Environmental ageing of polymer-based thin film transistors: influence of the semiconducting polymer chemical structure

Jorge Morgado1,2, Ana Charas1, Luís Alcácer1,2 1 Instituto de Telecomunicações, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal Phone: +351-218418451, Fax: +351-218418472, e-mail: jorge.morgado@lx.it.pt

2 Dep. Eng. Química e Biológica, Instituto Superior Técnico, Av. Rovisco Pais, P-1049-001 Lisboa, Portugal

Abstract1— At variance with a high stability of the inorganic materials for electronics under normal atmospheric conditions, organic materials are more sensitive, due to charge transfer doping by oxygen and water and degradation when exposed to the ultraviolet radiation. Here we show that the chemical structure of the conjugated polymer used as active semiconducting material of polymer field-effect transistors has a drastic effect on their stability. These results show that, by chemical manipulation, some of the above limitations, when envisaging the use of organic materials for electronics, can be reduced.

I. INTRODUCTION

Organic materials are now at the heart of a new technological development. While the exploitation of these materials at the single molecule level for electronics is at a very early stage, their use as bulk materials is promoting a new technological era. Both low molecular weight (small molecules) and polymers (usually known as “plastics”) can now be used as insulators, semiconducting and conducting materials. Furthermore, some of them are, in addition luminescent. This plethora of properties is at the origin of a wide range of electronic and optoelectronic devices using them as active components: diodes, memories, field-effect transistors, photovoltaic cells and photodiodes, light-emitting diodes and light-emitting electrochemical cells, electrochromic displays, photopumped lasers, sensors and actuators. Organic materials can be designed at the molecular level to tune their properties, and, in particular when solution processing techniques can be used, they can provide low cost fabrication processes.

Field-effect transistors (FETs) are key components concerning electronics applications. Polymers are being explored since the 1980’s [1] to form their semiconducting channel. Working in accumulation, they resemble the thin film transistors based on amorphous silicon, not only in their working principles but also in terms of the achieved field-effect mobilities. Being mainly disordered, the modest mobilities that have been achieved with polymer-based FETs (up to ca. 1 cm2/V.s) [2] will not allow their use for information processing but they can be used in devices and appliances with lower speed processing requirements, while, if a low price can be achieved, their use in disposable devices

Acknowledgments: This work was partially supported by IT under the internal project PLASTIC.

is also envisaged. Furthermore, there are several studies trying to incorporate them in clothing. A large investment was recently made in Dresden-Germany in a facility devoted to the fabrication of organic electronic components.

While chemists are devoting enormous efforts to develop new materials able to improve the performance of the various devices, they are also looking to materials with improved environmental stability, to reduce encapsulation needs. In fact, the modification of the chemical structure affects not only the device performance but also their environmental stability. In this communication we report on stability studies carried out in FETs based on two different polymers.

II. RESULTS AND DISCUSSION

This study involved two different semiconducting polymers: regio-regular poly(3-hexylthiophene), P3HT, and poly[(9,9-dioctylfluorene)-alt-(bithiophene)], F8T2. FETs were fabricated on silicon substrates, using doped silicon as gate and silicon dioxide as dielectric. To improve semiconducting polymer adhesion and ordering the surface of SiO2 was (in some cases) treated with HMDS (1,1,1,3,3,3-hexamethyldisilazane). Devices based on P3HT were of the bottom contact type: a thin film of P3HT (≈45 nm thick) was deposited, by spin coating from chloroform solution, on top of two interdigitated gold contacts (source and drain), defining a channel 10 mm wide (W) and 10µm long (L). Measurements were carried out using an HP4140B semiconducting parameter analyser. The devices based on F8T2 and derivatives are of the top contact type, being the gold source and drain contacts deposited by thermal evaporation, using a shadow mask, on top of the polymer films, defining channels with W=5.1 mm and L≈100 or 120 µm. Figure 1 shows a picture of a device.

Fig. 1. PFET inside a measurement chamber

A. FETs based on P3HT

Figure 2 shows the output characteristics of a P3HT-based transistor measured in the dark, under ambient atmosphere, soon after preparation. The results show a p-type behaviour, with the transistor working under accumulation. The device was kept in the dark, under ambient conditions and characterised again 20 h after preparation. It was then placed under vacuum (ca 10-5 mbar). It was measured after 4h and 24 h under vacuum. Figure 3 compares the effect of this ageing process on the transfer characteristics. The results of Figure 3 clearly show that, in air, the On/Off ratio (current at Vg=-20V/current at Vg=0V) degrades upon ageing. This effect is reversible as, under vacuum, it recovers the initial on/off ration and even improves with time. This effect is attributed to doping of the P3HT by atmospheric water and/or oxygen, which is particularly evident by the increase of the current at Vg=0V. The field effect mobility, µ, is estimated in the saturation regime, from the linear plot of Isd

1/2 vs Vg, according to (1),

!

