Stability of organic photovoltaic cells is the key issue for their commercialization. Despite intensive investigations to clarify the causes of failure of devices, the process of degradation is not yet well understood. In this work, we made use of the trap measurements by the charge-based deep-level transient spectroscopy to study devices that have been aged by continuous exposure to artificial sunlight for 20 h, and we compared the trap parameters to those obtained in freshly prepared samples. With regard to poly (3-hexylthiophene) (P3HT)-based devices, the P3HT:phenyl C61 butyric acid methyl ester blend cells showed an additional deep trap level and a higher trap density. In the degraded devices, all the existing traps in the fresh sample were found, and there was no creation of additional defect levels. The density of several trap levels in the polymer was strongly reduced after aging. Analysis of the results suggests that a phase separation in the photoactive blend has occurred, leading to a better organization of the polymer domains to lower defect states.
We have investigated the role of interfaces in the formation of traps in organic devices using poly(9,9-dihexylfluorene-co-N,N-di(9,9-dihexyl-2-fluorenyl)-N-phenylamine) (PF-N-Ph) as an emissive material. The basic structure of the studied diodes is ITO/PEDOT:PSS/PF/M where M is Al or Ca/Al. Trap parameters have been measured by Charge based Deep Level Transient Spectroscopy (Q-DLTS) in diodes having different electrode configurations. Five trap levels have been identified in the basic device structures with activation energies in the range of 0.1 - 0.6 eV and trap densities in the range of 1016 - 1017 cm-3. On the cathode side, no noticeable changes have been observed when changing the electrode from aluminium to calcium. On the anode side, comparing the trap parameters in devices with and without a PEDOT:PSS layer, we show that the hole injection layer introduced new trap levels, which are electron-traps. The density of these traps is of the order of 1016 cm-3 and their levels are at ~ 0.3 and ~ 0.5 eV from the band edges. The findings confirm and complete quantitatively earlier results by other groups on the role of the PEDOT:PSS /organic interface in the trap formation in OLEDs.
Charge based deep level transient spectroscopy (Q-DLTS) has been used to investigate the defect states of poly (p phenylene vinylene)(PPV) light emitting diodes. Studies in the temperature range 250-315K show the presence of two carrier trapping centers in the polymer bulk: a majority carrier trap at 0.5 eV with a cross section of 10-16cm2 and a minority carrier trap at 0.4eV with a cross section of 10-19cm2. The results are compared and discussed with those previously reported in PPV based diodes using other techniques to determine the trap parameters in the polymer.
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