Bulk Heterojunction (BHJ) solar cells have reached Power Conversion Efficiencies (PCE) over 10% but to be a competitive product long lifetimes are mandatory. In this view, guidelines for the prediction and optimization of the device stability are crucial to generate improved materials for efficient and stable BHJ devices. For encapsulated cells, degradation mechanisms can be mainly ascribed to external agents such as light and temperature. In particular, thermal degradation appears to be related not only to the BHJ morphology but also to the adjacent interfaces. Therefore, in order to have a complete description of the thermal stability of a BHJ cell, it is necessary to consider the entire stack degradation processes by using techniques enabling a direct investigation on working devices.
Here, five different donor polymers were selected and the OPV performance of the corresponding BHJ devices were monitored during the thermal degradation at 85°C, showing an exponential decay of the corresponding PCEs. In parallel, we measured the geometrical capacitance of analogous OPV devices as a function of temperature and we observed that at a defined temperature (TMAX), typical for each polymer-based device, the capacitance starts to decrease. Combining all these results we found an evident and direct correlation between TMAX and the PCE decay parameters (obtained from capacitance-temperature an thermal measurements, respectively). This implies that the capacitance-method here presented is a fast, reliable and relatively simple method to predict the thermal stability of BHJ solar cells without the need to perform time-consuming thermal degradation tests.
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.