Methylammonium lead iodide perovskite( MAPI) is ap romising materialf or highly efficient photovoltaic devices.H owever,i t suffers from photooxidation,whichimposes strict requirements for its protection from oxygen during processing and operation. Ah indered amine light stabilizer( HALS) has been found to exert as tabilization effect on methylammonium iodide (MAI) and MAPI against photooxidation. The HALS prevents the degradation of MAI by inhibiting the oxidation of iodide to iodine. Chemical modification of HALS allowsi ts incorporation in MAPI films, whiche xtendst he resistivity of MAPI against photodegradation in ambient air from ac ouple of hours to severald ays,w hile causing no significant changes in key properties, such as optical absorption and charge transport. These results represent an important advance in the stabilizationo f MAPI against decomposition and demonstrate for the first time that antioxidants improve the stability of MAPI.Methylammonium lead iodide perovskite (MAPI) solar cells constitute an emerging photovoltaict echnology that in recent years has shown an outstanding improvement in power conversion efficiency, surpassing 21 %.[1-6] However,d espite this significant progress, poor stabilityo fp erovskite devices remains am ajor issue. Factors such as light, oxygen, humidity, and temperature cause rapid degradation of perovskite solar cells (PSC). [7,8] This drawback hampers potential large-scale application of the devices, which must be able to operate for a sufficient length of time under real environmental conditions. Because PSC is ac omplexd evice, composed of MAPI absorber, charge extractionl ayers, and electrodes, multiple degradation phenomenao ccur in it. Most of the studies, relate the degradation of PSC with MAPI decomposition, yet degradation is also attributed to the role of the hole-transporting material, its additives,t he electron-extracting TiO 2 ,t he electrodes, or even the architecture of the device. [7][8][9][10] Recent studies have demonstrated that photooxidation (POX) of MAPI inducedb yl ight and oxygen is ad ominant factor limiting the lifetimeo fP SC under ambient conditions. [11] Photoexcited MAPI reacts with oxygen, resulting in the formation of superoxide anion,w hich in turn attacks the methylammoniumc ation thus initiating degradation of MAPI. Importantly,t he degradation rate may be reduced when effective electron extraction from the excited perovskite is ensured and the electron transfer from MAPI to oxygen is minimized. However, despite reduced photodegradationw hen TiO 2 is used as an electron acceptor,t he process cannot be completely suppressed, [12] since areas with poor electron extraction are present in ap olycrystalline MAPI layer.[13] These recent findings indicate unequivocally that MAPI is inherently unstable in the presence of oxygen andl ight. Furthermore,M API degradation in PSCs also occurs in the dark at forward bias when electrically injected electrons react with oxygen.[12] Photodegradation of one of the MAPI precursors, namely methyla...