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Bandtail broadening originating from increasing the polydispersity of colloidal quantum dots (CQDs) deteriorates open-circuit voltage (V OC) and hinders charge-carrier transport in CQD photovoltaics. The development of colloidal synthetic routes has enabled preparing monodisperse perovskite CQDs (Pe-CQDs) that have attracted attention as promising absorbers in CQD photovoltaics. However, polar-antisolvent-based purification induces the dissolution and agglomeration of Pe-CQDs, resulting in an irregular size distribution. Consequently, the photovoltaic performance decreases because of the increase in Pe-CQD polydispersity. Here, we demonstrate the preparation of well-purified monodisperse CsPbI3-Pe-CQDs via size selection on the basis of gel permeation chromatography. Well-purified monodisperse Pe-CQDs exhibit improved photovoltaic performance and achieve a low Pe-CQD polydispersity. Furthermore, these Pe-CQDs show higher photoluminescence quantum yields, narrower full-widths at half-maximum, and lower Urbach energies, in comparison to irregular-sized Pe-CQDs without size selection. Therefore, CsPbI3-Pe-CQD solar cells comprising monodisperse Pe-CQDs show the highest power conversion efficiency (15.3%) and V OC (1.27 V) among the fully inorganic CsPbI3-Pe-CQD solar cells reported so far.