Water-induced degradation of polymer solar cells studied by H ₂ ¹⁸O labeling

Water-induced degradation of polymer photovoltaics based on the active materials poly(3-hexylthiophene) (P3HT) or poly[2-methoxy-5-(2′- ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV) was studied. The solar cell devices comprised a bulk heterojunction formed by the active material and [6,6]-phenyl-C ₆₁-butyric acid methyl ester (PCBM) in a standard device geometry. The use of H ₂ ¹⁸O in conjunction with time-of-flight secondary ion mass spectrometry enabled mapping of the parts of the device that were induced by water. A comparison was made between the two active materials and between devices that were kept in the dark and devices that had been subjected to illumination under simulated sunlight. Devices that were exposed to ambient humidity were compared to devices exposed to saturated humidity. Finally, a comparison was made between results obtained using H ₂ ¹⁸O and earlier work involving ¹⁸O ₂. Water was found to have behavior similar to but not identical with molecular oxygen.