Analysis of the failure mechanism for a stable organic photovoltaic during 10000 h of testing

The degradation and failure mechanisms of a stable photovoltaic device comprising a bilayer heterojunction formed between poly(3-carboxythiophene-2,5-diyl-co-thiophene-2,5-diyl) (P3CT) and Buckminsterfullerene (C-60) sandwiched between indium tin oxide (ITO) and aluminium (Al) electrodes were elucidated by the time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis in conjunction with isotopic labelling using O-18(2) after a total testing time of 13 000 h. This experiment allowed us to understand the chemistry that takes place in three dimensions during degradation and failure of the device under accelerated testing conditions. The cell was subjected to continuous illumination with an incident light intensity of 1000 W m(-2) (AM1.5) at 72 +/- 2 degrees C under a vacuum of <10(-6) mBar. During the illumination period, IV-curves were recorded at regular intervals and the short circuit current of the device was monitored every 10 s for 10 760 h. The total illumination time was 12 200 h. During this period of time, the device performance degraded and the device was finally left in the dark at 25 degrees C in an atmosphere where the oxygen had been replaced with the isotope O-18(2). After 800 h in this atmosphere in the dark, the final IV-curves in the dark and under illumination were recorded. The main purpose of this work was the analysis using TOF-SIMS imaging and depth profiling of the degraded cell. The combined analyses correspond to the three-dimensional chemical imaging of the device showing specifically where the oxygen had reacted during exposure. Several general findings were made that are applicable to similar devices. It was found that the oxygen diffuses into the device through the Al electrode in between tire Al grains and through microscopic holes in the Al electrode. Once inside the device the oxygen diffuses in the lateral and vertical plane until the counter electrode is reached. C-60 was found to be susceptible to the incorporation of O-18 but P3CT was not under the conditions in question. The other prominent degradation pathway was found to be the diffusion of electrode materials into the device. Both electrode materials diffuse through the entire device to the counter electrode. Copyright (c) 2007 John Wiley & Sons, Ltd.