Metal halide perovskite materials have shown a relatively fast evolution in the power conversion efficiency (PCE) reaching the level exceeding those of CIGS and CdTe and approaching those of crystalline Si solar cells. However, low luminescence efficiency of metal halide perovskite in a complete device and non-radiative losses originating from sub gap charge carrier trap states on the grain surfaces (e.g. halide vacancies) are the main barriers against reaching the efficiency limit in solar cells. In addition, the long-term stability of perovskite solar cells (PSCs) remains a pressing challenge that hinders their commercialisation.

Here, we will detail several new and promising passivation approaches through compositional modification and interface engineering aimed at eliminating the sources of instability and loss processes in metal halide perovskites. We demonstrate substantial mitigation of both non-radiative losses and photoinduced ion migration in perovskite films and interfaces by decorating the surfaces and grain boundaries with passivating potassium halide layers. We find significant enhancement in both micro-photoluminescence and photoluminescence quantum efficiency (e.g. internal yields exceeding 95%) while maintaining high mobilities over 42 cm²V^-1s^-1, giving the elusive combination of both high luminescence and excellent charge transport translating into over 21% PCE of the PSCs with entire elimination of hysteresis and an order of magnitude enhancement in the external quantum efficiency and stability of perovskite based light emitting diodes.

One source of instability in PSCs is interfacial defects, in particular, those that exist between the perovskite and the hole transport layer (HTL). Here, we demonstrate that thermally evaporated dopant-free tetracene on top of the perovskite layer, capped with a doped Spiro-OMeTAD layer and top gold electrode offers an excellent hole-extracting stack with minimal interfacial defect levels.  However, we and others find that dopant-free organic semiconductor HTLs introduce undesirable injection barriers to the metal electrode. By capping 120 nm of tetracene with 200 nm solution-processed lithium TFSI - doped Spiro-OMeTAD, we demonstrate a graded hole injection interface to the top gold layer with enhanced ohmic extraction. For a perovskite layer interfaced between this graded HTLs structure and a mesoporous TiO₂ electron-extracting layer its external photoluminescence yield (PLQE) reaches 15%, compared to 5% for the perovskite layer interfaced between TiO₂ and Spiro-OMeTAD alone. For complete solar cell devices containing tetracene/Spiro-OMeTAD as the HTL with graded doping profile, we demonstrate PCEs of up to 21.5% and extended power output over 550 hours continuous illumination at AM1.5 retaining more than 90% of the initial performance, validating our approach. Our findings represent a breakthrough in the construction of stable PSCs with minimized non-radiative losses.