- Abstract We measure the diurnal dependence of the operating characteristics of tetraphenyldibenzoperiflanthene (DBP):C 70 planar-mixed heterojunction small-molecule organic photovoltaic (OPV) cells with 2,2′,2″-(1,3,5-benzenitryl tris-[1-phenyl-1 H -benzimidazole] (TPBi):C 70 electron-filtering cathode buffer layers. Over the course of a day, efficiency gradually increases as a result of a concomitant increase in short-circuit current, while the fill factor and open-circuit voltage remain constant. The results are analyzed on the basis of independent measurements of temperature- and intensity-dependent OPV performance. The power conversion efficiency is maximized slightly below 1 sun intensity and at 40 °C, which is beneficial for practical outdoor operation. We attribute the increased short circuit current with temperature to broadening of the absorption spectrum due to population of phonon states along with increased charge mobility, which also results in an increase in fill factor.