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Phys. Status Solidi A 206, No. 9, 2198–2201 (2009) / DOI 10.1002/pssa.200881788 p s sapplications and materials science
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Towards printed magnetic sensorsbased on organic diodes
Sayani Majumdar*,1,2, Himadri S. Majumdar1, Daniel Tobjork1, and Ronald Osterbacka1
1 Center for Functional Materials and Department of Physics, Abo Akademi University, Porthansgatan 3, 20500 Turku, Finland2 Wihuri Physical Laboratory, Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
Received 19 September 2008, revised 10 December 2008, accepted 26 February 2009Published online 28 July 2009
PACS 73.43.Qt, 73.61.Ph, 75.47.–m, 85.75.–d
∗ Corresponding author: e-mail [email protected], Phone: +358 2 333 6240, Fax: +358 2 215 4776
We report a study of magnetotransport properties of regio-regular poly(3-hexyl thiophene) (RRP3HT)-based organicdiodes. The devices were fabricated using two differenttechniques namely spin coating and inkjet printing. Positivemagnetoresistance (MR) effect was observed at roomtemperature in all the devices. The highest MR magnitudereached up to 16% for some spin-coated devices and up to 10%in inkjet printed devices. The MR magnitude and line shapes
were found to depend strongly on the measuring current. Weobserved deviation from the theoretically predicted Lorentzianor non-Lorentzian line shape of the MR traces, which isdiscussed in detail in the article. Although, the printed devicesexhibit MR response as high as for the spin-coated ones, theystill need to be optimised in terms of performance and yield forlarge scale applications as magnetic sensors.
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Conjugated or conductive polymersare subject of great research interest due to their rich chemicaland physical properties and also due to potential applicationin devices made using simple fabrication techniques suitablefor large scale applications [1, 2]. One such application isthe capability of sensing magnetic field. Reports from late1990s [3, 4] showed that organic semiconductor (OS) smallmolecules and polymers can have sizable magnetic fieldeffect due to change in spin state of the carriers. Very littleattention was paid in exploring the application possibilities ofsuch effect until in 2004, when Francis et al. [5] demonstratedthat organic diodes show 10–15% change in electricalresistance under an applied magnetic field of 10–15 mT atroom temperature. The physical understanding of the effectof magnetic field on the charge (spin) dynamics of the devicesis still strongly debated. Different models have been proposedbased on spin dynamics induced by hyperfine interaction,namely, electron–hole pair mechanism [6], triplet-excitonpolaron quenching model [7] and bipolaron model [8]. Thetwo former models explain the organic magnetoresistance(OMAR) as the effect of magnetic field on the singlet–tripletmixing [6] of excitons and scattering of charge carriersby triplet excitons together with quenching of tripletsby paramagnetic centres [7]. However, some experimental
results show that OMAR can be observed even in systemshaving almost single type of charge carriers and very limitedpossibility of exciton formation [9].
Earlier, we have investigated the magneto-transportresponse of regio-regular poly 3(hexyl-thiophene)(RRP3HT) diodes and bulk heterojunction solar cellsmade from RRP3HT:1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene (PCBM) blends and clarifiedthat a reduced probability of pair formation of electrons andholes lead to disappearance of magnetoresistance (MR) [10].Though in this article we will mainly report the observationof OMAR in inkjet-printed diodes, we would also indicatesome deviations of the experimental results of OMAR lineshapes from the proposed theoretical models.
Inkjet printing is very attractive for printing organicelectronic components due to advantages like low cost, lowmaterial consumption, non-contact patterning and roll-to-roll compatibility. Different organic electronics componentslike organic light-emitting diodes [11], organic photovoltaiccells [12] and organic thin film transistors [13] have alreadybeen fabricated and reported using the inkjet technique.Inspite of the inherent advantages there are some commonexperimental obstacles with this method namely, clogging ofthe nozzles and formation of uneven films on the substrate.
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Phys. Status Solidi A 206, No. 9 (2009) 2199
These problems can, however, be prevented by properformulations of the inks. Clogging of the nozzles can beavoided by using suitable printing parameters and an inkwith a suitable surface tension (around 28 mN/m2), a lowor moderate viscosity (8–15 mPa s) and a high boiling point(>100 ◦C) solvent.
