Super-exchange Transport Mechanism In Electron Donor Acceptor Complexs

Organic semiconductors have attracted much attention from both academy and industry due to their promising applications in low-cost,lightweight,flexible and solution-processable electronics.Recently,the advancements of the donor-acceptor(D-A)copolymers and cocrystals greatly improved the device performance of organic semiconductors.However,charge transport mechanism in this new type of organic material is not clear.In the D-A complexs,two donor(acceptor)groups are separated by one acceptor(donor)and are not directly coupled electronically.The effective coupling between adjacent donors(acceptors)is mediated by the in-between acceptor(donor),i.e.,via super-exchange(SE)coupling.In this dissertation,charge transport in eletron donor acceptor complexs was investigated based on density functional theory.Especially,SE mechanism was used to explain the origin of ultra-small effective masses and ambipolarity in D-A complex.Firstly,based on SE model and first-principles computations,we demonstrate that the donor-acceptor copolymers can intrinsically possess ultra-small effective mass due to the SE effect,rationalizing the recent experimental demonstration of ultra-high charge mobility.With the increase of the SE coupling,the effective mass decreases correspondingly.For the first time,we report that the SE coupling for holes is determined by the dihedral angle between the donor and acceptor moiety,HOMO charge density at the linkage site and the HOMO-LUMO gap of the bridge fragment.Thereby,we put forward molecular design strategy for large SE coupling to obtain ultra-small effective mass.Finally,through combining a variety of donor and acceptor groups,we predict several D-A copolymers can potentially possess ultra-low effective mass due to long range SE effect,which are expected to be potentially high mobility polymers.Experimentally,a remarkably high mobility,up to 1.57 cm~2/Vs for holes and 0.47cm~2/Vs for electrons,has been found in sulfur-bridged annulene-based donor-acceptor complexes with an alternate stacking motif,demonstrating efficient ambipolar transport properties.Through our computational studies,it was found that these ambipolar properties arise from the fact that the electronic couplings for both holes and electrons have the same SE nature along the alternate stacking direction.The magnitude of SE coupling depends not only on the intermolecular stacking distance and pattern,but also the energy level alignments between the adjacent D-A moieties.The concluded transport mechanism and structure-property relationship from this research will provide an important guideline for the future design of organic semiconductors based on D-A complexes.In general,it is believed that three-dimensional transport networks are preferable to those of lower dimensions.Nevertheless,it was found experimentally that the?-phase crystals of TiOPc with electronic couplings along two dimensions show a maximum mobility up to 26.8 cm~2/Vs.In sharp contrast,the?-phase crystals with extra significant inter-layer electronic couplings possess a maximum mobility of only 0.1 cm~2/Vs.Our theoretical calculations comparing both the bulk crystals and slab models reveal that the inter-layer electronic couplings for the?-phase devices are detrimental for charge transport owing to the coupling direction perpendicular to the current direction.This work provides new insights into the impact of the dimensionality and directionality of the packing arrangements on charge transport in organic semiconductors.Much attention has recently been focused on the ambipolar transport in?-conjugated materials.A common feature of the high-performing ambipolar semiconducting polymers is the incorporation of electron-poor unit DPP.It's speculated that the ambipolarity in DPP based D-A copolymers comes from the delocalized HOMO and LUMO along the entire polymer chain,in contrast to most donor-acceptor type copolymers.In this part,we understand the orgin of ambipolarity in DPP based D-A copolymers from SE perspective.Furthermore,donor length effect to the polarity is investigated,which can be used to design ambipolar DA copolymers.