Elsevier

Thin Solid Films

Volume 516, Issue 24, 31 October 2008, Pages 8949-8962
Thin Solid Films

Space-charge-limited currents in organic films: Some open problems

https://doi.org/10.1016/j.tsf.2007.11.070Get rights and content

Abstract

The theory of space-charge-limited currents (SCLC) is briefly reviewed and the spectroscopic character of the method is discussed. Shapes of current–voltage characteristics of thin films strongly depend on temperature, presence of charge carrier traps and their distribution in energy, spatial inhomogeneity of the sample and electrode configuration. The presence of traps influences generation-recombination noise which can be used as additional information for the determination of density-of-states function. The use of the technique is illustrated with experimental results obtained on thin films of phthalocyanines.

Introduction

In recent years, a rapid growth has been witnessed in the commercial prospects of electronic devices based on organic low-molecular weight materials and polymers, e.g., light emitting diodes, ambipolar light emitting transistors, field-effect transistors, solar cells, Schottky diodes, etc. This in turn stimulated interest in understanding electrical properties of the materials, namely charge carrier transport, trapping phenomena and energetic distribution of local states (traps). Traps may be present in the material bulk where they will act to reduce carrier mobility, or at interfaces where they may influence charge injection into the material.

A complete information about parameters of local states, e.g., their densities, energetic and spatial distributions and cross-section, is a crucial requirement in characterization of the transport and storage of charge carriers in low-conductivity materials. Various methods [1], [2], [3], [4] have been employed to study the localization (“trapping”) of charge carriers and the parameters of traps. Until mid-seventies, the determination of parameters of local states was considered to be a problem of merely academic interest. The number of papers dealing with the problem has increased rapidly due to the growing demand for reliable methods for characterization of local states in amorphous and polycrystalline semiconductors [5]. In practice, it is impossible to determine all trapping parameters from measurements using a single method. Thus, a complex characterization requires that several methods be employed independently. Among the methods commonly used to study the parameters of local states, the technique of space-charge-limited (SCL) currents occupies a prominent place. Both steady-state and transient SCL currents have been studied by several authors for over 40 years [1], [2], [3], [6], [7], [8], [9], [10], [11]. The spectroscopic character of SCLC technique [12], [13], [14], [15], [16], [17], [18], [19] allows determination of the electronic structure of local states and therefore to get some details concerning charge carrier transport.

Even though many problems concerning charge injection and SCLC have been solved, there are still some open questions and problems which need some discussions, e.g., influence of spatial trap distribution on the shape of current–voltage characteristics, current noise, polaron formation, etc. Some of these problems are mentioned in this paper.

Section snippets

Space-charge-limited currents — unipolar injection

In real insulators and wide band-gap semiconductors, where the concentration of thermally generated carriers (nf0) is small, most carriers come from external sources, i.e., they are injected from suitable contacts or are, e.g., generated by photons of suitable energy. The efficient injection of charge carriers requires an “injecting” contact, i.e., a contact which allows increasing the bulk carrier concentration (nf(x) > nf0). This contact, so-called “Ohmic” contact, does not limit the current

Electrical noise

Measurements of electrical noise can be used as a useful tool yielding results supplementary to those obtained from measurements of SCL current–voltage characteristics. Trapped charge carriers are in equilibrium with charges in the transport band; it means that the concentrations of free and localized charges are given by Boltzmann and Fermi–Dirac statistics but this does not imply a static equilibruim. There is an exchange of charges on transport and localized levels, the localization time

Ohmic contact

All equations and methods discussed in the preceding sections rely on the assumption of one of the electrodes being an Ohmic contact, i.e., an infinite reservoir of charge carriers. This means, in practice, that the contact must be capable of injecting charge whose density at a given voltage is at least three orders of magnitude higher than the thermodynamic equilibrium value. The interface barrier for injection must therefore be sufficiently low (lower than about 0.3 eV at room temperature).

Experimental examples

In this section, exemplary experimental results will be given, illustrating the procedures and techniques described in the preceding sections. Many of these results have already been published, the reader is thus referred to the original literature for technical details. The results obtained on thin films of phthalocyanines have been chosen.

Conclusions

The SCLC technique may be used as a spectroscopic method allowing one to determine the DOS function from j  U characteristics and voltage dependences of the activation energy of the current. To obtain correct values of DOS distribution and energies responsible for charge occupation statistics, a check of contact ohmicity and sample homogeneity must be done. Even though the theory of the SCL currents has been developed to minor details, there are several effects which must taken into account in

Acknowledgements

This work was supported by grants No. KAN 401770651 from the Grant Agency of the Academy of Sciences of the Czech Republic, No. 1041/2006-32 from the Ministry of Education, Youth and Sports of the Czech Republic, and grant No. 3 T08E 084 30 from the Ministry of Science and Higher Education of Poland.

