Hole injection efficiencies

Hole injection efficiencies (Jinj/Jscl) mobilities ratios in different energy barriers between phenylamine and PEDOT PSS. The open circles (O) and triangles (A) represent the ratios of mobility values from DISCLC (PDi/lt) and AS (PAs/mve)/ respectively. 

Space-charge-limited current is /scl- So, one natural way of evaluating the hole injection efficiency, ii, is to compute the ratio [71] ... 

Hence, r is equal to 1 if the hole injection efficiency is 100%, and the contact can be viewed as truly ohmic. On the other hand, t) should be less than 1 if the contact is Schottky. Figure 3.24 shows t) (solid squares) vs the nominal hole injection barrier at a field strength f=0.1MV/cm. The open circles are the ratios of mobilities derived from DI experiments (Xdi to the TOF-derived hole mobilities p. Moreover, the open triangles are ratios of AS-derived mobilities Pas to the average hole mobilities derived from TOF For the ohmic contact of PEDOTPSS/MTDATA, rj 1 and the hole injection approaches an efficiency of nearly 100%. Meanwhile, the injection efficiency for NPB is lowered to about 70%, but the nominal hole mobilities derived from Dl and AS reach 90% and 100% of the TOF-derived mobility value respectively. For the contact of TPD, the injection efficiency and the nominal hole mobilities are less than 80%. Therefore, PEDOTPSS forms the worst contact with TPD among those PA compounds. 

ZuppiroU, Nuesch, and coworkers demonstrated [108, 109, 134] that pretreatment of the transparent conductor indium tin oxide (ITO) with a few deriva-tized organic molecules can significantly improve (decrease) the turn-on characteristic (field) that denotes the field above which light emission is observed in the ITO/poly(paraphenylene)/Al LED system. The turn-on field was reduced from 200 MV m to 100 MV m after molecule adsorption. The decrease in the turn-on field was correlated with the dipole moment of the carboxylic group. The molecules modified (increased) the work function of the ITO. In this way they increased the hole injection efficiency in the device. Moreover, the device durability was increased. The stability under DC operation was increased from 15 minutes to 2.5 hours after the molecular treatment. 

A striking enhancement in hole injection efficiency for another type of carbon surface is illustrated in the upper right hand portion of Fig. 6. In this case the substrate contact consists of highly graphitiz carbon particles dispersed in a polymer binder at concentrations exceeding the percolation threshold. Carbon filled polymers are complex percolative systems whose electrical behavior is a function of the filler, the filler interparticle contacts, the host matrix and... 

Figure 6. Hole injection efficiency figure of merit for substrate contacts of varying work function vs. energy step across the contact polymer interface estimated from published work function data and electrochemical redox potential data. The height of each bar reflects the variability in injection efficiency due primarily to variation in substrate surface pretreatment and for the particular case of Au, diffusion to the interface of metal atoms from underlying binder layers.

Fig. 14 shows a comparison of the eariy time evolution of hole injection efficiency from evaporated Au top contacts (staged evaporation) into two specimens of 40 wt% TPD/polycarbonate. Note that sequentially evaporated Au contacts were employed in order to minimize the effect of interfacial damage. Fig. 14 shows a comparison of the temporal evolution of the injection efficiency from Au into a MDP specimen as prepared (curve a) with a separate specimen of the same thickness that was vapor doped for 30 minutes in a saturated atmosphere of methylene chloride vapor just prior to analysis (curve b). Fig. 14 dearly shows that prior vapor doping increases the initially observed injection dfidency Jai/ tfsclc ns... 

Figure 13. Plot of time dependence of the hole injection efficiency Jh /Jtfsclc droplet with a contact area of 0.316 cm on the top surface of 40 wt% TPD/polycarbonate. Steady state injection efficiency is emission limited at 0.02.

The phenomenon of evolving injection efficiency has been confirmed to apply for various metds under different falmcation conditions. Dilferently prepared Au, Ag and Hg contacts all show an initially severely limited hole injection efficiency into the MDP which however improves (increases) with time. Once again two processes governing the evolution in efficiency can be operationally distinguished for evaporated Au and Ag contacts. In the case of Au, whose workfunction is in close agreement with that of the MDP ( S.S eV) the injection cuirent evolves to J. 

The sensitivity data of Sq-1, -5, -8, -9, -10 and -11 are taken from a single publication [139]. These squaraines are pigmentary and exhibit similar solid state properties (Figures 10. lOa-10.10(f)). Thus, any negative effects resulting from device fabrication and morphology should be minimal. The most likely candidate to account for the sensitivity variation is the difference in hole-injection efficiency. This is indeed the case. Law et al. [139] reported the redox potentials of a series of squaraines and the use of the oxidation potentials to estimate the energy levels of the HOMOs of squaraines in devices. In the case of Sq-1, -5, -8, -10... 

The photoconductivity of USq-5 to -15 have been studied in bilayer xerographic devices [179]. Although fabrication effect, purity and hole-injection efficiency have been shown to be important factors that influence the photoconductivity, USq-13 was identified as the most outstanding squaraine so far. Specifically, USq-13 is shown to have a low dark-decay value (—15 V/s) and high sensitivity in xerographic devices, where Eo values of 3.1 and 1.9 ergs/cm at 600 nm and 790, respectively, are obtained [181]. The sensitivity appears to surpass all squaraines reported in the literature (Table 10.4). In fact, the sensitivity performance of USq-13... 

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