This journal is©The Royal Society of Chemistry 2016 J. Mater. Chem. C, 2016, 4, 5289--5296 | 5289

Cite this: J.Mater. Chem. C, 2016,

4, 5289

Photocontrol of charge injection/extractionat electrode/semiconductor interfaces forhigh-photoresponsivity organic transistors†

Hongtao Zhang,‡§a Hongliang Chen,§a Wei Ma,b Jingshu Hui,a Sheng Meng,b

Wei Xu,c Daoben Zhuc and Xuefeng Guo*a

Charge injection typically occurring at the electrode/semiconductor interface in organic field-effect

transistors (OFETs) is of great importance to the device performance and stability. Therefore, the

chemical modulation of electrode/semiconductor heterojunctions provides a promising approach to enhance

the performance and even incorporate new functionalities. In this study, we develop an efficient route for

constructing optically switchable OFETs featuring a photochromic spirothiopyran (SP) self-assembled

monolayer (SAM)-functionalized electrode/semiconductor interface. The photoisomerization of SPs induces a

reversible change in the dipole moment of SP-SAMs, which affects the work function of gold electrodes. This

change in the electrode work function enables the tuning of the contact resistance between organic

semiconductors and metal electrodes. Consequently, the channel conductance of these devices is

modulated in a nondestructive manner, thus leading to a new type of cost-effective OFET-based

photodetector with high photosensitivity. These results help us to better understand interfacial

phenomena and offer novel insights into developing a new generation of multifunctional interfaces and

ultrasensitive OFET-based sensors by interface engineering.

Introduction

Over the past 25 years, substantial efforts have been devotedto fabricating organic field-effect transistors (OFETs) withhigh mobility1 and specific functionalities.2 Very high carriermobilities exceeding 10 cm2 V�1 s�1 have been reported forconjugated polymers and small-molecule organic semiconductors,making them attractive for commercialization.3 In addition toimproving the carrier mobility of organic semiconductors, theengineering of interfaces that are ubiquitous in OFETs hasattracted considerable interest because of their influence ondevice performance and stability, and the drive to develop

functional high-performance OFETs.4 Among various interfacesin OFETs, the electrode/semiconductor interface is particularlyimportant because charge injection and extraction in opto-electronic devices generally occur at this interface under anapplied electric field.5 As reported previously, a well-ordered,two-dimensional self-assembled monolayer (SAM) with a desireddipole direction can be formed by using polar molecules on themetal surface as an electrode/semiconductor interface.6 Theinterfacial dipole associated with the molecular SAM affectsthe work function (FM) of metal contacts.7 For example, theSAMs of alkanethiols and perfluorinated alkanethiols6c,8

formed on metal electrodes exhibit opposite dipole directionsand can be used to decrease and increase, respectively, the FM

of the metal.5a This difference in FM can potentially inducechanges in the contact resistance (Rc), which is the mainparameter that determines the injection process in OFETs.1c,9

Therefore, the tunability of FM using the dipole of SAMsstrongly implies that if the SAM dipole is able to change underan external stimulus in a reversible manner, charge injection/extraction can be reversibly modulated at an electrode/semicon-ductor interface to control the device conductance.

In this study, we present a reliable approach to fabricatea new type of organic phototransistor capable of reversiblyphotomodulating the injection of carriers at the electrode/semi-conductor interface by using photochromic spirothiopyran (SP)

a Beijing National Laboratory for Molecular Sciences, State Key Laboratory for

Structural Chemistry of Unstable and Stable Species, College of Chemistry and

Molecular Engineering, Peking University, Beijing 100871, P. R. China.

E-mail: guoxf@pku.edu.cnb Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P. R. Chinac Key Laboratory of Organic Solids, Beijing National Laboratory for

Molecular Science, Institute of Chemistry, Chinese Academy of Sciences,

Beijing 100190, P. R. China

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6tc00387g‡ Current address: School of Materials Science and Engineering, State KeyLaboratory and Institute of Elemento-Organic Chemistry, Collaborative InnovationCenter of Chemical Science and Engineering (Tianjin), Nankai University,Tianjin 300071, P. R. China.§ These authors contributed equally to this work.

Received 27th January 2016,Accepted 27th April 2016

DOI: 10.1039/c6tc00387g

www.rsc.org/MaterialsC

Journal ofMaterials Chemistry C

PAPER

Publ

ishe

d on

27

Apr

il 20

16. D

ownl

oade

d by

Bei

jing

Uni

vers

ity o

n 15

/08/

2016

08:

51:1

2.

View Article OnlineView Journal | View Issue

5290 | J. Mater. Chem. C, 2016, 4, 5289--5296 This journal is©The Royal Society of Chemistry 2016

SAM-functionalized Au electrodes (Fig. 1). SP, a typical photo-chromic molecule,10 was selected as the light antenna becauseit can switch back-and-forth between a neutral colorless form(SP-closed) and zwitterionic colored form (SP-open) when itis triggered by light of different wavelengths (l). The photoiso-merization of SP molecules can substantially modulate the electricdipole moment (Pmol) of their SAMs.11 By taking advantage of thisunique property of SPs, reversible photoinduced shifts of the FM

of the electrode are accomplished. These shifts of FM change theSchottky barrier at the electrode/semiconductor interface, thusresulting in the modulation of Rc and device conductance in anondestructive and reversible manner. In fact, the incorporation ofphotochromic molecules into organic electronic devices has beenproven to be effective for constructing new generations of multi-functional interfaces2j such as dielectric/semiconductor inter-faces,12 environment/semiconductor interfaces,11a,b,13 andeven as a multicomponent mixture in semiconductor layers,14

with the aim of producing photoresponsive OFETs.

