Organic Semiconductor Materials Transport Mechanisms And Electrical Properties Of The Organic Diode

Because of merits that less expensive and very easy to fabricate, and many potential applications, such as organic light emitting diodes(OLEDs), organic field effect transistors(OFETs), organic solar cells, organic semiconductor lasers, organic thermoelectric devices, organic detectors and sensors and, and flexible electronics devices, the study of organic semiconductors has become hot topics in recent years. Although many materials have been synthesized, and many devices have been manufactured, some have been made; the transport mechanism of carriers has not been thoroughly determined which very important for synthesis of better materials and enhancement of performance.It has been recognized that the currents in organic semiconductors are space charge limited(SCL), and the traps is an important issue for SCL current characteristics to transport models. There are two types of models to consider trap effects in literature. The first type treating all carriers in the same way, the trap effects are considered by treating the mobility of carriers as a function of electric field or density of carriers. Two representative methods are the unified model of Pasveer et al., and the exponential model of Pai. The second type of models separating all carriers into free and trapped, the representative is the mobility edge(ME) model. The ME model divides the DOS into mobile states and trap states. Mobile states, with mobility μ0, are located above(below) the mobility edge in the case of an n-type(p-type) semiconductor. But all of these models have some shortcomings, so we proposed three improved models and improved analytic current-voltage formulae in literature.At first, a transport model with double Gaussian density of state(DOS) for organic semiconductors is proposed, with one Gaussian DOS for free carriers and one for trapped carriers. The importance of non-symmetric barriers at contacts is emphasized to quantitatively describe the current-voltage relationships of typical organic layers sandwiched in two metallic electrodes. It is shown that slopes of current-voltage curves at low bias are very sensitive to the values of right barriers. The slopes in all bias are sensitive to the values of left barriers, and would increase as the left barrier decreasing. As applying the modified model to three organic diodes, the excellent agreement between theoretical results and experimental data is obtainedAt second, the mobility edge(ME) model with single Gaussian DOS is simplified based on recent experimental results about Einstein relationship. The free holes are treated as non-degenerate, and the trapped holes are treated as degenerate. This makes integral for the trapped holes can be easily realized in program. The J-V curves are obtained through solving drift-diffusion equations. As applying the model to four organic diodes, obvious deviation between theoretical curves and experimental data is observed. In order to solve this problem, a new DOS with exponential tail is proposed. The results show that the agreement of J-V curves with experimental data based on new DOS is far better than the Gaussian DOS. And the variation of extracted mobility with temperature can be well described by the Arrhenius relationship.At third, it is pointed out that the important neutral condition has not been considered in most of transport models in literature. In order to consider the neutral condition, the trapped electrons in energy gap are introduced for p-type materials based on recent work of Nicolai et al. By expressing mobility as exponential function of electric field, and solving drift-diffusion equation, the experimental current-voltage data for four typical organic diodes can be well described.At last, the Poisson and drift-diffusion equations to describe SCL are very difficult to solve even in numerical solutions, in long times, the SCL current was described by using the analytic Mott-Gurney formulae at high bias voltages with the diffusion current being negligible. But the analytical formula is very important in device modeling and analysis of experimental data. Recently Bruyn et al. derived an analytic J-V formula for organic diodes by assuming mobility as constant. We then improve the J-V formula of Bruyn et al. by considering electric field dependence of mobility. The improved formula is applied to four devices. The results calculated from original formula cannot arrive at good agreement with experimental data. The results calculated from the improved analytic formula are in good agreement with the complete numerical solutions, and both agree with experimental data very good.