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  • U.P.B. Sci. Bull., Series C, Vol. 74, Iss. 4, 2012 ISSN 1454-234x

    BEHAVIOR OF ELECTRICAL STRESSED FLEXIBLE RESISITVE LAYER.

    PART II: INTRINSICALLY CONDUCTIVE POLYMER

    Detlef BONFERT1, Paul SVASTA2, Ciprian IONESCU3

    Includerea senzorilor rezistivi n designul dispozitivelor electronice organice este necesar pentru a extinde domeniul aplicaiilor posibile. Pentru aceasta este necesar identificarea materialelor rezistive, a proceselor i metodelor de structurare, precum i analiza proprietilor rezistive pe substrat flexibil ale acestora. Dou materiale cu aplicaie larg n electronica organic sunt polimerii umplui cu carbon (carbon filled polymer, CFP) i polimerii cu conducie intrinsec (ICP, poly (3, 4-ethylendioxythiophene) doped with polystyrene sulfonate acid, PEDOT:PSS). Lucrarea are ca scop prezentarea comportrii acestor polimeri conductivi pe suport flexibil sub influena curentului continuu i n impulsuri, subliniind variaiile rezultate ale proprietilor rezistive. Lucrarea este prezentat n dou pri, prima parte analizeaz comportarea polimerilor umplui cu carbon iar partea a doua pune accentul pe polimerii cu conducie intrinsec.

    There is a necessity to include sensors (resistors) in the design of organic electronic devices in order to extend the range of possible applications. It is essential to identify potential resistive materials, the processes and methods to structure them and to analyze their resistive properties on flexible substrates. Two materials widely used in organic electronics are carbon filled polymer (CFP) and the intrinsically conductive polymer (ICP) poly (3, 4-ethylendioxythiophene) doped with polystyrene sulfonate acid (PEDOT:PSS). In this paper we focus on the DC and pulsed stress behavior of these conductive polymers on flexible substrates and the resulting changes of their resistive properties. The paper is presented in two parts. Part one deals with carbon filled polymers (CFPs)and part two analyzes the behavior of intrinsically conductive polymesrs (ICPs).

    Keywords: Thick Film Flexible Resistors, Carbon filled polymer, PEDOT:PSS, DC- and Pulsed Stress

    1 Researcher, Polytronics Dept., Fraunhofer EMFT, Munich, Germany, e-mail:

    [email protected] 2 Prof., Electronics Faculty, CETTI, University POLITEHNICA of Bucharest, Romania, e-mail:

    [email protected] 3 Reder, Electronics Faculty, CETTI, University POLITEHNICA of Bucharest, Romania, e-mail:

    [email protected]

  • 124 Detlef Bonfert, Paul Svasta, Ciprian Ionescu

    NOTE The first part of the paper analyzed the behavior of electrical stressed carbon

    filled polymesrs (CFPs): - Part I: Carbon Filled Polymer. This part was published in the previous issue of the Scientific Bulletin of UPB.

    1. Intrinsically conductive polymer layer

    Fig. 1.1. Optical view and 3D scan of PEDOT:PSS resistive layer on PET foil, RS 100 /,

    resistor width W 1.4 mm, resistor length L 3.55 mm, L/W=2.5, resistor thickness 5 m.

    The intrinsically conductive resistive test structures were also realized in thick film technology in a screen printing roll-to-roll process. The polymer PEDOT:PSS paste (Orgacon EL-P 3040) is also designed for the use on polyimide (PI) and PET foils. Fig. 1.1 gives an optical view and a 3D scan of the used flexible PEDOT:PSS polymer conductive layer on polyimide (PI) foil, (Upilex S). The nominal sheet resistance of the processed devices was RS 2.5 k/, for a layer thickness of 5 m.

    10 m

    a) b)

    Ag

    PEDOT:PSS grains

    PEDOT:PSS

    Fig. 1.2. Optical view of cured PEDOT:PSS polymer paste, Orgacon 3040, a) resistive layer with

    polymer conductor (silver), b) zoomed resistive layer showing PEDOT:PSS grains.