Isd = µC0

W

2LVg "Vt( )

2 (1)

being Co the dielectric capacitance (1.72x10-8Fcm-2) and Vt the threshold voltage. The results are shown in Figure 3. In air, the mobility value did not change between the two measurements, while, under vacuum, it increases in time. Being the solution and the film both prepared in air, the film retains some residual doping, which is then removed under vacuum. We note that the best reported results (up to 0.1 cm2/V.s [3]) for P3HT refer to FETs prepared and characterised under inert atmosphere. These results show that, although this is the polymer exhibiting the highest hole mobilities, its use in air requires encapsulation to avoid the degradation of their properties.

0.0

1.0

2.0

3.0

4.0

5.0

6.0

-20-15-10-50

Curr

ent

( µA

)

Vsd

(V)

Vg = 0V

Vg = -10V

Vg = -20V

P3HT

S

S*

*

Fig. 2. Output characteristics of a P3HT-based FET

measured soon after preparation.

10-7

10-6

10-5

-20-15-10-50

Curr

ent

(A)

Vg (V)

Vsd

= -20V

t=0

t=20h, in air

t=4h, vacuum

t=24h, vacuum

µ=0.002 cm2/V.s

µ=0.0015 cm2/V.s

µ=0.0014 cm2/V.s

Fig. 3. Transfer characteristics of the P3HT device

measured upon ageing in air (in the dark) or under vacuum.

B. FETs based on F8T2 and derivatives

F8T2 is a polymer that has been widely studied [4]. Devices with 55nm thick F8T2 film were prepared and tested in air. During the measurements they were kept in the dark. Figure 4 shows the typical output of an FET. This device works in accumulation, being holes the charge carriers (p-type device).

0

5

10

15

20

25

30

-40-30-20-100

Curr

ent

(nA

)

Vsd

(V)

Vg = 0V

Vg = -20V

Vg = -30V

Vg = -40V

SS

n

F8T2

Fig. 4. Output of an FET based on F8T2, 55nm thick film,

obtained soon after preparation.

Figure 5 shows the effect of ageing of this FET under

ambient conditions in the dark. This typical behaviour found for the F8T2-based devices shows that after a first “stabilisation” which typically takes a couple of days, there is no monotonic effect of the shelf ageing. This stability contrasts with the behaviour found for the P3HT-based devices.

Figure 6a) shows the effect of measurement cycles. In general, we find a shift of the transfer curve towards more negative voltages (increase of the threshold voltage), with very little effect on the On/Off ratio, and the field effect mobility is nearly constant (increases slightly from 1.78 x10-3 to 1.86x10-3 cm2/V.s). The origin of this stress effect (attributed to impurities, mobile ions, humidity) is not yet

clear. In Figure 6b) we show an hysteresis behaviour, which is frequently found.

10-11

10-10

10-9

10-8

10-7

10-6

-40-30-20-100

Curr

ent

(A)

Vg(V)

Vsd

=-40V

t=0

t=172 days

t=477 days

t=129 days

µ=4x10-4

cm2/V.s

µ=1.8x10-3

cm2/V.s

µ=1.9x10-3

cm2/V.s

µ=1.6x10-3

cm2/V.s

Fig. 5. Ageing in the dark under normal atmosphere of the

FET based on F8T2.

10-12

10-11

10-10

10-9

10-8

10-7

10-6

-40-32-24-16-80

Curr

ent

(A)

Vg (V)

a)

10-12

10-11

10-10

10-9

10-8

10-7

10-6

-40-32-24-16-80

Curr

ent

(A)

Vg (V)

b)

Fig. 6. a) Effect of sequential driving on the transfer curve

of the FET based on F8T2; b) hysteresis upon cycling of the gate voltage on this FET

F8T2 is a soluble material and the films used in the FETs were deposited by spin coating. We have also synthesised soluble F8T2 derivatives which can become insoluble if, in the presence of an additive, they are exposed to UV light and cured at ca. 120 oC [5,6] (see Fig.7). This ability is being explored to prepared patterned films if the UV light is impinged in specific areas, using either a shadow mask or a scanning near-field optical microscope [5].

