Multilayer Epitaxial Growth of Lead Phthalocyanine and C₇₀ Using CuBr as a Templating Layer for Enhancing the Efficiency of Organic Photovoltaic Cells

The molecular orientation and crystallinity of donor and acceptor molecules are important for high-efficiency organic photovoltaic cells (OPVs) because they significantly influence both the absorption of light and charge-transport characteristics. We report that the templating effect extends to multilayers to increase the crystallinity and to modify the orientation of the crystals of lead phthalocyanine (PbPc) and C₇₀ layers at the same time by adopting CuBr as a new templating layer on indium tin oxide (ITO). The formation of a monoclinic phase with a preferred orientation of (320) for PbPc and a fcc phase with a preferred orientation of (220) for C₇₀ on the PbPc layer is revealed by X-ray diffraction (XRD) patterns. The multilayer epitaxy results in an increase of the exciton diffusion lengths from 5.6 to 8.8 nm for PbPc and from 6.9 to 13.8 nm for C₇₀ to enhance the power conversion efficiency (PCE) of the planar heterojunction OPVs composed of PbPc and C₇₀ from 1.4 to 2.3%. The quasi-epitaxy model is proposed to explain the multilayer epitaxy

Highly Efficient Vacuum-Processed Organic Solar Cells Containing Thieno[3,2‑<i>b</i>]thiophene-thiazole

Two novel electron-donor molecules based on donor−π-conjugated linker–acceptor structure with compact packing and intramolecular charge-transfer characteristics were synthesized for the preparation of efficient organic solar cells. The donor molecules featuring an electron-rich triphenyl amine as the electron-donor unit, dicyanovinylene as the acceptor unit, and π-conjugated linkers of thienothiophene, thiophene, and thiazole units were synthesized. The π-conjugated linkers were carefully designed to have a planar structure, an efficient conjugation length, and appropriate energy levels for a compact packing in the solid state. The vacuum-processed solar cells fabricated using the donor molecules of DTTh and DTTz exhibited average power-conversion efficiencies (PCEs) of 5.4 and 6.2% (the highest PCE obtained was 6.37%) under AM 1.5G illumination with an intensity of 100 mW cm^–2

Enhancement of the Fill Factor through an Increase of the Crystallinity in Fullerene-Based Small-Molecule Organic Photovoltaic Cells

We report that the crystallinity of C₇₀ is improved significantly if CuI is used as a templating layer, leading to remarkable enhancement of hole mobilities from 8.32 × 10^–6 to 3.26 × 10^–5 cm²/(V s). As a result, the use of the templating layer in C₇₀-based solar cells with low donor concentration resulted in significant improvement of the fill factor from 0.51 to 0.57 and the power conversion efficiency from 5.56% to 6.23% under simulated AM 1.5G, 1 sun irradiation. This result demonstrates that the CuI templating layer is effective at improving the crystallinity of the fullerene derivatives as well as the donor materials