A strategy to enhance both VOC and JS']JSC of A-D-A type small molecules based on diketopyrrolopyrrole for high efficient organic solar cells

Four different diketopyrrolopyrrole (DPP)-based small molecules (SMs) with A-D-A type structure were synthesized, where electron-donating unit (D) was systematically varied with different electron-donating power (thiophene vs. phenylene; thienothiophene vs. naphthalene) and different molecular planarity (bithiophene vs. thienothiophene; and biphenylene vs. naphthalene). The small molecules with weak donating unit (phenylene or naphthalene) have deeper HOMO energy levels than those with strong donating unit (thiophene or thienothiophene), and thus exhibit higher V-OC. When the fused aromatic ring (thienothiophene or naphthalene) with planar molecular structure is introduced in SMs, the SMs exhibit high hole mobility and thus afford high J(SC). As a result, the introduction of naphthalene (weak donating power and planar structure) enhances both V-OC and J(SC), resulting in a promising power conversion efficiency of 4.4%. This result provides a valuable guideline for rational design of conjugated small molecules for high performance organic solar cells. (C) 2013 Elsevier B.V. All rights reserved.