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 ${\mathrm{C}}_{61}$ butyric acid methyl ester with a low temperature annealed interfacial buffer layer, cesium carbonate $\left({\mathrm{Cs}}_2\mathrm{C}{\mathrm{O}}_3\right)$ . This approach improves the power conversion efficiency of the inverted cell from 2.3% to 4.2%, with short-circuit current of $11.17\mathrm{mA}∕{\mathrm{cm}}^2$ , open-circuit voltage of $0.59\mathrm{V}$ , and fill factor of 63% under AM1.5G $100\mathrm{mW}∕{\mathrm{cm}}^2$ 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 ${\mathrm{Cs}}_2\mathrm{C}{\mathrm{O}}_3$ layer decreases from $3.45\text{to}3.06\mathrm{eV}$ . Further x-ray photoelectron spectroscopy results reveal that ${\mathrm{Cs}}_2\mathrm{C}{\mathrm{O}}_3$ can decompose into low work function, doped cesium oxide ${\mathrm{Cs}}_2\mathrm{O}$ upon annealing, which is accountable for the work-function reduction and device efficiency improvement.