Synthesis and Search for Design Principles of New Electron Accepting Polymers for All-Polymer Solar Cells

Abstract

New electron withdrawing monomers, thieno-[2',3':5',6']pyrido[3,4-g] thieno [3,2-c] isoquinoline-5,11(4H,10H)-dione (TPTI) and fluorenedicyclopentathiophene dimalononitrile (CN), have been developed and used to form 12 alternating polymers having different monomer combinations: (a) weak donating monomer-strong accepting monomer, (b) weak accepting monomer-strong accepting monomer, (c) weak accepting monomer-weak accepting monomer, and (d) strong donating monomer-strong accepting monomer. It was found that lowest unoccupied molecular orbital (LUMO) energy levels of polymers are significantly determined by stronger electron accepting monomers and highest occupied molecular orbital (HOMO) energy levels by the weak electron accepting monomers. In addition, fluorescent quantum yields of the TPTI-based polymers in chloroform solution are significantly decreased as the LUMO energy levels of the TPTI series of polymers become deeper. The quantum yield was found to be closely related with the photovoltaic properties, which reflects the effect of internal polarization on the photovoltaic properties. Only the electron accepting polymers showing SCLC mobility higher than 10(-4) cm(2)/(Vs) exhibited photovoltaic performance in blend films with a donor polymer, and the PTB7:PNPDI (1:1.8 w/w) device exhibited the highest power conversion efficiency of 1.03% (V-oc = 0.69 V, J(cc) = -4.13 mA/cm(2), FF = 0.36) under AM 1.5G condition, 100 W/cm(2). We provide a large set of systematic structure-property relationships, which gives new perspectives for the design of electron accepting materials.