Results and discussionCharacterization of SP SAMs

The four-step synthesis of alkanethiol-terminated spirothiopyran(SP–SH) used here is provided in the ESI.† The formation of SPSAMs, device fabrication and characterization can be found inthe Experimental section. The reversible isomerization betweenthe colorless (SP-closed) and colored (SP-open) forms of SP–SHmolecules was investigated by UV-Vis absorption spectroscopyboth in dilute solution and in thin films. Fig. 2A and Fig. S1(ESI†) reveal that after sequential irradiation with UV (l = 365 nm)and visible lights (l 4 520 nm), a significant difference wasobserved in the absorption band (lmax, sol = 650 nm, lmax, film =675 nm), indicating that SP–SH molecules can reversibly switchbetween the closed and open forms both in solution and in thesolid state.10a Because of the interference of the surface plasmonresonance (SPR) of Au displayed as a broad absorption bandcentered at B510 nm,15 the UV-Vis absorption spectrum of a SPSAM on an Au electrode is not shown. The IR spectrum in Fig. 2Bshows that the sulfhydryl S–H shearing vibration of SP–SH at2558 cm�1 disappeared upon SAM construction, indicating theformation of S–Au bonds.6b Moreover, the IR spectrum exhibitedseveral features consistent with the structural characteristics ofSPs. The band observed at 1510 cm�1 is attributed to the typical

stretching absorption peak of the aryl nitro groups in SPs, whilethat at 1635 cm�1 is consistent with the CQC stretching bandsof the endocyclic double bond of SPs. Two bands at 1558 and1598 cm�1 are attributed to the stretching bands of aryl rings,and those at 1388 and 1459 cm�1 are assigned to alkyl bendingvibrations.12b These results confirm the successful modificationof the Au electrode surface with SPs, which is further verified bythe data obtained from X-ray photoelectron spectroscopy (XPS) asshown in Fig. 2C and D and Fig. S8 (ESI†). Peaks centered at399.6, 401.8, and 405.3 eV were observed, which are attributed toN 1s. The peak at 399.6 eV is consistent with the heterocyclicnitrogen atoms in a SP SAM; that at 401.8 eV is typical ofpositively charged amine groups, while that at 405.3 eV ischaracteristic of NO2 in SP.16 The S 2p peak at 161.8 eVcorresponds to the sulfur species (RS-Au) usually observed forthiolate-based self-assembled monolayers on gold; the othertwo peaks at 163.1 eV and 164.1 eV usually originate fromunbound organic sulfur compounds. These findings are in goodagreement with our previous results.12b For macroscopic charac-terization, water contact angle measurements were conductedby dropping water droplets with a volume of 1 mL on the SAMsurface. Fig. 2E reveals that a light-induced reversible watercontact angle variation of 121 was obtained, indicating that thesurface of the SAM was more polarizable under UV irradiation(in SP-open form) compared to that under visible irradiation(in SP-closed form).12b,17 This observation provides additionalevidence for the effective conversion of SP molecules betweentheir two forms in the SAM. The successful immobilization ofSPs on the Au surface sets the foundation for the development ofphotoresponsive devices.

Electrical and photoresponsive properties

After the characterization of SP SAMs, 40 nm-thick pentacenethin-film transistors with SAM-modified Au electrodes (55 mmin length and 2 mm in width) were fabricated (Fig. 1). The averagesaturation m of these devices was B0.02 � 0.002 cm2 V�1 s�1

(Fig. S2, ESI†). An obvious and reversible variation in ID occurred inthese photosensitive OFETs modified by SP SAMs (B80 devices)

Fig. 1 Schematic representation of the OFET architecture with Au electrodesmodified by SP SAMs. Photochromic SP molecules at the electrode/semiconductor interface undergo reversible photoisomerization betweenSP-closed and SP-open forms, modulating the device photocurrent. Vis:visible light; UV: UV light.

Fig. 2 Characterization of SP SAMs. (A) Solid-state UV-Vis absorptionspectra of SP–SH thin films on quartz substrates under irradiation withlight of different wavelengths (UV: red, and Vis: black). (B) IR spectrum ofSP SAM-modified Au thin films on silicon wafer substrates. (C) XPS N (1s)peaks of a SP SAM. (D) XPS S (2p) peaks of a SP SAM. (E) Changes of thewater contact angle of a SP SAM induced by light of different wavelengths.

Paper Journal of Materials Chemistry C

Publ

ishe

d on

27

Apr

il 20

16. D

ownl

oade

d by

Bei

jing

Uni

vers

ity o

n 15

/08/

2016

08:

51:1

2.

View Article Online

This journal is©The Royal Society of Chemistry 2016 J. Mater. Chem. C, 2016, 4, 5289--5296 | 5291

under alternating irradiation with UV and visible light. Fig. 3A andB display typical photoswitching behaviors of a SP SAM-modifiedtransistor. Noticeably, ID largely increased under UV irradiation forB1000 s, regardless of the gate bias. Furthermore, after B1600 sof visible-light irradiation, ID decreased, and the high-conductancestate returned to its original low-conductance state. The temporalevolution of the photoswitching behavior was observed; the corres-ponding current–voltage curves showing one complete switchingcycle under irradiation with UV and visible light are provided inthe inset of Fig. 3B. When the device was irradiated with UV light, aB3% increase in the total current variation was observed, whichis typically attributed to the photoexcitation of organicsemiconductors.

