Efficient molecular solar cells processed from green solvent mixtures

Abstract

Cyclopentyl methyl ether (CPME), a green solvent, can be used to replace toxic halogenated solvents in the production of efficient molecular solar cells. With CPME alone as the processing solvent, a power conversion efficiency (PCE) of 3.13% was achieved when using a two-dimensional conjugated small molecule (SMPV1) as the donor and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) as the acceptor to form the bulk heterojunction (BHJ) organic photovoltaic (OPV) device. This low PCE arose from the low solubility of PC₆₁BM in this green solvent. Accordingly, toluene (Tol) was introduced in various amounts as a co-solvent for CPME. The greater solubility of PC₆₁BM in these mixtures led to significant improvements in the short-circuit current density (J_sc) and fill factor (FF) of the device, achieving a PCE of 7% after processing in the optimized green solvent mixture of CPME : Tol (60 : 40). Furthermore, thermal annealing (TA), at 80 °C for 10 min, of the active layers processed from the 60 : 40 green solvent mixture enhanced the PCE to 8.10%—the highest ever reported for a molecular solar cell processed from a green solvent mixture. Large-area devices fabricated this way, having areas of 1 and 5.5 cm², exhibited PCEs of 6.20 and 3.73%, respectively. The morphological changes that occurred when applying the co-solvent and TA played key roles in achieving such high PCEs for molecular solar cells processed from green solvent mixtures—a promising step toward the upscaling of OPVs.