Organic solar cells have shown power conversion efficiencies approaching 20% yet their operational stability is relatively poor. One of the key factors driving device degradation is the morphological instability of the active layer, which is made of an interpenetrating mixture of an electron donor material and an electron acceptor material. Under illumination, bias or temperature, changes can occur in the intermixing, phase separation between donor and acceptor, and the molecular packing of the materials. These changes often reduce the ability of the device to generate and separate charges and thus lower the power conversion efficiency. However, the relationship between morphological changes and degradation of the device performance is still poorly understood, making it difficult to design stable materials. Now, Peter Müller-Buschbaum and a team across Europe and China examine the evolution of the active layer morphology during solar cell operation by means of in situ and operando grazing-incidence small angle X-ray scattering and find a correlation with the crystallinity of the acceptor materials.
The researchers analyse solar cells with different acceptor materials to understand how the initial morphological characteristics of the active layer blend impact on the device degradation. The selected acceptor materials all bear different degrees of crystallinity. Müller-Buschbaum and colleagues observe that the acceptor with the greatest intermixing with the donor material — which enables the higher power conversion efficiency — does not have the best ability to maintain its morphology during operation. Instead, acceptors slightly de-mixed from donor materials but with stronger intermolecular π–π stacking interactions are more resilient to changes in blend morphology during solar cell operation, resulting in a slower device performance degradation. The work provides useful insights that could inform the molecular design of acceptor and donor materials to achieve both high power conversion efficiency and device stability. Future systematic studies targeting operational stability and degradation modes of high-efficiency solar cells are needed to consolidate the viability of the technology.
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