most successful to date were complex lead halides comprising simultaneously several univalent cations (Cs + , CH 3 NH 3 + or MA + , [H 2 NCHNH 2 ] + or FA +) and halide anions (typically Br − , I −) in their crystal lattice. [2] However, these materials suffer from low photostability. In particular, Hoke et al. first demonstrated that the mixed-halide MAPb(I 1−x Br x) 3 absorbers undergo rapid light-induced halide segregation with the formation of I-rich and Br-rich phases leading to both structural and energetic disorder resulting in a significant decrease in solar cell performance. [3,4] While the effect of short light exposure was found to be essentially reversible in the dark, long-term irradiation of the mixed halide perovskite films results in their complete degradation. [5] Therefore, light-induced halide phase segregation is considered as a severe limitation for achieving long-term operational stability of perovskite solar cells based on the absorbers incorporating more than a single halide anion. [6] Overcoming this problem is crucially important for the development of tandem devices with the upper cell based on the perovskite absorber with the tailored optical properties realized through halide mixing. Since the discovery of the light-induced halide phase segregation in complex lead halides, many research groups have investigated this phenomenon in detail in an attempt to reveal its mechanism. Multiple models varying in the origin

2-nitrotoluene (2-NT) is a good model for both photolabile protecting groups for organic synthesis and the military explosive 2,4,6-trinitrotoluene (TNT). In addition to the direct C−NO 2 bond-cleavage reaction that initiates detonation in TNT, 2-NT undergoes an H-atom attack reaction common to the photolabile 2-nitrobenzyl group, which forms the aci-nitro tautomer. In this work, femtosecond pump-probe measurements with mass spectrometric detection and density functional theory (DFT) calculations demonstrate that the initially-prepared vibrational coherence in 2-NT radical cation (2-NT +) is preserved following H-atom attack. Strong-field adiabatic ionization is used to prepare 2-NT + , which can overcome a modest 0.76 eV energy barrier to Hatom attack to form the aci-nitro tautomer as soon as ∼ 20−60 fs after ionization. Once formed, the aci-nitro tautomer spontaneously loses −OH to form C 7 H 6 NO + , which exhibits distinctly faster oscillations in its ion yield (290 fs period) as compared to the 2-NT + ion (380 fs period). The fast oscillations are attributed to the coherent torsional motion of the aci-nitro tautomer, which has a significantly faster computed torsional frequency (86.9 cm −1) than the 2-NT + ion (47.9 cm −1). Additional DFT calculations identify reaction pathways leading to the formation of the dissociation products C 7 H 6 NO + , C 7 H + 7 , and C 6 H 6 N +. Collectively, these results reveal a rich picture of coherently-and incoherently-driven dissociation pathways in 2-NT + .

In this work, we report a comparative study of the gamma ray stability of perovskite solar cells based on a series of perovskite absorbers including MAPbI 3 (MA = methylammonium), MAPbBr 3 , Cs 0.15 FA 0.85 PbI 3 (FA = formamidinim), Cs 0.1 MA 0.15 FA 0.75 PbI 3 , CsPbI 3 , and CsPbBr 3. We reveal that the composition of the perovskite material strongly affects the radiation stability of the solar cells. In particular, solar cells based on the MAPbI 3 were found to be the most resistant to gamma rays since this perovskite undergoes rapid self-healing due to the special gas-phase chemistry analyzed with ab initio calculations. The fact that the solar cells based on MAPbI 3 can withstand a 1000 kRad gamma ray dose without any noticeable degradation of the photovoltaic properties is particularly exciting and shifts the paradigm of research in this field toward designing more dynamic rather than intrinsically robust (e.g., inorganic) materials.

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