Abstract
The aims of this study are (1) to understand charge transport mechanism in electroluminescent polymers grafted with charge transport moieties and (2) to evaluate the intrinsic charge mobility of poly(3-hexylthiophene) (P3HT). Four model systems based on spiropolyfluorene (sPF) and polycarbazole (PCzB) grafted with different charge transport moieties, including TPAsPF, G-sPF, PCzBOXD, and PCzBCz, are investigated. By uses of time-of-flight (TOF), thermally stimulated current (TSC), single carrier current characteristics measurement, together with analyses based on Gaussian Disorder Model (GDM), Correlated Disorder Model (CDM), and Gill model, we have proposed charge transport mechanisms and charge injection behaviors for these materials. We also demonstrate a facile method to evaluate the intrinsic charge mobility of poly(3-alkylthiophene) (P3HT) by measuring the change of microwave conductivity of P3HT in solution after doping. In spiropolyfluorene grafted with TPA moieties (termed as TPAsPF) and gradient-ionization-potential TPA-Cz moieties (termed as G-sPF), both TPA and Cz act as hole traps relative to the main chain (sPF) with trap depths of 0.4 and 0.2 eV, respectively. From the results of TOF measurements, hole current transient will be changed from a non-dispersive type in sPF to a dispersive type in the modified sPF, indicating hole transport is severely limited by the incorporation of TPA and Cz moieties. sPF exhibits a high hole mobility of 10-3 cm2/Vs and its energetic disorder and hopping activation energy extracted from GDM and Gill model are determined as 84 and 283 meV, respectively. Incorporating with 50 % side chain moieties, both TPA50-sPF and 50 G-sPF reduce their hole mobilities to 4×10-6 cm2/Vs, three orders of magnitude less than that of sPF, leading to an increase in their energetic disorder and hopping activation (σTPA50-sPF: 128 meV, σ50 G-sPF : 125 meV, Ea,TPA50-sPF : 622 meV, Ea, 50 G-sPF: 576 meV). Further increase in the content of side chain moieties, TPA and Cz will form hole channels to facilitate hole transport. Therefore, hole mobilities of TPA100-sPF and 100 G-sPF are enhanced to 10-4 and 10-5 cm2/Vs, respectively. And their energetic disorder and hopping activation are also reduced. The results of TSC measurements evidence that trapping current observed in TSC spectra is originated from TPA moieties. The detrapping mechanism can be considered as that thermal motion of side chain triggers the release of charges trapped by side chain moieties. In G-sPF system, a gradient ionization potential formed by TPA, Cz, and sPF can increase the probability of hopping among these three species and alleviate the trapping effect of side chain moieties. Only 5.5% trapping current in TPAsPF can be observed in G-sPF system. The incorporation of TPA moieties can promote hole injection exceeding more than 6,000 times. And the incorporation of gradient-ionization-potential TPA-Cz moieties can further improve hole injection by more than 70, 000 times. In polycarbazole grafted with OXD moieties (termed as PCzBOXD) and Cz moieties (termed as PCzBCz), both OXD and Cz serve as hole-blocking moieties relative to the main chain (PCzB) with barriers of 1.2 and 0.3 eV, respectively. Without modification, PCzB shows a non-dispersive hole transport and its hole mobility reaches 10-5 cm2/Vs. Dispersive current transients are observed in polycarbazole containing OXD and Cz moieties, indicating that the incorporation of OXD and Cz moieties brings a negative effect on hole transport. Their hole mobilities gradually decrease with increasing the content of side chain moieties. From the analyses of TOF results with GDM and Gill model, the energetic disorder and hopping activation energy vary little with the content of side chain moieties. The CDM analysis indicates that the average hopping distance will increase with increasing the content of OXD and Cz moieties. And these two systems hold a similar dependence on the relationship between hopping distance and volume ratio of main chain. Combining all information above, we conclude that inter-chain charge hopping dominates hole transport process. For charge injections, the presence of OXD moieties, which owns its hole-blocking and electron-transporting characteristics, can reduce hole injection ratio to only 2% and promote electron injection ratio by a factor of 200. However, the effect of Cz moieties on both electron and hole injection is not obvious in PCzBCz system. For measuring intrinsic hole mobility in P3HT, instead of using a laser or an electron accelerator as excitation sources in time-resolved microwave conductivity (TRMC) measurement, we generate moving charge carriers (polarons) of P3HT in its solutions by adding chemical dopants. The number of generated charge carriers can be accurately estimated from the doping level. P3HT solutions were prepared as 0.1 mg/ml with the doping level from 0 to 3%. The amount of microwave power absorbed by P3HT solutions will increase with increasing the doping levels, representing that the conductivity change increases with the doping levels. However, the deduced hole mobility decreases from 0.032 to 0.0064 cm2/Vs as the doping level increases from 0.1 to 3 %. The reasons are given as follows. The first, as the doping level increases, more and more polarons and counter ions are generated in P3HT chains, leading to stronger attractive forces between P3HT chains. And the conformation of doped P3HT exhibit rod-like characteristics, which favors P3HT chains to segregate together. The formation of segregation increases the probability of inter-chain transport and decreases hole mobility. The second, on average, more than one polarons per chain are generated in the high doping level greater than 0.3%. Coulombic repulsive force between moving polarons within a chain also reduce the moving capability of polarons. At the lowest doping level of 0.1%, the absence of P3HT segregation and coulombic repulsive force allows polarons moving freely along isolated P3HT chains. In this case, the deduced one-dimensional hole mobility (μ1D) of P3HT reaches 0.1 cm2/Vs, which can be considered as the intrinsic hole mobility of P3HT.