Dual-functional cathode buffer layer for power conversion efficiency enhancement of bulk-heterojunction solar cells

Figure (a) Schematic diagram showing down-conversion by ZnO:Al, Eu layer in PCDTBT based solar cell. (b) J–V curve for PCDTBT/PC70BM BHJ solar cell with ZnO and ZnO:Al,Eu as CBL, in dark and simulated AM 1.5 G irradiation (100 mWcm−2). [Display omitted] •Al, Eu doping in ZnO improves its electric and optical properties.•The role of the luminescent CBL layer is to down-shift the UV light into visible range.•Absorption overlapping between the CBL and PCDTBT polymer results in an energy transfer from ZnO:Al;Eu to PCDTBT polymer.•The luminescent ZnO:Al;Eu nanoparticle CBL has great potential in organic solar cells. Here, we report a luminescent cathode buffer layer (CBL) for power conversion efficiency (PCE) enhancement of organic solar cells (OSCs). ZnO doped with Aluminum (Al) and Europium (Eu) was cast as CBL by a solution-processed method. CBL serves a dual purpose by acting both as a spectral conversion and an electron transporting layer. The luminescent ZnO:Al,Eu CBL layer has broad absorption spanning the ultraviolet (300–400 nm) spectrum, contributing to absorption enhancement. Moreover, the emission of CBL layer overlaps with the absorption of poly [N -9′-heptadecanyl-2,7-carbazole- alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) polymer thus elevating the overall absorption of the active layer and improving the photocurrent. With this ZnO:Al,Eu CBL in the inverted device configuration (ITO/CBL/active layer/MoOx/Al), an enhanced power conversion efficiency (PCE) of 6.9% was obtained while the device with pristine ZnO as CBL showed PCE of 5.9%. A blend of PCDTBT donor and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) acceptor was used as an active layer in both the cases. In ZnO:Al,Eu CBL layer, Al doping improves the conductivity, while Eu doping significantly enhances the emission in the visible region by down-shifting the incoming solar UV light to the visible range which overlaps with the absorption of PCDTBT polymer resulting in energy transfer and improved overall device efficiency. The findings of the study show the significance of luminescent ZnO:Al,Eu nanoparticle CBL in enhancing the performance of organic solar cells.