Morphology control towards a greener, non-halogenated solvent system processed CH3NH3PbI3 film for high performance perovskite solar cells
One of the advantages of perovskite solar cells is that they are compatible with low-cost and large-scale solution processes via the roll-to-roll technique. Green solvent engineering plays an important role in large-scale manufacturing. N-Methyl pyrrolidone (NMP) is considered to be less toxic and a biodegradable compound, which is less harmful to the human body and the environment. However, it is difficult to achieve a high film quality for an NMP-processed perovskite film in a short time. In this communication, we developed a two-step green anti-solvent engineering technique to rapidly tune the morphology of the NMP-processed perovskite film. The first step was to deposit ethyl acetate (EA) on top of the precursor film to induce fast nucleation. Second, a methylammonium chloride (MACl) solution dissolved in isopropanol (IPA) was added to affect the crystal growth process and subsequently form a two-layer structured perovskite film including MAPbI3 as a bottom layer and mixed MAPbI3/MAPbIxCl3−x as an upper layer. The upper layer showed a significantly enhanced degree of crystallinity and reduced trap states. It has been recognized that MAPbIxCl3−x has a higher LUMO level than that of MAPbI3. Thus, the enrichment of a Cl-based perovskite near/at the surface can promote the efficient charge transfer. Furthermore, the limited content of MAPbIxCl3−x in MAPbI3 barely affects the entire absorption of the perovskite and thus the JSC of the device. The as-prepared inverted PSC yields a high PCE of 16%, mainly attributing to the obviously improved VOC and JSC, while the reference device containing the perovskite layer treated only by EA shows a PCE of 14.36%.