Two of such materials, designated as F8T2Ox1 and F8T2Ox2 were used in FETs. Figure 7 compares the characteristics of FETs based on F8T2Ox1 in the precursor, soluble form with those of FETs where the polymer was turned insoluble (cross-linked). In these devices the channel length was 120 µm. The field-effect mobility of the soluble form is 1.75x10-4 cm2/V.s, while that of the insoluble form is 8.3x10-5 cm2/V.s. This device, measured 570 days later, being kept in the dark under ambient atmosphere, shows very little effect of ageing, without noticeable changes of the On/Off ratio or the field-effect mobility, as shown in Fig. 8.

SS

O

O

n

SS

O

n

SS

O

O

n

Om

F8T2Ox1

r-F8T2Ox1

SS

O

O

n

F8T2Ox2

O

O

Fig. 7. Molecular structure of the F8T2 derivatives, F8T2Ox1 and F8T2Ox2 and scheme of the insoluble (cross-linked)

form r-F8T2Ox1.

The transfer characteristics of an FET based on F8T2Ox2 , deposited on SiO2 (untreated surface) are shown in Figure 8. From this data a field-effect mobility of 1.7x10-5 cm2/V.s is determined.

10-11

10-10

10-9

10-8

10-7

0

1 10-4

2 10-4

3 10-4

-70-60-50-40-30-20-100

Curr

ent

(A)

sqrt

[curr

ent(

A)]

Vg (V)

F8T2Ox1

r-F8T2Ox1

Fig. 8. Transfer characteristics of FETs based on F8T2Ox1

in the soluble and insoluble (cross-linked, r-F8T2Ox1) forms. For the insoluble form, the closed circles correspond to

measurements on a fresh device and the open circles correspond to the same device after being aged in the dark

under ambient atmosphere for 570 days.

10-11

10-10

10-9

10-8

0

2 10-5

4 10-5

6 10-5

8 10-5

1 10-4

-70-60-50-40-30-20-100

Curr

ent

(A)

sqrt

[curr

ent(

A)]

Vg (V)

Fig. 9. Transfer characteristics of an FET based on

F8T2Ox2.

III. CONCLUSIONS

We have shown that while FETs based on P3HT are very sensitive to ambient atmosphere (even in the dark), the characteristics of the devices based on F8T2 and derivatives are not significantly affected upon storage in the dark under ambient atmosphere for more than one year. This result shows that, upon design of novel polymer structures, new materials with improved performance and stability may be obtained.

The field-effect mobility values we obtained for the F8T2-based transistors, 1.8x10-3 cm2/V.s, is one order of magnitude lower than the value reported for optimised devices (0.02

cm2/V.s) [4], which means that, using these new materials, it should be possible to improve devices performance. The modification of F8T2 to obtain F8T2Ox1 and F8T2Ox2 promotes a decrease of the mobility to 1.75x10-4 cm2/V.s and 1.7x10-5 cm2/V.s, respectively. This decrease is attributed to the presence of the polar groups in these polymers (oxetane groups_Figure 7), which are necessary to promote the formation of an insoluble polymer network. These groups may likely induce a decrease of the order of the polymer chains when forming the film [6]. We attribute the decrease of the mobility to an increase of the disorder within the semiconducting film.

Upon formation of the insoluble network of the semiconducting polymer, starting from F8T2Ox1, there is a further reduction of the mobility (8x10-5 cm2/Vs) of the transistor.

These last results point out the need to, either design new cross-linkable polymers, which could lead to film with increased order, and/or to develop fabrication processes which could assist the polymers to form more ordered structures [4]. We believe that the search in this class of cross-linkable semiconducting polymers should be continued, in view of the possibilities they offer in terms of fabrication of patterned and other complex structures [5].

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[3] H. Sirringhaus, N. Tessler, R.H. Friend, “Integrated, high-mobility polymer field-effect transistors driving polymer light-emitting diodes”, Synthetic Metals, vol. 102, pp. 857-860, 1999.

[4] H. Sirringhaus, R. J. Wilson, R. H. Friend, M. Inbasekaran, W. Wu, E. P. Woo, M. Grell, D. D. C. Bradley, “Mobility enhancement in conjugated polymer field-effect transistors through chain alignment in a liquid-crystalline phase”, Applied Physics Letters, Vol. 77, pp. 406-408, 2000.

[5] A. Charas, H. Alves, J. M. G. Martinho, L. Alcácer, O. Fenwick, F. Cacialli, J. Morgado, “Photoacid cross-linkable polyfuorenes for optoelectronics applications”, Synthetic Metals, vol. 158, pp. 643-653, 2008.

[6] A. Charas, L. Alcácer, A. Pimentel, J. P. Conde, J. Moragdo, “Observation of field-effect in a cross-linked polyfluorene semiconductor”, Chemical Physics Letters, vol. 455, pp. 189-191, 2008.