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Synthetic Metals 160 (2010) 2061–2064

Contents lists available at ScienceDirect

Synthetic Metals

journa l homepage: www.e lsev ier .com/ locate /synmet

mproved device performance based on crosslinking of poly (3-hexylthiophene)

anoj Gaura, Jaya Lohanib, R. Ramanb, V.R. Balakrishnanb, P. Raghunathanc, S.V. Eswarana,∗

St. Stephen’s College, University of Delhi, Delhi 110007, IndiaSolid State Physics Laboratory, Lucknow Road, Delhi 110054, IndiaNational Brain Research Centre, Manesar 122050, India

r t i c l e i n f o

rticle history:eceived 6 April 2010eceived in revised form 14 July 2010

a b s t r a c t

Diode devices (glass/ITO/polymer/Al) have been fabricated using poly (3-hexylthiophene) (P3HT)crosslinked with two different biaryl crosslinkers. Crosslinking was performed by exposing the thin filmswith different wt% of crosslinker to UV irradiation and progress of crosslinking was monitored by IR spec-

ccepted 20 July 2010vailable online 21 August 2010

eywords:oly (3-hexylthiophene)rosslinkers

troscopy. An increase in hole mobility of two orders of magnitude has been observed after crosslinking.

© 2010 Elsevier B.V. All rights reserved.

isazidesiode devices

. Introduction

Poly (3-hexylthiophene) (P3HT) has emerged as one of the mostidely studied conjugated materials for electronic device applica-

ions due to its good film forming properties, strong absorption andigh hole mobilities. P3HT has been used for various device applica-ions such as fabrication of memory devices [1], thin film transistorsTFTs) [2,3], photovoltaic devices [4–8], optoelectronic devices [9]nd for the preparation of biosensors [10,11]. Hole mobility is anmportant parameter especially for photovoltaic applications. Hole

obilities of regioregular P3HT have been reported to be in orderf 10−5 cm2 V−1 s−1 as calculated from space charge limited cur-ent (SCLC) method [12–15]. However, in highly ordered films higheld-effect mobilities in the range of 0.1–1 cm2 V−1 s−1 have beenchieved [16].

The structural, optical and electrical properties of conjugatedaterials are governed by the extent of inter- as well as intra-

hain delocalization of �-orbitals. In solid state, dense packing ofhe polymer chains can allow overlap to occur in three-dimension.his 3D-electronic connectivity creates low resistivity pathways forhe charge carrier to travel and hence dense chain packing withetter overlap of �-orbitals can lead to high electrical conductiv-

ty. Since past few years, instead of synthesising new materials,he focus has shifted on the development of newer strategies toncrease hole mobilities. Some of the approaches employed in thisespect involves increasing molecular weight and decreasing poly-

∗ Corresponding author. Tel.: +91 11 27667462.E-mail address: sv.eswaran@gmail.com (S.V. Eswaran).

379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2010.07.023

dispersity [17,18], introduction of conjugated bridges in polymerbackbone [19,20], doping [21], blending with different materials[22,23] and thermal crosslinking by modifying the polymer sidechains and heating at high temperatures [24].

In continuation of our earlier work on crosslinking of aryl azides[25–27] and use of biaryls [28] for electronic device applications,we report here crosslinking of P3HT with two new biaryl basedbisazides. This simple and novel approach can be used as an alter-native strategy for better device performance.

2. Experimental

2.1. Materials and methods

Chemicals and solvents were purchased from Merck or S.D.Fine Chemicals. Regioregular P3HT (∼87 kDa) was purchased fromSigma–Aldrich Chemical Co. and used without any additionalpurification. Purification of compounds was done by column chro-matography over silica gel (60–120 mesh) with a gradient elutionusing hexane with increasing proportions of ethyl acetate. Meltingpoints were measured on an electrothermal melting point appa-ratus and are uncorrected. Optical measurements were done on aCARY-5E UV-Vis-NIR spectrophotometer on a quartz substrate. IRspectra were recorded on Spectrum BX series spectrophotometerusing KBr or Nujol (for compounds) and on GaAs substrate for film

state. 1H and 13C NMR spectra were recorded on Bruker AVANCE300 MHz instrument. Mass spectra were recorded on Waters Micro-analysis LCT Mass spectrometer (Model No. KC455). Film thicknesswas measured by SENTECH SE500 ellipsometer and AFM. AFMmeasurements were carried out on a Veeco CP II instrument in non-

