Current-voltage characteristics of polyspiroblue SB -based light-emitting diodes with the structure:
ITO/PEDOT:PSS/SB/cathode have been analyzed. Several cathodes were used (Al, LiF with different thicknesses, and
Ba) in order to change the barrier for electron injection. As expected, the inclusion of a thin (0.5-1 nm) LiF layer
between SB and Al, or the use of Ba, modifies the electron barrier as derived from the increment in the turn-on voltage
(related to the built-in potential) with respect to that observed for Al cathode. For hole-only devices (Au cathode) J-V
characteristics are interpreted in terms of bulk-limited SCLC transport with hole mobility of the order of 10-6 cm2/V s.
When J-V characteristics obtained using different cathodes are compared the current level observed are consistent with
the mobility observed for the hole only device. This implies that the device operation is mainly determined by the hole
conduction. However, the electroluminescence observed for these devices employing different cathodes differs over four
orders of magnitude. Our results suggest that the electron mobility is much smaller than the hole mobility and that the
recombination process is confined to a thin layer near the cathode. Additionally, the results obtained from simple device
modeling are also presented.
Charge injection in organic light emitting diodes (OLEDs) is studied by impedance spectroscopy on a solution processable polymer based OLED (PLED) using different metallic cathodes. A negative capacitance is observed in organic light-emitting diodes (OLEDs) at low frequencies which can be explained using a detailed kinetic model based on sequential injection through surface states at the metal/organic interface. In this paper the methodology used to derive this model and its application to experimental data is presented.
Impedance model of one-carrier space-charge limited current (SCLC) has often been applied to explain some experimental features in organic light-emitting diodes (OLED). However double injection current occurs in working devices and the impedance model has not been studied so far. We analyze the problem of double injection SCL current in the limit of infinite recombination. In order to obtain the ac response of a biased OLED we solve the equations for time dependent double-injection of space-charge-limited currents. We give an analytical expression for impedance as a function of frequency. Calculations predict values for the static capacitance C(ω→0) similar to those encountered in case of one-carrier SCLC, in which C/Cg=0.75 (being Cg the geometric capacitance), but shifted to higher frequencies. We give the equivalent circuits representing the limits at low frequencies. This model will help to understand the behavior of two carrier devices.
We discuss a generalized model for a solar cell, and the realization with heterogeneous photochemical photovoltaic converters such as the dye-sensitized solar cell. The different steps involved in the conversion of photon energy to electrical energy, indicate that a key point to consider is maintaining the separation of Fermi levels in the selective contacts to the absorber. In order to understand the irreversible processes limiting the efficient operation of the solar cell, it is necessary to obtain a precise description of the internal distribution of Fermi levels. We suggest the equivalent circuit as a central tool for obtaining such description, in relation with small perturbation measurement techniques. The fundamental steps of excitation and charge separation, and the losses by transport and charge transfer, can be represented by suitable circuit elements, and the overall circuit configuration indicates the operation of the selective contacts. The comparison of the equivalent circuits for heterogeneous dye solar cells and solid-state p-n junctions, shows the significant difference in the mechanisms of the selective contacts of these solar cells.