MODELLING HOLE TRANSPORT IN CONJUGATED POLYMERS FOR SOLAR CELL APPLICATIONS

摘要

We present Monte Carlo simulations of the time-of-flight (ToF) hole photocurrent transients and the field and temperature dependence of the hole mobility in two contrasting polymers, poly[2-methoxy-5-3(3’,7’- dimethyloctyloxy)-1-4-phenylene vinylene], (MDMO-PPV), and annealed poly(9,9-dioctylfluorene (PFO). Hopping transport is modelled as small polaron tunnelling between nearest neighbours in an energetically disordered medium. In the case of MDMO-PPV, where hole transport is dispersive, the observed behaviour is reproduced quantitatively using a density of states (DoS) with an exponential tail of trap states, which leads to strong field and temperature dependence. This DoS was previously extracted from transient optical measurements. In the case of PFO, where hole transport is non-dispersive and weakly field dependent, we find that correlations in the site energies must be incorporated. Excellent agreement between experiment and simulation is obtained for both materials, including the Poole-Frenkel like field dependence. In the case of dispersive transport, Monte Carlo simulations of the steady-state mobility are a poor approximation to the mobility obtained from transient simulations whereas for the non-dispersive transport the transient and steady state mobilities are in agreement.