    1.1. DC electrical stress

    To determine the DC behavior of the layer resistance, a DC sweep was performed. Fig. 1.3 shows the area where the resistance changes due to excessive

  • Behavior of electrical stressed []. Part II: Intrinsically conductive polymer 125

    voltage rating outside the normal operating values. The resistor is heated up and the resistance decreases because of the negative TCR of the polymer material.

    Between pulsing, the DC spot measurement was set in the safe operating area to 0.1 mA.

    PEDOT:PSS Resistor , (Tempered 105C 2h,)Double Current Sweep 0-1mA, R01, PET Foil 1

    0.0

    1.0

    2.0

    3.0

    4.0

    5.0

    6.0

    7.0

    0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

    Current / mA

    Volta

    ge /

    V

    6.20

    6.22

    6.24

    6.26

    6.28

    6.30

    Res

    ista

    nce

    / k

    VoltageResistance

    Fig. 1.3. DC resistance during current sweep, PEDOT:PSS resistive layer on PET foil,

    RS 2.5 k/ (reversible changes).

    a) b)

    PEDOT:PSS Resistor , (Temp.105C 2h,),Curr. Sampling 0.1 - 2.0 mA, R01, PET Foil 1, Meas. 9-14

    6300

    6350

    6400

    6450

    6500

    6550

    0 2 4 6 8 10 12 14 16 18 20 22 24Time / s

    R /

    T1 = 26 C T2 = 28 C

    T3 = 30 CT4 = 38 C

    T5 = 49 C

    T6 = 65 C

    Ih = 0.5 mAIh = 1.0 mA

    Ih = 1.5 mA

    Ih = 2 mA

    Ih = 0.1 mA Ih = 0.2 mA

    RDC, PEDOT:PSS Layer, Tempered, Current Sampling, 0.1-5mA, R13, PET Foil 1, M2-M10

    4150

    4200

    4250

    4300

    4350

    4400

    4450

    0 2 4 6 8 10 12 14 16 18 20 22 24Time / s

    R4P

    -K /

    0.1mA0.5mA1.0mA2.0mA3.0mA4.0mA5.0mA0.1mAb

    1.0mA

    0.5mA

    5.0mA

    0.1mA a

    0.1mA b

    2.0mA

    4.0mA 3.0mA

    Fig. 1.4. DC resistance during current sampling, PEDOT:PSS resistive layer on PET foil,

    a) reversible changes, b) irreversible changes.

    Fig. 1.5.Temperature distribution during self- heating of PEDOT:PSS resistive layer on PET foil,

    hot spot in the center of the resistive layer.

  • 126 Detlef Bonfert, Paul Svasta, Ciprian Ionescu

    For thermal measurements the current through the PEDOT:PSS layer on PET foil was increased up to 3 mA. For each sampling current value, represented in Fig. 1.4 a), a thermal image with an IR-camera was made and analyzed. Increasing the sampling current up to 5 mA will result in a permanent resistance decrease of about 5 %, indicated in Fig. 1.4 b). Infrared (IR) measurements during current sampling give the temperature distribution during self- heating of the polymer resistor, as represented in Fig. 1.5. It also shows the temperature profile across the length and width of the resistive layer. In the center of the resistor occurs again a hot- spot. For a sampling current of 3 mA the hot- spot reaches a temperature of 102C.

    DC-Resistance vs. Temperature, PEDOT:PSS on PET, 3550 m x 1400 m x 5 m

    0

    1000

    2000

    3000

    4000

    5000

    6000

    7000

    8000

    0 10 20 30 40 50 60 70 80 90 100T / C

    R /

    Simul-VRHMeasSimul-Lin

    R = Roexp(To/T)

    Ro = 6900To = 6500 = 0.25 = 1/1+dd=3

    R(T)= R0[1+(T-T0)]

    R0 = 6620 = -721 ppm/C

    Fig. 1.6. Resistance versus temperature during self- heating of PEDOT:PSS resistive layer on PET

    foil. Fig. 1.6 shows the resistance change of the PEDOT:PSS layer versus the

    corresponding measured layer temperature. Also represented in this figure is the simulated temperature dependence of the layer resistance, according to [30]. PEDOT:PSS is considered a disordered, grain sized semiconductor with localized states in the energy band gap and conduction can be described by Motts law for variable range hopping (VRH), [12], [13], [14] and [15].

