Over the last two-decades, several thin-film solar cell technologies such as polycrystalline silicon, CIGS, semiconductor nanoparticles, organic semiconductors have been explored. Among one of them, three-dimensional organic-inorganic perovskites have emerged as one of the most promising candidate due to its remarkable photo-physical properties, that have led to power conversion efficiencies exceeding 20% and can potentially approach the thermodynamic Shockley-Queisser limit for a single-junction solar cell (~33.5%). While constantly improving of PCE is important, a critical scientific bottleneck that will determine the fate of perovskite-based materials for photovoltaics and also other optoelectronic applications is the demonstration of both environmental and photo-stability under operating conditions. In contrast Ruddlesden-Popper phases, which are a form of layered two-dimensional (2D) perovskite films have exhibited promising stability against environment but poor PCE capped at 4.73%. This is attributed to a long-standing bottleneck where out-of-plane charge transport is inhibited by the presence of organic cations, which act like insulating spacing layers between the conducting inorganic components.We also examined the GIWAXS to confirm the crystal oreientaion evaluation with temperature and correlated with device performance. Here we overcome this fundamental issue in layered perovskites and report a record photovoltaic efficiency of 12.52 % with no hysteresis, more than two-times higher than previously reported values. The phenomenal increase in efficiency is attributed to the near single-crystalline quality thin-films with a strongly preferential out-of-plane alignment of the inorganic perovskite component that facilitates efficient charge transport. Layered 2D perovskite based photovoltaic devices exhibit much superior stability in comparison to their 3D counterparts when subjected to aggressive stress tests of light, humidity and heat. Unencapsulated 2D devices retain over 70% of their efficiency for over 800 hours under 1-Sun constant illumination and exhibit much higher threshold against 65% relative humidity. With simple encapsulation, the layered devices do not show any degradation under constant 1-Sun illumination or humidity. We anticipate Ruddlesden-Popper phases to play an important role in enabling for highly efficient and stable solar cells and other optoelectronic applications.