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

• Four TDPP-based small molecules, T2(TDPP)2, TT(TDPP)2, Ph2(TDPP)2, and NPT(TDPP)2 were synthesized.
• HOMO energy levels of small molecules were controlled by electron-donating unit with different electron-donating power.
• Charge carrier mobility of small molecules was improved by introducing electron-donating unit with planar structure.
• NPT(TDPP)2 showed the highest JSC and VOC due to planar and weak electron-donating property of naphthalene.
• A PCE of 4.4% was achieved from NPT(TDPP)2/PC71BM blend.

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 VOC. 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 JSC. As a result, the introduction of naphthalene (weak donating power and planar structure) enhances both VOC and JSC, 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.

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