Tin (Sn)‐based perovskites are promising materials for perovskite‐based light‐emitting diodes (PeLEDs) that avoid toxic lead. However, the performance of Sn‐based PeLEDs lags behind that of their lead analogues due to uncontrolled crystallization, the susceptibility of Sn(II) to oxidation, and high defect densities. In this study, ammonium thiocyanate (NH 4 SCN) is used as an additive in DMSO‐free perovskite precursor solution to afford Sn‐based perovskites that are evaluated in PeLEDs. NH 4 SCN modulates the crystallization process in the solution phase to improve the morphology and crystallinity of the resulting 2D perovskite films and inhibits the oxidation of Sn(II) to Sn(IV), a problem observed in DMSO. Transient terahertz spectroscopy reveals that NH 4 SCN‐based perovskite films have a higher free carrier density compared to the control films, indicative of reduced defect density. Consequently, pure‐red Sn‐based PeLEDs with an emissive peak of 629 nm achieve an external quantum efficiency of ≈1.4%, with an operating half‐life of >11 min. These findings provide valuable insights into the preparation of lead‐free perovskite materials avoiding DMSO for application in optoelectronic devices.