All that glitters is not gold: Recent progress of alternative counter electrodes for perovskite solar cells

Although ultrahigh power conversion efficiency (PCE) up to 22.7% has been achieved by solution-processable perovskite solar cells (PSCs), there are still some obstacles that hinder the way of this promising photovoltaic technology to its commercialization. One of the problems is that most of the state-of-art PSCs are dependent on the use of noble metal gold (Au) or silver (Ag) as a counter electrode (CE), while Au is costly, Ag is very unstable. CE plays a vital role in cell device efficiency and stability and affects the final cost of device or module. In this review, the efforts to seek low-cost and promising CEs for commercialization were first comprehensively summarized. Recent developments of non-(noble)-metal CEs used in PSCs were reviewed. An overview of materials, fabrication methods, and corresponding device performance of various non-(noble)-metal CEs are provided. A balanced discussion and comparison of efficiency, cost, and stability of different CEs are provided.