Luminescent cathode buffer layer for enhanced power conversion efficiency and stability of bulk-heterojunction solar cells

• Luminescent sodium doped zinc oxide quantum dots (L-ZnO QDs) as cathode buffer layer (CBL) in organic solar cell.
• The L-ZnO QDs absorbs ultraviolet (UV) light and its emission overlap with the absorption of p-DTS(FBTTh2)2.
• A significant enhancement from ∼8% to 9.2% in the efficiency of the solar cell device due to energy transfer.
• Enhanced UV stability of the devices with L-ZnO as compared to intrinsic ZnO, under constant irradiation of light.

We report high photovoltaic efficiency of over 9% in solution-processed, small-molecule (SPSM) 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c]1,2,5]thiadiazole) p-DTS(FBTTh2)2:[6-6]-phenyl C70 butyric acid methyl ester (PC70BM) blend based inverted BHJ solar cell by incorporating luminescent zinc oxide doped with sodium (ZnO:Na) quantum dots (QD) (l-ZnO) as a cathode buffer layer (CBL) in inverted bulk-heterojunction (BHJ) solar cells for the first time. The l-ZnO absorbs ultraviolet (UV) light and down-converts it to visible light. The l-ZnO layer's emission overlaps significantly with the absorption of p-DTS(FBTTh2)2, leading to an enhanced absorption by p-DTS(FBTTh2)2. This resulted in a significant enhancement of photo-current from 15.4 to 17.27 mA/cm2 and efficiency from 8% to 9.2% for ZnO and l-ZnO based devices, respectively. This is among one of the highest efficiency values reported so far in the case of SPSM based single junction BHJ solar cells. The luminescent ZnO layer also protects the active layer from UV-induced degradation as solar cells show high stability under constant solar light illumination retaining more than 90% (∼28 h) of its initial efficiency, whereas BHJ solar cells without the luminescent ZnO layer degraded to ∼50% of its initial value under same conditions. Since ZnO is an essential part of inverted organic solar cells, the luminescent l-ZnO CBL has great potential in inverted organic solar cells.

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