Improved photon-to-electron response of ternary blend organic solar cells with a low band gap polymer sensitizer and interfacial modification

Abstract

Incorporating two polymer donors with different bandgaps to compose a ternary blend bulk heterojunction (BHJ) is proved to be an effective approach to improve the device performance of BHJ polymer solar cells (PSCs). Here, we demonstrate an efficient ternary PSC consisting of a polythieno[3,4-b]-thiophene/benzodithiophene (PTB7):[6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) host blend sensitized by a low band gap (LBG) polymer poly[2,7-(5,5-bis-(3,7-dimethyloctyl)-5H-dithieno[3,2-b:20,30-d]pyran)-alt-4,7-(5,6-difluoro-2,1,3-benzothiadiazole)] (PDTP-DFBT). The addition of the PDTP-DFBT sensitizer remarkably extended the PSC photon to electron response from 750 to 900 nm, which increased the J_sc from 15.12 to 16.27 mA cm⁻², and the device performance from 8.08% to 8.63%. A study on the morphology involving the atomic force microscopy mapping and grazing incident X-ray diffraction showed that the incorporation of PDTP-DFBT improved the crystallinity of the PTB7 film with most of the sensitizers associated with the PTB7 domains when blending with a PC₇₁BM film. This observation, together with the unchanged V_oc of the ternary PSC, implies a sensitizing mechanism with addition of PDTP-DFBT. After further interfacial modification with a capronic acid self-assembling monolayer (C3-SAM), a higher PCE of 9.06% was achieved, which is among the highest values of efficient ternary PSCs. Our work suggests that a sensitizing mechanism of ternary blends compensates for the light absorbing limitation of binary blend PSCs for high device performance.