Boosting Up Performance of Inverted Photovoltaic Cells from Bis(alkylthien-2-yl)dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4′,5′-b′]di thiophene-Based Copolymers by Advantageous Vertical Phase Separation

The photovoltaic cells (PVCs) from conjugated copolymers of PDTBDT-BT and PDTBDT-FBT with 5,10-bis(4,5-didecylthien-2-yl)dithieno[2,3- d:2′,3′- d ′]benzo[1,2- b:4,5- b ′]dithiophene as electron donor moieties and benzothiadiazole and/or 5,6-difluorobenzothiadiazole as electron acceptor moieties are optimized by employing alcohol-soluble PFN (poly(9,9-bis(3′-( N, N -dimethylamino)propyl)-2,7-fluorene)- alt -2,7-(9,9-dioctylfluorene)) as cathode modification interlayer. The power conversion efficiencies (PCEs) of inverted PVCs ( i- PVCs) from PDTBDT-BT and PDTBDT-FBT with devices configuration as ITO/PFN/active layer/MoO 3 /Ag are increased from 4.97% to 8.54% and 5.92% to 8.74%, in contrast to those for the regular PVCs ( r- PVCs) with devices configuration as ITO/PEDOT:PSS/active layer/Ca/Al under 100 mW/cm 2 AM 1.5 illumination. The optical modeling calculations and X-ray photoelectron spectroscopy (XPS) investigations reveal that the r- PVCs and i- PVCs from the copolymers exhibit similar light harvesting characteristics, and the enhancements of the PCEs of the i- PVCs from the copolymers are mainly contributed to the favorable vertical phase separation as the strongly polymer-enriched top surface layers and slightly PC 71 BM (phenyl-C 71 -butyric acid methyl ester)-enriched bottom surface layers are correspondingly connected to the anodes and cathodes of the i- PVCs, while they are opposite in the r- PVCs. As we known, it is the first time to experimentally verify that the i- PVCs with alcohol-soluble conjugated polymers cathode modification layers enjoy favorable vertical phase separation.