The kinetics of each process shown in the inset of Fig. 3Bcan be fitted with a single exponential, indicating different rateconstants in each part: K(UV current) = B3.6 � 0.6 � 10�3 s�1,K(visible current) = B2.7 � 0.3 � 10�3 s�1, and K(dark) = B4.0 �0.2 � 10�4 s�1 (data obtained in the dark is not shown). Thesekinetic results determined from the transistor characteristicsare in good agreement with those obtained from UV-Vis absorptionspectra (Fig. S1, K(UV spectrum) = B4.4 � 0.5 � 10�3 s�1,K(visible spectrum) = B2.9 � 0.1 � 10�3 s�1, ESI†).12b,18 Therefore, thereversible photoswitching of the conductance of the SP SAM-functionalized devices is very similar to the reversible photo-isomerization of SP–SH in solution. This suggests that thephotoisomerization of SP is responsible for the modulation ofthe OFET characteristics under visible and UV irradiation. Thekinetic data also indicate that the conversion percentage (xe) ofSP–SH molecules from the SP-closed form to the SP-open onein the photostationary state in the devices is B52.9%.12c Toprove this, we have fabricated control devices based on pristine

pentacene without SP SAMs by using the same fabrication process.We measured the photoresponsive behaviors. The results shownin Fig. S5 (ESI†) revealed that neither UV nor Vis light illuminationcould change the device’s characteristics only except the photo-current jumps during both UV and visible illumination, whichshould be attributed to the intrinsic photoresponses of pentacene.As shown in Fig. 3C, the devices exhibited good operationalstability without obvious degradation by regulating the irradiationtime in an ambient atmosphere after operation over a period ofB2800 s. The slight increase of both on- and off-currents isprobably due to the incomplete conversion from SP-open toSP-closed under visible light irradiation.

We found that the devices retained a high conductance stateover a long retention time in the dark after programmingwith UV light (Fig. S3, ESI†). After over 2.5 h of continuousmeasurements in the dark, the devices maintained recordedinformation with slight current loss. Afterwards, we exposedthe devices to visible light and monitored again the currentchange over time in the dark (Fig. S3, ESI†). ID also shows only aslight decrease, which should be attributed to device degradation.In addition to ID, the threshold voltage (VT) is the second methodused to evaluate the retention ability of a device with memorycapability. We preset the devices with UV or Vis light, respectively,and kept them in the dark for a series periods of time. Then bymeasuring the transfer curves, we can extract the VT. We foundthat after keeping the devices in the dark for more than 24 h, theVT of SP-closed only decreased by 5%, while in the SP-open casethe change was as large as 50% (Fig. S4, ESI†). These results showthe relatively weak thermal stability of SP-open, which is notsuitable for practical non-volatile memory applications.

However, our photochromic system can be applied to high-responsivity organic phototransistors. Based on the ID curvesshown in Fig. 3B, the important parameters of photoresponsivity(R), expressed in A W�1, and the current change ratio (P) werecalculated using eqn (1) and (2), respectively,2k

R ¼ Ilight

Pill¼ Il � Idarkj j

Pill¼ Il � Idarkj j

IillLW(1)

P ¼ signal

noise¼ Ilight

Idark¼ Il � Idarkj j

Idark(2)

where Ilight is the drain current induced by the light, Il is the draincurrent under illumination, Idark is the drain current in the dark,Pill is the incident illumination power on the channel of thedevice, Iill is the intensity of the light, W is the channel width, andL is the channel length. Fig. 3D illustrates the bias-dependentphotoresponsivity of a device (source–drain voltage VD = �100 V).For SP SAM-functionalized devices, the best data achieved wereR = B800 A W�1 and P = B60 when VG was scanned from 10 to�100 V under UV irradiation using a very low effective light powerdensity of 7.4 mW cm�2. The R values are greater than thoseobtained for most organic phototransistors and amorphoussilicon.2k,19 Considering the low photoresponsivity of pentaceneitself (B10 A W�1),2i we conclude that the high photoresponsivityof our functional devices primarily originates from the photo-isomerization of SP molecules in SAMs (495% contribution).

Fig. 3 Photoresponsive characteristics of pentacene OFETs containingSP SAM-modified electrode/semiconductor interfaces. (A) Output and(B) transfer properties of a device before and after UV irradiation.VD = �100 V. The inset shows the change of ID over one completeswitching cycle, VD = �30 V, VG = �15 V. (C) Temporal evolution of ID forthe same device over a period of B2800 s, revealing reversible photo-switching modulation under alternating irradiation with UV and visiblelight. VD = �30 V, VG = �15 V. (D) Dependence of responsivity (R) andphotosensitivity (P) on the VG of a working device. VD = �100 V (effectiveirradiance power: 7.4 mW cm�2).

Journal of Materials Chemistry C Paper

Publ

ishe

d on

27

Apr

il 20

16. D

ownl

oade

d by

Bei

jing

Uni

vers

ity o

n 15

/08/

2016

08:

51:1

2.

View Article Online

5292 | J. Mater. Chem. C, 2016, 4, 5289--5296 This journal is©The Royal Society of Chemistry 2016

The P value is smaller than that obtained in a previous case wherephotochromic molecules were directly used as semiconductors20

and larger than that achieved in another case where photo-chromic molecules were incorporated into an organic semi-conductor matrix to form two component blends.21 Theseresults demonstrate the efficiency of our strategy to constructorganic phototransistors with high responsivity by modifyingelectrodes with photoactive layers composed of photochromicmolecules.

Characterization of the contact resistance

It is essential to investigate the mechanism of the photo-responsive behaviors of our devices containing a SP SAM-modified electrode/semiconductor interface. Charge injection

in OFETs is well known to be strongly affected by the propertiesof the electrode/semiconductor interface,5a,7a and Rc is themain parameter that influences the injection process in OFETs.4a

Therefore, we hypothesize that the reversible modulation of Rc

induced by the photoisomerization of SP molecules in the SAM isone of the reasons for the light-responsive ability. In a previouswork,4d where azobenzene-SAMs were used to modify Au electrodes,this modulation of Rc was not demonstrated. The transmissionline method (TLM) was used to obtain detailed informationabout the Rc of our bottom-gate, bottom-contact devices.5b

The OFET channel length (L), which is defined as the distancebetween the source and drain electrodes, was varied from 45 to120 mm, and the channel width (W) was maintained at 2 mm.The source–drain metal electrodes were modified with SP SAMs.

