Analysis of the failure mechanism for a stable organic photovoltaic during 10 000 h of testing

Abstract

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 18 O 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 S2(AM1Á5) at 72 W 2-C under a vacuum of <10 S6 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-C in an atmosphere where the oxygen had been replaced with the isotope 18 O 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 the 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 18 O 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.

Tunable molecular orientation and elevated thermal stability of vapor-deposited organic semiconductors

“…Because the three systems investigated here show the density and onset temperatures expected for stable glasses, we anticipate that organic semiconductors will generally have other stable glass properties that should provide control over some known failure modes (40)(41)(42). For example, high glass density reduces uptake of atmospheric gases which can cause chemical degradation (43).…”

Morphology and photochemical stability of P3HT:PCBM active layers of organic solar cells

“…The understanding of OSC failure mechanisms has recently built a foundation of knowledge. OSCs exhibit low resistance to the combined actions of sunlight, temperature, O 2 , and H 2 O. Ageing occurs at each layer and interface [6]. The photochemical stability of the polymer active layer is one aspect of the complex OSCs stability problem, and three points have to be taken into consideration:…”

Effects of concentrated sunlight on organic photovoltaics

“…4 Recently long-term-stable BHJ cells have been developed. 3,[5][6][7] This achievement opens various possibilities in OPV investigations ͑e.g., round robin test͒ 8 and development of practical applications. On the other hand, it raises problems, e.g., need for relevant accelerated tests of operational life-time.…”

“…Finally, the preliminary long-term experiments at high C ͑55-58 suns͒ and various temperatures proved that our setup can be used for accelerated tests of the OPV operational lifetime. Furthermore, one can propose a research strategy for future detailed experiments that may select light induced mechanisms in the OPV degradation from those ͑like interface diffusion, etc.͒ 5,6 controlled by the cell temperature.…”