Toward Bottom-up Optoelectronic Design of Increasing Fluorination Low Bandgap in <i>PDTPQ<SUB>X</SUB>-types</i> Copolymers for Organic Photovoltaics Devices

Abstract

Organic photovoltaic performance has been investigated about the fluorination effects as one part on the optoelectronic properties. The quantum chemical accuracy of the optoelectronic and structural properties based on D-A (Donor-Acceptor) conjugated copolymers as PDTPQ<SUB>X</SUB>-type (Poly-dithieno-pyrrol-Quinoxaline) has been tediously exposed. The Donor-Acceptor in the copolymers was in our case constitutes to the Donor part in the photovoltaic device, while the Acceptor starting is the PC₆₀BM in the same device, which composed the photovoltaic solar cells. The choice of the Donor part in the copolymers was obtained by their HOMO-LUMO bandgap and UV-visible absorption. The bandgap of the Donor part must be higher than that of the Acceptor part for an untroubled charges transfer from the Donor to the Acceptor according to the photovoltaic principle. The substitution of fluorine atoms (0F, 1F, 2F) on the quinoxaline constituents is an effective way to low the HOMO and LUMO energy levels of the alternating copolymers. This fluorine effect has been explored on the optoelectronic properties such as the HOMO-LUMO band gap <i>E_gap</i> energy, the fill factor <I>FF</I>, the open circuit voltage <i>V_oc</i>, the electron transfer energy Δ<i>E_et</i>, the excitation energy Δ<i>E_ex</i>, the absorption wave length <i>λ</i> and the oscillator strength <I>OS</I>. The equilibrium geometry at the ground state, the electronic structures as the frontier orbital isosurface have been obtained under the caster of the density functional theory (DFT) assist by the time-dependent density functional theory (TD-DFT) with M05 as exchange-correlation functional to come with 6-311G(d,p) basis set. Calculations were performed both in vaccuum and Chlorobenzene (CB) solvent with IEFPCM quantum model. All this has been done with the aim to enhance the energy gap, the <i>V_oc</i> values and the fill factor <I>FF</I>, which exposed the nanomorphology as the topology of the solar cells photoactive layers. The results of this study show that these promote compounds systems as in the fluorination order are excellent candidates to build photovoltaic device in aim to enhance the open-circuit voltage for donor-acceptor heterojunctions used in organic solar cells.