Fig. 4 Photoswitching mechanism of SP SAM-functionalized OFETs. (A) Relationship between RON and the channel length at VG = �40 V andVD = �50 V for pristine pentacene OFETs (black), SP SAM-functionalized OFETs after sufficient UV (red) and visible-light (blue) irradiation. (B) Packingmodels of SP SAMs on the Au(111) surface viewed along the normal to the surface (top) and the axis of the unit cell (bottom). (C) Electrostatic potentialprofiles averaged in the planes parallel to the normal axis for the SP SAM-modified gold surface. Insets are the magnified areas showing the interfacialdipole contribution to the work function shift (a 0.24 eV decrease for the SP-closed state and a 0.01 eV decrease for the SP-open state). (D–F) Energylevel diagrams of SP SAM-modified electrode/semiconductor interfaces. FAu is the work function of Au, which is located below the highest occupiedmolecular orbital (HOMO) of the organic semiconductor (pentacene). (D) Shows the pristine injection barriers of both electrons (Fe) and holes (Fh) for anunmodified interface (without a SP SAM); (E) shows that a SP SAM without UV irradiation (in the SP-closed form) has an interfacial dipole that decreasesFAu to 4.92 eV. (F) Shows that a SP SAM after UV irradiation (in the SP-open form) imposes an interfacial dipole that decreases FAu to 5.15 eV. Symbols (+)and (�) show the directions of the interfacial dipole. Evac: local vacuum energy level; IE: ionization energy; EA: electron affinity.

Paper Journal of Materials Chemistry C

Publ

ishe

d on

27

Apr

il 20

16. D

ownl

oade

d by

Bei

jing

Uni

vers

ity o

n 15

/08/

2016

08:

51:1

2.

View Article Online

This journal is©The Royal Society of Chemistry 2016 J. Mater. Chem. C, 2016, 4, 5289--5296 | 5293

For the linear operating regime of each OFET, the ON resistanceRON can be expressed as,

RON Lð Þ ¼ @VD

@ID

����VD!0;VG

¼ Rch Lð Þ þ Rc

¼ L

WmiCi VG � VTð Þ þ Rc (3)

where Rch is the channel resistance, Ci is the gate dielectriccapacitance, VT is the threshold voltage, and mi is the intrinsicmobility. Rc, which is associated with the contacts between thesource–drain electrodes and semiconductor layer, can be calcu-lated from RON in the linear region of the output characteristics.Fig. 4A shows a plot of RON as a function of L (VG = �40 V).A linear relationship was observed between RON and L, indicatingthat RON is well expressed by eqn (3). By extrapolating therelationship of RON against L to L = 0, Rc was determined. Basedon the data in Fig. 4A, the Rcs of a bare Au/pentacene interface,SP SAM-modified Au/pentacene interface before (SP-closed form)and after UV irradiation (SP-open form) are B8.6 MO, B26.9 MOand B12.5 MO, respectively. These results indicate that the Rc ofthe electrode/semiconductor interface with the SP-open form isB53% smaller than that of the interface with the SP-closed form.This demonstrates that the photoisomerization of SP moleculessandwiched between each Au electrode and organic semiconductorlayer induces a marked change in Rc, which modulated theinjection/extraction of the carrier, and thus the device conductance.

Physical mechanism

In general, Rc is derived from the Schottky barrier of theelectrode/semiconductor contact. This mechanism describesthe dependence of carrier injection and collection on theSchottky barrier height, fb. At an ideal metal electrode/organicsemiconductor interface, fb is simply the difference betweenthe work functions of metal electrodes (fm) and the highestoccupied molecular orbital (HOMO) or lowest unoccupiedmolecular orbital (LUMO) levels of the organic semiconductors.As reported previously, the electronic state of fAu can be modu-lated by the dipole alignment of molecules in a SAM.5a,7a,22

Similarly, in our photoswitchable SP SAM-modified OFETs, weinvestigated the change of fAu caused by the reversible trans-formations of the dipole moment of SP molecules during theirphotoisomerization.

The change of fAu upon the formation of SP SAMs for theopen and closed isomers was studied by first-principles densityfunctional theory calculations. In the calculations, the SAMon 5 layers of a (4 � 4) Au(111) surface with a lattice constantof 11.56 Å is chosen (Fig. 4B). Details about the theoreticalmethodology and geometry optimizations are reported in theExperimental section. Fig. 4C illustrates the calculated planeaveraged electrostatic potential profiles to the optimized struc-tures of SP-closed and SP-open isomers. After setting the zero ofenergy to the Fermi level of the system, the work function is directlyreadable from the profile value in the vacuum area showing4.92 and 5.15 eV in the SP-closed and open state, respectively.The calculated work functions of both SP-closed and open stateswere in consistence with the UPS measurements (Fig. S6, ESI†).

This corresponds to an upward shift of the work function by 0.24and 0.01 eV in both cases compared to the bare gold surface(theoretical value: 5.16 eV).

Since SP SAMs are supported on a monolayer of dodecanethiol(C12) SAM, and assuming a defect-free SAM formation, weattribute the gold work function change to dodecanethiol (C12)SAM and SP moieties.23

DfAu = DfC12+ DfSP (4)

In our system, the change of fAu originates from the dipolemoments at the interface, which can be decomposed into twocontributions. Theoretical calculations were performed toinvestigate the contribution of the molecular backbone ofdodecanethiol (C12) and SP moieties to their dipole moment.The details of these calculations are presented in Fig. S7 (ESI†).For the SP-closed isomer, alkanethiols adopt an approximateoff-normal tilt (a) of 151, the dipole along the normal axis isestimated to be 2.07 D. The normal axis component of theoptimized geometry for isolated SP-closed molecules is 2.06 D,with the same direction as dodecanethiol. For the SP-openisomer, considering a to be 211, the normal axis dipole ofalkanethiols is B1.89 D, slightly smaller than that in theSP-closed case. However, the contribution of SP-open moietiesis �0.93 D, which is a counter-acting contribution to alkanethiols.As a result, the total dipole moment of the optimized geometryfor SP-closed SAM is B4.13 D, and that for a SP-closed SAM isB0.96 D.

