Impact of donor halogenation on reorganization energies and voltage losses in bulk-heterojunction solar cells

Donor halogenation is a common molecular design strategy used to reduce voltage losses (ΔV_loss) and improve the power conversion efficiency (PCE) of bulk-heterojunction (BHJ) organic solar cells. Here, the impact of donor halogenation on the performance of organic donor-acceptor (DA) solar cells based on over 30 different materials systems is investigated, and the main reason for the improved performance of solar cells after donor halogenation is ascribed to the increased energy of the charge transfer (CT) state, and the reduced reorganization energy of the CT states (λ_CT). Also, the impact of donor halogenation on λ_CT is found to be stronger for the solar cells using the Y-series acceptors (Y5, Y6, etc.) than those using the non-Y-series acceptors (fullerene, ITIC, etc.), which is conducive to achieving lower ΔV_loss in organic solar cells. Finally, the impact of donor halogenation on the solar cell performance is demonstrated to be dependent on the halogen substitution position, as well as the number of halogen atoms added to the donor molecule: Halogen substitution on the side groups of the donor molecule is found to be more effective than substitution at the backbone in reducing ΔV_loss. These results suggest that future molecular design strategies focusing on the reduction of materials reorganization energy will be of great importance for further improving the performance of organic solar cells.