Efficiency improvement of organic solar cells with imprint of nanostructures by capillary force lithography

• Nanoimprints by capillary force lithography are developed to make nanostructures in the OPV.
• The nanostructures are made on either the electron transfer layer of ZnO or the active layer to improve cell performance.
• Both light trapping and charge transport can be improved to achieve a much higher cell performance.
• The imprint process is cost effective, easy to carry out and suitable for mass production.

Organic solar cells imprinted with nanostructures, by the capillary force lithography process using soft molding, on either the active layer or the electron transport layer (ZnO) of an inverted organic solar cell are made. With proper control of the wetness of the ZnO or the active layer, either of them can be readily driven by the capillary force into the cavities formed by the nanostructures on the soft mold during the imprint. The nanostructures demonstrated here can be either regular gratings or arrays of nanorods. Due to light scattering and intrusion of the nanostructures into the active layer, both light absorption and carrier transport can be significantly improved, which can effectively increase both the fill factor and the power conversion efficiency (PCE). For solar cells having nanostructures imprinted on the electron transport layer of ZnO, the PCE can increase from 2.39 to 2.65% for gratings or 3% for arrays of nanorods, respectively, and the fill factor can increase from 0.5 to 0.57 for arrays of nanorods. For solar cells having nanostructures of gratings imprinted on the active layer, the fill factor increases from 0.5 to 0.6, the current density increases from 9.02 mA/cm2 to 9.73 mA/cm2, and the PCE increases from 2.4 to 3.05%. The use of capillary force to imprint nanostructures eliminates the use of costly imprint equipments and possible damage of the active layer by contacting with air.