High-efficiency polymer solar cells with a cost-effective quinoxaline polymer through nanoscale morphology control induced by practical processing additives

In the quest to improve the performance of polymer solar cells (PSCs) with a view to realizing economic viability, various solvent additives such as 1,8-octanedithiol (ODT), 1,8-diiodooctane (DIO), diphenylether (DPE) and 1-chloronaphthalene (CN) are used in easily obtainable poly(2,3-bis-(3-octyloxyphenyl)-quinoxaline-5,8-dyl-alt-thiophene-2,5-diyl) (TQ1)-based systems with [6,6]-phenyl C-71-butyric acid methyl ester (PC71BM) as an acceptor to optimize the active layer nanomorphology. Utilizing a combination of X-ray diffraction (XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM), we find that the addition of 5% (v/v) CN leads to smoother films, less heterogeneous surface features, and well-distributed TQ1: PC71BM phases, resulting in more balanced charge transport in the devices and a highly efficient power conversion efficiency (PCE) of 7.08%. This is a record for quinoxaline-based PCSs and is also comparable with the hitherto reported highest efficiency of the PSCs in single junction devices. In addition, the PSCs using an inverted device structure show a satisfactory PCE of 5.83% with high stability to ambient exposure, maintaining over 80% of its initial PCE, even after storage in air for more than 1 month.