Effect of sol–gel derived ZnO annealing rate on light-trapping in inverted polymer solar cells

• Sol–gel derived zinc oxide (ZnO) thin films with different morphologies are prepared using two annealing rates.
• ZnO films with different morphologies and thus different scattering properties.
• Compared with the fast annealed film, ZnO film formed at 9 °C/min possesses the better scattering effect.
• The resulting inverted polymer solar cell (IPSC) shows a 12.6% improvement in short-current density.
• Performance of IPSCs could be effectively improved by controlling the morphology of ZnO films by a simple post-annealing process.

Different morphology zinc oxide (ZnO) thin films were prepared by a sol–gel method using two annealing rates and used as the electron transport layers in inverted polymer solar cells. The morphology, optical and structure properties were performed by AFM, UV–vis and XRD in order to study the effect of annealing rates. The undulating morphology of ZnO film fabricated at a slow heating rate of 9 °C/min possesses a rougher surface than that of ZnO film annealed at the fast heating rate of 56 °C/min, which provides efficient light-trapping and increases photon absorption.The resulting device shows 12.6% and 6.7% improvement in short current density and fill factor, respectively, compared with the device based on the rapidly annealed ZnO; a maximum power conversion efficiency of 2.55% was achieved.

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