Highly nonlinear transport phenomena in fullerene based diodes

G. J. Matt*, T. Fromhe, H. Neugebauer* and N. S. Sariciftci*

*LIOS, Physical Chemistry, Johannes Kepler University Linz, Altenbergerstrafie 69, A-4040Linz, Austria.
^Institute for Semiconductor and Solid State Physics, Johannes Kepler University Linz, Altenbergerstrafie 69, A-4040 Linz, Austria

Abstract. Voltage-current measurements of fullerene based diodes in the temperature range be¬tween 295 — 15 К are presented. At temperatures below 95 К and at high current densities the diodes exhibit a voltage instability with a voltage hysteresis for opposite current sweep directions. This ob¬servation is interpreted with the formation of highly conductive current filaments in the fullerene film.
Keywords: Fullerenes, electronic transport, hopping transport, mobility edges .
PACS: 71.20.Tx, 71.23.An, 71.23.Cq, 71.30.+h, 72.20.Ee, 73.61.Wp/

.

INTRODUCTION

Fullerenes and derivatives of fullerenes exhibit a very special case of organic com¬pounds which may possess semiconducting, metallic and even superconducting prop¬erties [1, 2, 3]. As in most organic compounds the mobility of the free charge carriers is rather low and is for spin cast fullerene films in the order of 10~3 cm2/Vs [4]. The hindered transport can be described with capturing and thermal remission of the charge carries in and out of localized states (traps) [6]. According to this model the effective measured mobility is expected to decrease with decreasing temperature. In contrast to these considerations we report the conversion of fullerene films into a highly conductive state at low temperatures and high current densities in fullerene based diodes. /

.

EXPERIMENTAL

Fullerene based diodes were prepared as follows: Indium-tin-oxide (ITO) cov-ered glass substrates were cleaned in three steps with toluene, acetone and methanol in an ultra-sonic bath. An approximately 70 nm thick film of poly(3,4-ethylenedioxythiophene/poly(styrene sulfonate) (PEDOT:PSS) (Bayer AG) as metal¬lic hole conductor was spin-coated on the ITO layer under ambient conditions. A methanofullerene [6,6]-phenyl C6i-butyric acid methyl ester (PCBM) film was then spin-coated from chlorobenzene solution (3% wt.) on top of the PEDOT:PPS covered substrate, resulting in a PCBM film of « 150 nm thickness. A top contact was evapo¬rated on the PCBM film under dynamic vacuum (p = 7 • 10~6 mbar) through a shadow mask and consists of a stack of lithium-fluoride (LiF) (0.7 nm), Al (70 nm) and Au (100 nm). .
Voltage-current (V — /) measurements via applying an certain current density and measuring the resulting voltage drop between the ITO and the LiF/Al/Au electrodes have been performed in the temperature range from room temperature to 15 K.

RESULTS AND DISCUSSION

From the device structure and the energy-level diagram given in Fig. 1, a built-in voltage Ум of « 1 V is estimated from the energy difference between the lowest unoccupied molecular orbital (LUMO) of PCBM at -4.2 eV [5] and the Fermi level of PEDOT:PSS at -5.2eV [5]. The low energy of the highest occupied molecular level (HOMO) of PCBM (—6.1 eV) [5] results in a large energy barrier of 0.9eV between the Fermi level in the PEDOT:PSS contact and the HOMO of PCBM. This energy barrier effectively suppresses hole injection from PEDOT:PSS into PCBM. Therefore, for biasing the diode in forward direction (i.e. biasing the PEDOT/ITO contact positive with respect to the LiF/Al/Au contact) only an electron current is injected from the LiF/Al/Au contact into the LUMO of PCBM and no hole current has to be considered for this device.

The current density for positive bias voltages features a power law dependency versus the effective applied bias voltage Vejf = V — Уы (see Fig. 2). Such an / — У behavior is due to the formation of a space charge upon electron injection (space charge limited currents - SCLC) [6]. c вцУе^;в = n/(Ef, l/T)/nt(Ef);Ef = f(V). (1) в is the fraction of free to trapped charge-carrier concentrations nj/nt, fx is the microscopic mobility, T the absolute temperature and Ef is the average Fermi level in the PCBM film. At room temperature в is a voltage independent value and the current density is proportional to the square of Veff (see Fig. 2a). Lowering the temperature в becomes voltage dependent and the current density is proportional to Veg with a higher power than 2 (e.g / oc y3-3 at T = 182 К in Fig. 2). Below 95 К the regime in the V - / dependence becomes temperature independent at high current densities (indicated by a (*) in Fig. 2a and Fig. 3). Below 61 К the V — I dependence features a voltage instability with a hysteresis for opposite current sweep directions (see Fig. 3). The hysteresis loops become more pronounced at lower temperatures and extend over larger area in the V — I diagram (see inset in Fig. 3).

We interpret this observation as being due to a voltage dependent Fermi energy in PCBM and the formation of highly conducting current filaments between the diode contacts at low temperatures and high current densities [7]. The total number of trapped charges Nt, contributing to the space charge, is proportional to the applied bias voltage given by Nte — С • Vejf, where e is the elementary charge and С is directly related to the geometric capacitance. With applied bias voltage the trapped charges shift the Fermi level from E/Q (the Fermi-level when no bias voltage is applied) within the DOS of the traps (T-DOS) closer to Ec (the bottom edge of the conduction band; see Fig. 2b). FIGURE 2. (a) Temperature dependent voltage-current density characteristics in double-logarithmic presentation. The dashed lines represent the / — V power law dependency at two temperatures. The region with temperature independent V — I characteristics is indicated by a (*). (b) Density of states and the respective average Fermi level in the PCBM film.

As the energy of the Fermi level is increasing with increasing bias voltage, в becomes voltage dependent and the current density is proportional to Vejf with a higher power than 2. At low temperatures and high current densities the Fermi level reaches Ec, the free charge carrier concentration is sharply increasing and the fullerene film forms a highly conductive state. This transition causes a voltage instability in the V — I diagram. As the value of Ef is directly related to the T-DOS and the T-DOS is nonuniform along the PCBM film, Ef and the free charge-carrier concentration is also nonuniform. These inhomogeneities lead to the formation of highly conductive filaments in the fullerene film. The process is meta-stable featuring a hysteresis for opposite current sweep directions.

SUMMARY

We have observed an anomalous voltage-current density behavior of fullerene based diodes at low temperatures. The process is interpreted in terms of space-charge limited currents which alters the Fermi level close to the conduction band of the fullerene film. At low temperatures and high current densities the Fermi level reaches the conduction band and the fullerene film forms highly conductive filaments. This filamented state, which is metastable, shows a hysteresis for opposite current sweep directions and leads to a temperature independent V — I behavior below 95 К and high current densities.