Side chain fluorination approach for enhancing efficiency of all polymer solar cells

Abstract

A new series of conjugated polymer donors (non-fluorinated P1 and fluorinated P2) were synthesized from an electron-rich benzodithiophene (BDT)-derived moiety in conjugation with electron-deficient 1,3-bis(4-octylthien-2-yl)-5-(2-ethylhexyl)thieno[3,4-c]pyrrole-4,6-dione unit via microwave assisted Stille polymerization, with a view to demonstrating fluorination effects of p-type polymers. We introduced the fluorine atoms into the 2D-conjugated side chains (alkoxyphenylthiophene) of the BDT unit instead of direct substitution into the polymer backbones to fine-tune the polymer properties, such as energy levels and interfacial interaction, without signifi-cantly affecting other polymer properties. As the fluorination of the polymers, the power conversion efficiency increased remarkably from 2.93% $(V_{OC} : 0.88 V, J_{SC} : 7.41 mA \dot cm^{-2}, and FF: 0.45)$ for the P1-based all-PSCs to 7.13% $(V_{OC} : 0.94 V, J_{SC} : 12.22 mA \dot cm^{-2}, and FF: 0.62)$ for the P2-based all-PSCs. This significant enhancement is achieved by synergistic improvements in open-circuit voltage, charge generation, and charge transport, as fluorination of the donor polymer optimized the band alignment and film morphology.