    a) b)

    PEDOT:PSS Resistor Orgacon 3040, Current Sweep 0-10mA, R13, PET Foil 01, M11

    0

    5

    10

    15

    20

    25

    30

    35

    40

    0 1 2 3 4 5 6 7 8Current / mA

    Volta

    ge /

    V

    3800400042004400460048005000520054005600

    R4P

    -K /

    VoltageResistance

    Voltage

    Resistance

    Resistorburnout

    PEDOT:PSS Resistor Orgacon 3040, Dissipated Power,Current Sweep 0-10mA, R13, PET Foil 01, M11

    0

    5

    10

    15

    20

    25

    30

    35

    40

    0 1 2 3 4 5 6 7 8Current / mA

    Volta

    ge /

    V

    0

    50

    100

    150

    200

    250

    300

    P / m

    W

    VoltagePower

    ResistorBurnout

    DissipatedPower

    Voltage

    Fig. 1.7. a) DC resistance during current sweep, PEDOT:PSS resistor on PI foil, b) power

    dissipation in the resistive layer (finally, resistor damage).

  • Behavior of electrical stressed []. Part II: Intrinsically conductive polymer 127

    Applying a current sweep up to 7 mA, as represented in Fig. 1.7 a), will finally lead to the burn- out of the resistive layer in the area of the hot spot. The dissipated power during burn out, shown in Fig. 1.7 b) reaches 0.25 W, corresponding to a power density of 5 W/cm2 of resistive area, which is less than the corresponding value of the carbon based resistor. Fig. 1.8 shows the burn out region in the center of the resistive layer. Because of the excessive heat the substrate PET foil is melted under the resistive layer. The image also reveals the straight cut across the resistors width in this area. Similar characteristic damage of DC- stressed resistive layer is presented in [34].

    a) b)

    Resistor burnout

    Fig. 1.8. a) Resistor burnout during DC stressing, b) caused by hot spot in the center of the flexible

    polymer PEDOT:PSS resistor on PET foil.

    1.2 Pulsed electrical stress

    The behavior of the PEDOT:PSS resistive layer during pulsed stress was studied applying 100 ns wide pulses from transmission line pulser with increasing and constant amplitudes.

    1.2.1 Influence of pulse amplitude on resistance

    To determine the influence of pulse amplitude on the resistance, TLP measurements were performed, similar to those on carbon based resistors, with stepwise increasing the open source TLP pulse amplitudes. Recording the measured voltage and current transients at the polymer layer during each voltage step, gives a 3D representation of the measurement, as shown in Fig. 1.9.

  • 128 Detlef Bonfert, Paul Svasta, Ciprian Ionescu

    Time

    P1

    Pulse

    num

    ber

    Pulse amplitude

    P2P3

    Pn

    Fig. 1.9. 3D representation of a) pulsed voltage at and b) pulsed current through the PEDOT:PSS

    resistive layer on PET foil, for TLP pulse width of 100 ns, TLP-voltage 0-1000 V, 2 V step.

    Transient Voltage and Current at DUT, TLP 100 ns, 900V, PEDOT:PSS on PET Foil 1, R13, 1300/, M1

    -1000

    100200300400500600700800

    -20 0 20 40 60 80 100 120Time / ns

    Pul

    sed

    Vol

    tage

    /V

    -0.30.00.30.50.81.01.31.51.82.0

    Pul

    sed

    Cur

    rent

    / A

    Pulsed current

    Pulsed voltage

    Fig. 1.10. Pulsed voltageat and pulsed current through PEDOT:PSS resistive layer on PET foil for

    TLP pulse width of 100 ns, TLP-voltage 900 V.

    Rpulsed , TLP 100 ns, 100 V and 900 V, PEDOT:PSS on PET Foil 1, R13, 1300/, M1

    2500

    3000

    3500

    4000

    4500

    20 30 40 50 60 70 80 90 100Time / ns

    Pul

    sed

    Res

    ista

    nce

    /

    TlpRes_900V_TlpTlpRes_100V_Tlp

    Calculated resistance during stress pulseTLP 900 V

    TLP 100 V

    Fig. 1.11. Pulsed resistance of PEDOT:PSS resistive layer on PET foil for TLP pulse width of

    100 ns, TLP-voltage 100 V and 900 V.

