All-Conjugated Block Copolymer Additives for Organic Solar Cells

Recent studies have demonstrated organic solar cells with power conversion efficiencies (PCEs) above 10%. However, the morphology of these devices is dictated by a kinetically trapped, metastable morphology, and strategies for achieving thermally-stable, co-continuous networks of donor and acceptor domains are desirable. Fully conjugated block copolymers (BCPs) with donor and acceptor blocks can be utilized as additives to both modulate the morphology and potentially improve the electronic properties of the active layer. The use of BCP additives represents an effective approach to control the donor-acceptor morphology, but prior studies mostly have focused primarily on block copolymers containing optically and electronically inactive polymer blocks or backbones. Here, we report a combined experimental and computational study of the impact of all-conjugated poly(thieno[3,4-b]-thiophene-co-benzodithiophene)-b-polynaphthalene diimide (PTB7-b-PNDI) BCP additives. We find that small amounts (2 wt%) of BCP additives lead to improved performance due to large increase in open circuit voltages (V_oc), and the V_oc is pinned to this higher value for higher BCP additive loadings. These results contrast prior results of ternary blend OPVs where V_oc is pinned to a lower value. We further demonstrate, by transmission electron microscopy and surface energy measurements, that voltage enhancement is likely due to the formation of a parallel bulk heterojunction which is formed after BCP addition to the PTB7/PCBM blends. In summary, this work demonstrates that all-conjugated block copolymers can be utilized as additives to both dictate morphology and enhance the electronic properties of organic photovoltaic solar cells.