Influence of Controlled Acidity of Hole-Collecting Buffer Layers on the Performance and Lifetime of Polymer:Fullerene Solar Cells

We report the influence of the controlled acidity of the hole-collecting buffer layer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), on the performance and lifetime of polymer:fullerene solar cells. The acidity was controlled by adding a strong base (NaOH) to the pristine PEDOT:PSS solutions. The NaOH-modified PEDOT:PSS layers were used for fabricating polymer:fullerene solar cells with active layers made from blend films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C-61-butyric acid methyl ester (PC61BM). The results showed that a small addition of NaOH (0.2 molar ratio) removed 23% of the sulfonic acid groups but did not change the device performance, even though further NaOH addition degraded the, device performance owing to an increased sheet resistance and lowered work function, as well as a changed surface morphology. Storage lifetime tests showed that the device with the modified PEDOT:PSS layer (0.2 molar ratio NaOH) was almost not degraded, whereas the pristine PEDOT:PSS layer might affect the deterioration at the interface with the active layer (P3HT: PCBM). Under 1 sun illumination for 10 h, the modified PEDOT:PSS layer (0.2 molar ratio NaOH) resulted in similar to 25% improved lifetime, which is in excellent agreement with the extent of reduction of the sulfonic acid groups (similar to 23%).