Vacuum-Deposited Small-Molecule Organic Solar Cells with High Power Conversion Efficiencies by Judicious Molecular Design and Device Optimization

Abstract

Three new tailor-made molecules (<b>DPDCTB</b>, <b>DPDCPB</b>, and <b>DTDCPB</b>) were strategically designed and convergently synthesized as donor materials for small-molecule organic solar cells. These compounds possess a donor–acceptor–acceptor molecular architecture, in which various electron-donating moieties are connected to an electron-withdrawing dicyanovinylene moiety through another electron-accepting 2,1,3-benzothiadiazole block. The molecular structures and crystal packings of <b>DTDCPB</b> and the previously reported <b>DTDCTB</b> were characterized by single-crystal X-ray crystallography. Photophysical and electrochemical properties as well as energy levels of this series of donor molecules were thoroughly investigated, affording clear structure–property relationships. By delicate manipulation of the trade-off between the photovoltage and the photocurrent via molecular structure engineering together with device optimizations, which included fine-tuning the layer thicknesses and the donor:acceptor blended ratio in the bulk heterojunction layer, vacuum-deposited hybrid planar-mixed heterojunction devices utilizing <b>DTDCPB</b> as the donor and C₇₀ as the acceptor showed the best performance with a power conversion efficiency (PCE) of 6.6 ± 0.2% (the highest PCE of 6.8%), along with an open-circuit voltage (<i>V</i>_oc) of 0.93 ± 0.02 V, a short-circuit current density (<i>J</i>_sc) of 13.48 ± 0.27 mA/cm², and a fill factor (FF) of 0.53 ± 0.02, under 1 sun (100 mW/cm²) AM 1.5G simulated solar illumination