2062 M. Gaur et al. / Synthetic Metals 160 (2010) 2061–2064

; (ii) C

csppt

2

lptatsFfwovomibdels

Fig. 1. Methodology for synthesis of crosslinkers (i) HNO3, 0–5 ◦C (for I)

ontact mode. A silicon nitride tip (sharpness 20 nm) was used andcan rate was kept 0.5–1 �ms−1. Nanoscope v 6.1.2 was used forrocessing of the images. Crosslinking was performed in Rayonethotoreactor fitted with eight UV tubes (254 nm) of 12 W each withhe fan of instrument kept on to avoid heating effect.

.2. Fabrication of devices

Devices were prepared on glass substrates, coated with a thinayer of indium tin oxide (ITO), which were patterned by etchingrocess and cleaned using standard wet cleaning method. Solu-ions of P3HT (2 wt%) were prepared in chlorobenzene after stirringt room temperature for 24 h in dark. The solutions were filteredhrough a 0.2 �m pore size PTFE membrane syringe filter beforepin coating at 2000 rpm to give a film with thickness ∼175 nm.or crosslinking experiment, the solutions were prepared with dif-erent concentrations of the crosslinker. The solvent from the filmsas removed in vacuo and metallic electrode contacts were formed

ver the film by thermal evaporation of aluminum (∼200 nm), in aacuum coating unit using a 10−6 mbar base pressure of the evap-ration chamber and a deposition rate > 1 A/s through a shadowask. The active device area was 0.4 cm2. The devices were kept

n vacuum till measurements were done in order to avoid dopingy oxygen. I–V measurements were performed on samples aftereposition of electrodes under normal class 10,000 clean room

nvironmental conditions (25 ◦C and RH ∼ 45–50%), using a Keith-ey 2410 source meter. For each experiment eight devices weretudied.

Fig. 2. UV–visible spectra of crosslinkers (I and II).

H3COOH/HNO3, 0 ◦C (for II); (iii) SnCl2/HCl, rt; (iv) HNO2/NaN3, 0–10 ◦C.

3. Results and discussion

Regular arrangement of the alkyl side chain in P3HT results ingood �-orbital overlap between neighboring chains possibly dueto formation of a lamellar stacking of the polymer chains [29].Better control of the structural order is expected to improve �–�interactions and hence efficiency of charge transport. In this con-text, we synthesised two new bisazides and used them to crosslinkP3HT as an alternative method to improve the �-overlap, which inturn could improve electronic interactions between the polymericstructures. The biaryl based bisazides (I and II) were synthesisedfrom their respective dinitro compounds (1b and 2b) via reduc-tion to diamino derivatives (3a and 3b) followed by diazotizationand displacement by azide ion (Fig. 1). All the compounds werefully characterized spectroscopically. For synthetic procedure andcharacterization data of compounds, see supporting information.

Both the crosslinkers show absorption maxima at ∼250 nm(Fig. 2). Crosslinking was performed at three different concen-trations (wt%) of the crosslinkers by irradiating the films to UVlight (254 nm) in a photoreactor. On photolysis the azides generatehighly reactive nitrenes which can insert into single bonds (C–H)or double bonds (thiophene ring) in the polymer. Though, the lattercould be energetically favoured over C–H insertion, but that woulddisturb �-overlap resulting in a decrease in mobility. The exactnature of crosslinking is difficult to pinpoint. Our results based onNMR studies (unpublished results) indicate the predominance ofC–H insertion in the alkyl side chains and not on the �-conjugatedpolymer backbone. No substantial change in the optical propertieswas observed by us after crosslinking, indicating that the polymer

retains its properties to be used as active material for device fab-rication. The progress of crosslinking (∼30–40 min) was followedby the disappearance of the characteristic peak for the azide group(∼2100 cm−1) in the IR spectrum, as shown in Fig. 3.

Fig. 3. IR of the films before and after crosslinking with crosslinker I (30 wt%).

