Search

Although degradation mechanisms in organic photovoltaic devices continue to receive increased attention, it is only recently that the initial light-induced failure, or so-called burn-in effect, has been considered. Both prototypical polythiophene:fullerene and polycarbazole:fullerene systems exhibit an exponential performance loss of ≈40% upon 150 h of continuous solar illumination. While the decrease in both the short-circuit current ( JSC) and open-circuit voltage ( VOC) is the origin of performance loss in poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC60BM), in poly(N-9′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)):[6,6]-phenyl-C71-butyric acid methyl ester (PCDTBT:PC70BM) the decline of the fill factor dominates. By systematic variation of the interface layers, active layer thickness, and acceptor in polythiophene:fullerene cells, the loss in JSC is ascribed to a degradation in the bulk of the P3HT:PC60BM, while the drop in VOC is reversible and arises from charge trapping at the contact interfaces. By replacing the C60 fullerene derivative with a C70 derivative, or by modifying the electron transport layer, the JSC or VOC, respectively, are stabilized. These insights prove that the burn-in process stems from multiple concurrent failure mechanisms. Comparing the ageing and recovery processes in P3HT and PCDTBT blends results in the conclusion that their interface failures differ in nature and that burn-in is a material dependent, rather than an intrinsic, failure mechanism. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we elucidate the role of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) in the degradation of polymer:PCBM ((6,6)-phenyl C61-butyric acid methyl ester) solar cells. The study is done on unencapsulated cells exposed to ambient conditions in dark. The cell degradation results from reduced carrier extraction, and an investigation of the various interfaces within the cell allows us to correlate this to oxidation of the low work function metal cathode. We further show that this oxidation is caused by water vapor diffusion from the edges through the hygroscopic PEDOT:PSS layer. We demonstrate that only the hygroscopic nature of PEDOT:PSS, and not its acidity, has a detrimental impact. The oxidation of the cathode progresses in synchrony with the water ingress into the PEDOT:PSS layer from the edges of the device towards the central part, and results in a progressive constriction of the active area. When the PEDOT:PSS layer is replaced by an evaporated layer of MoO3, the device lifetime is improved considerably even with highly reactive metal cathodes. Finally, we provide a quantitative relationship between device lifetime and the level of humidity in the ambient, thus establishing a suitable accelerated shelf-life test for organic solar cells and their encapsulation. [Copyright &y& Elsevier]

Stability studies of organic solar cells have recently revealed an initial light‐induced burn‐in process that calls into question the intrinsic stability of these devices. In article number 1400848, Eszter Voroshazi and co‐workers demonstrate that multiple concurrent failure mechanisms occur during the burn‐in, indicating that burn‐in is not intrinsic, but rather material dependent. This understanding allows the proposing of mitigation routes that are tested with the aim of solving this recognized issue. [ABSTRACT FROM AUTHOR]