Thin-film encapsulation of polymer-based bulk-heterojunction photovoltaic cells by atomic layer deposition

This study demonstrated thin-film encapsulation of bulk-heterojunction polymer photovoltaic cells, utilizing a process based on atomic layer deposition (ALD) that both prevented degradation caused by ambient gases and served as an annealing step that increased the initial efficiency of the cells. With the ALD temperature set at 140 °C and the total deposition time set at 1 h, the photovoltaic cells, based on blended poly-3-hexylthiophene (P3HT) and [6,6]-phenyl C61 butyric acid methylester (PCBM), were optimally annealed during encapsulation, achieving a power conversion efficiency (PCE) of 3.66%. Encapsulating the cells with a 26 nm Al2O3/HfO2 nanolaminated film overcoated with an epoxy resin protection layer enabled the cells to obtain an in-air degradation rate that was similar to cells that were stored in nominally O2/H2O-free atmosphere. The nanolaminated structure of the encapsulation film resolved the issue of hydrolysis-induced aging observed with Al2O3 films, owing to the hydrophobicity of the HfO2 layers. Additionally, extended exposure of the ALD precursors during the ALD process significantly improved the coverage of the ALD films over the P3HT/PCBM active layer at the perimeter of the cells.