Electron transport layer-free polymer solar cells show 40% higher efficiency than using ZnO transparent cathode

The poor compatibility of inorganic materials (electron transport layer) with the active layer and an ultrathin film of conjugated polymers becomes the great obstacle to producing high-quality polymer solar cells with high-throughput roll-to-roll (R2R) method. Novel electron transport layer-free polymer solar cells have been constructed by integrating the conjugated aminoalkyl-functionalized polymer, poly[3-(5-(9,9-bis(3-(dimethylamino)propyl)-7-methyl-9H-fluoren-2-yl)thiophen-2-yl)-2,5-bis(2-butyloctyl)-6-(5-methylthiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione] (PDTFN) into the active layer. PDTFN was synthesized by different Suzuki reaction conditions to gain different molecular weights- small molecular weight (PDTFN-L) and high molecular weight (PDTFN-H). It is noteworthy that PDTFN-H can significantly enhance the power conversion efficiency of the solar cells that incorporate a photoactive layer composed of poly[(3-hexylthiophene)-2,5-diyl] (P3HT) and the fullerene acceptor [6,6-phenyl-C₇₁-butyric acid methyl ester] (PC₇₁BM). The power conversion efficiency varies from 2.5% for ZnO transparent cathode based solar cells to 3.6% for PDTFN-H based electron transport layer-free solar cells. This improved performance can be attributed to the following reasons: the PDTFN-H can enhance cathode electron transporting by introducing the PDTFN-H into active layer without extra coating the cathode interfacial material; besides, a favorable vertical phase separation of active layer was formed due to the PDTFN-H excellent compatibility with the bulk-heterojunction. These experimental results revealed that the electron transport layer-free polymer solar cells based on PDTFN-H can be a promising novel effective fabrication with simplified manufacturing process and lower cost.