Low-Temperature, Solution-Processed Hole Selective Layers for Polymer Solar Cells

A new method is reported for preparing solution-processed molybdenum oxide (MoO3) hole selective layer (HSL). Via combustion processing at low annealing temperatures, the obtained MoO3 HSL exhibits a high charge-transporting performance similar to poly(ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) but overcoming its defect to device stability. The combustion precursor solution using ammonium heptamolybdate as the metal source, acetylacetone as a 'fuel', and nitric acid as an oxidizer can largely reduce the temperature for transformation of the polyoxomolybdate to alpha-phase MoO3. Furthermore, when a small amount of PEDOT:PSS has been introduced into the combustion precursor solution to improve the film morphology, the derived film can exhibit a flat and continuous surface morphology with coexistence of alpha- and beta-MoO3 after being annealed at a low temperature (150 degrees C). The simplicity, rapidness, and effectiveness of our method together with the low annealing temperature needed make it promising for the roll-to-roll manufacture of polymer solar cells.