For efficient hole injection into a p-type semiconductor,the fAu of an Au electrode has to match the HOMO energylevel (EHOMO) of the organic semiconductor. The hole injectionbarrier, fh, at a SP SAM-modified Au/pentacene interface wascalculated using an EHOMO, pentacene of �5.00 eV (Fig. 4D).24 Asshown in Fig. 4E, in the case of a SAM of SP-closed molecules,fh between fAu+SAM,closed and EHOMO, pentacene was 0.08 eV. Thiswill create a small injection barrier for holes, resulting inhigher contact resistance. When in the SP-open form, whichhas higher work functions for the gold electrode, there is noenergetic barrier for hole injection (Fig. 4F). This resultsin more efficient charge injection in the SP-open case, andtherefore increased device photocurrent. This prediction is ingood agreement with the experimental observations (Fig. 4A).

Ideally, small variations in the injection barrier typically resultin large current variations in OFETs, even exceeding one orderof magnitude.25 In our case, the current change of our devicesobtained experimentally is not as high as expected, which isprobably caused by the incomplete conversion of SP–SH moleculesunder illumination (B52.9% converted from SP-closed to SP-open)and the non-optimized operating conditions. The SP-open zwitter-ionic states are charged. Therefore, another possibility is thatcarrier trapping or carrier scattering induced by SP-open zwitter-ionic states could modulate the device photocurrent as demon-strated by previous reports,12b,14c where SP molecules were used tofunctionalize the dielectric/semiconductor interfaces. In that case,charges in SP can interact with the carriers in the channel ofOFETs. However, in the case of our current work, consideringthe relatively high source–drain voltage, the trapped charges in

Journal of Materials Chemistry C Paper

Publ

ishe

d on

27

Apr

il 20

16. D

ownl

oade

d by

Bei

jing

Uni

vers

ity o

n 15

/08/

2016

08:

51:1

2.

View Article Online

5294 | J. Mater. Chem. C, 2016, 4, 5289--5296 This journal is©The Royal Society of Chemistry 2016

SP-SAM modified electrodes should be negligible. Direct experi-mental evidence is still needed to distinguish between these twomechanisms.

Conclusions

We presented an efficient approach to achieve functional OFETswith the tunable channel conductance using the SP SAM-modifiedphotoactive electrode/semiconductor interfaces. The photoiso-merization of SP molecules initiates the reversible modulationof the dipole moment of SP SAMs, thus influencing the workfunction of Au electrodes to induce two distinct hole injectionbarriers at the electrode/semiconductor interface. This mechanismof photocontrolling charge injection barriers enables us to tuneRc and consequently modulate the channel conductance in anoninvasive manner, producing a new type of cost-effectiveOFET-based photodetector with high photosensitivity. Theseresults help us to better understand the interfacial phenomenain OFETs and reveal a novel strategy to construct multifunc-tional OFETs and low-cost sensors by interface engineering. Inaddition to their potential use as sensors and detectors, ourapproach of integrating molecular functionalities into electricalcircuits will aid the design and fabrication of a new generationof multifunctional interfaces and optoelectronic devices.

Experimental sectionFormation of SP SAMs

SP SAMs were formed on Au electrodes on SiO2 substrates.Au electrodes (50 nm thick) were deposited on the surface ofsilicon wafer substrates through a shadow mask. Before formingSAMs, the freshly-deposited gold electrodes were cleaned by O2

plasma. SP SAMs were formed by immersing cleaned substratesin an ethanolic solution of SP–SH (10�4 M) for 24 h. Afterimmersion, the resulting functionalized substrates were removedfrom the SP–SH solution, and immersed into ethanol for another24 h for eliminating the SP–SH molecules physically absorbedon the substrates. After that, the substrates were thoroughlyrinsed with ethanol and dried under a N2 stream. To keep SPin its closed form, the whole formation process of SP SAMs wasperformed in the dark. For IR, XPS and contact angle characteri-zation, we fabricated SP SAM samples on SiO2 substrates (1.5 cm�1.5 cm) that have been fully covered by 50 nm Au films. Theprocedure for forming SP SAMs on these substrates was the sameas that on Au electrodes.

Ultraviolet-visible (UV-Vis) spectra were recorded on a PerkinElmer Lambda35 UV-Vis spectrophotometer. X-ray photoelectronspectroscopy (XPS) data for SP SAMs were obtained using anESCALAB 220i-XL electron spectrometer (VG Scientific) using300 W AlKa radiation. The base pressure was about 3� 10�9 mbar.The binding energies were referenced to the C 1s line at284.8 eV from adventitious carbon. The water contact angleswere obtained using a Dataphysics OCA-20 contact angleinstrument. Infrared (IR) spectra were recorded on a Nicolet

ECTOR22 FT-IR. X-ray reflectivity data were obtained using aBruker D8-Advance diffractometer.

Device fabrication and characterization

After monolayer formation, pentacene films (40 nm) were fabricatedon the top of SP SAMs by thermal evaporation at a base pressure of2� 10�6 Torr and a rate of 0.1 A s�1. The channel length and widthwere 55 mm and 2 mm, respectively. The OFET devices for the TLMmeasurement were fabricated using shadow masks with variouschannel lengths from 45 to 120 mm and a fixed channel width of2 mm. Fig. 1 shows an OFET device with a bottom-gate, bottom-contact structure containing a 40 nm-thick pentacene semiconductorlayer and a 300 nm-thick SiO2 insulator layer.

