A Theoretical Study On Charge-transfer State In An Organic Polymer Heterojunction

As one of the new functional materials, organic semiconductors have always been the research hotspot in the interdisciplinary area of physics, chemistry and materials science. They possess the big π-conjugated structure and the excellent carrier transport performance, which make them having abundant electric, magnetic and optical properties. Easy processing and its unique mechanical property make organic materials easier to process than metal and crystal semiconductor, and the possibility of processing organic semiconductor materials in room temperature brings hope to cheap machining.During the past decades, the organic semiconductor materials have been used to fabricate various optoelectronic devices, such as organic solar cells, organic light emitting diodes and organic field effect transistors. This field needs interdisciplinary knowledge of physics, chemistry, polymer science, fluid mechanics, electrical and electronic engineering and optics.Over the past few decades, donor-acceptor polymer heterojunctions have been widely used as active components in organic photovoltaic cells, as well as organic light-emitting diodes. For such a system, a unique character is that both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of one polymer (Donor-type) lie higher in energy than the corresponding levels in the other polymer(Acceptor-type). In donor-acceptor polymer heterojunctions. electrons can transfer from HOMO of the donor to LUMO of the acceptor through optical excitation. Thus, electrons of the acceptor and holes of the donor are bound together by Coulomb attraction, forming the charge-transfer state.In this paper, in the framework of the extended SSH model, the charge-transfer state of donor-acceptor polymer heterojunctions was studied. Detailed research content and basic results are as follows,A crucial parameter of donor-acceptor polymer heterojunctions is the energy offset. A large energy offset may improve the dissociation probability of a CT state, or suppress its various types of recombination, but the energy offset is not as larger as expected, which will reduce the available open circuit voltage and limit the photovoltaic efficiency. Bittner’s theoretical investigation on the dependence of the charge transfer (or exciton dissociation) showed that the exciton dissociation requires the energy offset to be greater than the exciton binding energy. The interchain coupling of polymers is very important in the progress of charge transfer or exciton dissociation. Former research showed a large interchain coupling reduces the binding energy of exciton to a great extent.In this paper, a D-A polymer heterojunction consisting of two parallel molecules is modeled, in which the lift of on-site energy of one chain create an energy offset between the corresponding energy levels of the two chains. We firstly consider the formation of a CT state following a direct transition between the two molecules. Creation and binding energy of the CT state is sequentially calculated, by which we analyze its stability in a static picture. Through the discussions, we aim to give a light to choose the optimalizing energy offset in different applications of D-A polymer heteroj unctions.The results of this paper are displayed intuitively and transparently by figures. It is obtained that the binding energy decreases apparently with the increase of the energy offset. It indicates that a CT state is more easily dissociated in a D-A heterojunction with a large energy offset. Especially, when the energy offset is large enough, the binding energy will fall below the room temperature thermal energy, which means that the thermal fluctuation at room temperature is sufficient to dissociate a CT state into free charge carriers contributing to the photocurrent. The transferred charges will increase as the energy offset increases, and will decrease as the interchain coupling increases. The shows that with the energy offset increasing, the CT state will be like totally free carriers, and a little interchain coupling is beneficial to this.