Synthesis And Characterization Of Fluorinated Quinoxaline Based Donor-π-Acceptor Copolymers For Efficient Polymer Solar Cells

Against the background of energy crisis and environmental pollution, solar energy been a research hot spot as a clean energy. Compared with silicon solar cell, polymer solar cells (PSCs) have attracted tremendous attention because of their potential applications in flexible, light-weight, and low-cost large-area solar cells through roll-to-roll printing. However, the low power conversion efficiencies (PCE) and poor stability of PSCs limit its wide acceptance. In this paper, for the purpose of developing high efficiency polymer donor materials, we designed and synthesized a series of fluorinated quinoxaline(Qx) based D-n-A narrow band-gap polymers with varying π-conjugation structures and different conjugated side chains. The structure-property relationship of polymers and performance of corresponding photovoltaic devices were studied in detailed.To investigate the effect of the π-conjugated extension orientation on the photovoltaic properties of the copolymers, we designed three D-π-A type low band-gap copolymers, named PBDT-DFDTQX、PBDTT- DFDTQX and PNDT-DFDTQX, respectively, based on a new fluorinated alkoxyphenylquinoxaline (DFDTQX) as an acceptor unit and alkoxybenzodithiophene (BDT), alkylthienylbenzodithiophene (BDTT, with vertical π-conjugation extension compared with BDT), or alkoxynaphthodithiophene (NDT, with horizontal π-conjugation extension compared with BDT) moieties as the donor unit. Systematic characterizations of optoelectronic characteristics, and optimal results of field-effect transistors (FETs) and photovoltaic devices were made. The difference in π-conjugation structures of the donor units led to different properties of the three polymers. Compared with PBDT-DFDTQX, extended π-conjugation perpendicular to the main chain of PBDT-DFDTQX not only downshifted its HOMO energy level, but also enhanced π-π stacking. The PSC made from PBDTT-DFDTQX exhibited a promising PCE of 5.07%, which was 2.5 times higher than that from PBDT-DFDTQX (2.10%). On the other hand, the PSC made from PNDT-DFDTQX, which possessed extended π-conjugation along the main chain compared to PBDT-DFDTQX, showed a slightly improved PCE of 2.30%, but much higher carrier mobilities of approximately 0.1 cm2 V-1s-1.To broaden or intensify the absorption of the corresponding polymers, we developed a new type of two-dimensional conjugated copolymers with the D1-A (D2) repeating pattern, where D1 and A are used to construct the linear main-conjugated system and D2 is attached to the acceptor units of D1-A polymer main chains. As is well-known, the intramolecular charge transfer (ICT) effect from electron donor to acceptor is beneficial to broaden the absorption spectrum of the relative polymers. D1-A (D2) system can not only preserve the ICT from D1 to A, but can further enhance the ICT from D2 to A, resulting in better light harvesting. Prompted by the abovementioned considerations, we presented an approach to improve the optical absorption of the Qx-based polymers by introducing the electron-donating thiophene groups onto the Qx unit. The new Qx monomer displayed an additional strong absorption peak at about 430 nm. The corresponding PSCs based on PT-DFQX. PBDT-DFQX and PNDT-DFQX achieved an PCE of 5.03%、4.52% and 2.65% respectively.Base on the conclusion of that the polymers with a vertical π-conjugation extension possess higher PCE, in this part, four BDT-DFQX-based derivatives (PBDT-DFQX-PP, PBDT-DFQX-TP, PBDT-DFQX-PT and PBDT-DFQX-TT) with 2-D conjugated side chains on D- and A-units were synthesized to investigate the effect of conjugated side chains on the photovoltaic properties. In these four polymers, different side chain combinations were in turn varied from p-alkoxyphenyl (P) to 2-alkylthienyl (T) to tune the photoelectronic properties. As evidenced by UV/Vis absorption spectra and cyclic voltammetry, the HOMO energy levels and optical absorption of the polymers rise gradually as the phenyl conjugated side chains, either on donor or on acceptor units, were replaced by thienyl ones step by step. As for photovoltaic properties, lowering the ratio of thienyl in the conjugated side chains leads to lower performance, e.g., as the proportion of thienyl side chains decreased from 100% to 50% to 0, the maximum PCE dropped from 7.29% to 6.54% to 6.08% and then to 2.96%. The PBDT-DFQX-TT-based device exhibits the highest PCE of 7.29%, which is 2.5 times higher than that of the PBDT-DFQX-PP-based device (2.96%). This phenomenon indicates that thienyl side chains are beneficial to achieve high PCEs of relevant polymers. Meanwhile, we employ methanol to further optimize the performance of PBDT-DFQX-TT based PSCs. The PBDT-DFQX-TT obtained the best photovoltaic performance with an excellent PCE of 7.68%, which is much higher than the PCE of for those BDT-DFQX-based polymers reported in previous studies, the relative PSCs exhibited the highest PCE of 6.1%.