M. Gaur et al. / Synthetic Metals 160 (2010) 2061–2064 2063

Fig. 4. Current–voltage characteristics of the glass/ITO/polymer/Al devices.Inset shows the log–log plot. Hole mobilities were calculated to be P3HT:4 −5 2 −1 −1 −5 2 −1 −1

532

ssDwalitsdtafic

J

wεv

crouiasff1rfcdwp

a

.5 × 10 cm V s ; exposed P3HT: 2 × 10 cm V s ; crosslinker I (10%):× 10−4 cm2 V−1 s−1; crosslinker I (20%): 1.5 × 10−3 cm2 V−1 s−1; crosslinker I (30%):× 10−4 cm2 V−1 s−1; crosslinker II (10%): 5 × 10−4 cm2 V−1 s−1; crosslinker II (20%):.9 × 10−3 cm2 V−1 s−1; crosslinker II (30%): 1.9 × 10−4 cm2 V−1s−1.

The crosslinkers were added in appropriate ratio to P3HTolution and films were prepared over ITO patterned glass sub-trates and protected form light till crosslinking was performed.iode devices with structure glass/ITO/polymer/Al were fabricated,here P3HT is the organic semiconductor sandwiched between ITO

cting as positive electrode and a negative electrode made of theow work function metal, aluminum. As a blank experiment, thats without the crosslinker, films of pristine P3HT were exposedo similar conditions. A linear rise in current with voltage andubsequent transition at high voltages to the region towards V2

ependence was observed (Fig. 4). Considering the current flow inhe trap free limit, the hole mobility can be described by the Mottnd Gurney model of space charge limited. By applying Child’s Law,eld independent mobility, �, can be calculated from the followingorrelation

= 98

�εε0V2

d3

here J is the current density, � is the free carrier hole mobility,is the dielectric constant (taken as 3), ε0 is the permittivity in

acuum, V is the applied voltage and d is the film thickness.From the intercept of the corresponding lines on the axis of

urrent density hole mobility was calculated. Using the above J–Velationship, mobility (Fig. 5) of pristine P3HT was calculated to bef the order of ∼10−5 cm2 V−1 s−1, which is in accordance for val-es reported earlier [12–15]. Crosslinking resulted in an increase

n hole mobility an order of magnitude of two for crosslinker Ind magnitude of one for crosslinker II. The mobility increasedharply using 10 wt% of the crosslinker to 5 × 10−4 cm2 V−1 s−1

or both the crosslinkers and thereafter, a decrease was observedor crosslinker II with mobilities 2.9 × 10−4 cm2 V−1 s−1 and.9 × 10−4 cm2 V−1 s−1 for devices with 20 and 30 wt% crosslinker,espectively. For crosslinker I hole mobility further increasedor 20 wt% crosslinker and then started decreasing when 30 wt%rosslinker (3 × 10−4 cm2 V−1 s−1) was used. For the latter, theecrease in mobility values is not too substantial and the values

ere found to be still better as compared to devices prepared formristine P3HT (Fig. 5).

The formation of traps by decomposition of azides is ruled outs we have earlier reported that formation of traps leads to degra-

Fig. 5. Comparison of hole mobility values after crosslinking with P3HT.

dation of devices and does not show any SCLC region [30]. Ourblank experiments show that UV radiation has no effect on holemobility of pristine P3HT. Elsewhere, Tu et al. [31] have reportedthat inter-chain branching of P3HT (from thiophene ring) results indecrease in hole mobility. Crosslinking, in our case, occurs at thealkyl side chain and hence does not disrupt �-overlap of the thio-phene backbone. Further, our observations show that the increasein hole mobility is dependent on concentration. AFM studies (seesupporting information) show that the crosslinked film shows amore amorphous structure (formation of aggregation with largegrain size and visible boundaries) as compared to pristine P3HT.

4. Conclusions

To summarize, crosslinking of P3HT with two biaryl crosslinkersleads to better device performance. Further studies using this novelapproach for photovoltaic application are in progress.

Acknowledgements

We thank The Principal, St. Stephen’s College, Delhi and Direc-tor, Solid State Physics Laboratory (SSPL) for providing facilities. Wethank Defence Research Development Organisation (DRDO) andUniversity Grants Commission (UGC), Govt. of India, New Delhi forthe grant of research projects. We thank Dr. Vikram Kumar, IIT,Delhi for valuable discussions.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.synthmet.2010.07.023.

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