El modelo cognitivo, pieza fundamental en la enseñanza

First we tried to get a rough idea of the dynamics occurring at the interface to the elec- trolyte by performing transient measurements, i.e. applying subsequently potential steps (compare section 3.2.3). For this purpose we used the electrochemical cell described above with a leak free Ag/AgCl reference electrode and a Pt counter electrode. As electrolyte we used a 1mM N aCl solution. The contact area of the working electrode, i.e. the organic thin film, was 0.04cm2_{. In order to achieve an equilibrium state of the system we set the}

potential at 0V for 60s before the measurements started. The results for Au/P (50nm P), Au/T T C (50nm T T C) and Au/P/T T C (50nm P and 50nm T T C) thin films are presented in Fig. 5.3 a-c, respectively. The TTC thin films were evaporated at a substrate temperature of 300K and a deposition rate of 4.0 ˚A/s.

According to section 3.2.3 one should expect that the current IW E decays proportional to

5.2 Transient Measurements 79 potentiostat USB feed- through electrode-cable feedthrough electrode- stand external voltage feed- through

Figure 5.2: Photograph of the self-made shielding box for electrochemical measurements.

the functionIW E =A·t−1/2+IΩ (blue curve), where Ais a constant andIΩ is the current

offset caused by the ohmic drop. The reason is that the charging currents, which decay exponentially, can not be neglected. However, it is not possible to explain the data by charging currents alone (black curve). In contrast, if one assumes that the total current is composed by the sum of the faradaic current, the charging current and the ohmic drop, one obtains an excellent fit of the data (green curve). Using the function

IW E =A1·exp

−_τt

+A2·t−1/2+IΩ (5.1)

to fit the data in Fig. 5.3 a-c one obtains the following values for the fitting parameters A1, τ,A2 and IΩ: VW E [V] A1[nA] τ [s] A2[nA · s1/2] IΩ[nA] χ2/DoF [X] √ D_·10−12 _[O 2] [µM] -0.2 34.87 0.16 15.13 3.56 0.01 6.95 1.57 0.0 36.76 0.22 10.81 -2.49 0.04 4.96 1.12 0.2 56.47 0.21 24.57 -3.10 0.07 11.28 2.54 0.0 41.28 0.22 10.60 -1.14 0.04 4.87 1.10 -0.2 36.59 0.16 18.67 3.13 0.01 8.57 1.93 for Au/P,

P P TTC P+TTC (a) (c) (d) (b)

Figure 5.3: Transient measurements of (a) Au/P, (b) Au/T T C and (c) Au/P/T T C thin films. (d) Exemplary fits of the current obtained for the potential step of ₋0.2V in Fig. (a).

5.2 Transient Measurements 81 VW E [V] A1[nA] τ [s] A2[nA · s1/2] IΩ[nA] χ2/DoF [X] √ D_·10−12 _[O 2] [µM] -0.2 124.10 0.12 23.82 18.49 0.218 10.94 2.46 0.0 16.84 0.28 4.54 -3.88 0.219 2.09 0.47 0.2 197.09 0.21 10.19 -2.77 0.747 4.68 1.05 0.0 21.79 0.14 7.96 1.15 0.001 3.66 0.82

for Au/T T C and

VW E [V] A1[nA] τ [s] A2[nA · s1/2] IΩ[nA] χ2/DoF [X] √ D_·10−12 _[O 2] [µM] -0.2 2.69 0.20 4.26 0.72 0.001 1.96 0.44 0.0 5.82 0.28 4.35 -0.83 0.005 2.00 0.23 0.2 32.20 0.20 8.10 -0.57 0.017 3.72 0.84 0.0 10.39 0.22 2.37 -0.37 0.029 1.09 0.25 for Au/P/T T C.

In the tables also theχ2_{-values per degree of freedom (indicating the goodness of the fit) are}

presented, as well as the product of the bulk concentration of the reductant or the oxidant and the square root of the diffusion constant [X]√D (in units of [mol10−3 _cm−2 _s−1/2_])

calculated by the Cottrell equation Eq. 3.37 for a one electron reaction. Additionally, the value of the concentration for oxygen [O2] is calculated, assuming that oxygen is involved

in the redox reaction (D(O2) = 1.97 10−5cm−2s−1/2 [91]). The latter statement is under-

pinned by voltammetry measurements, see section 5.3. Note that the N a+ _{or Cl}− _does not contribute to the faradaic current as the standard electrode potentialV0 (see Eq. 3.8)

is much higher than the applied voltages. The differences in the values of [X]√Dindicates that the oxidants have other diffusion constants than the reductants.

For the pentacene thin film (Fig. 5.3 a) it is probable that one or more defects of pentacene (see section 4.1.1) involving oxygen, hydroxide or hydronium participate. The change of [X]√D for the initial reduction step from 0V to ₋0.2V and the same step in the end is probably caused by a non reversible electrode reaction in one of the foregoing steps. For the Au/T T C film (Fig. 5.3 b) apparently a very slow process is involved, causing that the current does not reach an equilibrium state in the span of time of 30s. A look on the table reveals that the diffusion of the reductant must be responsible for this behavior. As the TTC layer is chemically inert the electrode reaction must occur at the Au/T T C interface. Hence, it seems that the penetration of the reductant in the TTC layer is much slower than the penetration of the oxidant. For the Au/P/T T C film the values of [X]√D are much lower than for the other films, what confirms that TTC is chemically inert and significantly slow down the penetration of reactants. By changing the concentration of oxygen or the pH value it may be possible to determine which reactants are involved. The origin of the

faradaic currents will be further discussed in section 5.3.

As the second term in Eq. 5.1 describes the discharging of a capacitor the fit parameter τ can be set equal toRC, where R is the resistance and C is the capacitance of the system. Interestingly, RC differs for the Au/P film at VW E = −0.2 V and the other voltages.

This effect may result from a change of the resistivity of the pentacene layer due to the electric field. The changes ofRC of the other samples may be caused by the change of the resistance of the TTC film due to the penetration of charged particles. The capacitance and resistance of the organic thin films can be investigated more precisely by impedance spectroscopy and will be treated in section 5.4. In conclusion the transient measurements indicate that it is possible to minimize the faradaic currents at all samples by a low oxygen concentration.