Advancing Efficiency of Organic Solar Cells with ZnO Quantum Dots as Electron-Extraction Component

As one of the critical constituents in organic PV (OPV) solar cells, the electron transporting layer (ETL) ensures the effective extraction of electrons, suppressing charge recombination in the absorber layer. One of the most efficient ETL materials, nanocrystalline zinc oxide, is available commercially as liquid inks containing amorphous ZnO precursors. This requires obligatory annealing to crystallize the ZnO ETL and remove organic residuals, limiting the reliability of the control over the ETL quality and application of such inks for thermo-reactive substrates. In the present work, we develop an alternative approach to solution-processable ZnO ETLs, showing the feasibility of direct and mild synthesis of crystalline ZnO quantum dots (QDs) with a varied QD size in the form of stable and concentrated (ca. 2w.%) inks containing no additional ligands and/or stabilizers.
The OPV cells based on an advanced multinary absorber (PM6:L8BO:PC70BM:BTP-eC9) and ETL formed from ZnO QD inks demonstrated a notable improvement in efficiency, with the champion power conversion efficiency (PCE) of 18.85% and 18.24% achieved for ZnO QDs and commercial ZnO, respectively. The OPV cells based on ZnO QD ETL show notably higher operational stability as compared to the OPV devices with conventional ZnO ETLs, retaining 85% of original efficiency after ca. 200-h aging under 5-sun illumination at 75oC. The advanced efficiency and stability of OPV cells with ZnO QD ETLs are attributed to a low defect density evidenced by efficient photoluminescence emission shown by colloidal ZnO QDs, as well as to a denser contact between ZnO QDs and a lower light scattering from the ETL layer to smaller QD size of 5-6 nm as compared to 30-40 nm for commercial samples.
Overall, the proposed ZnO QD ETL materials are produced by an easily upscalable, cost-effective, and environmentally friendly solution approach, and show outstanding efficiency, advanced stability, and a large potential for low-temperature-processable rigid and flexible OPV solar cells.