Cesium-functionalized pectin as a cathode interlayer for polymer solar cells

Abstract

For polymer solar cells (PSCs), it remains challenging to develop an easily available cathode interlayer (CIL) with high transparency in the visible/near-infrared regions. Typical N-type conjugated polymeric/molecular backbones show strong light absorption, which interferes with the sunlight absorption of the photoactive layer and deteriorates the device efficiency. In this study, we report a functionalized biomass polymer material, cesium-functionalized pectin (PeCs), to serve as an efficient CIL for PSCs. PeCs can be readily prepared from pectin by a one-pot neutralization reaction. Due to the abundant hydrophilic groups, PeCs shows good solubility in water and limited solubility in CB or ODCB, which enables inverted PSC device fabrication. The cesium ions of PeCs form the desired interfacial dipole with the cathode, which decreases the work function and consequently improves electron extraction. The unconjugated structure yields high transparency in the visible/near-infrared regions (λ^abs_max = 282 nm), which benefits the light absorption of the photoactive layer. The high molecular weight of PeCs resulting from the pectin precursor ensures a film-forming capability. As a result, using a blend of poly[N-9-hepta-decanyl-2,7-carbazolealt-5,5-(4,7-di-thienyl-2,1,3-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) as the active layer, an inverted PSC with PeCs as the CIL shows a power conversion efficiency (PCE) of 7.14%, which is much higher than that of the PSCs with state-of-the-art ZnO as the CIL (PCE = 6.44%) or without a CIL (PCE = 3.65%). These results indicate that PeCs is a promising candidate as a cathode interlayer in low-cost and efficient polymer solar cells.