Structural Origins for Tunable Open-Circuit Voltage in Ternary-Blend Organic Solar Cells

Ternary-blend bulk-heterojunction solar cells have provided a unique opportunity for tuning the open-circuit voltage (V_oc) as the "effective" highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) energy levels shift with active-layer composition. Grazing-incidence X-ray diffraction (GIXD) measurements performed on such ternary-blend thin films reveal evidence that the two polymer donors interact intimately; their ionization potentials are thus reflections of the blend compositions. In ternary-blend thin films in which the two polymer donors do not interact physically, the polymer donors each retain their molecular electronic character; solar cells constructed with these ternary blends thus exhibit V_ocs that are pinned to the energy level difference between the highest of the two lying HOMO and the LUMO of the electron acceptor. These observations are consistent with the organic alloy model proposed earlier. Quantification of the square of the square-root differences of the surface energies of the components provides a proxy for the Flory-Huggins interaction parameter for polymer donor pairs in these ternary-blend systems. Of the three ternary-blend systems examined herein, this quantity has to be below 0.094 in order for ternary-blend solar cells to exhibit tunable V_oc. The first direct structural study of ternary-blend bulk heterojunction active layers in solar cells that demonstrate tunable open-circuit voltage (V_oc) is performed. Physical mixing of the polymer donors, quantified by the square of the square-root difference in surface energies, leads to ensemble-average electronic character that results in composition-dependent V_oc when these blends are incorporated in solar cells.