When cosmic rays (e.g. protons) passing through the atmosphere at low earth orbit (LEO) or spacecraft shielding, a certain amount of neutrons are created. Cacialliet al. reported that fast neutrons (energy ≥ 1 MeV) interact with perovskite, causing atomic displacement^[9], which induces Frenkel defects. These defects acted as unintentional dopants, increasing open-circuit voltage (V_oc) and reducing leakage current. Noticeably, unlike light-induced degradation, fast neutron-induced degradation is permanent and irreversible.

In space, the solar spectrum becomes AM0, which comprises additional UV irradiation than AM1.5. Generally, water and oxygen could participate in the degradation of perovskites under UV light^[15]. Recently, Liet al. indicated that the stability of PSCs with a TiO₂ layer was affected by UV light even in inert environment. The UV-induced degradation pathway is described inFig. 2(c)^[16]. Under UV light, Ti³⁺-V_O (oxygen vacancy) reacts with the photogenerated holes and transforms to active Ti⁴⁺-V_O trap states, leading to a loss of photocarriers. Ti⁴⁺-V_O oxidizes I− and produces I3−. I3− species further accelerate the decomposition of perovskite^[17]. Al₂O₃, Zn₂SnO₄, ZnTiO₃ and NiO_x serving as photocatalysts may follow a similar degradation mechanism.

PSCs demonstrate superior stability than silicon solar cells under cosmic rays, but they are certainly not indestructible. Some degradation mechanisms are proposed and more are yet to be explored.

By using 1486 eV X-ray to radiate MAPbI₃, the perovskite surface was damaged, causing the formation of crystalline PbI₂ and the evaporation of NH₃I^[13]. Cappelet al. indicated that different perovskites followed different decomposition modes under strong X-ray irradiation. For CsPbBr₃, X-rays caused the formation of metallic Pb, CsBr and Br₂, evidenced by a significant decrease in bromine content (Fig. 2(b)). For Cs_0.17FA_0.83PbI₃, X-rays caused the degradation of FA⁺, yielding a Pb/I ratio of 1 : 2 but without the formation of metallic Pb^[14].

Compared with electrons, protons have a higher mass and may cause severe damage. So far, the influence of proton irradiation has been studied mostly on unencapsulated PSCs under low-energy proton irradiation (50 or 150 keV) and encapsulated PSCs under high-energy irradiation (20 and 68 MeV). Low-energy protons could induce more damage because most of them stop in the device interior, while high-energy protons pass through the device^[5]. Nickelet al. studied the irradiation hardness of MAPbI₃ PSCs under 68 MeV proton irradiation with a total dose of 1.02 × 10¹³ p/cm². The device performance unexpectedly increased after irradiation, which was attributed to the reduced SRH recombination loss (Fig. 1(d))^[7]. They speculated that the proton-doping-induced shallow energy-level defects compensated the impact of MA-escape-induced deep energy-level defects. In their subsequent study, they conducted proton irradiation test on Cs_0.05MA_0.17FA_0.83Pb(I_0.83Br_0.17)₃ PSCs under the same condition. The photoluminescence (PL) decay curves indicated that the carrier lifetime was prolonged after 68 MeV proton irradiation. A kinetic model, in which the trap states induced by proton irradiation slowly release the trapped minority carriers, could perfectly fit the experimental results^[8].

High power-to-weight ratio, good radiation resistance, and low manufacturing cost enable perovskite solar cells (PSCs) to be possibly used in space^[1]. Different from terrestrial applications, PSCs used in space will face cosmic radiation, e.g., electrons, protons, neutrons, gamma rays, X-rays, ultraviolet (UV) rays, etc. (Fig. 1(a)), harming their long-term operational stability. Electrons are the most common high-energy particles in space. A series of terrestrial experiments mimicking electrons in the space environment were carried out, showing that PSCs can be deteriorated upon exposure. Al-Jassimet al. elucidated two failure pathways for perovskite films under electron-beam (e-beam) exposure through cathodoluminescence (CL) signals variation, which are knock-on-induced defect formation and heat-induced phase transformation^[2]. Gaoet al. revealed the decomposition process of MAPbI₃ under e-beam exposure by in-situ TEM^[3]. The structural change was initiated by the loss of I ions to form MAPbI_2.5, followed by the loss of MA ions to form MA_yPbI_2.5–z, and ended up with PbI₂ (Fig. 1(b)). Yanet al. observed severe short-circuit current density (J_sc) decrease after electron exposure, which was mainly attributed to the reduced substrate transmittance caused by glass “coloring” (Fig. 1(c))^[4]. Similar phenomenon was also observed when soda-lime glass was irradiated by protons^[5] and gamma rays^[6], thus the irradiation tolerance of space-used substrate is demanded.

Gamma rays, with energy from a few keV to ~8 MeV, have the strongest energy in the electromagnetic spectrum and the highest penetrating ability. Huanget al. used gamma rays of 0.42 rad/s to irradiate Cs_0.05FA_0.81MA_0.14PbI_2.55Br_0.45 solar cells^[6]. The power conversion efficiency (PCE) decreased to 85% of the initial value after first 2 h, declined slowly between 2–100 h, and kept stable between 100–1410 h. The initial PCE attenuation was attributed to ion displacement in the perovskite. Then, ions slowly returned to their original lattice positions, leading to the recovery of PCE. Yanget al. irradiated FA_0.945MA_0.025Cs_0.03Pb(I_0.975Br_0.025)₃ PSCs at a higher intensity (50 rad/s) and observed the formation of δ-phase FAPbI₃^[10]. Troshin et al. irradiated perovskite by using gamma rays of 4.2 rad/s with an accumulated dose up to 50 krad. PL spectra exhibited an increase in peak intensity and a red shift in peak position. They thought that gamma rays promote phase segregationvia forming I-rich and Br-rich domains^[11]. By excluding the influence of glass, they compared the stability of different perovskites upon gamma rays irradiation. MAPbI₃ presented the highest stability because of its unique self-healing nature (Fig. 2(a)). At the surface, MAPbI₃ can easily decompose into CH₃I, NH₃ and PbI₂. Under gamma rays irradiation, radicals of the volatile species will form and react to form MAI again, which will further react with PbI₂ to form MAPbI₃^[12].

Y. Jiang thanks the support from the Energy Materials and Optoelectronics Unit of Songshan Lake Materials Laboratory (Y0D1121E311). L. Ding thanks the open research fund of Songshan Lake Materials Laboratory (2021SLABFK02), the National Key Research and Development Program of China (2017YFA0206600), and the National Natural Science Foundation of China (51922032, 21961160720).