| Summary: | All-perovskite tandem solar cells have great potential to realize the efficiency beyond the Shockley-Queisser efficiency limitation of single junction solar cells. Here we investigate the photon redistribution effect induced by introduction of the optical coupling layer in two terminal (2 T) tandem solar cells. In this work, it is shown that the introduction of the optical coupling layer could not only reduce the reflection loss, but also would affect the redistribution of the absorbed photons in the tandem device due to the optical interference effect, which will disturb the previous balance and complicate the optical modulation. This photon redistribution phenomenon is systemically investigated by combining the optical and electrical aspects. By considering the coupling effect beyond the antireflection function after introducing the optical coupling layer, the new current match could be achieved, which would obviously improve the device performance further. Various materials such as LiF, MgF<sub>2</sub>, Al<sub>2</sub>O<sub>3</sub>, ZnO have been adopted as the optical coupling layer and it is demonstrated that this phenomenon is universal for all the cases. The improvement brought by photon redistribution effect may be even more obvious than antireflection function. Through analyzing the inner optical mechanism and modulating the structure, a higher performance can be achieved by carefully considering the optical coupling effect.
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