Perovskite solar cells (PSCs) represent a highly promising emerging photovoltaic technology with an extremely high potential for large-scale practical application since these devices combine the processability and scalability of organic photovoltaics (OPVs) with the high efficiency of crystalline silicon solar cells. Unfortunately, the low operational stability of perovskite solar cells remains the major obstacle to the transition of PSCs from research labs to industrial-scale production and mass application.

The stability of complex lead halides can be tuned by using different types of additives or modifiers capable of specific interactions with the perovskite absorber material, e.g. forming coordination bonds with coordinationally unsaturated Pb²⁺ cations exposed on the film surface. There are hundreds of various molecular modifiers or passivation coatings tested directly in photovoltaic cells, whereas the information on their action mechanisms is very scarce and controversial. One of the reasons is that these molecular modifiers were mostly screened towards improving ambient stability of perovskite solar cells, which is probably not the best approach since efficient encapsulation should solve the extrinsic stability problem. Surprisingly, there are very few studies on the influence of such additives on the intrinsic photochemical and thermal stability of perovskite films. Obviously, lack of such fundamental information complicates drawing any reliable conclusions about action mechanisms of certain molecular modifiers.

Herein, we investigated the impact of a broad range of molecular modifiers (>30 compounds) on intrinsic photochemical and thermal stability of MAPbI₃, FAPbI₃, Cs_0.12FA_0.88PbI₃ and Cs_0.1MA_0.15FA_0.85PbI₃ thin films, where MA⁺ and FA⁺ are methylammonium and formamidinium cations, respectively. All experiments were performed under well-controlled anoxic conditions. The obtained results allowed us to identify the most promising additives and establish correlations between the molecular structures of the modifiers and their stabilization effects induced in perovskite films and draw some conclusions about the action mechanisms of the most promising additives.

In particular, we explored 4,6,10-trihydroxy-3,5,7-trimethyl-1,4,6,10-tetraazaadamantane hydrochloride (NAdCl) as a molecular modifier for MAPbI₃ perovskite films. It was shown for the first time that molecular modifier can slow down the decomposition of perovskite films in the absence of oxygen and moisture. [1] Furthermore, we presented a comparative study of two azaadamantane-based molecular modifiers NAdCl and MAdI (an iodide of N-methylated 1,3,5,7-tetraazaadamantane) as stabilizing additives for perovskite films. [2] The designed absorber materials based on Cs_0.10MA_0.15FA_0.75PbI₃ with NAdCl modifier and Cs_0.12FA_0.88PbI₃ with both NAdCl and MAdI modifiers could withstand 5000 h and 16000 h of continuous light exposure, respectively, which are among the record values of the intrinsic stability of lead halide perovskites.

More recently, we have introduced the antibacterial drug octenidine dihydroiodide (OctI₂) as a highly promising molecular modifier for designing complex lead halides with spectacularly enhanced intrinsic photostability. [3] The Oct(FA)_n−1Pb_nI_3n+1 and Oct(Cs_0.12FA_0.88)_n−1Pb_nI_3n+1No material formulations showed no signs of decomposition under white light exposure for 9000 h and 20000 h, respectively, which to the best of our knowledge represent the record lifetimes of perovskite films reported to date.

The performed studies featured a tremendous potential of rationally designed molecular modifiers to be used for blocking the main intrinsic degradation pathways in complex lead halides and boosting the operational stability of perovskite solar cells.