Devices were characterized using the same probe stationand a semiconducting parameter analyzer (Agilent 4155C).Carrier mobilities (m) were calculated in the saturation regimeusing the standard method: ID = WCim(VG � VT)2/(2L), where ID

is the source–drain saturation current, Ci is the gate dielectriccapacitance (per area), VG is the gate voltage, and VT is thethreshold voltage. VT can be estimated as the x intercept of thelinear section of a plot of VG against (ID)1/2. Light irradiationwas performed using a handheld UV lamp (B10 mW cm�2,l = 365 nm) and a 150 W halogen incandescent lamp(Imax = B30 mW cm�2, l 4 520 nm). To avoid heating duringirradiation, visible light was focused and guided by a longoptical fiber to the probe station. To aid the analysis of theresults, we regulated the intensity of visible light so that thephotocurrents of the devices under visible irradiation wereequivalent to those induced by UV irradiation. By doing this,we could record the time traces of the drain currents of thedevices without obvious changes in current when UV andvisible light sources were switched. All measurements wereperformed under the same conditions at the same temperature.

Computational methods and models

First-principles density functional theory calculations werecarried out to study the molecular geometry and electronicstructure of the SAM/Au(111) systems. The static geometric andelectronic structures were performed using the VASP code26a

using a plane wave basis, the Perdew–Burke–Ernzerhof (PBE)density functional,26b,c and the projector augmented wave(PAW) potentials.26d,e An energy cutoff of 400 eV and G pointk-sampling were used. In the calculation, the SAM on 5 layers ofa (4 � 4) Au(111) surface with a lattice constant of 11.56 Å waschosen. The stacking density of SP molecules was one dye per116 Å2 or 1.07 mmol m�2. The vacuum region of more than 15 Åin the Z direction was applied, which is sufficiently large toeliminate the artificial periodic interaction.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (21225311, 91333102, 21373014 and21404060), 973 Project (2012CB921404), and Natural ScienceFoundation of Tianjin (14JCQNJC03800).

Paper Journal of Materials Chemistry C

Publ

ishe

d on

27

Apr

il 20

16. D

ownl

oade

d by

Bei

jing

Uni

vers

ity o

n 15

/08/

2016

08:

51:1

2.

View Article Online

This journal is©The Royal Society of Chemistry 2016 J. Mater. Chem. C, 2016, 4, 5289--5296 | 5295

Notes and references

1 (a) C. D. Dimitrakopoulos and P. R. L. Malenfant, Adv.Mater., 2002, 14, 99–117; (b) A. R. Murphy and J. M. J.Frechet, Chem. Rev., 2007, 107, 1066–1096; (c) H. Sirringhaus,Adv. Mater., 2014, 26, 1319–1335; (d) C. Wang, H. Dong, W. Hu,Y. Liu and D. Zhu, Chem. Rev., 2011, 112, 2208–2267;(e) M. Kaltenbrunner, T. Sekitani, J. Reeder, T. Yokota,K. Kuribara, T. Tokuhara, M. Drack, R. Schwodiauer, I. Graz,S. Bauer-Gogonea, S. Bauer and T. Someya, Nature, 2013, 499,458–463.

2 (a) T. Sekitani, T. Yokota, U. Zschieschang, H. Klauk,S. Bauer, K. Takeuchi, M. Takamiya, T. Sakurai andT. Someya, Science, 2009, 326, 1516–1519; (b) S.-T. Han,Y. Zhou and V. A. L. Roy, Adv. Mater., 2013, 25, 5425–5449;(c) R. C. G. Naber, K. Asadi, P. W. M. Blom, D. M. de Leeuwand B. de Boer, Adv. Mater., 2010, 22, 933–945; (d) Y. Guo,C. Du, G. Yu, C.-a. Di, S. Jiang, H. Xi, J. Zheng, S. Yan, C. Yu,W. Hu and Y. Liu, Adv. Funct. Mater., 2010, 20, 1019–1024;(e) X. Guo, S. Xiao, M. Myers, Q. Miao, M. L. Steigerwald andC. Nuckolls, Proc. Natl. Acad. Sci. U. S. A., 2009, 106,691–696; ( f ) X. Guo, M. Myers, S. Xiao, M. Lefenfeld,R. Steiner, G. S. Tulevski, J. Tang, J. Baumert, F. Leibfarth,J. T. Yardley, M. L. Steigerwald, P. Kim and C. Nuckolls,Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 11452–11456;(g) G. Schwartz, B. C. K. Tee, J. Mei, A. L. Appleton, D. H.Kim, H. Wang and Z. Bao, Nat. Commun., 2013, 4, 1859;(h) L. Torsi, G. M. Farinola, F. Marinelli, M. C. Tanese,O. H. Omar, L. Valli, F. Babudri, F. Palmisano, P. G.Zambonin and F. Naso, Nat. Mater., 2008, 7, 412–417;(i) K.-J. Baeg, M. Binda, D. Natali, M. Caironi and Y.-Y.Noh, Adv. Mater., 2013, 25, 4267–4295; ( j ) E. Orgiu andP. Samorı, Adv. Mater., 2014, 26, 1827–1845; (k) Y. Guo, G. Yuand Y. Liu, Adv. Mater., 2010, 22, 4427–4447; (l) Y. Cao,M. L. Steigerwald, C. Nuckolls and X. Guo, Adv. Mater., 2010,22, 20–32; (m) C. Liao, M. Zhang, M. Y. Yao, T. Hua, L. Li andF. Yan, Adv. Mater., 2015, 27, 7493–7527; (n) M. Magliulo,K. Manoli, E. Macchia, G. Palazzo and L. Torsi, Adv. Mater.,2015, 27, 7528–7551; (o) C.-W. Tseng, D.-C. Huang and Y.-T. Tao,ACS Appl. Mater. Interfaces, 2015, 7, 9767–9775.

