Bandgap and Molecular Level Control of the Low-Bandgap Polymers Based on 3,6-Dithiophen-2-yl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione toward Highly Efficient Polymer Solar Cells

A series of low-bandgap polymers based on a soluble chromophore of 3,6-dithiophen-2-yl-2, 5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP) unit were synthesized by introducing of different electron-rich building blocks copolymerized with DPP unit. Four new DPP-based polymers, PDPP-DTS, PDPP-F, PDPP-BDT, and PDPP-BDP, were characterized by GPC, TGA, NMR, UV-vis absorption, and electrochemical cyclic voltammetry. The results indicate that their bandgaps as well as their molecular energy levels are readily tuned by copolymerizing with different conjugated electron-donating units. In order to investigate their photovoltaic properties, polymer solar cell (PSC) devices based on PDPP-DTS, PDPP-F, PDPP-BDT, and PDPP-BDP were fabricated with a structure of ITO/PEDOT:PSS/polymers:PC70BM(1:2.w/w)/Ca/Al under the illumination of AM 1.5G. 100 mW/cm(2). The power conversion efficiencies (PCE) of the four DPP-based PSC devices were measured and shown in this paper. The best performance of the PSC device was obtained by using PDPP-BDP as the electron donor material, and a PCE of 4.45% with an open-circuit voltage(V-oc) of 0.72V, a short-circuit (J(sc)) of 10.0 mA/cm(2) and a fill factor (FF) of 61.8% was achieved, which is the best result among the DPP-based polymer materials. It is apparent that the PDPP-BDP-based device exhibits a very broad response range, covering from 300 to 850 nm. The results of the solar cells indicate that these types of materials are very promising candidates for highly efficient polymer solar cells.