2 Experimental The spin-coated diode devices usedin the experiment have the structure indium tin oxide(ITO) / poly (3, 4-ethylenedioxythiophene)–poly(styrenesul-phonate) (PEDOT:PSS)/RRP3HT/lithium fluoride (LiF)/aluminium (Al). The ITO-coated glass electrodes werecoated with a very thin layer of PEDOT:PSS and annealedat 120 ◦C for 15 min. The π-conjugated polymer RRP3HT(from Plextronics) was spin coated from an ortho-dichlorobenzene (oDCB) solution and annealed at 120 ◦Cfor 15 min. Finally, the LiF layer and the Al electrode werevacuum evaporated to complete the device structure.
All inkjet printing was performed on a Dimatix MaterialsPrinter (DMP-2831) in ambient room atmosphere. Thestructure of the inkjet printed diodes were Al/RRP3HT/Silver(Ag), where all layers except for the Al were inkjetted.The formulated inks were filtered through 0.45 and 0.2 �mfilters before filling the 1.5 ml cartridges (DMC-11610).The printed diodes were manufactured on flexible plasticsubstrates (PET-505) with pre-patterned (1.5 mm wide)Al contacts. RRP3HT 0.67 wt.% (from Plextronics) wasdissolved in toluene and oDCB (2:1) and was used as theOS. After printing three layers with a drop spacing of 20 �m,the substrates were heated on a hot plate at 100 ◦C for30 min to assure a complete evaporation of the solvents.The silver top electrodes were then inkjet printed fromSunTronic U5603 ink which has a silver content of 20 wt.%and suitable rheological properties for inkjet printing. Toachieve a continuously printed (∼1 mm wide) area on the lowsurface energy RRP3HT surface, while heating the substrateto 60 ◦C, ten layers were printed using a drop spacing of20 �m. The sample was finally annealed on a hot plate at120 ◦C for 20 min to evaporate solvents and surfactants andto slightly sinter the silver nanoparticles, resulting in a silvertop electrode with a surface resistivity of less than 1 �/�.
After fabrication, the devices were transferred into acryostat and placed in between the pole pieces of theelectromagnet capable of producing up to 300 mT magneticfield. The MR of the device was measured by sendinga constant current through the device and measuring thevoltage drop across it in a varying magnetic field.
3 Results and discussion We observed as high as16% positive MR response at room temperature in the spin-coated diodes, where MR is defined as:
MR = RB − R0
R0
= �RB
R0
,
where RB is the device resistance under external magneticfield B and R0 is the zero field resistance. Figure 1shows the MR response of a typical diode with the
Figure 1 Magnetoresistance response as a function of magneticfield (B) of a typical spin-coated RRP3HT diode at roomtemperature measured with 100 �A current.
structure ITO/PEDOT:PSS/RRP3HT/LiF/Al measured atroom temperature and with a bias current of 100 �A.The MR response of the devices start increasing rapidly,above the respective diode threshold voltages, as shownpreviously [10]. When the device current is less than or closeto 1 �A, the MR traces are quite sharp and above a certainmagnetic field value (typically 50 mT) the MR trace tendstowards saturation.
However, at higher current values, the MR line-shape starts changing clearly and saturation is no longerobserved in the MR traces. As shown in Fig. 2, whenthe device current is above 1 �A, the complete magneticfield range (0–300 mT) cannot be fitted to the theoreticallypredicted Lorentzian (�I(B)/I ∝ B2/(B2 + B2
0)) or specificnon-Lorentzian (�I(B)/I ∝ B2/(|B| + B0)2) line shapesanymore (black line), where B is the applied magneticfield and B0 is the full width at half maximum ofthe magnetoconductance curve. For higher currents, a
Figure 2 (online colour at: www.pss-a.com) Experimental MRdata of a typical spin-coated RRP3HT diode at 300 K measured with1 µA current and fitting of the experimental data with the Lorentzianfit for the whole range of B (black line) and for lower B values (redline) and with the power law fit (blue line).
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Figure 3 (online colour at: www.pss-a.com) Experimental MRdata of a typical spin-coated RRP3HT diode at room temperaturemeasured with 100 �A current and fitting of the experimental datawith the power law and Lorentzian fits show clear deviation of B′
to the low field side for measurements with a higher current.
Lorentzian line shape (red, dotted line) fitting is possibleonly at lower magnetic fields.
