Highly efficient inverted bulk-heterojunction solar cells with a gradiently-doped ZnO layer

In this study, we demonstrate a highly efficient inverted bulk heterojunction (BHJ) polymer solar cell using a wet-chemically prepared doped ZnO precursor with a self-organized ripple nanostructure as an electron extraction layer and a blend of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)-carbonyl]-thieno-[3,4-b]thiophenediyl]] (PTB7) and [6,6]-phenyl C71 butyric acid methyl ester (PC₇₁BM) as an active light absorbing layer. In order to enhance the electron extraction efficiency, the ZnO ripple surface was modified with various alkali metal carbonate materials including Li₂CO₃, K₂CO₃, Na₂CO₃, Cs₂CO₃, and (NH₄)₂CO₃. The inclusion of an additional metal carbonate layer led to gradient doping of the ZnO ripple layer and improved the electron extraction properties by modifying the energy levels without destroying the ZnO ripple structures. The highest performing solar cells were fabricated with Li₂CO₃ and yielded a maximum PCE of 10.08%; this value represents a ∼14% increase in the efficiency compared to solar cells without a metal carbonate treatment.