  • Behavior of electrical stressed []. Part II: Intrinsically conductive polymer 129

    During the pulses, the pulsed resistance is calculated from each corresponding point of the current- and voltage transients, represented in Fig. 1.10. Fig. 1.11 shows the pulsed resistance for two TLP voltages, 100 V to 900 V. The resistance decreases with increasing TLP voltage due to Joule heating during the pulse and of the negative TCR of the material.

    1.2.2 Influence of successive pulsing on resistance

    Fig. 1.12 depicts the pulsed current-voltage characteristic for four successive measurements, M1 M4. During each measurement the amplitude of the 100 ns wide pulses was stepwise increased, in 2 V steps, up to 1000 V of the TLP charging voltage. The different shapes of these characteristics indicate permanent and reversible changes that take place in the resistor during pulsing. This is confirmed by the absolute DC- resistance changes shown in Fig. 1.13 and Fig. 1.14, and by the pulsed resistance changes of Fig. 1.15.

    Pulsed I-V, TLP 100 ns, 2-1000V, step 2V, PEDOT:PSS layer on PET Foil 1, R 1300/, temp. R13, M1-M4

    0.0

    0.1

    0.1

    0.2

    0.2

    0.3

    0 100 200 300 400 500 600 700Pulsed Voltage at DUT / V

    Pul

    sed

    Cur

    rent

    / A

    M1M2M3M4

    Fig. 1.12. Pulsed current-voltage characteristic of PEDOT:PSS resistive layer on PET foil for

    successive measurements, pulse width 100 ns. RDC, TLP 100 ns, 2-1000V, step 2V, PEDOT:PSS layer on

    PET Foil 1, R13 1300/, temp., M1-M4

    3345

    3350

    3355

    3360

    3365

    3370

    0 100 200 300 400 500 600 700Pulsed Voltage at DUT / V

    DC

    Res

    ista

    nce

    /

    M1

    M2

    M4

    M3

    Fig. 1.13. DC-resistance change versus applied pulse voltage for successive measurements of

    PEDOT:PSS resistive layer on PET foil, pulse width 100 ns.

  • 130 Detlef Bonfert, Paul Svasta, Ciprian Ionescu

    RDC, PEDOT:PSS layer on PET Foil 1, R13 1300/, temp., M1-M3, before and after pulsing TLP 100 ns

    3340

    3350

    3360

    3370

    3380

    3390

    3400

    0 50 100 150 200 250 300 350 400 450 500V / mV

    RD

    C /

    R-M1-preR-M1-pstR-M2-pstR-M3-pst

    before TLP M1

    after TLP M1

    after TLP M2 after TLP M3

    Fig. 1.14. DC-resistance change versus applied DC voltage for successive measurements of

    PEDOT:PSS resistive layer on PET foil, before and after pulsed measurements M1-M3, pulse width 100 ns, pulse voltage 2-1000V, step 2 V.

    The final value of a measurement is the starting value for the following

    one, indicating irreversible changes in the resistor material.

    Rpulsed, TLP 100 ns, 2-1000V, step 2V, PEDOT:PSS layer on PET Foil 1, R13, 1300/, temp., M1-M4

    3000

    3100

    3200

    3300

    3400

    3500

    0 100 200 300 400 500 600 700Pulsed Voltage at DUT / V

    Pul

    sed

    Res

    ista

    nce

    /

    M1M2M3M4

    Fig. 1.15. Pulsed resistance change versus applied pulse voltage for successive measurements of

    PEDOT:PSS resistive layer on PET foil, pulse width 100 ns.

    1.2.3 Influence of pulse number on resistance

    Applying multiple high voltage pulses, below the breakdown value, and with constant amplitude gives information about the robustness of the resistor to the TLP stress. Up to 4000 pulses of 850 V amplitude were applied with 100 ns wide pulses, as can be seen from Fig. 1.16, for the applied first 1000 pulses. During the 100 ns pulses there is little change in voltage and current during transients, while sweeping the pulse amplitude, as shown in Fig. 1.17. The corresponding DC- and pulsed resistance during the applied number of 4000 pulses is depicted in Fig. 1.18 and Fig. 1.19, respectively, indicating a constant behavior of the intrinsic polymer layer.

