Highly efficient inverted polymer solar cell by low temperature annealing of Cs2CO3 interlayer

We demonstrate a highly efficient inverted bulk heterojunction polymer solar cell based on regioregular poly(3-hexylthiophene):[6,6]-phenyl C₆₁ butyric acid methyl ester with a low temperature annealed interfacial buffer layer, cesium carbonate (Cs₂CO₃). This approach improves the power conversion efficiency of the inverted cell from 2.3% to 4.2%, with short-circuit current of 11.17mA/cm², open-circuit voltage of 0.59V, and fill factor of 63% under AM1.5G 100mW/cm² irradiation. This result is comparable to the previous regular structure device on the same system. Ultraviolet photoelectron spectroscopy shows that the work function of annealed Cs₂CO₃ layer decreases from 3.45to3.06eV. Further x-ray photoelectron spectroscopy results reveal that Cs₂CO₃ can decompose into low work function, doped cesium oxide Cs₂O upon annealing, which is accountable for the work-function reduction and device efficiency improvement.