3 (a) Y. Yuan, G. Giri, A. L. Ayzner, A. P. Zoombelt, S. C. B.Mannsfeld, J. Chen, D. Nordlund, M. F. Toney, J. Huang andZ. Bao, Nat. Commun., 2014, 5, 35; (b) H. Minemawari,T. Yamada, H. Matsui, J. Tsutsumi, S. Haas, R. Chiba,R. Kumai and T. Hasegawa, Nature, 2011, 475, 364–367; (c) A. Y.Amin, A. Khassanov, K. Reuter, T. Meyer-Friedrichsen andM. Halik, J. Am. Chem. Soc., 2012, 134, 16548–16550; (d) H.-R.Tseng, H. Phan, C. Luo, M. Wang, L. A. Perez, S. N. Patel,L. Ying, E. J. Kramer, T.-Q. Nguyen, G. C. Bazan and A. J.Heeger, Adv. Mater., 2014, 26, 2993–2998; (e) G. Kim, S.-J. Kang,G. K. Dutta, Y.-K. Han, T. J. Shin, Y.-Y. Noh and C. Yang, J. Am.Chem. Soc., 2014, 136, 9477–9483; ( f ) X. Liu, Y. Guo, Y. Ma,H. Chen, Z. Mao, H. Wang, G. Yu and Y. Liu, Adv. Mater., 2014,26, 3631–3636.

4 (a) C.-A. Di, Y. Liu, G. Yu and D. Zhu, Acc. Chem. Res., 2009,42, 1573–1583; (b) C. Jia and X. Guo, Chem. Soc. Rev., 2013,

42, 5642–5660; (c) L. Miozzo, A. Yassar and G. Horowitz,J. Mater. Chem., 2010, 20, 2513–2538; (d) N. Crivillers,E. Orgiu, F. Reinders, M. Mayor and P. Samorı, Adv. Mater.,2011, 23, 1447–1452.

5 (a) B. de Boer, A. Hadipour, M. M. Mandoc, T. vanWoudenbergh and P. W. M. Blom, Adv. Mater., 2005, 17,621–625; (b) C.-a. Di, G. Yu, Y. Liu, X. Xu, D. Wei, Y. Song,Y. Sun, Y. Wang, D. Zhu, J. Liu, X. Liu and D. Wu, J. Am. Chem.Soc., 2006, 128, 16418–16419; (c) J. Youn, G. R. Dholakia,H. Huang, J. W. Hennek, A. Facchetti and T. J. Marks,Adv. Funct. Mater., 2012, 22, 1856–1869.

6 (a) L. Wang, G. M. Rangger, L. Romaner, G. Heimel,T. Bucko, Z. Ma, Q. Li, Z. Shuai and E. Zojer, Adv. Funct.Mater., 2009, 19, 3766–3775; (b) D. Kafer, G. Witte,P. Cyganik, A. Terfort and C. Woll, J. Am. Chem. Soc., 2006,128, 1723–1732; (c) D. M. Alloway, M. Hofmann, D. L. Smith,N. E. Gruhn, A. L. Graham, R. Colorado, V. H. Wysocki,T. R. Lee, P. A. Lee and N. R. Armstrong, J. Phys. Chem. B,2003, 107, 11690–11699.

7 (a) X. Cheng, Y.-Y. Noh, J. Wang, M. Tello, J. Frisch, R.-P.Blum, A. Vollmer, J. P. Rabe, N. Koch and H. Sirringhaus,Adv. Funct. Mater., 2009, 19, 2407–2415; (b) K. Asadi, Y. Wu,F. Gholamrezaie, P. Rudolf and P. W. M. Blom, Adv. Mater.,2009, 21, 4109–4114; (c) N. Crivillers, S. Osella, C. Van Dyck,G. M. Lazzerini, D. Cornil, A. Liscio, F. Di Stasio, S. Mian,O. Fenwick, F. Reinders, M. Neuburger, E. Treossi,M. Mayor, V. Palermo, F. Cacialli, J. Cornil and P. Samorı,Adv. Mater., 2013, 25, 432–436.

8 (a) C. Bock, D. V. Pham, U. Kunze, D. Kafer, G. Witte andC. Woll, J. Appl. Phys., 2006, 100, 114517; (b) I. H. Campbell,S. Rubin, T. A. Zawodzinski, J. D. Kress, R. L. Martin,D. L. Smith, N. N. Barashkov and J. P. Ferraris, Phys. Rev.B: Condens. Matter Mater. Phys., 1996, 54, 14321–14324.

9 O. Acton, M. Dubey, T. Weidner, K. M. O’Malley, T.-W. Kim,G. G. Ting, D. Hutchins, J. E. Baio, T. C. Lovejoy, A. H. Gage,D. G. Castner, H. Ma and A. K. Y. Jen, Adv. Funct. Mater.,2011, 21, 1476–1488.

10 (a) G. Berkovic, V. Krongauz and V. Weiss, Chem. Rev., 2000,100, 1741–1754; (b) V. I. Minkin, Chem. Rev., 2004, 104,2751–2776.

11 (a) Q. Shen, Y. Cao, S. Liu, M. L. Steigerwald and X. Guo,J. Phys. Chem. C, 2009, 113, 10807–10812; (b) B. C. Bunker,B. I. Kim, J. E. Houston, R. Rosario, A. A. Garcia, M. Hayes,D. Gust and S. T. Picraux, Nano Lett., 2003, 3, 1723–1727;(c) J. A. Hutchison, A. Liscio, T. Schwartz, A. Canaguier-Durand, C. Genet, V. Palermo, P. Samorı and T. W. Ebbesen,Adv. Mater., 2013, 25, 2481–2485.

