enhanced in the presence of oxygen and moisture. This degradation can also generate permanent defects in semiconductor layer [38e40]. Increased number of defects sites and decreased crystallinity due to prolonged UV exposure lead to permanently deteriorated quality of charge transport and therefore decreased drain current and charge carrier mobility, which renders the UV induced degradation irreversible. Fig. 4(a) shows the effect of applied gate bias during UV illumination on the transfer characteristics of the OFET for a constant illumination time of 100 s. An increasing VGS,bias during illumination causes a shift in the transfer curve towards positive gate voltages, resulting in a positive VTH shift. Fig. 4(b) shows the variation in msat and VTH as a function of VGS,bias. msat decreases whereas VTH shifts to positive values in proportion to the applied VGS,bias. A higher value of positive VGS,bias causes higher degree of trapping of photo-generated electrons at the dielectric-semiconductor interface, which results in larger VTH shift. However, decrease in the mobility may be attributed to the deteriorated quality of charge transport under cumulative effect of extended UV irradiation as discussed earlier. Please cite this article in press as: D. Bharti et al., Effect of UV irradiation on solution processed low voltage flexible organic fieldeffect transistors, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.05.041
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Fig. 4. (a) Effect of increasing gate bias during illumination on the transfer characteristics of an OFET for a constant illumination time of 100 s. (b) Dependence of msat and shift in VTH on the applied gate bias during illumination.
Fig. 5(a) shows the dynamic response of the OFETs under UV illumination. Under this study, the devices were irradiated with periodic UV illumination pulses (ON and OFF period ¼ 20 s), and the drain current was measured at VGS ¼ 5 V, VDS ¼ 5 V in the interval of 0.5 s. A repeatable switching phenomenon was observed with these irradiation pulses. On turning on the UV illumination, drain current begins to increase due to photo-generated holes, which reach quickly to drain terminals due to their higher mobility. After turning the illumination off, drain current starts to decrease due to de-trapping of electrons and their recombination with holes. Though the drain current responds quickly (with a response time of ~1 s) to UV illumination ON/OFF events, it does not attain a steady state value even for large illumination durations, making the response time apparently large of the order of a few tens of seconds. Response rate (rate of change in drain current) to UV irradiation was found to be ~1.5 nA/s. Trapping and release rates during illumination ON and OFF states can be calculated using following equation [41],
Rate ¼
dQ dV ðtÞ 2LC 1=2 dfIDS ðtÞg1=2 ¼ C TH ¼ ; dt dt msat W dt
(3)
Fig. 5. (a) UV Switching response of TIPS-pentacene OFETs. (b) Variation in {-IDS(t)}1/2 for a single cycle of UV illumination. Slopes have been extracted from the linear fit in the ON and OFF regions.
Please cite this article in press as: D. Bharti et al., Effect of UV irradiation on solution processed low voltage flexible organic fieldeffect transistors, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.05.041
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where Q is the charge density and C is the dielectric capacitance density. For p-type devices, -IDS(t) is used in place of IDS(t) to take square root, since IDS is negative during operation. Fig. 5(b) shows the plot of {-IDS(t)}1/2 for a single cycle of UV illumination. To estimate the trapping and release rates, the linear fit was applied on {-IDS(t)}1/2 in the ON and OFF regions in Fig. 5(b), and slope was extracted [42]. Trapping and release rates in UV ON and OFF regions were found to have values of 5.7 109 cm2 s1 and 12.1 109 cm2 s1 respectively. A higher release rate than the trapping rate signifies that de-trapping of electrons is faster in the absence of UV illumination than their trapping at PVP/TIPS-pentacene interface during illumination. 4. Conclusion Effect of UV irradiation (lpeak ¼ 365 nm) on solution processed, TIPS-pentacene flexible OFETs has been analyzed. Due to trapping of photo-generated electrons, a positive shift in VTH was observed, whereas a quick collection of photo-generated holes resulted in an increase in off current. Maximum current modulation of ~500 and maximum photo-responsivity of ~43 mA/W were obtained from these OFETs for UV illumination intensity of 1.8 mW/cm2 while operating at low voltage of 5 V. Increasing UV irradiation time resulted in an enhanced VTH shift and reduced mobility. The drain current at VDS ¼ 5 V and VGS ¼ 10 V was found to rise slightly for smaller values of irradiation time, however decreased for higher values of illumination time. This reduction in the drain current was associated with decrease in crystallinity of the semiconductor due to prolonged UV exposure. Similar trend of positive shifting of VTH and mobility roll-off was observed when gate bias during irradiation was increased. In addition, devices demonstrated repeatable switching response to periodic UV illumination pulses. 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Please cite this article in press as: D. Bharti et al., Effect of UV irradiation on solution processed low voltage flexible organic fieldeffect transistors, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.05.041