Relation between Microstructure and Charge Transport in Polymers of Different Regioregularity

Abstract

A methodology to link an atomistic description of a polymeric semiconductor with the experimental electrical characteristics of real devices is proposed. Microscopic models of poly(3-hexylthiophene) (P3HT) of different regioregularity are generated using molecular dynamics and their electronic structure determined via an approximate quantum chemistry scheme. The resulting density of trap states and distribution of localized and delocalized states is then compared with that obtained from thin film transistor measurements of P3HT at different regioregularities. The two complementary methodologies provide a converging description of the electron transport in semicrystalline P3HT and the role of regioregularity. States at the valence band edge are localized, but delocalized "band-like" states are thermally accessible and quantitatively characterized. Both theory and experiment agree that contrary to a commonly held belief the trap density and the DOS shape are little affected by the presence of regioregularity defects.

Long-range exciton dissociation in organic solar cells

“…The approximate value of D trap is known for semicrystalline polymers from device analysis (22,23) and from simulation (24); i.e., it is possible to set the level of disorder within a physically plausible range for a semiconducting polymer. The width of the random variables α ij and R ij was set to have D trap in the range 0.015-0.070 states per monomer; the experimental range is 0.028-0.045 (22,24).…”

Triplet transport in thin films: fundamentals and applications

“…As a result, many conjugated polymers designed to have good charge transport properties by increasing regioregularity, p stacking or crystallinity have been reported. [84][85][86][87] However, it is also well known that polymer processing conditions greatly affect the interactions between the conjugated core. More significantly, highly disordered or even seemingly amorphous polymers have field-effect mobilities as high as their crystalline counterparts.…”

Simulating charge transport in organic semiconductors and devices: a review

“…P3HT is an example of a polymer OSC whose electronic properties are strongly influenced by the positional (dis)order imposed by side chains [186,282]. Reducing the regioregularity of P3HT increases the bandgap and pushes quasi-band (high-mobility) states to less accessible energies [176]. Hence the degree of localization depends on energy, something that has been observed for other polymers [283], and is counter to the standard assumption in variable range hopping models.…”

“…The morphology of disordered OSCs is usually determined by a molecular dynamics (MD) simulation, in which atoms or sections of molecules are considered as points which interact via force-fields. MD simulations may resolve atomic detail [175,176], or be coarse-grained, in which case the molecular structure is simplified into structural units [177,178]. In both cases the force-field must be calibrated for the OSC in question [179].…”

“…This is counter to the expectation that slower, inter-chain hops act as a rate-limiting step in charge transport through an OSC film. Further studies have been published relating to specific, well-characterised OSC polymers such as P3HT [175,176,185,186,224,[280][281][282], poly(phenylenevinylene) (PPV) [283] and poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) [284,285]. An important theme of this work is the impact of solubilizing side-chains on the DoS and electron transfer integrals through the film.…”