References (66)

  • S. Nešpůrek et al.

    Radiat. Phys. Chem.

    (1990)
  • F. Schauer

    Sol. Energy Mater. Sol. Cells

    (2005)
  • G.T. Wright

    Solid-State Electron.

    (1961)
  • R.S. Muller

    Solid-State Electron.

    (1963)
  • M.S. El Naschie

    Chaos, Solitons Fractals

    (2005)
  • M.S. El Naschie

    Chaos, Solitons Fractals

    (2006)
  • O. Zmeškal et al.

    Chaos, Solitons & Fractals

    (2007)
  • A.M. Lampert et al.

    Current Injection in Solids

    (1970)
  • A.G. Milnes

    Deep Impurities in Semiconductors

    (1973)
  • K.C. Kao et al.

    Electrical Transport in Solids

    (1981)
  • R. Chen et al.

    Analysis of Thermally Stimulated Processes

    (1981)
  • N.F. Mott et al.

    Electron Processes in Non-crystalline Materials

    (1979)
  • E.A. Silinsh

    Organic Molecular Crystals. Their Electronic States

    (1980)
  • N. Karl

    Festkörperprobleme

    (1974)
  • M. Pope et al.

    Electronic Processes in Organic Crystals

    (1982)
  • J. Sworakowski

    J. Appl. Phys.

    (1970)
  • S. Nešpůrek et al.

    J. Appl. Phys.

    (1980)
  • S. Nešpůrek et al.

    Phys. Status Solidi, A Appl. Res.

    (1976)
  • S. Nešpůrek et al.

    Czechoslov. J. Phys. B

    (1972)
  • S. Nešpůrek et al.

    Phys. Status Solidi, A Appl. Res.

    (1977)
  • O. Zmeškal et al.

    J. Phys. C. Solid State Phys.

    (1985)
  • F. Schauer et al.

    J. Phys. C. Solid State Phys.

    (1986)
  • F. Stöckmann

    Phys. Status Solidi, A Appl. Res.

    (1981)
  • W. Helfrich
  • M.A. Lampert

    Phys. Rev.

    (1956)
  • A. Rose

    Phys. Rev.

    (1955)
  • P. Mark et al.

    J. Appl. Phys.

    (1962)
  • H. Baessler

    Phys. Status Solidi, B Basic Res.

    (1981)
  • H. Baessler

    Phys. Status Solidi, B Basic Res.

    (1993)
  • V.I. Arkhipov et al.

    Phys. Rev., B

    (2002)
  • I.I. Fishchuk et al.

    Phys. Rev., B

    (2003)
  • P.N. Murgatoroyd

    J. Phys., D, Appl. Phys.

    (1970)
  • Cited by (44)

    • Synthesis and investigation of charge transport properties in adducts of hole transporting carbazole derivatives and push-pull azobenzenes

      2019, Journal of Physics and Chemistry of Solids
      Citation Excerpt :

      The values of electron affinity energies in thin film (EAf) were calculated using the difference of experimentally determined values of Eth and If. Temperature modulated space-charge-limited current (TM-SCLC) method described by Nešpurek et al. [25]. was used for the measurements of the charge carrier activation energy which corresponds to the depth of the charge carrier trapping states.

    • Electrical and optical analyses of trapping phenomenon with temperature dependence of organic device

      2017, Organic Electronics
      Citation Excerpt :

      The C-F measurement was performed with applied biasing. Fig. 1(c) plots of the I-V characteristics in log-log scale, and this result shows that space charge field effect on the carrier injection and trapped condition in the TIPS-pentacene layer [20,21]. These I-V curves exhibit in proportion to V2 and trapped charge effects on a trapped condition and leads to higher conductivity at increased substrate temperatures [22–26].

    • The role of local potential minima on charge transport in thin organic semiconductor layers

      2017, Organic Electronics
      Citation Excerpt :

      At the core of the SCLC model is the concept of the distribution of localized states, which trap the carriers during charge transport. Based on the SCLC model, several experimental methods were developed in order to investigate space-charge effects on the charge transport [3]. Most of them focus on bulk materials, where space-charge screens externally applied electric field between a pair of parallel electrodes.

    View all citing articles on Scopus
    View full text