Calcium niobate nanosheets as a novel electron transport material for solution-processed multi-junction polymer solar cells

Solution-processed tandem polymer solar cells are demonstrated using stacked perovskite, (TBA,H) Ca2Nb3O10 (CNO), semiconductor nanosheets as an electron transport layer (ETL) within the recombination layers. Two poly(3-hexylthiophene):(6,6)-phenyl-C-61-butyric acid methyl ester, P3HT:PCBM, sub-cells connected in series via a CNO-poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate), Pedot:PSS, recombination layer provide a V-oc of 1.02 V. The V-oc is less than double that of a single junction due to leakage (dark) current. When TiOx is used as an additional hole-blocking layer within the recombination stack, the V-oc is improved to 1.16 V. Although further optimization of the CNO-layer is still required, the use of CNO nanosheets within the recombination stack shows clear advantages. In addition to electron extraction from photoactive layers, the presence of defect states in the CNO nanosheets with trapped electrons facilitates the recombination of holes from Pedot: PSS, enhancing the function of the recombination stack. The robust CNO-layer can be spin-coated on top of a P3HT:PCBM bulk-heterojunction photoactive layer and is stable toward subsequent processing and heat-treatment.