Abstract
Poly(phenylene vinylene) (PPV), polyfluorene (PF) and their derivatives are the most popular electroluminescent polymers due to their semi-conductive and good fluorescent properties. However, the charge transport, trapping, detrapping and recombination mechanisms have not been well known so far. Issues such as the assignment of trap polarity (hole or electron), the exact effect of the trap states on the charge mobility, and the relationships between detrapping carriers and radiative recombination… etc, are rarely discussed in documents. This is because that both the semi-conduction capabilities and chain relaxation complicate these questions. As a result, standardized instruments for these researches are not commercial available now. In this research, therefore, we used homemade apparatus assemblies for thermally stimulated current (TSC), thermally stimulated luminescence (TSL), time-of-flight (TOF), and time resolved electroluminescence spectroscopy (TREL) to investigate the charge transport and trapping mechanisms of conjugated polymers. By using TOF-based TSC, the trap states of hole and electron in MEH-PPV can be clearly assigned. The hole trap is located at about 210 K with an activation energy of 0.1-0.4 eV. This trap state is not affected by the ambient air and the change of morphology, which is attributed to the extrinsic impurities. The observed electron trap is located at about 300 K with an activation energy of 0.45-0.5 eV. Since the trap concentration increases by the exposure to oxygen and is reversible, it is attributed to the molecular oxygen. It is worth noting that the trap peak shifts to higher temperature as the drain field increases. This behavior is firstly observed by TSC measurement and indicates that electron mobility decreases as field increases. It can be attributed to the field-induced localization or positional-disorder dominant transport. The transport properties observed in TSC measurement are in agreement with those from TOF measurement. In MEH-PPV, electron transport is poor and electrons can not move across a 1.5-μm-thick film. Contrarily, holes can travel through a 8-μm-thick film, indicating a better transport. The relaxation currents for side chain and main chain can be observed unambiguously by TSC measurement, indicating that TSC is chain-relaxation-sensitive technique. The peak location and activation energies of detrapping current for electrons and holes are in agreement with those of side chain and main relaxations, implying that chain motion can induce carrier detrapping. The most possible form for molecular oxygen to catch an electron is “O2- “. Since the electron affinity of O2- (0.89eV) is smaller than that of MEH-PPV (2.8 eV), an electron prefers to stay on the conjugated main chain but slightly attracted by the adjacent molecular oxygen. A complex of MEH-PPV…e…O2 is likely to form. The onset temperature for electron detrapping is close to that of main chain relaxation, indicating that electron can escape from the attraction of oxygen as long as the main-chain relaxations start. The electron could be released at room temperature. However, serious retrapping makes these sites readily for charge recombination and then non-radiative decay. This will result in low efficiencies for electroluminescence devices. By the simultaneous measurement of TSC and TSL on MEH-PPV, the TSL emission contributed by geminate pairs (an intermediate between exciton and free carrier) can be investigated. In this research, the “two step excitation” method is firstly applied and we find that geminate pairs could be dissociated by the incident light. The wavelength dependent TSL is a result of dynamic balance. The incident light not only generates excitons to form geminate pairs but also dissociate them at the same time. This behavior implies that the geminate pairs may have characteristic absorption profile. This observation is very helpful to investigate carrier generation process. By fitting of photocurrent transient equation (PTE) to the non-dispersive, dispersive, and highly dispersive photocurrents, the charge mobility μfit and diffusion coefficient D can be obtained at the same time. PTE can perfectly fit the experimental data of different polymers and their films with different morphology. The thus obtained mobility is close to those determined by t1/2 (from the TOF results), which is close to the average mobility. By defining a deviation parameter Dv=Dq/μkT, large deviation from Einstein’s law is observed in all of the investigated polymers. The degree of this deviation is in agreement with the tail-broadening parameter W, which is widely used parameter for dispersion in photocurrent transients. Therefore, Dv can be regarded as an indicator of dispersion. This result also reveals that the larger deviation from Einstein’s law, the more dispersion in the transport behaviors. Since the D value characterizes the photocurrent transients in different type of polymers and films with different morphology, it lumps all factors together that cause the dispersion in carrier transport. From the TOF measurement, the copolymer PFOR1 (containing only 1% red Ir complex) has the hole mobility 2 orders of magnitude lower than PFO; and PFOR12 (with 12 % Ir complex) has an even lower hole mobility. These results indicate the occurrence of hole trapping on the side-chain Ir complex. In the TSC measurement, however, no trap currents were found. The reason is that Ir complex is an efficient recombination center because the HOMO and LUMO lay between those of the main chain (PFO), which permits both hole and electron to be trapped and recombine at these sites. Cabarzole is a hole-transport material. However, hole traps are formed and the hole mobility decreased about one order for carbazole-grafting polyfluorene (CzPF), as compared with that of PFO. By using photoexcitation and field-induction TSC, two hole traps were found and the distribution of trap currents and activation energies are in agreement with that in PVK. This two trap states are attributed to two types of Cz-Cz dimer. From the result of time-resolved electroluminescence (TREL) measurement, the monomeric emission from the annealed device increases with time in the initial operation period but decreases with time after the bias is off. This behavior indicates that aggregates are sites for charge trapping and recombination and function as dopant. Since the activation energy of aggregates is quite small so that the detrapping currents are difficult to be detected by TSC measurement.