Ultrafast laser-assisted synthesis of hydrogenated molybdenum oxides for flexible organic solar cells

Abstract

A novel method to synthesize a hydrogenated molybdenum oxide (H_yMoO_3−x) thin film by irradiation of photons using a KrF laser (λ = 248 nm) on an ammonium heptamolybdate ((NH₄)₆Mo₇O₂₄·4H₂O) precursor layer is demonstrated. The laser-assisted synthesis is simple, and can be conducted in an ambient atmosphere without damaging the underlying bottom electrode and plastic substrate. The exposure time (30 ns) is extremely short compared to thermal annealing (>3 min). Because the high-energy photons are absorbed by the MoO₃ layer and provide the activation energy for the reaction, the hydrogen atoms that dissociate from the ammonium molecules bond to the MoO₃; this process yields a H_yMoO_3−x thin-film. By controlling the laser energy, the stoichiometry of the H_yMoO_3−x layer can be manipulated to simultaneously obtain advantageous electrical properties of both high work function (5.6 eV) and electrical conductivity (9.9 μS cm⁻¹). The H_yMoO_3−x hole transport layer (HTL) is successfully demonstrated on flexible top-illuminated PTB7:PCBM organic solar cells (OSCs). This OSC has good mechanical flexibility, and 75% higher short-circuit current than the device with a PEDOT:PSS HTL. Finite-domain time-difference simulations were conducted to verify the enhancement of the photocurrent. The thin layer of H_yMoO_3−x was proven to be suitable for the microcavity condition which allows a resonant wavelength match to the PTB7:PCBM active layer.