Boosting Up Performance of Inverted Photovoltaic Cells from Bis(alkylthien-2-yl)dithieno[2,3‑<i>d</i>:2′,3′‑<i>d</i>′]benzo[1,2‑<i>b</i>:4′,5′‑<i>b</i>′]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-<i>d</i>:2′,3′-<i>d</i>′]­benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­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′-(<i>N</i>,<i>N</i>-dimethyl­amino)­propyl)-2,7-fluorene)-<i>alt</i>-2,7-(9,9-dioctyl­fluorene)) as cathode modification interlayer. The power conversion efficiencies (PCEs) of inverted PVCs (<i>i-</i>PVCs) from PDTBDT-BT and PDTBDT-FBT with devices configuration as ITO/PFN/active layer/MoO<sub>3</sub>/Ag are increased from 4.97% to 8.54% and 5.92% to 8.74%, in contrast to those for the regular PVCs (<i>r-</i>PVCs) with devices configuration as ITO/PEDOT:PSS/active layer/Ca/Al under 100 mW/cm<sup>2</sup> AM 1.5 illumination. The optical modeling calculations and X-ray photoelectron spectroscopy (XPS) investigations reveal that the <i>r-</i>PVCs and <i>i-</i>PVCs from the copolymers exhibit similar light harvesting characteristics, and the enhancements of the PCEs of the <i>i-</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<sub>71</sub>BM (phenyl-C<sub>71</sub>-butyric acid methyl ester)-enriched bottom surface layers are correspondingly connected to the anodes and cathodes of the <i>i-</i>PVCs, while they are opposite in the <i>r-</i>PVCs. As we known, it is the first time to experimentally verify that the <i>i-</i>PVCs with alcohol-soluble conjugated polymers cathode modification layers enjoy favorable vertical phase separation.