Above a certain magnetic field (B′) the MR responseshows a power law dependence of B0.5 (blue, dashed line).Earlier,
√B dependence of MR at higher B values has
been observed in different inorganic disordered electronicsystems and its origin was interpreted as an electron–electroninteraction effect [14]. The two distinct line shapes at low andhigh magnetic fields indicate that the total MR response isthe sum of multiple physical effects.
We further investigated the effect of different currentdensities on the MR properties. The MR line shapes deviatedmore from the Lorentzian or non-Lorentzian line shapeswhen the measuring current is increased. Figure 3 showsMR response for a bias current of 100 �A. The B′ value shiftstowards even lower values when the current is higher. Whenthe B′ value is plotted as a function of different measuring cur-rent, it is observed that B′ decreases exponentially as a func-tion of the measuring current at room temperature (Fig. 4).
From these observations we can conclude that thereare two distinct effects governing the MR response. Atlower current densities and lower B values, the Lorentzianline shape indicates MR due to spin splitting whereas withincreasing measuring current and higher B, the power lawdependence of MR suggest inelastic scattering of chargecarriers leading to increased positive MR values. Exponentialdependence (showed by the fitting line in Fig. 4) of B0
value with the measuring current indicates that as the currentdensity is sharply increased in the diodes in their operatingregion, the spin-splitting effect is no more important and onlythe scattering mechanism between the carriers control theMR response.
While the fundamental physics of the OMAR effect stillremain debatable, the very high MR response in almost allOS materials have given rise to an interest in possible sensorapplications. The emergence of printed electronics has also
Figure 4 (online colour at: www.pss-a.com) B′ as a function ofmeasuring current shows an exponential decrease with increasingcurrent at 300 K.
opened up the possibility of using such devices for low-endapplications. This motivated us towards the prospect of inkjetprinted organic diodes as magnetic sensors.
Figure 5 shows the MR response of two typicalinkjet printed diodes measured with 1 �A current at roomtemperature. The devices showed positive MR responsebetween 6 and 12%. Although the MR response of thesedevices are almost as good as the spin-coated devices, thedevice yield is still less than 50% and also the devices are notas stable as the spin-coated ones. In one of the device response(black curve), an oscillating behaviour of MR response wasobserved at higher magnetic fields. However, other similardevices (red curve) measured did not show any particularpattern of this nature. Further study of printed diodes isneeded to fully understand the origin of this phenomenon.
Among the difficulties with the printed devices, was thespreading of the RRP3HT solution on the substrate, as well assome observed ‘coffee-stain effects’ [15]. The problem withthe ‘coffee-stain effect’, i.e. uneven drying on the substrate, ofthe inkjetted semiconductor could probably be overcome byincreasing the concentration (and viscosity) and by changingthe solvents.
Figure 5 (online colour at: www.pss-a.com) %MR response oftwo inkjet printed diodes measured with 1 �A current at roomtemperature.
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-a.com
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Phys. Status Solidi A 206, No. 9 (2009) 2201
Another problem during inkjetting was the difference inwetting behaviour on the Al and the polyester substrate (andthe P3HT surface) when printing the P3HT solution and onthe P3HT surface and the polyester substrate when printingthe Ag-nanoparticle dispersion. Better wetting on the P3HTsurface would have been achieved, e.g. by a surface plasmatreatment, but could possibly also have detrimental effectson the diode characteristics.
In conclusion, we have observed OMAR effect inpolymeric diodes fabricated using both the conventionalspin-coating technique and the new inkjet printing technique.High enough MR was recorded in both types of devices,which is extremely promising for immediate applicationas roll-to-roll printed flexible magnetic sensors but thereis still large scope for improvement. MR lineshapes arediscussed as a function of the measuring current and differentcomponents of MR response was pointed out. Clearerphysical understanding of the OMAR phenomenon andbetter control of printing methods are required for furtherdevelopment of this area of research.
Acknowledgements The authors thank Harri Aarnio forthe fabrication of some of the spin-coated diodes. The authors alsogratefully acknowledge the Academy of Finland for funding fromproject 116995, 107684 and Centre of Excellence Programme. S.M.acknowledges the SITRA–CIMO fellowship. Planar InternationalLtd. is acknowledged for the patterned ITO substrates.
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