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

Three new tailor-made molecules (DPDCTB, DPDCPB, and DTDCPB) 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 DTDCPB and the previously reported DTDCTB 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 DTDCPB 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 (Voc) of 0.93 ± 0.02 V, a short-circuit current density
(Jsc) 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.