Comparing Polar and Non-Polar Polymer Additives for Moisture Stability of Organic-Inorganic Halide Perovskite Solar Cells

Abstract

Recently, power conversion efficiency (PCE) of lead halide perovskite solar cells (PeSCs) having rapidly increased from 3.8% to 23.3% within eight years. These results are comparable to state-of-the-art inorganic solar cells. Moreover, solution processability, superior optical, electrical properties such as long charge diffusion lengths, and very high absorption coefficient are also the strengths of the perovskite. However, despite of high efficiency and good processability, it is difficult to commercialize, due to its vulnerability to moisture. Common PeSCs structures are divided into n-i-p and p-i-n, and due to the characteristics of commonly used materials, n-i-p structures generally have better stability. Also most of high PCE PeSCs adapted n-i-p structure. In n-i-p structure, Metal oxide commonly used as Electron transporting layer (ETL). These metal oxides generally require high process temperatures. Herein, we demonstrated improving moisture stability, reproducibility and device performance of PeSCs via polymer which used additive and interface engineering material. PEIE and PFN commonly used interface engineering materials which help to improve surface uniformity and ITO work function tuning. In this paper, polyethylenimine ethoxylated (PEIE) and poly[(9,9-bis(3'-((N,N -dimethyl)-N -ethylammonium)- propyl)-2,7-fluorene)-alt -2,7-(9,9-dioctylfluorene)] (PFN-Br) were introduced to obtain the uniform SnO2 film. High performance solar cell need uniform and thin SnO¬2 layer. SnO2 is difficult to form thin and uniform film because they are well aggregate themselves in spin coating process. Therefore, pinhole and aggregation are well observed in SnO2 layers and these results ultimately affect the devices' performance and reproducibility. Through interface engineering, the uniform SnO2 layer was formed, which improved the reproducibility of the device. We introduced a polymer additives to solve the stability problem of perovskite solar cells. We have investigated the effect of two different polymers on the stability of perovskite by using polar and nonpolar polymers. Poly(2-vinylnapthalene) (PVN) was not well soluble in polar solvents such as N,N-Dimethylformamide (DMF) and Dimethyl sulfoxide (DMSO), and Poly(N-isopropylacrylamide) (PNIPAM) was highly soluble in polar solvents. Polymer additives such as PVN (nonpolar) and PNIPAM (polar) added perovskite solution, two different polymers did not significantly affect the performance of the device but improved device moisture stability. The nonpolar polymer adds hydrophobic properties to the film and the polar polymer slows the degradation through interaction with the MAI. As a result, we confirmed improving device reproducibility, stability, and performance with interface engineering and polymer additive method.