Tin perovskite solar cells are suffering from poor device performance and stability. However, their microscopic understanding is not sufficient at present. Here, we investigate charge states in charge transporting layers and degradation mechanisms in tin perovskite solar cells. By comparing the amount of electron transfer between PEDOT:PSS and perovskites, the suppression of oxidation by PEA substitution was proved by direct observation of charge transfer. Furthermore, the anions of ICBA were observed under dark conditions, which means the electron transfer from perovskite. The obtained understanding of charge states in charge transporting layers will accelerate future improvements in device performance and stability.