Organic-inorganic lead halide perovskites (CH₃NH₃PbI₃) are of utmost interest for the development of cost effective and efficient next generation solar cells [1]. Perovskite family is generally adopting the ABX₃ structure, where X is an anion (Br^-, I^-, Cl^-), A (MA⁺, FA⁺, Cs⁺) and B (Pb²⁺, Sn²⁺) are cations of different sizes. The PSC’s outstanding performance is determined by strong optical absorption of the perovskite layer in a visible range of solar spectrum. However the key challenge of the PSCs is associated with their poor operational stability, mainly caused by the rapid degradation in perovskite light absorbing layer which remains sensitive to air, high temperature and humidity [2]. Thus, numerous studies have been carried out aimed to overcome the issue of the long term device performance stability, particularly concerning the ion doping of the each of A-, B- and X-sites of the system. For alkali metal doped perovskites, several studies have been reported previously [3-4]. It was reported elsewhere [5] that the alkali metal halides could affect the perovskite layer crystallinity, due to re-crystallization of the small grains and passivation of the grain boundaries.

In this study we have fabricated air-processed perovskite solar cells based on pristine and potassium-doped (K-doped) CH₃NH₃PbI₃. We have provided long term and thermal stability tests of the PSCs which have been exposed to ambient conditions in the dark and to higher temperature of 60°C. We have shown that the incorporation of the additive metal ions into perovskite structure improved either the PV performance, or stability of PSCs.

PSCs were fabricated using a common one-step spin-coating deposition method. K-doping of CH₃NH₃PbI₃ was provided by adding 0.02M KI to 1.2M perovskite precursor solution. The long-term device performance of the PSCs was investigated by periodical measurements of the J-V curves under simulated light (AM1.5G, 1000 W/m²) using Keithley 4200-SCS Semiconductor Characterization System (USA).

The structure investigations of CH₃NH₃(Pb:K)I₃ layer have shown that the alkai doping results in the rearrangement of perovskite layer morphology, reducing grain boundaries and trap states, thus, retarding surface recombination processes.

Within the aging tests the first series of both K-doped and pristine PSC samples was exposed to normal temperature and pressure conditions (NTP) in the dark with the average humidity of 50%. The second series was exposed to higher temperature of 60°C in the dark. A drop in the PCE (resulting from J_SC and FF decrease) was observed for pristine PSCs. Due to the moisture influences, perovskite layer bonds are disrupted with the formation of highly corrosive HI and PbI₂. Rapid thermal degradation is probably attributed to the Au diffusion processes in PSCs. The results have shown that the incorporation of the additive metal ions into perovskite structure has a positive effect in terms of PSCs operation stability. K-doped PSCs exhibit improved stability both in air and when exposed to higher temperatures. Resistance to degradation in the doped structure was found to be 3 times greater than that in the pristine structure.

To conclude, the incorporation of alkai metal ions can affect the formation, phase stability and charge transport characteristics of the perovskite structures. Thus, we have succeeded in the development of PSCs with optimized materials, which led to the improvement of the PV performance and stability of the devices.