12 (a) Q. Shen, Y. Cao, S. Liu, L. Gan, J. Li, Z. Wang, J. Hui,X. Guo, D. Xu and Z. Liu, J. Phys. Chem. Lett., 2010, 1,1269–1276; (b) H. Zhang, X. Guo, J. Hui, S. Hu, W. Xu andD. Zhu, Nano Lett., 2011, 11, 4939–4946; (c) Q. Shen,L. Wang, S. Liu, Y. Cao, L. Gan, X. Guo, M. L. Steigerwald,Z. Shuai, Z. Liu and C. Nuckolls, Adv. Mater., 2010, 22,3282–3287; (d) C.-W. Tseng, D.-C. Huang and Y.-T. Tao, ACSAppl. Mater. Interfaces, 2012, 4, 5483–5491; (e) C.-W. Tseng,

Journal of Materials Chemistry C Paper

Publ

ishe

d on

27

Apr

il 20

16. D

ownl

oade

d by

Bei

jing

Uni

vers

ity o

n 15

/08/

2016

08:

51:1

2.

View Article Online

5296 | J. Mater. Chem. C, 2016, 4, 5289--5296 This journal is©The Royal Society of Chemistry 2016

D.-C. Huang and Y.-T. Tao, ACS Appl. Mater. Interfaces, 2013,5, 9528–9536.

13 Y. Ishiguro, R. Hayakawa, T. Yasuda, T. Chikyow andY. Wakayama, ACS Appl. Mater. Interfaces, 2013, 5, 9726–9731.

14 (a) C. Raimondo, N. Crivillers, F. Reinders, F. Sander,M. Mayor and P. Samorı, Proc. Natl. Acad. Sci. U. S. A.,2012, 109, 12375–12380; (b) E. Orgiu, N. Crivillers, M. Herder,L. Grubert, M. Patzel, J. Frisch, E. Pavlica, D. T. Duong,G. Bratina, A. Salleo, N. Koch, S. Hecht and P. Samorı,Nat. Chem., 2012, 4, 675–679; (c) Y. Li, H. Zhang, C. Qi andX. Guo, J. Mater. Chem., 2012, 22, 4261–4265.

15 G. W. Lu, B. L. Cheng, H. Shen, Z. H. Chen, G. Z. Yang, C. A.Marquette, L. J. Blum, O. Tillement, S. Roux, G. Ledoux,A. Descamps and P. Perriat, Appl. Phys. Lett., 2006, 88,023903.

16 (a) D. A. Stenger, J. H. Georger, C. S. Dulcey, J. J. Hickman,A. S. Rudolph, T. B. Nielsen, S. M. McCort and J. M. Calvert,J. Am. Chem. Soc., 1992, 114, 8435–8442; (b) D. Dattilo,L. Armelao, G. Fois, G. Mistura and M. Maggini, Langmuir,2007, 23, 12945–12950; (c) G. Beamson, D. Briggs, HighResolution XPS of Organic Polymers: The Scienta ESCA300Database, John Wiley & Sons, Chichester, ISBN 0471 935921,1992.

17 R. Rosario, D. Gust, M. Hayes, F. Jahnke, J. Springer andA. A. Garcia, Langmuir, 2002, 18, 8062–8069.

18 H. Zhang, J. Hui, H. Chen, J. Chen, W. Xu, Z. Shuai, D. Zhuand X. Guo, Adv. Electron. Mater., 2015, 1, 1500159.

19 Y. Kaneko, N. Koike, K. Tsutsui and T. Tsukada, Appl. Phys.Lett., 1990, 56, 650–652.

20 R. Hayakawa, M. Petit, K. Higashiguchi, K. Matsuda, T. Chikyowand Y. Wakayama, Org. Electron., 2015, 21, 149–154.

21 M. E. Gemayel, K. Borjesson, M. Herder, D. T. Duong, J. A.Hutchison, C. Ruzie, G. Schweicher, A. Salleo, Y. Geerts,S. Hecht, E. Orgiu and P. Samorı, Nat. Commun., 2015,6, 6330.

22 N. Crivillers, A. Liscio, F. Di Stasio, C. Van Dyck, S. Osella,D. Cornil, S. Mian, G. M. Lazzerini, O. Fenwick, E. Orgiu,F. Reinders, S. Braun, M. Fahlman, M. Mayor, J. Cornil,V. Palermo, F. Cacialli and P. Samori, Phys. Chem. Chem.Phys., 2011, 13, 14302–14310.

23 D. M. Alloway, M. Hofmann, D. L. Smith, N. E. Gruhn, A. L.Graham, R. Colorado, V. H. Wysocki, T. R. Lee, P. A. Lee andN. R. Armstrong, J. Phys. Chem. B, 2003, 107, 11690–11699;L. F. N. Ah Qune, H. Akiyama, T. Nagahiro, K. Tamada andA. T. S. Wee, Appl. Phys. Lett., 2008, 93, 083109.

24 J. H. Park, H. S. Lee, S. Park, S.-W. Min, Y. Yi, C.-G. Cho,J. Han, T. W. Kim and S. Im, Adv. Funct. Mater., 2014, 24,1109–1116.

25 K. Asadi, T. G. de Boer, P. W. M. Blom and D. M. de Leeuw,Adv. Funct. Mater., 2009, 19, 3173–3178.

26 (a) G. Kresse and J. Furthmuller, Phys. Rev. B: Condens.Matter Mater. Phys., 1996, 54, 11169–11186; (b) J. P. Perdew,K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77,3865–3868; (c) J. P. Perdew, K. Burke and M. Ernzerhof,Phys. Rev. Lett., 1997, 78, 1396; (d) P. E. Blochl, Phys. Rev. B:Condens. Matter Mater. Phys., 1994, 50, 17953–17979;(e) G. Kresse and D. Joubert, Phys. Rev. B: Condens. MatterMater. Phys., 1999, 59, 1758–1775.

Paper Journal of Materials Chemistry C

Publ

ishe

d on

27

Apr

il 20

16. D

ownl

oade

d by

Bei

jing

Uni

vers

ity o

n 15

/08/

2016

08:

51:1

2.

View Article Online