High-efficiency bulk heterojunction perovskite solar cell fabricated by one-step solution process using single solvent: synthesis and characterization of material and film formation mechanism

Abstract

Bulk heterojunction (BHJ) perovskite solar cells have recently attracted increased interest because of a significantly enhanced interface between perovskite and the n-type material in the active layer for efficient charge separation and increased power conversion efficiency (PCE). [6,6]-Phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) is the most commonly used n-type material in BHJ perovskite solar cells owing to its high electron mobility. However, it is very difficult to fabricate BHJ perovskite films because of the poor solubility of PC₆₁BM in the commonly used solvent dimethylformamide (DMF). In this study, we introduced two kinds of fluorinated PC₆₁BM (3F-PC₆₁BM and 5F-PC₆₁BM) as n-type materials in a BHJ perovskite film, which have higher solubility in DMF than that of PC₆₁BM. Thus, a BHJ perovskite film can be easily fabricated in one step using a single solvent in a BHJ precursor solution system for planar perovskite solar cells. A BHJ device with a high PCE of 16.17% can be obtained by adding 0.1 wt% of 3F-PC₆₁BM in a perovskite precursor solution to fabricate a solar cell, which outperforms the PCE of 14.12% of the pristine device. However, the addition of 5F-PC₆₁BM decreased the PCE to lower than that of the pristine device regardless of its amount. We systematically studied the effects of the amount and type of fluorinated PC₆₁BM on the morphology of BHJ perovskite films using SEM, AFM, GISAXS and GIWAXS. The results reveal that 3F-PC₆₁BM can fill the pinholes between perovskite grains and passivate the defects in the pristine film. Thus, the current density (J_sc) is greatly increased. On the other hand, the self-aggregation of 5F-PC₆₁BM in BHJ perovskite films caused the films to be full of large voids, which led to poor device performance. The dense and flat surface morphology of BHJ perovskite films containing 3F-PC₆₁BM can also prevent the permeation of moisture into grain boundaries and enhance the device stability. Therefore, the device could maintain 80% of its original efficiency over 550 hours without any encapsulation in comparison with 240 hours for the pristine device. Our results provide a novel strategy for fabricating high-PCE and high-stability BHJ perovskite solar cells for the production of low-cost solar cells in the near future.