Thioacetamide additive assisted crystallization of solution-processed perovskite films for high performance planar heterojunction solar cells

High quality perovskite films with uniform coverage and large grains are indispensable to enhance the performance of perovskite solar cells with high efficiency and stability. However, solution-processed perovskite films usually possess small grains associated with abundant grain boundaries, which induce high trap state density and then seriously degrade the device performance. In this paper, the volatile Lewis base, thioacetamide (TAA), is employed as an additive to fabricate high-quality methylammonium lead iodide (MAPbI(3)) films. The average grain size of perovskite films increases continuously with increasing TAA content and reaches a maximum value of 960 nm in the sample with 1.0% TAA. However, the average gain size drops dramatically to the value of samples without TAA when TAA content increases to 2.0%, and then the average gain size keeps nearly unchanged upon further increasing TAA content up to 10%. This unusual grain size variation tendency is attributed to the volatility of additive, and a mechanism is proposed based on various characterizations to illustrate how volatile TAA improves perovskite film crystallization. Furthermore, the device based on the MAPbI(3) film with 1.0% TAA shows a superior PCE of 18.9% and improved stability that the device with 1.0% TAA retains 88.9% of its initial performance after aging 816 h in the air with 25-35% relative humidity. The results strongly suggest that the TAA-modified MAPbI(3) films as absorber layers can significantly enhance the performance of the perovskite solar cell due to large grains, high crystallization and reduced trap state density of the high quality TAA-modified MAPbI(3) films.