Modelling of anomalous charge carriers transport in disordered organic semiconductors / Choo Kan Yeep

Choo, Kan Yeep (2017) Modelling of anomalous charge carriers transport in disordered organic semiconductors / Choo Kan Yeep. PhD thesis, University of Malaya.

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Abstract

Performance of organic devices is affected by material disorders, which yields low mobility, dispersive current and scaling noise behaviour. Anomalous transport and scaling noise behaviour are inadequately described by Fick’s law and characterised by low-frequency noise method. This work reports the study of (i) scaling behaviour of current noise in organic field-effect transistors (OFETs) using methods of fractal noise analysis and, (ii) the modelling of anomalous charge transports in disordered organic semiconductors based on fractional calculus. Current noises of Poly(3-hexylthiophene) (P3HT) OFETs were measured at various source-drain voltages (Vds) and characterised using the power spectral density method and detrended fluctuation analysis. Current noises were found to follow white noise, 1/f and Brownian noise characteristic at low, intermediate and high Vds, respectively. For Vds above 40 V, Brownian noise will be masked out by 1/f noise. Multiple-trapping mechanism is integrated with the drift-diffusion equation and then generalised to the time-fractional drift-diffusion equation (TFDDE) to model the anomalous transports and reproduce the transient photocurrents in regiorandom P3HT (RRa-P3HT) and regioregular P3HT (RR-P3HT). The TFDDE is solved by using finite difference scheme and Poisson solver is implemented to calculate the electric field. It is found that by acquiring extra energy from high electric field, charge carriers escape easily from trap centres and propagate with higher velocity resulting in higher current. Larger amount of charge carriers will be generated at higher illumination and they will be hopping near the mobility edges, hence encountering lesser capturing events. This explains why movement of charge carriers at higher illumination is less dispersive than the movement of charge carriers at lower illumination. It is also noted that transport dynamic of charge carriers in RR-P3HT is relatively less dispersive and has higher mobility than that of the RRa-P3HT since RR-P3HT has lower capturing rate and is less energetically disordered.

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