Drift-Diffusion Simulations of Charge Transport and Trap Dynamics in Organic Semiconductor Materials

Polymeric semiconductors have gained significant attention in the field of organic electronics due to their unique properties and potential applications. Through their ability to transport charge carriers and to absorb or emit light, they have attracted much attention in the display, sensing, and printed electronics industry. Particularly significant is the rather recent industrial success of organic semiconductor-based displays. Research on organic semiconductors has not ceased since its beginning in the middle of last century. Of special interest is the understanding of the disordered structure in these materials and its impact on electrical and optical properties. In this study, we have used drift-diffusion simulation to unravel intricate electronic mechanisms in three distinct areas related to polymeric semiconductors. First, we focused on the thermally stimulated current (TSC) measurement method. The electric current measured in TSC originates from trapped charge carrier which become mobile upon increasing the temperature. TSC thus directly probes key characteristics of trap states and was commonly used on inorganic semiconductors to study trap site density, trap energy and at-tempt-to-escape frequency. We investigate the applicability and reliability of the formulas originally developed for inorganic semiconductors for the organic counterpart by employing drift-diffusion simulation on a simple electrode-semiconductor-electrode stack. We identified the slow retrapping formula as the most accurate, albeit being the most complicated to implement for data analysis. However, for practical purposes, the initial rise formula emerged as the most robust simplification of the slow retrapping formula. Additionally, we discovered that not all trap states can be emptied due to diffusion, and a proportionality factor must be considered to obtain accurate values for the trap density.Second, we studied reversible trap states in a polymeric light-emitting diode (PLED) using the poly(p-phenylene vinylene) (PPV) derivative superyellow (SY) as the emitter. Trap states formed during PLED operation followed a third order kinetics, but after device switch-off slowly disappeared with a power law rate. Our investigation suggests that trap state formation and disaggregation may originate from a complex involving water and oxygen molecules. SY and other polymers commonly exhibit universal electron trap states, causing low electron mobility. Our research indicates that the observed reversible trap sites correspond to these known universal trap states, revealing their previously unexplored formation kinetics and reversibility.Last, we examined the operational principles of an upconverter device that employed superyellow as the emitter and a cyanine dye as the photodetector. This device converts near infra-red (NIR) light into visible light, enabling the detection of light that is invisible to the human eye. All-organic upconverter devices are still relatively unexplored, and the detailed mechanisms underlying the upconversion processes lack detailed understanding. Our investigation focused on understanding the cause behind the increased response time of the SY upconverter when the device voltage is elevated. We identified that the electron mobility within the emission layer plays a crucial role in influencing the behaviour of the upconverter device.