  • Behavior of electrical stressed []. Part II: Intrinsically conductive polymer 131

    a) b)Time

    Pulse

    num

    ber

    Pulse amplitude

    P1P2

    P3

    Pn

    Fig. 1.16. 3D representation of a) pulsed voltage at and b) pulsed current through the PEDOT:PSS

    resistive layer on PET foil, for TLP pulse width of 100 ns, constant TLP-voltage of 850 V, first 1000 pulses applied.

    Transient Voltage and Current at DUT, TLP 100 ns, 850V,

    PEDOT:PSS on PET Foil 1, R13, 1300/, M1

    -1000

    100200300400500600700800

    -20 0 20 40 60 80 100 120Time / ns

    Pul

    sed

    Vol

    tage

    /V

    -0.30.00.30.50.81.01.31.51.82.0

    Pul

    sed

    Cur

    rent

    / A

    Pulsed current

    Pulsed voltage

    Fig. 1.17. Pulsed voltage at and pulsed current through PEDOT:PSS resistive layer on PET foil for

    TLP pulse width of 100 ns, constant TLP-voltage 850 V.

    RDC, TLP 100ns, 850V, 4000 Pulses, PEDOT:PSS layer on PET Foil 1, R13 1300 /, temp., M1

    RDC = -5E-05n + 3597.8

    3585

    3590

    3595

    3600

    3605

    0 500 1000 1500 2000 2500 3000 3500 4000Number of Voltage Pulses at DUT / #

    DC

    Res

    ista

    nce

    /

    MeasLinear (Meas)

    Fig. 1.18. DC-resistance change versus number of applied pulses with constant voltage, TLP -

    voltage 850V, pulse width 100 ns, 4000 applied pulses.

  • 132 Detlef Bonfert, Paul Svasta, Ciprian Ionescu

    Rpulsed, TLP 100ns, 850V, 4000 Pulses, PEDOT:PSS layer on PET Foil 1, R13,1300 /, temp., M1

    Rpulsed = 6.8173E-04xn + 3.4529E+033450

    3475

    3500

    3525

    3550

    0 500 1000 1500 2000 2500 3000 3500 4000Number of Voltage Pulses at DUT / #

    Rpu

    lsed

    /

    MeasLinear (Meas)

    Fig. 1.19. Pulsed resistance change versus number of applied pulses with constant voltage, TLP -

    voltage 850V, pulse width 100 ns, 4000 applied pulses.

    2. Conclusions

    We have analyzed in part one of the paper the flexible carbon based polymer thick film resistors and in part two the intrinsically conductive polymer PEDOT:PSS layer, during DC- stress, single and multiple pulse stresses, showing the behavior before, during and after the stress.

    An applied DC- stress outside the safe operating area (SOA) can change the electrical properties of the layer permanently, by changing the physical structure of the resistive layer. The thermal simulation and the corresponding IR measurements revealed a hot spot in the center of the resistor, due to the low thermal conductivity of the flexible substrate. Increasing further the DC- stress, this hot spot will be the starting point of the resistors burn out, leading finally to a cut through the whole resistor width.

    The high current-voltage behavior of flexible thick film polymer resistors on polyimide foil has been investigated by means of rectangular TLP pulses. The amount of resistance change depends on pulse amplitude as well as on the number of applied pulses.

    The measurements show, that for the flexible thick film resistors, the resistance decreases with increasing pulse voltage. Multiple pulsing reduces this effect. Pulsing with constant amplitude produces also a decrease, but continuing pulsing stabilizes the resistance. Intrinsically conductive polymer PEDOT:PSS layer show little changes during this stress test, the resistance stabilizes from the very beginning.

    The tested flexible thick film polymer resistors are susceptible to high energy pulses and this can lead to irreversible changes in the resistor and its value. Nevertheless the effects are saturating leading to more stable resistors.

  • Behavior of electrical stressed []. Part II: Intrinsically conductive polymer 133

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